Abstract: Photomorphogenic responses triggered by low fluence rates of ultraviolet B radiation (UV-B; 280-315 nm) are mediated by the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8). Beyond our understanding of the molecular mechanisms of UV-B perception by UVR8, there is still limited information on how the UVR8 pathway functions under natural sunlight. Here, wild-type Arabidopsis (Arabidopsis thaliana) and the uvr8-2 mutant were used in an experiment outdoors where UV-A (315-400 nm) and UV-B irradiances were attenuated using plastic films. Gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation, and leaf metabolite signatures were analyzed. The results show that UVR8 is required for transcript accumulation of genes involved in UV protection, oxidative stress, hormone signal transduction, and defense against herbivores under solar UV. Under natural UV-A irradiance, UVR8 is likely to interact with UV-A/blue light signaling pathways to moderate UV-B-driven transcript and PDX1 accumulation. UVR8 both positively and negatively affects UV-A-regulated gene expression and metabolite accumulation but is required for the UV-B induction of phenolics. Moreover, UVR8-dependent UV-B acclimation during the early stages of plant development may enhance normal growth under long-term exposure to solar UV.
Abstract: Recent molecular and physiological studies have demonstrated that ultraviolet-B radiation (UV-B) can affect some of the processes involved in leaf growth, but the phases of leaf growth affected have not been clearly delimited. We used functional growth analysis to assess the effects of UV-B radiation on the time course of leaf growth in seedlings of two birch species (Betula pendula and Betula pubescens). Our aim was to identify the phase(s) of leaf development affected by UV-B radiation. In a greenhouse study, 1-year-old birch seedlings were subjected to three daily doses of supplemental UV-B radiation treatments (UV-Bâº) and no UV-B radiation controls (UV-Bâ»). Leaf growth measurements every 2 days were complemented by assessment of other functional traits over a 4-week period at the start of the growing season. Using fitted curves, we were able to determine that the rate of leaf expansion was slowed by the UV-B⺠treatment in leaves of B. pendula because of a slower maximum leaf growth rate compared with plants under the UV-Bâ» controls, but that compensation toward the end of the period of expansion negated this difference when leaves reached their final size. UV-B⺠had little effect on the rate of B. pubescens leaf growth despite a larger reduction in leaf final size due to UV-B⺠than occurred in B. pendula leaves. In conclusion, effective regulation ameliorated the effects of UV-B radiation on leaf and seedling growth in B. pendula, whereas in B. pubescens, reductions in leaf final size under UV-B⺠were consistent with a slightly reduced rate of height growth.
Abstract: Lolium perenne (cv. AberDart) was grown at 14 locations along a latitudinal gradient across Europe (37-68°N) to study the impact of ultraviolet radiation (UV) and climate on aboveground growth and foliar UV-B absorbing compounds. At each location, plants were grown outdoors for 5 weeks in a replicated UV-B filtration experiment consisting of open, UV-B transparent (cellulose diacetate) and UV-B opaque (polyester) environments. Fourier transform-infrared spectroscopy was used to compare plant metabolite profiles in relation to treatment and location. UV radiation and climatic parameters were determined for each location from online sources and the data were assessed using a combination of anova and multiple regression analyses. Most of the variation in growth between the locations was attributable to the combination of climatic parameters, with minimum temperature identified as an important growth constraint. However, no single environmental parameter could consistently account for the variability in plant growth. Concentrations of foliar UV-B absorbing compounds showed a positive trend with solar UV across the latitudinal gradient; however, this relationship was not consistent in all treatments. The most striking experimental outcome from this study was the effect of presence or absence of filtration frames on UV-absorbing compounds. Overall, the study demonstrates the value of an European approach in studying the impacts of natural UV across a large latitudinal gradient. We have shown the feasibility of coordinated UV filtration at multiple sites but have also highlighted the need for open controls and careful interpretation of plant responses.
Abstract: We measured the effect of elevated temperature, and its interaction with fruit load (exp. 1) and water supply (exp. 2), on the stomatal conductance (g(s)) of Vitis vinifera, cv. Shiraz. Thermal regimes (control vs. elevated temperature using open-top chambers) were initiated in spring, thus affecting leaf development, and were maintained during the whole growing season. We used a top-down approach where reaction norms were derived that relate gs for each treatment and the mean g(s) across treatments: the slopes of reaction norms quantify phenotypic plasticity. Stomatal conductance responded to neither the interaction between temperature and fruit load (P = 0.37) nor the interaction between temperature and water regime (P = 0.33). We therefore dealt with each factor separately. Reaction norms of g(s) under elevated temperature and control treatments diverged, i.e. elevated temperature had no effect on g(s) under conditions conducive to low conductance (below similar to 100 mmol m(-2) s(-1)) but increased g, in relation to controls under conditions favouring high conductance. Stomata density was 159 +/- 6.7 mm(-2), and was unaffected by temperature. Stomata length increased from 20.6 mu m in controls to 23.2 mu m in the heated treatment (P < 0.0001) and width increased from 14.4 mu m in controls to 16.8 mu m in the heated treatment (P < 0.0001). Thus, longer and wider stomata contributed to the enhanced plasticity of stomatal conductance under elevated temperature. Consistently, reaction norms of light-saturated photosynthesis were divergent, i.e. elevated temperature did not affect photosynthesis under conditions conducive to low g(s) and photosynthesis, but increased photosynthesis in relation to controls under more favourable conditions. A high source:sink ratio reduced the plasticity of stomatal conductance. Water regime had a minor effect on the plasticity of stomatal conductance, but the reaction norms for irrigated and water deficit treatments were offset by 60 mmol m(-2) s(-1) thus indicating a consistent effect of water deficit across environmental conditions. We conclude that the responses of stomata derived from transient temperature treatments in fully expanded leaves are unsuitable to model the dynamics of gas exchange in response to projected warming. Our experiments support a conceptual model where current stomatal conductance is a function of (i) the prevailing environmental conditions during early stages of leaf development that set the upper limit of conductance through the modulation of stomatal density and size, (ii) the current environmental conditions and (iii) the source:sink ratio. (c) 2012 Elsevier B.V. All rights reserved.
Abstract: Adansonia species, known by the common name â渂aobab,â have a very low regeneration rate in Madagascar. In order to determine if Malagasy Adansonia seedlingsâ vulnerability to drought may account for this low rate of regeneration, we compared growth, photosynthetic behavior and water use strategy of three species of Malagasy Adansonia (A. grandidieri, A. madagaseariensis, A. rubrostipa). Our results indicated that drought depressed the growth, net assimilation rate, stomatal conductance and transpiration rate of Adansonia seedlings but increased their water use efficiencies. Adansonia species are able to withstand drought by reducing water loss through stomatal closure and their ability to store water within roots. Interspecific differences were attributed to diversity in water-use strategies, relative water content and biomass allocation. A. rubrostipa and A. grandidieri appeared to be more adapted to arid environments than A. madagascariensis. Ecological implications of these results are discussed. (C) 2012 SAAB. Published by Elsevier B.V. All rights reserved.
Abstract: Limitations in the realism of currently available lamps mean that enhancement errors in outdoor experiments simulating UV-B radiation effects of stratospheric ozone depletion can be large. Here, we assess the magnitude of such errors at two Finnish locations, during May and June, under three cloud conditions. First we simulated solar radiation spectra for normal, compared with 10% and 20% ozone depletion, and convoluted the daily integrated solar spectra with eight biological spectral weighting functions (BSWFs) of relevance to effects of UV on plants. We also convoluted a measured spectrum from cellulose-acetate filtered UV-B lamps with the same eight BSWFs. From these intermediate results we calculated the enhancement errors. Differences between locations and months were small, cloudiness had only a minor effect. This assessment was based on the assumption that no extra enhancement compensating for shading of UV radiation by lamp frames is performed. Under this assumption errors between spectra are due to differences in the UV-B effectiveness rather than differences in the UV-A effectiveness. Hence, conclusions about plant growth from past UV-supplementation experiments should be valid. However, interpretation of the response of individual physiological processes is less secure, so results from some field experiments with lamps might need reassessment.
Abstract: The physiological mechanisms controlling plant responses to dynamic changes in ambient solar ultraviolet (UV) radiation are not fully understood: this information is important to further comprehend plant adaptation to their natural habitats. We used the fluorimeter Dualex to estimate in vivo the epidermal flavonoid contents by measuring epidermal UV absorbance (A(375) ) in Betula pendula Roth (silver birch) leaves of different ages under altered UV. Seedlings were grown in a greenhouse for 15 days without UV and transferred outdoors under three UV treatments (UV-0, UV-A and UV-A+B) created by three types of plastic film. After 7 and 13 days, Dualex measurements were taken at adaxial and abaxial epidermis of the first three leaves (L1, L2 and L3) of the seedlings. After 14 days, some of the seedlings were reciprocally swapped amongst the treatments to study the accumulation of epidermal flavonoids in the youngest unfolded leaves (L3) during leaf expansion under changing solar UV environments. A(375) of the leaves responded differently to the UV treatment depending on their position. UV-B increased the A(375) in the leaves independently of leaf position. L3 quickly adjusted A(375) in their epidermis according to the UV they received and these adjustments were affected by previous UV exposure. The initial absence of UV-A+B or UV-A, followed by exposure to UV-A+B, particularly enhanced leaf A(375) . Silver birch leaves modulate their protective pigments in response to changes in the UV environment during their expansion, and their previous UV exposure history affects the epidermal-absorbance achieved during later UV exposure.
Abstract: Fungi are usually thought not to have a boron (B) requirement. It is not known if mycorrhizas take up B from low concentrations that are common in forest soils, as fungi might also immobilise B. Here, we studied the B concentrations in sporophores of 49 ectomycorrhizal and 10 saprotrophic fungi to assess whether B is translocated in mycelium or not. Additionally, P and metal concentrations were measured for comparison.
Variability both within species and between species was very large, as the lowest measured B concentration was 0.01 mg kg-1 in Amanita muscaria, and the highest was 280 mg kg-1 in Paxillus involutus. No categorical difference was found between saprotrophic and mycorrhizal fungi. The majority of species did not accumulate B at more than 0.01-3 mg kg-1, but there were some species that consistently had median concentration values higher than 5-6 mg kg-1 and much higher maximum values, particularly Paxillus involutus, Lactarius necator and several Russula species. Most species increased their B concentration in B fertilised plots, but there were exceptions, particularly Rozites caperatus and Lactarius camphoratus. Boron concentrations did not correlate with those of other elements.
In conclusion, B is translocated in the mycelia of most of the studied species. The differences between species may be due to differences in their water use, or carbohydrates used in translocation. It remains to be studied, if B concentrations in mycorrhizas or mycelia in soil are in the same order of magnitude as the larger ones found here, and if this has any effects on the host plants.
Abstract: Abstract Adequate boron (B) nutrition may decrease concentrations of phenolic compounds and enhance structural integrity and lignification in plants, compared with suboptimal B. This could affect decomposition in areas where B deficiencies are common. The mass loss and changes in element concentrations in Norway spruce needle litter were studied with combinations of litter from high-B and low-B trees, incubated for 29 months, in either B fertilised or control plots without B addition. The litter originated from the same Norway spruce field experiments. Additionally, the field experiments included long-term N and P treatments. Initially, lowest lignin concentrations were found in Norway spruce litter from the treatment P and particularly in the combination Bâ+â塒, and highest in the Bâ+â塏 fertilised plots. The mass loss of Norway spruce litter was not affected by the treatments. However, Blitter increased Cu accumulation. The litter from the Bâ+â塒 fertilised plots accumulated considerably more Al, Ca, S and Zn than the other treatments, whereas B together with N reduced the remaining amounts of these elements. Reduced nutrient release from litter may have far-reaching consequences on nutrient cycles in forests.
Abstract: Abstract Inadequate boron (B) nutrition can affect the structural integrity and chemical composition of plant tissues. The changes in mass and element concentrations were studied using silver birch (Betula pendula Roth) leaf litter from seedlings grown with or without added B (Blitter+ or Blitterâ垝). The litter was produced in a growth room, and it was incubated in either B fertilised or control forest plots (Bsoil+ or Bsoilâ垝) between the moss and humus layers in two Norway spruce stands for 13 months. Additionally, the field decomposition experiment included long-term N and P application treatments (Nsoil and Psoil). Blitter+ somewhat reduced the remaining litter mass. In contrast, Bsoil+ increased it, possibly because of lower soil pH. The +Nsoil treatment reduced the remaining mass. Blitter+ increased the remaining P, S, Cu, Cd, Ni and Zn but reduced Pb. Remaining B was high in the Blitterâ which also accumulated B from soil. Bsoil increased remaining Ca, Cd, Mg, Na, Pb, and slightly reduced N (in N fertilised plots). These changes in decomposition and element release have a potential to affect nutrient, carbon, and heavy-metal cycles in areas where B deficiencies are common, and where B fertilisation is practised.
Abstract: Ultraviolet (UV) radiation is an important environmental factor for plant communities; however, plant responses to solar UV are not fully understood. Here, we report differential effects of solar UV-A and UV-B radiation on the expression of flavonoid pathway genes and phenolic accumulation in leaves of Betula pendula Roth (silver birch) seedlings grown outdoors. Plants were exposed for 30 days to six UV treatments created using three types of plastic film. Epidermal flavonoids measured in vivo decreased when UV-B was excluded. In addition, the concentrations of six flavonoids determined by high-performance liquid chromatography-mass spectrometry declined linearly with UV-B exclusion, and transcripts of PAL and HYH measured by quantitative real-time polymerase chain reaction were expressed at lower levels. UV-A linearly regulated the accumulation of quercetin-3-galactoside and quercetin-3-arabinopyranoside and had a quadratic effect on HYH expression. Furthermore, there were strong positive correlations between PAL expression and accumulation of four flavonols under the UV treatments. Our findings in silver birch contribute to a more detailed understanding of plant responses to solar UV radiation at both molecular and metabolite levels.
Abstract: To understand how plants respond to solar UV-B (280-315 nm) and UV-A (315-400 nm) radiation, wavelength specific studies must be performed. For a realistic assessment of these effects seasonal variations that can contribute to UV effects need to be taken into account. This has also implications to studies concerning stratospheric ozone depletion and resulting increased UV-B radiation. To this end, we established a field experiment using plastic films attenuating different parts of the solar UV spectrum. The concentration of individual phenolic compounds during one growing season in leaves of grey alder (Alnus incana) and white birch (Betula pubescens) trees was measured. Our results showed changes in concentration of e.g. hydrolyzable tannins in birch that suggest an effect of UV-A alone and e.g. chlorogenic acids in alder indicate a quadratic effect of UV-B irradiance and both linear and quadratic effect for UV-A in second degree polynomial fits. Further, there was interaction between treatment and sampling time for some individual metabolites, hence the UV response varied during the season. In addition to the UV effects, three temporal patterns emerged in the concentrations of particular groups of phenolics. Possible implications for both sampling methods and timing are discussed. Moreover, our results highlight differences in responses of the two tree species, which are taken to indicate differences in their ecological niche differentiation.
Abstract: <b><i>Background:</i></b> High nutrient supply has been frequently used in ultraviolet-B (UV-B) research, although it is unnatural for many species. <br /><br /><b><i>Aims:</i></b> To study the effect of increased irradiance of UV-B on the photosynthetic capability of silver birch (<i>Betula pendula</i> Roth.) seedlings. <br /><br /><b><i>Methods:</i></b> The seedlings were grown in a greenhouse in a substrate composed of birch forest top soil and sand, without additional supply of mineral nutrients. The increased daily integrated irradiance of UV-B (GPAS) was equivalent to 25% stratospheric ozone depletion under clear sky, and the treatment was started at seedling emergence. Leaf gas exchange was measured under saturating light conditions, and CO<sub>2</sub>-assimilation rate, stomatal conductance and intercellular CO<sub>2</sub> concentration were computed. Initial and total activity of RuBisCO and the concentrations of RuBisCO, chlorophylls, and soluble protein were measured in the same leaf. Total nitrogen concentration was also measured, as well as carbon isotope discrimination. <br /><br /><b><i>Results:</i></b> None of the variables measured were affected by UV-B at either of the two sampling dates (58 d and 76 d after the beginning of the treatment) except for stomatal conductance (interaction with time only). <br /><br /><b><i>Conclusions:</i></b> Consequently, it is unlikely that significant damage to photosynthesis of silver birch seedlings will occur in a natural setting under current and projected changes in solar UV-B.
Abstract: We examined the quality and decomposition of naturally abscised leaves of silver birch (<i>Betula pendula</i>) seedlings subjected to three different levels of fertilization under ambient and elevated levels of temperature and CO<sub>2</sub>. At the end of the second growing season, the chemical composition of the litter collected from the seedlings was analyzed. Whole-leaf samples from pooled litter from each of the four replicates from each treatment were put in mesh bags and transferred to ambient climate in the field. The remaining mass of litter was measured by sampling bags in May and October throughout the four-year incubation period. Fertilization with all nutrients decreased the initial carbon and tannin contents of litter, and increased the proportion of the fast-decomposing fraction, but still fertilization slowed down the decomposition of this fraction. Initially, the estimated proportion of the fast-decomposing fraction was smallest in elevated CO<sub>2</sub>Â +Â temperature, and largest in ambient climate. During decomposition, elevated growth-temperature slowed down decomposition of the fast fraction under ambient CO<sub>2</sub> but increased it under elevated CO<sub>2</sub>. The changes in litter decomposition rates found over four years were not very large. However, we conclude that the interactions of different factors lead to different results than if the factors had been studied separately, and future studies should take interactions into account.
Abstract: In research concerning stratospheric ozone depletion, action spectra are used as biological spectral weighting functions (BSWFs) for describing the effects of UV radiation on plant responses. Our aim was to evaluate the appropriateness of six frequently used BSWFs that differ in effectiveness with increasing wavelength. The evaluation of action spectra was based on calculating the effective UV radiation doses according to 1-2) two formulations of the generalized plant action spectrum, 3) a spectrum for ultraviolet induced erythema in human skin, 4) a spectrum for the accumulation of a flavonol in Mesembryanthemum crystallinum, 5) a spectrum for DNA damage in alfalfa seedlings and 6) the plant growth action spectrum. We monitored effects of UV radiation on the concentration of individual UV absorbing metabolites and chlorophyll concentrations in leaves and growth responses of silver birch (Betula pendula) seedlings. Experiments were conducted outdoors using plastic films attenuating different parts of the UV spectrum. Chlorophyll concentrations and growth were not affected by the UV treatments. The response to UV radiation varied between and within groups of phenolics. In general, the observed responses of phenolic groups and individual flavonoids were best predicted by action spectra extending into the UV-A region with moderate effectiveness.
Abstract: Solar ultraviolet (UV)-A and UV-B radiation were excluded from branches of grey alder (Alnus incana) and white birch (Betula pubescens) trees in a field experiment. Leaf litter collected from these trees was used in microcosm experiments under laboratory conditions. The aim was to evaluate the effects of the different UV treatments on litter chemical quality (phenolic compounds, C, N and lignin) and the subsequent effects of these changes on soil fauna and decomposition processes. We measured the decomposition rate of litter, growth of woodlice (Porcellio scaber), soil microbial respiration and abundance of nematodes and enchytraeid worms. In addition, the chemical quality of woodlice feces was analyzed. The exclusion of both UV-A and UV-B had several effects on litter chemistry. Exclusion of UV-B radiation decreased the C content in litter in both tree species. In alder litter, UV exclusion affected concentration of phenolic groups variably, whereas in birch litter there were no significant differences in phenolic compounds. Moreover, further effects on microbial respiration and chemical quality of woodlice feces were apparent. In both tree species, microbial CO(2) evolution was lower in soil with litter produced under exclusion of both UV-A and UV-B radiation when compared to soil with control litter. The N content was higher in the feces of woodlice eating alder litter produced under exclusion of both UV-A and UV-B compared to the control. In addition, there were small changes in the concentration of individual phenolic compounds analyzed from woodlice feces. Our results demonstrate that both UV-A and UV-B alter litter chemistry which in turn affects decomposition processes.
Abstract: We measured the concentrations of ultraviolet (UV)-absorbing phenolics varying
in response to exclusion of either solar UV-B or both solar UV-A and UV-B radiations in leaves of grey alder (Alnus incana) and white birch (Betula pubescens) trees under field conditions. In alder leaves 20 and in birch leaves 13 different phenolic metabolites were identified. The response to UVexclusion varied between and within groups of phenolics in both tree species. The changes in concentration for some metabolites suggest effects of only UV-A or UV-B, which band being effective depending on the metabolite. For some other metabolites, the results indicate that UV-A and UV-B affect concentrations in the
same direction, while for a few compounds there was evidence suggesting opposite effects of UV-A and UV-B radiation. Finally, the concentration of some phenolics did not significantly respond to solar UV. We observed only minor effects on the summed concentration of all determined phenolic metabolites in alder and birch leaves, thus indicating that measuring only total phenolics concentration may not reveal the effects of radiation. Here, we show that the appropriate biological spectral weighting functions for plant-protective responses against solar UV radiation extend in most cases â but not always â into the UV-A region and more importantly that accumulation of different phenolic metabolites follows different action spectra. This demonstrates under field conditions that some of the implicit assumptions of earlier research simulating ozone
depletion and studying the effects of UVradiation on plant secondary metabolites need to be reassessed.
Abstract: Abstract Loss of apical dominance is a well-known boron (B) deficiency symptom in trees. Recent field studies indicate that B deficiency may cause irreversible damage in emerging leader buds leading to bushy growth, and changes in developing needles in mature Norway spruce trees. We experimentally studied if timing of B application affects needles and buds of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings with low initial B levels. The treatments were: no B (B0); B supply from the beginning of the simulated summer (B1); starting soon after bud burst (B2) and starting at the occurrence of first needle primordia in new spruce buds (B3). At the end of the experiment, B concentration in B1 was 23 mg kgâÂÂ1 (pine) or 17 mg kgâÂÂ1 (spruce) and lower in the later applications. In B0 it was at deficiency limit. In B0, B2 and B3, there were fewer sclerenchyma cells, and cavities occurred in vascular cylinders in pine needles, and in spruce buds there were more tanniferous cells in the primordial shoots compared to B1. Furthermore, in all but B1 there was cell collapse in the bud apex of some spruce seedlings. The experimentally induced changes were the same as earlier reported in B deficient conifers in the field, and indicate, similarly as in the field that adequate B is necessary throughout the growing season for healthy growth, particularly for spruce. The differences between spruce and pines are due, at least partly, to the differences in time frame of needle development and in the differences in development of conducting tissues in the buds.
Abstract: Rainbow trout, Oneorhynchus mykiss, were infected concomitantly with Argulus coregoni and Flavobacterium columnare and their survival was compared with that of fish infected with either the parasite or the bacterium alone. The mortality of fish challenged with A. coregoni was negligible while infection with F columnare alone led to significantly lower survival. However, compared with single infections, the mortality was significantly higher and the onset of disease condition was earlier among fish, which were concomitantly infected by A. coregoni and F columnare. This data presents, for the first time, experimental support for the hypothesis that an ectoparasite infection increases susceptibility of fish to a bacterial pathogen. (c) 2006 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Silver birch (Betula pendula Roth) seedlings were grown individually in containers arranged in rows radiating from a central point (Nelder plot) at densities spanning the range from 207 to 891 plants m(-2). Height of one set of seedlings was measured at weekly intervals and additional seedlings were harvested each week for dry mass and leaf area measurements. Height and shoot:root dry mass ratio increased with plant density. Seedling-to-seedling variability in dry mass, but not height, increased with increasing plant density. The red to far-red (R:FR) photon ratio in horizontally propagated radiation decreased with increasing density, even when plant densities and leaf area index values were low. In a separate experiment, elongating internodes of seedlings were irradiated locally by red and far-red light emitting diodes and stem elongation measured. Far-red light markedly increased stem elongation, suggesting that changes in light quality sensed by growing internodes are involved in the observed responses to growth density.
Abstract: Two-year-old silver birch (Betula pendula) saplings were grown for a third growing season in controlled-environment rooms (dasotrons) at three soil temperatures (5, 10, and 20 °C). All trees grew the first flush of leaves, but the growth of the second flush was almost completely inhibited at the two lower temperatures. The dry weight of the second-flush leaves was 50 times larger at 20 °C than at 5 and 10 °C, with about 100 times more nitrogen. Root growth was less affected than shoot growth. Chlorophyll content, net assimilation rate and stomatal conductance were lower at low soil temperatures. The value of the cytoplasm resistance estimated from the electric impedance spectra was lower at 5 °C than at 10 or 20 °C. Leaf water potential was highest at the lowest soil temperature, and intercellular carbon dioxide concentration was only slightly lower in saplings growing in cooler soil. We conclude that the effect of long-term exposure to cold soil on net assimilation and growth was not caused by stomatal closure alone. It is likely to be additionally mediated by the limited nitrogen acquisition at the low soil temperatures, and perhaps additionally by some other factor. As the growth depression of aboveground parts in response to low soil temperature was more significant in silver birch than what has earlier been found in conifers, the relative changes in air and soil temperature may eventually determine whether birch will become more dominant in boreal forests with climate change.
Abstract: The effects of two boron (B) levels on growth, shoot water potential, gas exchange and nutrient accumulation in Norway spruce [Picea abies (L.) Karst.] seedlings were studied in a growth room experiment lasting 22 weeks which included well-watered control seedlings and seedlings exposed to one (8 days) or two (6+8 days) periods of drought and a rewatering period (8 days) at the end of the experiment. The effects of B and drought were monitored during drought and recovery. Needle B concentrations were 6 mg kg(-1) (-B treatment) and 34 mg kg(-1) (+B treatment) at the end of the experiment. The -B seedlings showed visible symptoms of damage in the upper shoot after repeated drought and had reduced height growth, root dry mass, allocation of biomass to roots and formation of root tips and mycorrhizas and reduced needle P, Ca, and Mg concentrations and contents. In contrast, N-15 uptake, shoot water potential and gas exchange were not markedly affected by B. It can be concluded that the visible symptoms of damage at low B were probably related to reduced B transport due to repeated drought. In contrast, the effects of low B on growth, particularly of the roots, and on nutrient uptake can be regarded as early effects which occur before any influence on shoot water potential or gas exchange. The positive effects of B on root biomass and nutrient accumulation are of particular importance regarding the establishment of young seedlings in the field.
Abstract: Reflection by waxy or resinous surface structures and hairs, repair reactions of biomolecules and induction of different sheltering components provide the means of plant protection from harmful solar UV-B radiation. Secondary products, especially flavonoids and phenolic acids as defense components are also important in plant tolerance to UV-B, fulfilling the dual role as screens that reduce UV-B penetration in plant tissues, and as antioxidants protecting from damage by reactive oxidant species. Plants are sensitive to UV-B radiation, and this sensitivity can be even more clone-specific than species- specific. The results available in the literature for deciduous trees and shrubs indicate that UV-B radiation may affect several directions in the interaction of woody species with biotic (herbivores) and abiotic (CO, and nutrition) factors depending on the specific interaction in question. These multilevel interactions should have moderate ecological significance via the overall changed performance of woody species and shrubs. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Soil temperature is a main factor limiting root growth in the boreal forest. To simulate the possible soil-warming effect of future climate change, 5-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were subjected to three simulated growing seasons in controlled environment rooms. The seedlings were acclimated to a soil temperature of 16 degreesC during the first (GSI) and third growing seasons (GSIII), but were assigned to random soil-temperature treatments of 9, 13, 18 and 21 degreesC during the second growing season (GSII). In GSII, shoot diameter growth was lowest in the 21 degreesC treatment and root growth was lowest in the 9 degreesC treatment. In GSIII, shoot height and root length growth improved in seedlings that had been kept at 9 degreesC during GSII, indicating compensatory growth in response to increased soil temperature. The temporary decrease in soil temperature had no long-lasting significant effect on seedling biomass or total nutrient uptake. At the end of GSIII, fine roots of seedlings exposed to a soil temperature of 21 degreesC in GSII were distributed more evenly between the organic and mineral soil layers than roots of seedlings in the other treatments. During GSII and GSIII, root growth started earlier than shoot growth, decreased during the rapid shoot elongation phase and increased again as shoot growth decreased.
Abstract: We previously traced B-10-enriched boric acid from shoots to roots to demonstrate the translocation of boron (B) in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings. To gain a more detailed understanding of B translocation, we sought: (1) to demonstrate B retranslocation directly, by showing that foliar-applied B-10 is located in the new growth after dormancy; and (2) to assess whether shoot-applied B affects growth in the long term. We applied B-10-enriched boric acid to needles of Scots pine and Norway spruce seedlings. After a dormancy period and 9 weeks of growth, small but significant increases in the B-10 isotope were found in the new stem and needles of both species. In Scots pine, the total B concentration of the new stem was also increased. Both species contained polyols, particularly pinitol and inositol. Boron-polyol complexes may provide a mechanism for mobilizing B in these species. To determine the long-term effects of applied B, seedlings were grown for two growing seasons after the application of B-10 to shoots. In Norway spruce, the proportion of B-10 in the root systems and current needles of the harvest year was slightly higher than in the controls, and in Scots pine root systems, marginally so. The B treatment had no effect on growth of Norway spruce seedlings. In Scots pine seedlings, the B treatment caused a 33% increase in total dry mass and significantly increased the number of side branches.
Abstract: Boron (B) is an essential micronutrient for plants but it is thought not to be essential for fungi. We studied whether the extraradical mycelia of Paxillus involutus in symbiosis with silver birch (Betula pendula) take up B and transport it to the host plant. We grew mycorrhizal plants in flat microcosms with a partitioning wall, below which there was only extraradical mycelium. A boric acid solution enriched in B-10 was applied to these mycelia. Increased B-10/B-11 isotope ratios were subsequently measured in birch leaves, stems, and roots plus mycorrhizas in the upper compartment. Boron was therefore taken up by the mycorrhizal mycelia and transported to the host plant in this species combination.
Abstract: Boron (B) has been found to be phloem mobile in species that translocate polyols, whereas it is almost immobile in other species. The objectives of the present study were to survey B mobility in deciduous trees, and to relate it to the presence of polyols. The stable isotope B-10 was applied as a tracer to mature leaves of seedlings, and growing leaves were subsequently harvested for B isotope analysis. Extensive B mobility was found in Sorbus aucuparia and Prunus padus, species with high sorbitol content, but also in Ulmus glabra, with only trace amounts of B-complexing polyols. Alnus incana, Fraxinus excelsior, Betula pubescens and Larix sibirica also translocated B-10 into new leaves. Mannitol in Fraxinus and pinitol in Larix probably explain this. A. glutinosa did not remobilize B, although the polyol concentrations were almost identical to A. incana, a closely related species. B mobility was not as closely related to the presence of polyols as expected, and it appears that to some degree remobilization occurs in many plant species. (C) New Phytologist (2004).
Abstract: The natural variation in quantity and quality of light modifies plant morphology, growth rate and concentration of biochemicals. The aim of two growth-room experiments was to study the combined effects of red (R) and far-red (FR) light and ultraviolet-B (UV-B) radiation on the concentrations of leaf phenolics and growth and morphology of silver birch (Betula pendula Roth) seedlings. Analysis by high-performance liquid chromatography showed that the leaves exposed to supplemental FR relative to R contained higher concentrations of total chlorogenic acids and a cinnamic acid derivative than the leaves treated with supplemental R relative to FR. In contrast, concentration of a flavonoid, quercetin 3-galactoside, was higher in the R + UV-B leaves than in the FR + UV-B leaves. The UV-B induced production of kaempferols, chlorogenic acids and most quercetins were not modified by the R :FR ratio. Growth measurements showed that the leaf petioles and stems of FR seedlings were clearly longer than those of R seedlings, but leaf area was reduced by UV-B radiation. Results of these experiments show that exposure of silver birch seedlings to supplemental FR compared to R leads to fast elongation growth and accumulation of phenolic acids in the leaves.
Abstract: We examined the effects of solar ultraviolet-B (UV-B) radiation on plant-insect interactions in Tierra del Fuego (55 degrees S), Argentina, an area strongly affected by ozone depletion because of its proximity to Antarctica. Solar UV-B under Nothofagus antarctica branches was manipulated using a polyester plastic film to attenuate UV-B (uvb-) and an Aclar film to provide near-ambient UV-B (uvb+). The plastic films were placed on both north-facing (i.e., high solar radiation in the Southern Hemisphere) and south-facing branches. Insects consumed 40% less leaf area from north- than from south-facing branches, and at least 30% less area from uvb+ branches than from uvb- branches. The reduced herbivory on leaves from uvb+ branches occurred for both branch orientations. Leaf mass per area increased and relative water content decreased on north- versus south-facing branches, while no differences were apparent between the UV-B treatments. Solar UV-B did lead to lower gallic acid concentration and higher flavonoid aglycone concentration in uvb+ leaves relative to uvb- leaves. Both the flavonoid aglycone and quercetin-3-arabinopyranoside were higher on north-facing branches. In laboratory preference experiments, larvae of the dominant insect in the natural community, Geometridae "Brown" (Lepidoptera), consumed less area from field-grown uvb+ leaves than from uvb- leaves in 1996-97, but not in 1997-98. Correlation analyses suggested that the reduction in insect herbivory in the field under solar UV-B may be mediated in part by the UV-B effects on gallic acid and flavonoid aglycone.
Abstract: The effects of increased ultraviolet-B (UV-B) radiation on the growth, mycorrhizas and mineral nutrition of silver birch (Betula pendula Roth) seedlings were studied in greenhouse conditions. Seedlings-planted in a birch-forest top soil and sand substrate-were grown without additional nutrient supply. Ultraviolet treatment started immediately after the seedlings emerged and the daily integrated biologically effective UV-B irradiance on the UV-B-treated plants was equivalent to a 25% depletion of stratospheric ozone under clear sky conditions. Visible symptoms of UV-B damage or nutrient deficiency were not observed throughout the experiment. Seedling height and dry weight (DW) (measured after 58 days and 76 days of treatment) were not affected by increased UV-B. However, a significant shift in DW allocation toward roots resulted in a lower shoot/root ratio and leaf area ratio in UV-B-treated plants compared to control plants. At the first harvest (after 58 days of treatment), the percentage of various mycorrhizal morphotypes and the number of short roots per unit of root length or weight were not affected by increased UV-B despite significantly increased DW allocation toward roots. Subtle reduction in the allocation of nitrogen (N) to leaves and increased allocation of phosphorus (P) to roots may suggest cumulative effects that could affect the plant performance over the long-term.
Abstract: In this study, the effects of elevated ultraviolet-B (UV-B, 280-320 nm) radiation on growth and leaf phenolics were evaluated in clones of dark-leaved willow (Salix myrsinifolia Salisb.) and tea-leaved willow (Salix phylicifolia L.). Willows were raised for one growing season in an irradiation field, where they were exposed either to a constant 50% increase in UV-B-CIE radiation simulating 20-25% ozone depletion or to a small increase in UV-A radiation (320-400 nm). Control willows were grown in the irradiation field under solar radiation (ambient control). Despite the high constitutive concentrations of a UV-absorbing leaf flavonoid, dihydromyricetin, UV-treatments clearly reduced the biomass and height growth of the shoots of one tea-leaved willow clone. In contrast, the growth of three other tea-leaved willow clones showed no significant sensitivity to UV-radiation. Under elevated UV-B radiation, the leaves of these clones were able to accumulate some of the UV-B-absorbing quercetins, myricetins or luteolins. In dark-leaved willows, biomass production and growth were not decreased by UV-exposures, although the concentrations of leaf flavonoids were clearly lower than those in tea-leaved willows; however, in all the dark-leaved willow clones, elevated UV-radiation increased the concentrations of certain quercetins, dihydromyricetin and phenolic acids. Other willow leaf phenolics, i.e. salicylates, condensed tannins and gallic acid derivatives, were either decreased by the UV-treatments or were unaffected. The results indicate that (1) the constitutive level and quality of secondary chemicals in native willow species or clones does not predict their sensitivity to elevated UV-radiation, (2) secondary chemical responses to UV-radiation in willows are more clone-specific than species-specific and (3) the leaves of field-grown willows treated with UV-B radiation accumulate only those phenolics that screen UV-B efficiently.
Abstract: We studied the effects of elevated ultraviolet-B radiation on interactions between insect herbivores and their host plants by exposing two species of phytochemically different willows, Salix myrsinifolia and S. phylicifolia, to a modulated increase in ultraviolet radiation in an outdoor experiment and monitoring the colonisation of insect herbivores on these willows. We examined the effect of increased ultraviolet-B (UV-B) radiation on (1) the quality of willow leaves, (2) the distribution and abundance of insect herbivores feeding on these willows, (3) the resulting amount of damage, and (4) the performance of insect larvae feeding on the exposed plant tissue. Six clones of each of the two willow species were grown in eight blocks for 12 weeks in the UV-B irradiation field. The clones were exposed to a constant 50% increase in UV-Bradiation (simulating 20-25% ozone depletion), to a small increase in UV-A radiation or to ambient solar irradiation. We allowed colonisation on the willows by naturally occurring insects, but also introduced adults of a leaf beetle, Phratora vitellinae, a specialist herbivore on S. myrsinifolia. Increased UV-B radiation did not affect any of the measured indices of plant quality. However, numbers of P. vitellinae on S. myrsinifolia were higher in plants with UV-B treatment compared with UV-A and shade controls. In laboratory tests, growth of the second-instar larva of P. vitellinae was not affected by UV-B treatment of S. myrsinifolia, but was retarded on UV-B treated leaves of S. phylicifolia. In addition, naturally occurring insect herbivores were more abundant on willows exposed to elevated UV-B radiation compared to those grown under control treatments. In spite of the increased abundance of insect herbivores, willows treated with elevated UV-B did not suffer more herbivore damage than willows exposed to ambient solar radiation (shade control). The observed effects of UV-B on herbivore abundance, feeding and growth varied significantly due to spatial variation in environment quality, as indicated by the UV-treatment x block interaction. The results suggest that (1) environmental variation modifies the effects of UV-B radiation on plant-insect interactions and (2) specialist herbivores might be more sensitive to chemical changes in their secondary host plants (S. phylicifolia) than to changes in their primary hosts (S. myrsinifolia).
Abstract: The effects of boron (13) fertilizer applied 10 growing seasons earlier were studied in mature Norway spruce (Picea abies (L.) Karst.) trees in long-term factorial fertilization experiments at two field sites. Needle nutrient status, above-ground and below-ground growth and delta(13)C and carbon concentrations in the annual rings were measured. Needle B concentrations varied between 4 and 19 mg kg(-1) on the plots that had not received B fertilization. On the B-fertilized plots they varied between 15 and 39 mg kg(-1). The lowest B concentrations were on the plots that had received N or NCa fertilization. Needle Mn and Zn concentrations were lower on the B plots than on the plots that had not received B fertilization, although not significantly. Mean annual volume growth was slightly higher on the B plots at the more fertile site, but not at the less fertile one. The living:dead fine root mass ratio and living fine root length were also higher on the B-fertilized plots than on the unfertilized plots, but delta(13)C was not significantly affected, suggesting that the water status of the trees was not markedly altered by the increase in root growth. The carbon concentration in the annual rings was higher in the B-fertilized trees than in the unfertilized ones, suggesting the importance of B for wood formation. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: Silver birch seedlings were grown in two types of trays differing in cell volume. For each kind of tray, three different spacings were achieved by growing seedlings in all, half or one quarter of the cells. The planting-stock so produced was out-planted at two forest sites and growth followed for five years. Container size had a large effect on seedling morphology and on field performance. Density also had a large effect on seedling morphology, but little influence on performance. The tightest relationship of field performance was to the mean dry weight of the stems of the planting stock (R-2 = 0.94). Mean height and â檚turdinessâ index, defined as diameter divided by height, were not correlated with performance. Of the non-destructive measurements, mean root-collar diameter was the most closely correlated with performance.
Abstract: Growth and pigment composition of plants can be affected by both UV-radiation and availability of nutrients. Therefore, the UV-induction in a plant as a protective response against UV-B radiation may depend on the UV-B dose encountered and the nutritional state of a plant. To test this, young Scots pine (Pinus sylvestris) seedlings were grown under two nutrient levels (relative nutrient addition fates, RAR, of 4 and 6%) and exposed to six different daily doses of UV-radiation (UV-A and UV-B) for one growing season. Both the growth and the accumulation of photosynthetic pigments were positively affected by the availability of nutrients, while the accumulation of condensed tannins and its precursor, (+)-catechin decreased significantly under higher nutrient level, which may imply some trade-off between primary and secondary metabolites. However, nutrient level did not markedly affect the total accumulation of flavonols. Since there was no decrease in the growth of the seedlings and the content of photosynthetic pigments in needles under increased UV-B levels, the seedlings had apparently good protection against UV-B radiation. Protection was probably dependent on the UV-induction of flavonoid compounds in needles, which differed between the two nutrient levels used. Under moderate nutrient availability (RAR 4%), the accumulation of flavonols and also chlorophylls was highest at the ambient or near to ambient levels of UV-radiation, the levels that pines are generally adapted in natural conditions. As the availability of nutrients and thereby, the resources in seedlings increased (RAR 6%), the UV-B levels higher than ambient increased specifically the accumulation of diacylated flavonols when dose responses were detected. Furthermore, the enhancement of UV-A radiation caused an accumulation of the dihydroflavonols, ampelopsin and taxifolin, under high nutrient availability. Since the flavonoid responses were very compound-specific, and in some cases were also dose-dependent and different between the two nutrient levels used, it can be suggested that the induction of secondary compounds in the flavonoid pathway is multi-step regulated by both environmental factors, UV-radiation and nutrients. Already the present variation in the ambient range of UV-B radiation at northern latitudes can influence the chemical composition of Scots pine seedlings and enhancement in nutrients may increase their responsiveness, particularly to high UV-intensities. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Long-term outdoor experiments were conducted to investigate the effects of elevated ultraviolet-B (UV-B, 280-320 nm) radiation on secondary metabolites (phenolics and terpenoids) and the main soluble sugars (sucrose, raffinose and glucose) in the bark of silver birch (Betula pendula Roth) saplings. Saplings were exposed to a constant 50% increase in erythemal UC irradiance (UV-B-CIE; based on the CIE (International Commission on Illumination) erythernal action spectrum) and a small increase in UV-A radiation (320-400 nm) for three growing seasons in an irradiation field in central Finland. Two control groups were used: saplings exposed to ambient radiation and saplings exposed to slightly increased UV-A radiation. Concentrations of sucrose, raffinose and glucose in bark were higher in UV-treated saplings than in saplings grown in ambient radiation, indicating that stem carbohydrate metabolism was changed by long-term elevated UV radiation. Saplings in the elevated UV-A + UV-B radiation treatment and the UV-A radiation control treatment had significantly increased concentrations of certain UV-absorbing phenolics, such as salidroside, 3,4â-dihydroxypropiophenone-3-glucoside, (+)-catechin and (-)-epicatechin compared with saplings in ambient radiation. In contrast, the radiation treatments had no effect on the non-UV-B-absorbing terpenoids, papyriferic acid and deacetylpapyriferic acid. We conclude that plant parts, in addition to leaves, accumulate specific phenolic UV-filters in response to UV radiation exposure.
Abstract: Nutrient conservation is considered important for the adaptation of plants to infertile environments. The importance of leaf life spans in controlling mean residence time of nutrients in plants has usually been analyzed in relation to nutrients that can be retranslocated within the plant. Longer leaf life spans increase the mean residence time of all mineral nutrients, but for non-mobile nutrients long leaf life spans concurrently cause concentrations in tissues to increase with leaf age, and consequently may reduce non-mobile nutrient use efficiency. Here we analyze how the role of leaf life span is related to the mobility of nutrients within the plant. We use optimality concepts to derive testable hypotheses, and preliminarily test them for boron (B), a nutrient for which mobility varies among plant species. We review published and unpublished data and use a simple model to assess the quantitative importance of B retranslocation for the B budget of mature conifer forests and as a mechanism for avoiding toxicity.
Abstract: Applications of growth retardants, daminozide and CCC (chlormequat chloride), were studied in order to determine their effects on morphology and post-planting growth of silver birch (Betula pendula Roth) container seedlings. Daminozide was sprayed once (on 28 June at concentrations of 1.0 - 6.0 g l(-1)) and CCC was sprayed twice (on 29 June and 27 July at concentrations of 0.5-3.0 g l(-1)). Height growth, morphology of seedlings at the end of the growing season, the shoot and root growth potential the following spring and field performance during the following seasons were measured. During the first summer in the nursery, both daminozide and CCC retarded height growth, but daminozide was more effective. The effects of compounds on stem diameter during the summer of application were small. Neither of the compounds affected the field performance of seedlings. The most suitable applications for retardation of height growth, without negative effects on other morphological variables, were 4 g l(-1) (32 mg per seedling) for daminozide and 2 g l(-1) (16 mg per seedling), sprayed in two applications, for CCC.
Abstract: Five-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were subjected to three simulated growing seasons in controlled environment chambers. Plants were acclimated to a soil temperature of 16 degreesC during the first and third growing seasons, but were allocated at random to soil temperature treatments of 9, 13, 18 and 21 degreesC during the second growing season. Low soil temperature during the second growing season depressed stomatal conductance and photosynthetic rate (A) per unit of projected leaf area, although intercellular CO2 concentrations did not differ significantly between treatments. At all soil temperatures, total chlorophyll concentration first decreased and then increased, although the rate of increase and the final concentration increased with soil temperature, which may explain the effect of soil temperature on A. Neither chlorophyll a/b ratio nor leaf nitrogen concentration was significantly affected by soil temperature. Treatment differences disappeared during the third simulated growing season when plants were again acclimated to a soil temperature of 16 degreesC.
Abstract: The effects of three boron levels on the growth dynamics and ectomycorrhizas of seedlings of Norway spruce [Picea abies (L.) Karst.] were studied in a growth room experiment. The seedlings were grown in forest humus mixed with quartz sand for 16 weeks. The B treatment was applied in the nutrient solution. Stem height, dry weight, number of root tips, mycorrhizas as well as B and N concentrations in the seedlings were monitored in sequential harvests. By the last harvest, in week 16, needle B concentrations were 6.6 mg kg(-1) at the lowest B level and 17.5 mg kg(-1) and 26.5 mg kg(-1) at the two higher levels. Boron slightly increased the stem hei-ht and the total dry weight, but did not affect N content of the seedlings. Low internal B reduced the number of root tips and mycorrhizas as well as mycorrhizal percentage and root dry weight, which indicates the importance of B for root growth in Norway spruce seedlings. The seedlings crown with adequate internal B had more root tips than those receiving the two lower B treatments as early as week 9, when needle B concentrations at the lowest B supply were 16.0-17.3 mg kg(-1), which has previously been considered a sufficient B level. Therefore, the critical needle B concentrations should perhaps be re-examined.
Abstract: Norway spruce (Picea abies (L.) Karst.) seedlings fertilized with boron (B) at three rates were grown in a 3:7 mix of forest humus and quartz sand for 18 weeks. Half of the seedlings were not watered during the last 9 days of the experiment (drought treatment). The role of B in drought resistance was assessed by comparing the effects of seedling internal B concentration on the water relations, photosynthesis, growth and nutrition of well-watered and drought-treated seedlings. At the end of the drought treatment, needle B concentrations were 7.0 mg kg(-1) at the lowest B supply rate and 17.5 and 23.5 mg kg(-1), respectively, at the higher supply rates. Seedlings at the lowest B supply rate had fewer root tips and mycorrhizas than seedlings at the higher B supply rates. Drought treatment had a more pronounced effect on the water relations and net photosynthetic rate of seedlings than B treatment. Although seedlings at the higher B supply rates lost water more rapidly than seedlings at the low B supply rate-leading to faster stomatal closure and decreased photosynthesis-drought did not affect their final height, whereas drought reduced height growth of seedlings at the low B supply rate.
Abstract: The effects of lateral far-red light (FR) and nutrient supply on the growth and nitrogen accumulation of silver birch (Betula pendula) seedlings were studied with the objective of testing the following hypotheses: (1) silver birch seedlings grow taller in response to increased FR irradiance received from the side; (2) this response is modulated by the nutritional status of the seedlings; and (3) an increase in lateral FR irradiance, and concomitant decrease in red to far-red photon ratio, affects the carbon and nitrogen economies of the seedlings. Two factorial experiments, each with two levels of mineral nutrient availability and two light treatments (background â檞hiteâ light with and without additional lateral FR simulating light reflected by neighbours in a sparse canopy) were done with small seedlings. The two experiments differed in PAR irradiance. The results of these experiments were that (1) stem elongation rate was increased by lateral FR addition, (2) there was no interaction between this effect and the nutritional status of the seedlings, and (3) neither whole plant accumulation of nitrogen nor dry weight increment was affected by lateral FR under either mineral nutrient supply regime.
Abstract: The effects of long-term elevated UV-B radiation on silver birch (Betula pendula Roth) seedlings were studied over three growing seasons in an outdoor experiment in Finland started 64 days after germination. One group of seedlings was exposed to a constant 50% increase in UV-B-CIE radiation, which corresponds to 20-25% of ozone depletion; another group received a small increase in UV-A radiation and a third (the control group) received ambient solar radiation. Changes in growth appeared during the third growing season; the stems of the UV-B treated seedlings were thinner and their height tended to be shorter compared with that of the control seedlings. In contrast, there were no UV-B effects on biomass, bud burst, bud dry weights, leaf area, rust frequency index or chlorophyll concentrations in any of the summers. During the three-year study, the flavonols were significantly increased by the elevated UV-B only in the first growing season. The responses varied greatly among individual compounds; the most induced were the quercetin glycosides, while the main flavonols, myricetins, were reduced by the UV-A control treatment. In the second summer phenolic acids, such as 3,4â-dihydroxypropiophenone-3-glucoside, neochlorogenic acid and 5-coumarylquinic acid, were increased by the UV-B treatment. In the third year, the constitutive concentrations of phenolics were not affected by the UV-B treatment.
Abstract: A new, controlled, environment facility for growing trees was built at Joensuu, Finland, between 1996 and 1998. It consists of four large rooms called dasotrons, with four large root pots in each. Each room is a separate unit, with independent control of air and soil temperature, air humidity and light. The environmental variables can be controlled to simulate conditions ranging from tropical to boreal. The controller set-points can be programmed locally or through a central control system running on a PC. The floor area and height of the rooms allows us to grow small trees (up to 3.7 m height) for several growing seasons. In each dasotron, there are four cylindrical pots with a removable upper section. There are access holes in the walls of the pots for the installation of sensors and minirhizotron tubes. Each pot has a drain, with valves, at the bottom to enable the removal of excess water or the collection of percolate samples. The operation of the facility was tested during one simulated annual growing cycle. During this test period, the dasotrons worked reliably and no systematic differences were found in the environmental conditions or in the growth of Norway spruce seedlings between the dasotrons. This new facility will enable diverse physiological and ecophysiological studies to be carried out on the responses of trees to their below- and above-ground environment.
Abstract: Changes in secondary metabolites in silver birch (Betula pendula) seedlings are presented in response to ultraviolet-B radiation (UV-B) and nutrient addition. Concentrations of individual secondary metabolites and nutrients were measured in leaves of greenhouse-grown silver birch seedlings exposed to five time-integrated irradiances of biologically effective UV-B (I-BE(UV-B)) and fertilized with two relative nutrient addition rates. Several phenolics were not only induced by UV-B, but their concentration was dependent on UV-B daily time-integrated irradiance. Relative nutrient addition rate also affected the concentration of phenolics but had little effect on the UV-B daily time-integrated irradiance-responses. The mineral nutrient concentration of leaves was affected by nutrient addition but not by increasing I-BE(UV-B). Possible threshold-exposures for the accumulation of secondary metabolites or nutrients were not detected. UV-B and relative nutrient addition rate have mainly an additive (rather than synergistic) effect in silver birch, and leaves respond to increasing UV-B by synthesizing metabolites (e.g. flavonols), which are important UV-B filters. This study reports the first UV-B-exposure-response curves for individual secondary metabolites and nutrients in leaves of a woody tree species.
Abstract: In plants, the allocation of carbon to secondary metabolites has been shown to be determined by both the availability of resources (e.g., CO2 concentration) and by specific stress factors (e.g., ultraviolet [UV]-radiation). It has been suggested that, in combination, CO2 and UV-B radiation may differentially affect plant growth and morphogenic parameters, and elevated CO2 may ameliorate the effects of UV-B radiation, In the present work, the effects of increased atmospheric CO2 concentration and UV-B radiation on growth and the accumulation of different types of secondary metabolites were studied in silver birch (Betula pendula Roth), Seedlings were exposed to 350 and 700 pmol mol(-1) of CO2 in a greenhouse. At both CO2 levels, additional UV-B was either present (8.16 kJ m(-2) day(-1) of biologically effective UV-B irradiance) or absent. The time course of accumulation of individual secondary compounds and the shifts in allocation of carbon between biomass and the secondary metabolites (phenolic acids, flavonoids, condensed tannins) were studied during a 1-month-long exposure. Additionally, the activities of enzymes (L-phenylalanine ammonia-lyase [PAL], EC 4.3.1.5; peroxidase, EC 1.11.1.7; polyphenol oxidase, EC 1.10.3.1) were determined for leaves, UV-B radiation significantly increased biomass, PAL activity, and the accumulation of phenolic acids and flavonoids in seedlings. Elevated CO2 concentration increased the activities of all the enzymes studied and the accumulation of condensed tannins in leaves, especially with UV-B radiation. Because the observed UV-B induction of flavonoids was smaller under a high CO2 concentration, it was suggested that the excess of carbon in the atmosphere may moderate the effect of UV-B by increasing the metabolic activity of leaves (high enzyme activities) and by changing the allocation of internal carbon between different primary and secondary metabolites in the plant. Our results demonstrate the significant increase in the allocation of carbon to secondary metabolites without any large change in growth due to the elevation of CO2 concentration and UV-B radiation. There also was a stronger impact of CO2 than UV-B on the phenolic metabolism of birch seedlings.
Abstract: In contrast to earlier beliefs, it is now known that baron (B) can be retranslocated complexed with sugar alcohols in some plant species. Conifers had been thought not to translocate sugar alcohols in the phloem. However, 1 d after applying B-10 enriched boric acid to shoots of Scots pine and Norway spruce seedlings, we found increases in both the amount and proportion of B-10 in the root systems in both species. We conclude that B is translocated in the phloem from shoots to roots in spruce and pine, and therefore it is possible that these species retranslocate B. (C) 2000 Annals of Botany Company.
Abstract: Plants generate and perceive informational signals. An interaction (between plants) is considered to be informational when it involves the exchange of an insignificant amount of matter or energy, in quantitative terms, but in spite of this has a profound effect on plants by modulating their developmental programme. This article discusses how plants â檜seâ light signals to detect neighbouring plants and outlines the implications of shade-avoidance responses for the development of size and fitness hierarchies in canopies, The role of shade avoidance as a stabilizing force in monospecific canopies is noted.
Abstract: We studied the response of Scots pine (Pinus sylvestris L.) to supplementary far-red sidelight in seedlings grown in a forest soil substrate without additional nutrient supply. Our aim was to determine possible changes in the accumulation and allocation of dry weight and mineral nutrients and the presence of mycorrhizas. Half of the seedlings were grown in light conditions simulating reflected far-red light (FR) from neighbouring plants and the other half were controls not receiving additional FR. PAR irradiance was kept constant in both treatments. At the first harvest (41 d of treatment), FR+ had no effect on stem height, biomass accumulation or allocation. However, at the end of the experiment (93 d of treatment), an increase in stem extension rate and stem dry weight was observed in FR+ seedlings when compared to controls. Both control and treated plants had several morphological types of ectomycorrhizas, but no effect of FR+ on the frequency of these morphotypes was observed. Nor was the concentration of ergosterol or estimated mycorrhizal fungal biomass affected. Nutrients were more responsive to the light quality treatment: P concentration in roots and N and P contents in stems and roots were higher in FR+ plants than in control seedlings. These results are in contrast to those of a previous study in which the root system of pine seedlings, which were fertilised and had less developed mycorrhizas, was reduced by FR+.
Abstract: The effects of supplementary far-red sidelight on the formation of mycorrhizas and on the accumulation and allocation of dry weight and mineral nutrients were studied in Scots pine (Pinus sylvestris L.) seedlings. Starting one week after germination the seedlings were subjected to two different light quality regimes: control and simulated sparse-canopy conditions (FR+). In the FR+ regime, light reflected by neighbouring plants was simulated by means of supplementary far-red light sources, which reduced the horizontal red/far-red photon ratio (R:FR) without affecting PAR. Seedlings were harvested after three months of treatment. FR+ increased stem height and decreased the total dry weight of seedlings. Dry weight allocation to needles was not affected, whereas dry weight allocation to roots was reduced and that to stems was increased in FR+ treated seedlings. The total number of short root tips and developing mycorrhizas per seedling were lower in FR+ than in control plants. Most short roots were developing mycorrhizas, while non-mycorrhizal short roots and mycorrhizas with mantle or external mycelium were very scarce. Changes in the allocation of nutrients in general followed the changes in dry weight allocation, and changes in nutrient content followed those in total dry weight. However, mismatches among these changes resulted in significant changes in nutrient concentrations in some organs: the concentrations of nitrogen and potassium in needles and the concentration of nitrogen in stems were higher in FR+ than in control seedlings. Changes in biomass and nutrient allocation under low R:FR may promote rapid height growth during early development in stands of Scots pine seedlings, but concomitant reductions in growth of the root system and mycorrhizas may negatively affect tree performance over the long term. [References: 42]
Abstract: Leaves of European silver birch (Betula pendula Roth) seedlings subjected both to ambient and increased levels of CO2 concentration (350 and 700 ppm) with no UV-B and supplementary UV-B radiation were offend to winter moth (Operophtera brumata L.) larvae in laboratory choice experiments. According to chemical analysis of the leaves, the high CO2 concentration decreased the levels of phenolic acids and two flavonoids. The UV-B treatment increased the content of flavonoids. Winter moth larvae consumed most of the leaves of birch seedlings which had been exposed to UV-B radiation in both CO2 environments. Therefore, the effect of the main leaf flavonoids, myricitrin and quercitrin, on the feeding of winter moth larvae was tested by an artificial diet choice experiment. The addition of the two main flavonoid glycosides into artificial diet had no stimulatory effect on larval feeding. Consequently, increases in flavonoid content may not be directly responsible for the larval preference for UV-B exposed leaves. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 54]
Abstract: We studied the effects of light quality and nutrient supply on growth and nitrogen accumulation in silver birch (Betula pendula Roth) seedlings to test three hypotheses: (1) growth Of birch seedlings is sensitive to changes in light quality; (2) the response of birch seedling growth to light quality depends on nutrient supply; and (3) assimilation and allocation of nitrogen by birch seedlings are affected by light quality. The two light regimes simulated the spectral quality of sunlight and shadelight, but did not differ in photosynthetic photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of mineral nutrients. + Accumulation and allocation of dry weight and nitrogen were strongly affected by nutrient supply regime, but light quality had little effect. During the first 15 days of the experiment, the largest effect of light quality was on height growth, which was greater in seedlings in simulated shadelight than in seedlings in simulated sunlight. Light quality had little effect on dry weight and nitrogen allocation to the stem during this period. However, at the end of the experiment (Day 29), there was an increase in N concentration per unit dry weight in leaves and stems of seedlings in the simulated shadelight plus high nutrient supply treatment. [References: 37]
Abstract: Birch (Betula pendula Roth) seedlings were grown under enhanced u.v.-B radiation and simulated forest-soil conditions, after which individual secondary metabolites were determined in the leaves. It was found that not all of the u.v.-absorbing secondary metabolites of the seedlings responded to supplementary u.v.-B radiation. Under increased u.v.-B radiation, significant increases in concentration were observed only for the major flavonoid, quercitrin, the minor flavonoid, myricetin-3-galactoside, and for chlorogenic acid. On the other hand, 3,4â檇ihydroxypropiophenone-3-beta-D-glucopyranoside decreased under u.v.-B irradiation. The concentration of phenolic compounds in the leaves changed during the growing season (between two harvests) but this change was not related to u.v.-B enhancement. A low availability of mineral nutrients did not impair the capacity of the seedlings to accumulate u.v.-protecting phenolic compounds under increased u.v.-B radiation. The growth conditions used might have affected the intracellular concentrations of secondary metabolites, and thus furnished the birch seedlings with an increased tolerance of u.v.-B radiation. These findings point to the significance of certain phenolic components in the protection of deciduous trees against u.v.-B radiation.
Abstract: The objective of the experiment described here was to assess the effect of the synthetic growth regulator CCC on the quality of containerised silver birch seedlings. The time course of stem elongation during the growing season, the morphology and size of the seedlings at the end of the growing season, and the growth potential after winter storage were measured and the following results obtained: (1) Stem height, root-collar diameter and total dry weight of the seedlings were decreased by CCC. (2) Stem-height:root-collar-diameter ratio was slightly decreased and root weight ratio was increased by CCC. (3) Stem flexibility was not significantly affected by CCC, although it was significantly correlated with total plant dry weight and with the stem-height:root-collar-diameter ratio. (4) Root-growth potential was decreased at high doses of CCC while shoot growth potential was not affected. [References: 14]
Abstract: The relationship between light saturated net photosynthesis (A(max)) and nitrogen concentration (N) was studied in needles of both Scots pine seedlings, grown at three relative growth rates (2, 6 and 8 %) controlled by nutrient addition rate, and Scots pine shoots collected from four sites with different fertility. In the seedlings, A(max) was measured on 14 different dates starting at the beginning of the second growing season and ending when growth of the new shoot and the secondary needles had finished. In shoots from the natural stands A(max) of the previous-year shoots was measured on 6 dates throughout the growing season. Both in seedlings and shoots, the correlation between A(max) and N was poor, when data from all sampling dates were taken together. However, A(max) was correlated with N in most instances when the age of the needles was considered and the data were examined either at weekly intervals (seedlings) or separately for each sampling date (shoots). The slope of the A(max) vs N relationship varied greatly between sampling dates. In the seedlings the correlation between A(max) and N was strongest by the time when the new needles were developing. In the shoots the correlation was significant from mid June until mid August, while no correlation was found in the beginning and at the end of the growing season. Our data indicate that in pine needles the photosynthesis-nitrogen relationship is more complex than in broadleaved species. Contrary to the broadleaved species, where the correlation is independent of sampling time, in this conifer the time of the year affects the correlation and there are phases during the growing season when the correlation is poor or nonexistent. [References: 17]
Abstract: The experiments and simulations reported in this paper show that, for stomata sensitive to both CO-2 and water vapour concentrations, responses of stomatal conductance (g-s-w) to boundary layer thickness have two components, one resulting from changes in intercellular CO-2 concentration (X-i-c) and another from changes in leaf surface water vapour saturation deficit (D-s-w). The experiments and simulations also show that the boundary layer conductance (g-b-w) can significantly alter the apparent response of g-s-w. to ambient air CO-2 mole fraction (X-a-c) and water vapour mole fraction (X-a-w). Because of the feedback loop involved the responses of g-s-w for X-a-c and X-a-w each include responses to both X-i-c and D-s-w. The boundary layer alters the state of the variables sensed by the guard cells-i.e. X-i-c and D-s-w-and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g-s-w for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g-s-w to X-i-c and D-s-w, the apparent responses of e-s-w to D-a-w and X-a-c depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.
Abstract: We analysed Ballâ檚 empirical model of stomatal conductance (Ball, Woodrow and Berry, 1987). The original interpretation was found to be flawed, and a new one is proposed which views the model as a description of the relationship between CO-2 flux rate and stomatal conductance, rather than as a model of stomatal conductance alone. It is shown that this model is useful for describing changes in intercellular CO-2 concentration. The model was tested against data from experiments in which the responses of stomatal conductance and CO-2 flux density to humidity and temperature were measured in leaves of Hedera helix. It was found that the responses to temperature and humidity are not treated in a satisfactory way in the model whereas the response of the model to other variables is realistic. An alternative to Ballâ檚 model is described and tested. It is concluded that Ballâ檚 model is a useful starting point for the development of simulation models to be used as submodels in canopy and regional models. However, as any empirical model, it is of no use for defining causal relationships.
Abstract: Previous work has shown that stomata respond directly to light, but it was not clear whether the only additional response is through CO2, or whether some other metabolite is involved in this response. Gas exchange experiments were done with normally positioned and inverted leaves of Hedera helix to investigate this problem. The macroscopic optical properties of the leaves and their anatomical structure were also studied. These experiments showed that there is no need to postulate the existence of a messenger other than CO2 to explain the indirect response of stomata to light. The experiments also showed that leaf inversion affects both stomatal conductance and photosynthesis, and highlight the difficulties involved in the interpretation of the effect of leaf inversion on stomata when stomatal conductance measurements are not done concurrently with measurements Of CO2 flux density and intercellular CO2 molar fraction.
Abstract: Previous work has shown that stomata respond directly to light, but it was not clear whether the only additional response is through CO2, or whether some other metabolite is involved in this response. Gas exchange experiments were done with normally positioned and inverted leaves of Hedera helix to investigate this problem. The macroscopic optical properties of the leaves and their anatomical structure were also studied. These experiments snowed that there is no need to postulate the existence of a messenger other than CO2 to explain the indirect response of stomata to light. The experiments also showed that leaf inversion affects both stomatal conductance and photosynthesis, and highlight the difficulties involved in the interpretation of the effect of leaf inversion on stomata when stomatal conductance measurements are not done concurrently with measurements of CO2 flux density and intercellular CO2 molar fraction
Abstract: The response of Fuchsia magellanica Lam. - a shade-tolerant perennial - to white light photon flux density and end-of-day red and far-red light irradiation was studied to evaluate the role that different morphological and functional alterations play in whole-plant acclimation to shade. The estimated relative growth rate of plants growing under a photon flux density of 30-mu-mol m-2 s-1 was 71% of that of those growing under 450-mu-mol m-2 s-1. The proportion of total dry weight allocated to shoots was higher in low light plants, but the proportion of shoot dry weight allocated to leaf blades was not affected by any of the treatments. The estimated relative growth rate, specific leaf area and leaf conductance were not affected by end-of-day irradiation. In low light plants the specific leaf area was higher, and at saturating photon flux density photosynthesis was lower on a leaf area basis, but was not on a leaf dry weight basis. Dark respiration was higher in high light plants when expressed on an area basis, but not when expressed on a dry weight basis. During the normal photoperiod, leaf conductance measured in darkness was higher in high light plants than in low light ones, which is consistent with the higher stomatal density observed in these plants. The cross-sectional area of the petioles was slightly reduced in low light in spite of a large increase in the area of individual leaves. Low irradiance affected the display of leaves, decreased leaf thickness and increased chlorophyll content per unit dry weight. End-of-day far-red resulted in plants with more erect branches. Both low photon flux density and far-red enhanced internode elongation. The effect of end-of-day irradiations was bigger in low light than in high light, which is the opposite to what is observed in shade-avoiders. The relatively small difference in mean growth rate can be explained by changes in specific leaf area, shoot:root dry weight partition ratio, and by the shape of the photosynthetic light response curve. However when comparing photosynthetic rates on a dry weight basis low light plants do not seem to be at a disadvantage under high light.
Abstract: The objective of the work reported was to answer the following questions: (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity.
Abstract: Plants were grown under white light (photoperiod 10 h). Pulses of far-red light, but not red light, given at the end of the day, slowed down chlorophyll accumulation in expanding, greening leaves and reduced chlorophyll content in mature leaves. The chlorophyll a/b ratio was unaffected. Low phytochrome photoequilibria reduced the chlorophyll content in mature leaves without affecting leaf area or DW. This effect occurred even before senescence was triggered in the control plants having high phytochrome photoequilibria.
Abstract: Far red light (FR)-treated plants showed low chlorophyll content and erect leaves, but higher leaf area and plant dry weight. Differences in area expansion were related to the late phase of leaf growth and were due, at least in part, to larger cells in FR-treated plants. Effects on length/width ratio, specific leaf area, net assimilation rate, shoot/root ratio and leaf number were small or non-existent. It is suggested that the lower chlorophyll content in FR-treated plants was not a consequence of scarcity of assimilates.
Abstract: The modelling of stomatal responses is hindered by gaps in our knowledge of the interactions between the effects of different environmental variables, and of the mechanistic basis for correlations between physiological variables. The objective of this thesis was to fill some of these gaps by studying short term stomatal responses to the environment, and by contrasting some current models against this new information. Four questions were addressed through simulation and gas-exchange experiments on Hc. (1) What is the relationship between stomatal responses and the rate of photosynthesis? The COtwo flux density and stomatal conductance are closely correlated, but there is not a simple causal link between them. This relationship is complex, and depends on both parallel but independent responses to light of stomata and photosynthesis, and indirect response of stomata mediated by photosynthesis. This indirect response occurs through COtwo depletion in the air spaces of the mesophyll and stomatal response to COtwo. No evidence was found in favour of the proposed effect of photosynthesis on stomata through an unknown messenger. (2) What is the nature of the interaction between stomatal responses to humidity and temperature? The hypothesis that these responses are brought about by a single response to relative humidity at the leaf surface was tested, and shown to be incompatible with the responses of Hh. It is suggested that the most appropriate variable for expressing humidity is, in this context, the water vapour deficit at the leaf surface. (3) What is the role of the boundary layer in the control of stomatal opening? Real world and simulation experiments were used to show that responses to bulk air water vapour and COtwo mol fractions are both dependent on stomatal responses to COtwo and humidity. It is also shown that a feedforward response to humidity requires feedback through another variable for stability under natural conditions. Response to wind speed is due to changes in humidity and COtwo mol fraction at the leaf surface. (4) Are our current knowledge, and the resulting models, good enough for predicting short-term stomatal responses to changes in the environment? The need for a careful analysis of simulation models is stressed. Ballâ檚 empirical model of stomatal conductance was analysed. The original interpretation was found to be flawed, and a new one was proposed. The new interpretation views the model as a description of the relationship between COtwo flux rate and stomatal conductance, rather than of stomatal conductance alone. It is shown that this model is useful for describing the behaviour of the intercellular COtwo concentration. The model was tested against data from the experiments. It was found that the responses to temperature and humidity are not treated in a satisfactory way. The response of the model to other variables is realistic. A modification to the model is described and tested. It is concluded that the model is a good starting point for the development of simulation models to be used as submodels in canopy and regional models. The modelling of stomatal responses is hindered by gaps in our knowledge of the interactions between the effects of different environmental variables, and of the mechanistic basis for correlations between physiological variables. The objective of this thesis was to fill some of these gaps by studying short term stomatal responses to the environment, and by contrasting some current models against this new information. Four questions were addressed through simulation and gas-exchange experiments on Hedere helix. (1) What is the relationship between stomatal responses and the rate of photosynthesis? The COtwo flux density and stomatal conductance are closely correlated, but there is not a simple causal link between them. This relationship is complex, and depends on both parallel but independent responses to light of stomata and photosynthesis, and indirect response of stomata mediated by photosynthesis. This indirect response occurs through COtwo depletion in the air spaces of the mesophyll and stomatal response to COtwo. No evidence was found in favour of the proposed effect of photosynthesis on stomata through an unknown messenger. (2) What is the nature of the interaction between stomatal responses to humidity and temperature? The hypothesis that these responses are brought about by a single response to relative humidity at the leaf surface was tested, and shown to be incompatible with the responses of Hh. It is suggested that the most appropriate variable for expressing humidity is, in this context, the water vapour deficit at the leaf surface. (3) What is the role of the boundary layer in the control of stomatal opening? Real world and simulation experiments were used to show that responses to bulk air water vapour and COtwo mol fractions are both dependent on stomatal responses to COtwo and humidity. It is also shown that a feedforward response to humidity requires feedback through another variable for stability under natural conditions. Response to wind speed is due to changes in humidity and COtwo mol fraction at the leaf surface. (4) Are our current knowledge, and the resulting models, good enough for predicting short-term stomatal responses to changes in the environment? The need for a careful analysis of simulation models is stressed. Ballâ檚 empirical model of stomatal conductance was analysed. The original interpretation was found to be flawed, and a new one was proposed. The new interpretation views the model as a description of the relationship between COtwo flux rate and stomatal conductance, rather than of stomatal conductance alone. It is shown that this model is useful for describing the behaviour of the intercellular COtwo concentration. The model was tested against data from the experiments. It was found that the responses to temperature and humidity are not treated in a satisfactory way. The response of the model to other variables is realistic. A modification to the model is described and tested. It is concluded that the model is a good starting point for the development of simulation models to be used as submodels in canopy and regional models.
