Krõõt Aasamaa

Abstract: The light sensitivity of the shoot hydraulic conductance in five temperate deciduous tree species was measured using two methods to clarify the role of light sensitivity and the suitability of the methods used to study it. The light sensitivity measured using a method that included an interruption of <10 min in shoot light acclimation did not differ from that measured using a method with continuous illumination. The ‘noncontinuous light’ methods are suitable for measuring hydraulic conductance and its light response. Light sensitivity correlated with other leaf water traits as follows: positively with the ionmediated increase in xylem hydraulic conductance; a relative decrease in the hydraulic conductance of the laminae in response to HgCl₂; a relative change in stomatal conductance in response to changes in PAR intensity or atmospheric CO₂ concentration, or to a decrease in air humidity or leaf water potential; and with instantaneous water use efficiency. The traits correlated negatively with shoot hydraulic conductance, stomatal conductance and relative increases in stomatal conductance in response to increases in leaf water potential. We suggest that high light sensitivity should be considered as one of the characteristics of conservative water use in trees. Low blue light increased shoot hydraulic conductance to a similar extent to moderate white light and twice as much as moderate red light. Blue light perception is important in the light sensitivity mechanism.

Abstract: To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined. Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO₂ concentration ([CO₂]) and light intensity, and to an increase in [CO₂] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought. Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO₂] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species. In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO₂], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO₂] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.

Abstract: To clarify interactions between stomatal responses to two simultaneous environmental changes, the rates of change in stomatal conductance were measured after simultaneously changing two environmental factors from the set of air humidity, leaf water potential (hydraulic environmental factors), air CO₂ concentration and light intensity (photosynthetic environmental factors). The stomatal responses to changes in leaf water potential were not significantly modified by any other simultaneous environmental change. A decrease in air humidity was followed by a decrease in stomatal conductance, and an increase in air humidity was followed by an increase in the conductance, irrespective of the character of the simultaneous change in the photosynthetic environmental factor. If the simultaneous change had an opposite effect on stomatal conductance, the rate of change in stomatal conductance was higher than the theoretical summed rate — the sum of the rate following one environmental change and the rate following another environmental change, measured separately. That is, the stomatal response to air humidity dominated over the responses to photosynthetic environmental factors. Yet, if the simultaneous change in photosynthetic factors had a codirectional effect on stomatal conductance, the rate of stomatal conductance change was lower than the theoretical summed rate. After a simultaneous change of two photosynthetic environmental factors, the rate of stomatal conductance change was very similar to the theoretical rate, if both the environmental changes had a codirectional effect on stomatal conductance. If the changes in the photosynthetic factors had opposite effects on stomatal conductance, the conductance increased, irrespective of the character of the increasing environmental factor. In drought-stressed trees, the rates of change in stomatal conductance tended to differ from the theoretical summed rates more than in well-watered trees. Stomatal closure following an increase in CO₂ concentration was the stomatal response that was most strongly suppressed by the response to another simultaneous environmental change. Six species of temperate deciduous trees were shown to be similar in their relations between the stomatal responses to two simultaneous environmental changes. The mechanism and ecological significance of the interactions between the two signal response pathways of stomata are discussed.

Abstract: Hydraulic conductance of stem and petioles increased in response to an increase in xylem sap ion concentration, and decreased in response to a decrease in the ion concentration in six temperate deciduous tree species. The ion sensitivity of hydraulic conductance of stem and petioles was higher than the ion sensitivity of the stem alone. The ion sensitivity was lowest in the earliest developmental stages of the xylem, and had a seasonal maximum in the second half of summer. The ion sensitivity was highest in slow-growing species and lowest in fast-growing species. The ion sensitivity correlated negatively with mean radius of xylem conduits, hydraulic conductance of stem and petioles, hydraulic conductance of leaf laminae, and stomatal conductance, and positively with response of the hydraulic conductance of leaf laminae to HgCl₂, and stomatal response to a decrease in leaf water potential or abscisic acid. It was concluded that the high ion sensitivity of xylem hydraulic conductance is a relevant characteristic of slow growth and a conservative water use strategy.

Abstract: An objective of the study was to identify leaf morphophysiological characteristics indicative of high aboveground woody biomass production of <i>Salix</i> clones growing in the vegetation filter of a wastewater purification system. It was hypothesized, that in these extra irrigated stands, the characteristics of photosynthesis are stronger determinants of the production than the characteristics of leaf water relations. The biomass production of the clones was correlated with numerous characteristics of leaf water relations and photosynthesis. Significant correlation was found between the biomass production and the following characteristics of top foliage leaves: daily decrease in water potential, stomatal conductance, stomatal sensitivity to increase in light intensity, and hydraulic conductance (positive); daily minimum and maximum water potentials, instantaneous water use efficiency, and stomatal sensitivity to decrease in leaf water potential (negative); maximum rate of photosynthetic electron transport, chlorophyll content, and the partitioning coefficient for leaf nitrogen into bioenergetics associated with electron transport (positive). In the most productive clones, the vertical gradient (through foliage) of values of almost all the characteristics was the smallest. The biomass correlated more strongly with the characteristics of photosynthesis light stage than with the characteristics of water relations. We conclude that the characteristics of photosynthesis light stage – high maximum rate of photosynthetic electron transport, high chlorophyll content, and also flat vertical gradient (through foliage) in values of the characteristics could be the strongest indicators of high biomass production of <i>Salix</i> clones growing in vegetation filter.

Abstract: Previous studies have shown a rapid enhancement in leaf hydraulic conductance (K_leaf) from low to high irradiance (from <10 to >1000 µmol photons m^−2s^−1), using the high-pressure flow meter (HPFM), for 7 of 14 tested woody species. However, theoretical suggestions have been made that this response might arise as an artifact of the HPFM. We tested the K_leaf light response for six evergreen species using refined versions of the rehydration kinetics method (RKM) and the evaporative flux method (EFM). We found new evidence for a rapid, 60% to 100% increase in K_leaf from low to high irradiance for three species. In the RKM, the leaf rehydration time constant declined by up to 70% under high irradiance relative to darkness. In the EFM, under higher irradiance, the flow rate increased disproportionately to the water potential gradient. Combining our data with those of previous studies, we found that heterobaric species, i.e. those with bundle sheath extensions (BSEs) showed a twofold greater K_leaf light response on average than homobaric species, i.e. those without BSEs. We suggest further research to characterize this substantial dynamic at the nexus of plant light- and water-relations.

Abstract: Leaf hydraulic conductance (K_leaf) and several characteristics of hydraulic architecture and physiology were measured during the first 10 weeks of leaf ontogeny in <i>Populus tremula</i> L. saplings growing under control, mild water deficit or elevated temperature conditions. During the initial 3 weeks of leaf ontogeny, most measured characteristics rapidly increased. Thereafter, a gradual decrease in K_leaf was correlated with a decrease in leaf osmotic potential under all conditions, and with increases in leaf dry mass per area and bulk modulus of elasticity under mild water deficit and control conditions. From about Week 3 onward, K_leaf was 33% lower in trees subjected to mild water deficit and 33% higher in trees held at an elevated temperature relative to control trees. Mild water deficit and elevated temperature treatment had significant and opposite effects on most of the other characteristics measured. The ontogenetic maximum in K_leaf was correlated positively with the width of xylem conduits in the midrib, but negatively with the overall width of the midrib xylem, number of lateral ribs, leaf dry mass per area and bulk modulus of elasticity. The ontogenetic maximum in K_leaf was also correlated positively with the proportion of intercellular spaces and leaf osmotic potential, but negatively with leaf thickness, volume of mesophyll cells and epidermis and number of cells per total mesophyll cell volume, the closest relationships being between leaf osmotic potential and number of cells per total mesophyll cell volume. It was concluded that differences in protoplast traits are more important than differences in xylem or parenchymal cell wall traits in determining the variability in K_leaf among leaves growing under different environmental conditions.

Abstract: The seasonal course of maximum hydraulic conductance of leaf laminae (K_lamina) of shoots correlated strongly with the seasonal course of the maximum hydraulic conductance of leaf laminae of HgCl₂-treated shoots (K_{lamina(HgCl₂)}), and with the seasonal course of the difference (dK_lamina) between K_lamina and K_{lamina(HgCl₂)}. However, it did not correlate strongly with the seasonal course of the hydraulic conductance of stem and petioles of the shoot (K_stpt) in six temperate deciduous tree species. The species ranked according to K_lamina as follows: <i>Populus tremula</i> L. > <i>Salix caprea</i> L. > <i>Padus avium</i> Mill. > <i>Quercus robur</i> L. > <i>Tilia cordata</i> Mill. > <i>Acer platanoides</i> L. The species-specific maxima of K_lamina correlated positively with the simultaneous values of K_{lamina(HgCl₂)}, dK_lamina and K_stpt; the correlation was strongest with K_{lamina(HgCl₂)}. It was concluded that the seasonal dynamics of maximum hydraulic conductance of leaf laminae was determined almost equally by the seasonal dynamics of the hydraulic conductance of foliar protoplasts and apoplast, but the inter-specific differences in K_lamina were mainly caused by the different apoplastic hydraulic conductance in leaves of these species. The relative contribution of dK_lamina (in K_lamina) was highest in slow-growing species (~55% in <i>A. platanoides</i>) and the lowest in fast-growers (~30% in <i>S. caprea</i>).

Abstract: Water-use strategies of <i>Populus tremula</i> and <i>Tilia cordata</i>, and the role of abscisic acid in these strategies, were analysed. <i>P. tremula</i> dominated in the overstorey and <i>T. cordata</i> in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (Y), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of <i>T. cordata</i> to the top of the foliage of <i>P. tremula</i>) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the Y, w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in <i>T. cordata</i>, and on Y, w and c in <i>P. tremula</i>. It was concluded that the water use of the species of the lower canopy layer — <i>T. cordata</i>, is more conservative than that of the species of the overstorey, <i>P. tremula</i>. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in <i>P. tremula</i>. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.

Abstract: Correlations between leaf abscisic acid concentration ([ABA]), stomatal conductance (g_s), rate of stomatal opening in response to an increase in leaf water potential (s_i), shoot hydraulic conductance (L) and photosynthetic characteristics were examined in saplings of six temperate deciduous tree species: <i>Acer platanoides</i> L., <i>Padus avium</i> Mill., <i>Populus tremula</i> L., <i>Quercus robur</i> L., <i>Salix caprea</i> L. and <i>Tilia cordata</i> Mill. Species-specific values of foliar [ABA] were negatively related to the mean values of g_s, s_i, L and light- and CO₂- saturated net photosynthesis (P_max), thus providing strong correlative evidence of a scaling of foliar gas exchange and hydraulic characteristics with leaf endogenous [ABA]. In addition, we suggest that mean g_s, s_i, L and P_max for mature leaves may partly be determined by the species-specific [ABA] during leaf growth. The most drought-intolerant species had the lowest [ABA] and the highest g_s, suggesting that interspecific differences in [ABA] may be linked to differences in species-specific water-use efficiency. Application of high concentrations of exogenous ABA led to large decreases in g_s, s_i and P_max, further underscoring the direct role of ABA in regulating stomatal opening and photosynthetic rate. Exogenous ABA also decreased L, but the decreases were considerably smaller than the decreases in g_s, s_i and P_max. Thus, exogenous ABA predominantly affected the stomata directly, but modification of L by ABA may also be an important mechanism of ABA action. We conclude that interspecific variability in endogenous [ABA] during foliage growth and in mature leaves provides an important factor explaining observed differences in L, g_s, s_i and P_max among temperate deciduous tree species.

Abstract: The relationship between shoot hydraulic conductance (L) and stomatal sensitivity to changes in leaf water status was studied in the saplings of six deciduous tree species. L increased significantly in sequence: <i>Acer platanoides</i> < <i>Tilia cordata</i> < <i>Padus avium</i> = <i>Quercus robur</i> < <i>Salix caprea</i> = <i>Populus tremula</i>. L was higher in the trees grown in soil with a higher nitrogen content and lower in the trees grown under mild water stress or kept in darkness for several days. L was higher in July than in September in all the species. L correlated positively with maximum photosynthesis, stomatal conductance and stomatal sensitivity to an increase in leaf water potential, but negatively with stomatal sensitivity to a decrease in leaf water potential. The correlations between L and any other parameter were approximated by three different curves: data for water-stressed plants fit to the first, data for plants kept in darkness fit to the second and all the other data fit to the third curve. The reasons of the differences of shoot hydraulic conductance in the different experimental sets and the mechanisms which may cause the correlation between L and the other characteristics are discussed.

Abstract: Some anatomical characteristics in leaves relating to hydraulic conductance and stomatal conductance were examined in six temperate deciduous tree species. The fourth power of the radius of the conducting elements in xylem (r⁴) and the area of mesophyll and epidermal cells per unit length of leaf cross-section (u) were high in leaves with high hydraulic conductance (L). Stomatal conductance (g_s) and stomatal sensitivity to an increase in leaf water potential (s_i) correlated positively with the length of stomatal pore (l), but negatively with the guard cell width (z) and the length of the dorsal side of the guard cells (l_d). Stomatal sensitivity to a decrease in leaf water potential (s_d) correlated negatively with l and positively with z and l_d. The anatomical characteristics associated with hydraulic conductance (r⁴ and u) and those associated with stomatal conductance and sensitivity to changes of leaf water potential (l, z and l_d) were correlated. We conclude that hydraulic conductance may depend on anatomical characteristics of xylem, mesophyll and epidermis, and stomatal conductance and its sensitivity to changing water potential may depend on anatomical characteristics of stomata. The correlation of shoot hydraulic conductance with stomatal conductance and its sensitivity may be based largely on the correlation between the anatomical characteristics of the water conducting system and stomata in these trees.