Abstract: The aims of the study were to gain a better understanding of fructan metabolism regulation during regrowth of Lolium perenne, and to evaluate the role of fructans of remaining tissues as well as carbon assimilation of new leaf tissues in refoliation. Two varieties that contrast for carbohydrate metabolism, Aurora and Perma, were subject to severe and frequent or infrequent defoliations before regrowth. Aurora, which had a greater content of fructans in leaf sheaths than Perma before defoliation, produced more leaf biomass within the 4 days following the. first cut. At the end of the regrowth period, Aurora produced more leaf biomass than Perma. Photosynthetic parameters, which were barely affected by defoliation frequency, could not explain these differences. Fructan synthesising activities [sucrose: sucrose 1-fructosyltransferase (1-SST) and fructan: fructan 6G-fructosyltransferase (6G-FFT)], declined after defoliation. In elongating leaf bases, corresponding transcript levels did not decline concomitantly, suggesting a post-transcriptional regulation of expression, while in leaf sheaths the gene expression pattern mostly followed the time-course of the enzyme activities. Regulation of Lp1-SST and Lp6G-FFT gene expression depends, therefore, on the sink-source status of the tissue after defoliation. During the phase of reserve accumulation, fructosyltransferase activities together with corresponding transcripts increased more in frequently defoliated plants than in infrequently defoliated plants.

Abstract: Caesium (Cs) is an alkali metal with chemical properties similar to potassium (K). It has no known role in plant nutrition and it is not toxic to plants at the micromolar concentrations occurring naturally in soil solutions. However, two radioisotopes of Cs (Cs-134 and Cs-137) are of environmental concern due to their relatively long half-lives, emissions of beta and gamma radiation during decay, and rapid incorporation into biological systems. There is considerable interest in remediating sites contaminated by these isotopes using phytoextraction and, since the produce from radiocaesium-contaminated areas may enter the food chain, the introduction of 'safe' crops that do not accumulate Cs. This article reviews the molecular mechanisms of Cs uptake by plants, and provides a perspective on strategies to develop: (1) plants that extract Cs efficiently from soils (for the phytoremediation of land), or (2) 'safe' crops that minimise the entry of radiocaesium directly into the human food chain.

Abstract: Quantifying and simulating the relationships between crop growth, total-nitrogen (total-N) and nitrate-N (NO3--N) concentration can improve crop nutritional husbandry. In this study, the relationship between shoot relative growth rate (RGR) and shoot total-N, organic-N and NO3--N concentration of hydroponically-grown lettuce (Lactuca sativa var. capitata L. cv. Kennedy) was described and simulated. Plants were grown hydroponically for up to 74 d. Nitrogen was supplied throughout (control; T1), or removed at 35 d (T2) and 54 d (T3), respectively, after sowing. The organic-N and NO3--N concentration declined in the shoots of control plants with growth, until commercial maturity approached when organic-N and NO3--N concentration increased. There were sub-linear relationships between both total-N and organic-N concentration, and shoot RGR, in the N-limited treatments, i.e. shoot RGR approached an asymptote at high shoot N concentration. The proportional effects of total-N and organic-N concentration on shoot RGR were independent of plant age. A dynamic simulation model ('Nicolet'), derived previously under different conditions, was used to simulate the growth, dry matter content, organic-N, and NO3--N concentration of lettuce grown under the extreme N-stress conditions experienced by the plants. In view of the largely successful fitting of the model to experimental data, the model was used to interpret the results. Suggestions for model improvement are made.

Abstract: Concerns about high nitrate levels in vegetable produce have led the European Union to introduce limits on nitrate concentrations in some salad crops including lettuce (Lactuca sativa L.), with the aim of decreasing nitrate intake by consumers. These limits are likely to create problems for lettuce growers in northern European countries such as the UK, where leafy vegetables can accumulate high nitrate concentrations in leaf tissues due to low light levels, especially during the winter. One option to overcome this problem is the use of low nitrate-accumulating genotypes. The objective of this work was to screen a selection of soil-grown glasshouse lettuce cultivars for head weight and nitrate concentration during the summer and the winter seasons. Two pairs of trials were carried out using a Trojan square design, following commercial cropping practices. Eight long-day commercial lettuce cultivars were grown for both early- and late-summer harvest, and eight short-day commercial cultivars for both early- and late-winter harvest. Both pairs of experiments included examples of cultivars belonging to the following types: Butterhead, Cos, Batavia, Curly, Oakleaf, Lollo Rosso and Lollo Bionda. It was found that the commercial fresh weight of the heads depends on season (summer or winter) and cultivar, with significant interaction between cultivar and experiment (i.e. harvest date within season). By comparison, nitrate concentration showed not only great variability between cultivars in general, but also between the main lettuce types and between cultivars within the Butterhead type. The data for the summer crops suggest that 'Vegus' (in particular) tended to accumulate less nitrate than the other cultivars of the Butterhead type. For the winter crops, although no single cultivar was found to exhibit consistently lower nitrate concentrations at harvest, the means for the Butterhead group were generally lower than the average of the cultivars for the other types. Several of the long-day cultivars were found to exceed the EU maximum summer nitrate limit of 3500 mg kg(-1) fresh weight, whereas relatively few of the short-day cultivars had nitrate concentrations greater than the corresponding 4500 mg kg(-1) limit for winter harvested crops.

Abstract: C assimilation (A) has been shown to limit the growth of young Lactuca sativa (lettuce) plants following an interruption in their external N supply. Further data from these plants were used to test two hypotheses: that N-limited growth of lettuce is associated with lower stomatal conductance (g(s)); and that reductions in g(s) result from adjustments to stomatal frequency or distribution. The photosynthetic characteristics, nitrate and organic N-concentrations, as well as epidermal and stomatal distributions, were determined in leaves of hydroponically grown lettuce plants supplied continuously with N or with N removed for up to 14 d. Although N-limited plants had lower maximum rates of A, comparisons at equivalent values of g(s) showed that A was not directly limited by organic-N but by g(s) Reductions in g(s) under N-limiting conditions did not associate with adjustments to stomatal frequency or distribution. Associations between plant N and A could arise either through stomata responding directly to signals induced by N deprivation or to increased CO2 partial pressure at the sites of carboxylation.

Abstract: Twenty-seven individual tree growth models are reviewed. The models take into account the same main physiological processes involved in carbon metabolism (photosynthate production, respiration, reserve dynamics, allocation of assimilates and growth) and share common rationales that are discussed. It is shown that the spatial resolution and representation of tree architecture used mainly depend on model objectives. Beyond common rationales, the models reviewed exhibit very different treatments of each process involved in carbon metabolism. The treatments of all these processes are presented and discussed in terms of formulation simplicity, ability to account for response to environment, and explanatory or predictive capacities. Representation of photosynthetic carbon gain ranges from merely empirical relationships that provide annual photosynthate production, to mechanistic models of instantaneous leaf photosynthesis that explicitly account for the effects of the major environmental variables. Respiration is often described empirically as the sum of two functional components (maintenance and growth). Maintenance demand is described by using temperature-dependent coefficients, while growth efficiency is described by using temperature-independent conversion coefficients. Carbohydrate reserve pools are generally represented as black boxes and their dynamics is rarely addressed. Storage and reserve mobilisation are often treated as passive phenomena, and reserve pools are assumed to behave like buffers that absorb the residual, excessive carbohydrate on a daily or seasonal basis. Various approaches to modelling carbon allocation have been applied, such as the use of empirical partitioning coefficients, balanced growth considerations and optimality principles, resistance mass-flow models, or the source-sink approach. The outputs of carbon-based models of individual tree growth are reviewed, and their implications for forestry and ecology are discussed. Three critical issues for these models to date are identified: (i) the representation of carbon allocation and of the effects of architecture on tree growth is Achilles' heel of most of tree growth models; (ii) reserve dynamics is always poorly accounted for; (iii) the representation of below ground processes and tree nutrient economy is lacking in most of the models reviewed. Addressing these critical issues could greatly enhance the reliability and predictive capacity of individual tree growth models in the near future.

Abstract: An extensive literature reports that Cs(+), an environmental contaminant, enters plant cells through K(+) transport systems. Several recently identified plant K(+) transport systems are permeable to Cs(+). Permeation models indicate that most Cs(+) uptake into plant roots under typical soil ionic conditions will be mediated by voltage-insensitive cation (VIC) channels in the plasma membrane and not by the inward rectifying K(+) (KIR) channels implicated in plant K nutrition. Cation fluxes through KIR channels are blocked by Cs(+). This paper tests directly the hypothesis that the dominant KIR channel in plant roots (AKT1) does not contribute significantly to Cs(+) uptake by comparing Cs(+) uptake into wild-type and the akt1 knockout mutant of Arabidopsis thaliana (L.) Heynh. Wild-type and akt1 plants were grown to comparable size and K(+) content on agar containing 10 mM K(+). Both Cs(+) influx to roots of intact plants and Cs(+) accumulation in roots and shoots were identical in wild-type and akt1 plants. These data indicate that AKT1 is unlikely to contribute significantly to Cs(+) uptake by wild-type Arabidopsis from 'single-salt' solutions. The influx of Cs(+) to roots of intact wild-type and akt1 plants was inhibited by 1 mM Ba(2+), Ca(2+) and La(3+), but not by 10 microM Br-cAMP. This pharmacology resembles that of VIC channels and is consistent with the hypothesis that VIC channels mediate most Cs(+) influx under 'single-salt' conditions.

Abstract: A model was developed that simulated photosynthesis, growth and allocation in tree seedlings. The model was parameterized with data from experiments on seedlings of sycamore (Acer pseudoplatanus L.), Sitka spruce (Picea sitchensis (Bong) Carr.) and young birch trees (Betula pendula Roth.). In these experiments, CO2 concentration ([CO2]) and nutrient addition rate were varied. Parameters quantifying nutrient uptake, translocation and starch synthesis were fitted, based on data from control treatments. Elevated [CO2] and low-nutrient treatments were then used to test the predicted response of growth and allocation against observations. The model accurately predicted total seedling growth in the elevated [CO2] treatments. A response of growth to elevated [CO2] was seen in the birch and sycamore experiments, but not in the Sitka spruce, because of photosynthetic down-regulation. Predictions of allocation were reasonably accurate in the birch and Sitka spruce experiments, but were notably poorer in the sycamore experiments, possibly because of differences in sink strength between root and shoot. In the birch and sycamore experiments, little change in allocation with elevated [CO2] was observed or predicted. This was ascribed to the relative values of K(Tc) and K(Tn), the translocation coefficients that determine the sensitivity of allocation to carbon and nitrogen uptake rates, respectively. Growth and allocation in the low-nutrient treatments were poorly predicted by the model. In Sitka spruce, it was suspected that the photosynthetic parameters measured in August 1994 had been higher earlier in the season, before nutrients became depleted. In sycamore, the discrepancies were thought to relate to differences in sink strength between root and shoot that could not be described by the model.

Abstract: Relationships between nitrogen (N) content and growth are routinely measured in plants. This study determined the effects of N on the separate morphological and physiological components of plant growth, to assess how N-limited growth is effected through these components. Lettuce (Lactuca sativa) plants were grown hydroponically under contrasting N-supply regimes, with the external N supply either maintained continuously throughout the period of study, or withdrawn for up to 14 d. Richards' growth functions, selected using an objective curve-fitting technique, accounted for 99.0 and 99.1% of the variation in plant dry weight for control and N-limited plants respectively. Sublinear relationships occurred between N and relative growth rates under restricted N-supply conditions, consistent with previous observations. There were effects of treatment on morphological and physiological components of growth. Leaf weight ratio increased over time in control plants and decreased in N-limited plants. Shoot:root ratio followed a similar pattern. On a whole-plant basis, assimilation of carbon decreased in N-limited plants, a response paralleled by differences in stomatal conductance between treatments. Changes in C assimilation, expressed as a function of stomatal conductance to water vapour, suggest that the effects of N limitation on growth did not result directly from a lack of photosynthetic enzymes. Relationships between plant N content and components of growth will depend on the availability of different N pools for remobilization and use within the plant.

Abstract: Photosynthesis, metabolic carbon partitioning and the contribution of sorbitol to the osmotic potential of mature peach (Prunus persica (L.) Batsch) leaves and phloem sap were examined in plants undergoing two levels of a short term drought stress. The relationship between osmotic potential at full turgor and water potential showed that neither mild nor severe drought stress induced a significant active osmotic adjustment in mature leaves. The osmotic potential of leaves at full turgor was -1.9 MPa; sorbitol was the major organic component (20%). Leaf sucrose and starch contents were significantly reduced by drought. The partitioning of newly-fixed carbon was also affected by stress. These changes appeared to originate from the inhibition of photosynthesis induced by drought stress. At low photosynthetic rates, the turnover of organic ions was low, and sorbitol synthesis was favoured over that of sucrose. Water stress did not affect the in vitro activity of sucrose phosphate synthase (EC 2.4.1.14), the key enzyme in sucrose synthesis. The in vitro activity of aldose-6-phosphate reductase (EC 1.1.1.200), the key enzyme in sorbitol synthesis, tended to increase linearly in response to drought stress. It is concluded that, contrary to some other polyol-synthesising species, peach did not seem to benefit from sorbitol synthesis during short term drought stress for active osmotic adjustment in mature leaves. However, in phloem sap, increases in sucrose and especially sorbitol concentration were observed in stressed plants.

Abstract:

Abstract: A deterministic and dynamic model of carbon allocation in walnut seedlings is described. Two experimental data sets were used to calibrate and validate the model. These data included: time course of the carbon content, chemical and isotope composition (12C and 13C) of the kernel and growing organs (roots, stem, leaves), and gas exchange rates during the first 55 days of the life of the plant with continuous 13CO2 feeding. The plant is modelled as a network with nodal organs acting as sources or sinks for carbohydrates. In a sink organ the demand for carbon is the sum of four elementary demands: maintenance respiration, structural growth, growth-associated respiration and carbon storage. The organs of the plant are assumed to be in exponential growth phase. The supply of carbon readily accessible for the organs is the store of soluble sugars present in a local reservoir. Carbon flow in the network is determined by the source/sink activities of the organs and local levels of demand and supply. Two carbon sources are considered: soluble sugars from the kernel, and gross photosynthesis. The rate of synthesis of soluble sugars in the kernel, and measured photosynthesis in the leaves are inputs for the plant model. The outputs are the predicted fluxes of carbon within the seedling; 13C composition,carbohydrate allocation to the growing organs, starch and soluble sugars accumulation, and respiration. The mathematical equations were translated into PSPICE software instructions. After optimisation of the parameter values, the model provided an accurate description of experimental observations in the seed-plant system during the critical transition from heterotrophy to autotrophy, especially C allocation to organs and C partitioning between storage, structural growth and respiration in each organ. The growth of the young plant is supply-limited, except at the earliest stages. A sensitivity analysis suggests that intense competition for carbohydrates dominates the relations among and within organs. Copyright 1998 Academic Press Limited

Abstract: Sorbitol, together with sucrose and starch, is a major final product of photosynthesis in most species of the Rosaceae. However, Little is known concerning the factors affecting the carbon fluxes into these three carbohydrates. The aim of this study was to investigate the effect of net photosynthetic CO2 assimilation rate (NAR) on primary carbon partitioning between sorbitol, sucrose, starch and other metabolites, and on the kinetic pattern of labelling in mature leaves of peach (Prunus persica [L.] Batsch). A pulse-chase labelling experiment was conducted on a group of peach seedlings showing a range of NAR's that were obtained by using bye photosynthetic photon flux density treatments. Mature leaves undergoing one of these treatments were subjected to a 4-min pulse of (CO2)-C-14 followed by a 2- to 120-min chase under (CO2)-C-12. The radioactivity in lipids, proteins and in a residual nonsoluble fraction was negligible. After a 2-min chase the organic ions were strongly labelled, accounting for up to 40% of the total incorporated radioactivity at NAR's below 10.4 mu mol CO2 m(-2)s(-1). After a 30-min chase, most label was found in sorbitol, sucrose and starch with very little radioactivity in the ionic fractions. The analysis of the partitioning of the newly-fixed carbon showed that at low NAR the C-14 flux was mostly into sorbitol rather than into sucrose (ratio = 2.6) or starch. When the NAR was lower than 6.1 mu mol CO2 m(-2) s(-1), starch was lightly labelled. As the NAR increased above 10.4 mu mol CO2 m(-2) s(-1) the (CO2)-C-14 diverted into starch increased dramatically. This study demonstrates that at low NAR's the newly-fixed CO2 is mainly used for sorbitol synthesis. However, as NAR increases, the participation of sucrose and starch in CO2 partitioning are favoured. These results support the hypothesis that primary carbon partitioning depends on photosynthetic rate.

Abstract: Sorbitol is one of the 17 sugar alcohols or alditols found in higher plants. It is a major final product of photosynthesis and, together with sucrose, represents the main form of carbon translocated in many species of the Rosaceae and Plantaginaceae families. Sorbitol accumulation is considered as an adaptative response of plants to drought, salinity or chilling stress. Biosynthesis of sorbitol is confined mainly to source leaves whereas metabolic utilization is restricted to sink tissues. Sorbitol synthesis is in competition with sucrose synthesis because they have a common pathway from triosephosphate to glucose-g-phosphate. The control mechanisms of cellular carbon partitioning into sorbitol versus sucrose and starch in source leaves remains unknown. However partitioning appears to be affected by environmental factors.

Abstract: The aim of this study was to investigate variability in the sorbitol : sucrose ratio (SSR) in source leaves of different peach [Prunus persica (L.) Batsch] cultivars. Four- and 5-year-old trees of 58 cultivars were examined. Mature leaves were sampled on three dates in middle to late summer and analyzed for neutral soluble sugars using high-performance liquid chromatography. Differences in SSRs were observed. In most cultivars, the sorbitol content was at least twice that of sucrose. The maximal range of SSR occurred on the third date and ranged from 1.5 to 4.3. There was a date x genotype interaction (P < 0.01). When cultivars were grouped by country of origin, the mean ratios of the Japanese group were lower than those Of the Italian and American groups for all three sampling dates. The SSRs bf nectarines were higher than those of peach and canning clingstone-type cultivars. In general, variations in SSR were due mostly to differences in sucrose content. The SSR was negatively correlated with flowering date. These results indicate variability in SSR in peach germplasm, variability that seems to be related to the geographical origin of the cultivars.

Abstract: The characteristics of sorbitol and sucrose export out of mature leaves in seedlings of peach (Prunus persica L. Batsch cv GF 305) were investigated by simulating carbon fluxes through the leaf. Three treatments were employed: a control treatment and two treatments modifying leaf export, the latter using either shading or girdling. Photosynthesis and 14C partitioning into sorbitol and sucrose were measured during carbohydrate pool buildup at the beginning of the photoperiod, and the export rate of sorbitol and sucrose was modeled using a PSPICE (Simulation Program with Integrated Circuit Emphasis) simulator. The simulation allowed prediction of the resulting sorbitol and sucrose contents, which were compared to experimental carbohydrate contents. The apparent Km for sorbitol and sucrose phloem loading, estimated by carbon flux modeling, was 6.6 and 4 mol m-3, respectively. The predicted export capacity of the leaf, characterized by the estimated Vmax values for phloem loading of sorbitol and sucrose, was similar to the photosynthetic carbon flux measured under the leaf growth conditions. This export capacity was enhanced in plants in which all leaves except those studied were shaded. The mature leaf had a higher storage capacity for sorbitol than for sucrose in control plants, especially in the girdled treatment. Sucrose content appears to be tightly regulated.