ALBERT

Notes: Many plants accumulate organic osmolytes in response to the imposition of environmental stresses that cause cellular dehydration. Although an adaptive role for these compounds in mediating osmotic adjustment and protecting subcellular structure has become a central dogma in stress physiology, the evidence in favour of this hypothesis is largely correlative. Transgenic plants engineered to accumulate proline, mannitol, fructans, trehalose, glycine betaine or ononitol exhibit marginal improvements in salt and/or drought tolerance. While these studies do not dismiss causative relationships between osmolyte levels and stress tolerance, the absolute osmolyte concentrations in these plants are unlikely to mediate osmotic adjustment. Metabolic benefits of osmolyte accumulation may augment the classically accepted roles of these compounds. In re-assessing the functional significance of compatible solute accumulation, it is suggested that proline and glycine betaine synthesis may buffer cellular redox potential. Disturbances in hexose sensing in transgenic plants engineered to produce trehalose, fructans or mannitol may be an important contributory factor to the stress-tolerant phenotypes observed. Associated effects on photoassimilate allocation between root and shoot tissues may also be involved. Whether or not osmolyte transport between subcellular compartments or different organs represents a bottleneck that limits stress tolerance at the whole-plant level is presently unclear. None the less, if osmolyte metabolism impinges on hexose or redox signalling, then it may be important in long-range signal transmission throughout the plant.

Notes: The evergreen species Yucca glauca was characterized at the end of September and following exposure to low temperatures at the end of November. In November the diurnal pattern of xanthophyll cycle-dependent energy dissipation was altered such that this thermal dissipation process was engaged at a high level throughout the day, whereas in September it only became engaged when leaves received direct sunlight. An analysis of the diurnal partitioning of the absorbed excitation energy into photochemistry versus thermal dissipation suggested that a smaller fraction of that energy was utilized in photochemistry and a greater fraction was dissipated thermally at the end of November compared to September. Lower ratios of Chl a/b and β-carotene/xanthophylls both suggested a decrease in the ratio of reaction centre plus core antenna proteins compared to light-harvesting proteins, and a lower leaf chlorophyll content suggested a decrease in light-harvesting capacity in November versus September. Thus adjustments to the photosynthetic apparatus occurred on several levels in response to the increase in excess excitation energy that Y. glauca experienced during the onset of winter.

Notes: Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO2 concentrations in the proximity of a natural CO2 spring. Trees at the spring site have been growing for generations in elevated atmospheric CO2 concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO2 spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO2 was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO2 spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO2 on plant water use at the organ and whole-tree scale.

Notes: Water relations of tomato fruit and the epidermal and pericarp activities of the putative cell wall loosening and tightening enzymes Xyloglucan endotransglycosylase (XET) and peroxidase were investigated, to determine whether tomato fruit growth is principally regulated in the epidermis or pericarp. Analysis of the fruit water relations and observation of the pattern of expansion of tomato fruit slices in vitro, has shown that the pericarp exerts tissue pressure on the epidermis in tomato fruit, suggesting that the rate of growth of tomato fruit is determined by the physical properties of the epidermal cell walls. The epidermal activities of XET and peroxidase were assayed throughout fruit development. Temporal changes in these enzyme activities were found to correspond well with putative cell wall loosening and stiffening during fruit development. XET activity was found to be proportional to the relative expansion rate of the fruit until growth ceased, and a peroxidase activity weakly bound to the epidermal cell wall appeared shortly before cessation of fruit expansion. No equivalent peroxidase activity was detected in pericarp tissue of any age. It is therefore plausible that the expansion of tomato fruit is regulated by the combined action of these enzyme activities in the fruit epidermis.

Notes: We explore the extent to which a simple mechanistic model of short-term plant carbon (C) dynamics can account for a number of generally observed plant phenomena. For an individual, fully expanded leaf, the model predicts that the fast-turnover labile C, starch and protein pools are driven into an approximate or moving steady state that is proportional to the average leaf absorbed irradiance on a time-scale of days to weeks, even under realistic variable light conditions, in qualitative agreement with general patterns of leaf acclimation to light observed both temporally within the growing season and spatially within plant canopies. When the fast-turnover pools throughout the whole plant (including stems and roots) also follow this moving steady state, the model predicts that the time-averaged whole-plant net primary productivity is proportional to the time-averaged canopy absorbed irradiance and to gross canopy photosynthesis, and thus suggests a mechanistic explanation of the observed approximate constancy of plant light-use efficiency (LUE) and carbon-use efficiency. Under variable light conditions, the fast-turnover pool sizes and the LUE are predicted to depend negatively on the coefficient of variation of irradiance. We also show that the LUE has a maximum with respect to the fraction of leaf labile C allocated to leaf protein synthesis (alp), reflecting a trade-off between leaf photosynthesis and leaf respiration. The optimal value of alp is predicted to decrease at elevated [CO2]a, suggesting an adaptive interpretation of leaf acclimation to CO2. The model therefore brings together a number of empirical observations within a common mechanistic framework.

Notes: To assess the possible physiological function of chlorogenic acid (CGA, 3-O-caffeoylquinic acid) in vivo, we characterized the free radical scavenging properties of pure phenylpropanoids and leaf extracts against two free radicals, superoxide and the 2,2’-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical cation. CGA was found to be a highly efficient scavenger of these free radicals, surpassing the activity of all other phenylpropanoids tested, as well as the ‘classical’ antioxidant ascorbate. Seasonal differences in the leaf content of CGA were examined in field populations of the broadleaf evergreen Mahonia repens growing in different light environments. Leaves of fully sun-exposed plants contained significantly more (74 ± 10 mmol m–2) CGA in winter than leaves from plants growing under deeply shaded conditions (17 ± 2 mmol m–2). Sun-acclimated, but not shade-acclimated, leaves also produced high levels of anthocyanins in winter, suggesting a simultaneous increase in carbon flow through the phenylpropanoid and flavonoid pathways in response to high light and seasonal low temperature stress. In summer, high light-acclimated leaves contained ≈ 2-fold less CGA than in winter, whereas CGA levels were similar between seasons in shaded leaves. Consistent with the strong scavenging capacity of CGA measured in vitro, a linear correlation was observed between CGA content and the antioxidant activity of leaf extracts in both scavenging assays. On the basis of these results, we propose that CGA is a powerful hydrogen-donating antioxidant that may play an important role in mitigating the effects of oxidative stress in plants.

Notes: Pulses of blue light cause stimulation of red light saturated photosynthesis in Ectocarpus siliculosus, because blue light activates the operation of a pathway for inorganic carbon (Ci) acquisition by inducing the mobilization of CO2 from an intermediate metabolite. In the absence of exogenous Ci, photosynthetic rates roughly equal those of CO2 release by respiration. In seawater of pH 9·5 (2·3 mol m–3 total Ci, but concentrations of free CO2 below 0·2 mmol m–3), photosynthesis was clearly above these rates, although they were only ≈ 30% of those in normal seawater (≈ pH 8). The degree and the time course of the stimulations of photosynthesis by pulses of blue light were unaltered at high pH. Essentially the same characteristics were found after buffering or in the presence of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity. Therefore, it is concluded that Ectocarpus is able to directly take up HCO3– in addition to CO2 (uptake of CO32– cannot be excluded). The dependence of photosynthesis on Ci at pH 9·5 was biphasic, with Ci below 0·2 mol m–3 having no effect at all. In Ci-free seawater, the shapes of the stimulations after blue light pulses differed for pH 6, pH 8 and pH 9·5. At low pH, only the fast peak (maximum ≈ 5 min after blue light) was detected, whereas at high pH mainly the slow peak (maximum ≈ 20 min after blue light) was observed. At the intermediate pH 8, both peaks were present. As inhibition of total carbonic anhydrase by ethoxyzolamide brought out the fast peak of the stimulations at pH 9·5 it is concluded that the fast component was due to a transient disequilibrium of an intracellular pool of Ci which, after blue light, was fed by CO2 released from the postulated storage intermediate.

Notes: The combined effects of carbon dioxide (CO2) enrichment and water deficits on nodulation and N2 fixation were analysed in soybean [Glycine max (L.) Merr.]. Two short-term experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO2 atmospheres of either 360 or 700 μmol CO2 mol–1. Under drought-stressed conditions, elevated [CO2] resulted in a delay in the decrease in N2 fixation rates associated with drying of the soil used in these experiments. The elevated [CO2] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO2]. The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO2 had much higher TNC levels than those under ambient CO2. For both [CO2] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N2 fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO2] resulted in substantially less drought-induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO2] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N2 fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO2 enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N2 rates under soil water deficits. We suggest that future [CO2] increases are likely to benefit soybean production by increasing the drought tolerance of N2 fixation.

Notes: The aim of the present study was to test the four commonly used models to predict the dates of flowering of temperate-zone trees, the spring warming, sequential, parallel and alternating models. Previous studies concerning the performance of these models have shown that they were unable to make accurate predictions based on external data. One of the reasons for such inaccuracy may be wrong estimations of the parameters of each model due to the non-convergence of the optimization algorithm towards their maximum likelihood. We proposed to fit these four models using a simulated annealing method which is known to avoid local extrema of any kind of function, and thus is particularly well adapted to fit budburst models, as their likelihood function presents many local maxima. We tested this method using a phenological dataset deduced from aeropalynological data. Annual pollen spectra were used to estimate the dates of flowering of the populations around the sampling station. The results show that simulated annealing provides a better fit than traditional methods. Despite this improvement, classical models still failed to predict external data. We expect the simulated annealing method to allow reliable comparisons among models, leading to a selection of biologically relevant ones.

Notes: Carbon isotope composition (δ13C) was measured in a glasshouse experiment with N2-fixing and NO3–- or NH4+-fed Casuarina equisetifolia Forst. & Forst plants, both under well-watered and drought conditions. The abundance of 13C was higher (more positive δ13C) for NH4+- than for NO3– -grown plants and was lowest for N2-fixing plants. NH4+-fed plants had more leaf area and dry weight and higher water use efficiency (on a biomass basis) than N2- and NO3–-grown plants and had lower water consumption than plants supplied with NO3–, either with high or low water supply. Specific leaf areas and leaf area ratios were higher with NH4+ than with NO3– or N2 as the N source. The difference observed in δ13C between plants grown with different N sources was higher than that predicted by theory and was not in the right direction (NH4+-grown plants with a more negative δ13C) to be explained by differences in plant composition and engagement of the various carboxylation reactions. The more positive δ13C in NH4+- than in NO3–-grown plants is probably due to a decreased ratio of stomatal to carboxylation conductances, which accounts for the lower water cost of C assimilation in NH4+-grown plants.