Showing papers in "Functional Plant Biology in 2003"
TL;DR: Attention is drawn to the perception and signalling processes (chemical and hydraulic) of water deficits, which are essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques.
Abstract: In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.
3,287 citations
TL;DR: It is shown that acclimation to anoxia in plants involves integration of a set of sophisticated characteristics, as a consequence of which the habitat within the anoxic cell is a very different world to that of the aerobic cell.
Abstract: Anoxia can be one consequence of waterlogging and submergence of plants. Anoxia in plant tissues reduces the rate of energy production by 65-97% compared with the rate in air. Thus, adaptation to anoxia always includes coping with an energy crisis. Tolerance to anoxia is relevant to wetland species, rice cultivation and transient waterlogging of other agricultural and horticultural crops. This perspective, in two parts, examines mechanisms of anoxia tolerance in plants. Part 1 covers anoxia tolerance in terms of growth and survival, the interaction of anoxia tolerance with other environmental factors, and the development of anoxic cores within plant tissues. Equally importantly, Part 1 also examines anaerobic carbohydrate catabolism (principally ethanolic fermentation in plants) and its regulation. We put forward two modes of anoxia tolerance, one based on reduced rates of anaerobic carbohydrate catabolism and the other on accelerated rates (Pasteur effect). Further, Part 1 examines mechanisms of post-anoxic injury. In Part 2 (Greenway and Gibbs, manuscript in preparation) we consider flow of the limited amount of energy produced under anoxia to processes essential for cell survival. We show that acclimation to anoxia in plants involves integration of a set of sophisticated characteristics, as a consequence of which the habitat within the anoxic cell is a very different world to that of the aerobic cell.
513 citations
TL;DR: Current knowledge of soil organic carbon (SOC) dynamics with respect to physical protection, soil moisture and temperature, and recalcitrant carbon fractions (such as charcoal) in predominantly agricultural soils are reviewed.
Abstract: This paper reviews current knowledge of soil organic carbon (SOC) dynamics with respect to physical protection, soil moisture and temperature, and recalcitrant carbon fractions (such as charcoal) in predominantly agricultural soils. These factors are discussed within the framework of current soil organic matter models. The importance of soil structure in the stabilisation of organic residues through physical protection has been documented previously in various studies. In addition, changes in soil structure associated with tillage can significantly affect soil organic matter decomposition rates. The concept of physical protection has been incorporated into several soil carbon models as a function of soil texture. While soil texture can affect the soil's capacity for aggregation and adsorption, factors such as soil moisture and temperature may further enhance or reduce the extent of physical protection. While adsorption and aggregation can slow decomposition processes, it is unlikely that these processes are solely responsible for the high mean residence times measured in biologically active surface soils. Accordingly, chemical recalcitrance appears to be the only mechanism by which soil organic carbon can be protected for long periods of time.
509 citations
TL;DR: Review of this literature suggests that the assumption of conservative R : P ratio is an effective and practicable approach in the context of C-cycle modelling for global change research and documentation, requiring minimal ecosystem-specific data on respiration.
Abstract: Plant respiratory regulation is too complex for a mechanistic representation in current terrestrial productivity models for carbon accounting and global change research. Accordingly, simpler approaches that attempt to capture the essence of respiration are commonly adopted. Several approaches have been used in the literature: respiration may be embedded implicitly in growth algorithms; assumed values for specific respiration rates may be adopted; respiration may be calculated in terms of growth and maintenance components; conservatism in the ratio of respiration to photosynthesis (R:P) may be assumed; or a more complex process or residual approach may be adopted. Review of this literature suggests that the assumption of conservative R:P ratio is an effective and practicable approach in the context of C-cycle modelling for global change research and documentation, requiring minimal ecosystem-specific data on respiration. Some long-standing controversies in respiration are now becoming resolved. The apparently wasteful process of cyanide-resistant respiration by the alternative oxidase may not be wasteful, as it is thought to be involved in protecting the plant from 'reactive oxygen species'. It is now clear that short-term respiratory response coefficients of plants (e.g. the Q10) do not predict their long-term temperature response. A new experimental approach suggests that leaf respiration is not suppressed by light as previously thought. Careful experiments, taking account of several proposed measurement artefacts, indicate that plant respiration is not suppressed by elevated CO2 concentration in a short-term reversible way.
384 citations
TL;DR: colourless and blue tautomers of cyanidin 3-(6-malonyl)glucoside at pH 7 removed up to 17% of O2•- generated by chloroplasts, indicating that cytosolic anthocyanins can serve as effective antioxidants.
Abstract: Anthocyanins have the potential to mitigate photooxidative injury in leaves, both by shielding chloroplasts from excess high-energy quanta, and by scavenging reactive oxygen species. To distinguish between the impacts of these two putative mechanisms, superoxide (O2•–) concentration and chlorophyll oxidation were measured for Lactuca sativa L. chloroplast suspensions under various light and antioxidant-supplemented environments. A red cellulose filter, the optical properties of which approximated that of anthocyanin, effected a 33% decline in rate of O2•– generation and 37% reduction in chlorophyll bleaching, when used to shield irradiated chloroplasts. Colourless and blue tautomers of cyanidin 3-(6-malonyl)glucoside at pH 7 removed up to 17% of O2•– generated by chloroplasts, indicating that cytosolic anthocyanins can serve as effective antioxidants. Red tautomers, typical of vacuolar anthocyanins, also showed strong reducing potentials as indicated by cyclic voltammetry. These potentials declined by 40% after 15 min exposure to O2•–. Maximum quantum efficiencies of photosynthesis were similar for red and green portions of intact L. sativa leaves, but the red regions were less photoinhibited, and recovered more extensively after exposures to strong light. Anthocyanins evidently offer effective and versatile protection to leaves without significantly compromising photosynthesis.
317 citations
TL;DR: It is concluded that the likely causes of death under anoxia are firstly, a decrease in pHcyt below 7.0, and secondly, deterioration in membrane selectivity culminating in loss of membrane integrity would be fatal.
Abstract: Anoxia in plant tissues results in an energy crisis (Gibbs and Greenway 2003). How anoxia-tolerant tissues cope with such an energy crisis is relevant not only to anoxia tolerance, but also to adverse conditions in air that cause an energy crisis.To survive an energy crisis, plant cells need to reduce their energy requirements for maintenance, and also direct the limited amount of energy produced during anaerobic catabolism to the energy-consuming processes that are critical to survival.We postulate that during anoxia, reductions in ion fluxes and protein turnover achieve economies in energy consumption. Processes receiving energy from the limited supply available under anoxia include synthesis of anaerobic proteins and energy-dependent substrate transport. Energy would also be required for maintenance of membrane integrity and for regulation of cytoplasmic pH (pHcyt). We suggest that a moderate decrease in the set point of pHcyt, from approximately 7.5 to approximately 7.0 is an acclimation to the energy crisis in anoxia-tolerant tissues. This decrease in the set point of pHcyt would favour metabolism of acclimative value, such as reduction in protein synthesis and stimulation of ethanolic fermentation. During anoxia lasting several days, a proportion of the scarce energy produced may need to be spent to mitigate the acidifying effect on pHcyt arising from fluxes of undissociated organic acids across the tonoplast as a consequence of high concentrations of organic acids in the vacuole. Increases in vacuolar pH (pHvac), with concomitant decreases in the vacuolar concentrations of undissociated acids, would mitigate such an 'acid load' on the cytoplasm. We present evidence that a preferential engagement of V-PPiases, over that of V-ATPases, may direct energy flow at the tonoplast to maintain pHcyt.We conclude that the likely causes of death under anoxia are firstly, a decrease in pHcyt below 7.0. Cytoplasmic acidosis occurs in several anoxia-intolerant tissues and may contribute to their death. Such adverse decreases in pHcyt can be mitigated by the biochemical pH stat. Secondly, deterioration in membrane selectivity culminating in loss of membrane integrity would be fatal. We suggest these two causes are not mutually exclusive but may act in concert.
307 citations
TL;DR: The results suggest that the H2O2-induced chilling tolerance in these plants might be mediated by an elevation of glutathione content and is independent of the ABA mechanism of chilling protection.
Abstract: Transient oxidative shock induced by pretreatment of leaves with H2O2 effectively increased chilling tolerance in mung bean and Phalaenopsis. Seedlings of the chilling-tolerant (V3327) cultivar of mung bean (Vignaradiata L.) were employed to study the mechanism of H2O2-induced chilling tolerance. Pretreatment with 200 mM H2O2 increased survival rates of seedlings chilled at 4°C for 36 h from 30% to 70%. The same treatment also lowered the electrolyte leakage from 86% to 21%. Time-course analysis immediately after the treatment demonstrated that exogenous application of H2O2 did not alter the endogenous H2O2 level of the plants. This observation suggests that the primary receptor for the exogenous H2O2 is localized on the leaf surface or in some other way isolated from the endogenous H2O2 pool. Oxidative shock inhibited the induction of the antioxidant enzymes, ascorbate peroxidase and catalase; however, it substantially increased glutathione content both under chilling and control conditions. Combined pretreatment of mung bean plants with abscisic acid and H2O2 showed no synergistic effect on glutathione content and decreased survival rate relative to treatment with either compound alone. These results suggest that the H2O2-induced chilling tolerance in these plants might be mediated by an elevation of glutathione content and is independent of the ABA mechanism of chilling protection.
281 citations
TL;DR: Monitoring transmission changes at 820 nm, a measure of the redox states of plastocyanin (PC) and P700, is a good complementary technique for chlorophyll a fluorescence induction measurements and it is shown that the ratio between the potential contributions of PC : P700 is 50 : 50 in pea and Camellia leaves and 40 : 60 in sugar beet leaves.
Abstract: Monitoring transmission changes at 820 nm, a measure of the redox states of plastocyanin (PC) and P700, is a good complementary technique for chlorophyll (chl) a fluorescence induction measurements. A thorough characterization of the properties of the 820-nm transmission kinetics during the first second after a dark-to-light transition is provided here for pea (Pisum sativum L.) leaves. The data indicate that plastocyanin in a dark-adapted leaf is in the reduced state. Three photosystem I (PSI)-related components, PC, P700 and ferredoxin, can contribute to the 820-nm transmission signal. The contribution of ferredoxin, however, is only approximately 5%, thus, it can be neglected for further analysis. Here, we show that by monitoring the sequential oxidation of PC and P700 during a far-red pulse and analysing the re-reduction kinetics it is possible to assign the three re-reduction components to PC (τ = 7-14 s) and P700 (τ = 35-55 ms and 1.2-1.6 s). Our data indicate that the faster re-reduction phase (τ =35-55 ms) may represent a recombination reaction between P700+ and the acceptor side of PSI. This information made it possible to show that the ratio between the potential contributions of PC : P700 is 50 : 50 in pea and Camellia leaves and 40 : 60 in sugar beet leaves.
266 citations
TL;DR: It is suggested that an extensive collection of annual wild Cicer species, based on ecogeographic principles to maximise the probability of collecting diverse ecotypes, should provide a better understanding of the biology and adaptation in this ancient crop and lead to improved productivity.
Abstract: Chickpea (Cicer arietinum L.) is characterised by a different adaptation profile from the other crops of West Asian origin such as pea, barley, and wheat. In this paper we suggest that a series of four evolutionary bottlenecks occur in chickpea: (1) the scarcity and limited distribution of the wild progenitor, C. reticulatum Ladiz., (2) the founder effect associated with domestication, (3) the shift, early in the crop's history, from winter to spring sowing, and the attendant change from using rainfall as it occurs to a reliance on residual soil moisture, and (4) the replacement of locally evolving landraces by elite cultivars produced by modern plant breeding. While two of the bottlenecks are common to all species, the limited distribution of the wild progenitor and shift of cropping from utilisation of current rainfall to stored soil moisture is unique to chickpea. In this paper we suggest that in order to widen the genetic base of cultivated chickpea it is imperative to reintroduce traits from across the primary gene pool. Moreover, a comparative physiological approach to the study of adaptation among the annual wild relatives of chickpea may reveal adaptive strategies within the genus currently obscured by monomorphic loci. The poor state of the world collection of annual wild Cicer species severely constrains the implementation of both these imperatives. We suggest that an extensive collection of annual wild Cicer species, based on ecogeographic principles to maximise the probability of collecting diverse ecotypes, should provide a better understanding of the biology and adaptation in this ancient crop and lead to improved productivity.
259 citations
TL;DR: There is a close link between water availability and grape yield, mostly through water stress effects on photosynthesis, while drought effects on grape quality are linked to water availability but not to photosynthesis or yield.
Abstract: The effects of moderate irrigation, compared with non-irrigation, on leaf photosynthesis and transpiration, grape yield, and quality parameters, were studied over ten years in two Spanish cultivars (Tempranillo and Manto Negro) of field-grown grapevines (Vitis vinifera L.). The aim was to increase our knowledge of the relationships between water availability, canopy water losses, photosynthesis, and fruit yield and quality. A second aim was to analyse some of the mechanisms of photosynthetic down-regulation under drought, such as the capacity for RuBP regeneration and Rubisco activity. Moderate irrigation improved plant water status, leaf photosynthesis and transpiration. Considering the results over ten years, soil water availability (estimated as pre-dawn leaf water potential, ΨPD) largely determined leaf photosynthesis and leaf transpiration. Decreased photosynthesis was due to both stomatal and non-stomatal factors. The latter were related to decayed electron transport rate and reduced RuBP regeneration capacity, but not to decreased Rubisco activity. Moderate irrigation also improved grape yield, although this effect was much larger in Tempranillo than in Manto Negro. Moreover, the correlation between photosynthesis and grape yield was significant in Tempranillo, but not in Manto Negro. In contrast, the correlation between ΨPD and several parameters reflecting fruit quality (such as soluble solids and total polyphenol content) was significant only in Manto Negro. These results suggest that there is a close link between water availability and grape yield, mostly through water stress effects on photosynthesis. Drought effects on grape quality are linked to water availability but not to photosynthesis or yield.
256 citations
TL;DR: It is demonstrated that the concentration of endogenous ABA at the time of pathogen challenge is important for the development of susceptibility in Arabidopsis.
Abstract: The phytohormone abscisic acid (ABA) plays a major role in the regulation of many physiological stresses although its role in pathogen-induced stress remains poorly understood. We examined the influence of ABA on interactions of Arabidopsis thaliana (L.) Heynh. (Arabidopsis) with a bacterial pathogen, Pseudomonas syringae pv. tomato and an Oomycete, Peronospora parasitica. Both addition of 100 μM ABA to plants and drought stress stimulated increased susceptibility of Arabidopsis to an avirulent isolate of P. syringae pv. tomato. In contrast, an ABA-deficient mutant of Arabidopsis, aba1-1, displayed reduced susceptibility to virulent isolates of P. parasitica. An ABA-insensitive mutant, abi1-1, that is impaired in ABA signal transduction did not alter in susceptibility to either P. syringae pv. tomato or P. parasitica. These results demonstrate that the concentration of endogenous ABA at the time of pathogen challenge is important for the development of susceptibility in Arabidopsis.
TL;DR: Water use efficiency was increased by about 80% in PRD and DI when compared with FI, as a result of almost similar yields in the three treatments, while yield gains of irrigated plants in relation to NI were modest.
Abstract: A study to assess the effects of the Partial Rootzone Drying (PRD) irrigation strategy in comparison to other irrigation systems was carried out in southern Portugal in two field-grown grapevines varieties, Moscatel and Castelao. We addressed the question of whether by regulating growth and plant water use, the PRD system would enable an equilibrated vegetative development, leading to a favourable capture of solar radiation for photoassimilate production and, at the same time to provide an optimum environment for fruit maturation. Three irrigation schemes were applied in addition to the non-irrigated (NI) vines: partial root drying (PRD), 50% of crop evapotranspiration (ETc), supplied to only one side of the root system while the other one was allowed to dry, alternating sides every 15 days; deficit irrigated (DI), 50% ETc supplied, half to each side of the root system and full irrigated (FI, 100% ETc). During the whole season FI plants of both varieties exhibited a high leaf predawn water potential (ψ pd, ca -0.2 MPa) while a progressive decline was observed in NI plants, reaching ψ pd values near -0.7 MPa at the end of August. PRD and DI presented intermediate values. PRD vines exhibited a stronger control over vegetative growth as compared with DI and FI plants. This was expressed by lower values of total leaf area at harvest, leaf layer number, canopy wideness and water shoots number, allowing a higher light interception at the cluster zone that induced an improvement in some berry quality characteristics. Watering had no significant effects on sugar accumulation in the berries but led to a favourable increase in the must titratable acidity, mainly in Castelao. Whereas in DI and FI treatments berry skin anthocyanins and phenols content were always lower than in NI, in PRD there was either no reduction or the reduction was much lower than in the other irrigation treatments. Water use efficiency (WUE) was increased by about 80% in PRD and DI when compared with FI, as a result of almost similar yields in the three treatments. Yield gains of irrigated plants in relation to NI were modest, explained by the rainy spring in both years.
TL;DR: Non-invasive, microelectrode ion-flux (MIFE) measurements showed that the onset of salt stress caused rapid and prolonged efflux of H+, K+ and NH4+ from the root epidermis, which could be significantly reversed, or completely prevented, by the presence of high Ca2+ concentration in the bath solution, even after several days of salt Stress.
Abstract: The effects of various Na / Ca ratios on root growth, development, and ion acquisition patterns were studied in hydroponic experiments with barley (Hordeum vulgare L.) plants. In total, interactions between three different levels of salinity (1, 50 and 100 mM NaCl) and three different levels of Ca2+ (0.1, 1 and 10 mM) were studied (a full factorial experiment). Growth rate and biomass accumulation were significantly lower in salinised roots. In addition to reduction in extension growth, salinity also significantly affected plant developmental processes (for example reduced root hair density and root thickening). Supplemental Ca2+ significantly ameliorated those detrimental effects of salinity. Non-invasive, microelectrode ion-flux (MIFE) measurements showed that the onset of salt stress caused rapid and prolonged efflux of H+, K+ and NH4+ from the root epidermis. This efflux could be significantly reversed, or completely prevented, by the presence of high Ca2+ concentration in the bath solution, even after several days of salt stress. Membrane potential measurements in root epidermal cells showed that high Ca2+ levels in the bath were able to restore (otherwise depolarised) membrane potential back to control level (–120 to –130 mV). At the same time, no significant impact of Ca2+ on net Na+ uptake in plant roots was found. Some limitations of the MIFE technique for study of Na+ uptake kinetics under saline conditions, as well as possible ionic mechanisms underlying the ameliorating Ca2+ effects on ion fluxes in roots of salt-stressed plants, are discussed.
TL;DR: The hypothesis that aerenchyma formation reduces the respiratory and phosphorus requirements of soil exploration by roots, and thus, represents a useful adaptation to low phosphorus availability, is supported.
Abstract: Low phosphorus availability induces the formation of cortical aerenchyma in roots. The adaptive significance of this response is unknown. We hypothesized that aerenchyma may be helpful to low-phosphorus plants by reducing root respiratory and phosphorus requirements, thereby increasing the metabolic efficiency of soil exploration. To test this hypothesis we investigated aerenchyma formation, root respiration and tissue phosphorus concentration in maize and common bean genotypes in response to phosphorus availability and ethylene treatments. Genotypes differed substantially in their ability to form aerenchyma in response to low phosphorus. Aerenchyma formation was disproportionately correlated with reduced root respiration; roots with 30% cross-sectional area as aerenchyma had 70% less respiration than roots without aerenchyma. Aerenchyma formation was also proportionally correlated with reduced root phosphorus concentration. Variation in aerenchyma formation was correlated with root respiration and phosphorus concentration, regardless of whether such variation was caused genetically or by ethylene or phosphorus treatments. Results with isolated roots were confirmed by measurement of whole root respiration of intact maize plants. Our results support the hypothesis that aerenchyma formation reduces the respiratory and phosphorus requirements of soil exploration by roots, and thus, represents a useful adaptation to low phosphorus availability.
TL;DR: It is suggested that drought-induced decrease in water potential and increase in ABA content in flowers and pods at critical developmental stage (3-5 DAA) contribute to pod abortion in soybean.
Abstract: Drought stress occurring during flowering and early pod expansion decreases pod set in soybean (Glycine max L. Merr.). The failure of pod set may be associated with changes in water status and ABA content in soybean reproductive structures under drought stress. To test this, pot experiments in an environmentally-controlled greenhouse were conducted, in which soybeans were exposed to drought stress around anthesis. In a preliminary experiment (Expt. I), irrigation was withheld at -6 (D1), -4 (D2) and -2 (D3) to 11 days after anthesis (DAA), then the droughted plants were re-watered to control levels until physiological maturity. Pod set percentage, seed yield and yield components were recorded. In the main experiment (Expt. II), irrigation was withheld from -11 to 10DAA. During the drying cycle, parts of the droughted plants were re-watered at 0, 3, 5, 7 and 10 DAA and kept well-watered until physiological maturity. In Expt. II, water status, ABA contents in xylem sap, leaves, flowers and pods were measured at 0, 3, 5, 7 and 10 DAA. The water potential in the flowers and pods was always lower than the leaf water potential. Turgor was decreased in leaves by drought 3 DAA, but remained at control levels in flowers and pods. Compared with well-watered plants, in severely droughted plants (10 DAA), xylem [ABA] increased about 60-fold; leaf [ABA] increased 9-fold; pod [ABA] increased 6-fold. During soil drying, flower and pod [ABA] was linearly correlated with xylem [ABA] and leaf [ABA], indicating that root-originated ABA and/or leaf ABA were the likely sources of ABA accumulated in the flowers and pods. In Expt. I, pod set and seed number per pod was unaffected by drought stress, while seed yield and individual seed weight was significantly decreased by drought. In Expt. II, significant reductions in pod set and seed yield were observed when re-watering the droughted plants at 3-5 DAA, re-watering the droughted plants later than this stage resulted in a similar pod set. Collectively, these results suggest that drought-induced decrease in water potential and increase in ABA content in flowers and pods at critical developmental stage (3-5 DAA) contribute to pod abortion in soybean.
TL;DR: It is shown that PRD irrigation was able to maintain a vine water status closed to FI, but with double water use efficiency, which was due to a reduction of stomatal conductance with no significant decrease in carbon assimilation.
Abstract: The effects of 'partial rootzone drying' (PRD) irrigation compared with other irrigation systems, namely non-irrigated (NI), full irrigation (FI) and deficit irrigation (DI), on stomatal conductance and carbon assimilation were evaluated in field-grown grapevines (Vitis vinifera L. cv. Moscatel). At the end of the growing season, pre-dawn leaf water potential was highest in FI (-0.18 ± 0.01 MPa; mean ± s.e.), intermediate in PRD (-0.30 ± 0.01 MPa) and DI (-0.36 ± 0.02 MPa), and lowest in NI vines (-0.64 ± 0.03 MPa). Stomatal conductance measured under controlled conditions of light and temperature was reduced in NI (ca 60%) and PRD (ca 30%) vines compared with DI and FI vines. Under ambient conditions, NI vines had lower rates of stomatal conductance (ca 26%), net CO2 assimilation (ca 28%) and light-adapted PSII quantum yields (ca 47%) than PRD, DI and FI vines. No significant differences were found among the three irrigated treatments. Both maximum electron transport rate (Jmax; ca 30%) and triose-phosphate utilization rates (TPU; ca 20%) were significantly lower in NI and PRD vines than in DI and FI vines. Carbon isotope composition (δ 13 C) of grape berries was highest in NI vines (-24.3‰), followed by PRD (-25.4‰) and DI (-25.8‰) and lowest in FI (-26.4‰) vines, suggesting a long-term increase in the efficiency of leaf gas exchange in NI compared with PRD, DI and FI vines. Sap-flow data and estimates of relative stomatal limitation are in accordance with the observed stomatal closure in PRD vines. In this study, we show that PRD irrigation was able to maintain a vine water status closed to FI, but with double water use efficiency, which was due to a reduction of stomatal conductance with no significant decrease in carbon assimilation.
TL;DR: It is proposed that adventitious rooting is a useful adaptation to low phosphorus availability, because adventitious roots explore topsoil horizons more efficiently than other root types.
Abstract: We hypothesized that adventitious roots may improve crop adaptation to low-phosphorus soils by enhancing topsoil foraging. In a tropical field study, phosphorus stress stimulated adventitious rooting in two phosphorus-efficient genotypes of common bean (Phaseolus vulgaris L.) but not in two phosphorus-inefficient genotypes. Although phosphorus availability had no consistent effects on the length or biomass of whole root systems, it had differential effects on adventitious, basal, and taproots within root systems in a genotype-dependent manner, resulting in increased allocation to adventitious roots in efficient genotypes. Adventitious roots had greater length per unit biomass than other root types, especially under phosphorus stress. Adventitious roots had less construction cost than basal roots, despite having similar tissue nitrogen content. Phosphorus stress reduced lateral root density, and adventitious roots had less lateral root density than basal roots. Lateral roots formed further from the root tip in adventitious roots compared with basal roots, especially under phosphorus stress. Field results were confirmed in controlled environments in solid and liquid media. Stimulation of adventitious rooting by phosphorus stress tended to be greater in wild genotypes than in cultivated genotypes. We propose that adventitious rooting is a useful adaptation to low phosphorus availability, because adventitious roots explore topsoil horizons more efficiently than other root types.
TL;DR: Investigation of field pea experiments in southern France found a single linear relationship between final seed number and plant growth rate during the critical period for seed set, reflecting the reproductive plasticity of pea, which adjusts the number of reproductive sinks in an apparent balance with assimilate availability in the plant.
Abstract: Seed number, the most variable yield component of legumes is strongly affected by heat stress (HS) and water deficit (WD). The objective of this paper is to investigate whether HS and WD reduced seed number in field pea through their negative effects on biomass production rather than by specific effects on the developing reproductive organs. Several field and glasshouse experiments were carried out in southern France, in which HS and / or WD of various intensities, durations and positions in the plant lifecycle were imposed on several pea cultivars. WD and HS reduced seed number, in an intensity-dependent manner. They also changed the distribution of seeds along the stem. Plants subjected to WD and mild HS had more seeds on the basal phytomers than did control plants, making it possible to exclude direct effects of stress on seed development. In contrast, severe HS resulted in the immediate abortion of reproductive organs. WD and HS also decreased net photosynthesis (Pn), but only during the period of constraint. Quantitative relationships between Pn and soil water status and between Pn and leaf temperature were established. Nevertheless, in all cases there was a single linear relationship between final seed number and plant growth rate during the critical period for seed set (from the beginning of flowering to the beginning of seed fill for the last seed-bearing phytomer). This reflects the reproductive plasticity of pea, which adjusts the number of reproductive sinks in an apparent balance with assimilate availability in the plant.
TL;DR: A close relationship is found between field-average soil water storage and pre-dawn water potential, a relationship which could be used to couple physiological models of growth and / or photosynthesis to the soil water dynamics.
Abstract: A geometrical canopy model describing radiation absorption (Riou et al. 1989, Agronomie 9, 441-450) and partitioning between grapevines (Vitis vinifera L.) and soil was coupled to a soil water balance routine describing a bilinear change in relative transpiration rate as a function of the fraction of soil transpirable water (FTSW). The model was amended to account for changes in soil evaporation after precipitation events and subsequent dry-down of the top soil layer. It was tested on two experimental vineyards in the Alsace region, France, varying in soil type, water-holding capacity and rooting depth. Simulations were run over four seasons (1992-1993, 1995-1996) and compared with measurements of FTSW conducted with a neutron probe. For three out of four years, the model simulated the dynamics in seasonal soil water balance adequately. For the 1996 season soil water content was overestimated for one vineyard and underestimated for the other. Sensitivity analyses revealed that the model responded strongly to changes in canopy parameters, and that soil evaporation was particularly sensitive to water storage of the top soil layer after rainfall. We found a close relationship between field-average soil water storage and pre-dawn water potential, a relationship which could be used to couple physiological models of growth and / or photosynthesis to the soil water dynamics.
TL;DR: Decreased stomatal conductance coincided with an increase in xylem [ABA] and occurred before any significant change of leaf turgor could be detected, indicating that chemical signals (seemingly root-originated ABA) controlStomatal behaviour at moderate soil water deficits.
Abstract: Both hydraulic and chemical signals are probably important in regulating leaf growth and stomatal conductance of soybean (Glycine max L. Merr.) under drought stress. However, until now they have not been investigated concomitantly in this species. To explore this, a pot experiment in a temperature-regulated greenhouse was conducted, in which plants were subjected to progressive drought during early reproductive stages. Biophysical parameters, viz. relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were followed in control and stressed plants. Drought stress decreased relative leaf expansion rate, stomatal conductance and leaf turgor, whereas it increased leaf [ABA], xylem pH and xylem [ABA]. As soil dried, significant differences between water treatments for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were observed at 14, 9, 14, 14, 14 and 9 d after imposition of stress, respectively. The relationships of relative values for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA] and xylem pH to the fraction of transpirable soil water (FTSW) were well described by linear-plateau functions that allowed calculation of the soil-water thresholds at which processes in stressed plants began to diverge from well-watered controls. The soil-water threshold for stomatal conductance (0.64) was significantly higher than that for relative leaf expansion rate (0.29), xylem pH (0.28), leaf [ABA] (0.27) and leaf turgor (0.25). Relative xylem [ABA] increased, first linearly (when FTSW > 0.5) and then exponentially (when FTSW < 0.5) with decreasing FTSW. Relative stomatal conductance decreased exponentially with increasing relative xylem [ABA] (r2=0.98). Decreased stomatal conductance coincided with an increase in xylem [ABA] and occurred before any significant change of leaf turgor could be detected, indicating that chemical signals (seemingly root-originated ABA) control stomatal behaviour at moderate soil water deficits. Relative relative leaf expansion rate was linearly correlated with relative leaf turgor (r2=0.93), relative xylem pH (r2=0.97) and relative leaf [ABA] (r2=0.98), implying that both hydraulic and chemical signals were probably involved in regulation of leaf expansion at severe soil water deficits.
TL;DR: Treatment with Cd2+ affects nitrogen assimilation and metabolism to a greater extent in soybean roots than in nodules, and changes in protease activity were found in the two tissues treated with 50μMCd2+.
Abstract: The nitrogen metabolism of soybean (Glycine max L.) nodules and roots was studied in plants subjected to two different concentrations (50 and 200 μM) of CdCl2. Nitrogenase activity was decreased in nodules treated with 200 μM Cd2+. In 50 μM Cd2+-treated plants, NH4+ content showed similar values to controls in nodules, but increased by 55% in roots. However, after treatment with 200 μM Cd2+, NH4+ levels increased in both tissues. Glutamate (Glu) and protein contents remained unaltered in nodules treated with 50 μM Cd2+, while at the higher Cd2+ concentration both were decreased. Nevertheless, polyamine content was increased at the two Cd2+ concentrations. In roots, Glu, polyamine and protein levels were significantly diminished at 50 and 200 μM CdCl2. For nitrogen-assimilation enzymes, glutamate dehydrogenase activity was moderately increased in nodules and roots following the lower Cd2+ treatment, though at the higher Cd2+ concentration root enzyme activity returned to control levels. An impressive increase in enzyme activity was found in nodules. In roots, the glutamine synthetase / glutamate synthase pathway was decreased at the two Cd2+ concentrations, though in nodules it was diminished only at 200 μM Cd2+. No changes in protease activity were found in the two tissues treated with 50μMCd2+. However, at 200 μM Cd2+, nodule and root protease activities decreased and increased, respectively. These results suggest that, in general, treatment with Cd2+ affects nitrogen assimilation and metabolism to a greater extent in soybean roots than in nodules.
TL;DR: The identification of defined transcription factor genes provides tools for modulating both the amount and distribution of secondary metabolites in plants, and the validity of this approach has been well established by transgenic plants with modified flavonoid accumulation patterns.
Abstract: Plants produce secondary metabolites during development and in response to environmental stimuli such as light or pathogen attack. Transcriptional regulation provides the most important control point for the secondary metabolic pathways studied to date. In this article we review the data on the transcription factors that modulate this regulation. For the phenylpropanoid pathway, much is understood about both the specific sequences in the target genes (cis-elements) that are involved in responses to environmental and developmental stimuli, and the transcription factors involved. Most information is available for the light induction of the genes for hydroxycinnamic acid production, the production of anthocyanins in leaves and floral tissues, and the production of proanthocyanidins in seeds. Some of the functional interactions between the different types of transcription factor are now being elucidated, and upstream regulators of the genes encoding the transcription factors identified. For other secondary metabolic pathways much less is known, although good progress has been made on identifying transcription factors involved in controlling terpenoid indole alkaloid production. The identification of defined transcription factor genes provides tools for modulating both the amount and distribution of secondary metabolites in plants, and the validity of this approach has been well established by transgenic plants with modified flavonoid accumulation patterns.
TL;DR: An improvement in the understanding of seed chemistry would permit manipulation of these molecules for effective control of pathogens, insect pests, Striga and destructive weeds, as well as for enhanced acquisition of N and P via symbioses with soil rhizobia and AM fungi.
Abstract: Flavonoids and nitrogenous metabolites such as alkaloids, terpenoids, peptides and amino acids are major components of plant seeds. Conjugated forms of these compounds are soluble in water, and therefore, are easily released as chemical signals following imbibition. Once in the soil, these metabolites are first in line to serve as eco-sensing signals for suitable rhizobia and arbuscular mycorrhizal (AM) fungal partners required for the establishment of symbiotic mutualisms. They may also serve as defence molecules against pathogens and insect pests, as well as playing a role in the control of parasitic members of the family Scrophulariaceae, especially Striga, a major plant pest of cereal crops in Africa. Seed metabolites such as flavonoids, alkaloids, terpenoids, peptides and amino acids define seedling growth and, ultimately, crop yields. Thus, an improvement in our understanding of seed chemistry would permit manipulation of these molecules for effective control of pathogens, insect pests, Striga and destructive weeds, as well as for enhanced acquisition of N and P via symbioses with soil rhizobia and AM fungi.
TL;DR: Comparison of fluorimetrically-determined skin absorbance with absorbance of extracted flavonols or hydroxycinnamic acids suggested that acclimation of UV screening depends almost exclusively on flavonol synthesis.
Abstract: The role of phenolics in UV-screening was investigated in berries of a white grape cultivar (Vitis vinifera L. cv. Bacchus). Fluorescence microscopy revealed accumulation of phenolics in the skin of berries and, by high performance liquid chromatography and mass spectrometry, flavonols and hydroxycinnamic acids were identified as the main groups of UV-absorbing phenolics. Relationships between natural radiation and the synthesis of phenolics were studied in plants that were cultivated in the absence of UV radiation in a greenhouse before outdoor exposure to three different light regimes: the entire solar spectrum, the solar spectrum minus UV-B radiation and only visible radiation. During six days of exposure, flavonol synthesis was significantly stimulated by natural UV, in particular UV-B, but concentrations of hydroxycinnamic acids decreased under all conditions. Direct comparison of fluorimetrically-determined skin absorbance with absorbance of extracted flavonols or hydroxycinnamic acids suggested that acclimation of UV screening depends almost exclusively on flavonol synthesis. While increased flavonol levels resulted in efficient UV-A shielding, UV-B shielding was incomplete, probably due to decreased levels of the UV-B-absorbing hydroxycinnamic acids during exposure.
TL;DR: The combination of aerenchyma and a barrier to ROL enhances the longitudinal diffusion of O2 within roots towards the apex, and the possibility of a link between having a barrierto ROL and the X or H genomes in Hordeum species might enable a genetic analysis of this important trait.
Abstract: Growth, root aerenchyma, and profiles of radial O2 loss (ROL) along adventitious roots were evaluated in 35 'wild' Hordeum accessions and cultivated barley (H. vulgare L. ssp. vulgare) when grown in stagnant nutrient solution (deoxygenated and containing 0.1% agar). When grown in stagnant solution, accessions from wetland and 'intermediate' habitats were superior, compared with accessions from non-wetland habitats, in maintaining relative growth rate, tillering, and adventitious root mass. Constitutive aerenchyma formation in adventitious roots was ≥ 10% in 22 accessions (cf. H. vulgare at 2%). When grown in stagnant solution, aerenchyma was ≥ 20% in the adventitious roots of 14 accessions (cf. H. vulgare at 12%). Variation among the accessions in the volume of aerenchyma formed when grown in aerated or stagnant solution was not determined by the waterlogging regime of the species' natural habitat. However, the genus Hordeum comprises four genomes and when grown in stagnant solution accessions with the X genome formed, on average, 22% aerenchyma in adventitious roots (50 mm behind apex), whereas those with the H genome averaged 19%, and those with the Y or I genomes averaged 16 and 15%, respectively. Sixteen accessions formed a barrier to ROL in the basal region of adventitious roots when grown in stagnant solution. The formation of a barrier to radial O2 loss was predominant in accessions from wet habitats, and absent in accessions from non-wetland habitats. In addition, this trait was only present in accessions with the X or H genomes. The combination of aerenchyma and a barrier to ROL enhances the longitudinal diffusion of O2 within roots towards the apex. The possibility of a link between having a barrier to ROL and the X or H genomes in Hordeum species might, in future studies, enable a genetic analysis of this important trait.
TL;DR: A comparative analysis of waterlogging effects on leaf photosynthesis, pigment composition, PSII photochemistry, and plant growth characteristics was undertaken using four different lucerne cultivars and concluded that the Fv/Fm ratio is the most appropriate for practical screening purposes.
Abstract: Waterlogging is a serious environmental stress on lucerne (Medicago sativa) affecting its agronomic performance. To facilitate the breeding process, efficient tools to screen a population of lucerne cultivars are needed. In this study, a comparative analysis of waterlogging effects on leaf photosynthesis, pigment composition, PSII photochemistry, and plant growth characteristics was undertaken using four different lucerne cultivars (Aurora, Hunter River, L153 and Sequel HR). Two-month-old plants, grown in half-strength Hoagland nutrient solution, were waterlogged for 16 days, and plant physiological characteristics were monitored at regular intervals (every few days). All cultivars had significantly reduced fresh and dry weight for both shoots and roots after 16 days of waterlogging. Root biomass showed a greater percentage of reduction than did shoot biomass. As waterlogging stress developed, chlorophyll content, CO2 assimilation rate, transpiration rate, stomatal conductance and maximal quantum efficiency of PSII (Fv/Fm) decreased significantly. Chlorophyll a and b content gradually decreased over the time of the experiment in the stressed cultivars, and leaf chlorosis became increasingly evident. Although most of the parameters showed significant changes as waterlogging progressed, limitations render some of them inapplicable for screening. It is concluded that for practical screening purposes, the Fv/Fm ratio is the most appropriate. A significant difference between control and waterlogged plants became evident as early as day 7. Possible physiological mechanisms involved are discussed.
TL;DR: This article concludes that all four approaches can be successfully used to model photosynthetic uptake and that the best approach in a given situation will depend on model objectives and data availability.
Abstract: A fundamental component of most models of terrestrial carbon balance is an estimate of plant canopy photosynthetic uptake driven by radiation interception by the canopy. In this article, we review approaches used to model the conversion of radiation into photosynthate. As this process is well understood at the leaf-scale, the modelling problem is essentially one of up-scaling, to canopy, regional or global scale. Our review therefore focuses on issues of scaling, including model identification, parameterisation and validation at large scales. Four different approaches are commonly taken to modelling photosynthate production at large scales: the maximum productivity, resource-use efficiency, big-leaf, and sun-shade models. Models representing each of these approaches are discussed and model predictions compared with estimates of gross primary productivity derived from eddy covariance data measured above a Sitka spruce forest. The sun-shade model was found to perform best at all time scales considered. However, other models had significant advantages including simplicity of implementation and the ability to combine the model with remotely-sensed information on vegetation radiation interception. We conclude that all four approaches can be successfully used to model photosynthetic uptake and that the best approach in a given situation will depend on model objectives and data availability.
TL;DR: Cross-validation shows that these mechanistic models based on solar radiation and VPD provide good predictions of canopy conductance and transpiration under the conditions of the study.
Abstract: Whole-vine transpiration was estimated for well-watered nine-year-old Sultana grapevines (Vitis vinifera L. cv. Sultana) from xylem sap flow measured with Granier's heat-dissipation probes. Canopy conductance of the grapevine was calculated by inverting the Penman–Monteith equation. Transpiration from grapevine canopies was strongly controlled by the canopy conductance. Canopy conductance decreased exponentially with increasing vapour pressure deficit (VPD) except in the morning when solar radiation was less than 200 W m–2 and the canopy conductance was predominantly limited by the solar radiation. A non-linear model of canopy conductance as a function of the solar radiation and VPD explained > 90% of the variation observed in canopy conductance. Under contrasting VPD conditions (daytime maximum of 3 kPa vs 8 kPa), grapevines were able to regulate their canopy conductance from 0.006 to 0.001 m s–1 to maintain a near constant transpiration. Whole-canopy transpiration calculated from modelled canopy conductance using the Penman–Monteith equation was highly correlated with the measured transpiration (sap flow) values over the range of 0–0.20 mm h–1 (R2 > 0.85). Cross-validation shows that these mechanistic models based on solar radiation and VPD provide good predictions of canopy conductance and transpiration under the conditions of the study.
TL;DR: Differences of inner-integument cell number and endosperm cell number were related to a difference of endos perments size between superior and inferior caryopses, implying the coordinated development of the endOSperm with maternal tissues.
Abstract: Rice caryopses show different patterns of grain filling depending on position within a panicle. Caryopses located on the upper primary rachis branches generally accumulate larger amounts of starch at maturity than caryopses located on the secondary rachis branches of the lower primary rachis. In this study, the former and latter types of caryopses were defined as superior and inferior caryopses, respectively. Superior caryopses elongated soon after flowering, whereas inferior caryopses hardly elongated and were morphologically stagnant until the first 4 d after flowering (DAF). However, once inferior caryopses began elongation, their morphological development was the same as superior caryopses until the middle stage of grain filling. Cell division of the inner integument ceased before endosperm cellularization, pericarp functioned as a transient starch storage tissue until endosperm accumulated starch, and endosperm cell number was determined concomitantly with nucellus disintegration. These results implied the coordinated development of the endosperm with maternal tissues. In addition, differences of inner-integument cell number and endosperm cell number were related to a difference of endosperm size between superior and inferior caryopses.
TL;DR: Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water and suggest that the potential influence of enriched phloem water should be considered when interpreting the 18O / 16O signatures of plant organic material and plant cellulose.
Abstract: We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The 18O / 16O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of αo=1.027, where αo=Ro / Rw , and Ro and Rw are 18O / 16O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14–22‰ compared with source water under our experimental conditions, and depleted by 4–7‰, compared with evaporative site water. This showed that it is the average lamina leaf water 18O / 16O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the 18O / 16O signatures of plant organic material and plant cellulose.