Arabidopsis growth under prolonged high temperature and water deficit: independent or interactive effects?

High temperature (HT) and water deficit (WD) are frequent environmental constraints restricting plant growth and productivity. These stresses often occur simultaneously in the field, but little is known about their combined impacts on plant growth, development and physiology. We evaluated the responses of 10 Arabidopsis thaliana natural accessions to prolonged elevated air temperature (30 °C) and soil WD applied separately or in combination. Plant growth was significantly reduced under both stresses and their combination was even more detrimental to plant performance. The effects of the two stresses were globally additive, but some traits responded specifically to one but not the other stress. Root allocation increased in response to WD, while reproductive allocation, hyponasty and specific leaf area increased under HT. All the traits that varied in response to combined stresses also responded to at least one of them. Tolerance to WD was higher in small-sized accessions under control temperature and HT and in accessions with high biomass allocation to root under control conditions. Accessions that originate from sites with higher temperature have less stomatal density and allocate less biomass to the roots when cultivated under HT. Independence and interaction between stresses as well as the relationships between traits and stress responses are discussed.     

Leaf carbohydrate controls over Arabidopsis growth and response to elevated CO2: an experimentally based model

Transient starch production is thought to strongly control plant growth and response to elevated CO2. We tested this hypothesis with an experimentally based mechanistic model in Arabidopsis thaliana. Experiments were conducted on wild-type (WT) A. thaliana, starch-excess (sex1) and starchless (pgm) mutants under ambient and elevated CO2 conditions to determine parameters and validate the model. The model correctly predicted that mutant growth is approx. 20% of that in WT, and the absolute response of both mutants to elevated CO2 is an order of magnitude lower than in WT. For sex1, direct starch unavailability explained the growth responses. For pgm, we demonstrated experimentally that maintenance respiration is proportional to leaf soluble sugar concentration, which gave the necessary feedback mechanism on modelled growth. Our study suggests that the effects of sugar-starch cycling on growth can be explained by simple allocation processes, and the maximum rate of leaf growth (sink capacity) exerts a strong control over the response to elevated CO2 of herbaceous plants such as A. thaliana.     

Canopy studies on ethylene-insensitive tobacco identify ethylene as a novel element in blue light and plant-plant signalling

Plants growing at high densities express shade avoidance traits as a response to the presence of neighbours. Enhanced shoot elongation is one of the best researched shade avoidance components and increases light capture in dense stands. We show here that also leaf movements, leading to a more vertical leaf orientation (hyponasty), may be crucial in the early phase of competition. The initiation of shade avoidance responses is classically attributed to the action of phytochrome photoreceptors that sense red:far-red (R:FR) ratios in light reflected by neighbours, but also other signals may be involved. It was recently shown that ethylene-insensitive, transgenic (Tetr) tobacco plants, which are insensitive to the gaseous plant hormone ethylene, have reduced shade avoidance responses to neighbours. Here, we report that this is not related to a reduced response to low R:FR ratio, but that Tetr tobacco plants are unresponsive to a reduced photon fluence rate of blue light, which normally suppresses growth inhibition in wild-type (WT) plants. In addition to these light signals, ethylene levels in the canopy atmosphere increased to concentrations that could induce shade avoidance responses in WT plants. Together, these data show that neighbour detection signals other than the R:FR ratio are more important than previously anticipated and argue for a particularly important role for ethylene in determining plant responses to neighbours.     

Plastic responses in the metabolome and functional traits of maize plants to temperature variations

Environmentally inducible phenotypic plasticity is a major player in plant responses to climate change. However, metabolic responses and their role in determining the phenotypic plasticity of plants that are subjected to temperature variations remain poorly understood. The metabolomic profiles and metabolite levels in the leaves of three maize inbred lines grown in different temperature conditions were examined with a nuclear magnetic resonance metabolomic technique. The relationship of functional traits to metabolome profiles and the metabolic mechanism underlying temperature variations were then explored. A comparative analysis showed that during heat and cold stress, maize plants shared common plastic responses in biomass accumulation, carbon, nitrogen, sugars, some amino acids and compatible solutes. We also found that the plastic response of maize plants to heat stress was different from that under cold stress, mainly involving biomass allocation, shikimate and its aromatic amino acid derivatives, and other non‐polar metabolites. The plastic responsiveness of functional traits of maize lines to temperature variations was low, while the metabolic responsiveness in plasticity was high, indicating that functional and metabolic plasticity may play different roles in maize plant adaptation to temperature variations. A linear regression analysis revealed that the maize lines could adapt to growth temperature variations through the interrelation of plastic responses in the metabolomes and functional traits, such as biomass allocation and the status of carbon and nitrogen. We provide valuable insight into the plastic response strategy of maize plants to temperature variations that will permit the optimisation of crop cultivation in an increasingly variable environment.     

Physiological and morphological short-term responses to light and temperature in two Nothofagus species of Patagonia, South America

The study of plant responses to environmental stress factors is essential for management of plant systems and for anticipating their response to climate change. The main goal of this study was to determine morphological and physiological responses of Nothofagus obliqua and N. nervosa seedlings to light and temperature, two of the main stress factors acting in their current natural distribution in NW Patagonia. Responses to light were evaluated analyzing growth and survival, as well as morphological and physiological traits related to them, in seedlings subjected to three contrasting light conditions (full-sun conditions, 50% of sunlight and 20% of sunlight) during one growth season. Temperature photosynthetic responses were evaluated in seedlings subjected to temperature treatments between ?5 and 40°C for 2 and 4 h. Growth rate and biomass partition were similar between light treatments in both species. High apical meristem damage and decreased photosynthetic capacity of preformed leaves were observed under full-sun conditions, suggesting that high light levels have a deleterious effect on plant yield. Both species produced neoformed leaves during the growing season with better photosynthetic capacity than preformed leaves under full sun conditions, contributing to plant acclimation. Almost no plasticity was observed in morphological traits in response to shade. Both species differed in optimum temperature for photosynthesis, with a wider temperature range at which high photosynthesis is maintained in N. obliqua. In both species the higher values of net photosynthetic rate were found at higher temperatures than the mean annual temperature of its current natural distribution range. Under no water-stress conditions, future higher temperatures could increase carbon fixation of these species, with a little advantage of N. obliqua if temperature variance is high. Synergy effect of various environmental stress factors, particularly considering cultivation of these species outside their current natural distribution sites require further studies.     

Effects of long‐term individual and combined water and temperature stress on the growth of rice,wheat and maize: relationship with morphological and physiological acclimation

This study evaluates the long‐term individual and combined effects of high temperature (HT) and water deficit (WD) stress on plant growth, leaf gas‐exchange and water use efficiency in cultivars of the three most important crops worldwide, rice, wheat and maize. Total plant biomass (B_t) accumulation decreased under all treatments, being the combined HT–WD treatment the most detrimental in all three species. Although decreases in B_t correlated with adjustments in biomass allocation patterns (i.e. the leaf area ratio), most of the variation observed in B_t was explained by changes in leaf gas exchange parameters. Thus, integrated values of leaf carbon balance obtained from daily course measurements of photosynthesis and respiration were better predictors of plant growth than the instantaneous measurements of leaf gas exchange. Leaf water use efficiency, assessed both by gas exchange and carbon isotope measurements, was negatively correlated with B_t under WD, but not under the combined WD and HT treatment. A comparative analysis of the negative effects of single and combined stresses on the main parameters showed an additive component for WD and HT in rice and maize, in contrast to wheat. Overall, the results of the specific cultivars included in the study suggest that the species native climate plays a role shaping the species acclimation potential to the applied stresses. In this regard, wheat, originated in a cold climate, was the most affected species, which foretells a higher affectation of this crop due to climate change.     

Effects of elevated ozone and low light on diurnal and seasonal carbon gain in sugar maple

The long‐term interactive effects of ozone and light on whole‐tree carbon balance of sugar maple (Acer saccharum Marsh.) seedlings were examined, with an emphasis on carbon acquisition, foliar partitioning into starch and soluble sugars, and allocation to growth. Sugar maple seedlings were fumigated with ambient, 1·7 × ambient and 3·0 × ambient ozone in open‐top chambers for 3 years under low and high light (15 and 35% full sunlight, respectively). Three years of ozone fumigation reduced the total biomass of seedlings in the low‐ and high‐light treatments by 64 and 41%, respectively, but had no effect on whole‐plant biomass allocation. Ozone had no effect on net photosynthesis until late in the growing season, with low‐light seedlings generally exhibiting more pronounced reductions in photosynthesis. The late‐season reduction in photosynthesis was not due to impaired stomatal function, but was associated more with accelerated senescence or senescence‐like injury. In contrast, the 3·0 × ambient ozone treatment immediately reduced diurnal starch accumulation in leaves by over 50% and increased partitioning of total non‐structural carbohydrates into soluble sugars, suggesting that injury repair processes may be maintaining photosynthesis in late spring and early summer at the expense of storage carbon. The results in the present study indicate that changes in leaf‐level photosynthesis may not accurately predict the growth response of sugar maple to ozone in different light environments. The larger reduction in seedling growth under low‐light conditions suggests that seedlings in gap or closed‐canopy environments are more susceptible to ozone than those in a clearing. Similarly, understanding the effects of tropospheric ozone on net carbon gain of a mature tree will require scaling of leaf‐level responses to heterogeneous light environments, where some leaves may be more susceptible than others.     

Ecosystem assembly and terrestrial carbon balance under elevated CO(2)

Research aimed at understanding how the global carbon balance will change with elevated CO(2) has largely ignored the responses of individual species and genotypes. Yet, plant traits strongly influence the biogeochemical cycling of carbon. Here, we illustrate how differences in inter- and intraspecific responses to elevated CO(2) affect not only physiology and growth, but also higher order biotic interactions and lifetime fitness, ultimately leading to new ecosystem assemblages. We assert that the unique combination of inter- and intraspecific traits in these ecosystem assemblages ultimately determine how ecosystems respond to elevated atmospheric CO(2). Thus, the identity of species and genotypes in an ecosystem is a crucial element to consider in forecasts of global carbon balance.     

Interactions between ethylene and gibberellins in phytochrome-mediated shade avoidance responses in tobacco

Plants respond to proximate neighbors with a suite of responses that comprise the shade avoidance syndrome. These phytochrome-mediated responses include hyponasty (i.e. a more vertical orientation of leaves) and enhanced stem and petiole elongation. We showed recently that ethylene-insensitive tobacco (Nicotiana tabacum) plants (Tetr) have reduced responses to neighbors, showing an important role for this gaseous plant hormone in shade avoidance. Here, we investigate interactions between phytochrome signaling and ethylene action in shade avoidance responses. Furthermore, we investigate if ethylene acts in these responses through an interaction with the GA class of hormones. Low red to far-red light ratios (R:FR) enhanced ethylene production in wild-type tobacco, resulting in shade avoidance responses, whereas ethylene-insensitive plants showed reduced shade avoidance responses. Plants with inhibited GA production showed hardly any shade avoidance responses at all to either a low R:FR or increased ethylene concentrations. Furthermore, low R:FR enhanced the responsiveness of hyponasty and stem elongation in both wild-type and Tetr plants to applied GA(3), with the stem elongation process being more responsive to GA(3) in the wild type than in Tetr. We conclude that phytochrome-mediated shade avoidance responses involve ethylene action, at least partly by modulating GA action.     

Ecophysiological and bioclimatic foundations for a global plant functional classification

Question: What plant properties might define plant functional types (PFTs) for the analysis of global vegetation responses to climate change, and what aspects of the physical environment might be expected to predict the distributions of PFTs? Methods: We review principles to explain the distribution of key plant traits as a function of bioclimatic variables. We focus on those whole‐plant and leaf traits that are commonly used to define biomes and PFTs in global maps and models. Results: Raunkiær's plant life forms (underlying most later classifications) describe different adaptive strategies for surviving low temperature or drought, while satisfying requirements for reproduction and growth. Simple conceptual models and published observations are used to quantify the adaptive significance of leaf size for temperature regulation, leaf consistency for maintaining transpiration under drought, and phenology for the optimization of annual carbon balance. A new compilation of experimental data supports the functional definition of tropical, warm‐temperate, temperate and boreal phanerophytes based on mechanisms for withstanding low temperature extremes. Chilling requirements are less well quantified, but are a necessary adjunct to cold tolerance. Functional traits generally confer both advantages and restrictions; the existence of trade‐offs contributes to the diversity of plants along bioclimatic gradients. Conclusions: Quantitative analysis of plant trait distributions against bioclimatic variables is becoming possible; this opens up new opportunities for PFT classification. A PFT classification based on bioclimatic responses will need to be enhanced by information on traits related to competition, successional dynamics and disturbance.     

Foraging for nutrients, responses to changes in light, and competition in tropical deciduous tree seedlings

We evaluated (1) the responses of two co-occurring tropical tree species, Heliocarpuspallidus and Caesalpiniaeriostachys, to changes in light, (2) the ability of these species to search for and exploit a fertilized soil patch, (3) the relationship between the capacity to forage for a fertilized patch and the capacity to respond to changes in light availability and (4) how the relationship between light and nutrient acquisition influenced the competitive interactions between these species. Plants of the two species were exposed to a factorial combination of high (H) and low (L) light intensity and fertilized (+Fp) and unfertilized (−Fp) nutrient patches for 50 days. Half of the plants from H were then transferred to L (HL treatment), and half of the plants from L were transferred to H (LH). The remaining plants were kept in their original light condition and grown for another 50 days. Plants were grown in these light and patch treatments alone (one plant per pot) and in interspecific competition (one plant per species resulting in two plants per pot). Both species exploited fertilized patches by increasing root biomass and length in the patch. This enhanced plant productivity and growth rate mainly under LH and HH conditions for Heliocarpus and the HH condition for Caesalpinia). When plants in the HH light environment were grown with an unfertilized patch, plant biomass and relative growth rates (RGRs) were even lower than␣under the LL light environment [(HH–Fp)<LL]. However, the combined activity of shoot and roots when above- and below-ground resources were temporally and spatially heterogeneous influenced plant productivity and growth rate. The benefit from light increase (LH) was reduced when grown with an unfertilized patch. Larger reductions in root biomass, length and density in the patch, and in plant biomass and RGR, were exhibited by Heliocarpus than by Caesalpinia. These results suggest a close relationship between root foraging and light capture, where the benefit of the exploitation of the patch will be reflected in whole-plant benefit, if enough light is captured above-ground. In addition, the results suggest a change in the expected plant responses to light due to heterogeneity in soil nutrients, even though the fertilized patch was only a small proportion of the total soil volume. Leaf characteristics such as specific leaf area responded only to light conditions and not to patchily distributed nutrients. Root characteristics responded more strongly to nutrient heterogeneity. Competition modified the pattern of foraging under both high- and low-light conditions in Heliocarpus by 50 days, and the ability to forage for a fertilized patch under LL after 100 days of growth for Caesalpinia. Even though plant growth and productivity are greatly reduced under low-light conditions (HL and LL), competition modifies the ability of species to forage for a rich patch (especially for the fast-growing species Heliocarpus). Received: 24 November 1997 / Accepted: 15 June 1998     

Distinct responses of the mitochondrial respiratory chain to long- and short-term high-light environments in Arabidopsis thaliana

In order to ensure the cooperative function with the photosynthetic system, the mitochondrial respiratory chain needs to flexibly acclimate to a fluctuating light environment. The non-phosphorylating alternative oxidase (AOX) is a notable respiratory component that may support a cellular redox homeostasis under high-light (HL) conditions. Here we report the distinct acclimatory manner of the respiratory chain to long- and short-term HL conditions and the crucial function of AOX in Arabidopsis thaliana leaves. Plants grown under HL conditions (HL plants) possessed a larger ubiquinone (UQ) pool and a higher amount of cytochrome c oxidase than plants grown under low light conditions (LL plants). These responses in HL plants may be functional for efficient ATP production and sustain the fast plant growth. When LL plants were exposed to short-term HL stress (sHL), the UQ reduction level was transiently elevated. In the wild-type plant, the UQ pool was re-oxidized concomitantly with an up-regulation of AOX. On the other hand, the UQ reduction level of the AOX-deficient aox1a mutant remained high. Furthermore, the plastoquinone pool was also more reduced in the aox1a mutant under such conditions. These results suggest that AOX plays an important role in rapid acclimation of the respiratory chain to sHL, which may support efficient photosynthetic performance.     

Photosynthetic acclimation in the context of structural constraints to carbon export from leaves

The potential role of foliar carbon export features in the acclimation of photosynthetic capacity to differences and changes in light environment was evaluated. These features included apoplastic vs. symplastic phloem loading, density of loading veins, plasmodesmatal frequency in intermediary cells, and the ratio of loading cells to sieve elements. In initial studies, three apoplastic loaders (spinach, pea, Arabidopsis thaliana) exhibited a completely flexible photosynthetic response to changing light conditions, while two symplastic loaders (pumpkin, Verbascum phoeniceum), although able to adjust to different long-term growth conditions, were more limited in their response when transferred from low (LL) to high (HL) light. This suggested that constraints imposed by the completely physical pathway of sugar export might act as a bottleneck in the export of carbon from LL-acclimated leaves of symplastic loaders. While both symplastic loaders exhibited variable loading vein densities (low in LL and high in HL), none of the three apoplastic loaders initially characterized exhibited such differences. However, an additional apoplastic species (tomato) exhibited similar differences in vein density during continuous growth in different light environments. Furthermore, in contrast to the other apoplastic loaders, photosynthetic acclimation in tomato was not complete following a transfer from LL to HL. This suggests that loading vein density and loading cells per sieve element, and thus apparent loading surface capacity, play a major role in the potential for photosynthetic acclimation to changes in light environment. Photosynthetic acclimation and vein density acclimation were also characterized in the slow-growing, sclerophytic evergreen Monstera deliciosa. This evergreen possessed a lower vein density during growth in LL compared to HL and exhibited a more severely limited potential for photosynthetic acclimation to increases in light environment than the rapidly-growing, mesophytic annuals.     

Molecular mechanisms governing plant responses to high temperatures

The increased prevalence of high temperatures(HTs) around the world is a major global concern, as they dramatically affect agronomic productivity. Upon HT exposure, plants sense the temperature change and initiate cellular and metabolic responses that enable them to adapt to their new environmental conditions.Decoding the mechanisms by which plants cope with HT will facilitate the development of molecular markers to enable the production of plants with improved thermotolerance. In recent decades, genetic, physiological, molecular, and biochemical studies have revealed a number of vital cellular components and processes involved in thermoresponsive growth and the acquisition of thermotolerance in plants. This review summarizes the major mechanisms involved in plant HT responses, with a special focus on recent discoveries related to plant thermosensing, heat stress signaling, and HT-regulated gene expression networks that promote plant adaptation to elevated environmental temperatures.     

Photomorphogenesis and photoassimilation in soybean and sorghum grown under broad spectrum or blue-deficient light sources

The role of blue light in plant growth and development was investigated in soybean (Glycine max [L.] Merr. cv Williams) and sorghum (Sorghum bicolor [L.] Moench. cv Rio) grown under equal photosynthetic photon fluxes (approximately 500 micromoles per square meter per second) from broad spectrum daylight fluorescent or blue-deficient, narrow-band (589 nanometers) low pressure sodium (LPS) lamps. Between 14 and 18 days after sowing, it was possible to relate adaptations in photosynthesis and leaf growth to dry matter accumulation. Soybean development under LPS light was similar in several respects to that of shaded plants, consistent with an important role for blue light photoreceptors in regulation of growth response to irradiance. Thus, soybeans from LPS conditions partitioned relatively more growth to leaves and maintained higher average leaf area ratios (mean LAR) that compensated lower net assimilation rates (mean NAR). Relative growth rates were therefore comparable to plants from daylight fluorescent lamps. Reductions in mean NAR were matched by lower rates of net photosynthesis (A) on an area basis in the major photosynthetic source (first trifoliolate) leaf. Lower A in soybean resulted from reduced leaf dry matter per unit leaf area, but lower A under LPS conditions in sorghum correlated with leaf chlorosis and reduced total nitrogen (not observed in soybean). In spite of a lower A, mean NAR was larger in sorghum from LPS conditions, resulting in significantly greater relative growth rates (mean LAR was approximately equal for both light conditions). Leaf starch accumulation rate was higher for both species and starch content at the end of the dark period was elevated two- and three-fold for sorghum and soybean, respectively, under LPS conditions. Possible relations between starch accumulation, leaf export, and plant growth in response to spectral quality were considered.     

Natural variation for carbohydrate content in Arabidopsis. Interaction with complex traits dissected by quantitative genetics

Besides being a metabolic fuel, carbohydrates play important roles in plant growth and development, in stress responses, and as signal molecules. We exploited natural variation in Arabidopsis (Arabidopsis thaliana) to decipher the genetic architecture determining carbohydrate content. A quantitative trait locus (QTL) approach in the Bay-0 x Shahdara progeny grown in two contrasting nitrogen environments led to the identification of 39 QTLs for starch, glucose, fructose, and sucrose contents representing at least 14 distinct polymorphic loci. A major QTL for fructose content (FR3.4) and a QTL for starch content (ST3.4) were confirmed in heterogeneous inbred families. Several genes associated with carbon (C) metabolism colocalize with the identified QTL. QTLs for senescence-related traits, and for flowering time, water status, and nitrogen-related traits, previously detected with the same genetic material, colocalize with C-related QTLs. These colocalizations reflect the complex interactions of C metabolism with other physiological processes. QTL fine-mapping and cloning could thus lead soon to the identification of genes potentially involved in the control of different connected physiological processes.