Contrasting responses to drought stress in herbaceous perennial legumes

Background and aims Medicago sativa L. is widely grown in southern Australia, but is poorly adapted to dry, hot summers. This study aimed to identify perennial herbaceous legumes with greater resistance to drought stress and explore their adaptive strategies. Methods Ten herbaceous perennial legume species/accessions were grown in deep pots in a sandy, low-phosphorus field soil in a glasshouse. Drought stress was imposed by ceasing to water. A companion M. sativa plant in each pot minimised differences in leaf area and water consumption among species. Plants were harvested when stomatal conductance of stressed plants decreased to around 10% of well watered plants. Results A range of responses to drought stress were identified, including: reduced shoot growth; leaf curling; thicker pubescence on leaves and stems; an increased root:shoot ratio; an increase, decrease or no change in root distribution with depth; reductions in specific leaf area or leaf water potential; and osmotic adjustment. The suite of changes differed substantially among species and, less so, among accessions. Conclusions The inter- and intra-specific variability of responses to drought-stress in the plants examined suggests a wide range of strategies are available in perennial legumes to cope with drying conditions, and these could be harnessed in breeding/selection programs.

Seasonal drought stress in the Amazon: Reconciling models and observations

The Amazon Basin is crucial to global circulatory and carbon patterns due to the large areal extent and large flux magnitude. Biogeophysical models have had difficulty reproducing the annual cycle of net ecosystem exchange (NEE) of carbon in some regions of the Amazon, generally simulating uptake during the wet season and efflux during seasonal drought. In reality, the opposite occurs. Observational and modeling studies have identified several mechanisms that explain the observed annual cycle, including: (1) deep soil columns that can store large water amount, (2) the ability of deep roots to access moisture at depth when near-surface soil dries during annual drought, (3) movement of water in the soil via hydraulic redistribution, allowing for more efficient uptake of water during the wet season, and moistening of near-surface soil during the annual drought, and (4) photosynthetic response to elevated light levels as cloudiness decreases during the dry season. We incorporate these mechanisms into the third version of the Simple Biosphere model (SiB3) both singly and collectively, and confront the results with observations. For the forest to maintain function through seasonal drought, there must be sufficient water storage in the soil to sustain transpiration through the dry season in addition to the ability of the roots to access the stored water. We find that individually, none of these mechanisms by themselves produces a simulation of the annual cycle of NEE that matches the observed. When these mechanisms are combined into the model, NEE follows the general trend of the observations, showing efflux during the wet season and uptake during seasonal drought.

Effects of Silicon and Drought Stress on Tuber Yield and Leaf Biochemical Characteristics in Potato

Silicon has beneficial effects on many crops, mainly under biotic and abiotic stresses. Silicon can affect biochemical, physiological, and photosynthetic processes and, consequently, alleviates drought stress. However, the effects of Si on potato (Solanum tuberosum L.) plants under drought stress are still unknown. The objective of this study was to evaluate the effect of Si supply on some biochemical characteristics and yield of potato tubers, either exposed or not exposed to drought stress. The experiment was conducted in pots containing 50 dm(3) of a Typic Acrortox soil (33% clay, 4% silt, and 63% sand). The treatments consisted of the absence or presence of Si application (0 and 284.4 mg dm(-3)), through soil amelioration with dolomitic lime and Ca and Mg silicate, and in the absence or presence of water deficit (-0.020 MPa and -0.050 MPa soil water potential, respectively), with eight replications. Silicon application and water deficit resulted in the greatest Si concentration in potato leaves. Proline concentrations increased under lower water availability and higher Si availability in the soil, which indicates that Si may be associated with plant osmotic adjustment. Water deficit and Si application decreased total sugars and soluble proteins concentrations in the leaves. Silicon application reduced stalk lodging and increased mean tuber weight and, consequently, tuber yield, especially in the absence of water stress.

Transcription factors expressed in soybean roots under drought stress

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Root niche separation can explain avoidance of seasonal drought stress and vulnerability of overstory trees to extended drought in a mature Amazonian forest

Large areas of Amazonian evergreen forest experience seasonal droughts extending for three or more months, yet show maximum rates of photosynthesis and evapotranspiration during dry intervals. This apparent resilience is belied by disproportionate mortality of the large trees in manipulations that reduce wet season rainfall, occurring after 2-3 years of treatment. The goal of this study is to characterize the mechanisms that produce these contrasting ecosystem responses. A mechanistic model is developed based on the ecohydrological framework of TIN (Triangulated Irregular Network)-based Real Time Integrated Basin Simulator + Vegetation Generator for Interactive Evolution (tRIBS+VEGGIE). The model is used to test the roles of deep roots and soil capillary flux to provide water to the forest during the dry season. Also examined is the importance of "root niche separation," in which roots of overstory trees extend to depth, where during the dry season they use water stored from wet season precipitation, while roots of understory trees are concentrated in shallow layers that access dry season precipitation directly. Observational data from the Tapajo's National Forest, Brazil, were used as meteorological forcing and provided comprehensive observational constraints on the model. Results strongly suggest that deep roots with root niche separation adaptations explain both the observed resilience during seasonal drought and the vulnerability of canopy-dominant trees to extended deficits of wet season rainfall. These mechanisms appear to provide an adaptive strategy that enhances productivity of the largest trees in the face of their disproportionate heat loads and water demand in the dry season. A sensitivity analysis exploring how wet season rainfall affects the stability of the rainforest system is presented. Citation: Ivanov, V. Y., L. R. Hutyra, S. C. Wofsy, J. W. Munger, S. R. Saleska, R. C. de Oliveira Jr., and P. B. de Camargo (2012), Root niche separation can explain avoidance of seasonal drought stress and vulnerability of overstory trees to extended drought in a mature Amazonian forest, Water Resour. Res., 48, W12507, doi:10.1029/2012WR011972.

Potential drought stress in a Swiss mountain catchment-Ensemble forecasting of high mountain soil moisture reveals a drastic decrease, despite major uncertainties

Climate change is expected to profoundly influence the hydrosphere of mountain ecosystems. The focus of current process-based research is centered on the reaction of glaciers and runoff to climate change; spatially explicit impacts on soil moisture remain widely neglected. We spatio-temporally analyzed the impact of the climate on soil moisture in a mesoscale high mountain catchment to facilitate the development of mitigation and adaptation strategies at the level of vegetation patterns. Two regional climate models were downscaled using three different approaches (statistical downscaling, delta change, and direct use) to drive a hydrological model (WaSiM-ETH) for reference and scenario period (1960–1990 and 2070–2100), resulting in an ensemble forecast of six members. For all ensembles members we found large changes in temperature, resulting in decreasing snow and ice storage and earlier runoff, but only small changes in evapotranspiration. The occurrence of downscaled dry spells was found to fluctuate greatly, causing soil moisture depletion and drought stress potential to show high variability in both space and time. In general, the choice of the downscaling approach had a stronger influence on the results than the applied regional climate model. All of the results indicate that summer soil moisture decreases, which leads to more frequent declines below a critical soil moisture level and an advanced evapotranspiration deficit. Forests up to an elevation of 1800 m a.s.l. are likely to be threatened the most, while alpine areas and most pastures remain nearly unaffected. Nevertheless, the ensemble variability was found to be extremely high and should be interpreted as a bandwidth of possible future drought stress situations.

Drought Stress alters Solute Allocation in Broadleaf Dock (Rumex obtusifolius)

According to climate models, drier summers must be expected more frequently in Central Europe during the next decades, which may influence plant performance and competition in grassland. The overall source–sink relations in plants, especially allocation of solutes to above- and below-ground parts, may be affected by drought. To investigate solute export from a given leaf of broadleaf dock, a solution containing 57Co and 65Zn was introduced through a leaf flap. The export from this leaf was detected by analysing radionuclide contents in various plant parts. Less label was allocated to new leaves and more to roots under drought. The observed alterations of source–sink relations in broadleaf dock were reversible during a subsequent short period of rewatering. These findings suggest an increased resource allocation to roots under drought improving the functionality of the plants.

Impacts of drought stress on water relations and carbon assimilation in grassland species at different altitudes

Grassland is an important ecosystem type which is not only used agriculturally, but also covers sites which cannot be used for other purposes, e.g. in very steep locations or above timberlines. Prolonged summer droughts in the mid-term future, as are predicted for Central Europe, are expected to have a major impact on such ecosystems. To address this topic, rainfall exclusion via shelters was performed on three grassland sites at different altitudes (393, 982 and 1978 m above sea level) in Switzerland. Diurnal drought treatment effects were studied at each study site on a completely sunny day towards the end of an 8–10 week shelter period. Ecophysiological parameters including gas exchange (An, gs and intrinsic WUE) and chlorophyll a fluorescence (Fv/Fm, ΦPSII and NPQ) were considered for several species. The lowland and the Alpine field site were more strongly affected by soil drought than the pre-Alpine site. At all sites, grasses showed different patterns of reductions in stomatal conductance under soil drought compared to legumes and forbs. In addition, grasses were significantly more affected by reductions in assimilation rates at all sites. Time courses of reductions in assimilation rates relative to controls differed between species at the Alpine site, as some species showed reduced assimilation rates at this site in the early morning. Thus, similar rainfall exclusion treatments can trigger different reactions in various species at different sites, which might not become obvious during mere midday measurements. Overall, results suggest strong impacts of prolonged summer drought on grassland net photosynthesis especially at the Alpine site and, within sites, for grasses