Severe Soil Water Deficit Research Articles

Divergent responses of deep SOC sequestration to large-scale revegetation on China’s Loess Plateau

Dryland revegetation can mitigate land degradation, increase vegetation coverage and terrestrial C sinks, and substantially decrease soil water storage. However, variation in soil organic C (SOC) sequestration with precipitation under large-scale ecological restoration is unclear, and the trade-off between SOC sequestration and water consumption under different vegetation types remain controversial, particularly in deep soils. This study investigated SOC storage (SOCS) and soil water storage (SWS) in 0–1000 cm profiles under various vegetation types (woodland, shrubland, grassland, and farmland) along a precipitation gradient from 406 mm to 606 mm on China’s Loess Plateau. The four vegetation types exhibited similar trends in the spatial distribution of SOCS and SWS, significantly increasing as mean annual precipitation (MAP) increased, except for SWS in shrubland. In contrast, deep SOC sequestration and water deficit effects varied with vegetation type and MAP. Specifically, ΔSOCS (SOCS difference between restored vegetation and farmland) of woodland significantly decreased with increasing MAP, whereas no significant relationship occurred for shrubland and grassland; significant positive and negative linear correlations occurred between ΔSWS (SWS difference between restored vegetation and farmland) and MAP for woodland and shrubland, respectively, with no apparent relationship for grassland. Moreover, the three restored vegetation had positive correlations between ΔSOCS and ΔSWS in deep soils. Woodland had the highest SOC sequestration efficiency (ΔSOCS/ΔSWS) but also caused the most severe soil water deficit, while grassland had the lowest C and water effect. Therefore, shrublands are better for revegetation in semiarid regions, with relatively lower soil water deficit than woodland and higher SOC sequestration effect than grassland; multispecies forest plantations dominated by R. pseudoacacia may be more suitable than single-species plantations in semihumid regions. These findings highlight the SOC sequestration potential of deep soils and offer insights into dryland revegetation.

Arbuscular mycorrhizal fungi increase the interspecific competition between two forage plant species and stabilize the soil microbial network during a drought event: Evidence from the field

Arbuscular mycorrhizal fungi (AMF) have been found to help plants adapt to water deficit conditions. However, it still lacks filed evidence on how AMF affect plant physiology and soil microbial communities under natural extreme drought events. Here, we conducted an AMF-inoculation and legume-intercropping (Medicago sativa) field experiment in a forage (Broussonetia papyrifera) monoculture ecosystem, and assessed the effects of AMF on plants and soil microbes during a severe drought event lasting for 40 d. The results showed that the mycorrhizal colonization rate was significantly higher for M. sativa than B. papyrifera. In plots with both B. papyrifera and M. sativa, the physiological properties (e.g., diurnal range of water potential and malondialdehyde concentration) of M. sativa were better in the AMF-inoculation plots than in non-inoculation plots; the trend was opposite for B. papyrifera. AMF-inoculation significantly increased the aboveground productivity of M. sativa by increasing diurnal range of water potential and stem water content during the drought event, resulting in more severe soil water deficits. AMF-inoculation or/and legume-intercropping weakened taxonomic interactions (i.e., edges) and increased the modularity of co-occurrence networks compared with the control. The structural equation model analysis indicated that soil nutrients had a direct positive correlation with soil microbial community composition, whereas soil nutrients and moisture had an indirect negative correlation with soil microbial community diversity by inducing changes in aboveground biomass of B. papyrifera. Overall, AMF-inoculation helps M. sativa acquire water during the drought event, which exacerbates soil water deficits and thereby may increase interspecific water competition between B. papyrifera and M. sativa. AMF-inoculation or/and legume-intercropping maintain the stability of microbial networks by weakening taxonomic interactions and increasing the modularity under the extreme drought event. Our findings improve the understanding of AMF effects on plant-plant and plant-soil interactions under climate extremes.

Relative Water Content, Proline, and Antioxidant Enzymes in Leaves of Long Shelf-Life Tomatoes under Drought Stress and Rewatering.

Some physiological, oxidative, and antioxidant enzymatic patterns were assessed in plants of three local Sicilian landraces of long shelf-life tomatoes ('Custonaci', 'Salina', and 'Vulcano'), as compared to the commercial tomato hybrid 'Faino' (control). Three water treatments were considered in open-field: DRY (no irrigation); IRR (long-season full irrigation); REW (drought/rewatering cycles). During the growing season, soil water deficit (SWD) was estimated, and relative water content (RWC), specific leaf area (SLA), proline and malondialdehyde (MDA) content, and glutathione peroxidase (GPX), catalase (CAT), and superoxide dismutase (SOD) activities were measured in leaves. Differently from control, RWC in local landraces exhibited a similar pattern in REW and DRY, indicating a low capacity to re-hydrate after rewatering. Positive correlation of proline content vs. SWD in all local landraces highlights an osmotic adjustment occurring in these tomatoes in response to limited soil water content. Long shelf-life tomatoes suffered minor oxidative stress due to severe soil water deficit, as revealed by the lower levels of MDA with respect to the control. Significant correlation of CAT vs. SWD for all tomatoes indicates that this antioxidant enzyme, among those analyzed, may be considered as a biomarker for a water stress condition more than for oxidative stress due to water deficit.

Response of dew and hydraulic redistribution to soil water in a rainfed dryland jujube plantation in China’s Hilly Loess Region

Soil desiccation occurs widely across China’s Loess Plateau due to long-term water demand in excess of water supply. Hydraulic redistribution (HR) - the movement of water from wet to dry soil via roots - and canopy dew (CD) are crucial to water replenishment in dryland ecosystems. However, much is still not known about the response of CD and HR to soil water deficits (including no deficit, slight deficit, moderate deficit, and severe deficit water conditions) in dryland jujube (Ziziphus jujuba Mill.) plantations in China’s Hilly Loess Region. To this end, the duration and intensity of dew were monitored using dielectric leaf wetness sensors to better understand the effects of soil water status on dew. Also, soil water content, sap flow, and meteorological variables were measured during the jujube growing seasons in 2017–2019. The results showed that the time of occurrence of CD under the no deficit condition came earlier than the time of occurrence of CD under slight, moderate, and severe soil water deficit conditions. Peak CD appeared at 5:00–6:30 am and ended at 6:30–9:00 am local time under the four soil water conditions. The amount, duration, and frequency of CD decreased with increasing soil water deficit. The amount and duration of CD under the no deficit condition were significantly higher than under the slight, moderate, and severe soil water deficit conditions (p < 0.05). The amount and frequency of HR (in the 20–60 cm soil layer) increased with increasing soil water deficit. Under the no deficit condition, HR was significantly lower than under the slight, moderate, and severe soil water deficit conditions (p < 0.05). While HR was influenced by CD, CD enhanced HR under the no deficit condition, but inhibited HR under slight, moderate, and severe soil water deficit conditions. For the growing season, the contribution of CD to total actual evapotranspiration (ETc act, CD/ ETc act ratio) was highest under the no deficit condition (maximum CD/ ETc act ratio of 0.21). The contribution of HR to ETc act (HR/ ETc act ratio) was higher under slight, moderate, and severe soil water deficit conditions (maximum of 0.11). The mutual effects of CD and HR lessened soil water deficit, making them important ecohydrological processes affecting the response of dryland jujube plantations to soil desiccation.

Effects of Soil Water Deficit on Three Tree Species of the Arid Environment: Variations in Growth, Physiology, and Antioxidant Enzyme Activities

Low water availability predicted under climate change is a major abiotic factor limiting plants growth and productivity. In this study a greenhouse experiment was conducted on three important tree species of arid environment: Conocarpus erectus (CE), Acacia modesta (AM), and Salix tetrasperma (ST). Young saplings were subjected to control (C), medium (MWD) and severe soil water deficit (SWD) treatments and response was evaluated. Results showed that in all the three species leaf, stem and root dry weight production remained similar to C under MWD treatment but decreased significantly under SWD. The highest decrease in total dry weight was noticed in ST and the lowest was evidenced in AM under SWD. Root:shoot ratio increased significantly in both CE and AM under MWD and SWD. Furthermore, chlorophyll content decreased while proline content increased significantly in both MWD and SWD treatments. The production of oxidants (hydrogen peroxide and superoxide anions) and antioxidants (superoxide dismutase, catalase, peroxidase and ascorbate peroxidase) increased significantly under both MWD and SWD treatments and were the highest in AM in both MWD and SWD treatments. Therefore, we may conclude that all the three species can tolerate medium water stress due to increased root production and an effective antioxidant defense mechanism.

Transgenic chickpea (Cicer arietinum L.) harbouring AtDREB1a are physiologically better adapted to water deficit

BackgroundChickpea (Cicer arietinum L.) is the second most widely grown pulse and drought (limiting water) is one of the major constraints leading to about 40–50% yield losses annually. Dehydration responsive element binding proteins (DREBs) are important plant transcription factors that regulate the expression of many stress-inducible genes and play a critical role in improving the abiotic stress tolerance. Transgenic chickpea lines harbouring transcription factor, Dehydration Responsive Element-Binding protein 1A from Arabidopsis thaliana (AtDREB1a gene) driven by stress inducible promoter rd29a were developed, with the intent of enhancing drought tolerance in chickpea. Performance of the progenies of one transgenic event and control were assessed based on key physiological traits imparting drought tolerance such as plant water relation characteristics, chlorophyll retention, photosynthesis, membrane stability and water use efficiency under water stressed conditions.ResultsFour transgenic chickpea lines harbouring stress inducible AtDREB1a were generated with transformation efficiency of 0.1%. The integration, transmission and regulated expression were confirmed by Polymerase Chain Reaction (PCR), Southern Blot hybridization and Reverse Transcriptase polymerase chain reaction (RT-PCR), respectively. Transgenic chickpea lines exhibited higher relative water content, longer chlorophyll retention capacity and higher osmotic adjustment under severe drought stress (stress level 4), as compared to control. The enhanced drought tolerance in transgenic chickpea lines were also manifested by undeterred photosynthesis involving enhanced quantum yield of PSII, electron transport rate at saturated irradiance levels and maintaining higher relative water content in leaves under relatively severe soil water deficit. Further, lower values of carbon isotope discrimination in some transgenic chickpea lines indicated higher water use efficiency. Transgenic chickpea lines exhibiting better OA resulted in higher seed yield, with progressive increase in water stress, as compared to control.ConclusionsBased on precise phenotyping, involving non-invasive chlorophyll fluorescence imaging, carbon isotope discrimination, osmotic adjustment, higher chlorophyll retention and membrane stability index, it can be concluded that AtDREB1a transgenic chickpea lines were better adapted to water deficit by modifying important physiological traits. The selected transgenic chickpea event would be a valuable resource that can be used in pre-breeding or directly in varietal development programs for enhanced drought tolerance under parched conditions.

Effects of manipulated precipitation on aboveground net primary productivity of grassland fields: Controlled rainfall experiments in Inner Mongolia, China

AbstractRecovery from grassland degradation caused by climate change and human disturbance is critical to the sustainable development of drylands. Grassland in China accounts for 13% of the global grassland area, and more than half of it is still recovering from degradation. How climate change will contribute to the recovery of degraded grassland has not been clarified through field experiments. Here, we report the effects of manipulated precipitation regimes on above ground net primary productivity (ANPP) in five grassland fields along a degradation gradient using a 3‐year controlled rainfall manipulation experiment in Inner Mongolia, China. We found that differences of approximately 20–30% in growing season precipitation have limited impacts on the ANPP of grasslands, and the responses can depend crucially on the degradation status indicated by vegetation and soil features. ANPP sensitivity to spring drought increased with increasing degradation level, which was mainly determined by vegetation composition. ANPP was generally insensitive to summer drought except for mature grassland, in which drought led to severe soil water deficit, mainly mediated by soil water holding capacity. Our results suggest that the recovery of degraded grassland is expected to be delayed by increasing drought early in the growing season, and mature grassland would be more vulnerable to extreme summer drought. These findings highlight the negative impacts of seasonal drought on degraded grassland and emphasize the importance of accounting for climate change and degradation status in future grassland ecosystem restoration and management policies.

Physiological and Agronomic Responses of Processing Tomatoes to Deficit Irrigation at Critical Stages in a Semi-Arid Environment

Deficit irrigation is a valid alternative to conventional irrigation to save water while maintaining high productivity in tomatoes. However, crop sensitivity to water stress due to deficit irrigation may change with the growth stage. To assess the physiological and agronomic responses of processing tomatoes to deficit irrigation applied at critical stages, a field experiment was conducted in a coastal site of Southern Italy, where seven irrigation treatments differing for daily evapotranspiration (ETc) restored (100%—full or 50%—deficit) and the time of watering (long-season or limited to the vegetative period or to flowering) were applied to processing tomatoes cv. Hypeel F1. Plants continuously irrigated and those irrigated only at flowering maintained higher rates of leaf transpiration (E) and stomatal conductance (gs) over those irrigated only during the vegetative period. Fruit yield was the greatest under long-season full irrigation (51 t ha−1). Severe soil water deficit during flowering, more than during the vegetative period, adversely affected crop productivity. Irrigation water use efficiency (IWUE) was maximized under long-season deficit irrigation (&gt;19 kg m−3) or deficit irrigation during flowering (&gt;16 kg m−3). E and gs measured at early or mid-flowering may be adopted as valuable indicators to predict crop productivity; however, they may be altered under high vapor pressure deficit (VPD). Predawn water potential, being little affected by VPD, is a more reliable parameter than leaf transpiration and stomatal conductance under these climatic conditions.

The Effect of Insect Defoliations and Seed Production on the Dynamics of Radial Growth Synchrony among Scots Pine Pinus sylvestris L. Provenances

The intraspecific variation of climate–growth relationships observed on provenance trials results from among–provenance differences in phenotypic plasticity. Temporal variation in radial growth synchrony among provenances may be modified by adverse climatic/biotic conditions such as drought or insect defoliation. However, these factors can potentially diminish provenance–specific growth reactions and, consequently, prevent the identification of provenances with the highest adaptive potential. Thus, understanding the influence of major biotic conditions on provenance–specific climate–growth relationships seems to be important to anticipate climate change. To determine provenance–specific growth patterns in relation to climate conditions (drought), seed production (reproductive effort), and insect defoliation in a common garden of Scots pine (Pinus sylvestris L.), we applied dendroecological techniques to time–series of tree–ring widths and basal area increments. The long–term records of seed production and insect outbreaks from the local Scots pine stands were used to explain the potential effect of biotic factors on the temporal dynamics of radial growth synchrony. During a period of favorable growth conditions, Scots pine provenances showed a decline in inter–provenance synchronicity in growth patterns, while during years affected by severe soil water deficit and insect defoliation, they manifested high uniformity in growth dynamics. The long–term trend in growth synchrony among P. sylvestris provenances depend on both abiotic and biotic environmental factors. This gains significance following an introduction of the appropriate selection of tree provenances for climate–smart forestry.

Estimation of Actual Evapotranspiration in a Semiarid Region Based on GRACE Gravity Satellite Data—A Case Study in Loess Plateau

Jointly influenced by natural factors and artificial protection measures in recent years, the vegetation coverage of the Loess Plateau has significantly increased. However, extensive vegetation recovery can result in massive water consumption and a severe soil water deficit, which poses a great threat to the sustainable development of the regional ecological system. Maintaining the balance between precipitation and water consumption is an important foundation of ecological security in the Loess Plateau. Based on this, the present study used the GRACE (Gravity Recovery and Climate Experiment) gravity satellite data to simulate the annual actual water consumption from 2003 to 2014 and to analyze the temporal and spatial evolution of the regional precipitation and the actual evapotranspiration (AET). This study also applied the newly developed rainwater utilization potential index (IRUP) to quantify the sustainability of the water balance in the Loess Plateau. The spatial-temporal patterns of precipitation, potential evapotranspiration, and AET from 2003 to 2014 in the Loess Plateau were all analyzed in this study. Based on the results, the annual average precipitation (AAP) and AET in the entire Loess Plateau had significant increasing trends. The analysis of the spatial distribution reveals that the AET was decreasing from the southeast to the northwest in the Loess Plateau. However, the average values of potential evapotranspiration did not obviously change. Based on the estimated AET result, it was determined that the average IRUP had an increasing trend. The increase in the IRUP is due to an increased rate of precipitation that is statistically higher than that of the AET. Consequently, the Loess Plateau experienced a wetting trend during the period of 2003–2014, especially after the Grain for Green project was implemented. The results in this paper were proven by using three different depths of ERA-Interim (a global atmospheric reanalysis product created by the European Centre for Medium-Range Weather Forecasts) soil water content data from the same period and the observed runoff data from 18 different hydrological sites. Consequently, it seems that the vegetation could maintain a sustainable growth with the implementation of the Grain for Green Project.

Soil fertilization transiently increases radial growth in sessile oaks but does not change their resilience to severe soil water deficit

The future climate is projected to become increasingly problematic for European forest ecosystems, likely leading to the decline of many tree species. Forest management must adapt to cope with the negative effects of these changes. Despite the close interactions between nutrients and water in plant physiology, very few studies have investigated the link between soil fertility and tree response to soil water availability and vapour pressure deficit. The objective of this study was to test whether fertilization would modify forest tree resilience to soil water deficit.The potential impact of fertilization on trees during soil water deficit was assessed in a fertilization experiment carried out on sessile oaks. Three fertilization treatments (NCa, NPKCaMg and an unfertilized control) were applied to 40-year-old oaks that were cored almost 40 years later. The responses to drought in the fertilized and control trees were compared by analysing the radial growth chronologies following two complementary approaches. Firstly, the relationships between radial growth and inter-annual climate variations were investigated through dendroclimatic modelling including water deficit indices computed with the daily soil water balance model Biljou©. Secondly, resistance, recovery and resilience to drought-induced crises were evaluated from tree-ring width and compared among treatments. In addition, the carbon isotope composition (δ13C) of the tree rings was measured to evaluate potential differential gas-exchange processes between fertilized and control trees.Fertilization had an immediate, strong positive effect on oak growth which disappeared after nine years. NCa and NPKCaMg fertilizations had similar effects, suggesting that adding P, K and Mg did not increase radial growth. Whatever the treatment, ring width variations appeared to be identically constrained by climate, especially by the summer water deficits of the current and previous years. Tree resistance to soil water deficit was negatively affected by the severity of the drought event but did not significantly differ between control and fertilized trees. Tree resilience was not affected by fertilization and remained stable across crises. The tree-ring δ13C clearly increased during the severe 1976 water deficit but did not vary among treatments. We discuss the possible mechanisms involved in changes in growth and the lack of any significant effect of fertilization on resilience to drought and conclude that, under the relatively dry conditions and moderately poor soil fertility of our study site, fertilization could temporarily stimulate aboveground growth without inducing imbalances that would modify resilience and resistance to severe drought episodes.

Exploring adaptations of groundnut cropping to prevailing climate variability and extremes in Limpopo Province, South Africa

Adapting groundnut production to the high climatic variability and extremes prevailing in Limpopo (South Africa) requires season- and site-specific exploitation of genotype x environment x management interactions. Notably, the El Niño-Southern Oscillation phenomenon (ENSO) causes distinctive seasonal rainfall anomalies in the province: Weather records over the period 1955–2010, showed El Niño years with an average of 296 mm growing season rainfall, neutral years with 349 mm and La Niña with 434 mm. We hypothesized that (i) early planting date is crucial for ensuring high productivity of groundnut and (ii) early maturing cultivars can ensure satisfactory production even in El Niño years, both options aiming at escaping prolonged periods of drought. To evaluate our hypotheses, we conducted a three year on-station trial with three planting dates and two cultivars, a one year trial at two sites comparing the two cultivars, tested crop model APSIM against field trial data, and, finally, performed long-term gridded crop model simulation runs for all cultivated land (i.e. 14,739 grid cells at a resolution 0.01°) in Limpopo exploring the effect of the cultivars and planting dates on groundnut yield. Field trials showed that early planting was superior to later plantings, irrespective of the weather year. Even in the very dry El Niño year 2016, yields of more than 1000 kg/ha were achieved with early planting, while planting later was not possible due to severe soil water deficits. Cultivar ICGV 03796 performed slightly better than ICGV 99257 due to faster ground cover closure, but no difference in phenology was observed. After evaluation of the model against the field trial data was found satisfactory, APSIM simulations showed that highest yields were achieved in La Niña years (median 996 kg/ha across all grids) and smallest in El Niño years (654 kg/ha across all grids). However, as in the field trials, we observed on average over all grid cells higher grain yields for early planting for all weather years as effective rainfall captured was higher than for later plantings. Longer growth duration by cultivar ICGV 03796 appeared to be more productive than an earlier maturing cultivar that we added into the simulation analysis as a third cultivar. However, a closer look at the spatial pattern of the simulation analysis revealed that for the generally drier regions of Limpopo, this cultivar with shorter growth duration would indeed be more productive in El Niño years. It showed that such season-specific adaptation would increase yields only by 2.9% for Limpopo province in total, but yield gains of more than 10% could be obtained for 11.3% of the grid cells in El Niño years. Hence, this study demonstrated that management recommendations at provincial scale explicitly need to consider in more detail a site- and season-specific approach.

Stress-Related Gene Expression Reflects Morphophysiological Responses to Water Deficit.

Acclimation to water deficit (WD) enables plants to maintain growth under unfavorable environmental conditions, although the mechanisms are not completely understood. In this study, the natural variation of long-term acclimation to moderate and severe soil WD was investigated in 18 Arabidopsis (Arabidopsis thaliana) accessions using PHENOPSIS, an automated phenotyping platform. Soil water content was adjusted at an early stage of plant development and maintained at a constant level until reproductive age was achieved. The accessions were selected based on the expression levels of ANNEXIN1, a drought-related marker. Severe WD conditions had a greater effect on most of the measured morphophysiological traits than moderate WD conditions. Multivariate analyses indicated that trait responses associated with plant size and water management drove most of the variation. Accessions with similar responses at these two levels were grouped in clusters that displayed different response strategies to WD The expression levels of selected stress-response genes revealed large natural variation under WD conditions. Responses of morphophysiological traits, such as projected rosette area, transpiration rate, and rosette water content, were correlated with changes in the expression of stress-related genes, such as NINE-CIS-EPOXYCAROTENOID DIOXYGENASE3 and N-MYC DOWNREGULATED-LIKE1 (NDL1), in response to WD Interestingly, the morphophysiological acclimation response to WD also was reflected in the gene expression levels (most notably those of NDL1, CHALCONE SYNTHASE, and MYB DOMAIN PROTEIN44) in plants cultivated under well-watered conditions. Our results may lead to the development of biomarkers and predictors of plant morphophysiological responses based on gene expression patterns.

Summer drought survival and recovery in Microlaena stipoides

Microlaena stipoides (microlaena) is an important perennial grass in over 7 million hectares of native pastures in southern Australia and can survive and persist despite severe soil water deficits during summer. Many other pasture species survive similar conditions by relying on summer dormancy, which raises the possibility that microlaena may behave similarly. A field experiment using rainout shelters was conducted on an existing microlaena pasture in north-east Victoria. The experiment was a split-plot design with two watering treatments (‘summer storm’ or ‘summer dry’) as main plots and three defoliation treatments (nil, intense defoliation, strategic defoliation) as subplots. The ‘summer storm’ treatment resulted in the formation of new buds and tillers and increased basal cover from 1% in February to 18% in March. A glasshouse pot experiment examined the recovery of microlaena after different periods of drought and subsequent rewatering. In the pot experiment, microlaena withstood relatively short (up to 30 days) dry periods and then recovered when rewatered. Thus, it appears that microlaena is a persistent, perennial pasture plant that, although it survives very dry summers in Mediterranean areas, is not summer dormant. Microlaena does not exhibit summer dormancy in response to moisture stress and enter a quiescent stage, because normal growth is prevented by the lack of water, but it quickly recommences growth when soil water becomes available. The ability of microlaena to withstand summer soil water deficits and to recruit from seedlings make it a valuable pasture species across drought-prone environments, and this undoubtedly partly explains its very broad adaptation across eastern Australia.

Yield and economic return response of silage maize to different levels of irrigation water in a sub-humid zone

Field studies were conducted during the summers of 2007 and 2008 to determine the response of silage maize (Zea mays L.) to different levels of irrigation water to guide programs for the development of improved irrigation management practices for sub-humid zones. The experiments were carried out in Bursa, Marmara region, Turkey. Silage maize plants (cv. ‘Ada-523’) were subjected to different levels of irrigation using a drip system in the field on a clay-loam soil. Fully irrigated (FI = 100%) plants were irrigated at 100% pan evaporation (Epan) replenishment with 7-day intervals. In other treatments, irrigation was applied as excessive (EI = 125% Epan) and deficit (DI-75 = 75% Epan, DI-50 = 50% Epan, DI-25 = 25% Epan, NI – no irrigation). Plant height, first ear height, stem diameter, number of ears per plant and net income decreased with decreases in the amount of irrigation, but the effect of soil water deficit on the number of leaves per plant and ear ratio in forage was minor. The highest forage yields were obtained with EI (125% Epan), FI (100% Epan) and DI-75 (75% Epan) treatments. FI treatment produced the highest net income based forage yield. Severe soil water deficit substantially reduced forage yields and net income in both years. The results showed that full irrigation during the whole growing season is preferable for higher forage yield and net income. However, in regions of water scarcity, farmers should adopt the deficit irrigation (DI75 = 75%, DI-50 = 50% and DI-25 = 25% Epan) approach to achieve economically sustainable crop production. As an alternative to full irrigation during the entire growing season, the irrigation at a rate of DI-75% Epan can be recommended as optimal level because it achieved irrigation water savings of 25%, an increase of 16% in forage yield irrigation water use efficiency, satisfactory crop morphological characters and an acceptable net income with a yield loss of only approximately 7% compared with full irrigation.

Water-use advantage for lianas over trees in tropical seasonal forests.

Lianas exhibit peak abundance in tropical forests with strong seasonal droughts, the eco-physiological mechanisms associated with lianas coping with water deficits are poorly understood. We examined soil water partitioning, sap flow, and canopy eco-physiological properties for 99 individuals of 15 liana and 34 co-occurring tree species in three tropical forests that differed in soil water availability. In the dry season, lianas used a higher proportion of deep soil water in the karst forest (KF; an area with severe seasonal soil water deficit (SSWD)) and in the tropical seasonal forest (TSF, moderate SSWD), permitting them to maintain a comparable leaf water status than trees in the TSF or a better status than trees in the KF. Lianas exhibited strong stomatal control to maximize carbon fixation while minimizing dry season water loss. During the dry period, lianas significantly decreased water consumption in the TSF and the KF. Additionally, lianas had a much higher maximum photosynthetic rates and sap flux density in the wet season and a lower proportional decline in photosynthesis in the dry season compared with those of trees. Our results indicated that access to deep soil water and strong physiological adjustments in the dry season together with active wet-season photosynthesis may explain the high abundance of lianas in seasonally dry forests.

Genetic Variation for Epidermal Conductance in Peanut

ABSTRACTPeanut (Arachis hypogaea L.) is an important legume that is often grown in drought‐prone areas. Low epidermal conductance (EC) may delay the dehydration of leaves and improve plant survival of severe water‐deficit stress. No reports of genetic variation for EC in peanut have been found in the literature. Therefore, the main objective of this study was to document genetic variation for EC among 21 peanut (Arachis spp.) genotypes in two greenhouse experiments and a field experiment. The effects of water‐deficit stress on EC and the association between EC and stomata density (SD) were also investigated. Tests for an influence of water‐deficit stress on EC were inconclusive. Comparison of EC and SD among genotypes failed to show any significant correlations in the various test environments. However, genetic variation for EC in peanut was found in both greenhouse experiments, with genotypes TMV 2, PI 298639, and VA 98R having very low EC. In the field, genotype SPT 06‐07 was found to have low EC. These results indicate that selection of genotypes with low EC is a viable approach in identifying peanut genotypes with improved capacity to survive severe soil water deficits.

Transpiration of silver Fir (Abies alba mill.) during and after drought in relation to soil properties in a Mediterranean mountain area

Silver fir is declining and dying at its southern margin on the Mediterranean area, where climate is expected to be warmer and drier. At a regional scale, silver fir seems to be vulnerable to drought, and at a forest stand scale, tree death seems to be distributed according to soil water availability. To understand the vulnerability of silver fir to drought, factors involved in the regulation of transpiration were assessed with respect to soil properties in order to document the spatial distribution of death rates. Soil properties were characterized by electric resistivity measurements. Sap flow density and predawn needle water potential were recorded on sampled trees during several years, and crown specific transpiration was estimated. In addition, the vulnerability of coarse roots and branches to cavitation was quantified. Trees growing on soils with a large water storage capacity were the most vulnerable to drought induced soil water deficits. Transpiration was down-regulated as soon as predawn water potential decreased. The vulnerability to cavitation was low, which protected the trees from run-away xylem embolism. Severe soil water deficits led to a rapid decrease of transpiration, which was still visible the following year. The drop-off in transpiration was mainly due to inner sap flow that almost ceased after the drought on all monitored trees. Our results suggest that root dynamic and the ability of roots to take up water were modified by soil water deficit over several years. Such a regulation scheme needs to be better documented and included in models to address silver fir forest responses to drought.

Water deficits promote flowering in Rhododendron via regulation of pre and post initiation development

Flowering is generally considered to be advanced by water deficits in many woody perennial species. A long-standing paradigm being that as a plant senses severe environmental conditions resources are diverted away from vegetative growth and towards reproduction before death. It is demonstrated that in Rhododendron flowering is promoted under water deficit treatments. However, the promotion of flowering is not achieved via an increase in floral initiation, but through separate developmental responses. If regulated deficit irrigation (RDI) is imposed prior to the time of initiation, fewer vegetative nodes are formed before the apical meristems switch to floral initiation, and chronologically, floral initiation occurs earlier. Both RDI and partial rootzone drying (PRD) treatments stimulate the development of more flowers on each inflorescence if the treatments are continued after the plant has undergone floral initiation. However, floral initiation is inhibited by soil water deficits. If the soil water deficit continues beyond the stages of floral development then anthesis can occur prematurely on the fully formed floral buds without a need for a winter chilling treatment. It is hypothesised that inhibition of floral initiation in plants experiencing severe soil water deficits results from the inhibitory action of ABA transportation to the apical meristem from stressed roots. It is demonstrated that ABA applications to well-watered Rhododendron inhibit floral initiation.

The role of plant physiology in hydrology: looking backwards and forwards

Abstract. The implementation of plant physiological studies at the Institute of Hydrology focussed both on examining and understanding the physiological controls of transpiration as well as evaluating the value of using physiological methods to measure transpiration. Transpiration measurement by physiological methods would be particularly valuable where this could not be achieved by micrometeorological and soil physics methods. The principal physiological measurements used were determinations of leaf stomatal conductance and leaf water relations to monitor plant water stress. In this paper the value of these approaches is illustrated by describing a few case studies in which plant physiological insight, provided both as new measurements and existing knowledge, would aid in the interpretation of the hydrological behaviour of important vegetation. Woody vegetation figured largely in these studies, conducted in the UK and overseas. Each of these case studies is formulated as a quest to answer a particular question. A collaborative comparison of conifer forest transpiration in Thetford forest using micrometeorological and soil physics techniques exhibited a substantially larger (~1 mm day−1) estimate from the micrometeorological approach. So the question – Why is there a disagreement in the estimates of forest transpiration made using micrometeorological and soil physics approaches? A range of physiological studies followed that suggested that there was no one simple answer but that the larger estimate from the micrometeorology technique might include contributions of water taken up by deep roots, from shallow-rooted vegetation and possibly also from water previously stored in trees. These sources of water were probably not included in the soil physics estimate of transpiration. The annual transpiration from woodlands in NW Europe shows a low magnitude and notable similarity between different sites raising the question – Why is transpiration from European forests low and conservative? An important contribution both to the similar and low transpiration is the likely reduction of stomatal conductance of the foliage associated with increasing air humidity deficit. A greater response is usually found when initial conductances are highest. Also contributing to similarities in transpiration from forest stands would be a compensatory role of understories and that deficits in soil moisture may not come into play until severe soil water deficits occur. Physiological studies have been conducted in many locations overseas. The modest transpiration of tropical rainforest is intriguing – Why is tropical rainforest transpiration so low? In common with temperate trees the reduction of stomatal conductance of tropical trees in association with increasing air humidity deficit will limit transpiration. In addition the high leaf area index of tropical rainforest creates conditions in the lower canopy layers that mean transpiration from those layers is much reduced from what might be possible. As well as being used to quantify and understand transpiration, physiological techniques might be used to assess when plants require water. What is the first signal that plants need water? Studies on sugar cane in Mauritius indicated that leaf growth was the most sensitive measure. A look forward to the future suggests that there will be a continued need for physiological measurements particularly where other techniques more suited to extensive vegetation are not appropriate. There are many unresolved issues about water use from fragmented, heterogeneous vegetation and physiological approaches are best suited to these. The measurement of sap flow in individual stems will be an important methodology in the future but there are still methodological issues to resolve.