Partial Stomatal Closure Research Articles

Effects of drought stress on the activities of some photosynthetic enzymes and gas exchange parameters in the leaves of Chenopodium album

We have studied the effects of drought stress on the activities of some enzymes of the Calvin-Benson cycle and their degree of activation in the leaves of Chenopodium album and photosynthetic gas exchange parameters. The decrease in the rate of photosynthetic CO2 assimilation under moderate drought stress initially occurs due to the partial stomatal closure and under severe drought. It is assumed that the negative effect of drought on the activities of some enzymes of the Calvin-Benson cycle may cause the weakening of photosynthesis in the leaves of Chenopodium album. Keywords: Chenopodium album, drought stress, photosynthetic enzymes, photosynthetic rate

Decreased irrigation volume, not irrigation placement, promotes accumulation of multiple hormones in cotton leaves during partial rootzone drying

While ABA is often assumed to mediate partial stomatal closure as the soil dries, other plant hormones and hydraulic signals may also be involved. We tested whether irrigation volume (% of crop evapotranspiration, ET) and placement (partial rootzone drying [PRD] or deficit irrigation [DI], which irrigate part or all of the rootzone respectively) affect this signalling by measuring stomatal conductance (gs), leaf and shoot water potential (Ψleaf, Ψshoot), shoot xylem sap ABA concentration ([X-ABA]shoot) and various foliar hormones (ABA, IAA, SA, JA, JA-Ile and cis-OPDA) in cotton plants exposed to different irrigation volumes (100 %ET or 50 %ET) and placements (DI or PRD). Partial rootzone drying caused stomatal closure coincident with sustained foliar ABA accumulation and minimal changes in Ψshoot, but continued soil drying of the dry compartment reversed partial stomatal closure (with gs of 100 %ET PRD plants sometimes greater than well-watered plants). With 100 %ET PRD, partial stomatal closure correlated with decreased soil moisture of the dry compartment and increased [ABA]leaf, but neither Ψleaf nor [X-ABA]shoot. Irrespective of irrigation placement, 50 %ET significantly decreased gs, Ψleaf and Ψshoot, but significantly increased [ABA]leaf, [X-ABA]shoot, [SA]leaf, [IAA]leaf and [cis-OPDA]leaf, with stomatal closure of 50 %ET PRD plants occurring earlier than 50 %ET DI plants. While stomatal closure at 50 %ET correlated with foliar accumulation of multiple plant hormones, foliar ABA dynamics best explained transient stomatal closure at 100 %ET PRD but not stomatal re-opening with prolonged soil drying. Thus, stomatal sensitivity to drying soil (and putative regulatory signals such as ABA) depended on irrigation volume and placement.

Sap Flow Responses of the Endangered Species Juniperus drupacea Labill. to Environmental Variables in Parnon Mountain, Greece

In the face of ongoing climatic changes, understanding the species’ sap flow responses is of crucial importance for adaptation and resilience of ecosystems. This study investigated diurnal variability and radial sap flux density (Js) in a natural Juniperus drupacea forest on Mt Parnon and determined the climatic factors affecting its total sap flow (Qs). Between July 2021 and March 2022, Granier-type sensors and automatic weather stations monitored Js of J. drupacea trees and environmental factors. Utilizing a multi-point sensor for Js radial profile variability, correction factors were applied to calculate (Qs), ranging from 4.78 to 16.18 L day−1. In drier months of the study period (July–September), Qs progressively increased with increasing PAR and soil temperature, reaching a plateau at maximum values (app. 600 µmol m−2 s−1 and 26 °C respectively) indicating partial stomatal closure. Whereas, during the wetter period (October–March), when water was no longer a limiting factor, VPD and PAR emerged as significant controllers of stand transpiration. In this period, Qs responded positively to increasing soil water content (θ) only on days with high VPD (>0.5 kPa). The studied J. drupacea stand demonstrated adaptability to varying environmental conditions, crucial for the species’ survival, considering anticipated climate change scenarios.

Effects of dry spells on soil moisture and yield anomalies at a montane managed grassland site: A lysimeter climate experiment

AbstractThe frequency and severity of droughts in the Alps are expected to increase due to rising air temperatures and changes in precipitation regimes. Although biomass production in humid mountain areas tends to be energy limited rather than water limited, an increase in droughts may have negative impacts on the water availability and thus agricultural yields. This study aimed to analyse the impacts of dry spells on soil moisture and yield anomalies at a montane permanent grassland site in Austria. Dry spells in the time period from 2018 to 2020 were identified using the Standardized Precipitation Index, Palmer Drought Severity Index and the Soil Moisture Anomaly Index. Data from a lysimeter climate experiment were used to evaluate drought impacts on soil water storage and grassland yield under ambient and manipulated conditions. The results indicated the occurrence of three extreme droughts between 2018 and 2020. Although the studied grassland is generally considered a nonwater‐limited ecosystem, the most extreme drought in summer 2019 caused severe and extreme yield anomalies under ambient and heated conditions. Only mild yield anomalies were observed on plots with elevated atmospheric carbon dioxide concentration. This drought‐mitigating effect was attributed to the water savings enabled by partial stomatal closure under elevated CO2. The shorter dry spells in spring and late summer 2018 led to more diverse effects; mildly to moderately negative yield anomalies were found on the heated plots, whereas the anomalies tended to be less negative or even positive on plots under ambient temperature. In contrast, some time periods without water stress showed positive effects of heating on yield. These findings suggest that drought impacts on a humid montane grassland depend on both water availability and air temperature. Higher air temperature can have positive effects on yield if the ecosystem is energy limited. However, global warming suggests a tendency from energy to water limitation, in which the increased evaporative demand of the atmosphere aggravates soil moisture droughts and thus has potentially negative effects on yield.

Sensitivity of evapotranspiration and seepage to elevated atmospheric [formula omitted] from lysimeter experiments in a montane grassland

The response of vegetation to elevated atmospheric carbon dioxide (eCO2) has opposing effects on terrestrial water fluxes, especially evapotranspiration. In particular, CO2 fertilization may enhance plant growth, therefore increasing canopy transpiration, while a partial stomatal closure decreases leaf transpiration. In this study, the effects of eCO2 on water fluxes at a managed montane grassland were monitored using two high precision weighable lysimeters exposed to ambient and elevated (+300 ppm relative to ambient conditions) CO2 concentrations. During the growing season, eCO2 led to a general decrease in evapotranspiration and an increase in seepage. It was shown that eCO2 influences evapotranspiration primarily through the change in stomatal resistance, as no effect of eCO2 on the leaf area index was confirmed. A CO2-dependent Penman–Monteith equation and actual evapotranspiration data from the lysimeters made it possible to estimate the effect of eCO2 on stomatal resistance. eCO2 was found to increase stomatal resistance by 50% relative to ambient conditions. A HYDRUS-1D model allowed a temporal extension of the experimental findings to the period of 1990–2021 and revealed high variability in the relative change in annual seepage under eCO2. The modeling exercise demonstrated that the effect of eCO2 can be easily implemented in the Penman–Monteith equation, and if neglected, leads to overestimation of transpiration and underestimation of seepage, especially in the first post-drought seepage events. Our findings show that eCO2 positively affects the water balance in montane grasslands, thus eCO2 might be an important factor in mitigating the effects of warming and droughts in the future. We propose that the vegetation response to eCO2, in particular the effect on stomatal resistance, should be taken into account when assessing the effects of climate change on grassland water fluxes. This may also have large implications when studying the impact of climate change at a watershed scale.

Stable isotopes reveal differences in climate sensitivity and physiological responses between dieback and healthy trees in a shelter forest

Tree-ring stable isotopes provide insights into the drought-induced eco-physiological mechanisms of forests. We investigated the growth of poplar and Mongolian pine trees decline in the Three-North Shelter Forest, and analyzed the differences in tree-ring width (TW), δ13C, δ18O, and intrinsic water-use efficiency (iWUE) between dieback and healthy trees. An R package was used to quantify the relative contributions of climatic and physiological factors to growth. Both TW and basal area increment were lower for the dieback poplar and Mongolian pine than for the healthy trees.δ13C, δ18O, and iWUE were higher for the dieback poplar than for the healthy individuals; there were no significant differences in Mongolian pine. The measured iWUE for healthy poplar, healthy and dieback Mongolian pine were consistent with the modeled iWUE for Ci/ Ca scenario, indicating the stomata were open and Ci increased along with Ca. In contrast, the measured iWUE was higher for the dieback poplar than for the healthy individuals, which was consistent with the constant Ci scenario and partial stomatal closure. A significant decreasing Ci/Ca was detected only for the dieback poplar, reflecting the diversity in the leaf gas-exchange strategies of the two species. Pearson coefficients showed that on the annual timescale, tree growth was significantly negatively correlated with temperature, VPD, and iWUE, but positively correlated with SPEI. Moreover, the relationships between TW and stable isotopes and stomatal regulation, which differed in the two species, were affected by the dieback process. Furthermore, iWUE andδ18O had the highest and lowest relative contributions to TW, respectively. The relative contribution of meteorological factors (Tem, VPD and SPEI) to the growth was significantly lower than that of physiological factors (iWUE and δ18O). Compared with the healthy controls of both species, the dieback trees were less sensitive to climate conditions, but the effects of physiological activities on growth were greater. These results provide a better understanding of the climate sensitivity and physiological responses of trees under long-term drought conditions.

Amplified warming from physiological responses to carbon dioxide reduces the potential of vegetation for climate change mitigation

Global warming is increasing due to the ongoing rise in atmospheric greenhouse gases, and has the potential to threaten humans and ecosystems severely. Carbon dioxide, the primary rising greenhouse gas, also enhances vegetation carbon uptake, partially offsetting emissions. The vegetation physiological response to rising carbon dioxide, through partial stomatal closure and leaf area increase, can also amplify global warming, yet this is rarely accounted for in climate mitigation assessments. Using six Earth System Models, we show that vegetation physiological response consistently amplifies warming as carbon dioxide rises, primarily due to stomatal closure-induced evapotranspiration reductions. Importantly, such warming partially offsets cooling through enhanced carbon storage. We also find a stronger warming with higher leaf area and less warming with lower leaf area. Our study shows that the vegetation physiological response to elevated carbon dioxide influences local climate, which may reduce the extent of expected climate benefits offered by terrestrial ecosystems.

Ultraviolet radiation causes leaf warming due to partial stomatal closure.

Variation in solar ultraviolet radiation induces a wide-range of plant responses from the cellular to whole-plant scale. We demonstrate here for the first time that partial stomatal closure caused by ultraviolet radiation exposure results in significant increases in leaf temperature. Significant leaf warming in response to ultraviolet radiation was consistent in tomato (Solanum lycopersicum L.) across different experimental approaches. In field experiments where solar ultraviolet radiation was attenuated using filters, exposure to ultraviolet radiation significantly decreased stomatal conductance and increased leaf temperature by up to 1.5°C. Using fluorescent lamps to provide ultraviolet radiation treatments, smaller but significant increases in leaf temperature due to decreases in stomatal conductance occurred in both multi-day controlled environment growth room experiments and short-term (<2 hours) climate cabinet irradiance response experiments. We show that leaf warming due to partial stomatal closure is independent of any direct warming effects of ultraviolet radiation manipulations. We discuss the implications of ultraviolet radiation-induced warming both for horticultural crop production and understanding broader plant responses to ultraviolet radiation.

Girdling changes root and shoot hormonal balance but does not alter drought-induced stomatal closure in soybean

To investigate the importance of shoot-to-root communication in modulating tissue phytohormone balance as the soil dries, root and leaf hormonal profiles were determined in soybean plants (Glycine max L. Merr. cv. Siverka) exposed to factorial girdling (achieved surgically on Day 0 by excising the phloem with a sharp razor blade thereby disrupting basipetal hormone transport) and soil drying treatments. Plants were randomized into 2 groups: girdled and intact plants, and each girdled group was further separated into two groups: well-watered and soil drying. Thus the experiment was a two (± girdling) x two (± soil drying) factorial experiment. Within 26 h while the soil was still moist, girdling increased root ACC (by 11-fold) and decreased root ABA concentrations (by 28 %), decreased foliar iP, tZ and GA3 concentrations (by up to 90 %), and increased foliar ABA and JA concentrations (by 1.6- and 4.9-fold respectively) compared to intact plants, but only leaf ABA concentration correlated with partial stomatal closure. Two days of soil drying increased root ABA and JA concentrations in intact plants, and both hormones significantly decreased in roots of girdled plants compared to intact plants. Soil drying increased foliar iP, ABA, JA and SA concentrations by 1.3-, 2.5-, 1.3- and 1.4-fold respectively compared to well-watered intact plants by the end of the experiment, but girdling prevented this foliar iP, ABA, JA accumulation compared to droughted intact plants. Thus, girdling affects the co-ordination of root and leaf phytohormone concentrations such as JA and ABA, which may transiently modulate stomatal closure. However, leaf ABA concentration varied consistently with root ABA concentration across all treatments, and explained more of the variation in stomatal conductance than any other hormone or leaf water potential.

Salinity-Induced Changes of Photosynthetic Performance, Lawsone, VOCs, and Antioxidant Metabolism in Lawsonia inermis L.

The present study aimed to elucidate the salinity influence on the bioactive metabolites of Lawsonia inermis L. (henna) plants. Young henna plants were cultivated under salinity stress with two NaCl concentrations (75 mM and 150 mM) in controlled environmental conditions and the leaves were investigated to check their adaptative responses. The modulation of photosynthetic performance to salinity stress was demonstrated by gas exchange and chlorophyll fluorescence parameters. The partial stomatal closure triggered an enhanced water-use efficiency, and a proline accumulation was observed, leading to an osmotic adjustment. The increased capacity to dissipate the excess excitation energy at photosystem II as heat was associated with changes in chlorophylls, anthocyanins, and carotenoids. The higher antioxidant activity at 150 mM salt level suggested its scavenger role on reactive oxygen species (ROS) dissipation and photoprotection. The reduced CO2 uptake and the higher metabolic costs necessary to sustain the henna tolerance mechanism against high NaCl concentration negatively affected lawsone production. Leaf volatile organic compounds (VOCs) showed changes in the amount and composition of VOCs with increasing salinity level. Overall, this study revealed efficient physiological and biochemical adaptations of henna leaves to salt stress despite an altered production of important economic metabolites such as lawsone.

Transpiration response to vapor pressure deficit and soil drying among quinoa genotypes (Chenopodium quinoa Willd.)

ABSTRACT Water-deficit conditions limit increasing crop yield around the world. In order to improve crop yield it has been proposed to decrease water use early in the season so more water will be available later in the season to support seed growth during reproductive development. To achieve this, there are two water-conservation traits of special interest: partial stomatal closure under high vapor pressure deficit (VPD) and early in the soil drying cycle. Quinoa (Chenopodium quinoa Willd.) is well known for its ability to grow in poor soils and extreme climatic environments. Therefore, quinoa may especially benefit from expression of water-conservation for water-limited conditions. These traits have not been previously studied in quinoa. This study reported the response of eight quinoa genotypes. Genotypes Red head, CICA-17, Salcedo, Ollague, Good Afternoon, and Pasankalla expressed a VPD breakpoint (BP) but Titicaca and French Vanilla not. All genotypes expressed a FTSW threshold with soil drying as expected. French Vanilla had the highest threshold, so it would be a candidate as a water-conserving genotype. The results of this study can be applied directly in field tests comparing cultivars under water-deficit conditions, and selection of genotypes to be used in breeding for improved cultivars specifically for drought.

Alternation of wet and dry sides during partial rootzone drying irrigation enhances leaf ethylene evolution

Soil drying increases endogenous ABA and ACC concentrations in planta, but how these compounds interact to regulate stomatal responses to soil drying and re-watering is still unclear. To determine the temporal dynamics and physiological significance of root, xylem and leaf ABA and ACC concentrations in response to deficit irrigation (DI) or partial rootzone drying (PRD-F) and re-watering, these variables were measured in plants exposed to similar whole pot soil water contents. Both DI and PRD-F plants received only a fraction of the irrigation supplied to well-watered (WW) plants, either to all (DI) or part (PRD-F) of the rootzone of plants grown in split-pots. Both DI and PRD-F induced partial stomatal closure, increased root ABA and ACC accumulation consistent with local soil water content, but did not affect xylem or leaf concentrations of these compounds compared to WW plants. Two hours after re-watering all (DI-RW) or part of the rootzone (PRD-A) to the same soil water content, stomatal conductance returned to WW values or further decreased respectively. Re-watering the whole rootzone had no effect on xylem and leaf ABA and ACC concentrations, while re-watering the dry side of the pot in PRD plants had no effect on xylem and leaf ABA concentrations but increased xylem and leaf ACC concentrations and leaf ethylene evolution. Leaf water potential was similar between all irrigation treatments, with stomatal conductance declining as xylem ABA concentrations and leaf ACC concentrations increased. Prior to re-watering PRD plants, accounting for the spatial differences in soil water uptake best explained variation in xylem ACC concentration suggesting root-to-shoot ACC signalling, but this model did not account for variation in xylem ACC concentration after re-watering the dry side of PRD plants. Thus local (foliar) and long-distance (root-to-shoot) variation in ACC status both seem important in regulating the temporal dynamics of foliar ethylene evolution in plants exposed to PRD.

Environmental and biophysical controls of evapotranspiration from Seasonally Dry Tropical Forests (Caatinga) in the Brazilian Semiarid

Seasonally dry tropical forests are among the most important biomes regarding regional and global hydrological and carbon fluxes. Thus, the objective of this study was to evaluate the seasonal and interannual variability of evapotranspiration (ET) and its biophysical control and characteristics (surface conductance—Gs; decoupling coefficient—Ω; ratio between actual evapotranspiration and equilibrium evapotranspiration—ET/ETeq) in a preserved Caatinga Biome environment during two dry years in the Northeast Brazil region. A study on this subject with this level of detail in this biome is unprecedent. Measurements were carried out using an eddy covariance system during the period from 1st January 2014 to 31st December 2015. The lowest ET values were observed in the dry season of both experiment years (0.3 and 0.2 mm day−1) as a consequence of poor water availability, which favored partial stomatal closure and reduced Gs values (0.22 and 0.13 mm s−1). The opposite occurred in the wet season, when ET (2.6 and 1.7 mm day−1) and Gs (3.74 and 2.13 mm s−1) means reached higher values. Regarding annual values, differences between total annual rainfall in both years is the most probable cause for the differences observed in annual ET values. In 2014, annual ET was of 473.3 mm while in 2015 it was 283.4 mm, which incurred in an overall decrease in Gs, Ω and ET/ETeq values. Leaf senescence and extremely low Gs values during the dry season suggest that the trees of the Caatinga Biome are more resilient regarding the use of water and are avoiding water stress caused under low water availability

Ultraviolet (UV) transparent plastic claddings warm crops and improve water use efficiency

Advances in the manufacturing of plastic cladding for protected crop cultivation have resulted in wavelength selective plastics capable of manipulating the transmission of solar radiation to include ultraviolet (UV: 280-400 nm). Commercial growers already utilising these plastics report early maturity associated with warmer crops. We hypothesised that UV-B radiation causes partial stomatal closure that reduces stomatal conductance and transpiration rate, thereby increasing leaf temperature (relative to air temperature). We tested this hypothesis by investigating leaf gas exchange and temperature responses of individual tomato leaves to UV-B and UV-A radiation provided by UV lamps in a controlled environment. Transient (90 min) exposure to UV-B radiation decreased stomatal conductance but had minimal impact on photosynthesis, thus increasing leaf temperature and instantaneous water use efficiency. Should this enhanced water use efficiency also occur at a whole plant/canopy scale, these responses may benefit growers of protected crops in arid climates where plastic clad polytunnels are often utilised. © 2020 International Society for Horticultural Science. All rights reserved.

Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest.

Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.

Wheat drought-tolerance to enhance food security in Tunisia, birthplace of the Arab Spring

Abstract The beginning of the ‘Arab Spring’ in 2011, a regional revolution which started in the Tunisian city of Sidi Bouzid in late 2010, occurred in part as a result of drought-triggered high wheat prices, which in the past led to ‘bread riots’ across several Middle East and North Africa (MENA) nations. Here we present, for the first time, an analysis of possible amelioration of wheat yield loss and greater stability in bread supply resulting from the incorporation of putative drought-tolerant traits into wheat cultivars grown in Tunisia. To this end, we used a simulation crop modeling approach using SSM-Wheat to evaluate yield loss or gain resulting from three types of water-saving traits that have been recently identified in wheat. These consisted in partial stomatal closure at high soil water content, overall decrease in transpiration rate (TR), and partial stomatal closure under elevated vapor pressure deficit (VPD). To capture large gradients in seasonal precipitation across wheat growing areas over a small country such as Tunisia, a grid pattern of 29 × 29 km was established as a basis for the geospatial simulation. Surprisingly, the simulation reflected opposite strategies in terms of water use (water-saving vs aggressive water use). The highest yield gain (30%) resulting from water-saving modification was found to occur in the food-insecure region of Sidi Bouzid. Traits enabling aggressive water use were found to be generally favorable across Tunisia, with one trait leading to up to 80% and 40% increases in yield and its stability in the food-challenged south of the country. However, major yield penalties were found to occur if water-saving traits were to be deployed in the ‘wrong’ region. Those findings could be used as a blueprint to navigate complex trait × environment interactions and to better inform local breeding and management programs to improve wheat yield and it stability in Tunisia and the MENA region in general.

Distinctive phytohormonal and metabolic profiles of Arabidopsis thaliana and Eutrema salsugineum under similar soil drying.

Arabidopsis and Eutrema show similar stomatal sensitivity to drying soil. In Arabidopsis, larger metabolic adjustments than in Eutrema occurred, with considerable differences in the phytohormonal responses of the two species. Although plants respond to soil drying via a series of concurrent physiological and molecular events, drought tolerance differs greatly within the plant kingdom. While Eutrema salsugineum (formerly Thellungiella salsuginea) is regarded as more stress tolerant than its close relative Arabidopsis thaliana, their responses to soil water deficit have not previously been directly compared. To ensure a similar rate of soil drying for the two species, daily soil water depletion was controlled to 5-10% of the soil water content. While partial stomatal closure occurred earlier in Arabidopsis (Day 4) than Eutrema (from Day 6 onwards), thereafter both species showed similar stomatal sensitivity to drying soil. However, both targeted and untargeted metabolite analysis revealed greater response to drought in Arabidopsis than Eutrema. Early peaks in foliar phytohormone concentrations and different sugar profiles between species were accompanied by opposing patterns in the bioactive cytokinin profiles. Untargeted analysis showed greater metabolic adjustment in Arabidopsis with more statistically significant changes in both early and severe drought stress. The distinct metabolic responses of each species during early drought, which occurred prior to leaf water status declining, seemed independent of later stomatal closure in response to drought. The two species also showed distinct water usage, with earlier reduction in water consumption in Eutrema (Day 3) than Arabidopsis (Day 6), likely reflecting temporal differences in growth responses. We propose Arabidopsis as a promising model to evaluate the mechanisms responsible for stress-induced growth inhibition under the mild/moderate soil drying that crop plants are typically exposed to.

Limited-transpiration rate under high atmospheric vapour pressure deficit: a trait for developing genotypes for water-deficit conditions.

Abstract Water deficits are the major limitation in increasing crop yields in many regions of the world. Various plant traits that might result in yield increases in water-limited environments have been discussed for decades. Conservative use of soil water is an important breeding strategy in drought-prone environments that can be achieved by traits based on partial stomatal closure under specific environmental conditions to limit transpiration rate (TR). The focus of this review is on a specific trait for conservative soil water that results in partial stomatal closure that can be supported by a plant. This limit on TR is expressed in terms of the atmospheric vapour pressure deficit (VPD) at which partial stomatal closure occurs. The review provides the physiological background of partial stomatal closure under elevated VPD. Simulation studies that analyse the potential benefits of this trait are also discussed. Finally, we provide a review of the various research that has been made in the identification of genetic variability of this trait in major crops.

Effective Water Use Required for Improving Crop Growth Rather Than Transpiration Efficiency.

The phenomenological expression showing crop yield to be directly dependent on crop transpiration use efficiency (TE) has encouraged continued focus on TE as a viable approach to increasing crop yields. The difficulty in the phenomenological perspective is that research tends not to match up with the underlying mechanistic variables defining TE. Experimental evidence and the mechanistic derivation of TE by Tanner and Sinclair showed that the common focus on increasing the intrinsic ratio of leaf CO2/H2O exchange has limited opportunities for improvement. On the other hand, the derivation showed that daily vapor pressure deficit (VPD) weighted for the daily cycle of transpiration rate has a large, direct impact on TE. While VPD is often viewed as an environmental variable, daily weighted VPD can be under plant control as a result of partial stomatal closure during the midday. A critical feature of the partial stomatal closure is that transpiration rate is decreased resulting in conservation of soil water. The conserved soil water allows late-season, sustained physiological activity during subsequent periods of developing water deficits, which can be especially beneficial during reproductive development. The shift in the temporal dynamics of water use by water conservations traits has been shown in simulation studies to result in substantial yield increases. It is suggested from this analysis that effective water use through the growing season is more important for increasing crop yield than attempts focused on improving the static, intrinsic TE ratio.

Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland.

Elevated atmospheric CO2 concentration (eCa ) might reduce forest water-use, due to decreased transpiration, following partial stomatal closure, thus enhancing water-use efficiency and productivity at low water availability. If evapotranspiration (Et ) is reduced, it may subsequently increase soil water storage (ΔS) or surface runoff (R) and drainage (Dg ), although these could be offset or even reversed by changes in vegetation structure, mainly increased leaf area index (L). To understand the effect of eCa in a water-limited ecosystem, we tested whether 2years of eCa (~40% increase) affected the hydrological partitioning in a mature water-limited Eucalyptus woodland exposed to Free-Air CO2 Enrichment (FACE). This timeframe allowed us to evaluate whether physiological effects of eCa reduced stand water-use irrespective of L, which was unaffected by eCa in this timeframe. We hypothesized that eCa would reduce tree-canopy transpiration (Etree ), but excess water from reduced Etree would be lost via increased soil evaporation and understory transpiration (Efloor ) with no increase in ΔS, R or Dg . We computed Et , ΔS, R and Dg from measurements of sapflow velocity, L, soil water content (θ), understory micrometeorology, throughfall and stemflow. We found that eCa did not affect Etree , Efloor , ΔS or θ at any depth (to 4.5m) over the experimental period. We closed the water balance for dry seasons with no differences in the partitioning to R and Dg between Ca levels. Soil temperature and θ were the main drivers of Efloor while vapour pressure deficit-controlled Etree , though eCa did not significantly affect any of these relationships. Our results suggest that in the short-term, eCa does not significantly affect ecosystem water-use at this site. We conclude that water-savings under eCa mediated by either direct effects on plant transpiration or by indirect effects via changes in L or soil moisture availability are unlikely in water-limited mature eucalypt woodlands.