Sleep tight and wake-up early: nocturnal transpiration traits to increase wheat drought tolerance in a Mediterranean environment

Identifying plant traits to increase wheat yield under irrigated conditions

Crop models have frequently been used to identify desired plant traits for rainfed wheat (Triticum aestivum L.). However, efforts to apply these models to irrigated wheat grown under non-limiting water and nitrogen conditions have been rare. Using simulation models to identify plant traits that impact yield can facilitate more targeted cultivar improvement and reduce time and cost. In this study, the SSM-iCrop model was employed to identify effective plant traits for increasing the yield of irrigated wheat in four distinct environments in Iran. A comprehensive range of traits related to phenology, leaf area development, dry matter production, and yield formation, which exhibited reported genetic variation, were tested. The impact of these traits on yield showed slight variation across different environmental zones due to genetic × environment interaction. However, across all environments, modifying current cultivars to increase radiation use efficiency (RUE) resulted in a 19 % increase in yield, accelerating leaf area development led to a 10 %-15 % increase, lengthening the grain filling period resulted in a 14 % increase, and extending the vegetative period led to a 6 % increase. These improvements were all statistically significant. Considering that longer duration cultivars may disrupt cropping systems and the need to develop simple methods for targeting and phenotyping RUE, faster leaf area development was found as the most promising option to increase irrigated wheat yield under optimal water and nitrogen management within a short time frame. It should be noted that cultivars with modified traits needed higher water and nitrogen inputs to support increased yields. These findings can be applied to select desirable key traits for targeted breeding and expedite the production of high-yielding cultivars of irrigated wheat in various environmental zones. The potential for further improvement through combined traits requires further investigation.

Night-time water relations and gas exchange in cut shoots of five boreal dwarf shrub species: impact of soil water availability

Recent findings suggest that drought may affect plants' daytime and night-time stomatal regulation differently. However, knowledge of night-time stomatal behaviour in dwarf shrubs growing in boreal ecosystems is lacking. We sampled cut shoots from dwarf shrub species to elucidate their capacity to transpire at night and the effect of drought on stomatal regulation. The shoots' water relations and gas exchange were measured under controlled conditions in a growth chamber. The studied species demonstrated considerable differences in their diurnal water use. The night-time water use percentage of daytime water use (NWU) reached up to 90% in Andromeda polifolia and Vaccinium uliginosum. In Rhododendron tomentosum, Vaccinium myrtillus and Chamaedaphne calyculata, the NWU was 62, 27 and 26%, respectively. The shoots of C. calyculata showed a significant increase (P < 0.001) in the transpiration rate (E) during the night. However, in R. tomentosum, a decrease (P < 0.05) in nightly E was observed. The shoot conductance (g) at the end of the night was lower than daytime g in all studied species, but the difference was not significant for V. uliginosum. Across the species, NWU was negatively related (P < 0.001) to the soil volumetric water content (SWC) in the plant habitat. However, daytime E and g were positively related (P < 0.05) to the habitat SWC. Only in V. myrtillus was night-time E higher (P < 0.05) in dry conditions than in wet conditions. Our results demonstrate high variability in diurnal water relations in dwarf shrubs, which can keep stomata open in the dark even when drought limits daytime g and E.

Nocturnal Transpiration May Be Associated with Foliar Nutrient Uptake

Aerosols can contribute to plant nutrition via foliar uptake. The conditions for this are best at night because the humidity is high and hygroscopic, saline deposits can deliquesce as a result. Still, stomata tend to be closed at night to avoid unproductive water loss. However, if needed, nutrients are on the leaf surface, and plants could benefit from nocturnal stomatal opening because it further increases humidity in the leaf boundary layer and allows for stomatal nutrient uptake. We tested this hypothesis on P-deficient soil by comparing the influence of ambient aerosols and additional foliar P application on nocturnal transpiration. We measured various related leaf parameters, such as the foliar water loss, minimum leaf conductance (g_min), turgor loss point, carbon isotope ratio, contact angle, specific leaf area (SLA), tissue element concentration, and stomatal and cuticular characteristics. For untreated leaves grown in filtered, aerosol-free air (FA), nocturnal transpiration consistently decreased overnight, which was not observed for leaves grown in unfiltered ambient air (AA). Foliar application of a soluble P salt increased nocturnal transpiration for AA and FA leaves. Crusts on stomatal rims were shown by scanning electron microscopy, supporting the idea of stomatal uptake of deliquescent salts. Turgor loss point and leaf moisture content indicated a higher accumulation of solutes, due to foliar uptake by AA plants than FA plants. The hypothesis that deliquescent leaf surface salts may play a role in triggering nocturnal transpiration was supported by the results. Still, further experiments are required to characterize this phenomenon better.

Atmospheric Vapor Impact on Desert Vegetation and Desert Ecohydrological System

The ability of plants to absorb unsaturated atmospheric water vapor is a controversial topic. To study how vegetation in arid areas survives under limited water resources, this study uses Tamarisk in the Ulan Buh Desert of China as an example. The in-situ observation of a newly designed Lysimeter and sap flow meter system were used to monitor the precipitation infiltration and the utilization efficiency of Tamarisk of atmospheric vapor. The results show that the annual precipitation of 84 mm in arid areas could still result in deep soil recharge (DSR) with a recharge rate of 5 mm/year. Furthermore, DSR is detectable even in the winter, and the 5-year average DSR was 5.77% of the annual precipitation. It appears that the small precipitation events are critically important for the survival of Tamarisk. When the atmospheric relative humidity reaches 70%, Tamarisk leaves can absorb the unsaturated atmospheric vapor, which accounts for 13.2% of the annual precipitation amount. To adapt to the arid environment, Tamarisk can harvest its water supply from several sources including atmospheric vapor and micro-precipitation events (whose precipitation is below the measurement limit of 0.2 mm of the precipitation gauge) and can still permit a certain amount of recharge to replenish the deep soil moisture. Such an ecohydrological dynamic is of great significance to desert vegetation.

Environment-oriented selection criteria to overcome controversies in breeding for drought resistance in wheat

Wheat is one of the most important cereal crops, representing a fundamental source of calories and protein for the global human population. Drought stress (DS) is a widespread phenomenon, already affecting large wheat-growing areas worldwide, and a major threat for cereal productivity, resulting in consistent losses in average grain yield (GY). Climate change is projected to exacerbate DS incidence and severity by increasing temperatures and changing rainfall patterns. Estimating that wheat production has to substantially increase to guarantee food security to a demographically expanding human population, the need for breeding programs focused on improving wheat drought resistance is manifest. Drought occurrence, in terms of time of appearance, duration, frequency, and severity, along the plant's life cycle varies significantly among different environments and different agricultural years, making it difficult to identify reliable phenological, morphological, and functional traits to be used as effective breeding tools. The situation is further complicated by the presence of confounding factors, e.g., other concomitant abiotic stresses, in an open-field context. Consequently, the relationship between morpho-functional traits and GY under water deficit is often contradictory; moreover, controversies have emerged not only on which traits are to be preferred, but also on how one specific trait should be desired. In this review, we attempt to identify the possible causes of these disputes and propose the most suitable selection criteria in different target environments and, thus, the best trait combinations for breeders in different drought contexts. In fact, an environment-oriented approach could be a valuable solution to overcome controversies in identifying the proper selection criteria for improving wheat drought resistance.

Drivers of nocturnal stomatal conductance in C3 and C4 plants

Nocturnal water losses were for long considered negligible, but it is now known that incomplete stomatal closure during the night leads to significant water losses at leaf, plant and ecosystem scales. However, only daytime transpiration is currently accounted for in evapotranspiration studies. Important uncertainties on the drivers of nocturnal water fluxes hinder its incorporation within modelling frameworks because some studies indicate that night-time stomatal drivers may differ from day-time responses. Here, we synthesise the studies on nocturnal stomatal conductance (g_n) to determine underlying drivers through a systematic literature review and, whenever possible, meta-analytical techniques. Similar to daytime responses, we found negative effects of vapour pressure deficit, predawn water potential, air temperature, and salinity on g_n across the plant species. However, the most apparent trend was an increase of g_n from the beginning until the end of the night, indicating significant and widespread endogenous regulation by the circadian clock. We further observed how neither elevated CO₂ nor nutrient status affected g_n significantly across species. We also did not find any significant associations between g_n and elevated ozone or increasing plant age. There was a paucity of studies on climatic extremes such heat waves and also few studies connected g_n with anatomical features such as leaf specific area or stomatal density. Further studies are also needed to address the effects of plant sex, abscisic acid concentrations and genotypic variations on g_n. Our findings solve the long-term conundrum on whether stomatal responses to daytime drivers are the same as those that during the nighttime.

Nocturnal stomatal conductance in wheat is growth-stage specific and shows genotypic variation

Nocturnal stomatal conductance (g_sn ) represents a significant source of water loss, with implications for metabolism, thermal regulation and water-use efficiency. With increasing nocturnal temperatures due to climate change, it is vital to identify and understand variation in the magnitude and responses of g_sn in major crops. We assessed interspecific variation in g_sn and daytime stomatal conductance (g_s ) in a wild relative and modern spring wheat genotype. To investigate intraspecific variation, we grew six modern wheat genotypes and two landraces under well watered, simulated field conditions. For the diurnal data, higher g_sn in the wild relative was associated with significantly lower nocturnal respiration and higher daytime CO₂ assimilation while both species exhibited declines in g_sn post-dusk and pre-dawn. Lifetime g_sn achieved rates of 5.7-18.9% of g_s . Magnitude of g_sn was genotype specific 'and positively correlated with g_s . g_sn and g_s were significantly higher on the adaxial surface. No relationship was determined between harvest characteristics, stomatal morphology and g_sn , while cuticular conductance was genotype specific. Finally, for the majority of genotypes, g_sn declined with age. Here we present the discovery that variation in g_sn occurs across developmental, morphological and temporal scales in nonstressed wheat, presenting opportunities for exploiting intrinsic variation under heat or water stressed conditions.

Dynamics of Nocturnal Evapotranspiration and Its Biophysical Controls over a Desert Shrubland of Northwest China

Knowledge about the dynamics and biophysical controlling mechanism of nocturnal evapotranspiration (ETN) in desert-dwelling shrub ecosystem is still lacking. Using the eddy covariance measurements of latent heat flux in a dried shrubland in northwest China, we examined the dynamics of ETN and its biophysical controls at multiple timescales during growing-seasons from 2012 to 2014. The ETN was larger in the mid-growing season (usually in mid-summer) than in spring and autumn. The maximum daily ETN was 0.21, 0.17, and 0.14 mm night−1 in years 2012–2014, respectively. At the diel scale, ETN decreased from 21:00 to 5:00, then began to increase. ETN were mainly controlled by soil volumetric water content at 30 cm depth (VWC30), by vapor pressure deficit (VPD) and normalized difference vegetation index (NDVI) at leaf expanding and expanded stage, and by air temperature (Ta) and wind speed (Ws) at the leaf coloring stage. At the seasonal scale, variations of ETN were mainly driven by Ta, VPD, and VWC10. Averaged annual ETN was 4% of daytime ET. The summer drought in 2013 and the spring drought in 2014 caused the decline of daily evapotranspiration (ET). The present results demonstrated that ETN is a significant part of the water cycle and needs to be seriously considered in ET and related studies. The findings here can help with the sustainable management of water in desert ecosystems undergoing climate change.

Chronoculture, harnessing the circadian clock to improve crop yield and sustainability

Human health is dependent on a plentiful and nutritious supply of food, primarily derived from crop plants. Rhythmic supply of light as a result of the day and night cycle led to the evolution of circadian clocks that modulate most plant physiology, photosynthesis, metabolism, and development. To regulate crop traits and adaptation, breeders have indirectly selected for variation at circadian genes. The pervasive impact of the circadian system on crops suggests that future food production might be improved by modifying circadian rhythms, engineering the timing of transgene expression, and applying agricultural treatments at the most effective time of day. We describe the applied research required to take advantage of circadian biology in agriculture to increase production and reduce inputs.

Harnessing nighttime transpiration dynamics for drought tolerance in grasses

Non-negligible nighttime transpiration rates (TR_N) have been identified in grasses such as wheat and barley. Evidence from the last 30 years indicate that in drought-prone environments with high evaporative demand, TR_N could amount to 8-55% of daytime TR, leading several investigators to hypothesize that reducing TR_N might represent a viable water-saving strategy that minimizes seemingly 'wasteful' water loss that is not traded for CO₂ fixation. More recently however, evidence suggests that actual increases in TR_N during pre-dawn hours, which are presumably controlled by the circadian clock, mediate drought tolerance - not through water conservation - but by enabling maximized gas exchange early in the morning before midday depression sets in. Finally, new findings point to a previously undocumented role for leaf sheaths as substantial contributors (up to 45%) of canopy TR_N, although the extent of their involvement in these two strategies remains unknown. In this paper, we synthesize and reconcile key results from experimental and simulation-based modeling efforts conducted at scales ranging from the leaf tissue to the field plot on wheat and barley to show that both strategies could in fact concomitantly enable yield gains under limited water supply. We propose a simple framework highlighting the role played by TR_N dynamics in drought tolerance and provide a synthesis of potential research directions, with an emphasis on the need for further examining the role played by the circadian clock and leaf sheath gas exchange.