Helianthus nighttime conductance and transpiration respond to soil water but not nutrient availability

Nocturnal stomatal behaviour and its impact on water use strategies of desert herbs in the Gurbantunggut Desert, Northwest China

Nocturnal stomatal behaviour has the potential to exert a profound influence on plant-water relations, especially water use efficiency. However, we know very less about plant functional type differences in nocturnal stomatal conductance and their roles in plant stress adaptation, especially drought adaptation. To address this critical knowledge gap, we assessed diel leaf gas exchanges in eight ephemeral and perennial herbs growing on the southern edge of the Gurbantunggut Desert, Northwest China. For both ephemeral and perennial herbs, the nocturnal stomatal conductance (gs) exceeded 30 % of daytime gs, except for an ephemeral herb (Malcolmia scorpioides). The nocturnal gs in the studied perennial herbs were significantly higher than it in the ephemeral herbs. The results suggest that circadian-driven stomatal priming plays a pivotal role in accelerating the attainment of steady-state gs during the morning for perennial herbs, thereby enhancing their capacity for carbon capture. Moreover, the nocturnal stomatal behaviour of the ephemeral herbs favored water retention in the morning, consequently enhanced intrinsic water use efficiency and long-term water use efficiency. In summary, plant functional type differences in the magnitude of nocturnal stomatal opening were related to differences in water acquisition and utilization and highlighted diverse water use strategies in the desert plants.

Night-time warming in the field reduces nocturnal stomatal conductance and grain yield but does not alter daytime physiological responses

Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn ) is surprisingly poorly understood and has not been investigated despite constituting a significant proportion of overall canopy water loss. Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum genotypes which were grown in NW Mexico and subjected to an artificial increase in night-time temperatures of 2°C. Under nocturnal heating, grain yields decreased (1.9% per 1°C) without significant changes in daytime leaf-level physiological responses. Under warmer nights, there were significant differences in the magnitude and decrease in gsn , values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype-specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights. We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype-specific nocturnal heat tolerance.

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.

Salt Stress-Regulation of Root Water Uptake in a Whole-Plant and Diurnal Context

This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

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.

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.

Variation in nocturnal stomatal conductance and development of predawn disequilibrium between soil and leaf water potentials in nine temperate deciduous tree species

It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m-2 s-1); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species.

Climate and stomatal traits drive covariation in nighttime stomatal conductance and daytime gas exchange rates in a widespread C4 grass

Nighttime stomatal conductance (g_sn ) varies among plant functional types and species, but factors shaping the evolution of g_sn remain unclear. Examinations of intraspecific variation in g_sn as a function of climate and co-varying leaf traits may provide new insight into the evolution of g_sn and its adaptive significance. We grew 11 genotypes of Panicum virgatum (switchgrass) representing differing home-climates in a common garden experiment and measured nighttime and daytime leaf gas exchange, as well as stomatal density (SD) and size during early-, mid-, and late-summer. We used piecewise structural equation modelling to determine direct and indirect relationships between home-climate, gas exchange, and stomatal traits. We found no direct relationship between home-climate and g_sn . However, genotypes from hotter climates possessed higher SD, which resulted in higher g_sn . Across genotypes, higher g_sn was associated with higher daytime stomatal conductance and net photosynthesis. Our results indicate that higher g_sn may arise in genotypes from hotter climates via increased SD. High SD may provide benefits to genotypes from hotter climates through enhanced daytime transpirational cooling or by permitting maximal gas exchange when conditions are suitable. These results highlight the role of climate and trait coordination in shaping genetic differentiation in g_sn .

The Hidden Costs of Nighttime Warming on Yields

Nighttime warming poses a threat to global food security as it is driving yield declines worldwide, but our understanding of the physiological basis of this phenomenon remains very limited. Furthermore, it is often assumed that such declines are driven solely by increases in nighttime temperature (T_Night). Here we argue that, in addition to temperature, increases in nighttime evaporative demand may 'conspire' to penalize yields and end-use quality traits. We propose an ecophysiological framework outlining the possible mechanistic basis of such declines in yield and quality. We suggest ways to use the proposed framework as a guide to future efforts aimed at alleviating productivity losses by integrating crop ecophysiology with modeling, breeding, and management.

Ozone-induced impairment of night-time stomatal closure in O3-sensitive poplar clone is affected by nitrogen but not by phosphorus enrichment

Nocturnal transpiration may be a key factor influencing water use in plants. Tropospheric ozone (O₃) and availability of nutrients such as nitrogen (N) and phosphorus (P) in the soil can affect daytime water use through stomata, but the combined effects of O₃, N and P on night-time stomatal conductance (g_s) are not known. We investigated the effects of O₃ and soil availability of N and P on nocturnal g_s and the dynamics of stomatal response after leaf severing in an O₃-sensitive poplar clone (Oxford) subjected to combined treatments over a growing season in an O₃ free air controlled exposure (FACE) facility. The treatments were two soil N levels (0 and 80 kg N ha^-1; N0 and N80), three soil P levels (0, 40 and 80 kg P ha^-1; P0, P40 and P80) and three O₃ levels (ambient concentration, AA [35.0 ppb as hourly mean]; 1.5 × AA; 2.0 × AA). The analysis of stomatal dynamics after leaf severing suggested that O₃ impaired stomatal closure execution. As a result, nocturnal g_s was increased by 2.0 × AA O₃ in August (+39%) and September (+108%). Night-time g_s was correlated with POD₀ (phytotoxic O₃ dose) and increased exponentially after 40 mmol m^-2 POD₀. Such increase of nocturnal g_s was attributed to the emission of ethylene due to 2.0 × AA O₃ exposure, while foliar abscisic acid (ABA) or indole-3-acetic acid (IAA) did not affect g_s at night. Interestingly, the O₃-induced stomatal opening at night was limited by N treatments in August, but not limited in September. Phosphorus decreased nocturnal g_s, although P did not modify the O₃-induced stomatal dysfunction. The results suggest that the increased nocturnal g_s may be associated with a need to improve N acquisition to cope with O₃ stress.

Phylogenetic and biogeographic controls of plant nighttime stomatal conductance

The widely documented phenomenon of nighttime stomatal conductance g_sn could lead to substantial water loss with no carbon gain, and thus it remains unclear whether nighttime stomatal conductance confers a functional advantage. Given that studies of g_sn have focused on controlled environments or small numbers of species in natural environments, a broad phylogenetic and biogeographic context could provide insights into potential adaptive benefits of g_sn . We measured g_sn on a diverse suite of species (n = 73) across various functional groups and climates-of-origin in a common garden to study the phylogenetic and biogeographic/climatic controls on g_sn and further assessed the degree to which g_sn co-varied with leaf functional traits and daytime gas-exchange rates. Closely related species were more similar in g_sn than expected by chance. Herbaceous species had higher g_sn than woody species. Species that typically grow in climates with lower mean annual precipitation - where the fitness cost of water loss should be the highest - generally had higher g_sn . Our results reveal the highest g_sn rates in species from environments where neighboring plants compete most strongly for water, suggesting a possible role for the competitive advantage of g_sn .

Kaolin particle film modulates morphological, physiological and biochemical olive tree responses to drought and rewatering

Regarding the foreseeing climate change is reasonable to expect harmful consequences to olive tree (Olea europaea L.), an iconic species of Mediterranean region. Thus, the selection of practices that allow a better drought resistance and recovery capacity needs the immediate attention of scientific community. This study evaluates the strategies adopted by young potted olive trees, subjected to three cycles of drought and rewatering, in the presence of a reflective clay, kaolin (KL). The results demonstrated that KL induced shade-related leaf structural changes and was effective in keeping leaf water status during the most stressful periods. In general, photosynthetic activity of sprayed plants was improved by the alleviation of drought-induced stomatal and non-stomatal limitations. Moreover, during stress imposition sprayed leaves showed reduced oxidative damages, allowing lower investment in antioxidant defences. Furthermore, sprayed plants also had lower nighttime water losses due to inferior nighttime stomatal conductance, and are able to maintain higher respiration rates. Upon rewatering, the shaded effect conferred by KL limited gas exchange restauration, but improved the plants' capacity to restore the metabolic functions. In spite of the induced physiological and biochemical changes, no significant differences were found in whole-plant water use efficiency and plant biomass accumulation, possibly by the attenuation of photosynthesis restauration during the recovery events. In conclusion, the changes induced by KL might be beneficial under severe conditions, as on realistic Mediterranean field environments.

The role of nighttime water balance on Olea europaea plants subjected to contrasting water regimes

The climate change scenarios besides foreseeing a severe drought imposition also emphasize the temperature rising in the Mediterranean region, with special prominence at nighttime. Despite the high olive tree tolerance to severe environmental conditions, stomatal nighttime water loss can change plant water relations, and the related consequences and opportunities, especially under water scarcity, must be clarified. A set of 3-year-old potted olive trees were subjected to three cycles of drought, imposed by withholding irrigation, while another group were continuously irrigated. At the end of the latter and more severe drought cycle, daytime gas exchange parameters, water status and membrane integrity was negatively affected by drought imposition. Moreover, the nighttime transpiration rate was far above cuticular water loss, suggesting sustained stomatal aperture during nighttime, leading to substantial water losses, which was higher under drought in the first hours of darkness. The higher nighttime stomatal conductance of droughted plants were related with higher starch concentration in their leaves, a thicker trichome layer and a lower intercellular CO₂ concentration, in a closely association with an inferior nighttime respiration. Still, whole-plant transpiration on droughted plants were much lower than leaf transpiration-based estimates, which is interpreted as compensation by water inputs due to dew deposition on leaves. Although unexpected, the increased of stomatal conductance in the first hours of the night, until a certain level of water deficit intensity, could be linked with potential benefits to the plants.

Biotic- and abiotic-driven variations of the night-time sap flux of three co-occurring tree species in a low subtropical secondary broadleaf forest

Although several studies on the night-time water use of different plant species have been reported, comparative studies under the same climatic conditions of a region are scarce. This study aimed to analyse the inter- and intraspecific variations in night-time water use in relation to environmental factors and to tree morphological features to understand and elucidate the possible underlying mechanisms. The sap flow of three co-occurring tree species in a low subtropical secondary broadleaf forest in South China was monitored using Granier-style sap flux sensors. All examined environmental factors except wind speed exerted significant influence on the daytime sap flows of Schima superba, Castanopsis hystrix and Michelia macclurei, but the impacts of all factors, including wind speed, on the night-time sap flux were trivial. These results indicated that sap flow was mainly used for water recharge at night. The morphological features of the trees, except tree height, significantly affected the daytime water use, but no morphological features significantly affected the night-time water use. We found that night-time water recharge was strongly affected by the maximum flux density. A principal component analysis showed that there were more intraspecific than interspecific variations in water transport. The results also revealed that the night-time water use and the percentage of night/day (Qn/Qd) of photosynthetic stem species (C. hystrix and M. macclurei) were greater than those of non-photosynthetic stem species (S. superba).

Shifts in gene expression profiles are associated with weak and strong Crassulacean acid metabolism

PREMISE OF THE STUDY

The relative ease of high throughput sequencing is facilitating comprehensive phylogenomic and gene expression studies, even for nonmodel groups. To date, however, these two approaches have not been merged; while phylogenomic methods might use transcriptome sequences to resolve relationships, assessment of gene expression patterns in a phylogenetic context is less common. Here we analyzed both carbon assimilation and gene expression patterns of closely related species within the Agavoideae (Asparagaceae) to elucidate changes in gene expression across weak and strong phenotypes for Crassulacean acid metabolism (CAM).

METHODS

Gene expression patterns were compared across four genera: Agave (CAM), which is paraphyletic with Polianthes (weak CAM) and Manfreda (CAM), and Beschorneria (weak CAM). RNA-sequencing was paired with measures of gas exchange and titratable acidity. Climate niche space was compared across the four lineages to examine abiotic factors and their correlation to CAM.

KEY RESULTS

Expression of homologous genes showed both shared and variable patterns in weak and strong CAM species. Network analysis highlights that despite shared expression patterns, highly connected genes differ between weak and strong CAM, implicating shifts in regulatory gene function as key for the evolution of CAM. Variation in carbohydrate metabolism between weak and strong CAM supports the importance of sugar turnovers for CAM physiology.

CONCLUSIONS

Integration of phylogenetics and RNA-sequencing provides a powerful tool to study the evolution of CAM photosynthesis across closely related but photosynthetically variable species. Our findings regarding shared or shifted gene expression and regulation of CAM via carbohydrate metabolism have important implications for efforts to engineer the CAM pathway into C₃ food and biofuel crops.

Nitrogen species coupled with transpiration enhance Fe plaque assisted aquatic uranium removal via rhizofiltration of Phragmites australis Trin ex Steud

The influences of N species and transpiration on the Fe plaque (IP) formation and related aquatic U rhizofiltration had not revealed yet, especially when these factors were co-existed. It was evaluated in a mesocosm experiment in the condition of respective ammonium (NH₄⁺)/nitrate (NO₃^-) cultivation of Phragmites australis Trin ex Steud. coupled with different transpiration rates (TRs). The results suggested that the enhanced transpiration of P. australis improved the aquatic U rhizofiltration in both NO₃^- and NH₄⁺ rich milieus. However, the NO₃^- dependent oxidizing milieu restricted aquatic U uptake by the root of P. australis (up to 47.6 ± 1.8 mg kg^-1 under high TR) via IP assisted rhizofiltration. The high aquatic U availability and limited IP formation in NO₃^- rich milieu benefited the U retention within root tissue. On the contrary, the aquatic U rhizofiltration (up to 62.1 ± 1.0 mg kg^-1 under high TR) was enhanced under NH₄⁺ dependent reductive milieu. It was mainly contributed by U retention within IP. The area related U accumulation in different N species cultured roots was enhanced but did not significantly different under higher TR condition. The result suggested that the supplied NH₄⁺ coupled with enhanced transpiration was supposed to be more optimized option for IP assisted aquatic U rhizofiltration via P. australis.

The ecophysiology of leaf cuticular transpiration: are cuticular water permeabilities adapted to ecological conditions?

When the stomata are closed under drought, the only route for water loss from the leaf interior to the atmosphere is across the cuticle. Thus, the extent of cuticular transpiration in relation to the reservoirs of water in the plant and the water acquisition from the soil determines the fitness and survival of the plant. It is, therefore, widely assumed that the cuticular water permeability of plants regularly experiencing drought is comparatively low and, thus, adapted to the environment. To test this hypothesis, 382 measurements of cuticular permeability from 160 species were extracted from the literature published between 1996 and 2017. The data had been produced either by using isolated cuticles and astomatous leaf sides or by measuring the minimum leaf conductance under conditions assumed to induce maximum stomatal closure. The species were assigned to 11 life form groups. Except for two particular cases (epiphytes, and climbers and lianas), the cuticular permeabilities of all groups either did not differ significantly or the available data did not allow a statistical test. In conclusion, present knowledge either does not support the hypothesis that ecological adaptions of cuticular water permeability exist or the available data are insufficient to test it.

Seasonal variability of the parameters of the Ball-Berry model of stomatal conductance in maize (Zea mays L.) and sunflower (Helianthus annuus L.) under well-watered and water-stressed conditions

The Ball-Berry (BB) model of stomatal conductance (g_s ) is frequently coupled with a model of assimilation to estimate water and carbon exchanges in plant canopies. The empirical slope (m) and 'residual' g_s (g₀ ) parameters of the BB model influence transpiration estimates, but the time-intensive nature of measurement limits species-specific data on seasonal and stress responses. We measured m and g₀ seasonally and under different water availability for maize and sunflower. The statistical method used to estimate parameters impacted values nominally when inter-plant variability was low, but had substantial impact with larger inter-plant variability. Values for maize (m = 4.53 ± 0.65; g₀  = 0.017 ± 0.016 mol m^-2 s^-1 ) were 40% higher than other published values. In maize, we found no seasonal changes in m or g₀ , supporting the use of constant seasonal values, but water stress reduced both parameters. In sunflower, inter-plant variability of m and g₀ was large (m = 8.84 ± 3.77; g₀  = 0.354 ± 0.226 mol m^-2 s^-1 ), presenting a challenge to clear interpretation of seasonal and water stress responses - m values were stable seasonally, even as g₀ values trended downward, and m values trended downward with water stress while g₀ values declined substantially.

Nighttime stomatal conductance differs with nutrient availability in two temperate floodplain tree species

Nighttime water flow varies between plant species and is a phenomenon for which the magnitude, purpose and consequences are widely discussed. A potential benefit of nighttime stomata opening may be increased nutrient availability during the night since transpiration affects the mass flow of soil water towards plant roots. We investigated how nitrogen (N) and phosphorus (P) fertilization, and short-term drought affected stomatal conductance of Fraxinus excelsior L. and Ulmus laevis Pallas during the day (gs) and night (gn), and how these factors affected growth for a period of 18 weeks. Both species were found to open their stomata during the night, and gn responded to nutrients and water in a different manner than gs. Under N-deficiency, F. excelsior had higher gn, especially when P was sufficient, and lower pre-dawn leaf water potential (Ψpd), supporting our assumption that nutrient limitation leads to increases in nighttime water uptake. Under P-deficiency, F. excelsior had higher relative root production and, thus, adjusted its biomass allocation under P shortage, while sufficient N but not P contributed to overall higher biomasses. In contrast, U. laevis had higher gn and lower root:shoot ratio under high nutrient (especially N) availability, whereas both sufficient N and P produced higher biomasses. Compared with well-watered trees, the drought treatment did not affect any growth parameter but it resulted in lower gn, minimum stomatal conductance and Ψpd of F. excelsior. For U. laevis, only gs during July was lower when drought-treated. In summary, the responses of gs and gn to nutrients and drought depended on the species and its nutrient uptake strategy, and also the timing of measurement during the growing season. Eutrophication of floodplain forests dominated by F. excelsior and U. laevis may, therefore, considerably change nighttime transpiration rates, leading to ecosystem-level changes in plant-water dynamics. Such changes may have more severe consequences in the future as a higher frequency of drought events is predicted under climate change.

Controlling stomatal aperture in semi-arid regions-The dilemma of saving water or being cool?

Stomatal regulation of leaf gas exchange with the atmosphere is a key process in plant adaptation to the environment, particularly in semi-arid regions with high atmospheric evaporative demand. Development of stomata, integrating internal signaling and environmental cues sets the limit for maximum diffusive capacity of stomata, through size and density and is under a complex genetic control, thus providing multiple levels of regulation. Operational stomatal conductance to water vapor and CO2 results from feed-back and/or feed-forward mechanisms and is the end-result of a plethora of signals originated in leaves and/or in roots at each moment. CO2 assimilation versus water vapor loss, proposed to be the subject of optimal regulation, is species dependent and defines the water use efficiency (WUE). WUE has been a topic of intense research involving areas from genetics to physiology. In crop plants, especially in semi-arid regions, the question that arises is how the compromise of reducing transpiration to save water will impact on plant performance through leaf temperature. Indeed, plant transpiration by providing evaporative cooling, is a major component of the leaf energy balance. In this paper we discuss the dilemma of 'saving water or being cool' bringing about recent findings from molecular genetics, to development and physiology of stomata. The question of 'how relevant is screening for high/low WUE in crops for semi-arid regions, where drought and heat co-occur' is discussed.

Reduced nighttime transpiration is a relevant breeding target for high water-use efficiency in grapevine

Increasing water scarcity challenges crop sustainability in many regions. As a consequence, the enhancement of transpiration efficiency (TE)-that is, the biomass produced per unit of water transpired-has become crucial in breeding programs. This could be achieved by reducing plant transpiration through a better closure of the stomatal pores at the leaf surface. However, this strategy generally also lowers growth, as stomatal opening is necessary for the capture of atmospheric CO2 that feeds daytime photosynthesis. Here, we considered the reduction in transpiration rate at night (En) as a possible strategy to limit water use without altering growth. For this purpose, we carried out a genetic analysis for En and TE in grapevine, a major crop in drought-prone areas. Using recently developed phenotyping facilities, potted plants of a cross between Syrah and Grenache cultivars were screened for 2 y under well-watered and moderate soil water deficit scenarios. High genetic variability was found for En under both scenarios and was primarily associated with residual diffusion through the stomata. Five quantitative trait loci (QTLs) were detected that underlay genetic variability in En Interestingly, four of them colocalized with QTLs for TE. Moreover, genotypes with favorable alleles on these common QTLs exhibited reduced En without altered growth. These results demonstrate the interest of breeding grapevine for lower water loss at night and pave the way to breeding other crops with this underexploited trait for higher TE.

Suppression of nighttime sap flux with lower stem photosynthesis in Eucalyptus trees

It is widely accepted that substantial nighttime sap flux (J s,n) or transpiration (E) occurs in most plants, but the physiological implications are poorly known. It has been hypothesized that J s,n or E serves to enhance nitrogen uptake or deliver oxygen; however, no clear evidence is currently available. In this study, sap flux (J s) in Eucalyptus grandis × urophylla with apparent stem photosynthesis was measured, including control trees which were covered by aluminum foil (approximately 1/3 of tree height) to block stem photosynthesis. We hypothesized that the nighttime water flux would be suppressed in trees with lower stem photosynthesis. The results showed that the green tissue degraded after 3 months, demonstrating a decrease in stem photosynthesis. The daytime J s decreased by 21.47%, while J s,n decreased by 12.03% in covered trees as compared to that of control, and the difference was statistically significant (P < 0.01). The linear quantile regression model showed that J s,n decreased for a given daytime transpiration water loss, indicating that J s,n was suppressed by lower stem photosynthesis in covered trees. Predawn (ψ pd) of covered trees was marginally higher than that of control while lower at predawn stomatal conductance (g s), indicating a suppressed water flux in covered trees. There was no difference in leaf carbon content and δ(13)C between the two groups, while leaf nitrogen content and δ(15)N were significantly higher in covered trees than that of the control (P < 0.05), indicating that J s,n was not used for nitrogen uptake. These results suggest that J s,n may act as an oxygen pathway since green tissue has a higher respiration or oxygen demand than non-green tissue. Thus, this study demonstrated the physiological implications of J s,n and the possible benefits of nighttime water use or E by the tree.

Populus species from diverse habitats maintain high night-time conductance under drought

We investigated the interspecific variability in nocturnal whole-plant stomatal conductance under well-watered and drought conditions in seedlings of four species of Populus from habitats characterized by abundant water supply (mesic and riparian) or from drier upland sites. The study was carried out to determine whether (i) nocturnal conductance varies across different species of Populus according to their natural habitat, (ii) nocturnal conductance is affected by water stress similarly to daytime conductance based on species habitat and (iii) differences in conductance among species could be explained partly by differences in stomatal traits. We measured whole-plant transpiration and conductance (G) of greenhouse-grown seedlings using an automated high-resolution gravimetric technique. No relationship was found between habitat preference and daytime G (GD), but night-time G (GN) was on average 1.5 times higher in riparian and mesic species (P. deltoides Bartr. ex Marsh. and P. trichocarpa Torr. & Gray) than in those from drier environments (P. tremuloides Michx. and P. × petrowskyana Schr.). GN was not significantly reduced under drought in riparian species. Upland species restricted GN significantly in response to drought, but it was still at least one order of magnitude greater that the cuticular conductance until leaf death was imminent. Under both well-watered and drought conditions, GN declined with increasing vapour pressure deficit (D). Also, a small increase in GN towards the end of the night period was observed in P. deltoides and P. × petrowskyana, suggesting the involvement of endogenous regulation. The anatomical analyses indicated a positive correlation between G and variable stomatal pore index among species and revealed that stomata are not likely to be leaky but instead seem capable of complete occlusion, which raises the question of the possible physiological role of the significant GN observed under drought. Further comparisons among closely related species that occupy ecologically diverse habitats may provide a better understanding of the genetic versus environmental regulations of nocturnal water loss.

Phylogenetic and ecological patterns in nighttime transpiration among five members of the genus Rubus co-occurring in western Oregon

Nighttime transpiration is a substantial portion of ecosystem water budgets, but few studies compare water use of closely related co-occurring species in a phylogenetic context. Nighttime transpiration can range up to 69% of daytime rates and vary between species, ecosystem, and functional type. We examined leaf-level daytime and nighttime gas exchange of five species of the genus Rubus co-occurring in the Pacific Northwest of western North America in a greenhouse common garden. Contrary to expectations, nighttime transpiration was not correlated to daytime water use. Nighttime transpiration showed pronounced phylogenetic signals, but the proportion of variation explained by different phylogenetic groupings varied across datasets. Leaf osmotic water potential, water potential at turgor loss point, stomatal size, and specific leaf area were correlated with phylogeny but did not readily explain variation in nighttime transpiration. Patterns in interspecific variation as well as a disconnect between rates of daytime and nighttime transpiration suggest that variation in nighttime water use may be at least partly driven by genetic factors independent of those that control daytime water use. Future work with co-occurring congeneric systems is needed to establish the generality of these results and may help determine the mechanism driving interspecific variation in nighttime water use.

Consequences of nocturnal water loss: a synthesis of regulating factors and implications for capacitance, embolism and use in models

Total daily water use is a key factor influencing the growth of many terrestrial plants, and reflects both day-time and nocturnal water fluxes. However, while nocturnal sap flow (En) and stomatal conductance (gs,n) have been reported across a range of species, ecosystems and microclimatic conditions, the regulation of these fluxes remains poorly understood. Here, we present a framework describing the role of abiotic and biotic factors in regulating En and gs,n highlighting recent developments in this field. Across ecosystems, En and gs,n generally increased with increasing soil water content and vapor pressure deficit, but the interactive effects of these factors and the potential roles of wind speed and other abiotic factors remain unclear. On average, gs,n and En are higher in broad-leaved compared with needle-leaved plants, in C3 compared with C4 plants, and in tropical compared with temperate species. We discuss the impacts of leaf age, elevated [CO2] and refilling of capacitance on night-time water loss, and how nocturnal gs,n may be included in vegetation models. Younger leaves may have higher gs,n than older leaves. Embolism refilling and recharge of capacitance may affect sap flow such that total plant water loss at night may be less than estimated solely from En measurements. Our estimates of gs,n for typical plant functional types, based on the published literature, suggest that nocturnal water loss may be a significant fraction (10-25%) of total daily water loss. Counter-intuitively, elevated [CO2] may increase nocturnal water loss. Assumptions in process-based ecophysiological models and dynamic global vegetation models that gs is zero when solar radiation is zero are likely to be incorrect. Consequently, failure to adequately consider nocturnal water loss may lead to substantial under-estimation of total plant water use and inaccurate estimation of ecosystem level water balance.

Pre-dawn stomatal opening does not substantially enhance early-morning photosynthesis in Helianthus annuus

Most C3 plant species have partially open stomata during the night especially in the 3-5 h before dawn. This pre-dawn stomatal opening has been hypothesized to enhance early-morning photosynthesis (A) by reducing diffusion limitations to CO2 at dawn. We tested this hypothesis in cultivated Helianthus annuus using whole-shoot gas exchange, leaf level gas exchange and modelling approaches. One hour pre-dawn low-humidity treatments were used to reduce pre-dawn stomatal conductance (g). At the whole-shoot level, a difference of pre-dawn g (0.40 versus 0.17 mol m(-2) s(-1)) did not significantly affect A during the first hour after dawn. Shorter term effects were investigated with leaf level gas exchange measurements and a difference of pre-dawn g (0.10 versus 0.04 mol m(-2) s(-1)) affected g and A for only 5 min after dawn. The potential effects of a wider range of stomatal apertures were explored with an empirical model of the relationship between A and intercellular CO2 concentration during the half-hour after dawn. Modelling results demonstrated that even extremely low pre-dawn stomatal conductance values have only a minimal effect on early-morning A for a few minutes after dawn. Thus, we found no evidence that pre-dawn stomatal opening enhances A.

Modelled hydraulic redistribution by sunflower (Helianthus annuus L.) matches observed data only after including night-time transpiration

The movement of water from moist to dry soil layers through the root systems of plants, referred to as hydraulic redistribution (HR), occurs throughout the world and is thought to influence carbon and water budgets and ecosystem functioning. The realized hydrologic, biogeochemical and ecological consequences of HR depend on the amount of redistributed water, whereas the ability to assess these impacts requires models that correctly capture HR magnitude and timing. Using several soil types and two ecotypes of sunflower (Helianthus annuus L.) in split-pot experiments, we examined how well the widely used HR modelling formulation developed by Ryel et al. matched experimental determination of HR across a range of water potential driving gradients. H. annuus carries out extensive night-time transpiration, and although over the last decade it has become more widely recognized that night-time transpiration occurs in multiple species and many ecosystems, the original Ryel et al. formulation does not include the effect of night-time transpiration on HR. We developed and added a representation of night-time transpiration into the formulation, and only then was the model able to capture the dynamics and magnitude of HR we observed as soils dried and night-time stomatal behaviour changed, both influencing HR.

Soil phosphorous and endogenous rhythms exert a larger impact than CO2 or temperature on nocturnal stomatal conductance in Eucalyptus tereticornis

High nocturnal transpiration rates (5-15% of total water loss in terrestrial plants) may be adaptive under limited fertility, by increasing nutrient uptake or transport via transpiration-induced mass flow, but the response of stomata in the dark to environmental variables is poorly understood. Here we tested the impact of soil phosphorous (P) concentration, atmospheric CO2 concentration and air temperature on stomatal conductance (gs) during early and late periods in the night, as well as at midday in naturally, sun-lit glasshouse-grown Eucalyptus tereticornis Sm. seedlings. Soil P was the main driver of nocturnal gs, which was consistently higher in low soil P (37.3-79.9 mmol m(-2) s(-1)) than in high soil P (17.7-49.3 mmol m(-2)(-1)). Elevated temperature had only a marginal (P = 0.07) effect on gs early in the night (gs decreased from 34.7 to 25.8 mmol m(-2) s(-1) with an increase in temperature of 4 °C). The effect of CO2 depended on its interaction with temperature. Stomatal conductance responses to soil P were apparently driven by indirect effects of soil P on plant anatomy, since gs was significantly and negatively correlated with wood density. However, the relationship of gs with environmental factors became weaker late in the night, relative to early in the night, likely due to apparent endogenous processes; gs late in the night was two times larger than gs observed early in the night. Time-dependent controls over nocturnal gs suggest that daytime stomatal models may not apply during the night, and that different types of regulation may occur even within a single night. We conclude that the enhancement of nocturnal gs under low soil P availability is unlikely to be adaptive in our species because of the relatively small amount of transpiration-induced mass flow that can be achieved through rates of nocturnal water loss (3-6% of daytime mass flow).

Ontogeny strongly and differentially alters leaf economic and other key traits in three diverse Helianthus species

The leaf economics spectrum (LES) describes large cross-species variation in suites of leaf functional traits ranging from resource-acquisitive to resource-conservative strategies. Such strategies have been integral in explaining plant adaptation to diverse environments, and have been linked to numerous ecosystem processes. The LES has previously been found to be significantly modulated by climate, soil fertility, biogeography, growth form, and life history. One largely unexplored aspect of LES variation, whole-plant ontogeny, is investigated here using multiple populations of three very different species of sunflower: Helianthus annuus, Helianthus mollis, and Helianthus radula. Plants were grown under environmentally controlled conditions and assessed for LES and related traits at four key developmental stages, using recently matured leaves to standardize for leaf age. Nearly every trait exhibited a significant ontogenetic shift in one or more species, with trait patterns differing among populations and species. Photosynthetic rate, leaf nitrogen concentration, and leaf mass per area exhibited surprisingly large changes, spanning over two-thirds of the original cross-species LES variation and shifting from resource-acquisitive to resource-conservative strategies as the plants matured. Other traits being investigated in relation to the LES, such as leaf water content, pH, and vein density, also showed large changes. The finding that ontogenetic variation in LES strategy can be substantial leads to a recommendation of standardization by developmental stage when assessing 'species values' of labile traits for comparative approaches. Additionally, the substantial ontogenetic trait shifts seen within single individuals provide an opportunity to uncover the contribution of gene regulatory changes to variation in LES traits.

Nocturnal and daytime stomatal conductance respond to root-zone temperature in 'Shiraz' grapevines

BACKGROUND AND AIMS

Daytime root-zone temperature may be a significant factor regulating water flux through plants. Water flux can also occur during the night but nocturnal stomatal response to environmental drivers such as root-zone temperature remains largely unknown.

METHODS

Here nocturnal and daytime leaf gas exchange was quantified in 'Shiraz' grapevines (Vitis vinifera) exposed to three root-zone temperatures from budburst to fruit-set, for a total of 8 weeks in spring.

KEY RESULTS

Despite lower stomatal density, night-time stomatal conductance and transpiration rates were greater for plants grown in warm root-zones. Elevated root-zone temperature resulted in higher daytime stomatal conductance, transpiration and net assimilation rates across a range of leaf-to-air vapour pressure deficits, air temperatures and light levels. Intrinsic water-use efficiency was, however, lowest in those plants with warm root-zones. CO(2) response curves of foliar gas exchange indicated that the maximum rate of electron transport and the maximum rate of Rubisco activity did not differ between the root-zone treatments, and therefore it was likely that the lower photosynthesis in cool root-zones was predominantly the result of a stomatal limitation. One week after discontinuation of the temperature treatments, gas exchange was similar between the plants, indicating a reversible physiological response to soil temperature.

CONCLUSIONS

In this anisohydric grapevine variety both night-time and daytime stomatal conductance were responsive to root-zone temperature. Because nocturnal transpiration has implications for overall plant water status, predictive climate change models using stomatal conductance will need to factor in this root-zone variable.

Reduced drought tolerance during domestication and the evolution of weediness results from tolerance-growth trade-offs

The increased reproductive potential, size, shoot allocation, and growth rate of weedy plants may result from reduced resource allocation to other aspects of plant growth and defense. To investigate whether changes in resource allocation occurred during domestication or the evolution of weediness, we compared the mycorrhizal responsiveness, growth, and drought tolerance of nine native ruderal, nine agriculturally weedy (four U.S. weedy and five Australian weedy), and 14 domesticated populations (eight ancient landraces and six improved cultivars) of the common sunflower (Helianthus annuus). Domesticated sunflower cultivars were less drought tolerant, but had higher plant growth and fecundity and coarser roots than wild populations. There were no changes in level of drought tolerance between improved cultivars and ancient landrace plants, but there was an increase in allocation to flowers with recent selection. Weedy populations were intermediate between domesticated cultivars and native ruderal populations for plant growth rate, root architecture, and drought tolerance. Weedy populations benefited most from mycorrhizal inoculation by having fewer wilted leaves and wetter soil. Overall, we found that trade-offs between drought tolerance and several aspects of plant growth, including growth rate, allocation to flowering, and root architecture, govern evolution during sunflower domestication and the invasion of disturbed habitat.

Nocturnal stomatal conductance responses to rising [CO2], temperature and drought

The response of nocturnal stomatal conductance (g(s,n)) to rising atmospheric CO(2) concentration ([CO(2)]) is currently unknown, and may differ from responses of daytime stomatal conductance (g(s,d)). Because night-time water fluxes can have a significant impact on landscape water budgets, an understanding of the effects of [CO(2)] and temperature on g(s,n) is crucial for predicting water fluxes under future climates. Here, we examined the effects of [CO(2)] (280, 400 and 640 μmol mol(-1)), temperature (ambient and ambient + 4°C) and drought on g(s,n,) and g(s,d) in Eucalyptus sideroxylon saplings. g(s,n) was substantially higher than zero, averaging 34% of g(s,d). Before the onset of drought, g(s,n) increased by 85% when [CO(2)] increased from 280 to 640 μmol mol(-1), averaged across both temperature treatments. g(s,n) declined with drought, but an increase in [CO(2)] slowed this decline. Consequently, the soil water potential at which g(s,n) was zero (Ψ(0)) was significantly more negative in elevated [CO(2)] and temperature treatments. g(s,d) showed inconsistent responses to [CO(2)] and temperature. g (s,n) may be higher in future climates, potentially increasing nocturnal water loss and susceptibility to drought, but cannot be predicted easily from g(s,d). Therefore, predictive models using stomatal conductance must account for both g(s,n) and g(s,d) when estimating ecosystem water fluxes.

Interactive effects of elevated CO2 and drought on nocturnal water fluxes in Eucalyptus saligna

Nocturnal water flux has been observed in trees under a variety of environmental conditions and can be a significant contributor to diel canopy water flux. Elevated atmospheric CO(2) (elevated [CO(2)]) can have an important effect on day-time plant water fluxes, but it is not known whether it also affects nocturnal water fluxes. We examined the effects of elevated [CO(2)] on nocturnal water flux of field-grown Eucalyptus saligna trees using sap flux through the tree stem expressed on a sapwood area (J(s)) and leaf area (E(t)) basis. After 19 months growth under well-watered conditions, drought was imposed by withholding water for 5 months in the summer, ending with a rain event that restored soil moisture. Reductions in J(s) and E(t) were observed during the severe drought period in the dry treatment under elevated [CO(2)], but not during moderate- and post-drought periods. Elevated [CO(2)] affected night-time sap flux density which included the stem recharge period, called 'total night flux' (19:00 to 05:00, J(s,r)), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00, J(s,c)). Elevated [CO(2)] wet (EW) trees exhibited higher J(s,r) than ambient [CO(2)] wet trees (AW) indicating greater water flux in elevated [CO(2)] under well-watered conditions. However, under drought conditions, elevated [CO(2)] dry (ED) trees exhibited significantly lower J(s,r) than ambient [CO(2)] dry trees (AD), indicating less water flux during stem recharge under elevated [CO(2)]. J(s,c) did not differ between ambient and elevated [CO(2)]. Vapour pressure deficit (D) was clearly the major influence on night-time sap flux. D was positively correlated with J(s,r) and had its greatest impact on J(s,r) at high D in ambient [CO(2)]. Our results suggest that elevated [CO(2)] may reduce night-time water flux in E. saligna when soil water content is low and D is high. While elevated [CO(2)] affected J(s,r), it did not affect day-time water flux in wet soil, suggesting that the responses of J(s,r) to environmental factors cannot be directly inferred from day-time patterns. Changes in J(s,r) are likely to influence pre-dawn leaf water potential, and plant responses to water stress. Nocturnal fluxes are clearly important for predicting effects of climate change on forest physiology and hydrology.

Rates of nocturnal transpiration in two evergreen temperate woodland species with differing water-use strategies

Nocturnal fluxes may be a significant factor in the annual water budget of forested ecosystems. Here, we assessed sap flow in two co-occurring evergreen species (Eucalyptus parramattensis and Angophora bakeri) in a temperate woodland for 2 years in order to quantify the magnitude of seasonal nocturnal sap flow (E(n)) under different environmental conditions. The two species showed different diurnal water relations, demonstrated by different diurnal curves of stomatal conductance, sap flow and leaf water potential. The relative influence of several microclimatic variables, including wind speed (U), vapour pressure deficit (D), the product of U and D (UD) and soil moisture content, were quantified. D exerted the strongest influence on E(n) (r² = 0.59-0.86), soil moisture content influenced E(n) when D was constant, but U and UD did not generally influence E(n). In both species, cuticular conductance (G(c)) was a small proportion of total leaf conductance (G(s)) and was not a major pathway for E(n). We found that E(n) was primarily a function of transpiration from the canopy rather than refilling of stem storage, with canopy transpiration accounting for 50-70% of nocturnal flows. Mean E(n) was 6-8% of the 24-h flux across seasons (spring, summer and winter), but was up to 19% of the 24-h flux on some days in both species. Despite different daytime strategies in water use of the two species, both species demonstrated low night-time water loss, suggesting similar controls on water loss at night. In order to account for the impact of E(n) on pre-dawn leaf water potential arising from the influence of disequilibria between root zone and leaf water potential, we also developed a simple model to more accurately predict soil water potential (ψ(s)).

Contrasting drought tolerance strategies in two desert annuals of hybrid origin

Woody plants native to mesic habitats tend to be more vulnerable to drought-induced cavitation than those in xeric habitats. Cavitation resistance in herbaceous plants, however, is rarely studied and whether or not annual plants in arid habitats conform to the trends observed in woody plants is unknown. This question is addressed by comparing the hydraulic properties of annual plants endemic to relatively mesic and seasonally xeric habitats in the Great Basin Desert, in both native and experimental settings. Vulnerability to cavitation between species differed as predicted when vulnerability curves of similar-sized native individuals were compared. Contrary to expectations, Helianthus anomalus from the relatively mesic dune sites, on average, exhibited higher native embolism, lower soil-to-leaf hydraulic conductance (k(L)) and lower transpiration rates, than its xeric analogue, H. deserticola. In transplant gardens, H. anomalus' vulnerability to cavitation was unaffected by transplant location or watering treatment. In H. deserticola, however, vulnerability to cavitation varied significantly in response to watering in transplant gardens and varied as a function of stem water potential (Psi(stem)). H. deserticola largely avoided cavitation through its higher water status and generally more resistant xylem, traits consistent with a short life cycle and typical drought-escape strategy. By contrast, H. anomalus' higher native embolism is likely to be adaptive by lowering plant conductance and transpiration rate, thus preventing the loss of root-to-soil hydraulic contact in the coarse sand dune soils. For H. anomalus this dehydration avoidance strategy is consistent with its relatively long 3-4 month life cycle and low-competition habitat. We conclude that variance of hydraulic parameters in herbaceous plants is a function of soil moisture heterogeneity and is consistent with the notion that trait plasticity to fine-grained environmental variation can be adaptive.

Soil nitrogen limitation does not impact nighttime water loss in Populus

Nighttime transpirational water loss from C(3) trees occurs without carbon gain and is both common and substantial. However, the magnitude of this water loss varies and a better understanding of the environmental factors driving this variation is needed. We investigated the response of nighttime conductance (g(night)) and transpiration (E(night)) to soil nitrogen limitation. We used instantaneous gas exchange measurements in greenhouse studies of Populus angustifolia James (narrowleaf cottonwood) and Populus balsamifera L. spp. trichocarpa (Torr. & A. Gray ex Hook.) Brayshaw (black cottonwood). g(night) for sufficiently watered plants ranged from 0.045 to 0.308 mol m(-2) s(-1) for P. balsamifera and 0.037 to 0.188 mol m(-2) s(-1) for P. angustifolia, which was much larger than minimum leaf conductance (g(min); up to 0.005 mol m(-2) s(-1) in the dark). Long-term nitrogen limitation sufficient to substantially reduce biomass did not affect g(night) or E(night) when potentially confounding water stress effects were eliminated. We conclude that nighttime water loss from two Populus species is large and although it is under stomatal control is not regulated at night in response to soil nitrogen availability.

K+ deprivation induces xylem water and K+ transport in sunflower: evidence for a co-ordinated control

The effect of K+ deprivation on water and K+ transport in roots was studied in sunflower plants. Deprivation was achieved in two different ways: by removing K+ from the growth medium for varying intervals; and by growing plants permanently in a low-K+ medium. Removal of K+ from the growth medium for a few hours prompted a significant increase in xylem sap exudation, associated with an increase in root hydraulic conductivity; however, it did not give rise to any significant change in plant K+ content, nor did it favour root K+ exudation. By contrast, prolonged K+ deprivation led to a decline in the internal K+ content and stimulated water and K+ transport in roots. Leaf application of K+ (Rb+) in plants grown permanently in a low-K+ medium inhibited the effect of K+ deprivation on root water and K+ transport, without significantly modifying the internal K+ content of the plants. This treatment had no effect on normal-K+ plants. These results suggest the existence of mechanisms enabling perception of plant K+ status and/or K+ availability in the medium, which trigger transduction processes governing the transport of water and K+ from the root to the shoot.

Night-time transpiration can decrease hydraulic redistribution

C(3) plants dominate many landscapes and are critically important for ecosystem water cycling. At night, plant water losses can include transpiration (E(night)) from the canopy and hydraulic redistribution (HR) from roots. We tested whether E(night) limits the magnitude of HR in a greenhouse study using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Plants were grown with their roots split between two compartments. HR was initiated by briefly withholding all water, followed by watering only one rooting compartment. Under study conditions, all species showed substantial E(night) and HR (highest minus lowest soil water potential [Psi(s)] during a specified diel period). Suppressing E(night) by canopy bagging increased HR during the nightly bagging period (HR(N)) for A. tridentata and H. anomalus by 73 and 33% respectively, but did not affect HR(N) by Q. laevis. Total daily HR (HR(T)) was positively correlated with the Psi(s) gradient between the rooting compartments, which was correlated with light and/or atmospheric vapour pressure deficit (VPDa) the prior day. For A. tridentata, HR(T) was negatively correlated with night-time VPDa. Ecological implications of the impact of E(night) on HR may include decreased plant productivity during dry seasons, altered ecosystem water flux patterns and reduced nutrient cycling in drying soils.

Magnitude of nighttime transpiration does not affect plant growth or nutrition in well-watered Arabidopsis

Significant water loss occurs throughout the night via partially open stomata in many C(3) and C(4) plant species. Although apparently wasteful in terms of water use, nighttime transpiration (E(night)) is hypothesized to benefit plants by enhancing nutrient supply. We tested the hypothesis that plants with greater E(night) would have improved plant nutrient status and greater fitness, estimated as pre-bolting biomass, for Arabidopsis thaliana. Two very different levels of E(night) were generated in plants by exposing them to high vs low nighttime leaf-to-air vapor pressure deficits (VPD(leaf)) in controlled environment chambers. An assessment of responses of nighttime leaf conductance (g(night)) to VPD(leaf) indicated that E(night) differed by at least 80% between the treatments. This large difference in E(night), imposed over the entire vegetative growth phase of Arabidopsis, had no effect on leaf nutrient content (N, Ca, K) or pre-bolting rosette biomass. The lack of response to differences in E(night) held true for both a high and a low nitrogen (N) treatment, even though the low N treatment decreased leaf N and biomass by 40-60%. The N treatment had no effect on g(night). Thus, higher E(night) did not provide a nutrient or growth benefit to Arabidopsis, even when the plants were N-limited.

Super-elevated CO2 interferes with stomatal response to ABA and night closure in soybean (Glycine max)

Studies have shown stomatal conductance (g(s)) of plants exposed to super-elevated CO2 (>5000micromol mol(-1)) increases in several species, in contrast to a decrease of g(s) caused by moderate CO2 enrichment. We conducted a series of experiments to determine whether super-elevated CO2 alters stomatal development and/or interferes with stomatal closure in soybean (Glycine max). Plants were grown at nominal ambient (400), elevated (1200) and super-elevated (10,000micromol mol(-1)) CO2 in controlled environmental chambers. Stomatal density of the plant leaf was examined by a scanning electron microscope (SEM), while the stomatal response to the application of exogenous abscisic acid (ABA), a phytohormone associated with water stress and stomatal control, was investigated in intact growing plants by measuring the g(s) of abaxial leaf surfaces using a steady-state porometer. Relative to the control (400micromol mol(-1) CO2) plants, daytime stomatal conductance (g(s,day)) of the plants grown under 1200 and 10,000micromol mol(-1) CO2 was reduced by 38% and 15%, respectively. Dark period stomatal conductance (g(s,night)) was unaffected by growing under 1200mumol mol(-1) CO2) but dramatically increased under 10,000micromol mol(-1) CO2. Stomatal density increased by 10% in the leaves of 10,000micromol mol(-1) CO2-grown plants, which in part contributed to the higher g(s,night) values. Elevating [CO2] to 1200micromol mol(-1) enhanced ABA-induced stomatal closure, but further increasing CO2 to 10,000micromol mol(-1) significantly reduced ABA-induced stomatal closure. These results demonstrated that stomatal response to ABA is CO2 dependent. Hence, a stomatal failure to effectively respond to an ABA signal and to close at night under extremely high CO2 may increase plants susceptibility to other abiotic stresses.

The importance of nutritional regulation of plant water flux

Transpiration is generally considered a wasteful but unavoidable consequence of photosynthesis, occurring because water is lost when stomata open for CO(2) uptake. Additionally, transpiration has been ascribed the functions of cooling leaves, driving root to shoot xylem transport and mass flow of nutrients through the soil to the rhizosphere. As a consequence of the link between nutrient mass flow and transpiration, nutrient availability, particularly that of NO(3)(-), partially regulates plant water flux. Nutrient regulation of transpiration may function through the concerted regulation of: (1) root hydraulic conductance through control of aquaporins by NO(3)(-), (2) shoot stomatal conductance (g(s)) through NO production, and (3) pH and phytohormone regulation of g(s). These mechanisms result in biphasic responses of water flux to NO(3)(-) availability. The consequent trade-off between water and nutrient flux has important implications for understanding plant distributions, for production of water use-efficient crops and for understanding the consequences of global-change-linked CO(2) suppression of transpiration for plant nutrient acquisition.

Environmental stress and genetics influence night-time leaf conductance in the C4 grass Distichlis spicata

Growing awareness of night-time leaf conductance (g_night) in many species, as well as genetic variation in g_night within several species, has raised questions about how genetic variation and environmental stress interact to influence the magnitude of g_night. The objective of this study was to investigate how genotype salt tolerance and salinity stress affect g_night for saltgrass [Distichlis spicata (L.) Greene]. Across genotypes and treatments, night-time water loss rates were 5-20% of daytime rates. Despite growth declining 37-87% in the high salinity treatments (300 mm and 600 mm NaCl), neither treatment had any effect on g_night in four of the six genotypes compared with the control treatment (7 mm NaCl). Daytime leaf conductance (g_day) also was not affected by salinity treatment in three of the six genotypes. There was no evidence that more salt tolerant genotypes (assessed as ability to maintain growth with increasing salinity) had a greater capacity to maintain g_night or g_day at high salinity. In addition, g_night as a percentage of g_day was unaffected by treatment in the three most salt tolerant genotypes. Although g_night in the 7 mm treatment was always highest or not different compared with the 300 mm and 600 mm treatments, g_day was generally highest in the 300 mm treatment, indicating separate regulation of g_night and g_day in response to an environmental stress. Thus, it is clear that genetics and environment both influence the magnitude of g_night for this species. Combined effects of genetic and environmental factors are likely to impact our interpretation of variation of g_night in natural populations.

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C(3) plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO(2) resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO(2) levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO(2). After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.

Distinct light responses of the adaxial and abaxial stomata in intact leaves of Helianthus annuus L

Using a laboratory-constructed system that can measure the gas exchange rates of two leaf surfaces separately, the light responses of the adaxial and abaxial stomata in intact leaves of sunflower (Helianthus annuus L.) were investigated, keeping the intercellular CO(2) concentration (C(i)) at 300 microL L(-1). When evenly illuminating both sides of the leaf, the stomatal conductance (g(s)) of the abaxial surface was higher than that of the adaxial surface at any light intensity. When each surface of the leaf was illuminated separately, both the adaxial and abaxial stomata were more sensitive to the light transmitted through the leaf (self-transmitted light) than to direct illumination. Relationships between the whole leaf photosynthetic rate (A(n)) and the g(s) for each side highlighted a strong dependence of stomatal opening on mesophyll photosynthesis. Light transmitted through another leaf was more effective than the direct white light for the abaxial stomata, but not for the adaxial stomata. Moreover, green monochromatic light induced an opening of the abaxial stomata, but not of the adaxial stomata. As the proportion of blue light in the transmitted light is less than that in the white light, there may be some uncharacterized light responses, which are responsible for the opening of the abaxial stomata by the transmitted, green light.

Genetic variation in Arabidopsis thaliana for night-time leaf conductance

Night-time leaf conductance (g(night)) and transpiration may have several adaptive benefits related to plant water, nutrient and carbon relations. Little is known, however, about genetic variation in g(night) and whether this variation correlates with other gas exchange traits related to water use and/or native habitat climate. We investigated g(night) in 12 natural accessions and three near isogenic lines (NILs) of Arabidopsis thaliana. Genetic variation in g(night) was found for the natural accessions, and g(night) was negatively correlated with native habitat atmospheric vapour pressure deficit (VPD(air)), suggesting lower g(night) may be favoured by natural selection in drier habitats. However, there were also significant genetic correlations of g(night) with daytime gas exchange traits expected to affect plant fitness [i.e. daytime leaf conductance, photosynthesis and intrinsic water-use efficiency (WUE(i))], indicating that selection on daytime gas exchange traits may result in indirect selection on g(night). The comparison of three NILs to their parental genotypes identified one quantitative trait locus (QTL) contributing to variation in g(night). Further characterization of genetic variation in g(night) within and among populations and species, and of associations with other traits and native habitats will be needed to understand g(night) as a putatively adaptive trait.