Nighttime Stomatal Conductance Research Articles

Nitrogen control of transpiration in grapevine.

Transpiration per unit of leaf area is the end-product of the root-to-leaf water transport within the plant, and it is regulated by a series of morpho-physiological resistances and hierarchical signals. The rate of water transpired sustains a series of processes such as nutrient absorption and leaf evaporative cooling, with stomata being the end-valves that maintain the optimal water loss under specific degrees of evaporative demand and soil moisture conditions. Previous work provided evidence of a partial modulation of water flux following nitrogen availability linking high nitrate availability with tight stomatal control of transpiration in several species. In this work, we tested the hypothesis that stomatal control of transpiration, among others signals, is partially modulated by soil nitrate ( ) availability in grapevine, with reduced availability (alkaline soil pH, reduced fertilization, and distancing source) associated with decreased water-use efficiency and higher transpiration. We observed a general trend when was limiting with plants increasing either stomatal conductance or root-shoot ratio in four independent experiments with strong associations between leaf water status, stomatal behavior, root aquaporins expression, and xylem sap pH. Carbon and oxygen isotopic signatures confirm the proximal measurements, suggesting the robustness of the signal that persists over weeks and under different gradients of availability and leaf nitrogen content. Nighttime stomatal conductance was unaffected by manipulation treatments, while application of high vapor pressure deficit conditions nullifies the differences between treatments. Genotypic variation for transpiration increase under limited availability was observed between rootstocks indicating that breeding (e.g., for high soil pH tolerance) unintentionally selected for enhanced mass flow nutrient acquisition under restrictive or nutrient-buffered conditions. We provide evidence of a series of specific traits modulated by availability and suggest that fertilization is a potential candidate for optimizing grapevine water-use efficiency and root exploration under the climate-change scenario.

Nocturnal pollutant uptake contributes significantly to the total stomatal uptake of Mangifera indica

DO3SE (Deposition of Ozone for Stomatal Exchange), is a dry deposition model, designed to assess tropospheric ozone risk to vegetation, and is based on two alternative algorithms to estimate stomatal conductance: multiplicative and photosynthetic. The multiplicative model has been argued to perform better for leaf-level and regional-level application. In this study, we demonstrate that the photosynthetic model is superior to the multiplicative model even for leaf-level studies using measurements performed on Mangifera indica. We find that the multiplicative model overestimates the daytime stomatal conductance, when compared with measured stomatal conductance and prescribes zero conductance at night while measurements show an average conductance of 100 mmol(H2O)m−2s−1 between 9 p.m. and 4 a.m. The daytime overestimation of the multiplicative model can be significantly reduced when the model is modified to include a response function for ozone-induced stomatal closure. However, nighttime pollutant uptake fluxes can only be accurately assessed with the photosynthetic model which includes the stomatal opening at night during respiration and is capable of reproducing the measured nighttime stomatal conductance. At our site, the nocturnal flux contributes 64%, 39%, 46%, and 88% of the total for NO2 uptake in winter, summer, monsoon, and post-monsoon, respectively. For SO2, nocturnal uptake amounts to 35%, 28%, 28%, and 44% in winter, summer, monsoon, and post-monsoon, respectively while for ozone the nighttime uptake contributes 30%, 17%, 18%, and 29% of the total stomatal uptake in winter, summer, monsoon, and post-monsoon respectively.

Predawn leaf conductance depends on previous day irradiance but is not related to growth in aspen saplings grown under artificially manipulated air humidity.

Recent studies have suggested that predawn stomatal opening may enhance early-morning photosynthesis (A) and improve the relative growth rate of trees. However, the causality between night-time stomatal conductance, A, and tree growth is disputable because stomatal opening in darkness can be mediated by previous day photosynthate loads and might be a consequence of growth-related processes like dark respiration (R). To identify linkages between night-time leaf conductance (gl_night), A, R, and tree growth, we conducted an experiment in hybrid aspen saplings grown under different air relative humidity (RH) conditions and previous day irradiance level (IR_pday). Predawn leaf conductance (gl_predawn) depended on RH, IR_pday and R (P < 0.05), whereas early-morning gross A (Agross_PAR500) depended on IR_pday and gl_predawn (P < 0.001). Daytime net A was positively related to Agross_PAR500 and leaf [N] (P < 0.05). Tree diameter and height increment correlated positively with gl at the beginning and middle of the night (P < 0.05) but not before dawn. Although our results demonstrate that gl_night was related to tree growth, the relationship was not determined by R. The linkage between gl_predawn and Agross_PAR500 was modified by IR_pday, indicating that daily CO2 assimilation probably provides feedback for stomatal opening before dawn.

Nighttime transpiration represents a negligible part of water loss and does not increase the risk of water stress in grapevine.

Nighttime transpiration has been previously reported as a significant source of water loss in many species; however, there is a need to determine if this trait plays a key role in the response to drought. This study aimed to determine the magnitude, regulation and relative contribution to whole plant water‐use, of nighttime stomatal conductance (g night) and transpiration (E night) in grapevine (Vitis vinifera L.). Our results showed that nighttime water loss was relatively low compared to daytime transpiration, and that decreases in soil and plant water potentials were mainly explained by daytime stomatal conductance (g day) and transpiration (E day). Contrary to E day, E night did not respond to VPD and possible effects of an innate circadian regulation were observed. Plants with higher g night also exhibited higher daytime transpiration and carbon assimilation at midday, and total leaf area, suggesting that increased g night may be linked with daytime behaviors that promote productivity. Modeling simulations indicated that g night was not a significant factor in reaching critical hydraulic thresholds under scenarios of either extreme drought, or time to 20% of soil relative water content. Overall, this study suggests that g night is not significant in exacerbating the risk of water stress and hydraulic failure in grapevine.

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

Nighttime stomatal conductance (gsn ) varies among plant functional types and species, but factors shaping the evolution of gsn remain unclear. Examinations of intraspecific variation in gsn as a function of climate and co-varying leaf traits may provide new insight into the evolution of gsn 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 gsn . However, genotypes from hotter climates possessed higher SD, which resulted in higher gsn . Across genotypes, higher gsn was associated with higher daytime stomatal conductance and net photosynthesis. Our results indicate that higher gsn 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 gsn .

Spatial examination of leaf-boundary-layer conductance using artificial leaves for assessment of light airflow within a plant canopy under different controlled greenhouse conditions

Leaf-boundary-layer conductance (ga), which is mainly affected by airflow near the leaf surface, is an important limiting factor on energy budgets, transpiration and photosynthesis, especially under very light-wind conditions. However, little research has been done, with a focus on the direct evaluation of ga under such conditions, because of the difficulty in measuring slower wind speeds that continuously vary in space and time. Here we propose a reasonable airflow assessment using spatiotemporal analysis of ga, with the aid of numerous artificial leaves facilitating a continuous and multipoint evaluation of ga. In our testing and development of the method, the artificial leaves consisted of thin brass sheets that sandwiched constantan micro heaters, and ga was evaluated on the basis of energy balance of the electrically heated leaves. The analysis was performed within a tomato canopy in a greenhouse under different regimes of environmental controls (air circulation, forced and natural ventilation, heating and air ductwork), thereby allowing the spatiotemporal distributions of ga within the canopy to be determined. The artificial leaves successfully captured the fluctuations in ga affected by the light airflow within the canopy, although ga was overestimated by only 2% (at most 5%) as compared to that of actual leaves owing to the buoyancy effect of electrical heating of the artificial leaves. Thus, without excessive electrical heating, the artificial leaves are considered reliable tools for airflow assessment in the greenhouse. Daytime ga values were small and equivalent to daytime stomatal conductance, even under environmental controls, thus limiting the heat transfer, transpiration and photosynthesis of the leaf. In contrast, night-time ga values were higher values than night-time stomatal conductance, which indicates the small impact of ga on heat and mass exchange via the stomata during the night-time. Spatiotemporal changes in ga substantially varied within the canopy, owing to the different environmental controls. Consequently, remarkable non-uniformities in ga appeared within the canopy, with implications for variable heat and mass exchange. These results indicate that airflow management in the greenhouse can still be improved by spatiotemporal analysis of ga.

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 (gs) are not known. We investigated the effects of O3 and soil availability of N and P on nocturnal gs 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 gs was increased by 2.0 × AA O3 in August (+39%) and September (+108%). Night-time gs was correlated with POD0 (phytotoxic O3 dose) and increased exponentially after 40 mmol m-2 POD0. Such increase of nocturnal gs 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 gs 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 gs, although P did not modify the O3-induced stomatal dysfunction. The results suggest that the increased nocturnal gs may be associated with a need to improve N acquisition to cope with O3 stress.

Embolism recovery strategies and nocturnal water loss across species influenced by biogeographic origin.

Drought‐induced tree mortality is expected to increase in future climates with the potential for significant consequences to global carbon, water, and energy cycles. Xylem embolism can accumulate to lethal levels during drought, but species that can refill embolized xylem and recover hydraulic function may be able to avoid mortality. Yet the potential controls of embolism recovery, including cross‐biome patterns and plant traits such as nonstructural carbohydrates (NSCs), hydraulic traits, and nocturnal stomatal conductance, are unknown. We exposed eight plant species, originating from mesic (tropical and temperate) and semi‐arid environments, to drought under ambient and elevated CO2 levels, and assessed recovery from embolism following rewatering. We found a positive association between xylem recovery and NSCs, and, surprisingly, a positive relationship between xylem recovery and nocturnal stomatal conductance. Arid‐zone species exhibited greater embolism recovery than mesic zone species. Our results indicate that nighttime stomatal conductance often assumed to be a wasteful use of water, may in fact be a key part of plant drought responses, and contribute to drought survival. Findings suggested distinct biome‐specific responses that partially depended on species climate‐of‐origin precipitation or aridity index, which allowed some species to recover from xylem embolism. These findings provide improved understanding required to predict the response of diverse plant communities to drought. Our results provide a framework for predicting future vegetation shifts in response to climate change.

Phylogenetic and biogeographic controls of plant nighttime stomatal conductance.

The widely documented phenomenon of nighttime stomatal conductance gsn 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 gsn 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 gsn . We measured gsn 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 gsn and further assessed the degree to which gsn co-varied with leaf functional traits and daytime gas-exchange rates. Closely related species were more similar in gsn than expected by chance. Herbaceous species had higher gsn 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 gsn . Our results reveal the highest gsn rates in species from environments where neighboring plants compete most strongly for water, suggesting a possible role for the competitive advantage of gsn .

Functional roles of ammonium (NH4+) and nitrate (NO3-) in regulation of day- and night-time transpiration in Phaseolus vulgaris.

Nitrogen may regulate transpiration and consequently the delivery of nutrients to root surfaces. It remains unclear whether both NO3- and NH4+ regulate transpiration and nutrient acquisition. In this study we investigated the functional role of NO3- or NH4+ in regulating both day- and night-time transpiration for potential 'mass-flow' delivery of nutrients. Phaseolus vulgaris L. plants were grown in troughs with a poly vinyl chloride (PVC) 'root-barrier' with a 25-µm mesh window designed to create an N-availability gradient by restricting roots from intercepting a slow-release NO3- or NH4+ fertiliser. 'Root-barrier' plants had their fertiliser placed at one of four distances behind the mesh from which nutrient acquisition was by diffusion or mass-flow. Control plants had direct access to fertiliser, termed 'no root-barrier' plants. NO3--fed 'root-barrier' plants closest to the N source had 2-fold higher stomatal conductance, 2.6-fold higher transpiration, 1.8-fold higher night-time stomatal conductance, and 1.5-fold higher night-time transpiration than NO3--fed 'no-barrier' plants, despite having comparable photosynthetic rates and biomass. Day- and night-time transpiration of NO3--fed 'root-barrier' plants, however, was downregulated with further distance from the N source. All NH4+-fed plants displayed ammoniacal toxicity symptoms. NH4+-fed 'no root-barrier' plants had higher root biomass, 2-fold higher stomatal conductance and photosynthetic rate, 1.7-fold higher transpiration but had the same night-time stomatal conductance and transpiration compared with NH4+-fed 'root-barrier' plants closest to the N source. Unlike with NO3--fed 'root-barrier' plants, NH4+-fed 'root-barrier' plants continuously increased their water fluxes with distance of N source. Thus, under N-limited conditions plants may be opportunistic in their water uptake, transpiring more when the water is available, in order to acquire nutrients through mass-flow. NH4+-fed plants prone to ammoniacal toxicity allocated their biomass towards the roots at non-limiting N levels, which may be linked to their extensive root system and enhanced photosynthetic rate. Thus, root hydraulic conductivity in response to NH4+-N requires further scrutiny, given that previous studies indicated that NH4+ may not alter the expression of root aquaporins or root hydraulic conductance.

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.

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

Abstract. Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61°51′ N, 24°17′ E; 181 m a.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m−2 s−1 for FCOS-Rn and FCOS-EC, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222Rn (of which the source is the soil) with COS (average R2 = 0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34–40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.

Representing nighttime and minimum conductance in CLM4.5: global hydrology and carbon sensitivity analysis using observational constraints

Abstract. The terrestrial biosphere regulates climate through carbon, water, and energy exchanges with the atmosphere. Land-surface models estimate plant transpiration, which is actively regulated by stomatal pores, and provide projections essential for understanding Earth's carbon and water resources. Empirical evidence from 204 species suggests that significant amounts of water are lost through leaves at night, though land-surface models typically reduce stomatal conductance to nearly zero at night. Here, we test the sensitivity of carbon and water budgets in a global land-surface model, the Community Land Model (CLM) version 4.5, to three different methods of incorporating observed nighttime stomatal conductance values. We find that our modifications increase transpiration by up to 5 % globally, reduce modeled available soil moisture by up to 50 % in semi-arid regions, and increase the importance of the land surface in modulating energy fluxes. Carbon gain declines by up to ∼ 4 % globally and &gt; 25 % in semi-arid regions. We advocate for realistic constraints of minimum stomatal conductance in future climate simulations, and widespread field observations to improve parameterizations.

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.

Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir

The frequency and intensity of climatic extremes, such as heat waves, are predicted to increase globally, with severe implications for terrestrial carbon and water cycling. Temperatures may rise above critical thresholds that allow trees to function optimally, with unknown long-term consequences for forest ecosystems. In this context, we investigated how photosynthetic traits and the water balance in Douglas-fir are affected by exposure to three heat waves with temperatures about 12°C above ambient. Photosynthetic carboxylation efficiency (Vcmax) was mostly unaffected, but electron transport (Jmax) and photosynthetic rates under saturating light (Asat) were strongly influenced by the heat waves, with lagging limitations on photosynthesis still being observed six weeks after the last heat wave. We also observed lingering heat-induced inhibitions on transpiration, minimum stomatal conductance, and night-time stomatal conductance (gs-night). Results from the stomatal models used to calculate minimum stomatal conductance were similar to gs-night and indicated changes in leaf morphology, e.g. stomatal occlusions and alterations in epicuticular wax. Our results show Douglas-fir’s ability to restrict water loss following heat stress, but at the price of reduced photosynthetic performance. Such limitations indicate potential long-term restrictions that heat waves can impose on tree development and functioning under extreme climatic conditions.

Evaluating stomatal ozone fluxes in WRF-Chem: Comparing ozone uptake in Mediterranean ecosystems

The development of modelling tools for estimating stomatal uptake of surface ozone in vegetation is important for the assessment of potential damage induced due to both current and future near surface ozone concentrations. In this study, we investigate the skill in estimating ozone uptake in plants by the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) V3.6.1, with the Wesely dry deposition scheme. To validate the stomatal uptake of ozone, the model simulations were compared with field measurements of three types of Mediterranean vegetation, over seven different periods representing various meteorological conditions. Some systematic biases in modelled ozone fluxes are revealed; the lack of an explicit and time varying dependency on plants’ water availability results in overestimated daytime ozone stomatal fluxes particularly in dry periods. The optimal temperature in the temperature response function is likely too low for the woody species tested here. Also, too low nighttime stomatal conductance leads to underestimation of ozone uptake during night. We demonstrate that modelled stomatal ozone flux is improved by accounting for vapor pressure deficit in the ambient air. Based on the results of the overall comparison to measured fluxes, we propose that additional improvements to the stomatal conductance parameterization should be implemented before applying the modelling system for estimating ozone doses and potential damage to vegetation.

Woody clockworks: circadian regulation of night‐time water use in Eucalyptus globulus

The role of the circadian clock in controlling the metabolism of entire trees has seldom been considered. We tested whether the clock influences nocturnal whole-tree water use. Whole-tree chambers allowed the control of environmental variables (temperature, relative humidity). Night-time stomatal conductance (gs ) and sap flow (Q) were monitored in 6- to 8-m-tall Eucalyptus globulus trees during nights when environmental variables were kept constant, and also when conditions varied with time. Artificial neural networks were used to quantify the relative importance of circadian regulation of gs and Q. Under a constant environment, gs and Q declined from 0 to 6 h after dusk, but increased from 6 to 12 h after dusk. While the initial decline could be attributed to multiple processes, the subsequent increase is most consistent with circadian regulation of gs and Q. We conclude that endogenous regulation of gs is an important driver of night-time Q under natural environmental variability. The proportion of nocturnal Q variation associated with circadian regulation (23-56%) was comparable to that attributed to vapor pressure deficit variation (25-58%). This study contributes to our understanding of the linkages between molecular and cellular processes related to circadian regulation, and whole-tree processes related to ecosystem gas exchange in the field.

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

Background and AimsDaytime 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.MethodsHere 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 ResultsDespite 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. CO2 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.ConclusionsIn 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.

Differential daytime and night‐time stomatal behavior in plants from North American deserts

Night-time stomatal conductance (g(night)) occurs in many ecosystems, but the g(night) response to environmental drivers is relatively unknown, especially in deserts. Here, we conducted a Bayesian analysis of stomatal conductance (g) (N=5013) from 16 species in the Sonoran, Chihuahuan, Mojave and Great Basin Deserts (North America). We partitioned daytime g (g(day)) and g(night) responses by describing g as a mixture of two extreme (dark vs high light) behaviors. Significant g(night) was observed across 15 species, and the g(night) and g(day) behavior differed according to species, functional type and desert. The transition between extreme behaviors was determined by light environment, with the transition behavior differing between functional types and deserts. Sonoran and Chihuahuan C(4) grasses were more sensitive to vapor pressure difference (D) at night and soil water potential (Ψ(soil)) during the day, Great Basin C(3) shrubs were highly sensitive to D and Ψ(soil) during the day, and Mojave C(3) shrubs were equally sensitive to D and Ψ(soil) during the day and night. Species were split between the exhibition of isohydric or anisohydric behavior during the day. Three species switched from anisohydric to isohydric behavior at night. Such behavior, combined with differential D, Ψ(soil) and light responses, suggests that different mechanisms underlie g(day) and g(night) regulation.