Hydraulic Plant Research Articles

Plant hydraulics and measurement of vulnerability to embolism formation: a guide for beginners

Summary Prolonged and/or intense drought leads to the deterioration of plant water balance, inducing embolism formation in the water conducting system, the xylem. The consequent loss of water transport capacity from roots to leaves (hydraulic failure) has been proposed as a main driver of plant mortality. Substantial inter- and intraspecific variation of resistance to embolism formation has been reported in plants. Hence, screening of different species/individuals is key to project the impact of future climate on ecosystems, while supporting breeding and reforestation. This review seeks to explain the mechanisms of water transport in plants and the phenomenon of embolism formation under drought stress by using concise and straightforward scholarly language. The main aim is to introduce non-expert readers (students, nonacademics, and academics from different scientific fields) to plant hydraulics and the controversial world of methods for measuring the vulnerability to embolism formation. To convey the message in full, we provide ranges of water potential values and widely used drought resistance indexes characterizing plants from different biomes. Various established methods used worldwide to monitor hydraulic efficiency under stress and measure hydraulic vulnerability by means of curves of different plant organs are introduced. Both classical widely used destructive methods and current non-destructive techniques, which have been gaining momentum in the last decade, are described. The main advantages and disadvantages of each method are briefly discussed to support decisions and selection of the most suitable method in experimental practice.

Adaptive strategies to freeze-thaw cycles in branch hydraulics of tree species coexisting in a temperate forest

Freeze-thaw cycles (FTCs) limit the distribution and survival of temperate tree species. Tree species with different wood types coexist in temperate forests and are subjected to the same FTCs. It is essential to understand how these trees differentially cope with xylem hydraulic failure induced by FTCs in the field. The branch hydraulic traits and nonstructural carbohydrate concentration of six coexisting tree species in a temperate forest were measured from mid-winter to early spring when the FTCs occurred from January to April. The percentage loss of hydraulic conductivity (PLC) was lower, and the water potential inducing a 50% loss of hydraulic conductivity (P50) was more negative in tracheid trees than in ring- and diffuse-porous trees, suggesting tracheid trees with narrow tracheid diameters showed less vulnerable to embolism and provided a lower degree of hydraulic failure during FTCs (stronger resistance). Ring-porous trees always showed lower hydraulic conductivity and higher PLC and P50, and these traits did not change during FTCs, suggesting that they might lose the hydraulic functions in winter and abandon the last year xylem. The P50 in diffuse-porous increased after several FTCs (frost fatigue), but that in tracheid species continued to increase (or even decrease) until the end of FTCs (69 cycles), suggesting that tracheid trees were less sensitive to frost fatigue than diffuse-porous trees. Soluble sugar concentration in deciduous trees negatively correlated with PLC at the end of FTCs, indicating that the effect of soluble sugar on refilling embolism occurred in early spring. While the soluble sugar concentration of deciduous trees decreased, that of two evergreen tracheid trees gradually increased, possibly due to the winter photosynthesis of evergreen leaves. Our results suggest temperate trees adopt different strategies to cope with the same FTCs. These findings enrich the understanding of plant hydraulics and carbon physiology in winter and provide insights into the response of different species coexisting in temperate forests under climate change.

Drought effects on water status, plant hydraulics and growth in Eucalyptus grandis and clonal hybrids with red gums and E. urophylla

Afforestation with Eucalyptus species occupies over a million hectares in Uruguay, and E. grandis is one of the most cultivated species, mainly for the timber industry. However, the increasing frequency of drought and heat waves in the region raises the need to evaluate hybrids with red gums (E. camaldulensis and E. tereticornis), since they are known to sustain gas exchange and growth even under severe drought conditions. Hybrids with E. urophylla are also evaluated for plantations in subtropical regions. This study aimed to compare the effects of drought on plant water status, hydraulics, and growth of E. grandis (one clone; G), E. grandis × camaldulensis (one clone; GC), E. grandis × tereticornis (one clone; GT), and E. grandis × urophylla (two clones; GU1 and GU2). Two drought-stress cycles were applied to six-month rooted cuttings from mid-spring to early fall under greenhouse environmental conditions. Predawn leaf water potential (?pd), midday leaf water potential (?md), relative leaf water content (RWC), stomatal conductance (gs), and plant growth were measured across each cycle. Daily fluctuation of Ψ was calculated as ΔΨ = Ψpd - Ψmd. Specific (kS) and leaf-specific (kL) hydraulic conductivities and specific leaf area (SLA) were measured at the end of the experiment. Percentage loss of hydraulic conductivity (PLC) was calculated after removing the xylem embolism. Preliminary results suggest that both red-gum hybrids had higher PLC under moderate-to-severe drought, as a consequence of maintaining high water potential gradients to sustain transpiration and carbon fixation throughout the experiment. Under the well-watered condition, no clonal difference was observed. Also, GT showed a three-fold increase in cavitation and had 1.5 times stomatal opening (gs) than the most drought-sensitive clone (E. grandis, G), and exhibited the lowest reduction (17%) in diameter growth under drought. Also, GC attained the highest height at the end of the study, 40% higher than E. grandis, and increased by 25% the SLA under drought. GU clones reached the highest diameter at the end of the study. Correlations among functional variables are explored, and the water supply capacity to the foliage among clones is discussed for a better understanding of drought resistance strategies and growth performance of the different taxa.

Coordination of rooting, xylem, and stomatal strategies explains the response of conifer forest stands to multi-year drought in the southern Sierra Nevada of California

Abstract. Extreme droughts are a major determinant of ecosystem disturbance that impacts plant communities and feeds back into climate change through changes in plant functioning. However, the complex relationships between aboveground and belowground plant hydraulic traits and their role in governing plant responses to drought are not fully understood. In this study, we use a model, the Functionally Assembled Terrestrial Ecosystem Simulator in a configuration that includes plant hydraulics (FATES-Hydro), to investigate ecosystem responses to the 2012–2015 California drought in comparison with observations at a site in the southern Sierra Nevada that experienced widespread tree mortality during this drought. We conduct a sensitivity analysis to explore how different plant water sourcing and hydraulic strategies lead to differential responses during normal and drought conditions. The analysis shows the following. Deep roots that sustain productivity through the dry season are needed for the model to capture observed seasonal cycles of evapotranspiration (ET) and gross primary productivity (GPP) in normal years, and deep-rooted strategies are nonetheless subject to large reductions in ET and GPP when the deep soil reservoir is depleted during extreme droughts, in agreement with observations. Risky stomatal strategies lead to greater productivity during normal years as compared to safer stomatal control, but they also lead to a high risk of xylem embolism during the 2012–2015 drought. For a given stand density, stomatal and xylem traits have a stronger impact on plant water status than on ecosystem-level fluxes. Our study highlights the significance of resolving plant water sourcing strategies to represent drought impacts on plants and consequent feedbacks in models.

When do plant hydraulics matter in terrestrial biosphere modelling?

The ascent of water from the soil to the leaves of vascular plants, described by the study of plant hydraulics, regulates ecosystem responses to environmental forcing and recovery from stress periods. Several approaches to model plant hydraulics have been proposed. In this study, we introduce four different versions of plant hydraulics representations in the terrestrial biosphere model T&C to understand the significance of plant hydraulics to ecosystem functioning. We tested representations of plant hydraulics, investigating plant water capacitance, and long-term xylem damages following drought. The four models we tested were a combination of representations including or neglecting capacitance and including or neglecting xylem damage legacies. Using the models at six case studies spanning semiarid to tropical ecosystems, we quantify how plant xylem flow, plant water storage and long-term xylem damage can modulate overall water and carbon dynamics across multiple time scales. We show that as drought develops, models with plant hydraulics predict a slower onset of plant water stress, and a diurnal variability of water and carbon fluxes closer to observations. Plant water storage was found to be particularly important for the diurnal dynamics of water and carbon fluxes, with models that include plant water capacitance yielding better results. Models including permanent damage to conducting plant tissues show an additional significant drought legacy effect, limiting plant productivity during the recovery phase following major droughts. However, when considering ecosystem responses to the observed climate variability, plant hydraulic modules alone cannot significantly improve the overall model performance, even though they reproduce more realistic water and carbon dynamics. This opens new avenues for model development, explicitly linking plant hydraulics with additional ecosystem processes, such as plant phenology and improved carbon allocation algorithms.

Constraining Plant Hydraulics With Microwave Radiometry in a Land Surface Model: Impacts of Temporal Resolution

AbstractVegetation water content (VWC) plays a key role in transpiration, plant mortality, and wildfire risk. Although land surface models now often contain plant hydraulics schemes, there are few direct VWC measurements to constrain these models at global scale. One proposed solution to this data gap is passive microwave remote sensing, which is sensitive to temporal changes in VWC. Here, we test that approach by using synthetic microwave observations to constrain VWC and surface soil moisture within the Climate Modeling Alliance Land model. We further investigate the possible utility of sub‐daily observations of VWC, which could be obtained through a satellite in geostationary orbit or combinations of multiple satellites. These high‐temporal‐resolution observations could allow for improved determination of ecosystem parameters, carbon and water fluxes, and subsurface hydraulics, relative to the currently available twice‐daily sun‐synchronous observational patterns. We find that incorporating observations at four different times in the diurnal cycle (such as could be available from two sun‐synchronous satellites) provides a significantly better constraint on water and carbon fluxes than twice‐daily observations do. For example, the root mean square error of projected evapotranspiration and gross primary productivity during drought periods was reduced by approximately 40%, when using four‐times‐daily relative to twice‐daily observations. Adding hourly observations of the entire diurnal cycle did not further improve the inferred parameters and fluxes. Our comparison of observational strategies may be informative in the design of future satellite missions to study plant hydraulics, as well as when using existing remotely sensed data to study vegetation water stress response.

Comparative anatomy vs mechanistic understanding: how to interpret the diameter-vulnerability link

Summary Results from comparative and ecological wood anatomy combined with a number of experimental studies on plant hydraulics have led to a pervasive and longstanding assumption that wider-diameter vessels are more vulnerable to drought-induced embolism than narrower vessels. Although we agree that wider vessels tend to be more vulnerable than narrower vessels within stems and within roots across most species, our current understanding of the diameter-vulnerability link does not offer a mechanistic explanation for why increased vessel diameter should consistently lead to greater vulnerability or vice versa. Causes of drought-induced embolism formation and spread likely operate at the nano-level, especially at gas-liquid-surfactant interfaces inside intervessel pit membranes. We evaluate here new perspectives on drought-induced embolism and its key anatomical and physico-chemical drivers, of which vessel diameter is one of the parameters involved, although its linkage to embolism vulnerability is likely indirect. As such, the diameter-vulnerability link does not imply that species with on average wider vessels are consistently more susceptible to drought-induced embolism compared to species with narrower vessels. Scientific priorities for future progress should focus on more accurate predictions of how water transport in plants is affected by drought, which requires a better mechanistic understanding of xylem network topology and biophysical processes at the nano-scale level in individual vessels that determine embolism formation and spread.

Wind-hydro pumped storage systems to meet lebanese electricity demand

Lebanon is facing currently anacute energy crisis, due tolack of domestic energy resources, reduced production capacities and a growing demand for energy. Relying on wind energy could reduce the impact of this crisis. However, continuous change in wind speed from calm to stormy introduces challenges. One possible solution to address these challenges is to introduce new storage facilities or to use storage capabilities already available in the power generation system. Existing hydro power plants with large reservoirs or pumped storage hydro power plants are suitable for this purpose. Furthermore, Lebanon has a fairly high wind energy potential and hydro power resources. This paper is an attempt to analyze the design of a pumping station and the performance of a hybrid wind-hydro power plant, in three hydraulic plants to produce electricity in Lebanon (Markabi, Awali and Joun), in order to choose the most suitable plant to store electrical power. An economic analysis and an evaluation of the amount of water that could be pumped in each of the hydraulic plants are carried out in this study, in order to select the most suitable hydraulic plant to store the electricity surplus. Moreover, this study presents an economic evaluation of the implementation of wind turbines in Lebanon. It is shown that Markabi is the most suitable plant to store electrical power.

Inovasi Terraplanter Baby Tears Sebagai Bisnis Kekinian Paket Hadiah (Hampers) Florikultura Indoor

<p><em>Reduction of green land in urban areas from 2017 which was before 7.75 million hectares to be only 7.1 million hectares in 2018. This condition is a problem for people who want to grow ornamental plants. </em><em>D</em><em>uring the pandemic, the trend of ornamental plants is on the rise in the market. Therefore, the presence of an innovative Plant.et product in the form of plant hampers consisting of terraplanter baby tears that apply hydraulic planting technology by growing outside the terraplanter tube without using soil planting media and QR code system. The purpose of this innovation is to be a solution to the problem of reducing green land, developing unique and superior businesses based on post-pandemic technology.The method of implementation on Plant.et product is carried out by determining product design, product quality testing, determining product uniqueness and excellence, market surveys, and marketing strategies. The results showed that Plant.et products are in great demand among teenagers with a female gender and has been sale more 100 pcs. The advantages and uniqueness of Plant.et product in the form of a hydraulic plant outside the terraplanter tube with a QR code system packed with hampers. Therefore, Plant.et product has great potential to be developed.</em></p>

Pipe Cavitation Parameters Reveal Bubble Embolism Dynamics in Maize Xylem Vessels across Water Potential Gradients

Maize, a crop of international relevance, frequently undergoes xylem embolism due to water shortage, negatively impacting growth, yield, and quality. Consequently, a refined comprehension of xylem embolism is vital for enhancing maize cultivation. Notwithstanding extensive research and the generation of analytical models for embolism mechanisms, prevalent models often disregard crop-specific hydraulic processes and the formation of embolisms via air bubbles in the xylem conduit. In this research, we present an inventive model applying pipe cavitation parameters to discern water potential and bubble formation in maize leaf xylem. The model integrates pivotal physiological traits of the maize–leaf count, leaf vein count, and diameter of xylem vessels—demonstrating robust correlations. Furthermore, we constructed Percent Loss of Conductivity (PLC) curve based on water potential and compared it with our model, offering interval data to observe embolization events triggered by air bubbles. Utilizing experimental data, our novel cavitation-parameter-based model effectively corresponds with observed bubble phenomena and appropriately characterizes water transport in plant xylem conduits. This method enabled us to observe the transition from bubble occurrence to cavitation embolism microscopically, which aligned with the embolism intervals provided by the model. This procedure reveals potential trends in bubble-induced embolism and deepens our knowledge of microscopic plant hydraulics and crop embolism. This work establishes a basis for understanding the generation of bubble embolisms in maize, assists in evaluating maize-plant water status for efficient water supply management throughout the growth cycle, and contributes towards potential water management strategies for maize.

Evaporation-driven internal hydraulic redistribution alleviates root drought stress: Mechanisms and modeling.

Many tree species have developed extensive root systems that allow them to survive in arid environments by obtaining water from a large soil volume. These root systems can transport and redistribute soil water during drought by hydraulic redistribution (HR). A recent study revealed the phenomenon of evaporation-driven hydraulic redistribution (EDHR), which is driven by evaporative demand (transpiration). In this study, we confirmed the occurrence of EDHR in Chinese white poplar (Populus tomentosa) through root sap flow measurements. We utilized microcomputed tomography technology to reconstruct the xylem network of woody lateral roots and proposed conceptual models to verify EDHR from a physical perspective. Our results indicated that EDHR is driven by the internal water potential gradient within the plant xylem network, which requires 3 conditions: high evaporative demand, soil water potential gradient, and special xylem structure of the root junction. The simulations demonstrated that during periods of extreme drought, EDHR could replenish water to dry roots and improve root water potential up to 38.9% to 41.6%. This highlights the crucial eco-physiological importance of EDHR in drought tolerance. Our proposed models provide insights into the complex structure of root junctions and their impact on water movement, thus enhancing our understanding of the relationship between xylem structure and plant hydraulics.

How can ecosystem engineer plants boost productivity in east Mediterranean drylands

BackgroundWater availability is the key limiting factor for plant productivity in drylands covering ca. 40% of Earth’s land surface. For such ecosystems to retain productivity and biodiversity under climatic change, it would be valuable to identify/promote keystone plant species that (i) have developed strategies to more efficiently utilize moisture resources not easily accessible and (ii) improve moisture conditions for neighboring plants. The very deep-rooted Ziziphus lotus, considered an ecosystem engineer, is one such example. However, it is not known which biotic traits: (a) canopy interception of moisture/rainfall, (b) hydraulic redistribution of deep ground moisture by roots, or non-biotic factors: (c) soil’s volume, and (d) organic matter content, Z. lotus activates/modulates to play such a role. We, thus, selected dryland ecosystems where the plant dominates and measured for potential effects on the less deep-rooted Thymbra capitata. For assessing impacts on ecosystem productivity, we measured the spatial aggregation of ca. 3600 T. capitata plants. As a proxy for soil moisture availability and its spatial variability, we conducted a 7-year-long study using thymes’ nighttime rehydration. Sampling extended up to 15 m away from Z. lotus.ResultsThe density of T. capitata plants growing up to 5 m around Z. lotus vs. thymes growing 10–15 m away was found significantly increased (2.5–4.5 times), while their stem/leaf moisture was ca. 10% higher at predawn compared to nightfall during the dry season. This suggests that ecosystem productivity is driven by a greater soil moisture availability around Z. lotus permitting more thyme daytime transpiration, in contrast to thymes growing further away. The phenomenon appeared only under dry topsoil (during the dry season; becoming stronger during dry years). Morning dew/rainfall interception from the canopy or soil depth/organic matter did not show significant effects, leaving only the hydraulic lift properties of Z. lotus as the most likely driver for soil moisture availability.ConclusionsThe deep-rooting properties and hydraulic lift potential of Z. lotus may be the key in permitting it to boost ecosystem productivity. Such hydraulic plant traits require more attention as they may prove valuable in combating desertification and restoring ecosystems in arid/semiarid regions threatened by climate change.

Is green stem photosynthesis in avocado related to plant hydraulics?

Is green stem photosynthesis in avocado related to plant hydraulics?

Plant Hydraulics Improves Predictions of ET and GPP Responses to Drought

AbstractLand surface models typically ignore plant hydraulic processes and use empirical soil moisture stress functions to limit transpiration and photosynthesis. How plant hydraulics impacts the predictions of carbon and water fluxes and their responses to drought remains unclear. Here, we developed an analytical plant hydraulic scheme wherein hydraulic capacitances at the leaf level and stem level were considered, and incorporated it into the vegetation interface processes model (hereafter, VIP‐PHS). Then, we calibrated the VIP‐PHS parameters using a Markov Chain Monte Carlo (MCMC) method and conducted an ecosystem‐scale evaluation of the VIP‐PHS at 30 FLUXNET sites. The results showed that VIP‐PHS improved evapotranspiration (ET) and gross primary production (GPP) simulations across the studied sites, reducing the root mean square error (RMSE) by 0.02–0.27 mm day−1 and 0.04–0.52 g C m−2 day−1 in relation to the empirical soil hydraulic scheme (hereafter, VIP‐SHS). VIP‐PHS decreased the RMSE in ET and GPP by up to 45% and 55% in evergreen needleleaf forests. Compared to VIP‐SHS, VIP‐PHS decreased the sensitivities of ET and GPP to soil moisture stress, and significantly improved ET and GPP predictions under low soil moisture conditions. The sensitivity analysis showed that larger stem hydraulic capacitance better relieved xylem water stress, resulting in greater transpiration and GPP and less‐negative daily minimum stem water potential. These findings highlight the need to incorporate plant hydraulics into the next generation of Earth system models under future drought conditions.

Supporting data for "Constraining plant hydraulics with microwave radiometry in a land surface model: Impacts of temporal resolution"

Supporting data for "Constraining plant hydraulics with microwave radiometry in a land surface model: Impacts of temporal resolution"

Coordination of hydraulic and morphological traits across dominant grasses in eastern Australia

Abstract Leaf hydraulic traits characterize plant drought tolerance and responses to climate change. Yet, plant hydraulics are biased towards northern hemisphere woody species. We collected rhizomes of several perennial grass species along a precipitation gradient in eastern Australia and grew them in an experimental pot study to investigate potential trade‐offs between drought tolerance and plant morphology. We measured the following leaf hydraulic traits: the leaf water potential (Ψleaf) at 50% and 88% loss of leaf hydraulic conductance (P50Kleaf and P88Kleaf), the Ψleaf at 50% loss of stomatal conductance (P50gs), leaf turgor loss point (TLP), leaf dry matter content (LDMC), leaf modulus of elasticity (ε), and the slope of the relationship between predawn and midday Ψleaf. We also measured basal area, tiller density, seed head density, root collar diameter, plant height, and aboveground biomass of each individual. As expected, grass species varied widely in leaf‐level drought tolerance, with loss of 88% hydraulic conductance occurring at a Ψleaf ranging from −1.52 to −4.01 MPa. However, all but one species lost leaf turgor, and most reached P50gs before this critical threshold. Taller more productive grass species tended to have drought vulnerable leaves characterized by low LDMC and less negative P88Kleaf. Species with greater tiller production experienced stomatal closure and lost turgor at more negative Ψleaf. Although our sample size was limited, we found no relationships between these species' traits and their climate of origin. Overall, we identified important hydraulic and morphological trade‐offs in Australian grasses that were surprisingly similar to those observed for woody plants: (1) xylem of taller species was less drought tolerant and (2) turgor loss occurs and stomatal closure begins before significant loss of Kleaf. These data build upon a small yet growing field of grass hydraulics and may be informative of species responses to further drought intensification in Australia. Read the free Plain Language Summary for this article on the Journal blog.

Further Test of Pneumatic Method in Constructing Vulnerability Curves Using Six Tree Species with Contrasting Xylem Anatomy

The pneumatic method is a novel method determining vulnerability to embolism in plants, yet it remains unclear whether this method is suitable for all species with different xylem anatomy. In this study, using six tree species with contrasting xylem anatomy, including four vessel-bearing species (diffuse-porous wood and ring-porous wood) and two tracheid-bearing species (non-porous wood), we test the reliability of the pneumatic method by comparing to hydraulic methods and also considering turgor loss point and native embolism. Vessel length distribution and cut-open vessel volume were also evaluated using the silicone injection technique. Additionally, we also synthesized published data to find out the consistency between the pneumatic method and hydraulic methods. Results showed that there was a maximum 10-folds difference in mean vessel length and mean vessel diameter varying from 30 to 56 μm among species. The estimated open vessel volume ranges from 0.064 to 0.397 mL, with a maximum of 14% of the tube vacuum reservoir. For four vessel-bearing species, the pneumatic method showed good consistency with hydraulic methods, and this consistency was evidenced by turgor loss point and native embolism. For two tracheid-bearing species, the pneumatic method significantly overestimated vulnerability because of the bad consistencies with hydraulic methods and plant water relations. Data synthesis of 56 species also suggested that the pneumatic method can accurately measure the embolism vulnerability of vessel-bearing species but not for tracheid-bearing species. Our study provided further evidence that the pneumatic method is accurate for most vessel-bearing species and thus has the potential to be widely used in the plant hydraulics field. However, we proposed that the precise calculation of air discharge volume should take into account the volume of open vessels for species with wide and long vessels.

Xylem anatomical responses of Larix Gmelinii and Pinus Sylvestris influenced by the climate of Daxing'an mountains in Northeastern China.

Wood anatomy and plant hydraulics play a significant role in understanding species-specific responses and their ability to manage rapid environmental changes. This study used the dendro-anatomical approach to assess the anatomical characteristics and their relation to local climate variability in the boreal coniferous tree species Larix gmelinii (Dahurian larch) and Pinus sylvestris var. mongolica (Scots pine) at an altitude range of 660m to 842m. We measured the xylem anatomical traits (lumen area (LA), cell wall thickness (CWt), cell counts per ring (CN), ring width (RW), and cell sizes in rings) of both species at four different sites Mangui (MG), Wuerqihan (WEQH), Moredagha (MEDG) and Alihe (ALH) and investigated their relationship with temperature and precipitation of those sites along a latitude gradient. Results showed that all chronologies have strong summer temperature correlations. LA extremes were mostly associated with climatic variation than CWt and RWt. MEDG site species showed an inverse correlation in different growing seasons. The correlation coefficient with temperature indicated significant variations in the May-September months at MG, WEQH, and ALH sites. These results suggest that climatic seasonality changes in the selected sites positively affect hydraulic efficiency (increase in the diameter of the earlywood cells) and the width of the latewood produced in P. sylvestris. In contrast, L. gmelinii showed the opposite response to warm temperatures. It is concluded that xylem anatomical responses of L. gmelinii and P. sylvestris showed varied responses to different climatic factors at different sites. These differences between the two species responses to climate are due to the change of site condition on a large spatial and temporal scale.

How the interactions between atmospheric and soil drought affect the functionality of plant hydraulics.

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Guard cell activity of PIF4 and HY5 control transpiration

Whole-plant transpiration, controlled by plant hydraulics and stomatal movement, is regulated by endogenous and environmental signals, with the light playing a dominant role. Stomatal pore size continuously adjusts to changes in light intensity and quality to ensure optimal CO2 intake for photosynthesis on the one hand, together with minimal water loss on the other. The link between light and transpiration is well established, but the genetic knowledge of how guard cells perceive those signals to affect stomatal conductance is still somewhat limited. In the current study, we evaluated the role of two central light-responsive transcription factors; a bZIP-family transcription factor ELONGATED HYPOCOTYL5 (HY5) and the basic helix-loop-helix (BHLH) transcription factor PHYTOCHROME INTERACTING FACTOR4 (PIF4), in the regulation of steady-state transpiration. We show that overexpression of PIF4 exclusively in guard cells (GCPIF4) decreases transpiration, and can restrain the high transpiration of the pif4 mutant. Expression of HY5 specifically in guard cells (GCHY5) had the opposite effect of enhancing transpiration rates of WT- Arabidopsis and tobacco plants and of the hy5 mutant in Arabidopsis. In addition, we show that GCHY5 can reverse the low transpiration caused by guard cell overexpression of the sugar sensor HEXOKINASE1 (HXK1, GCHXK), an established low transpiring genotype. Finally, we suggest that the GCHY5 reversion of low transpiration by GCHXK requires the auto-activation of the endogenous HY5 in other tissues. These findings support the existence of an ongoing diurnal regulation of transpiration by the light-responsive transcription factors HY5 and PIF4 in the stomata, which ultimately determine the whole-plant water use efficiency.