Plant Gas Exchange Research Articles

Foliage applied proline induces salt tolerance in chili genotypes by regulating photosynthetic attributes, ionic homeostasis, and antioxidant defense mechanisms

Globally, soil salinity is a serious threat to horticultural crop productivity. Chili (Capsicum annum L.) is a major spice horticultural crop. Its growth and production is severely affected by salt stress. To address this problem, a sand culture experiment was conducted in which two contrasting chili genotypes ‘Plahi’ (salt tolerant) and ‘A-120’ (salt sensitive) were grown under salt stress (50 mM NaCl) with foliar application of proline (0.8 mM). Proline application enhanced salt tolerance in both genotypes by osmoregulation of sodium and potassium. It enhanced chili growth and fruit yield by improving plant water relations and gas exchange attributes under salt stress. Antioxidants were increased with proline application under salt stress. There was also a significant positive correlation found among glycinebetaine and proline contents. In conclusion, protein-treated chili plants performed better than untreated plants as evidenced by normal function of photosynthetic machinery and antioxidant mechanisms under salt stress.

Hardware Validation of the Advanced Plant Habitat on ISS: Canopy Photosynthesis in Reduced Gravity.

The Advanced Plant Habitat (APH) is the largest research plant growth facility deployed on the International Space Station (ISS). APH is a fully enclosed, closed-loop plant life support system with an environmentally controlled growth chamber designed for conducting both fundamental and applied plant research during experiments extending as long as 135 days. APH was delivered to the ISS in parts aboard two commercial resupply missions: OA-7 in April 2017 and SpaceX-11 in June 2017, and was assembled and installed in the Japanese Experiment Module Kibo in November 2018. We report here on a 7-week-long hardware validation test that utilized a root module planted with both Arabidopsis (cv. Col 0) and wheat (cv. Apogee) plants. The validation test examined the APH’s ability to control light intensity, spectral quality, humidity, CO2 concentration, photoperiod, temperature, and root zone moisture using commanding from ground facilities at the Kennedy Space Center (KSC). The test also demonstrated the execution of programmed experiment profiles that scheduled: (1) changes in environmental combinations (e.g., a daily photoperiod at constant relative humidity), (2) predetermined photographic events using the three APH cameras [overhead, sideview, and sideview near-infrared (NIR)], and (3) execution of experimental sequences during the life cycle of a crop (e.g., measure photosynthetic CO2 drawdown experiments). Arabidopsis and wheat were grown in microgravity to demonstrate crew procedures, planting protocols and watering schemes within APH. The ability of APH to contain plant debris was assessed during the harvest of mature Arabidopsis plants. Wheat provided a large evaporative load that tested root zone moisture control and the recovery of transpired water by condensation. The wheat canopy was also used to validate the ability of APH to measure gas exchange of plants from non-invasive gas exchange measurements (i.e., canopy photosynthesis and respiration). These features were evaluated by executing experiment profiles that utilized the CO2 drawdown technique to measure daily rates of canopy photosynthesis and dark-period CO2 increase for respiration. This hardware validation test confirmed that APH can measure fundamental plant responses to spaceflight conditions.

Action potential propagation effect on gas exchange of ABA-mutant microtomato after re-irrigation stimulus

During their life cycles, plants are often subjected to periods of fluctuation between water deficit and re-irrigation events. Efficient communication and rapid signaling between the shoot and root systems are needed for adaptive stress responses. Abscisic acid (ABA) and the hydraulic signal are the main long-distance signals responsive to water deficit. Nevertheless, ABA is susceptible to interactions with electrical signals and may not be sufficient as a sign of stress in plants. Therefore, we aimed to investigate the relationship between hydraulic, chemical and electrical signals after a re-irrigation stimulus to induce changes in gas exchange of ABA mutant microtomato plants. Three genotypes of tomato, cultivar Micro-Tom (MT) (MTwt), the isogenic mutant MTnotabilis (MTnot) and transgenic MTsp12::NCED (MTNCED), were exposed to 7, 4 and 11 days under simulated drought conditions, respectively. Then, the plants were re-irrigated and we simultaneously evaluated gas exchange, electric potential and the propagation of action potentials. We found that water deficit affected the water potential of the plants and induced increased depolarization of the plasma membrane electric potential in Mtwt and MTnot. In response to re-irrigation, all genotypes propagated action potentials from root to shoot. In MTwt, the electrical signal induced increases in photosynthesis, stomatal conductance and transpiration rate. By contrast, in ABA-mutants, the gas exchange variables were reduced. These results were related to the effect of ABA and possibly JA on stomatal control and pH changes inactivating photosynthesis.

Gas exchanges and chlorophyll fluorescence of young coffee plants submitted to water and nitrogen stresses

This research aims to evaluate the impact of nitrogen deprivation and water stress on gas exchange and chlorophyll fluorescence in young plants of five cultivars of Arabic coffee. A factorial experiment 5 (cultivars) × 3 (treatments: control without stress, water stress of −1.5 MPa and stress of N – 0.0 mmol L−1 N) was carried out in a complete randomized block design with three replicates. Before being submitted to the treatments, the plants were grown in a greenhouse for 240 days, and then transferred to a growth chamber under controlled conditions. Subsequently, after the experimental period of 96 h we measured photosynthetic rate (A), stomatal conductance to water vapor (gs), transpiratory rate (E), internal and external carbon ratio (Ci/Ca), water use efficiency (A/E), electron transport rate (ETR), actual quantum yield of PS II electron transport (φFSII), and maximum photochemical efficiency of PS II (Fv/Fm). Water stress reduced A, gs, E, A/E, ETR, φFSII, and Fv/Fm. The nitrogen deficiency reduced ETR, φFSII, and Fv/Fm. Under short-term water stress Catuaí Vermelho maintain the A values due to better stomatal control, reduced water lost by transpiration (E) and better water use efficiency A/E, while Mundo Novo and Acauã show lower damage to Fv/Fm. Short-term nitrogen stress has low impact on A of young plants of Coffea arabica cultivars with adequate N-nutrition.

Irrigation frequency transiently alters whole plant gas exchange, water and hormone status, but irrigation volume determines cumulative growth in two herbaceous crops

Physiological effects of irrigation frequency, at the same irrigation volume, have received little attention but might determine crop yield and water use efficiency. Potted plants of two species, tomato and basil, received two irrigation treatments that both supplied the same irrigation volume (75% of that received by a well-watered treatment—WW), but either frequently (once or twice per day-FDI) or infrequently (every three days-IDI). Stem diameter variations, whole-plant gas exchange, root and leaf water potential, and foliar hormones were monitored for 11 days after applying the treatments, and whole-plant biomass accumulation determined at the end of that period. Treatments showed temporal and spatial differences in soil moisture, with FDI resulting in a wet upper layer and dry lower layer. In both species, water stress integral in IDI was three-fold higher than in FDI, and gas exchange lower than FDI plants. Despite these differences, both treatments accumulated biomass and stem diameter growth similarly. In tomato, IDI induced compensatory stem growth (higher than WW plants) after re-watering, and attenuated hormone accumulation (lower jasmonic, gibberellic, and salicylic acid concentrations than FDI plants) that maintained growth. In basil, stem growth of IDI plants only recovered to WW and FDI levels upon re-watering, but lower sensitivity of stem growth to water deficits explained similar final biomass accumulation to FDI plants. Although both deficit irrigation treatments showed similar cumulative growth, temporal differences in physiological responses suggest that irrigation frequency could be tailored to specific crop species depending on their sensitivity to soil water deficits and re-hydration.

Morphophysiological characteristics of okra plants submitted to saline stress in soil with organic fertilizer

The excess of salts may compromise the growth and physiological functions of plants, but the use of bovine organic fertilizer can mitigate these effects. It was intended was to evaluate the saline stress in soil with and without bovine organic fertilizer on the initial growth, the accumulation of biomass and the gas exchanges of the okra culture. The test was conducted at the UNILAB’s experimental farm, in Piroás, in Redenção – CE city. The treatments were distributed in a completely randomized design with five replications, in a 5 x 2 factorial concerning five irrigation water salinity levels (1.0; 2.0; 3.0; 4.0 and 5.0 dS m-1), in the presence and absence of bovine organic fertilizer. Plant height, stem diameter, root length, a dry mass of shoot, photosynthesis, transpiration and stomatal conductance were evaluated. The bovine organic fertilizer promoted better means in initial growth and biomass in okra plants compared to the control treatment, thus showing efficiency in the attenuation of the salts present in irrigation water. The increase in the saline concentration of the irrigation water reduced the gas exchange (photosynthesis, stomatal conductance and transpiration) in okra plants. Nonetheless, in smaller proportions in the soil with the bovine organic fertilizer.

Physiological responses of beet plants irrigated with saline water and silicon application

Although not considered an essential element, silicon can be used to increase crop productivity, especially under stress conditions. In this sense, the objective was to evaluate the gas exchange of beet plants irrigated with saline water depending on the application of silicon. The experiment was conducted in a randomized block design, in a 5 x 5 factorial, referring to five levels of electrical conductivity of irrigation water (ECw): (0.5; 1.3; 3.25; 5.2 and 6.0 dS m-1) and five doses of silicon (0.00; 2.64; 9.08; 15.52 and 18.16 mL L-1), with six beet plants as an experimental unit. The effect of treatments on beet culture was evaluated at 30 and 60 days after irrigation with saline water from measurements of internal carbon concentration, stomatal conductance, net photosynthesis rate, instantaneous water use efficiency and instantaneous carboxylation efficiency using the LCpro+Sistem infrared gas analyzer (IRGA). Irrigation with saline water reduced the gas exchange of beet plants at 60 days after irrigation, but at 30 days after irrigation, the use of saline water increased stomatal conductance, transpiration rate and internal carbon concentration. The application of silicon decreased stomatal conductance, internal carbon concentration and efficiency in the use of water, but increased the rate of net photosynthesis, the rate of transpiration and instantaneous efficiency of carboxylation at 30 and 60 days after irrigation.

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat.

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (Kleaf ) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and Kleaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψleaf ) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψleaf - T relationship partially accounted for the differing gm -T relationships, where gm was less sensitive to T in Yannong 19 than in Shanyou 63. With different gm -T and Ψleaf -T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with gm . With the increasing T, Kleaf increased in Shanyou 63 while it decreased in Yannong 19. The different Kleaf -T relationships were related to different Ψleaf -T relationships. When excluding the effects of water viscosity and Ψleaf , Kleaf was insensitive to T in both lines. gm and Kleaf were generally not coordinated across different temperatures. This study highlights the importance of Ψleaf on leaf carbon and water exchanges, and the mechanisms for the gm -T and Kleaf -T relationships were discussed.

Optimized Protocol for OnGuard2 Software in Studying Guard Cell Membrane Transport and Stomatal Physiology.

Stomata are key innovation in plants that drives the global carbon and water cycle. In the past few decades, many stomatal models have been developed for studying gas exchange, photosynthesis, and transpirational characteristics of plants, but they provide limited information on stomatal mechanisms at the molecular and cellular levels. Quantitative mathematical modeling offers an effective in silico approach to explore the link between microscopic transporter functioning and the macroscopic stomatal characteristics. As a first step, a dynamic system model based on the guard cell membrane transport system was developed and encoded in the OnGuard software. This software has already generated a wealth of testable predictions and outcomes sufficient to guide phenotypic and mutational studies. It has a user-friendly interface, which can be easily accessed by researchers to manipulate the key elements and parameters in the system for guard cell simulation in plants. To promote the adoption of this OnGuard application, here we outline a standard protocol that will enable users with experience in basic plant physiology, cell biology, and membrane transport to advance quickly in learning to use it.

Optimizing Substrate Available Water and Coir Amendment Rate in Pine Bark Substrates

Water resources can be used more efficiently by including sustainable substrate components like coir that increase water-holding capacity. The first objective of this study was to evaluate the impact of coir amendment rate on plant available water and plant gas exchange, with the goal of optimizing substrate available water and determining the optimum coir amendment rate in a greenhouse environment. The second objective was to establish the optimum method of determining plant available water using either plant gas exchange parameters or substrate physical properties. Greenhouse experiments were conducted with Hydrangea paniculata ‘Jane’ (Little Lime® hardy hydrangea) potted with one of five different coir rates (0%, 10%, 25%, 40% and 65%) mixed with pine bark on a volume basis. Plant gas exchange parameters and substrate water content were measured daily over a range of increasingly drier substrate moisture contents. Actual photosynthetic rates increased with increasing coir amendment rate and were highest with 65% coir amendment. Amending pine bark with coir increased the water storage capacity, plant available water, and plant gas exchange parameters. Results suggest that 65% coir amendment rate was the optimum amendment rate among those tested in a greenhouse environment and plant photosynthetic rate was the better method of determining plant available water.

Exogenous salicylic acid modifies gas exchange and biomass production of Mentha x piperita L.

Peppermint (Mentha x piperita L.) is a medicinal plant that produces an essential oil used as an additive in food and pharmaceutical formulations. Salicylic acid is a phenolic compound widely distributed throughout the plant kingdom, being able to modify plant physiological and metabolic responses. The objective of this study was to evaluate the effects of the application of salicylic acid on the production of biomass, pigments, as well as gas exchange of peppermint plants. For this purpose, a completely randomized design composed of four replications was applied. Pigment evaluations were carried out in a 4x4 factorial scheme, consisting of the application treatments: 0 (control), 75, 150, and 225 mg L-1 of salicylic acid and four harvest periods. Gas exchange and physiological growth rates were evaluated in a 4x3 factorial scheme. Data were submitted to analysis of variance and regression. The application of salicylic acid increased CO2 assimilation and use, stomatal conductance, water use efficiency, and transpiration, which in turn, resulted in increased height, and dry stem and root mass production. It is concluded that the application of salicylic acid influences the primary metabolism, increasing gas exchange and biomass production of peppermint plants.

Gas exchanges and growth of maize as affected by aeration porosity and soil compaction

Based on the hypothesis that less than 10% aeration porosity is sufficient to ensure plant development, the objective of this study was to evaluate the effect of levels of aeration porosity and soil density on the gas exchange and growth of maize plants. The experiment was conducted in a greenhouse, in randomized blocks in a 2 × 5 factorial arrangement: two densities (1.6 and 1.7 Mg m-3) and five aeration porosities (0.07, 0.08, 0.09, 0.10 and 0.12 m3 m-3), with five replicates. The interaction between the factors did not have an effect on the analyzed variables. Aeration porosity had a significant effect on photosynthesis, stomatal conductance, relative chlorophyll index (SPAD), plant height and biomass production, whose data were fitted by quadratic models. The highest aeration porosity reduced conductance by 11%, followed by 7% reduction in photosynthesis and 6.4% in SPAD. Aeration porosities lower than 0.10 m3 m-3 did not cause significant reduction in the values of gas exchange and growth of maize. At soil density of 1.7 Mg m-3, photosynthesis, SPAD, chlorophyll b, total chlorophyll and carotenoids were higher, but with no increments in biomass and leaf area. It is concluded that aeration porosities greater than 0.10 m3 m-3 are more limiting to plants due to changes in soil attributes, such as penetration resistance and water content. The highest contents of chlorophyll b and carotenoids at the highest density are acclimatization responses to the stress condition.

CHARACTERISTICS OF WHEAT PLANT RESPONSES TO SHORT-TERM AND PROLONGED EXPOSURE TO SALICYLIC ACID UNDER OPTIMAL AND LOW TEMPERATURE

The effect of salicylic acid (SA, 100 μM) of various duration on some indexes of CO 2 -gas exchange, water regime, growth and cold resistance of wheat plants ( Triticum aestivum L.) under optimal (22 °C) and low hardening (4 °C) temperatures were studied. For this, week-long seedlings throughout the whole experiment (7 days) were kept on a nutrient solution with added SA at 22 and 4 °C (prolonged SA action) or kept for 1 day at 22 °C on a nutrient solution with SA and then on a solution without SA (within 6 days) exposed to temperatures of 22 and 4 °C (pre-treatment of SA). It was shown that at a temperature of 22 °C, the prolonged action of SA led to a decrease in the rate of visible photosynthesis and the ratio of respiration in the process of CO 2 -gas exchange, did not affect water use efficiency (WUE), increased the biomass of shoot and root. On the contrary, diurnal exposure to SA had virtually no effect on the dynamics of CO 2 assimilation, reduced the ratio of respiration in CO 2 -gas exchange and transpiration, but increased the WUE. At a temperature of 4 °C, the continuous action of the SA contributed to maintaining the rate of photosynthesis at a higher level than in the control, reduced the ratio of respiration in CO 2 -gas exchange, increased WUE, and also stabilized the accumulation of plant biomass. In contrast, the diurnal pretreatment of the SA increased the negative effect of low temperature on the assimilation of CO 2 , led to increase in expenses of respiration, did not affect the level of transpiration, caused a decrease in WUE. It is important, that the exogenous SA contributed to the growth of the cold tolerance of wheat in conditions of normal and low hardening temperatures. Thus, the response of wheat plants to the effect of SA depends on its duration, while the nature of changes in physiological processes differ under optimal and low temperatures.

Effects of elevated atmospheric CO2 on leaf gas exchange response to progressive drought in barley and tomato plants with different endogenous ABA levels

ABA plays an important role in modulating stomatal response to drought and elevated atmospheric CO2 (e [CO2]). This study aimed to investigate the effect of e[CO2] on the response of leaf gas exchange and plant water relations of barley and tomato plants with different endogenous ABA levels to progressive soil drying. Barley and tomato plants were grown in ambient (a[CO2], 400 ppm) and e[CO2] (800 ppm) and subjected to progressive drought stress. Wild type (WT) genotypes (Steptoe barley and AC tomato) and their ABA-deficient mutants (Az34 barley and flacca) were examined. e[CO2] sensitized the photosynthetic decline with soil drying. Soil-drying induced stomatal closure was affected by [CO2] in WT genotypes, where e[CO2] sensitized stomatal closure in barley but retarded it in tomato, whereas such effects were absent in mutants. Compared to a[CO2], e[CO2] maintained leaf water potential and improved turgor pressure except in the flacca mutant. For the WT genotypes, the stomata became less sensitive to an increase in leaf ABA concentration ([ABA]leaf) under e[CO2] than a[CO2]; while for both mutants, the stomata was predominately controlled by leaf turgor and not an increase in [ABA]leaf during soil drying. Endogenous ABA level played an important role in modulating the effect of e[CO2] on stomatal response to soil drying. These findings improve our understanding of the mechanisms of stomatal control in monocot and dicot species responding to a future drier and CO2-enriched environment.

Morphophysiological aspects of young Calotropis procera plants submitted to different shading levels

The silk flower (Calotropis procera) is a plant found practically in the whole Northeast region of Brazil and can be used for several purposes. However, its morphophysiological characteristics may be influenced by the degree of brightness. This study aimed to evaluate the morphophysiological behavior of silk flower under different shading levels. The experiment was carried out in a greenhouse belonging to the Federal University of Paraíba, Areia, Paraíba, Brazil. The experimental design was completely randomized, with five treatments and eight repetitions. The treatments used were different shading levels: 0 (full sun), 30%, 50%, 70% and 90% of shading. Growth characteristics, morphofunctional attributes, gas exchange and chlorophyll a fluorescence were evaluated. The data were submitted to analysis of variance by the F-test, and in cases of significance, a polynomial regression analysis was performed. The best result for the plant height was found at the shading level of 68.4%. Gas exchange and chlorophyll a fluorescence variables presented higher values in the levels of higher light availability. Silk flower plants grow best under full sun conditions. Shading negatively affects the gas exchange and quantum efficiency of silk flower plants.

A functional-structural plant model that simulates whole- canopy gas exchange of grapevine plants (Vitis vinifera L.) under different training systems.

Scaling from single-leaf to whole-canopy photosynthesis faces several complexities related to variations in light interception and leaf properties. To evaluate the impact of canopy strucuture on gas exchange, we developed a functional-structural plant model to upscale leaf processes to the whole canopy based on leaf N content. The model integrates different models that calculate intercepted radiation, leaf traits and gas exchange for each leaf in the canopy. Our main objectives were (1) to introduce the gas exchange model developed at the plant level by integrating the leaf-level responses related to canopy structure, (2) to test the model against an independent canopy gas exchange dataset recorded on different plant architectures, and (3) to quantify the impact of intra-canopy N distribution on crop photosynthesis. The model combined a 3D reconstruction of grapevine (Vitis vinifera) canopy architecture, a light interception model, and a coupled photosynthesis and stomatal conductance model that considers light-driven variations in N distribution. A portable chamber device was constructed to measure whole-plant gas exchange to validate the model outputs with data collected on different training systems. Finally, a sensitivity analysis was performed to evaluate the impact on C assimilation of different N content distributions within the canopy. By considering a non-uniform leaf N distribution within the canopy, our model accurately reproduced the daily pattern of gas exchange of different canopy architectures. The gain in photosynthesis permitted by the non-uniform compared with a theoretical uniform N distribution was about 18 %, thereby contributing to the maximization of C assimilation. By contrast, considering a maximal N content for all leaves in the canopy overestimated net CO2 exchange by 28 % when compared with the non-uniform distribution. The model reproduced the gas exchange of plants under different training systems with a low error (10 %). It appears to be a reliable tool to evaluate the impact of a grapevine training system on water use efficiency at the plant level.

Vegetative and Physiological Responses of “Emerald” Blueberry to Ammoniacal Sources with a Nitrification Inhibitor

The nitrogen nutrition in blueberry has been studied by some authors; however, the effect of N-nitrate or N-ammoniacal fertilizers with nitrification inhibitors on plant growth and physiology has not been known. The aim of this investigation was to study the effectiveness and physiological implications of ammoniacal fertilizers, with or without nitrification inhibitors in blueberry. An experiment was conducted on 1-year “Emerald” blueberries grown in 20-L plastic pots. Our data indicate that ammonium-containing fertilizers promote vegetative growth and increase the leaf nitrogen concentration and gas exchange in plants, possibly due to higher nitrogen root absorption compared with nitrate. On the other hand, fertilization with ammonium with a nitrification inhibitor increases the leaf chlorophyll concentration compared with the addition of ammonium without a nitrification inhibitor and nitrate. However, the ammonium supply decreases the concentration of calcium and potassium in leaves. Our data suggest, for the first time, that fertilization with ammonium accompanied by a nitrification inhibitor is an effective strategy to improve the nitrogen status and promote plant development in “Emerald” blueberry.

A Role for Plant KASH Proteins in Regulating Stomatal Dynamics.

Stomatal movement, which regulates gas exchange in plants, is controlled by a variety of environmental factors, including biotic and abiotic stresses. The stress hormone abscisic acid (ABA) initiates a signaling cascade, which leads to increased H2O2 and Ca2+ levels and F-actin reorganization, but the mechanism of, and connection between, these events is unclear. SINE1, an outer nuclear envelope component of a plant Linker of Nucleoskeleton and Cytoskeleton complex, associates with F-actin and is, along with its putative paralog SINE2, expressed in guard cells. Here, we have determined that Arabidopsis (Arabidopsis thaliana) SINE1 and SINE2 play an important role in stomatal opening and closing. Loss of SINE1 or SINE2 results in ABA hyposensitivity and impaired stomatal dynamics but does not affect stomatal closure induced by the bacterial elicitor flg22. The ABA-induced stomatal closure phenotype is, in part, attributed to impairments in Ca2+ and F-actin regulation. Together, the data suggest that SINE1 and SINE2 act downstream of ABA but upstream of Ca2+ and F-actin. While there is a large degree of functional overlap between the two proteins, there are also critical differences. Our study makes an unanticipated connection between stomatal regulation and nuclear envelope-associated proteins, and adds two new players to the increasingly complex system of guard cell regulation.

From reproduction to production, stomata are the master regulators.

The best predictor of leaf level photosynthetic rate is the porosity of the leaf surface, as determined by the number and aperture of stomata on the leaf. This remarkable correlation between stomatal porosity (or diffusive conductance to water vapour gs ) and CO2 assimilation rate (A) applies to all major lineages of vascular plants (Figure 1) and is sufficiently predictable that it provides the basis for the model most widely used to predict water and CO2 fluxes from leaves and canopies. Yet the Ball-Berry formulation is only a phenomenological approximation that captures the emergent character of stomatal behaviour. Progressing to a more mechanistic prediction of plant gas exchange is challenging because of the diversity of biological components regulating stomatal action. These processes are the product of more than 400 million years of co-evolution between stomatal, vascular and photosynthetic tissues. Both molecular and structural components link the abiotic world of the whole plant with the turgor pressure of the epidermis and guard cells, which ultimately determine stomatal pore size and porosity to water and CO2 exchange (New Phytol., 168, 2005, 275). In this review we seek to simplify stomatal behaviour by using an evolutionary perspective to understand the principal selective pressures involved in stomatal evolution, thus identifying the primary regulators of stomatal aperture. We start by considering the adaptive process that has locked together the regulation of water and carbon fluxes in vascular plants, finally examining specific evidence for evolution in the proteins responsible for regulating guard cell turgor.

Optimal stomatal drought response shaped by competition for water and hydraulic risk can explain plant trait covariation.

The classical theory of stomatal optimization stipulates that stomata should act to maximize photosynthesis while minimizing transpiration. This theory, despite its remarkable success in reproducing empirical patterns, does not account for the fact that the available water to plants is dynamically regulated by plants themselves, and that plants compete for water in most locations. Here, we develop an alternative theory in which plants maximize the expected carbon gain under stochastic rainfall in a competitive environment. We further incorporate xylem hydraulic limitation as an additional constraint to transpiration and evaluate its impacts on stomatal optimization by incorporating the direct carbon cost of xylem recovery and the opportunity cost of reduced future photosynthesis as a result of irrecoverable xylem damage. We predict stomatal behaviour to be more conservative with a higher cost induced by xylem damage. By varying the unit carbon cost and extent of xylem recovery, characterizing the direct and opportunity cost of xylem damage, respectively, our model can reproduce several key patterns of stomatal-hydraulic trait covariations. By addressing the key elements of water limitation in plant gas exchange simultaneously, including plants' self-regulation of water availability, competition for water and hydraulic risk, our study provides a comprehensive theoretical basis for understanding stomatal behaviour.