Effects of kinetics of light-induced stomatal responses on photosynthesis and water-use efficiency

Viewing Stomata in Action: Autonomous in Planta Imaging of Individual Stomatal Movement

Stomata regulate plant gas exchange under changing environments, but observations of single stomata dynamics in planta are sparse. We developed a compact microscope system that can measure the kinetics of tens of stomata in planta simultaneously, with sub-minute time resolution. Darkfield imaging with green light was used to create 3D stacks from which 2D surface projections were constructed to resolve stomatal apertures. Stomatal dynamics of Chrysanthemum morifolium (Chrysanthemum) and Zea mays (Maize) under changing light intensity were categorized, and a kinetic model was fitted to the data for quantitative comparison. Maize stomata transitioned frequently between open and closed states under constant growth light and these 'opening and closing' stomata, when closed, responded faster to a change to saturating light than steady-state closed stomata under the constant growth light. The faster opening response benefits CO2 uptake under saturating light. The slow closure of Chrysanthemum stomata reduced water use efficiency (WUE). Over 50% showed delayed or partial closure, leading to unnecessarily large apertures after reduced light. Stomata with larger apertures had more lag and similar closure speeds compared to those with smaller apertures and lag, further reducing WUE. In contrast, maize stomata with larger apertures closed faster, with no lag.

Impact of climate-driven changes in temperature on stomatal anatomy and physiology

Climate-driven changes in temperature are likely to have major implications for plant performance including impacts on stomatal conductance (gs), gaseous exchange, photosynthesis, leaf temperature and plant water use. Stomatal conductance is not only vital for carbon assimilation but also plays a key role in maintaining optimum leaf temperatures for cellular processes. Higher gs facilitates both CO2 uptake and enhanced evaporative leaf cooling, however, most likely at the cost of greater water loss and lower water-use efficiency. Lower gs helps to maintain overall plant water status but at the expense of C uptake with reduced evaporative cooling which, at elevated temperatures, could be lethal. It is therefore important for gs to balance these competing demands; however, with rapid changes in temperature due to climate change, stomatal engineering may be required to ensure that this balance is achieved and different strategies for different crops in different environments may be needed. Here, we review current knowledge of stomatal anatomical and behavioural responses to temperature-driven changes, focusing on both rising temperatures and extreme heat events and potential genetic targets for manipulation of relevant stomatal characteristics.This article is part of the theme issue 'Crops under stress: can we mitigate the impacts of climate change on agriculture and launch the 'Resilience Revolution'?'.

Metabolic engineering of stomatal precursor cells enhances photosynthetic water-use efficiency and vegetative growth under water-deficit conditions in Arabidopsis thaliana

Stomata are epidermal pores that control the exchange of gaseous CO2 and H2O between plants and their environment. Modulating stomatal density can alter this exchange and thus presents a viable target for engineering improved crop productivity and climate resilience. Here, we show that stomatal density in Arabidopsis thaliana can be decreased by the expression of a water-forming NAD(P)H oxidase targeted to stomatal precursor cells. We demonstrate that this reduction in stomatal density occurs irrespective of whether the expressed enzyme is localized to the cytosol, chloroplast stroma or chloroplast intermembrane space of these cells. We also reveal that this decrease in stomatal density occurs in the absence of any measurable impact on the efficiency and thermal sensitivity of photosynthesis, or on stomatal dynamics. Consequently, overexpression plants exhibit a higher intrinsic water-use efficiency due to an increase in CO2 fixed per unit water transpired. Finally, we demonstrate that this enhanced water-use efficiency translates to an improvement in vegetative growth and biomass accumulation under water-deficit conditions. Together, these results thus provide a novel approach for enhancing plant productivity through metabolic engineering of stomatal density.

Stomatal and Hydraulic Redundancy Allows Woody Species to Adapt to Arid Environments

Functional redundancy is considered a pivotal mechanism for maintaining the adaptability of species by preventing the loss of key functions in response to dehydration. However, we still lack a comprehensive understanding of the redundancy of leaf hydraulic systems along aridity gradients. Here, photosynthesis (An), stomatal conductance (gs) and leaf hydraulic conductance (Kleaf) during dehydration were measured in 20 woody species from a range of aridity index (AI) conditions and growing in a common garden to quantify stomatal redundancy (SR), the extent of stomatal opening beyond the optimum required for maximum photosynthesis (Amax), leaf hydraulic redundancy (HR), and the extent of leaf hydraulic conductance (Kleaf) beyond the optimum required for maximum gs (gs-max). The findings revealed that species from arid habitats tended to have higher SRs but lower HRs than did species from humid habitats. The relatively high SR in arid species arose from relatively high gs-max values. The relatively low HR arose from the relatively high Kleaf value at a 5% reduction in gs-max (Kleaf-gs). Our results suggest that greater stomatal redundancy and lower hydraulic redundancy prevent the loss of photosynthesis and water transportation, respectively, and thus might be the key adaptive mechanisms for plants to adapt to drought conditions.

Genetic diversity of leaf photosynthesis under fluctuating light conditions among temperate japonica rice varieties

Under field conditions, solar radiation on a crop canopy fluctuates according to clouds, wind, and self-shading. The slower response of photosynthesis compared with the rate of irradiation changes leads to loss of photosynthetic carbon gain. Although some genetic differences in the rate of photosynthetic induction have been reported, the diversity among rice varieties is largely unknown. Here we evaluated genetic variation in the response of photosynthesis to a step increase in light intensity among 166 temperate japonica varieties including landraces and modern varieties. Large genetic variation in photosynthetic induction and less evidence of improvement across modern breeding programmes were acknowledged. In the correlation analysis between physiological traits for all varieties, the efficiency of non-stomatal processes was the major factor affecting the rate of induction. The landrace Aikokumochi, which has intermediate photosynthetic capacity, showed rapid photosynthetic induction-eight times that of the slowest variety. This was attributed to smaller non-stomatal limitation in the initial phase of induction and smaller stomatal limitation in the later phase than in reference varieties. Aikokumochi also had a greater photosynthetic CO2 gain without reduced water use efficiency under repeated fluctuating light. These findings demonstrate the importance of genetic resources to improve photosynthesis while maintaining water use efficiency under fluctuating light conditions.

Revealing stable SNPs and genomic prediction insights across environments enhance breeding strategies of productivity, defense, and climate-adaptability traits in white spruce

Exploring the relationship between phenotype, genotype, and environment is essential in quantitative genetics. Considering the complex genetic architecture of economically important traits, integrating genotype-by-environment interactions in a genome-wide association (GWAS) and genomic prediction (GP) framework is imperative. This integration is crucial for identifying robust markers with stability across diverse environments and improving the predictive accuracy of individuals' performance within specific target environments. We conducted a multi-environment GWAS and GP analysis for 30 productivity, defense, and climate-adaptability traits on 1540 white spruce trees from Alberta, Canada, genotyped for 467,224 SNPs and growing across three environments. We identified 563 significant associations (p-value < 1.07 ×10-05) across the studied traits and environments, with 105 SNPs showing overlapping associations in two or three environments. Wood density, myrcene, total monoterpenes, α-pinene, and catechin exhibited the highest overlap (>50%) across environments. Gas exchange traits, including intercellular CO2 concentration and intrinsic water use efficiency, showed the highest number of significant associations (>38%) but less stability (<1.2%) across environments. Predictive ability (PA) varied significantly (0.03-0.41) across environments for 20 traits, with stable carbon isotope ratio having the highest average PA (0.36) and gas exchange traits the lowest (0.07). Only two traits showed differences in prediction bias (PB) across environments, with 80% of site-trait PB falling within a narrow range (0.90 to 1.10). Integrating multi-environment GWAS and GP analyses proved useful in identifying site-specific markers, understanding environmental impacts on PA and PB, and ultimately providing indirect insights into the environmental factors that influenced this white spruce breeding program.

Symplastic guard cell connections buffer pressure fluctuations to promote stomatal function in grasses

Stomata regulate plant gas exchange via repeated turgor-driven changes of guard cell shape, thereby adjusting pore apertures. Grasses, which are among the most widespread plant families on the planet, are distinguished by their unique stomatal structure, which is proposed to have significantly contributed to their evolutionary and agricultural success. One component of their structure, which has received little attention, is the presence of a discontinuous adjoining cell wall of the guard cell pair. Here, we demonstrate the presence of these symplastic connections in a range of grasses and use finite element method simulations to assess hypotheses for their functional significance. Our results show that opening of the stomatal pore is maximal when the turgor pressure in dumbbell-shaped grass guard cells is equal, especially under the low pressure conditions that occur during the early phase of stomatal opening. By contrast, we demonstrate that turgor pressure differences have less effect on the opening of kidney-shaped guard cells, characteristic of the majority of land plants, where guard cell connections are rarely or not observed. Our data describe a functional mechanism based on cellular mechanics, which plausibly facilitated a major transition in plant evolution and crop development.

Nitrogen Supply Mitigates Temperature Stress Effects on Rice Photosynthetic Nitrogen Use Efficiency and Water Relations

Climate-change-induced temperature fluctuations pose significant threats to global rice production, particularly through their impact on photosynthetic efficiency. The differential mechanisms by which low and high temperatures affect leaf photosynthetic processes in rice remain poorly understood. Here, we investigate the effects of temperature stress (15 °C, 30 °C, 45 °C) on rice photosynthetic performance across a gradient of nitrogen supply levels: low nitrogen (LN), medium nitrogen (MN), and high nitrogen (HN). The low temperature exhibited stronger negative impacts on photosynthesis than the high temperature, primarily through increased mesophyll limitation and disrupted cellular CO2 diffusion, while the high temperature showed less pronounced effects, particularly under HN and MN conditions. While photosynthetic nitrogen use efficiency (PNUE) decreased with increasing nitrogen under the optimal temperature, moderate nitrogen supply maintained optimal PNUE under temperature stress, suggesting that a balanced nitrogen level is crucial for maximizing both photosynthetic capacity and nitrogen use efficiency. Plants with adequate nitrogen maintained higher intrinsic water use efficiency (iWUE) under both temperature extremes through improved coordination between CO2 uptake and water loss. Our findings reveal distinct mechanisms underlying low- and high-temperature stress effects on photosynthesis and highlight the importance of optimizing nitrogen management for enhancing crop resilience to temperature extremes under climate change.

The Kinetics of Mesophyll Conductance and Photorespiration During Light Induction

Mesophyll conductance to CO2 (gm) act as a significant limiting factor influencing the CO2 assimilation rate (AN) during photosynthetic induction. However, the effect of vapor pressure deficit (VPD) on gm kinetics during light induction is not well clarified. We combined gas exchange with chlorophyll fluorescence measurements to assess the induction kinetics of gm during light induction under contrasting vapor pressure deficit (VPD) in two tree species with different stomatal conductance (gs) behavior, Catalpa fargesii and Pterocarya stenoptera. Our results revealed three key findings: (1) the coordination of gm and gs kinetics during light induction occurred in C. fargesii but not in P. stenoptera, and the model of gs kinetics largely determines whether the coordination of gs and gm exist in a given species; (2) a high VPD induced simultaneous changes in gs and gm kinetics in C. fargesii but had separated effects on gs and gm kinetics in P. stenoptera, indicating that the response of gm kinetics during light induction to VPD differs between species; and (3) the relative contribution of photorespiration to total electron flow was flexible in response to the change in relative diffusional and biochemical limitations, pointing out that photorespiration has a significant role in the regulation of photosynthetic electron flow during light induction. These results provide new sight into the species-dependent kinetics of gm and photorespiration during light induction.

Translational photobiology: towards dynamic lighting in indoor horticulture

Crop productivity depends on the ability of plants to thrive across different growth environments. In nature, light conditions fluctuate due to diurnal and seasonal changes in direction, duration, intensity, and spectrum. Laboratory studies, predominantly conducted with arabidopsis (Arabidopsis thaliana), have provided valuable insights into the metabolic and regulatory strategies that plants employ to cope with varying light intensities. However, there has been less focus on how horticultural crops tolerate dynamically changing light conditions during the photoperiod. In this review we connect insights from photobiology in model plants to the application of dynamic lighting in indoor horticulture. We explore how model species respond to fluctuating light intensities and discuss how this knowledge could be translated for new lighting solutions in controlled environment agriculture.

Interannual Variation of Stomatal Traits Impacts the Environmental Responses of Apple Trees

Stomata are fundamental to plant-water relations and represent promising targets to enhance crop water-use efficiency and climate resilience. Here, we investigated stomatal density (SD) variation in 269 apple accessions across 3 years (2019-2021), which demonstrated significant differences between accessions but consistency over time. We selected 2 subsets of 20 accessions, each with contrasting SD: high stomatal density (HSD; 370-500 mm-2) and low stomatal density (LSD; 192-316 mm-2). SD groups were compared in stomatal function, leaf physiology and crop productivity across two seasons (2021-2022). LSD had lower stomatal conductance (gs) and higher intrinsic water-use efficiency in both years (p < 0.05). Hotter and drier conditions in 2022 reduced gs similarly in both groups (-22% HSD, -21% LSD), but also created a difference in net carbon assimilation (Anet) that was not present in 2021 (HSD + 1.7 μmol CO2 m-2 s-1, p < 0.05). LSD constraints on Anet were reflected in carbon isotope discrimination (δ13C, p < 0.001) and annual decline in fruit yield (-35%, p < 0.001). Our results demonstrate the suitability of SD as a trait to improve WUE, but also identifies a trade-off between water savings and productivity, which requires consideration for breeding.

A C4 plant K+ channel accelerates stomata to enhance C3 photosynthesis and water use efficiency

Accelerating stomatal kinetics through synthetic optogenetics and mutations that enhance guard cell K+ flux has proven a viable strategy to improve water use efficiency and biomass production. Stomata of the model C4 species Gynandropsis gynandra, a relative of the C3 plant Arabidopsis thaliana, are similarly fast to open and close. We identified and cloned the guard cell rectifying outward K+ channel (GROK) of Gynandropsis and showed that GROK is preferentially expressed in stomatal guard cells. GROK is homologous to the Arabidopsis guard cell K+ channel GORK and, expressed in oocytes, yields a K+ current consistent with that of Gynandropsis guard cells. Complementing the Arabidopsis gork mutant with GROK promoted K+ channel gating and K+ flux, increasing stomatal kinetics and yielding gains in water use efficiency and biomass with varying light, especially under water limitation. Our findings demonstrate the potential for engineering a C4 K+ channel into guard cells of a C3 species, and they speak to the puzzle of how C4 species have evolved mechanisms that enhance water use efficiency and growth under stress.

Have We Selected for Higher Mesophyll Conductance in Domesticating Soybean?

Soybean (Glycine max) is the single most important global source of vegetable protein. Yield improvements per unit land area are needed to avoid further expansion onto natural systems. Mesophyll conductance (gm) quantifies the ease with which CO2 can diffuse from the sub-stomatal cavity to Rubisco. Increasing gm is attractive since it increases photosynthesis without increasing water use. Most measurements of gm have been made during steady-state light saturated photosynthesis. In field crop canopies, light fluctuations are frequent and the speed with which gm can increase following shade to sun transitions affects crop carbon gain. Is there variability in gm within soybean germplasm? If so, indirect selection may have indirectly increased gm during domestication and subsequent breeding for sustainability and yield. A modern elite cultivar (LD11) was compared with four ancestor accessions of Glycine soja from the assumed area of domestication by concurrent measurements of gas exchange and carbon isotope discrimination (∆13C). gm was a significant limitation to soybean photosynthesis both at steady state and through light induction but was twice the value of the ancestors in LD11. This corresponded to a substantial increase in leaf photosynthetic CO2 uptake and water use efficiency.

Boosting photosynthesis opens new opportunities for agriculture sustainability and circular economy: The BEST-CROP research and innovation action

There is a need for ground-breaking technologies to boost crop yield, both grains and biomass, and their processing into economically competitive materials. Novel cereals with enhanced photosynthesis and assimilation of greenhouse gasses, such as carbon dioxide and ozone, and tailored straw suitable for industrial manufacturing, open a new perspective for the circular economy. Here we describe the vision, strategies, and objectives of BEST-CROP, a Horizon-Europe and United Kingdom Research and Innovation (UKRI) funded project that relies on an alliance of academic plant scientists teaming up with plant breeding companies and straw processing companies to use the major advances in photosynthetic knowledge to improve barley biomass and to exploit the variability of barley straw quality and composition. We adopt the most promising strategies to improve the photosynthetic properties and ozone assimilation capacity of barley: (i) tuning leaf chlorophyll content and modifying canopy architecture; (ii) increasing the kinetics of photosynthetic responses to changes in irradiance; (iii) introducing photorespiration bypasses; (iv) modulating stomatal opening, thus increasing the rate of carbon dioxide fixation and ozone assimilation. We expect that by improving our targeted traits we will achieve increases in aboveground total biomass production without modification of the harvest index, with added benefits in sustainability via better resource-use efficiency of water and nitrogen. In parallel, the resulting barley straw is tailored to: (i) increase straw protein content to make it suitable for the development of alternative biolubricants and feed sources; (ii) control cellulose/lignin contents and lignin properties to develop straw-based construction panels and polymer composites. Overall, by exploiting natural- and induced-genetic variability as well as gene editing and transgenic engineering, BEST-CROP will lead to multi-purpose next generation barley cultivars supporting sustainable agriculture and capable of straw-based applications.

Elevated CO2 Shifts Photosynthetic Constraint from Stomatal to Biochemical Limitations During Induction in Populus tomentosa and Eucalyptus robusta

The relative impacts of biochemical and stomatal limitations on photosynthesis during photosynthetic induction have been well studied for diverse plants under ambient CO2 concentration (Ca). However, a knowledge gap remains regarding how the various photosynthetic components limit duction efficiency under elevated CO2. In this study, we experimentally investigated the influence of elevated CO2 (from 400 to 800 μmol mol-1) on photosynthetic induction dynamics and its associated limitation components in two broadleaved tree species, Populus tomentosa and Eucalyptus robusta. The results show that elevated CO2 increased the steady-state photosynthesis rate (A) and decreased stomatal conductance (gs) and the maximum carboxylation rate (Vcmax) in both species. While E. robusta exhibited a decrease in the linear electron transport rate (J) and the fraction of open reaction centers in photosynthesis II (qL), P. tomentosa showed a significant increase in non-photochemical quenching (NPQ). With respect to non-steady-state photosynthesis, elevated CO2 significantly reduced the induction time of A following a shift from low to high light intensity in both species. Time-integrated limitation analysis during induction revealed that elevated CO2 reduces the relative impacts of stomatal limitations in both species, consequently shifting the predominant limitation on induction efficiency from stomatal to biochemical components. Additionally, species-specific changes in qL and NPQ suggest that elevated CO2 may increase biochemical limitation by affecting energy allocation between carbon fixation and photoprotection. These findings suggest that, in a future CO2-rich atmosphere, plants productivity under fluctuating light may be primarily constrained by photochemical and non-photochemical quenching.

Photosynthesis and photoprotection in top leaves respond faster to irradiance fluctuations than bottom leaves in a tomato canopy

Accounting for the dynamic responses of photosynthesis and photoprotection to naturally fluctuating irradiance can improve predictions of plant performance in the field, but the variation of these dynamics within crop canopies is poorly understood. We conducted a detailed study of dynamic and steady-state photosynthesis, photoprotection, leaf pigmentation, and stomatal anatomy in four leaf layers (100, 150, 200, and 250 cm from the floor) of a fully grown tomato (Solanum lycopersicum cv. Foundation) canopy in a greenhouse. We found that leaves at the top of the canopy exhibited higher photosynthetic capacity and slightly faster photosynthetic induction compared with lower-canopy leaves, accompanied by higher stomatal conductance and a faster activation of carboxylation and linear electron transport capacities. In upper-canopy leaves, non-photochemical quenching showed faster induction and relaxation after increases and decreases in irradiance, allowing for more effective photoprotection in these leaves. Despite these observed differences in transient responses between leaf layers, steady-state rather than dynamic photosynthesis traits were more influential for predicting photosynthesis under fluctuating irradiance. Also, a model analysis revealed that time-averaged photosynthesis under fluctuating irradiance could be accurately predicted by one set of Rubisco activation/deactivation parameters across all four leaf layers, thereby greatly simplifying future modelling efforts of whole-canopy photosynthesis.

Biochemical versus stomatal acclimation of dynamic photosynthetic gas exchange to elevated CO2 in three horticultural species with contrasting stomatal morphology

Understanding photosynthetic acclimation to elevated CO2 (eCO2) is important for predicting plant physiology and optimizing management decisions under global climate change, but is underexplored in important horticultural crops. We grew three crops differing in stomatal density-namely chrysanthemum, tomato, and cucumber-at near-ambient CO2 (450 μmol mol-1) and eCO2 (900 μmol mol-1) for 6 weeks. Steady-state and dynamic photosynthetic and stomatal conductance (gs) responses were quantified by gas exchange measurements. Opening and closure of individual stomata were imaged in situ, using a novel custom-made microscope. The three crop species acclimated to eCO2 with very different strategies: Cucumber (with the highest stomatal density) acclimated to eCO2 mostly via dynamic gs responses, whereas chrysanthemum (with the lowest stomatal density) acclimated to eCO2 mostly via photosynthetic biochemistry. Tomato exhibited acclimation in both photosynthesis and gs kinetics. eCO2 acclimation in individual stomatal pore movement increased rates of pore aperture changes in chrysanthemum, but such acclimation responses resulted in no changes in gs responses. Although eCO2 acclimation occurred in all three crops, photosynthesis under fluctuating irradiance was hardly affected. Our study stresses the importance of quantifying eCO2 acclimatory responses at different integration levels to understand photosynthetic performance under future eCO2 environments.

Stomatal dynamics in Alloteropsis semialata arise from the evolving interplay between photosynthetic physiology, stomatal size and biochemistry

C4 plants are expected to have faster stomatal movements than C3 species because they tend to have smaller guard cells. However, little is known about how the evolution of C4 photosynthesis influences stomatal dynamics in relation to guard cell size and environmental factors. We studied photosynthetically diverse populations of the grass Alloteropsis semialata, showing that the origin of C4 photosynthesis in this species was associated with a shortening of stomatal guard and subsidiary cells. However, for a given cell size, C4 and C3-C4 intermediate individuals had similar or slower light-induced stomatal opening speeds than C3 individuals. Conversely, when exposed to decreasing light, stomata in C4 plants closed as fast as those in non-C4 plants. Polyploid formation in some C4 plants led to larger stomatal cells and was associated with slower stomatal opening. Conversely, diversification of C4 diploid plants into wetter environments was associated with an acceleration of stomatal opening. Overall, there was significant relationship between light-saturated photosynthesis and stomatal opening speed in the C4 plants, implying that photosynthetic energy production was limiting for stomatal opening. Stomatal dynamics in this wild grass therefore arise from the evolving interplay between photosynthetic physiology and the size and biochemical function of stomatal complexes.

Microcontroller-based water control system for evaluating crop water use characteristics

BACKGROUND

Climate change and the growing demand for agricultural water threaten global food security. Understanding water use characteristics of major crops from leaf to field scale is critical, particularly for identifying crop varieties with enhanced water-use efficiency (WUE) and stress tolerance. Traditional methods to assess WUE are either by gas exchange measurements at the leaf level or labor-intensive manual pot weighing at the whole-plant level, both of which have limited throughput.

RESULTS

Here, we developed a microcontroller-based low-cost system that integrates pot weighing, automated water supply, and real-time monitoring of plant water consumption via Wi-Fi. We validated the system using major crops (rice soybean, maize) under diverse stress conditions (salt, waterlogging, drought). Salt-tolerant rice maintained higher water consumption and growth under salinity than salt-sensitive rice. Waterlogged soybean exhibited reduced water use and growth. Long-term experiments revealed significant WUE differences between rice varieties and morphological adaptations represented by altered shoot-to-root ratios under constant drought conditions in maize.

CONCLUSIONS

We demonstrate that the system can be used for varietal differences between major crops in their response to drought, waterlogging, and salinity stress. This system enables high-throughput, long-term evaluation of water use characteristics, facilitating the selection and development of water-saving and stress-tolerant crop varieties.

Fast stomatal kinetics in sorghum enable tight coordination with photosynthetic responses to dynamic light intensity and safeguard high water use efficiency

In this study, we assessed 43 accessions of sorghum from 16 countries across three continents. Our objective was to identify stomatal and photosynthetic traits that could be exploited in breeding programmes to increase photosynthesis without increasing water use under dynamic light environments. Under field conditions, sorghum crops often have limited water availability and are exposed to rapidly fluctuating light intensities, which influences both photosynthesis and stomatal behaviour. Our results highlight a tight coupling between photosynthetic rate (A) and stomatal conductance (gs) even under dynamic light conditions that results in steady intrinsic water use efficiency (Wi). This was mainly due to rapid stomatal responses, with the majority of sorghum accessions responding within ≤5 min. The maintenance of the ratio of the concentration of CO2 inside the leaf (Ci) and the surrounding atmospheric concentration (Ca) over a large range of accessions suggests high stomatal sensitivity to changes in Ci, that could underlie the rapid gs responses and extremely close relationship between A and gs under both dynamic and steady-state conditions. Therefore, sorghum represents a prime candidate for uncovering the elusive mechanisms that coordinate A and gs, and such information could be used to design crops with high A without incurring significant water losses and eroding Wi.

Exploring natural genetic diversity in a bread wheat multi-founder population: dual imaging of photosynthesis and stomatal kinetics

Recent research has shown that optimizing photosynthetic and stomatal traits holds promise for improved crop performance. However, standard phenotyping tools such as gas exchange systems have limited throughput. In this work, a novel approach based on a bespoke gas exchange chamber allowing combined measurement of the quantum yield of PSII (Fq'/Fm'), with an estimation of stomatal conductance via thermal imaging was used to phenotype a range of bread wheat (Triticum aestivum L.) genotypes. Using the dual-imaging methods and traditional approaches, we found broad and significant variation in key traits, including photosynthetic CO2 uptake at saturating light and ambient CO2 concentration (Asat), photosynthetic CO2 uptake at saturating light and elevated CO2 concentration (Amax), the maximum velocity of Rubisco for carboxylation (Vcmax), time for stomatal opening (Ki), and leaf evaporative cooling. Anatomical analysis revealed significant variation in flag leaf adaxial stomatal density. Associations between traits highlighted significant relationships between leaf evaporative cooling, leaf stomatal conductance, and Fq'/Fm', highlighting the importance of stomatal conductance and stomatal rapidity in maintaining optimal leaf temperature for photosynthesis in wheat. Additionally, gsmin and gsmax were positively associated, indicating that potential combinations of preferable traits (i.e. inherently high gsmax, low Ki, and maintained leaf evaporative cooling) are present in wheat. This work highlights the effectiveness of thermal imaging in screening dynamic gs in a panel of wheat genotypes. The wide phenotypic variation observed suggested the presence of exploitable genetic variability in bread wheat for dynamic stomatal conductance traits and photosynthetic capacity for targeted optimization within future breeding programmes.

Natural genetic variation in dynamic photosynthesis is correlated with stomatal anatomical traits in diverse tomato species across geographical habitats

Plants grown under field conditions experience fluctuating light. Understanding the natural genetic variations for a similarly dynamic photosynthetic response among untapped germplasm resources, as well as the underlying mechanisms, may offer breeding strategies to improve production using molecular approaches. Here, we measured gas exchange under fluctuating light, along with stomatal density and size, in eight wild tomato species and two tomato cultivars. The photosynthetic induction response showed significant diversity, with some wild species having faster induction rates than the two cultivars. Species with faster photosynthetic induction rates had higher daily integrated photosynthesis, but lower average water use efficiency because of high stomatal conductance under natural fluctuating light. The variation in photosynthetic induction was closely associated with the speed of stomatal responses, highlighting its critical role in maximizing photosynthesis under fluctuating light conditions. Moreover, stomatal size was negatively correlated with stomatal density within a species, and plants with smaller stomata at a higher density had a quicker photosynthetic response than those with larger stomata at lower density. Our findings show that the response of stomatal conductance plays a pivotal role in photosynthetic induction, with smaller stomata at higher density proving advantageous for photosynthesis under fluctuating light in tomato species. The interspecific variation in the rate of stomatal responses could offer an untapped resource for optimizing dynamic photosynthetic responses under field conditions.

Differences in stomatal sensitivity to CO2 and light influence variation in water use efficiency and leaf carbon isotope composition in two genotypes of the C4 plant Zea mays

The ratio of net CO2 uptake (Anet) and stomatal conductance (gs) is an intrinsic measurement of leaf water use efficiency (WUEi); however, its measurement can be challenging for large phenotypic screens. Measurements of the leaf carbon isotope composition (δ13Cleaf) may be a scalable tool to approximate WUEi for screening because it in part reflects the competing influences of Anet and gs on the CO2 partial pressure (pCO2) inside the leaf over time. However, in C4 photosynthesis, the CO2-concentrating mechanism complicates the relationship between δ13Cleaf and WUEi. Despite this complicated relationship, several studies have shown genetic variation in δ13Cleaf across C4 plants. Yet there has not been a clear demonstration of if Anet or gs are the causal mechanisms controlling WUEi and δ13Cleaf. Our approach was to characterize leaf photosynthetic traits of two Zea mays recombinant inbred lines (Z007E0067 and Z007E0150) which consistently differ for δ13Cleaf even though they have minimal confounding genetic differences. We demonstrate that these two genotypes contrasted in WUEi driven by differences in the speed of stomatal responses to changes in pCO2 and light that lead to unproductive leaf water loss. These findings provide support that differences in δ13Cleaf in closely related genotypes do reflect greater WUEi and further suggest that differences in stomatal kinetic response to changing environmental conditions is a key target to improve WUEi.

Initial stomatal conductance increases photosynthetic induction of trees leaves more from sunlit than from shaded environments: a meta-analysis

It has long been held that tree species/leaves from shaded environments show faster rate of photosynthetic induction than species/leaves from sunlit environments, but the evidence so far is conflicting and the underlying mechanisms are still under debate. To address the debate, we compiled a dataset for 87 tree species and compared the initial increasing slope during the first 2-min induction (SA) and stomatal and biochemical characteristics between sun and shade species from the same study, and those between sun and shade leaves within the same species. In 77% of between-species comparisons, the species with high steady-state photosynthetic rate in the high light (Af) exhibited a larger SA than the species with low Af. In 67% within-species comparisons, the sun leaves exhibited a larger SA than the shade leaves. However, in only a few instances did the sun species/leaves more rapidly achieve 50% of full induction, with an even smaller SA, than the shade species/leaves. At both the species and leaf level, SA increased with increasing initial stomatal conductance before induction (gsi). Despite exhibiting reduced intrinsic water-use efficiency in low light, a large SA proportionally enhances photosynthetic carbon gain during the first 2-min induction in the sun species and leaves. Thus, in terms of the increase in absolute rate of photosynthesis, tree species/leaves from sunlit environments display faster photosynthetic induction responses than those from shaded environments. Our results call for re-consideration of contrasting photosynthetic strategies in photosynthetic adaption/acclimation to dynamic light environments across species.

Starch metabolism in guard cells: At the intersection of environmental stimuli and stomatal movement

Starch metabolism in guard cells plays a central role in regulating stomatal movement in response to light, elevated ambient CO2 and potentially other abiotic and biotic factors. Here, we discuss how various guard cell signal transduction pathways converge to promote rearrangements in guard cell starch metabolism for efficient stomatal responses, an essential physiological process that sustains plant productivity and stress tolerance. We suggest manipulation of guard cell starch dynamics as a previously overlooked strategy to improve stomatal behavior under changing environmental conditions.

C4 cereal and biofuel crop microbiomes

Microbiomes provide multiple life-support functions for plants, including nutrient acquisition and tolerance to abiotic and biotic stresses. Considering the importance of C4 cereal and biofuel crops for food security under climate change conditions, more attention has been given recently to C4 plant microbiome assembly and functions. Here, we review the current status of C4 cereal and biofuel crop microbiome research with a focus on beneficial microbial traits for crop growth and health. We highlight the importance of environmental factors and plant genetics in C4 crop microbiome assembly and pinpoint current knowledge gaps. Finally, we discuss the potential of foxtail millet as a C4 model species and outline future perspectives of C4 plant microbiome research.

Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees

Water-use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well-documented. Here, we investigated the relationship between stomatal morphology, intrinsic water-use efficiency (iWUE) and leaf carbon isotope ratio (δ13C). We sampled 190 individuals, including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), iWUE and bulk leaf δ13C as a proxy for long-term WUE. Leaf δ13C correlated positively with iWUE across species in both juvenile and mature trees, while GCL showed a negative and SD a positive effect on iWUE and leaf δ13C. Within species, however, only GCL was significantly associated with iWUE and leaf δ13C. SLA had a minor negative influence on iWUE and leaf δ13C, but this effect was inconsistent between juvenile and mature trees. We conclude that GCL and SD can be considered functional morphological traits related to the iWUE and leaf δ13C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.

Dual role of BdMUTE during stomatal development in the model grass Brachypodium distachyon

Grasses form morphologically derived, four-celled stomata, where two dumbbell-shaped guard cells (GCs) are flanked by two lateral subsidiary cells (SCs). This innovative form enables rapid opening and closing kinetics and efficient plant-atmosphere gas exchange. The mobile bHLH transcription factor MUTE is required for SC formation in grasses. Yet whether and how MUTE also regulates GC development and whether MUTE mobility is required for SC recruitment is unclear. Here, we transgenically impaired BdMUTE mobility from GC to SC precursors in the emerging model grass Brachypodium distachyon. Our data indicate that reduced BdMUTE mobility severely affected the spatiotemporal coordination of GC and SC development. Furthermore, although BdMUTE has a cell-autonomous role in GC division orientation, complete dumbbell morphogenesis of GCs required SC recruitment. Finally, leaf-level gas exchange measurements showed that dosage-dependent complementation of the four-celled grass morphology was mirrored in a gradual physiological complementation of stomatal kinetics. Together, our work revealed a dual role of grass MUTE in regulating GC division orientation and SC recruitment, which in turn is required for GC morphogenesis and the rapid kinetics of grass stomata.

Perspectives on improving photosynthesis to increase crop yield

Improving photosynthesis, the fundamental process by which plants convert light energy into chemical energy, is a key area of research with great potential for enhancing sustainable agricultural productivity and addressing global food security challenges. This perspective delves into the latest advancements and approaches aimed at optimizing photosynthetic efficiency. Our discussion encompasses the entire process, beginning with light harvesting and its regulation and progressing through the bottleneck of electron transfer. We then delve into the carbon reactions of photosynthesis, focusing on strategies targeting the enzymes of the Calvin-Benson-Bassham (CBB) cycle. Additionally, we explore methods to increase carbon dioxide (CO2) concentration near the Rubisco, the enzyme responsible for the first step of CBB cycle, drawing inspiration from various photosynthetic organisms, and conclude this section by examining ways to enhance CO2 delivery into leaves. Moving beyond individual processes, we discuss two approaches to identifying key targets for photosynthesis improvement: systems modeling and the study of natural variation. Finally, we revisit some of the strategies mentioned above to provide a holistic view of the improvements, analyzing their impact on nitrogen use efficiency and on canopy photosynthesis.

Effects of drought and moisture stress on the growth and ecophysiological traits of Schima superba seedlings

Changes in rainfall patterns are important environmental factors affecting plant growth, especially when larger precipitation events and prolonged drought periods occur in subtropical regions. There are many studies on how drought reduces plant biomass through drought-sensitive functional traits, but how excess water affects plant growth and ecophysiology is still poorly understood. Therefore, a greenhouse experiment was conducted on Schima superba (Theaceae), a dominant tree species in subtropical forests and commonly used in forestry, in a closed chamber under control (25% soil water content (SWC) as in local forests), drought stress (D, 15% SWC) and moisture stress (W, 35% SWC). Plant growth and ecophysiological traits related to morphology, leaf gas exchange, water potential and structural traits were measured. Compared to control, S. suberba under dry conditions significantly decreased its aboveground biomass, photosynthetic rate (A), leaf water potential and nitrogen use efficiency, but increased intrinsic water use efficiency, root to shoot ratio and specific root length. S. superba under wet conditions also significantly decreased its total biomass, aboveground biomass and specific root length, while W had no effect on A and leaf water potential. Our results indicate that S. superba shows a decrease in carbon gain under drought stress, but less response under wet conditions. This emphasizes the need to consider the strength and frequency of rainfall pattern changes in future studies because rainfall may either alleviate or intensify the effects of drought stress depending on the moisture level, thus suitable water conditions is important for better management of this tree species in subtropical China.

Cell size differences affect photosynthetic capacity in a Mesoamerican and an Andean genotype of Phaseolus vulgaris L

The efficiency of CO2 flux in the leaf is hindered by a several structural and biochemical barriers which affect the overall net photosynthesis. However, the dearth of information about the genetic control of these features is limiting our ability for genetic manipulation. We performed a comparative analysis between three-week-old plants of a Mesoamerican and an Andean cultivar of Phaseolus vulgaris at variable light and CO2 levels. The Mesoamerican bean had higher photosynthetic rate, maximum rate of rubisco carboxylase activity and maximum rate of photosynthetic electron transport at light saturation conditions than its Andean counterpart. Leaf anatomy comparison between genotypes showed that the Mesoamerican bean had smaller cell sizes than the Andean bean. Smaller epidermal cells in the Mesoamerican bean resulted in higher stomata density and consequently higher stomatal conductance for water vapor and CO2 than in the Andean bean. Likewise, smaller palisade and spongy mesophyll cells in the Mesoamerican than in the Andean bean increased the cell surface area per unit of volume and consequently increased mesophyll conductance. Finally, smaller cells in the Mesoamerican also increased chlorophyll and protein content per unit of leaf area. In summary, we show that different cell sizes controls the overall net photosynthesis and could be used as a target for genetic manipulation to improve photosynthesis.

CO2 response screen in grass Brachypodium reveals the key role of a MAP kinase in CO2-triggered stomatal closure

Plants respond to increased CO2 concentrations through stomatal closure, which can contribute to increased water use efficiency. Grasses display faster stomatal responses than eudicots due to dumbbell-shaped guard cells flanked by subsidiary cells working in opposition. However, forward genetic screening for stomatal CO2 signal transduction mutants in grasses has yet to be reported. The grass model Brachypodium distachyon is closely related to agronomically important cereal crops, sharing largely collinear genomes. To gain insights into CO2 control mechanisms of stomatal movements in grasses, we developed an unbiased forward genetic screen with an EMS-mutagenized B. distachyon M5 generation population using infrared imaging to identify plants with altered leaf temperatures at elevated CO2. Among isolated mutants, a "chill1" mutant exhibited cooler leaf temperatures than wild-type Bd21-3 parent control plants after exposure to increased CO2. chill1 plants showed strongly impaired high CO2-induced stomatal closure despite retaining a robust abscisic acid-induced stomatal closing response. Through bulked segregant whole-genome sequencing analyses followed by analyses of further backcrossed F4 generation plants and generation and characterization of sodium azide and CRISPR-cas9 mutants, chill1 was mapped to a protein kinase, Mitogen-Activated Protein Kinase 5 (BdMPK5). The chill1 mutation impaired BdMPK5 protein-mediated CO2/HCO3- sensing together with the High Temperature 1 (HT1) Raf-like kinase in vitro. Furthermore, AlphaFold2-directed structural modeling predicted that the identified BdMPK5-D90N chill1 mutant residue is located at the interface of BdMPK5 with the BdHT1 Raf-like kinase. BdMPK5 is a key signaling component that mediates CO2-induced stomatal movements and is proposed to function as a component of the primary CO2 sensor in grasses.

Surrounded by luxury: The necessities of subsidiary cells

The evolution of stomata marks one of the key advances that enabled plants to colonise dry land while allowing gas exchange for photosynthesis. In large measure, stomata retain a common design across species that incorporates paired guard cells with little variation in structure. By contrast, the cells of the stomatal complex immediately surrounding the guard cells vary widely in shape, size and count. Their origins in development are similarly diverse. Thus, the surrounding cells are likely a luxury that the necessity of stomatal control cannot do without (with apologies to Oscar Wilde). Surrounding cells are thought to support stomatal movements as solute reservoirs and to shape stomatal kinetics through backpressure on the guard cells. Their variety may also reflect a substantial diversity in function. Certainly modelling, kinetic analysis and the few electrophysiological studies to date give hints of much more complex contributions in stomatal physiology. Even so, our knowledge of the cells surrounding the guard cells in the stomatal complex is far from complete.

Photorespiratory glycine contributes to photosynthetic induction during low to high light transition

Leaves experience near-constant light fluctuations daily. Past studies have identified many limiting factors of slow photosynthetic induction when leaves transition from low light to high light. However, the contribution of photorespiration in influencing photosynthesis during transient light conditions is largely unknown. This study employs dynamic measurements of gas exchange and metabolic responses to examine the contribution of photorespiration in constraining net rates of carbon assimilation during light induction. This work indicates that photorespiratory glycine accumulation during the early light induction contributes 5-7% to the additional carbon fixed relative to the low light conditions. Mutants with large glycine pools under photorespiratory conditions (5-formyl THF cycloligase and hydroxypyruvate reductase 1) showed a transient spike in net CO2 assimilation during light induction, with glycine buildup accounting for 22-36% of the extra carbon assimilated. Interestingly, levels of many C3 cycle intermediates remained relatively constant in both mutants and wild-type throughout the light induction period where glycine accumulated, indicating that recycling of carbon into the C3 cycle via photorespiration is not needed to maintain C3 cycle activity under transient conditions. Furthermore, our data show that oxygen transient experiments can be used as a proxy to identify the photorespiratory component of light-induced photosynthetic changes.

Nitrogen addition alleviates water loss of Moso bamboo (Phyllostachys edulis) under drought by affecting light-induced stomatal responses

The combined climate-change-evoked drought and nitrogen (N) deposition have severely affected plant carbon and water relations governed by stomata. However, the interplay between steady-state and dynamic stomatal behavior responses to light remains unclear regarding its impact on plant water and carbon relations. The objective here was to investigate whether light-induced stomatal dynamics could mitigate the adverse effects of steady-state gas exchange on water conservation or photosynthesis under drought and N addition conditions. We conducted a manipulative experiment to investigate the impacts of throughfall reduction, N addition, and their combination on light-induced stomatal and photosynthetic dynamics in a Moso bamboo (Phyllostachys edulis) forest. We determined the influence of stomal response rate on water loss and photosynthesis, and further assessed whether it mitigated the effects of steady-state gas exchange (gs). We found that Moso bamboo decreased gs under throughfall reduction, while accelerated stomatal opening and biochemical activation when irradiance increased, which reduced the lag in photosynthesis during the induction period. In contrast, under the combined throughfall reduction and N addition condition, Moso bamboo increased gs but showed faster stomatal closure, which decreased the percentage of transpiration following a decrease in light intensity. Our findings indicate that stomatal dynamic behavior may depend on the effects of steady-state gas exchange on water conservation and carbon uptake under different soil water and N conditions. These discoveries contribute to our understanding of the coupling mechanisms of plant water use and carbon uptake in the context of global changes.

Stomatal behaviour and water relations in ferns and lycophytes across habits and habitats

Stomatal anatomy and behaviour are key to managing gas exchange fluxes, which require coordination with the plant vascular system to adequately supply leaves with water. Stomatal response times and regulation of water loss are generally understudied in ferns, especially across habits (i.e. epiphytic and terrestrial) and habitats (i.e. wet mesic and dry xeric environments). Our objectives were to (i) determine if hydraulic and anatomical traits that control water use are correlated with their habitats (i.e. xeric, mesic) and habits (i.e. epiphytic, terrestrial) for ferns and lycophytes across taxa, and (ii) explore how those traits and others like average leaf water residence time correlate with stomatal function using a subset of closely related species. Epiphytic species had lower vein densities than terrestrial species, while xeric species had higher vein densities than mesic species. Xeric ferns also had smaller stomata than mesic ferns but had similar stomatal densities. Further, in a subset of mesic and xeric ferns, the xeric ferns had higher maximum stomatal conductance and water content, as well as shorter average stomatal opening responses to light intensity, but stomatal closing times did not differ. Finally, shorter stomatal opening and closing responses were correlated with shorter water residence time. Our study highlights anatomical and physiological differences between ferns and lycophytes, which may partially explain habitat preference based on their optimization of light and water.

C4 monocots and C4 dicots exhibit rapid photosynthetic induction response in contrast to C3 plants

Considering the prevalence of ever-changing conditions in the natural world, investigation of photosynthetic responses in C4 plants under fluctuating light is needed. Here, we studied the effect of dynamic illumination on photosynthesis in totally 10 C3, C3-C4 intermediate, C4-like and C4 dicots and monocots at CO2 concentrations of 400 and 800 μmol mol-1. C4 and C4-like plants had faster photosynthetic induction and light-induced stomatal dynamics than C3 plants at 400 μmol mol-1, but not at 800 μmol mol-1 CO2, at which the CO2 supply rarely limits photosynthesis. C4 and C4-like plants had a higher water use efficiency than C3 plants at both CO2 concentrations. There were positive correlations between photosynthetic induction and light-induced stomatal response, together with CO2 compensation point, which was a parameter of the CO2-concentrating mechanism of C4 photosynthesis. These results clearly show that C4 photosynthesis in both monocots and dicots adapts to fluctuating light conditions more efficiently than C3 photosynthesis. The rapid photosynthetic induction response in C4 plants can be attributed to the rapid stomatal dynamics, the CO2-concentrating mechanism or both.

A review on strategies for crop improvement against drought stress through molecular insights

The demand for food goods is rising along with the world population growth, which is directly related to the yield of agricultural crops around the world. However, a number of environmental factors, including floods, salinity, moisture, and drought, have a detrimental effect on agricultural production around the world. Among all of these stresses, drought stress (DS) poses a constant threat to agricultural crops and is a significant impediment to global agricultural productivity. Its potency and severity are expected to increase in the future years. A variety of techniques have been used to generate drought-resistant plants in order to get around this restriction. Different crop plants exhibit specific traits that contribute to drought resistance (DR), such as early flowering, drought escape (DE), and leaf traits. We are highlighting numerous methods that can be used to overcome the effects of DS in this review. Agronomic methods, transgenic methods, the use of sufficient fertilizers, and molecular methods such as clustered regularly interspaced short palindromic repeats (CRISPRs)-associated nuclease 9 (Cas9), virus-induced gene silencing (VIGS), quantitative trait loci (QTL) mapping, microRNA (miRNA) technology, and OMICS-based approaches make up the majority of these techniques. CRISPR technology has rapidly become an increasingly popular choice among researchers exploring natural tolerance to abiotic stresses although, only a few plants have been produced so far using this technique. In order to address the difficulties imposed by DS, new plants utilizing the CRISPR technology must be developed.

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

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

Estimation of Photosynthetic Induction Is Significantly Affected by Light Environments of Local Leaves and Whole Plants in Oryza Genus

Photosynthetic induction and stomatal kinetics are acknowledged as pivotal factors in regulating both plant growth and water use efficiency under fluctuating light conditions. However, the considerable variability in methodologies and light regimes used to assess the dynamics of photosynthesis (A) and stomatal conductance (gs) during light induction across studies poses challenges for comparison across species. Moreover, the influence of stomatal morphology on both steady-state and non-steady-state gs remains poorly understood. In this study, we show the strong impact of IRGA Chamber Illumination and Whole Plant Illumination on the photosynthetic induction of two rice species. Our findings reveal that these illuminations significantly enhance photosynthetic induction by modulating both stomatal and biochemical processes. Moreover, we observed that a higher density of smaller stomata plays a critical role in enhancing the stomatal opening and photosynthetic induction to fluctuating light conditions, although it exerts minimal influence on steady-state gs and A under constant light conditions. Therefore, future studies aiming to estimate photosynthetic induction and stomatal kinetics should consider the light environments at both the leaf and whole plant levels.

Supplemental greenhouse lighting increased the water use efficiency, crop growth, and cutting production in Cannabis sativa

The expanding cannabis production sector faces economic challenges, intensified by freshwater scarcity in the main US production areas. Greenhouse cultivation harnesses sunlight to reduce production costs, yet the impact of greenhouse light levels on crucial production components, such as plant growth, branching, and water use efficiency (WUE), remains poorly understood. This study aimed to assess the effects of combined sunlight and supplemental lighting on the crop's main production components and leaf gas exchange of Cannabis sativa 'Suver Haze' in the vegetative stage. Within a greenhouse, LED lighting provided at intensities of ~150, 300, 500, and 700 µmol m-2 s-1 (18-hour photoperiod), combined with solar radiation, resulted in average daily light integrals of 17.9, 29.8, 39.5, and 51.8 mol m-2 d-1. Increasing light levels linearly increased biomass, leaf area, and the number of branches per plant and square meter, with respective rates of 0.26 g, 32.5 cm2, and 0.41 branches per mole of additional light. As anticipated, crop evapotranspiration increased by 1.8-fold with the increase in light intensity yet crop WUE improved by 1.6-fold when comparing the lowest and highest light treatments. Moreover, water requirements per unit of plant biomass decreased from 0.37 to 0.24 liters per gram when lighting increased from ~18 to 52 mol m-2 d-1, marking a 35% reduction in evapotranspiration. These results were supported by increments in leaf photosynthesis and WUE with light enhancement. Furthermore, our findings indicate that even 52 mol m-2 d-1 of supplemental lighting did not saturate any of the crop responses to light and can be economically viable for cannabis nurseries. In conclusion, light supplementation strongly enhanced photosynthesis and plant growth while increasing WUE. Additionally, a comprehensive discussion highlights the shared physiological mechanisms governing WUE in diverse plant species and their potential for water conservation under enhanced lighting conditions.

Multiplexed CRISPR-Cas9 mutagenesis of rice PSBS1 noncoding sequences for transgene-free overexpression

Understanding CRISPR-Cas9's capacity to produce native overexpression (OX) alleles would accelerate agronomic gains achievable by gene editing. To generate OX alleles with increased RNA and protein abundance, we leveraged multiplexed CRISPR-Cas9 mutagenesis of noncoding sequences upstream of the rice PSBS1 gene. We isolated 120 gene-edited alleles with varying non-photochemical quenching (NPQ) capacity in vivo-from knockout to overexpression-using a high-throughput screening pipeline. Overexpression increased OsPsbS1 protein abundance two- to threefold, matching fold changes obtained by transgenesis. Increased PsbS protein abundance enhanced NPQ capacity and water-use efficiency. Across our resolved genetic variation, we identify the role of 5'UTR indels and inversions in driving knockout/knockdown and overexpression phenotypes, respectively. Complex structural variants, such as the 252-kb duplication/inversion generated here, evidence the potential of CRISPR-Cas9 to facilitate significant genomic changes with negligible off-target transcriptomic perturbations. Our results may inform future gene-editing strategies for hypermorphic alleles and have advanced the pursuit of gene-edited, non-transgenic rice plants with accelerated relaxation of photoprotection.

Guard cell and subsidiary cell sizes are key determinants for stomatal kinetics and drought adaptation in cereal crops

Climate change-induced drought is a major threat to agriculture. C4 crops have a higher water use efficiency (WUE) and better adaptability to drought than C3 crops due to their smaller stomatal morphology and faster response. However, our understanding of stomatal behaviours in both C3 and C4 Poaceae crops is limited by knowledge gaps in physical traits of guard cell (GC) and subsidiary cell (SC). We employed infrared gas exchange analysis and a stomatal assay to explore the relationship between GC/SC sizes and stomatal kinetics across diverse drought conditions in two C3 (wheat and barley) and three C4 (maize, sorghum and foxtail millet) upland Poaceae crops. Through statistical analyses, we proposed a GCSC-τ model to demonstrate how morphological differences affect stomatal kinetics in C4 Poaceae crops. Our findings reveal that morphological variations specifically correlate with stomatal kinetics in C4 Poaceae crops, but not in C3 ones. Subsequent modelling and experimental validation provide further evidence that GC/SC sizes significantly impact stomatal kinetics, which affects stomatal responses to different drought conditions and thereby WUE in C4 Poaceae crops. These findings emphasize the crucial advantage of GC/SC morphological characteristics and stomatal kinetics for the drought adaptability of C4 Poaceae crops, highlighting their potential as future climate-resilient crops.

Stomatal dynamics are regulated by leaf hydraulic traits and guard cell anatomy in nine true mangrove species

Stomatal regulation is critical for mangroves to survive in the hyper-saline intertidal zone where water stress is severe and water availability is highly fluctuant. However, very little is known about the stomatal sensitivity to vapour pressure deficit (VPD) in mangroves, and its co-ordination with stomatal morphology and leaf hydraulic traits. We measured the stomatal response to a step increase in VPD in situ, stomatal anatomy, leaf hydraulic vulnerability and pressure-volume traits in nine true mangrove species of five families and collected the data of genome size. We aimed to answer two questions: (1) Does stomatal morphology influence stomatal dynamics in response to a high VPD in mangroves? with a consideration of possible influence of genome size on stomatal morphology; and (2) do leaf hydraulic traits influence stomatal sensitivity to VPD in mangroves? We found that the stomata of mangrove plants were highly sensitive to a step rise in VPD and the stomatal responses were directly affected by stomatal anatomy and hydraulic traits. Smaller, denser stomata was correlated with faster stomatal closure at high VPD across the species of Rhizophoraceae, and stomata size negatively and vein density positively correlated with genome size. Less negative leaf osmotic pressure at the full turgor (πo) was related to higher operating steady-state stomatal conductance (gs); and a higher leaf capacitance (Cleaf) and more embolism resistant leaf xylem were associated with slower stomatal responses to an increase in VPD. In addition, stomatal responsiveness to VPD was indirectly affected by leaf morphological traits, which were affected by site salinity and consequently leaf water status. Our results demonstrate that mangroves display a unique relationship between genome size, stomatal size and vein packing, and that stomatal responsiveness to VPD is regulated by leaf hydraulic traits and stomatal morphology. Our work provides a quantitative framework to better understand of stomatal regulation in mangroves in an environment with high salinity and dynamic water availability.

Mechanical advantage makes stomatal opening speed a function of evaporative demand

Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals, particularly evaporative demand driven by vapor pressure deficit (VPD) influences the kinetics of this response. In angiosperms, and some fern species from the family Marsileaceae, a mechanical interaction between the guard cells and the epidermal cells determines the aperture of the pore. Here, we examine whether this mechanical interaction influences the speed of stomatal opening in the light. To test this, we investigated the speed of stomatal opening in response to light across a range of VPDs in seven plant species spanning the evolutionary diversity of guard cell and epidermal cell mechanical interactions. We found that stomatal opening speed is a function of evaporative demand in angiosperm species and Marsilea, which have guard cell and epidermal cell mechanical interactions. Stomatal opening speeds did not change across a range of VPD in species of gymnosperm and fern, which do not have guard cell mechanical interactions with the epidermis. We find that guard cell and epidermal cell mechanical interactions may play a key role in regulating stomatal responsiveness to light. These results provide valuable insight into the adaptive relevance of mechanical advantage.

Simulating short-term light responses of photosynthesis and water use efficiency in sweet sorghum under varying temperature and CO2 conditions

Climate change, characterized by rising atmospheric CO2 levels and temperatures, poses significant challenges to global crop production. Sweet sorghum, a prominent C4 cereal extensively grown in arid areas, emerges as a promising candidate for sustainable bioenergy production. This study investigated the responses of photosynthesis and leaf-scale water use efficiency (WUE) to varying light intensity (I) in sweet sorghum under different temperature and CO2 conditions. Comparative analyses were conducted between the A n-I, g s-I, T r-I, WUEi-I, and WUEinst-I models proposed by Ye et al. and the widely utilized the non-rectangular hyperbolic (NRH) model for fitting light response curves. The Ye's models effectively replicated the light response curves of sweet sorghum, accurately capturing the diminishing intrinsic WUE (WUEi) and instantaneous WUE (WUEinst) trends with increasing I. The fitted maximum values of A n, g s, T r, WUEi, and WUEinst and their saturation light intensities closely matched observations, unlike the NRH model. Despite the NRH model demonstrating high R 2 values for A n-I, g s-I, and T r-I modelling, it returned the maximum values significantly deviating from observed values and failed to generate saturation light intensities. It also inadequately represented WUE responses to I, overestimating WUE. Across different leaf temperatures, A n, g s, and T r of sweet sorghum displayed comparable light response patterns. Elevated temperatures increased maximum A n, g s, and T r but consistently declined maximum WUEi and WUEinst. However, WUEinst declined more sharply due to the disproportionate transpiration increase over carbon assimilation. Critically, sweet sorghum A n saturated at current atmospheric CO2 levels, with no significant gains under 550 μmol mol-1. Instead, stomatal closure enhanced WUE under elevated CO2 by coordinated g s and T r reductions rather than improved carbon assimilation. Nonetheless, this response diminished under simultaneously high temperature, suggesting intricate interplay between CO2 and temperature in modulating plant responses. These findings provide valuable insights into photosynthetic dynamics of sweet sorghum, aiding predictions of yield and optimization of cultivation practices. Moreover, our methodology serves as a valuable reference for evaluating leaf photosynthesis and WUE dynamics in diverse plant species.

P, Todd J. Gas exchange and time to reach maximum rate of photosynthetic rate and their relationship with whole-plant traits in sugarcane in water abundant Louisiana, USA

Variety development of sugarcane (Saccharum spp. hybrids) is necessary to continue improving sugar yields and selecting photosynthetic traits can improve sugar production through increased carbon inputs. In this study, gas exchange and whole-plant measurements were made on 55 sugarcane genotypes in Louisiana. Variation in the relationship between photosynthetic rate and stomatal conductance suggests that sugarcane exhibits variation in both photosynthetic capacity and CO2 substrate availability. Genotypes that reached maximum photosynthetic rate (TRMPR) in the gas-exchange cuvette more quickly had greater CO2 assimilation during transitory periods. Temporary shading and fluctuating light are common transitory conditions in the field, so increasing TRMPR can improve photosynthesis in water-abundant regions. Canopy leaf area was positively correlated with stalk mass, but gas-exchange traits were not correlated with whole-plant traits. A better understanding of the relationship between leaf and whole-plant traits is necessary to identify physiological traits that lead to increased genetic gain.

Calvin cycle and guard cell metabolism impact stomatal function

Stomatal conductance (gs) determines CO2 uptake for photosynthesis (A) and water loss through transpiration, which is essential for evaporative cooling and maintenance of optimal leaf temperature as well as nutrient uptake. Stomata adjust their aperture to maintain an appropriate balance between CO2 uptake and water loss and are therefore critical to overall plant water status and productivity. Although there is considerable knowledge regarding guard cell (GC) osmoregulation (which drives differences in GC volume and therefore stomatal opening and closing), as well as the various signal transduction pathways that enable GCs to sense and respond to different environmental stimuli, little is known about the signals that coordinate mesophyll demands for CO2. Furthermore, chloroplasts are a key feature in GCs of many species, however, their role in stomatal function is unclear and a subject of debate. In this review we explore the current evidence regarding the role of these organelles in stomatal behaviour, including GC electron transport and Calvin-Benson-Bassham (CBB) cycle activity as well as their possible involvement correlating gs and A along with other potential mesophyll signals. We also examine the roles of other GC metabolic processes in stomatal function.

Speedy stomata of a C4 plant correlate with enhanced K+ channel gating

Stomata are microscopic pores at the surface of plant leaves that facilitate gaseous diffusion to support photosynthesis. The guard cells around each stoma regulate the pore aperture. Plants that carry out C4 photosynthesis are usually more resilient than C3 plants to stress, and their stomata operate over a lower dynamic range of CO2 within the leaf. What makes guard cells of C4 plants more responsive than those of C3 plants? We used gas exchange and electrophysiology, comparing stomatal kinetics of the C4 plant Gynandropsis gynandra and the phylogenetically related C3 plant Arabidopsis thaliana. We found, with varying CO2 and light, that Gynandropsis showed faster changes in stomata conductance and greater water use efficiency when compared with Arabidopsis. Electrophysiological analysis of the dominant K+ channels showed that the outward-rectifying channels, responsible for K+ loss during stomatal closing, were characterised by a greater maximum conductance and substantial negative shift in the voltage dependence of gating, indicating a reduced inhibition by extracellular K+ and enhanced capacity for K+ flux. These differences correlated with the accelerated stomata kinetics of Gynandropsis, suggesting that subtle changes in the biophysical properties of a key transporter may prove a target for future efforts to engineer C4 stomatal kinetics.