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The control of stomatal aperture involves reversible changes in the concentration of osmolytes in guard cells. Sucrose has long been proposed to have an osmolytic role in guard cells. However, direct evidence for such a role is lacking. Furthermore, recent evidence suggests that sucrosemay perform additional roles in guard cells. Here, we provide an update covering the multiple roles of sucrose in guard cell regulation, highlighting the knowledge accumulated regarding spatiotemporal differences in the synthesis, accumulation, and degradation of sucrose as well as reviewing the role of sucrose as a metabolic connector between mesophyll and guard cells. Analysis of transcriptomic data from previous studies reveals that several genes encoding sucrose and hexose transporters and genes involved in gluconeogenesis, sucrose and trehalose metabolism are highly expressed in guard cells compared with mesophyll cells. Interestingly, this analysis also showed that guard cells have considerably higher expression of C4-marker genes than mesophyll cells.Wediscuss the possible roles of these genes in guard cell function and the role of sucrose in stomatal opening and closure. Finally, we provide a perspective for future experiments which are required to fill gaps in our understanding of both guard cell metabolism and stomatal regulation.

Stomatal responses to environmental signals differ substantially between ferns and angiosperms. However, the mechanisms that lead to such different responses remain unclear. Here we investigated the extent to which leaf metabolism contributes to coordinate the differential stomatal behaviour among ferns and angiosperms. Stomata from all species were responsive to light and CO₂ transitions. However, fern stomatal responses were slower and minor in both absolute and relative terms. Angiosperms have higher stomatal density, but this is not correlated with speed of stomatal closure. The metabolic responses throughout the diel course and under different CO₂ conditions differ substantially among ferns and angiosperms. Higher sucrose content and an increased sucrose-to-malate ratio during high CO₂-induced stomatal closure was observed in angiosperms compared to ferns. Furthermore, the speed of stomatal closure was positively and negatively correlated with sugars and organic acids, respectively, suggesting that the balance between sugars and organic acids aids in explaining the faster stomatal responses of angiosperms. Our results suggest that mesophyll-derived metabolic signals, especially those associated with sucrose and malate, may also be important to modulate the differential stomatal behaviour between ferns and angiosperms, providing important new information that helps in understanding the metabolism-mediated mechanisms regulating stomatal movements across land plant evolution.

Contents 1018 I. 1018 II. 1019 III. 1022 IV. 1025 V. 1026 VI. 1029 1030 References 1030 SUMMARY: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water-use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll-derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter-ion of potassium, a well-established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.

Plant calcium sensing receptor (CAS) optimizes photosynthesis by its effect on the formation of photosynthetic electron transport. CAS also regulates transpiration under water stress. A novel correlation between CAS and plant water use efficiency is revealed

Stomata are central players in the hydrological and carbon cycles, regulating the uptake of carbon dioxide (CO₂) for photo-synthesis and transpirative loss of water (H₂O) between plants and the atmosphere. The necessity to balance water-loss and CO₂-uptake has played a key role in the evolution of plants, and is increasingly important in a hotter and drier world. The conductance of CO₂ and water vapour across the leaf surface is determined by epidermal and stomatal morphology (the number, size, and spacing of stomatal pores) and stomatal physiology (the regulation of stomatal pore aperture in response to environmental conditions). The proportion of the epidermis allocated to stomata and the evolution of amphistomaty are linked to the physiological function of stomata. Moreover, the relationship between stomatal density and [CO₂] is mediated by physiological stomatal behaviour; species with less responsive stomata to light and [CO₂] are most likely to adjust stomatal initiation. These differences in the sensitivity of the stomatal density—[CO₂] relationship between species influence the efficacy of the ‘stomatal method’ that is widely used to infer the palaeo-atmospheric [CO₂] in which fossil leaves developed. Many studies have investigated stomatal physiology or morphology in isolation, which may result in the loss of the ‘overall picture’ as these traits operate in a coordinated manner to produce distinct mechanisms for stomatal control. Consideration of the interaction between stomatal morphology and physiology is critical to our understanding of plant evolutionary history, plant responses to on-going climate change and the production of more efficient and climate-resilient food and bio-fuel crops.

Both photosynthesis (A) and stomatal conductance (g s) respond to changing irradiance, yet stomatal responses are an order of magnitude slower than photosynthesis, resulting in noncoordination between A and g s in dynamic light environments. Infrared gas exchange analysis was used to examine the temporal responses and coordination of A and g s to a step increase and decrease in light in a range of different species, and the impact on intrinsic water use efficiency was evaluated. The temporal responses revealed a large range of strategies to save water or maximize photosynthesis in the different species used in this study but also displayed an uncoupling of A and g s in most of the species. The shape of the guard cells influenced the rapidity of response and the overall g s values achieved, with different impacts on A and W i. The rapidity of g s in dumbbell-shaped guard cells could be attributed to size, whilst in elliptical-shaped guard cells features other than anatomy were more important for kinetics. Our findings suggest significant variation in the rapidity of stomatal responses amongst species, providing a novel target for improving photosynthesis and water use.

Stomatal movements control CO2 uptake for photosynthesis and water loss through transpiration, and therefore play a key role in plant productivity and water use efficiency. The predicted doubling of global water usage by 2030 mean that stomatal behaviour is central to current efforts to increase photosynthesis and crop yields, particularly under conditions of reduced water availability. In the field, slow stomatal responses to dynamic environmental conditions add a temporal dimension to gaseous fluxes between the leaf and atmosphere. Here, we review recent work on the rapidity of stomatal responses and present some of the possible anatomical and biochemical mechanisms that influence the rapidity of stomatal movements.

Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (g op) correlates with minimum stomatal conductance prior to morning light [g min(dawn)]; (ii) stomatal size (S) is negatively correlated with g op and the maximum rate of stomatal opening in response to light, (dg/dt)max; and (iii) g op correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc max) and light-saturated rate of electron transport (J max). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g op can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk.

• Although xylem embolism resistance is traditionally considered as static, we hypothesized that in grapevine (Vitis vinifera) leaf xylem becomes more embolism-resistant over the growing season. • We evaluated xylem architecture, turgor loss point (ΨTLP) and water potentials leading to 25% of maximal stomatal conductance (g s25) or 50% embolism in the leaf xylem (P50) in three irrigation treatments and at three time points during the growing season, while separating the effects of leaf age and time of season. • Hydraulic traits acclimated over the growing season in a coordinated manner. Without irrigation, ΨTLP, g s25, and P50 decreased between late May and late August by 0.95, 0.77 and 0.71 MPa, respectively. A seasonal shift in P50 occurred even in mature leaves, while irrigation had only a mild effect (< 0.2 MPa) on P50. Vessel size and pit membrane thickness were also seasonally dynamic, providing a plausible explanation for the shift in P50. • Our findings provide clear evidence that grapevines can modify their hydraulic traits along a growing season to allow lower xylem water potential, without compromising gas exchange, leaf turgor or xylem integrity. Seasonal changes should be considered when modeling ecosystem vulnerability to drought or comparing datasets acquired at different phenological stages.

Stomatal responses to humidity, soil moisture and other factors that influence plant water status are critical drivers of photosynthesis, productivity, water yield, ecohydrology and climate forcing, yet we still lack a thorough mechanistic understanding of these responses. Here I review historical and recent advances in stomatal water relations. Clear evidence now implicates a metabolically mediated response to leaf water status (‘hydroactive feedback’) in stomatal responses to evaporative demand and soil drought, possibly involving abscisic acid production in leaves. Other hypothetical mechanisms involving vapor and heat transport within leaves may contribute to humidity, light and temperature responses, but require further theoretical clarification and experimental validation. Variation and dynamics in hydraulic conductance, particularly within leaves, may contribute to water status responses. Continuing research to fully resolve mechanisms of stomatal responses to water status should focus on several areas: validating and quantifying the mechanism of leaf-based hydroactive feedback, identifying where in leaves water status is actively sensed, clarifying the role of leaf vapor and energy transport in humidity and temperature responses, and verifying foundational but minimally replicated results of stomatal hydromechanics across species. Clarity on these matters promises to deliver modelers with a tractable and reliable mechanistic model of stomatal responses to water status.