Spatial and temporal responses in stomatal behaviour, photosynthesis and implications for water-use efficiency

Stomatal conductance (gs) determines CO₂ uptake for photosynthesis (A) and controls the amount of water loss via transpiration, promoting evaporative cooling and nutrient uptake. It is therefore clear that stomatal behaviour has major implications for plant productivity which can be assessed by water-use efficiency (WUE) (Wi = A/gs). However, stomatal responses to changes in environmental stimuli or intra-cellular cues are rarely synchronised under the continually fluctuating environmental conditions experienced in the field, leading to unnecessary water loss or limiting A required for promoting high yields. In addition, stomatal responses are intra-and inter-specific, varying both temporally and spatially over a single leaf and at the whole-plant level with further implications for canopy and ecosystem fluxes of CO₂ and water vapour. The effect of this variation in gs responses on both A and Wi is discussed, along with the underlying anatomical characteristics which determine the potential of these physiological responses. Recent advances in both imaging techniques and modelling of guard cell membrane transporters have provided the means to more accurately assess the sensitivity, speed and magnitude of dynamic stomatal behaviours to environmental stimuli and the further impact on A and Wi at a range of scales. Use of these technologies provides a route to targeted manipulation of stomatal function to promote responses, which will improve whole-plant Wi without compromising CO₂ uptake for productivity. Here, we discuss how scaling processes are currently used to predict gs, and the importance in considering the heterogeneous nature of stomatal dynamics, to improve the representation of how stomata influence the local and global fluxes of CO₂ and water along the soil-plant-atmosphere continuum.