Integrating stomatal physiology and morphology: evolution of stomatal control and development of future crops.

Stomata are central players in the hydrological and carbon cycles, regulating the uptake of carbon dioxide (CO2) for photosynthesis 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.