A game of two leaf sides: Abaxial vs Adaxial stomata

Stomata are of the utmost importance to plant function, growth and productivity, as they enable gas exchange for photosynthesis, as well as facilitate transpiration for evaporative cooling and nutrient uptake. The balance between these two processes depends on stomatal density, size and behaviour, which together determine stomatal conductance and intrinsic water use efficiency. There has been great progress in elucidating the genetic pathways that govern the development of stomatal patterning and density, however, although many species have stomata on both surfaces of the leaf, traditionally the importance of adaxial stomata has been largely overlooked in the literature. Recent studies have started to evaluate the impact of the varying contribution of adaxial (AD) vs abaxial (AB) stomata to gas exchange, the costs and benefits of different AD:AB ratios for different species and growth conditions, as well as differences in sensitivity to environmental cues between the two surfaces. Our review focuses on the environmental drivers of differential stomatal patterning and physiological function and the potential to manipulate surface-specific stomatal patterning independently to optimize leaf gas exchange for crops growing in different environments.