The red and blue light dialogue of stomatal regulation for water-efficient crops

Stomatal responses to changing irradiance have significant impacts on photosynthetic carbon gain as well as on plant water use efficiency. These responses can be broken down into the red or mesophyll response which links stomatal conductance with photosynthesis and the specific blue light response that saturates at low fluence levels and is not linked to photosynthesis. Although the two pathways are thought to operate independently, there is growing evidence that the two are more closely linked than we first appreciated. Understanding both the independence and interplay between red light (RL) and blue light (BL) responses in stomatal behaviour could reveal unexploited but critical targets for sustaining or enhancing intrinsic water-use efficiency (Wi) and crop productivity. In this update review, we synthesise the recent advances in our understanding of the RL and BL stomatal responses and propose future research directions to deepen our mechanistic understanding and inform crop improvement strategies.