Guard cell and subsidiary cell sizes are key determinants for stomatal kinetics and drought adaptation in cereal crops.

Climate change-induced drought is a major threat to agriculture. C 4 crops have a higher water use efficiency (WUE) and better adaptability to drought than C 3 crops due to their smaller stomatal morphology and faster response. However, our understanding of stomatal behaviours in both C 3 and C 4 Poaceae crops is limited by knowledge gaps in physical traits of guard cell (GC) and subsidiary cell (SC). We employed infrared gas exchange analysis and a stomatal assay to explore the relationship between GC/SC sizes and stomatal kinetics across diverse drought conditions in two C 3 (wheat and barley) and three C 4 (maize, sorghum and foxtail millet) upland Poaceae crops. Through statistical analyses, we proposed a GCSC-τ model to demonstrate how morphological differences affect stomatal kinetics in C 4 Poaceae crops. Our findings reveal that morphological variations specifically correlate with stomatal kinetics in C 4 Poaceae crops, but not in C 3 ones. Subsequent modelling and experimental validation provide further evidence that GC/SC sizes significantly impact stomatal kinetics, which affects stomatal responses to different drought conditions and thereby WUE in C 4 Poaceae crops. These findings emphasize the crucial advantage of GC/SC morphological characteristics and stomatal kinetics for the drought adaptability of C 4 Poaceae crops, highlighting their potential as future climate-resilient crops.