Contrasting effects of high stomatal density on rice photosynthesis and water use efficiency: synergistic enhancement under drought but not under well‐watered conditions

Synergistically enhancing both photosynthetic rate (A) and intrinsic water use efficiency (iWUE) in rice remains a major challenge for achieving high productivity in the future. In this study, 37 cultivated rice (Oryza sativa) varieties with significant variation in stomatal morphological traits were selected for pot experiments. Among these, the two highest and two lowest stomatal density varieties were further subjected to drought treatments. Under well-watered conditions, stomatal density was identified as a key factor coordinating stomatal conductance (g_s) and mesophyll conductance (g_m) among rice varieties by influencing mesophyll cell arrangement through stomatal development. Although increased stomatal density enhanced A, it did not synergistically improve iWUE. Under water stress, however, stomatal aperture decreased rapidly as drought intensified, gradually diminishing the positive effect of high stomatal density on g_s until it disappeared. Notably, varieties with high stomatal density maintained higher g_m than those with low stomatal density across the entire range of leaf water potentials measured, enabling simultaneous enhancement of both A and iWUE under drought conditions. Our study demonstrates that high stomatal density can synergistically enhance both A and iWUE under drought conditions, underscoring its potential utility in breeding drought-tolerant rice varieties.