Reduced leaf width increases photosynthetic rate without improving water-use efficiency in rice.

Enhancing rice photosynthesis is essential for increasing yield, yet the specific leaf morphological characteristics associated with high photosynthetic efficiency remain unclear. This study aims to investigate how reducing leaf width (LW) influences photosynthetic rate (A) and water-use efficiency (iWUE) in rice. Pot experiments were performed using 14 cultivated rice genotypes exhibiting considerable LW variations and genetically modified rice lines carrying the NARROW LEAF 1 (NAL1) gene. We observed a significant negative correlation between LW and A among the 14 cultivated rice varieties. Simultaneously, a 48.2% reduction in the LW of NAL1-K was accompanied by a 49.9% significant increase in A. Narrower leaves increased leaf hydraulic conductance and stomatal density, thereby synergistically augmenting stomatal conductance (gs). Furthermore, increased stomatal density enhances mesophyll conductance (gm) by facilitating airspace formation and less resistance to CO2 transfer. Reduced LW also increased leaf nitrogen content and enhanced the maximum carboxylation rate of RuBisCO (Vcmax). Although reduced LW synergistically increased gs, gm, Vcmax, and ultimately A, it did not concurrently achieve a coordinated improvement in iWUE. Our findings provide valuable insights into the physiological mechanisms underlying photosynthetic efficiency in rice, suggesting that optimizing LW may be a potential strategy for enhancing A without necessarily improving iWUE.