The coordinated increase in stomatal density and vein dimensions during genetic improvement in rice

Rice (Oryza sativa L. ssp. indica) has experienced three distinct phases of considerable yield increases: Green Revolution, utilization of heterosis, and the combination of ideotype and inter-subspecific hybrid breeding. Crop breeding and selection for high yield have increased radiation use efficiency in modern indica rice varieties. However, the underlying leaf morphological and physiological changes have not been established. Field and pot experiments were conducted in 2016 and 2017. We investigated the relationships between the anatomical maximum stomatal conductance (g(max)), operational stomatal conductance (g(op)), and the anatomy of the stomata and vein in relation to leaf-level transpiration and photosynthesis across historical indica rice varieties. The results showed that flag leaf temperature of new varieties was reduced relative to the temperature of older varieties due to increased g(op) and leaf transpirational cooling. Both high stomatal density and larger veins were increased in new varieties with improved yield potential, while no change was observed in stomatal length and vein density. There was a significant correlation between stomatal density and g(op) as well as between g(op) and the light-saturated photosynthetic rate. The present study reveals that historical selection for high yield is accompanied by leaf morphological changes that contribute to enhanced g(op), leaf cooling, and photosynthesis of irrigated rice inhabiting hot, high light environments.