The mesophyll anatomy enhancing CO₂ diffusion is a key trait for improving rice photosynthesis

Increases in rates of individual leaf photosynthesis (P_n) are critical for future increases in yields of rice plants. Although many efforts have been made to improve rice P_n with transgenic technology, the desired increases in P_n have not yet been achieved. Two rice lines with extremely high values of P_n were identified among the backcrossed inbred lines derived from the indica variety Takanari, one of the most productive varieties in Japan, and the elite japonica variety Koshihikari (Koshihikari/Takanari//Takanari). The P_n values of the two lines at an ambient CO₂ concentration of 370μmol mol^-1 as well as at a saturating concentration of CO₂ were 20-50% higher than those of the parental varieties. Compared with Takanari, these lines had neither a higher content nor a higher activity of ribulose 1,5-bisphosphate carboxylase/oxygenase when the leaf nitrogen contents were similar, but they did have high mesophyll conductance with respect to CO₂ flux due to their higher density and more highly developed lobes of mesophyll cells. These lines also had higher electron transport rates. The plant growth rates of these lines were higher than that of Takanari. The findings show that it is possible to increase P_n significantly, both at the current atmospheric concentration of CO₂ and at the increased concentration of CO ₂ expected in the future, using appropriate combinations of genetic resources that are available at present.