Introgression of two chromosome regions for leaf photosynthesis from an indica rice into the genetic background of a japonica rice

Increases in rates of individual leaf photosynthesis (P_n) are critical for future increases of rice yields. A previous study, using introgression lines derived from a cross between indica cultivar Habataki, with one of the highest recorded values of P_n, and the Japanese elite cultivar Koshihikari, identified four QTLs (qCAR4, qCAR5, qCAR8, and qCAR11) that affect P_n. The present study examined the combined effect of qCAR4 and qCAR8 on P_n in the genetic background of Koshihikari. The pyramided near-isogenic line NIL(qCAR4+qCAR8) showed higher P_n than both NIL(qCAR4) and NIL(qCAR8), equivalent to that of Habataki despite being due to only two out of the four QTLs. The high P_n of NIL(qCAR4+qCAR8) may be attributable to the high leaf nitrogen content, which may have been inherited from NIL(qCAR4), to the large hydraulic conductance due to the large root surface area from NIL(qCAR4), and to the high hydraulic conductivity from NIL(qCAR8). It might be also attributable to high mesophyll conductance, which may have been inherited from NIL(qCAR4). The induction of mesophyll conductance and the high leaf nitrogen content and high hydraulic conductivity could not be explained in isolation from the Koshihikari background. These results suggest that QTL pyramiding is a useful approach in rice breeding aimed at increasing P_n.