Alleles of high-yielding indica rice that improve root hydraulic conductance also increase flag leaf photosynthesis, biomass, and grain production of japonica rice in the paddy field

Improving net leaf photosynthetic rate (A_n) by molecular breeding is an important research challenge in improving rice productivity. For the effective use of a locus or gene that increases A_n in breeding, we must not only elucidate its roles in traits that increase A_n but also evaluate its effects on phenology, leaf area, and canopy architecture and light-interception characteristics, all of which affect biomass and grain production. Alleles of the high-yielding indica rice cultivars ‘Habataki’ on chromosome (Chr.) 5 and ‘Takanari’ on Chr. 10 increase A_n of rice with a japonica ‘Koshihikari’ genetic background. Here, we developed near isogenic lines (NILs) carrying these alleles (NIL5 and NIL10) and elucidated their performance in photosynthesis and in biomass and grain production in plants grown in the paddy field and pots. NIL5 and NIL10 had flag leaves with 12–13% higher A_n than those of Koshihikari from full heading to ripening. Their higher A_n of the flag leaf was attributable to higher stomatal conductance (g_s). NIL5 and NIL10 had a larger whole-plant hydraulic conductance than Koshihikari owing to the increased root surface area and root hydraulic conductance. NIL5 and NIL10 produced 6–10% more biomass at harvest and 8–14% higher grain yield than Koshihikari. Their crop growth rate was higher via higher net assimilation rate (NAR) during ripening. Heading date, ripening duration, biomass at booting (or heading), leaf area index, canopy architecture and canopy extinction coefficient at ripening and harvest index did not differ between Koshihikari and either NIL. Thus, the higher NAR and the consequent higher biomass and grain production in NIL5 and NIL10 were attributed to their increased flag leaf A_n. We conclude that both alleles are involved in improving root hydraulic conductance, which in turn increase flag leaf A_n, biomass and grain production, and they can be used in breeding high-yielding rice.