Objective Genetics related to the photosynthesis and water use efficiency (WUE) of indica-japonica hybrids was studied.

Methods Sixty indica-japonica hybrids were obtained by crossing 5 indica-type cytoplasmic male-sterile (CMS) lines with 12 indica-japonica derived restorer lines using the North Carolina II (NCII) mating design. Photosynthetic parameters and WUE were measured on flag leaves at full heading stage of the plants.

Results Substantial genetic variations existed on all target traits of the hybrids. The non-additive effects accounted predominately for 63.30%-75.92% of the total genetic variance. The remaining 24.08%-36.70% additive effects were primarily derived from the restorer lines and insignificantly from the sterile lines. The broad-sense heritability was high at 83.46%-98.08%, while the narrow-sense heritability was low at 20.79%-36.03%. The path coefficient analysis showed T_r to exert the strongest positive direct effect on P_n of 0.532, and WUE a weak negative direct effect of −0.131. P_n significantly and positively correlated to the seed-setting rate with a Pearson correlation coefficient (r) of 0.352, to the filled grains per panicle with an r of 0.277, and to the grain yield per plant with an r of 0.341.

Conclusion Targeting the physiological indicators of a high C_i, a low-to-moderate G_s, and strong Tr direct effect on P_n, a high-yield rice variety efficient in photosynthesis and water utilization could possibly be bred by crossing Minghui 398 or Minghui 417 with Minghui 502.