Grain and dry matter yields and partitioning of assimilates in Japonica/Indica hybrid rice

Poor grain filling is a major constraint in utilizing the heterosis of japonica/indica hybrid rice (Oryza sativa L.). The objective of this study was to investigate potential causes of poor grain filling of japonica/indica hybrids (J/IHs) by examining the source-sink relations in 36 J/IHs. Thirty-six J/IHs and their parents and two intervarietal hybrids (IVHs) were grown at the Yangzhou University farm during the 1998, 1999, and 2000 rice growing seasons. Growth analyses were performed at heading and during grain filling period, and yield and yield components were determined at maturity. Results showed that J/IHs had 18.9% greater spikelet number per square meter than the two IVHs because of the difference in spikelet number per panicle, but grain yields of the two kinds of hybrids did not differ significantly as a result of low spikelet filling percentage in J/IHs. Above-ground dry matter accumulation of J/IHs was 73.8% greater than that of IVHs during the grain filling period. The above-ground dry matter per spikelet of the J/IHs was 12.7 and 4.1% higher than that of their parents and IVHs, respectively, indicating that source limitation was not the cause of poor grain filling of J/IHs. Translocation of assimilates and remobilization of stored assimilates from the straw to the grains during the grain filling period in J/IHs was about 64%, which was significantly less than that of IVHs. At maturity, only 44.1% of [sup.14]C fed to the flag leaves of J/IHs was partitioned into grains and the rest remained in stems and leaves. Poor translocation and partitioning of assimilates to the grain of J/IHs resulted in low harvest index. The results suggest that poor transport of assimilates to grains account for poor grain filling of J/IHs.