Varietal differences in sink production and grain-filling ability in recently developed high-yielding rice (Oryza sativa L.) varieties in Japan

Abstract

In Japan, the rice production oriented toward use for flour and animal feed to substitute for imported wheat flour and maize has been promoting, and many high-yielding varieties (HYVs) for the above purpose have been developed and released recently. In order to establish cultivation methods for newly released high-yielding varieties (HYVs) and to develop breeding targets for further increases in yield, evaluation of their yield potential and growth characteristics is essential. The objective of this study was to evaluate yield potential and the constraints of the recently developed HYVs in Japan. Field trials were conducted for two years using five Japanese varieties including reference variety Nipponbare (NIP); japonica-dominant HYVs Bekoaoba (BEKO) and Momiroman (MOMI); indica-dominant HYVs Takanari (TAKA) and Hokuriku 193 (H193). We compared the sink capacity (total number of spikelet per unit area×a thousand grain (brown rice) weight) and grain-filling ability in HYVs by the analysis of the characteristics related to them. The hulled grain yields of the japonica-dominant HYVs were 9–23% greater, and those of indica-dominant HYVs were 24–34% greater than that of reference variety NIP, respectively. The main factor for the high yield was remarkably large sink capacity in HYVs due to the high sink production efficiency per unit of absorbed nitrogen (SPE). Although all HYVs showed high SPE, the yields showed varietal differences, in that the yields of japonica-dominant HYVs were lower than those of indica-dominant HYVs. The yield constraint was the grain-filling ability, which indicated larger difference especially at high sink capacity. In the present study, three factors were suggested to be involved with grain-filling. First one is dry matter production during the ripening period, in that the CGRs of the indica-dominant varieties were greater during this period, especially the CGR of H193, which achieved the highest yield and had the highest CGR. Second one is the unfertilized spikelets due to the large number of spikelets per panicle. In MOMI, the relatively high percentage of unfertilized spikelets on secondary rachis-branches on the lower part of the panicle can explain the lower grain-filling. On the other hand, the grain-filling ability of another japonica-dominant variety BEKO was superior to that of MOMI. The better grain-filling was attributed to the lower spikelet number and lower ratio of spikelets on secondary rachis-branches, which may be advantageous for a variety with large grain size. Another factor is the mobilization of the non-structural carbohydrate (NSC). The net decreased amount of NSC (difference of the maximum and minimum accumulated NSC in the post-heading) of H193 was largest among used varieties. However, in the case of TAKA which indicated high grain-filling ability as well as H193, the net amount of translocated NSC was similar to those of japonica-dominant HYVs. On the other hand, both of the indica-dominant HYVs showed an earlier decrease of NSC and a larger increase of panicle weight at earlier ripening period. From these results, the rapid translocation of NSC during the ripening period was assumed to contribute to the high grain-filling ability in indica-dominant HYVs. These results indicate that the improvement of grain-filling by the stimulation of source activity and translocation of NSC is important for japonica-dominant varieties, and the improvement of sink production efficiency would result in the increase of the yield potential in the indica-dominant varieties.