Critical Factors for Grain Filling in Low Grain‐Ripening Rice Cultivars

Abstract

Some improved rice (Oryza sativa L.) cultivars such as the new plant type (NPT) lines of IRRI or Japanese indica 3 japonica cultivars can attach a greater density of spikelets, and hence have higher yield potential (YP); however, low grain ripening means that their higher YP is not fully used. Our objective was to clarify why their grain ripening is lower than in other cultivars, and whether low assimilate supply to grains or insufficient sink capability of spikelets restricts grain ripening in these cases. IR65564-44-2-2 (NPT) and Akenohoshi (indica 3 japonica) cultivars yield lower percentages of ripened grain (PRG) and have lower spikelet filling percentages (F%) than other cultivars in field trials. When plant density was halved during the grain-filling period (GFP) to improve plant dry-matter increase (DMI), PRG and F% in Akenohoshi increased, but scarcely changed in IR65564-44-2-2 and other cultivars. Inferior assimilate supply to YP limited the PRG in most cultivars due to close relationships between PRG or F% and DMI/YP; however, in IR65564-44-2-2 increased DMI/YP did not improve PRG and F%. Among the higher YP cultivars, F% in IR65564-44-2-2 started to decrease earlier with accumulated temperature. When panicles detached during the early GFP were cultured in a complete nutrient medium for 1 wk, F% in cultured Akenohoshi and IR65564-44-2-2 was significantly greater than that under field conditions. These results suggest that inferior assimilate supply is the main cause of lower grain ripening in Akenohoshi, whereas the earlier decrease in grain ripening, probably due to higher spikelet nonfertilization, is the factor responsible in IR65564-44-2-2.

High Risk of the Formation of Milky White Rice Kernels in Cultivars with Higher Potential Grain Growth Rate under Elevated Temperatures

“…As a result, there was a clear cultivar difference in the rate of MWK among the five cultivars examined, although the rate in Nipponbare in 2006 was low due to the lower temperature in the grain-filling period (Table 1). The trend in cultivar differences of the rate of MWK coincides well with the results of the incubated temperature in 2005 should be appropriate to the estimation of the potential GIR more than in 2006, because the F% in diverse rice cultivars was the highest between 20 and 70% in the middle of the grain filling period (Kobata and Uemuki, 2004;Kobata et al, 2006) and the result of solution culture in 2005 well covered the range. However, in 2006 spike culture in several cultivars started from an F% much lower than 20% and hence these F% would be affected by lower rate in the fi rst phase of the grain-fi lling.…”

“…Between 0 and 7d after the start of culture, four panicles or one panicle per replication were harvested, dried in an oven at 80ºC for 48 h, divided into spikelets and other parts, and weighed. The grain dry matter increase was indicated as the filling percentage of grains (F%) because the cultivars used have different grain sizes (Kobata et al, 2006). The F% was defined as the ratio of the grain dry weight (G) to the potential grain weight (GP):…”

“…Based on these findings, we hypothesized that cultivars with a higher potential GIR under high temperature conditions are more sensitive to a high temperature than other cultivars in developing MWK. The potential GIR can be estimated using the spike culture system, with an adequate nutritional supply (Kobata et al, 2001;Kobata et al, 2006). Our objectives were to clarify the relationship between the rate of MWK and the potential GIR in rice cultivars differing in sensitivity to the high temperature.…”

Convenient Estimation of Unfertilized Grains in Rice

“…Unfertilized spikelets (%UFS) and %FLS occupied 64 and 68% of the un-ripened grains (100-%RG), respectively. Hence, the lower percentage of ripened grain in Takanari than in other cultivars (Kobata and Uemuki, 2006) resulted dominantly from unfertilized spikelets. (2) In different cultivars with diverse variations in spikelet sizes and weights The solution necessary to obtain unfertilized floating spikelets was 80% ethanol in Takanari but the optimal ethanol concentration for the other cultivars with different spikelet sizes and weights should be tested.…”

“…Sterility of rice in response to cool and high temperature conditions (Satake and Yoshida, 1978;Nishiyama, 1983;Matsui et al, 2001;Jagadish et al, 2007) or to desiccated soils (Kobata et al, 1994) would vary with the cultivar, and the cultivar difference is expected to be useful as a determining factor in the breeding of stressresistant cultivars. Even under non-stress conditions, in high-yielding rice cultivars bearing heavy spikelets such as some indica x japonica or new plant type rice cultivars, sterility would be an important yield determination factor (Kobata and Uemuki, 2006).…”