Abstract
Cultivars with different spikelet weight were compared with the ripening characteristics of primary and secondary spikelets at different positions in panicle and discussed the relation of the ripening process and dry matter production. The objectives were to know the variations of grain filling process of both primary and secondary spikelets among different grain size cultivars. Second, to find out the main ripening period (MRP, the accumulation period from 15.9-84.1% of the maximum grain weight) with the rate of grain filling and which factor between these two are important for the determination of final grain weight. The MRP of different grain size cultivars groups varied from 14-21 (small), 15-21 (medium), and 16-21 d (large) . The dry weight accumulation rate of a spikelet (DWAR-S) during MRP ranged 0.404-0.720, 0.624-0.981 and 0.703-1.297 mg/spikelet/d in small, medium and large grain cultivars, respectively. Among all cultivars MRP of primary and secondary branch spikelets were 13-16 and 14-23 d where DWAR-S were 0.672-1.544 and 0.334-0.939 mg/spikelet/d, respectively. The larger spikelet weight was accompanied with the faster rate of accumulation and a slower rate enhances the MRP. The relations between DWAR-S during MRP and a final spikelet dry weight showed significant positive correlation (r=0.834***) but no significant correlation was observed between the days of MRP and the final spikelet dry weight (FSDW) . Further, the temperature and radiation showed insignificant relationship with DWAR-S during MRP and for the same temperature the DWAR-S varied much among the cultivars which indicated that the FSDW was controlled mainly genetically with a partial impact of environment. It could be concluded that the large grain cultivars gained the higher spikelet weight at a faster rate of accumulation and not to the long period of accumulation.
