Seed filling determines seed vigour of superior and inferior spikelets during hybrid rice (Oryza sativa) seed production

Growth and development of spikelets are asynchronous during hybrid rice seed production. However, knowledge is limited about the variation in seed vigour between superior and inferior spikelets. The main objective of this study was to evaluate the differences in seed vigour between superior and inferior spikelets of hybrid rice, and to explore the mechanisms associated with such variations with regards to seed filling characteristics and starch accumulation. Field experiments were conducted in 2017 and 2018, and seed vigour parameters, seed filling characteristics and starch accumulation were determined. We found that significant differences were recorded in seed vigour parameters between superior and inferior spikelets. Germination percentage (GP) and vigour index (VI) of superior spikelets were higher than that of inferior spikelets, irrespective of year or variety. Moreover, significant differences were observed in seed filling characteristics and starch accumulation between superior and inferior spikelets. Lower total starch content, amylose content and amylopectin content were recorded in inferior spikelets with low seed filling rate and long seed filling duration. Superior spikelets with high seed filling rate and short seed filling duration increased final seed weight by 23.9 and 22.3% for each variety, respectively. Therefore, these results raise the possibility that seed filling influences the starch accumulation and seed weight, and is closely associated with differences in the vigour of seeds from superior and inferior spikelets during hybrid rice seed production.