Limited yield advantage of early-season rice hybrids over inbreds in middle-lower reaches of the Yangtze River

Early-season rice plays a significant role in double rice-cropping system in China to improve the grain yield per unit of area. But recently, the stagnation of grain yield increase has been witnessed for early-season rice varieties in middle-lower reaches of the Yangtze River, particular in the national rice regional adaption trials of southern China. Further, in the early cropping season, hybrid rice did not always perform better than inbred rice in grain production. It is relevant to question whether yield ceilings have been reached in early-season indica rice before a new breakthrough on the germplasm exploitation. Field experiments were conducted to evaluate the yield performance of five inbreds and their 12 derived hybrid rice varieties in 2017 and 2018 in Hangzhou, China. Limited yield advantage of early-season hybrids over inbreds was observed, commonly less than 5.00% and averaged as 0.62% in 2017 and 3.20% in 2018. The hybrids always produced more panicles per plant but less grains per panicle with similar growing periods compared with the inbreds. Then, the genetic distance of the hybrid parents was surveyed using 48 selected molecular markers, showing low polymorphisms. Further, they were genotyped for four critical quantitative trait loci for both yield traits and heading date and three ones only for yield traits. So did 60 major early-season indica rice varieties at the four pleiotropic loci. Evidence was shown for the high uniform of their haplotype combinations across the tested materials, which could lead to early heading and low yield potential. Our work revealed the low genetic diversity, or called genetic bottleneck, for early-season hybrids to further improve grain yield potential. Moreover, along with the popularization of low-input requirements, the inbred rice varieties of cheap seed price might be a more suitable alternative for this cropping season in middle-lower reaches of the Yangtze River.