Abstract:Middle indica cultivars are planted in the middle to lower regions of the Yangtze River. Hybrid combinations with these cultivars have a high yield potential but the presence of late-transgressive-segregants limits the further exploitation of heterosis. To understand the genetic basis of the heading date in these middle maturing cultivars, we carried out a genetic analysis of 10 typical middle-season cultivars using a number of heading date isogenic lines under both long and short day conditions. The results showed that Teqing, 752, CDR22, Bo B, 9311 and II-32B carry two photoperiod sensitive genes E1 or Se-1 and E3, and Teqing, 752 and CDR22 carry a dominant early-heading gene Ef-1 while Bo B, 9311 and II-32B carry a recessive late-heading gene ef-1. Based on the findings of the present and previous works on Guichao 2, Minghui 63, Nanjing 11 and Pei‘ai64S, it was concluded that all of these middle indica cultivars carried the recessive allele hd2, which could inhibit the expression of E1 or Se-1, and they formed rational combinations of genotype for heading date during a long period of evolution. The effects of the different combinations of genotype for heading date on rice cultivation and extension were discussed, and genetic basis of broad adaptability of hybrid middle indica cultivars was analyzed.