Rice is a major crop, and its heading date is a critical trait for adaptation to different cultivation areas and cropping seasons. Photoperiod sensitivity (PS) and basic vegetative growth are the major determinants of heading date in rice. Several major genes controlling these traits have been identified in rice, including Ef-1 (early flowering) (Yamagata et al. 1986), E₁-E₃ and Se-1 (Ichitani et al. 1997, 1998). In particular, E₁ and Se-1 have proven to be the most common alleles controlling heading time (Yamagata et al. 1986, Ichitani et al. 1997, 1998). The recessive inhibitor of PS i-Se-1 confers early heading even under long days. Due to the ubiquitous presence of i-Se-1, most early indica rice cultivars exhibit no PS even if they carry PS genes (Ohshima et al. 1993, 1994, Luo et al. 2002).

We employed three groups of genetic stocks: (1) tester lines EG0-7 (Yamagata et al. 1986; Ichitani et al. 1998), ER~LR and T65-T65m (Yamagata et al. 1986), (2) heading time near isogenic lines Nipponbare Hd1-Hd4 (the Hd genes were extracted from the cultivar Kasalath (Yano et al. 1997) and (3) 10 mid-indica materials from the region of the middle and lower of Yangzi River in China(Table 1). These cultivars was crossed with the testers in 2002, and a sample of the resulting F₁ seed was grown in Hainan Province in winter 2003 to generate F₂ populations.

F₁ (Table 1) and derived F₂ results showed that Teqing, 752, CDR22, Nanjing 11, Minghui 63, Guichao 2, Pei'ai64S and II-32B all carried dominant photoperiod sensitive gene E₁ whereas II-32B ( Xu et al. 2006). Guichao 2 and Bo B carried the dominant Se-1ⁿ. In addition, all tested cultivars carried the dominant early heading gene Ef-1 at the Ef-1 locus with the exception of II-32B and Bo B. These photoperiod sensitive genes (E₁ or Se-1ⁿ) in middle indica cultivars might ensure a desirable basic vegetative growth phase for rice plants and an abundant photosynthesis accumulation for high yield and quality. While all these indica type cultivars of middle season rice plant in the middle-lower regions of the Yangtze river carried the recessive allele hd2 which inhibits the expression of E₁ or Se-1 to prevent late heading and enable hybrid rice to normally mature. Therefore, these cultivars, such as Minghui 63, Nanjing 11, Teqing, 752 and CDR-22(E₁Ef-1hd2), showing weak photoperiod sensitivity and short basic vegetative phase, can be planted in Provinces Fujian, Jiangxi, Anhui and Guangxi as late indica cultivars. Other cultivars, such as II-32B and 9311(Se-1ⁿef-1hd2), showing weak photoperiod sensitivity and long basic vegetative phase, can only be planted as middle indica cultivars. Furthermore, the reasonable recombination of these genes might possibly result in a high yield and proper growth duration rice type regarding to specific environments.

The presence of PS genes could ensure a desirable duration of vegetative growth in hybrid rice, while the inhibitor gene for PS could prevent late-heading to enable hybrid rice to mature normally. The definitude of these types rice cultivars is useful to the propagation of hybrid rice and further selecting combinations with strong heterosis.

References

Yamagata H, Y. Okumoto and T. Tanisaka, 1986. Analysis of genes controlling heading time in Japanese rice. In: Rice Genetics. Manila, International Rice Research Institute: 351-359.

Ichitani K, Y. Okumoto and T. Tanisaka, 1997. Photoperiod sensitivity gene of Se-1 locus found in photoperiod insensitive rice cultivars of the northern limit region of rice cultivation. Breeding Sci. 47: 145-152.

Ichitani K., Y. Okumoto and T. Tanisaka, 1998, Genetic analysis of the rice cultivar Kasalath with special reference to two photoperiod sensitivity loci E₁ and Se-1. Breeding Sci. 48: 51-57.

Ohshima I., Y. Watanabe and C. Asahic, 1993, Genetic analysis of heading time in cross between two Indica varieties with two inhibitor genes for photoperiod sensitivity. Jpn. J. Breed 43: 101-106.

Xu J. F., L. Jiang, X. J. Wei, J. M. Wan. 2006. Genotyping the heading date of male-sterile rice line II-32A. J. Integrative Plant Biol. 48(4): 440-446.

Yano M., Y. Harushima, Y. Nagamura et al., 1997. Identification of quantitative trait loci controlling heading date in rice using a high-density linkage map. Theor. Appl. Genet. 95: 1025-1032.