K.H. tsai

Department of Agronomy, National Chung-Hsing University, Taichung, 40227 Taiwan

Genetic analyses for heading behavior have been carried out in the genetic-background of a rice variety, Taichung 65 (abbr. T65), which is a representative Japonica (Ponlai) rice cultivar in Taiwan. The early heading gene, Efl, (previous symbol was Ef-1) located on chromosome 10 (Sato et al. 1988), is known to be present in various cultivars and wild strains, and T65 has its recessive allele, ef1 (previously called e₁) (Tsai 1985; 1986b, c 1987: 1992; 1995; 1996; Tsai and Oka 1966). The different Ef1 (abbr. E₁) alleles were considered to be isoalleles (Tsai 1976; Tsai and Oka 1968). Among E₁ alleles, the Efl-a (E₁^a) was derived from a Northern Chinese variety, Tatung-tsailai (Ttg). A carrier of E^a₁ (20) E^a, moves up floral initiation and heading about 10 days as compared with T65 of the e₁ carrier (Tsai and Oka 1965; 1968). At the m-Efl locus, a recessive allele, m-Efl (m), and a dominant allele, m⁺carried by T65, were found. The m locus belongs to the 7th chromosome (Tsai 1986a). The m alleles were obtained from two donors, Ttg and Bozu 5 (from Hokkaido), and they were named as m^a in line (7) m^a and m^b in (7)m^b, respectively.

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When combined with e₁ the m genes had no effect in the second crop season, but moved up the heading time a few days in the first crop season (Tsai and Oka 1970). The main effect of m to emphasize the heading advancing effects of the E₁ alleles which is called the E₁-m epistasis (Tsai 1980). The lines with E₁ and m^a (line Em) were for 20 to 10 days earlier heading than those with E^a and m⁺ ((20)E^a ). The E₁-m epistasis was intensified under high temperature when planted in May. The two donors, Ttg and Bozu 5 both having E₁ and m genes in common, had very short heading days when planted in the second crop season. This suggests that varieties grown in high latitude areas have a high temperature response. The recessive gene, ef2 (e₂) retarding the heading time of T65 for about 20 days, was obtained from an induced late-heading mutant of T65, and its dominant

Table 1. Changes in genic effects on heading days according to crop seasons,

Research Notes 71

allele, Ef2 (E₂), was carried by T65 (Tsai 1986b, 1990). The e₂ locus is independent of both E₁ and m loci. The heading time of plants with e₁ e₂ (2) e₂ was much delayed than that of e₁ (T65) when planted in May or June under long-days. When e₂, was combined with e₁, m, e₂ was semiepistatic to e₁, m, it was hypostatic to E₁m indicated the nonallelic interaction between the e₂ and m genes which differed according to the presence of E₁, or e₁ (Tsai 1986d).

The explanation mentioned above is supported by the experimental results shown in Table 1. For testing the photoperiodic responses on heading genes, T65 and its different near-isogenic lines with established genotypes shown in Table 1, were seeded on May 23 to the natural field (N plot) and short-day (S plot, a 9h photoperiod treated from transplanting to heading) plots, respectively, to observe their heading variations. T65 (e₁) showed a 9.7% of shortening rate of heading days for the S-plot compared with the N-plot, which can be regarded as the scale of photoperiodic response. But, the line (7) m^a had no such response. A line (2) e; with the late heading gene, e₁, showed a shortening rate of heading days similar to that of T65. The line e₁ me₂ advanced the shortening rate to 20.9% that must have been caused by a nonallelic interaction between m and e₂ genes.

As for the E₁ gene, it increased the heading advancing effect strongly in the S plot compared with its other experiments. It also showed a high photoperiodic response. A line E₁ m had the same heading days in both N and S plots showing no photoperidoic response (Table 1). Genes e₂ and E₁ compensated each other in the first and the second crop seasons, but e₂ was epistatic to E₁ in the N plot. In the S plot, E₁ e₂ line had the heading days similar to that of E₁ line indicated that there were an epistatic effect of E₁ gene to e₂ gene. The behavior of line E₁ me₂ was also similar to that did in the E₁ line on heading days. The three lines, E₁m, E₁e₂ and E₁me₂ had the similar heading days as that had in E₁-line in the S plot. These results showed that E₁ gene had a strong epistatic effects to cover the actions of m, e₂ and me₂, respectively, under the short-day conditions. For convenience, in this note there are no descriptions concerned with the interaction between day-lengths and temperatures that affects the heading times.

(Editor's comment: E₁, e₁, etc used in the present note are the abbreviations of Ef1, ef1, etc, and not identical to E₁, e₁, used by Okumoto et al. (1992))

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