• Rgn12

    Aromatic rices like Basmati and BR5 are strongly dormant. The dormancy provides protection against pre-harvest sprouting. The aroma and dormancy are important traits for breeding quality rice. In the previous studies, for the detection of aroma, either grain or leaf samples were used (Berner and Hoff 1986) while the dormancy was studied by analyzing seeds of F₁ plants (Mitra et al. 1975; Seshu and Sorrells 1986). The expression of dormancy is influenced by variations in environment, date of maturity, moisture content, and storage temperature of seeds. Therefore, this study was conducted in a uniform environment with control over the external factors. BR₅ a strongly dormant (SD) and scented (S) while AC3828 a non-dormant (ND) and non-scented (NS) parents along with their F₁, BC₁, BC₂ and F₂ plant generations were evaluated. Germination tests (GT) were conducted at 5, 35 and 55 days after harvest (DAH) to classify progenies for dormancy. For the detection of aroma, fresh leaf samples were treated with potassium hydroxide solution and then smelled to classify as S and NS progenies. The detailed procedures are given elsewhere (Das 1989).
    The results indicated that both the F₁ and BC₁ populations were NS while BC₂ showed 1S:1NS ratio. Also the F₂ segregation pattern (3NS : 1S) clearly indicated monogenic recessive nature of aroma in BR5. On the other hand, both the F₁ and BC₁ appeared weakly dormant (WD); responding as D at 5 DAH and ND at 35 DAH.
    Although the F₁ showed incomplete or partial dominance of dormancy, but the absence of transgressive segregants in F₂ indicated that the dormancy was not a polygenic trait. The F₂ segregated into an array of ND, weakly dormant (WD), moderately dormant (MD) and SD phenotypes, respectively, at 5, 35 and 55 DAH. Das (1989) observed that the WD and the MD types were conditioned, respectively, by the pericarp (monogenic dominant) and the hull (monogenic recessive), while the SD type was the cumulative product of both the factors. On this basis, the F₂ progenies (Table 1) were comprised of four possible phenotypes in the ratio of 3 ND:9WD: IMD:3SD showing an epistatic ratio of 13 D:3 ND at 5 DAH. Since the WD phenotypes appeared as ND at 35 DAH due to their termination of dormancy, the F₂ showed 3 ND: 1 D ratio at 35 DAH. But at 55 DAH only the SD phenotype remained dormant (D) showing 13 ND:3D ratio. The BC1 and the BC₂ segregation patterns further proved digenic (dominant and recessive) control of dormancy in BR₅ as revealed in the F₂ generation (Table 1).
    The joint segregation analyses (Table 1) both in BC₂ and F₂ showed non-significant chi-square (X²) value, indicating that the dormancy and the aroma are independent of each other. The F₂ showed significant value X² only at 35 DAH, because the segregants rated as D were actually comprised of the MD and SD, while the segregants rated as ND at 35 DAH were comprised of the ND and the WD phenotypes. The aroma and the dormancy can be readily combined as revealed from both the BC₂ and the F₂ populations (Table 1 ).

Table 1. The segregation patterns for dormancy and aroma at certain DAH and generations of

GT=germination test: DAH=days after harvest; D=Dormant; ND=Non-dormant; S=scented :
NS=non-scented; (a)=D and S: (b)=D and NS; (c)=ND and S; (d)=ND and NS; Seg.=Segregating; and
P=probability. The linkage between dormancy and a suitable marker needs to be established for proper identification of the
locus(s) for dormancy.

References

Bemer, D. K. and B. J. Hoff, 1986. Inheritance of scent in American long grain rice. Crop Sci. 26: 876-878.
Das, T. 1989. Inheritance of seed dormancy in rice. Ph.D. thesis. BRRI, Gazipur 1701, Bangladesh.
Mitra, A. K., D. K. Mukherji and P. Mukherji, 1975. Inheritance of dormancy in rice. SABRAO J. 7(2):
        197-200.
Seshu, D. V. and M. E. Sorrells. 1986, Genetic studies on seed dormancy in rice. IRRI (ed.). Rice Genetics,
        pp.369-382.

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