Monton POONYARIT, D. J. MACKILL and B. S. VERGARA
International Rice Research Institute, P. O. Box 933, Manila, Philippines
The inheritance of photoperiod sensitivity and critical daylength was studied in four Indica rices (Table 1). The parents, F1 and F2 of the six crosses were planted under natural daylength on April 6-7, 1986, at Los Banos, Philippines (14°N latitude).
A bimodal distribution in F2 progenies was observed in all crosses except PR2/SN, using different cut-off points depending upon the distribution (Table 2). F2 progeny of IR26760-27/NS19 showed a good fit to a 9: 7 ratio (late: early) (Table 2). The F2 of the crosses of PR2 or SN with NS 19 or IR26760-27 showed a good fit to the ratio 3:1 (late: early) (Table 2). The F2 progeny of crosses with NS19 were later flowering compared to the progeny of crosses with IR26760-27. It seems that all the F2 Plants of crosses with NS 19 were photoperiod-sensitive and the differences among these genotypes were due to critical daylength.
Two genes that confer photoperiod sensitivity, one dominant (Se-1) and another recessive (se-2), have been identified in rice (Yu and Yao 1968; Yokoo and Fujimaki 1971; Yokoo et al. 1980). Since the photoperiod sensitivity gene Se-1 is closely linked with the isozyme gene Pgi-2 (Kinoshita 1986), the Pgi-2 alleles were used to determine if any of the genes conferring photoperiod sensitivity were allelic to Se-1. Results indicated that PR2, SN and NS19 all had a dominant gene conferring sensitivity at the Se-1 locus. IR26760-27 has a recessive allele at this locus, but has a dominant gene at another locus independent of Se-1 which also delays flowering. Since NS19 has the recessive allele at the same locus, here designated as se-3(t), it flowers much earlier than PR2 or SN. It appears that Se-3(t), when present with the dominant allele at Se-1, confers strong photoperiod sensitivity and short critical daylength. When tillers of F2 plants of PR2/NS19 and SN/NS19 divided during the basic vegetative phase were treated under 5 photoperiods, a bimodal distribution was observed (Fig. 1). The F2 distribution showed a good fit to a 3: 1 ratio indicating that this dominant gene conferred short critical daylength.
Yokoo and Kikuchi (1977) found evidence that different alleles of the Se-1 (Lm) locus control flowering date. A study by Li (1970) suggested that a single gene controlled critical daylength. In the recent study, a single gene independent of Se-1, designated as Se-3(t), affected critical daylength. lt is not clear if this gene is the same as E1, which also affects photoperiod sensitivity and is independent of Se-1 (Yamagata et al. 1986).
Acknowledgement. The authors express their gratitude to Drs. E. Guiderdoni and J. C. Glaszmann, and Mr. B. de los Reyes for helpful comments and assistance in the isozyme studies.
Table 1.Parents used in genetic study of photoperiod-sensitivity ________________________________________________________________ PSP2 Regression Proposed Parent Origin Duration1 (days) Coefficient3 genotype ________________________________________________________________ IR26760 -27 IRRI 145 75 -0.12** se-1se-1Se-3(t)Se-3(t) Nam Sa- gui 19 Thailand 122 114 -0.26** Se-1Se-1se-3(t)se-3(t) Puang Rai 2 Thailand 193 >135 -0.64** Se-1Se-1Se-3(t)Se-3(t) Sac Nau Vietnam 190 >141 -0.57** Se-1Se-1Se-3(t)Se-3(t) ________________________________________________________________1Seeded 18 June 1986.
2 Photoperiod sensitive phase measured as the difference in days to flower under 14 hour photoperiod and the days to flower under 10 hour photoperiod.
3 Regression of flowering duration on seeding dates for 11 dates of plating (May 14-Oct.17) IRRI.
** Significant at the 1% level of probability.
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