HEADING DISTURBANCE BY LONG-DAYLENGTH TRE ATMENT DURING REPRODUCTIVE STAGE IN PHOTOPERIODSENSITIVE RICE

The Lm or Se-1 on the rice chromosome 6 is a principal locus controlli ng heading time of native and improved rice varieties (Yokoo et al. 19 80), and its photoperiod-sensitivity has been intensively studied unde r appropriate cycles of short-daylength (Yokoo and Kikuchi 1982, Yokoo et al. 1982) . This report deals with the effects of long-daylength c ycles on heading time of a rice line with the photoperiod-sensitive al lele Lm(u). SL and SE are near-isogenic lines which were developed fro m a cross between a Malaysian late-heading variety ''Morak Sepilai'' a nd a Japanese early-heading variety ''Fujisaka 5'' with successive bac kcrossing by Fujisaka 5 for four generations and heterozygote maintena nce for 10 more generations on the Lm locus.SL with the late-heading a llele Lm(u) headed 105 days after sowing and 18 days later than SE wit h the early-heading allele Lm(e) under the natural daylength condition s, while SL headed 13 days earlier than SE under the 9-hour, daylength (Table 1). This reverse order of heading in the two lines was due to the nature of Lm that controls the degree of photoperiod-sensitivity a nd the duration of basic vegetative growth (Yokoo and Kikuchi 1982). S L plants were first grown under the 9-hour day-length and then exposed to various cycles of 24-hour daylength at three panicle growth stages (Fig. 1). When plants were treated by one to five cycles of 24-hour d aylength from the 26th and the 33rd day after sowing, they headed a li ttle later than the control and had short or loose panicles with a few er spikelets. More than seven cycles at these two stages and any cycle s given from the 40th day caused the developed panicles to non-heading and other tillers to later heading., Since panicle initiation was con sidered to occur about 30 days before heading (Table 2), the three tim es when 24-hour daylength treatments started seemed to be the early, t he medium and the late stages of panicle development, respectively. Th e present results revealed that a few cycles of 24-hour daylength may cause panicles malformation at the early and medium stages, and more c ycles at these two stages and any cycles at the late stage may inhibit the internodes from elongating and the developed panicles from emergi ng.