PHOTOPERIODICALLY SENSITIVE AND INSENSITIVE PHASES OF PREFLOWERING DEVELOPMENT IN RICE

The time interval between sowing and flowering in rice (Oryza sativa L .) comprises three successive phases: a basic vegetative phase (BVP), a photoperiod-sensitive phase (PSP), and a post-PSP phase (PPP). The o bjective of this study was to estimate the length of the three phases in diverse rice cultivars to provide the basis for accurately predicti ng rice flowering dates. In greenhouse experiments, plants of 20 culti vars were transferred from short day (SD) (10 h d(-1)) to long day (LD ) (12.5 or 14 h d(-1)) or from LD to SD at various times after sowing. The duration of BVP varied greatly among cultivars ranging from 16.7 to 45.4 d. The indica cultivars exhibited an apparently longer BVP tha n the japonica cultivars. The duration of PPP also varied among cultiv ars, from 18.0 to 37.2 d. For all cultivars, the length of the interve ning PSP was shorter under SD conditions (3.6-24.1 d) than under LD co nditions (10.5-76.5 d); the difference in PSP between SD and LD depend ed on the photoperiod sensitivity of the cultivar. The PSP under both SD and LD conditions did not necessarily end at panicle initiation (PI ) but, on average, 4 to 5 d after PI. This study confirms the necessit y to divide the entire preflowering period into the three phases for m odeling purposes. Further studies are needed to elucidate the phenomen on in some cultivars of a significant delay of flowering by early SD-t o-LD transfers before PI.