Durations of the Photoperiod-sensitive and Photoperiod-insensitive Phases of Development to Flowering in Four Cultivars of Rice (Oryza sativa L.)

The durations of the photoperiod-sensitive and photoperiod-insensitivephases of development to panicle emergence were estimated infour contrasting indica cultivars of rice (Oryza sativa L.)in a reciprocal-transfer experiment. Plants were grown in potsin glasshouses maintained at warmer (32/26 C) or cooler (28/20C) day/night temperatures, and the durations from sowing topanicle emergence were determined for plants moved from relativelyshort (11 h) to relatively long (13.5 h) days and vice versaat various times after sowing. Panicle emergence was delayedby long days in all cultivars, but the traditional cvs Carreonand Peta were much more sensitive to photoperiod than the moderncvs IR8 and IR36 The durations of the photoperiod-insensitivepre-inductive phase (equivalent to some definitions of the basicvegetative phase) varied from 14.4 d in cv. Carreon at 32/26C to 42.0 d in cv. IR8 at 28/20 C. In all cultivars this initialphase was of a longer duration in the cool than in the warmregime. The duration of the photoperiod-insensitive post-inductivephase was also consistently greater, but usually only slightso, at cool than relatively warm temperatures; it varied from6.8 d in cv. IR8 at 32/26 C to 272 d in cv. Carreon at 28/20C. As expected, the length of the intervening photoperiod-sensitiveinductive phase was greater in long days, but the effect oftemperature on these durations was not consistent; for example,these durations were longer in warm than in cool temperaturesin cv. 1R8 but, if anything, they were slightly longer in coolthan in warm temperatures in cv. IR36. This difference is compatiblewith previous findings that cv. IR36 has a warmer optimum temperaturefor rate of progress towards panicle emergence than cv. IR8.A subsequent reciprocal-transfer experiment with cv. Peta providedestimates of the durations of the photoperiod-insensitive andphotoperiod-sensitive phases of pre-flowering development whichwere compatible with our earlier estimates. Furthermore, panicleinitiation was found to occur after about 80% of the photoperiod-sensitiveinductive phase had elapsed. We conclude that although the durationof the photoperiod-insensitive pre-inductive phase in rice isgreater than in many other annual crops, genotypic variationin this duration may well be less than was previously deduced.We also conclude that, despite common assumptions to the contrary,photoperiod-sensitivity during rice plant development does notend at panicle initiation. Oryza sativa L., rice, panicle initiation, panicle emergence, photoperiodism, temperature