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-insensitive phases of development to panicle emergence were estimated in four contrasting indica cultivars of rice (Oryza sativa L.) in a reciprocal-transfer experiment. Plants were grown in pots in glasshouses maintained at warmer (32/26 °C) or cooler (28/20 °C) day/night temperatures, and the durations from sowing to panicle emergence were determined for plants moved from relatively short (11 h) to relatively long (13.5 h) days and vice versa at various times after sowing. Panicle emergence was delayed by long days in all cultivars, but the traditional cvs Carreon and Peta were much more sensitive to photoperiod than the modern cvs IR8 and IR36 The durations of the photoperiod-insensitive pre-inductive phase (equivalent to some definitions of the basic vegetative phase) varied from 14.4 d in cv. Carreon at 32/26 °C to 42.0 d in cv. IR8 at 28/20 °C. In all cultivars this initial phase was of a longer duration in the cool than in the warm regime. The duration of the photoperiod-insensitive post-inductive phase was also consistently greater, but usually only slight so, at cool than relatively warm temperatures; it varied from 6.8 d in cv. IR8 at 32/26 °C to 272 d in cv. Carreon at 28/20 °C. As expected, the length of the intervening photoperiod-sensitive inductive phase was greater in long days, but the effect of temperature on these durations was not consistent; for example, these durations were longer in warm than in cool temperatures in cv. 1R8 but, if anything, they were slightly longer in cool than in warm temperatures in cv. IR36. This difference is compatible with previous findings that cv. IR36 has a warmer optimum temperature for rate of progress towards panicle emergence than cv. IR8. A subsequent reciprocal-transfer experiment with cv. Peta provided estimates of the durations of the photoperiod-insensitive and photoperiod-sensitive phases of pre-flowering development which were compatible with our earlier estimates. Furthermore, panicle initiation was found to occur after about 80% of the photoperiod-sensitive inductive phase had elapsed. We conclude that although the duration of the photoperiod-insensitive pre-inductive phase in rice is greater than in many other annual crops, genotypic variation in 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 not e