PHOTOTHERMAL RESPONSES OF FLOWERING IN RICE (ORYZA-SATIVA)

Durations from sowing to panicle emergence in 16 diverse genotypes of rice (Oryza sativa L.) were recorded in 13 different photothermal regimes, comprising constant and diurnally alternating temperatures between 16 and 32 °C and photoperiods between 10.5 and 15.0 h d-1-all provided by controlled-environment growth cabinets. In 11.5 h days and at sub-optimal temperatures, relations between the rate of progress towards panicle emergence and mean temperature were linear in all genotypes, and amongst these the base temperature at that photoperiod varied between 6.6 and 11.9 °C. In most cases progress was most rapid at 24-26 °C, i.e. the optimum temperature was much cooler than expected from previously published values of times to panicle emergence in a less extensive range of photothermal regimes. Only in three cultivars was it warmer than 28 °C, and in these there were sufficient data to establish that relations between rates of progress to panicle emergence and photoperiod in the diurnally alternating temperature regime of 28-20 °C are also linear. Also, the responses of these three cultivars provide no evidence of any interaction between the effects of photoperiod and temperature. We conclude, then, that the model in which rate of development is a linear function of both temperature and photoperiod with no interaction, which has been shown to be common to many other species, also applies to rice. Differences among genotypes in relative sensitivity of rate of progress towards panicle emergence to both temperature and to photoperiod were considerable; japonica cultivars tended to be more sensitive to temperature and less sensitive to photoperiod than indica cultivars. Four indica cultivars bred and selected at The International Rice Research Institute (IRRI) in the Philippines did not differ (P > 0.10) in their relations between rate of progress towards panicle emergence and sub-optimal temperatures in a daylength of 11.5 h, but the optimum temperature for cv. IR 36 was appreciably warmer than that for the cvs IR 5, IR 8 and IR 42. © 1992 Annals of Botany Company.