Abstract
Under the hypothesis that environmental responses of ontogenetic development of rice are different before and after panicle initiation (PI), a model for simulating rice panicle development and heading date was synthesized. In this model, the values of the developmental index (DVI) which is a continuous variable representing rice development stage are defined to be 0 at the emergence, 1.0 at Pl and 2.0 at the heading, respectively. Under this defining condition of DVI, we determined, for each developmental phase before and after Pl, the values of parameters of the equation giving the developmental rate (DVR) as a function of daily mean temperature and photoperiod, by using the Simplex method and rice phenology data. Field experiments on three cultivars of Nipponbare, Koshihikari and IR36, in Kyoto over four years using a range of sowing dates in each year, provided data for the parameter estimation and model validation. The values of parameters thus obtained suggested that DVR responses to temperature and photoperiod are different between the vegetative and reproductive development phases. The model explained dates of PI and heading of the three cultivars more accurately than the previous model, in which DVR responses to the environment were assumed to be unchanged before and after the PI. A close correspondence was observed between the DVI values calculated by the model and the morphological development stage of rice panicle, irrespective of environmental conditions and cultivars, suggesting that the model is applicable for the prediction of the panicle development from environmental conditions.
