Phenology of sunflower cultivars. III. Models for prediction in field environments

Models of the daily rate of development for the stages emergence to head visible and head visible to first anthesis were formed for two sunflower cultivars (Helianthus annuus cvv. Sunfola 68-2 and Hysun 30). The models relate rate of development to photoperiod, daily mean temperature and relative cultivar sensitivity to photoperiod. They were derived from controlled-environment and field studies reported in the first two papers of this series. The two cultivars were found not to differ in sensitivity to temperature. The base temperature for development was found to decrease as the life cycle progressed. The models were validated on an independent data set and are applicable to the entire Australian continent, with one possible limitation on the temperature range at long photoperiods (14-18 h). The relationship of these models to the cultivar groups defined in the first paper of this series is discussed and a rapid method of determining relative cultivar sensitivity factors for new cultivars is outlined. The application of the models in planning for avoidance of frost by selecting the time of planting and the cultivar to be planted was demonstrated for three major sunflower-growing districts in Queensland. The possible use of these phenology models in conjunction with growth simulation models for studies of crop adaptation is discussed