Applying Thermal Time Scales to Sunflower Development

Received for publication July 6, 2004. Knowledge of sunflower (Helianthus annuus L.) development can support integrated pest management and cultural practices to achieve crop yield potential while reducing production costs. The objective of this research was to establish continuous, quantitative relationships among thermal time (two computation methods), photoperiod, and the progression of vegetative leaf appearance and reproductive development in sunflower. Empirical models were fit to phenostage observations for an oilseed hybrid during five planting periods; two analogous sets of coefficients corresponded to calculations of thermal time that assumed linear or optimized developmental responses to temperature. The resulting relationships were tested for predictive value by using similar observations for another oilseed and for a confection hybrid. Predictive accuracy ranged from 67 to 91% for leaf appearance and from 90 to 95% for reproductive phenostage. Field observations confirm earlier reports of long-day photoperiod response for thermal time requirements to bud-visible phenostage. Evidence for short-day response for thermal time to maturity was also detected. The relationships are consistent with recent published reports of sunflower development and are suitable for forecasting sunflower phenostages, given knowledge of thermal time requirements to R1 and R9 phenostages and photoperiod sensitivity.