Modeling of sorghum response to photoperiod : a threshod-hyperbolic approach

High photoperiod sensitivity is a singular trait for adaptation of sorghum to environmental constraints in sudano-sahelian West Africa. Difficulties encountered by selected models such as CERES-sorghum and STICS to simulate crop development may result from the representation of sorghum response to daylength during the photoperiod inductive phase. Four modeling approaches combining two temperature and photoperiod responses (linear, hyperbolic) and two calculation methods for development rates (cumulative, threshold) were evaluated to simulate time to panicle initiation (PI) in highly photoperiod sensitive Guinea sorghum variety CSM388. In the cumulative method, development rates were computed as summations of daily photothermal ratios, whereas in the threshold method accumulated degree days were tested against thermal time requirement to PI modulated by current photoperiod. Each model was calibrated based on observations from a Sotuba, Mali (12°39'N) planting date experiment spanning a 2-month period in 1996. Observed time from emergence to PI decreased from 54 to 22 days for a 20 min variation in daylength. Apparent higher performance by threshold methods was further tested against a 1994 independent dataset featuring three latitudes and a much wider range of sowing dates extending from February to September. Results validate the superiority of threshold over cumulative methods and confirm the better fit of a hyperbolic temperature and photoperiod response. A threshold-hyperbolic modeling approach is believed to be more consistent with crop physiology as it associates cumulative (temperature) processes and trigger (photoperiod) events that better reflect the concepts of quantitative plant growth and qualitative plant development. Its mathematical form and computational simplicity should ensure wide applicability for varietal screening over a large range of photoperiod sensitivities including neutral cultivars, and easy implementation into existing models. (Résumé d'auteur