Photothermal adaptation of sorghum (Sorghum bicolour) in Nigeria

Sorghum is an important crop of the seasonally dry savannas of West Africa adapted to growing periods of <100 to >200 days. Locally adapted cvs flower at the end of the rains irrespective of sowing date. Research reported here; (i) reanalysed a sowing date experiment planted at Samaru, Nigeria by Kassam and Andrews in the early seventies to test whether phenology and homeostasis of flowering date in sorghum can be explained by a photothermal model; and (ii) investigated phenological adaptation in Nigeria at four locations between 8 and 13°N by simulating using a photothermal model the duration from sowing to flowering of genotypes originating from latitudes between 6 and 14°N in West Africa using photoperiod and 20 years of daily mean temperature and rainfall data. Phenology was separated into four phases: pre-inductive or juvenile; panicle initiation to flowering; and flowering to maturity, all modulated by temperature; and an inductive phase, modulated by both temperature and photoperiod. The duration of the inductive phase was the major determinant of variation in duration from sowing to maturity. Cultivar SK5912 sown by Kassam and Andrews was acutely sensitive to photoperiod and the thermal duration of the inductive phase was increased by 2115 growing degree days (GDD)/h photoperiod when mean photoperiod is >13 h. The simulations explained how flowering is timed to occur shortly before the end of the rains at the latitudes of cultivar origin, irrespective of sowing date.