Photothermal zoning of castor (Ricinus communis L.) growing season in the semi-arid Mediterranean area

Abstract Castor (Ricinus communis L.) is an oilseed crop holding great promise for semi-arid environments. In regions where minimum temperature during winter is high enough to allow plants to survive, it would be worth to explore the adaptability of the species by performing autumn sowings, thus enabling rainfed cropping systems. A two-year field experiment under different sowing times (from November to June) was conducted in Southern Italy using castor Hy. Hazera. Autumnal sowings extended the plant vegetative period; nevertheless, flowering was simultaneous with all sowing dates. Only with the latest spring sowing (that of June) both floral induction and seed ripening were postponed. Seed yield with January and February sowings was the highest (3.92 Mg/ha on average). Using the wide variability of temperature and photoperiod conditions of the different growing periods, thermal and photothermal sums to predict the length of the main phenological intervals were calculated using different models. Overall, the best predicting model was that using sums of thermal units from minimum temperatures > 8 °C and maximum temperatures of 30 °C minus the degrees exceeding 30 °C, multiplied by photoperiod. The model allowed to develop zonation maps predicting the latest sowing date and the earliest seed ripening date in Sicily for autumn-spring and spring-summer growing season. The zonation procedure indicates that castor can be sown in autumn in coastal areas, while spring sowings are more suited to colder inland and hilly areas of Sicily. The work might find practical application to individuate suitable Mediterranean areas to grow castor in winter sowings.