Determination of a suitable formula for the calculation of sum growing degree days in watermelon [Citrullus lanatus (Thunb.) Mansf.].

The purpose of this study was to explore the possibility of adaptation to watermelon of the temperature sum methods by the calculation of the sum growing degree days (SGDD), which was already used successfully in other crops. In the experiments, ten watermelon cultivars which express various degrees of earliness were used. These genotypes were sown at various dates and the experiments were carried out in 1990 and 1991. Variations in the daily temperatures were created under different growing conditions, i.e. greenhouse, high tunnel, low tunnel, and the open field. The duration of the period from sowing to harvesting dates, as well as temperatures encountered were recorded for all the experiments. All the total SGDDs were calculated by three different models using the temperatures of three daily maximums, i.e., 30, 32 and 35°C and four bases, i.e., 10, 13, 15 and 18°C, being 36 equations altogether. The F test was applied to the data obtained and CVs were calculated. The formula, i.e. SGDD =  (T max+T min)/2-Tb, [(T max+ T min)/2] > Tb, using 18°C as the base temperature (Tb) was determined to be the most suitable for the calculation of the SGDDs from sowing to harvesting dates in the watermelon compared to the rest.