Air temperature is one of the most important environmental factors that drive development in watermelon. Thermal time is usually used for representing the effect of the air temperature on plant development because it is a simple method and a better time descriptor than calendar days or days after sowing. The objective of this study was to calculate the thermal time of three developmental phases in watermelon using different methods. A field experiment was carried out in Santa Maria, RS, Brazil, with two sowing dates during the 2006-2007 growing seasons (09/05/2006 and 09/21/2006). The cultivar 'Crimson Sweet' was used and the experimental design was a complete randomized block with six replications. Developmental phases were sowing-emergence (SO-EM), emergence-flowering (EM-FL) and flowering-harvest (FL-HA). Daily degree-days (DDD, °C day) were calculated with three methods using the cardinal temperatures of 10°C, 33°C and 42°C. Thermal time (TT, °C day) during the three developmental phases was calculated by accumulating DDD. There was no difference in the duration, in TT, for the total cycle and the developmental phases among the three methods of thermal time used. The duration (in TT) of the EM-FL phase was similar (417°C day) in the two sowing dates, but the TT of the SO-EM and FL-HA phases varied with sowing date from 98 to 130°C day and from 770 to 840°C day, respectively.
Citrullus lanatus; thermal time; degree-days; cardinal temperatures
