USING DIF TO SHORTEN INTERNODES AND INCREASE HEAT TOLERANCE IN GREENHOUSE GROWN WATERMELONS (CITRULLUS LANATUS THUNB. 'BINKNA' AND 'KAMRO')

Abstract

Watermelon plants were grown for 30 days in all combinations of day and night temperatures of 25°C, 30°C, and 35°C; yielding 9 separate treatments with DIFs from -10 to +10. At harvest, plant internode length, dry weight, total chlorophyll, number of flowers and heat tolerance were measured. Dry weight accumulation was very sensitive to average daily temperature with plants grown at 35/30°C (DT/NT) being the most sensitive. However, within the same average daily temperatures, a negative DIF had no effect on total dry matter accumulation. A negative DIF did shorten the average internode length as compared to the plants with no DIF or plants grown under a positive DIF. Flowering of these cultivars appeared to be promoted by increasing day temperature, while high night temperature regimes of 35°C significantly reduced it as compared to 30°C night temperature regimes. Growth at a higher average daily temperature with DIF significantly increased the heat tolerance as measured by chlorophyll fluorescence. There was no significant difference in heat tolerance between negative DIF and positive DIF grown watermelon. However, these results suggest that watermelon morphology can be controlled by DIF treatment during the seedling stage. The shortening of distance between the first 14 internodes where fruits are set, will result in more compact plants which are easier to harvest.