Watermelon Seedling Quality, Growth, and Development as Affected by Grafting and Chilling Exposure During Simulated Transportation

Abstract

Grafted watermelon plants available in the United States are typically transported for a long distance from a specialized nursery to the production field. To investigate the effects of chilling stress during transportation on the early plant growth and development, grafted and nongrafted ‘Tri-X-313’ seedless watermelon (Citrullus lanatus) seedlings were subjected to low-temperature treatments applied over a 72-hour period. The first experiment exposed grafted and nongrafted seedlings to 0, 6, 12, 24, or 48 hours of 1 °C chilling, and then were moved to a 12 °C growth chamber for the remainder of the chilling treatment period. The second experiment exposed nongrafted seedlings to seven different combinations of chilling duration (0, 24, 32, 41, 44, or 48 hours) to create varied chilling degree hours (CDH) at different temperatures (between −0.4 °C and 1.2 °C). After 72 hours, seedlings were transplanted in pots filled with a commercial substrate in a greenhouse to evaluate the early plant growth and floral development. Each experiment had two repeats (spring and summer) with a randomized complete block design (n = 10). Although greater exposure to chilling negatively affected visual quality and photosynthetic capacity [measured by chlorophyll fluorescence parameter, variable fluorescence/maximum fluorescence (Fv/Fm)] in both repeats, delay in flowering after transplanting was significant in spring only and increased with increasing CDH (up to 6 days with 48 hours of 1 °C exposure). Grafting was found to mitigate the degree of flowering delay when the same chilling exposure was applied. When chilling temperatures were varied, visual damage of leaves, decrease in Fv/Fm, and delays in female flower development were best correlated with CDH at a base temperature of 15 °C, 3 °C, and 4 °C, respectively. Our experiments and further analyses with available literature data suggest that 50 to 70 CDH4 [CDH with base temperature (BT) = 4 °C] seems to be a critical threshold to cause significant delay in female flower development (3.5 days for grafted and 1.3 days for nongrafted plants). Therefore, if temperatures lower than 4 °C are expected during transportation of seedlings, we suggest mitigation measures be taken so that CDH4 do not reach greater than 50 degree hours.