Transplant quality and growth of grafted and non-grafted watermelon seedlings as affected by chilling during simulated long-distance transportation

Grafting of fruiting vegetable crops is quickly gaining traction in the United States as a means for disease suppression and crop performance improvement. Producers typically purchase grafted seedlings from specialized nurseries, which may be distantly located due to limited availability. During long-distance shipment, plants can be exposed to low temperature as a source of injury that reduces transplant quality, including visual quality and growth capacity. However, limited information is available regarding effects of low temperature on grafted vegetable seedlings and their performance after transplanting. In this preliminary study, watermelon (Citrullus lanatus), a most chilling sensitive member of the cucurbitaceous family, was tested. 'SSX-8585' seeded watermelon seedlings were grafted onto 'Strong Tosa' interspecific hybrid squash (Cucurbita maxima × C. moschata) rootstocks. Grafted and non-grafted seedlings were subjected to a 72-h shipping temperature simulation of 12°C, within which seedlings were exposed to 3°C for 0-48 h. Seedlings were rated for visual damage on true leaves before the 72-h treatments, and then 24 h after treatment. Regardless of grafting, watermelon seedlings showed only minor necrosis after 24 h of treatments and were considered tolerant to an exposure of 3°C up to 48 h. This temperature is lower than previously reported thresholds (12-15°C) for growing or storing watermelon plants without damages. Seedlings were transplanted to individual pots filled with substrate in a greenhouse for assessing post-transplanting growth. Nodal positions of flower development were only affected by grafting, and not by chilling. Regardless of grafting or exposure to chilling temperatures, no significant differences were observed in days to first male and female flower opening. Our results suggest that watermelon seedlings can be shipped under moderately cold conditions (3-12°C) for 2-3 days without causing notable impact on transplant quality.