Developmental consequences of cold temperature stress at transplanting on seedling and field growth aad yield. I. Watermelon

Watermelon (Citrullus lanatus (Thunb,) Matsum. and Nakai) seedlings transpl anted before the last frost date may be exposed to temperatures alternating between freezing and optimal until field temperatures finally stabilize. C old stress may ultimately reduce growth and yield. To simulate such tempera ture alternations that occur naturally after field transplanting, diploid ' Carnival' watermelon seedlings were exposed immediately before field planti ng to cyclic cold temperature stress at 2 +/- 1 degreesC then transferred t o a greenhouse at 29 +/- 5 degreesC. In 1997, transplants were exposed to 2 degreesC from 3 to 81 hours and in 1998, exposure ranged from 9 to 81 hour s. Cold-stressed seedlings were field planted after all potential risk of c old stress in the field had passed. In 1997, cold stress decreased seedling shoot and root fresh and dry weights, leaf area, chlorophyll and carbohydr ate contents but not seedling height. In 1998, all seedling growth variable s decreased in response to longer durations of cold stress. Plants cold str essed for up to 81 hours transpired more for I week after transplanting tha n those exposed to shorter periods of cold stress. In both years, vining (d ate first runner touched the ground), flowering, and fruit set were delayed significantly as cold stress hours increased. Although early yields were u naffected, total yields decreased linearly in both years with increasing ho urs of cold, with 38 to 40 hours of cold stress reducing yield 10% in both years. Data indicate that 'Carnival' watermelon transplants exposed to cold stress soon after transplanting may suffer yield reductions.