Differential cold stress duration and frequency treatment effects on muskmelon seedling and field growth and yield

In attempts to produce the earliest yields of spring melons, muskmelon fields are transplanted in late winter before the last frost date. Seedlings may then be repeatedly exposed to temperatures cycling between almost freezing and optimal temperatures for weeks until the weather stabilizes. To simulate this potential cold stress possible in the field, 'Athena' Cucumis melo L. (Reticulatus Group) muskmelon seedlings were subjected to 2+/-1 degreesC in a walk-in cooler then to 29+/-5 degreesC in a greenhouse before field transplanting. In 1997, the durations of cold exposure were 3, 6, and 9 hours which were repeated (frequency) for 1, 3, 6, and 9 days before transplanting. In 1998, duration levels were the same as in 1997 but frequencies were 3, 6, and 9 days. In both years, cold stress duration interacted with frequency, significantly reducing almost all seedling growth characteristics of muskmelon, indicating that these two stress factors are not acting independently and that they need to be considered together. Muskmelon flowering and vining were delayed significantly by increasing cold stress duration and frequency in both years but fruit set was delayed only in 1998. Muskmelon yields decreased with increasing cold stress frequency in both years but not with cold stress duration indicating that frequency was the driving factor behind the yield reduction. Therefore, muskmelon plants should not be planted early in the season without protective tunnels if there is a chance of repeated low temperature exposure.