Rate of Natural Outcrossing In Watermelon as Affected by In-Row Spacing

Watermelon (Citrullus lanatus) is a cross-pollinated crop. Estimation of natural outcrossing rate is important to plant breeders to determine the minimum isolation distance required for seed increase and to design a suitable breeding strategy for crop improvement. There is little inbreeding depression in watermelon, indicating a lack of dominance variance, and possibly a high rate of self-pollination. Hence, the objective of this study was to determine the natural outcrossing rate in watermelon to determine the distance that pollen can be transferred by honeybee (Apis mellifera). The experiment was a split plot in a randomized complete block design with 7 replications. Whole plots were 2 locations (Kinston and Clinton, NC), sub-plots were the 8 in-row spacing treatments. Spacing treatments were 0.610, 1.219, 1.829, 2.438, 3.048, 3.658, 4.267, and 4.877 m. Sub-sub plots were 2 cultivars (‘Allsweet' and ‘Mickylee'). The cultivar ‘Moon and Stars' was used as a pollenizer. ‘Moon and Stars' has bright yellow spots on leaves, fruit and cotyledon due to the Sp gene that is dominant to uniform color. Analysis of variance showed significant differences in natural outcrossing rate due to in-row spacing. Natural outcrossing rate was low (< 1.8%) where a wide in-row spacing (> 4.267 m) was used. At any spacing greater than 4.267 m, natural outcrossing rate was low. On the other hand, close in-row spacing had a significantly higher rate of natural outcrossing. A high rate of self-pollination can be achieved in watermelon plants trained in a tight spiral and spaced more than 4.267 m (14 ft) apart.