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Sensitivity of Watermelon Cultigens to Ambient Ozone in North Carolina

Foliar ozone (O₃) injury is common on watermelon grown in eastern North Carolina. During the 2000 and 2001 growing seasons, 93 cultivars and breeding lines (i.e., cultigens) were evaluated for foliar injury caused by ambient levels of O₃. One tetraploid, 42 diploid, and 50 triploid cultigens were evaluated. Injury developed on all cultigens in both years, but severity of injury varied among cultigens and between years. Visible foliar injury (percent surface area necrotic or chlorotic) was rated 1 week after the first harvest in both years. Mean injury for all cultigens was 39% in 2000 (range = 16 to 66%) and 20% in 2001 (range = 2.5 to 60%). This corresponded to greater mean seasonal O₃ levels (12-h [0900 to 2100 h EST] mean daily O₃ concentration between early May and mid-June) in 2000 (58 ppb) than in 2001 (52 ppb). Triploid cultigens were less sensitive to injury than diploid cultigens. Mean injury for all triploids was 31% in 2000 and 16% in 2001, whereas injury for diploids was 47% in 2000 and 25% in 2001. Injury level was consistently negatively correlated with average number of days between transplanting and harvest. Correlations between injury level and yield, fruit size, fruit shape, and fruit sweetness were inconsistent between years and/or insignificant. O₃ tolerance in watermelon cultivars should receive increased attention in watermelon breeding programs and in cultivar selection in areas of high O₃ pollution.