Physiological effects of ozone on cultivars of watermelon (Citrullus lanatus) and muskmelon (Cucumis melo) widely grown in Spain

Two cultivars of watermelon (Citrullus lanatus) and muskmelon (Cucumis melo), which are widely grown in Spain, were exposed to ozone (70 nl litre(-1), 6 h d(-1)) for 21 days. Ozone sensitivity was assessed by recording the extent of visible injury, changes in fast-fluorescence kinetics, the relative-growth rate (R) of root (RR) and shoot (RS), and effects on the number of flowers produced per plant. Leaf gas exchange was measured in order to provide some indication of the factors underlying differential response to ozone. After 9-10 days of fumigation, all the cultivars developed typical visible symptoms of zone injury on the older leaves. However, significant (P < 0.05) changes in fast-fluorescence kinetics were detected prior to the development of visible foliar injury, indicating that detectable effects of ozone on primary photochemical processes supersede the appearance of visible symptoms. In both muskmelon and watermelon, there was a marked reduction in the rate of CO(2) assimilation as a result of exposure to ozone, and this was accompanied by a parallel decrease in stomatal conductance. Mean plant-relative-growth rate (R) was markedly (P < 0.01) suppressed by ozone in the two cultivars of watermelon, but there were no significant effects on R in muskmelon. Ozone reduced root growth relative to the shoot in three out of four cultivars-an effect that may be of considerable ecological significance. Moreover, exposure to ozone reduced flower production in both muskmelon and watermelon, which indicated effects on yield. There was no correlation between a variety of methods used to assess ozone sensitivity and visible injury, and reasons for this are discussed. This observation draws clear attention to the dangers in ranking plants for ozone sensitivity purely on the basis of visible symptoms. It is concluded from this study that ozone-insensitive genotypes should be identified and considered for planting in the major areas of melon production concentrated on the Mediterranean coast of Spain.