Abstract
An Egyptian cultivar of tomato (Lycopersicon esculentum Mill. cv. Baladey) was exposed to 68 nl l(-1) ozone (O-3) for 8 h d(-1) for 75 days in the open-top chambers at Braunschweig, Germany. Half of the plants were either fully irrigated or were subjected to a controlled drought stress treatment. O-3 exposure caused visible injuries on the foliage of the tomato plants and decreased pigment content in the leaves. Drought stress alone had no effect on these parameters, but reduced the extent of foliar injury under O-3 stress. Photosynthetic rate (A) and stomatal conductance (g(s)) of the plants were measured at regular intervals from the time when O-3 and drought stress treatment were imposed. It was found that drought stress or O-3 alone caused a reduction in A by 21 and 17 %, respectively, while exposure to both stresses resulted in the highest reduction in A. In contrast, O-3 caused a mean increase in g(s) of 18 % in well-watered plants, while drought stress decreased g(s) by 22 %, compared to the well-watered control treatment. This stimulating effect of O-3 On g(s) was also seen in the combination with drought stress. Both O-3 and drought stress significantly suppressed biomass and fruit yield. Yield reductions by O-3 and drought stress alone were 48 and 29 %, respectively. How ever,suppression of yield was greatest in plants exposed to the combination of O-3 and drought stress. The results indicated that 'Baladey' tomato is particular sensitive to O-3 exposures. It is suggested that this sensitivity may even increase under water deficit conditions.