Ecophysiological and biochemical strategies of response to ozone in Mediterranean evergreen broadleaf species.

Three Mediterranean shrubs, Phillyrea latifolia L. (phillyrea), Arbutus unedo L. (strawberry tree), and Laurus nobilis L. (laurel), differing in their morphological and ecological response to water shortage, were exposed for 90 days to 0 or 110 ppb of ozone (O3), 5 h each day. This yielded an accumulated exposure over of a threshold of 40 ppb (AOT40) of 31.5 ppm h over the 3 months experiment. These species showed differing responses to O3: laurel and phillyrea developed foliar chlorotic mottles on the adaxial surface of leaves, whereas strawberry tree leaves showed reddish interveinal stipple-like necrotic lesions. In all cases, however, foliar injury did not exceed 8% of the sampled leaf area. At the end of the exposure period, O3-induced stomatal limitation caused significant decreases of net photosynthesis in strawberry tree and laurel, but not in phillyrea. The relative water content of the leaves was significantly decreased by O3, especially in laurel and strawberry tree, suggesting the occurrence of drought stress. Electrical conductivity of leachates from foliar discs increased in response to the treatment, much more strongly in laurel and in strawberry tree than in phillyrea, suggesting an O3-dependent alteration of the membrane retention capacity. At the end of the experimental period, the activity of superoxide dismutase and the content of reduced glutathione, but not that of reduced ascorbate, were significantly increased in the ozonated leaves of strawberry tree and phillyrea, but not in laurel. The evergreen broadleaves studied here maybe relatively tolerant to realistic O3 levels, at least in terms of visible injury and gas exchange. Such tolerance might overlap with their level of tolerance to drought stress. High constitutive levels, and/or O3-induced increases in antioxidants, might contribute to O3 tolerance in these Mediterranean evergreen broadleaf species.