Ultrastructural response of a Mediterranean shrub species to O3

Mediterranean vegetation (especially evergreen shrubs) is regarded as resistant to ozone (O-3) because of its avoidance mechanisms (summer stomatal closure, low stomatal conductance, low gaseous exchange rates, slow growth). Nevertheless, visible symptoms of ozone damage were observed on some species (Arbutus unedo, Pistacia lentiscus) both in natural and controlled conditions. This paper reports the results of a survey on the ultrastructural characterization of the visible symptoms (red stippling) occurring on strawberry trees (Arbutus unedo) in controlled conditions (fumigation chambers), i.e., treated with a chronic exposure to two realistic O-3 doses (50 and 100 ppb for 21 days, 5 h day(-1)) on recently mature leaves. The findings, obtained with light and transmission electron microscopy, show that the most important responses are in plants treated with 100 ppb O-3, and are localized in the epidermis-cuticle complex. The thickness of the cuticle increases and changes also occur in the reticular structure of its lower layer. In the epidermal cells of 100 ppb O-3-treated leaves, tannins embed the outer primary wall. In the mesophyll, we observe the alteration of tannins contained in the vacuoles. Results suggest that Arbutus unedo increases its defenses as an active response to ozone stress.