In this review we analyze the research results about biochemical, physiological and productive effects of ozone pollution on vegetation, with particular attention to vegetable and fruit crops. In Italy, ozone tropospheric concentrations are very high from April to September and overcome the phytotoxicity threshold (40 ppb). The daily trend is well correlated with temperature trend (i.e. the daily peaks are in the first afternoon). Ozone penetrates into leaf tissues through stomata, then stimulates the synthesis of antioxidant metabolites and finally damages cell membranes. Main physiological consequences are the decrease in stomatal conductance, photosynthetic rate and assimilate partitioning. In Mediterranean area, the typical environmental stresses (drought and salinity) can cause cross resistance. A lot of researches have pointed out that a moderate water stress, reducing stomatal conductance just during the central hours of the day (i.e. when the ozone concentrations are higher), is able to reduce plant responses to ozone because of the lower uptake of ozone into leaf tissues. In several cases water stressed plants resulted quite resistant to this pollutant. Similarly, soil salinity, reducing soil water potential, also reduces root water uptake, thus causing water deficit. Therefore, plants respond also to salinity by reducing stomatal conductance that reduces ozone uptake by plants. Furthermore, both these stresses increase antioxidant synthesis, thus improving scavenging and detoxifying capacity of plants. As regards researches about horticultural and fruit crops, strong genotypic differences have been found in different species. Tomato: yield losses ranged from 0 (cv San Marzano) to 30% (cv Oxheart); the ozone resistance was related to a higher antioxidant content; irrigation with saline water reduced ozone effects. Potato: yield losses were from 5% (cv Bintje) to 44% (cv Desiree), mainly as regards higher size class of tubers; organic fertilization was able to avoid ozone effects. Leaf vegetables: the damage can be very high, since necrotic spots on leaves could make non-marketable the entire yield; yield losses were from 11 to 50% in relation to species. Beans: yield losses were from 15 (cv Groffy) to 41% (cv Borlotto nano lingua di fuoco); commercial antitranspirants were able to reduce ozone injuries. Fruit trees: ozone was reported to reduce photosynthesis and chlorophyll b levels and to increase dark respiration, but such physiological disturbance did not result in yield differences but only in an early ripening, possibly related to an O3 stimulated ethylene metabolism. This possibility was confirmed by a decreased vegetative strength observed in the year following the O3 treatment. This means that O3 effects on tree crops must be analyzed in terms of both short- and longterm damages since they may be delayed over time, as a consequence of cumulated effects. Shelf life: ozone improved preservability of a lot of products thanks to antimicrobic action (mushrooms), ethylene scavenging (apple, pear), antioxidant content increase (strawberry, tomato), protection against Botrytis cinerea (strawberry, tomato, grape, plum).
Keywords: O3, cross resistance, stomata conductance, antioxidants, yield, quality