Effects of Ozone on Tobacco (Nicotiana tabacum L.) Varieties

The well-known ozone bioindicator tobacco ( Nicotiana tabacum L.) variety Bel-W3 and six varieties cultivated in Greece were tested for their relative sensitivities to ozone by exposures conducted in controlled-environment chambers (0, 90, 135 and 180 p.p.b. for 20 days at 8 h day^?1). Based on the extent of macroscopic symptoms, Bel-W3 was found to be the most sensitive variety, followed by Zichnomirodata (KK6/5), while K63 was the most resistant. There was a differentiation between Bel-W3 and KK6/5, in terms of both magnitude and visual appearance of symptoms. Ozone reduced the plant height, leaf area and dry weights of leaves, stems and roots. The root dry weight reduction was greater than that of leaves and stems. Moreover, gross photosynthesis was reduced by ozone exposure, mainly in the lower leaves. The ozone-related sensitivity could not be attributed to greater stomatal density because (a) the most tolerant and the most resistant varieties had almost the same number of stomata per mm² and (b) the most resistant leaves (upper) had greater stomatal density than the most sensitive (lower) ones, even in the same plant. Stomatal conductance was greater in the abaxial surface (P < 0.001) than in the adaxial surface and in the upper leaves (P=0.013) than in the lower leaves, but it did not differ significantly between the Bel-W3 and K63 varieties (P=0.3417). The fumigation of grafted plants – Bel-W3 (ozone-sensitive) on K63 (ozone-resistant) and vice versa – suggested that stocks did not affect the ozone sensitivity of scions.