Abstract:

Changes in stoma density and stomatal conductance of 10 tobacco (Nicotiana tabacum) genotypes and its relationships to weather-fleck caused by ozone were studied in controlled field experiments in 2001 and 2002. Under controlled growing conditions in 2002, changes in the activities of 3 antioxidant enzymes in leaves of 5 genotypes were analyzed in relation to the degree of ozone injury following a combined treatment of ozone and low temperatures (10 ℃). The results showed that there were significant differences in leaf resistance to ozone injury among genotypes. Stoma density and stomatal conductance on the lower epidermis of tobacco leaves were related to weather-flecks caused by ozone. The lower leaves on the plants were especially sensitive to ozone injury. Correlation coefficients between stoma density and weather-flecks and between stomatal conductance and weather-flecks were 0.65 -0.68 and 0.80-0.87, respectively. Therefore, it is proposed that stoma density and stomatal conductance of the lower leaves on tobacco plant be used as a selection marker for disease-resistance breeding. The results of controlled experiments showed that ozone combined with low temperatures of 10 ℃ induced weather-flecks in leaves of 5 flue-cured tobacco genotypes. The activity of SOD increased in ozone-injured leaves and increasing SOD activity was negatively correlated to the weather-fleck index. Activity of CAT was slightly enhanced but activity of POD decreased remarkably. Activity of antioxidant enzymes was low and hence weather-fleck injury was great in tobacco genotypes with little resistance to ozone, whereas when the activity of antioxidant enzymes was high in genotypes, there was strong resistance to ozone injury. The results indicated that ozone combined with low temperatures of 10 ℃ markedly affected active oxygen-scavenging systems in tobacco leaves caused by an imbalance in activities of 3 antioxidant enzymes. Sharp decreases in POD activity might be one of the major reasons why leaves were injured by ozone, whereas increases in the activities both of SOD and CAT helped to protect against ozone injury.