EVALUATION OF COMPONENTS OF OXIDATIVE STRESS METABOLISM FOR USE IN SELECTION OF DROUGHT TOLERANT CULTIVARS OF NICOTIANA-TABACUM-L

Effect of drought stress on lipid peroxidation, chlorophyll stability and several antioxidant enzyme activities was evaluated under controll ed environmental conditions, for possible use as drought tolerance sel ection criteria, in four tobacco (Nicotiana tabacum L.) cultivars of d ifferent, but known, drought tolerance. A progressive highly significa nt (p <0.01), but differential (in correlation with their individual d rought tolerance), increase in glutathione reductase (EC 1.6.4.2) acti vity was detected in all four cultivars as their leaf water potential (Psi(L)) decreased. In spite of an initial lag (up to a Psi(L) of -1.5 MPa), on average, the superoxide dismutase (SOD, EC 1.15.1.1) activit y of the drought-tolerant cultivars increased by 244 % while that of t he drought-sensitive cultivars only increased by 161 %. Contrary to th e other enzyme activities monitored, only a moderate increase in catal ase (EC 1.11.1.6) activity (p <0.05) relative to their respective cont rols was observed in all four cultivars. Increased ascorbate peroxidas e (EC 1.11.1.7) activity was not only observed to be ca. 300-400% high er (p <0.01 already at a Psi(L) of -1.5 MPa) in the drought-tolerant c ultivars under stress, but was also more pronounced than the increase in catalase activity. This indicated that ascorbate peroxidase rather than catalase might be mainly responsible for scavenging drought-stres s produced H2O2. On reaching a Psi(L) of -2.5 MPa, the glutathione red uctase activity of the drought-sensitive cultivars increased by only 1 59 % (TL33) and 187 % (CDL28), opposed to the 233 % (GS46) and 250 % ( ELSOMA) in the drought-tolerant cultivars. A differential (significant ly higher in the drought-tolerant cultivars at a Psi(L) of -2.5 MPa) d rought stress-induced increase in the level of lipid peroxidation occu rred in all cultivars which decreased faster in the drought-tolerant c ultivars upon rehydration. The latter was also observed for catalase a nd glutathione reductase activity, but of which the activity returned to levels comparable with that of their respective controls. The level s of SOD and ascorbate peroxidase activity on the other hand remained higher (p <0.01) than their respective controls after rehydration, but did not differ statistically significantly between the respective cul tivars. No destruction of chlorophyll, due to photoperoxidation was de tected in any cultivar, as reflected by a stable chlorophyll a/b ratio . These results are discussed in relation to the potential drought tol erance adaptive advantage of an effective antioxidant system. In this regard the use of monitoring the capacity to increase ascorbate peroxi dase activity and/or glutathione reductase activity as possible drough t tolerance selection criteria in tobacco is advocated.