Drought has been limiting the productivity of agricultural crops by increasing alarmingly in recent years. This situation has increased the importance of research on the determination of plants resistant to arid conditions. In this study, two cultivars (Yalova and STF-4) of Stevia rebaudiana Bertoni plant were investigated physiological and biochemical responses of under drought stress. Plants were grown in controlled greenhouse conditions for 3 months and then exposed to drought for 3 weeks. The plants harvested after three weeks were measured leaf length, osmotic potential, relative water content (RWC), chlorophyll fluorescence (Fv/Fm), proline amount, hydrogen peroxide (H2O2) amount and lipid peroxidation level. In both cultivars of S. rebaudiana plant, when compared to the plants in the control group under drought stress, while a decrease in leaf length was detected, the highest decrease was found in Yalova cultivar (25.7%). However, both cultivars maintained their water status under drought stress. While Fv/Fm value was not affected by drought in STF-4 cultivar, Yalova cultivar decreased compared to the control. In the amount of proline was a difference between the cultivars. While STF-4 cultivar under arid conditions no change was observed in the amount of proline, increase of 42.9% occurred in Yalova cultivar. On the other hand, drought stress significantly increased the lipid peroxidation level in leaves. While this increase was 41.2% in Yalova cultivar, it was 21.1% in STF-4 variety. Differential response to drought stress between the two cultivars, was observed in H2O2 content. The amount of H2O2 under drought stress, while decreased by 42.7% in Yalova cultivar it increased by 5.5% in STF-4 variety. As a result, in the light of the measured parameters the STF-4 cultivar of S. rebaudiana plant was revealed that more drought tolerant.
This study investigated the effects of drought stress on the physiological and biochemical responses of the medicinal and aromatic plant Hypericum perforatum (St. John's Wort). Changes were determined in leaf length, relative water content (RWC), osmotic potential, chlorophyll fluorescence (Fv/Fm), lipid peroxidation (TBARS), hydrogen peroxide (H2O2), and proline content as well as in the antioxidant system enzyme activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), and glutathione reductase (GR). These responses were examined in relation to the tolerance of drought stress in H. perforatum. Ninety-dayold seedlings were subjected to drought for three weeks. The physiological parameters of leaf length, RWC, Fv/Fm, and osmotic potential were reduced under drought. The H2O2, TBARS, and proline levels were increased significantly under drought stress. Moreover, the proline content increase was greatly pronounced (25.9-fold) compared to the control groups. The high accumulation of proline may have resulted from the 83.8% leaf RWC still remaining under drought stress. On the other hand, the SOD, CAT, and GR enzyme activities were enhanced, whereas the POX and APX activities were reduced. The results indicate that improved tolerance to drought stress in H. perforatum plants may be accomplished through increased capacity of the antioxidative defense system.
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