Effects of growth regulators on enzymatic and non-enzymatic antioxidants in leaves of two contrasting wheat cultivars under water stress

Manipulating the levels of antioxidative defense system may affect leaf senescence and increase yield. Phytohormones, reactive oxygen species, and antioxidants interact in the regulation of plant metabolism. Therefore, the aim of this study was to find out whether the exogenous applications of BAP (a synthetic Cytokinin) and ABA to wheat cultivars subjected to water stress could influence the rate of H2O2, MDA (Malondialdehyde), and enzymatic and non-enzymatic antioxidative defense systems. For this purpose, a factorial experiment based on randomized complete block design was carried out with three replications. The experiment was performed on two Iranian wheat cultivars (Pishtaz and Karaj3), under water stress, using hormones. Plants were also exposed to water stress after anthesis with daily pot weighing. BAP and ABA were applied at the stage of initial grain filling, starting 2 days post anthesis. Water stress resulted in reduction of total Chl and AsA (Total Ascorbate) in both cultivars. The reduction was lower in the tolerant cultivar Pishtaz. In addition, water stress led to an enhancement in MDA, H2O2, Car (Carotenoid), alpha-tocopherol, the activity of CAT (Catalase), and POX (Peroxidase) and APX (Ascorbate Peroxidase) enzymes. The tolerance of Pishtaz cultivar under water stress is associated with low MDA and H2O2 contents as well as high Chl (Chlorophyll), Car, alpha-tocopherol, and AsA contents, which are closely related to the activity of its enzymatic antioxidants (including CAT, POX, and APX). BAP application alleviated oxidative stress (H2O2 and MDA contents) in both cultivars. However, the response was higher in the tolerant cultivar Pishtaz than that in Karaj3. Our results showed that ABA application resulted in an increase in the content of H2O2 and MDA, and the activities of CAT, POX, and APX enzymes; moreover, it enhanced the contents of non-enzymatic antioxidants such as AsA, Car, and alpha-tocopherol. Consequently, it was found that hormones regulate leaf senescence mainly by modulating H2O2 through different mechanisms such as enzymatic antioxidant and non-enzymatic antioxidant defense systems.