Pattern of antioxidant enzyme activities under drought stress and exogenous application of proline in sunflower

Drought stress disturbs metabolic pathways in plant cells that leads to increase in reactive oxygen species (ROS) abundance. Plants have high efficient defensive system that prevents oxidative stress and can eliminate free radicals. Therefore high activity of antioxidant enzymes is essential for drought stress tolerance. Knowledge of antioxidant enzymes function and variation can be suitable marker to identification of morphological characteristics linked to this enzyme activities in plant breeding programs under drought conditions.

In order to study the effect of drought stress and endogenous treatment of proline on the anti-oxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX), drought treatments (0, -4, -8 bar created by adding polyethylene glycol-6000) and 3 levels of proline (0,5, 10 mM) were applied on sunflower in seedling stage. This research was conducted in a factorial experiment based on completely randomized design, including 10 sunflower genotypes (in two susceptible and tolerant groups).

Based on the changes in proline content and shoot dry weight, two inbred lines BGK 329 and RGK 221 were identified as the most susceptible and most sensitive among 10 inbred lines. Then, stripe pattern of enzymes was studied in 7.5 percent horizontal acrylamide gel electrophoresis. Quantitate evaluation of activity of observed isozymes and statistical analysis of the data indicated that there is a significant difference among the activity of four enzymes SOD1, SOD2, CAT1, and POX1, out of all 7 enzymes. No significant change was observed in activity of isozyme SOD1 with increasing water stress and proline levels. Interactions of genotype× proline was significant (p<0.05) for POX1 and SOD2. Drought stress had significant effect on isozyme CAT1 and interaction of stress× genotype affected the activity of isozyme SOD2. While the activity of isozyme POX1 was significant at stress× proline interaction, the triple effect of triple stress× proline× genotype was significant (p<0.01) in the activity of isozyme POX1. SOD2 activity was largely influenced by the genotype so that at different levels of stress and proline, some genotypes increased and some others showed decreased activity. The exogenous 5mM proline treatment reduced the activity of isozyme POX1 and CAT1 under stress. Data analysis based on two susceptible and tolerant sunflower groups under drought stress showed that only POX1 activity was significant between the two groups and therefore.

Proline as an important amino acid induces drought stress tolerance. An efficient antioxidant defensive system with inducible expression system to increase the accumulation of proline have a vital role in the tolerance of drought stress environments. Isozyme of POX could be suggested to identify drought tolerant drought genotypes in seedling growth stage.