Study the effect of Epibrassinolide application on the activity of some antioxidant enzymes, nitrate reductase, and photosynthetic pigments of common bean under drought stress

Drought is one of the most important environmental stresses and is one of the most common causes of plant growth retardation and yields, and usually decreases the productivity of plants along with other environmental stresses, including salinity and heat. One of the reasons that environmental stresses such as drought, reduces growth and plant photosynthesis ability, is a disturbance in the balance between the production of free oxygen radicals and the protective mechanisms that remedy these radicals which results in the accumulation of reactive oxygen species (ROS), induction of oxidative stress, damage to proteins, membrane lipids, and other cellular components. Under adverse environmental conditions, the role of antioxidant defense system in protecting cellular membranes and other growing organs against oxidative damage seems to be very important. Brassinosteroids (BRs) comprise a group of steroidal hormones that have been implicated in a wide range of physiological responses in plants, including stem elongation, growth of pollen tubes, ethylene biosynthesis, proton pump activation, the regulation of gene expression, activation of enzymes, response to different stresses, nucleic acid and protein synthesis, and photosynthesis. In addition, BRs can protect against damage from stresses such as drought, salinity, and high temperature by activating various mechanisms in plants and increasing the activity of enzymatic antioxidants such as catalase, superoxide dismutase, peroxidase, and glutathione reductase. The purpose of this study was to investigate the possibility of increasing the activity of some antioxidant enzymes, nitrate reductase, photosynthetic pigments, and finally, the seed yield of common beans with the use of Epibrassinolide under drought stress conditions.

A split factorial experiment was conducted based on randomized complete block design with three replications at the research farm of Faculty of Agriculture, the University of Zanjan during the 2016-2017 cropping season. In this experiment, two irrigation conditions included optimal irrigation and drought stress were applied to main plots and two common bean genotypes including Kusha cultivar and COS16 genotype, and four levels of brassinosteroid including of no-application (control), two, four, and six μM were allocated to subplots as factorial. Drought stress was applied at the flowering stage, and common bean plants were sprayed with brassinosteroid (Epibrassinolide) simultaneously with drought stress. In this study, the activity of catalase, guaiacol peroxidase, ascorbate peroxidase, and superoxide dismutase, nitrate reductase, chlorophyll and carotenoid contents, and seed yield were studied.

The results showed that drought stress increased the activity of catalase, guaiacol peroxidase, ascorbate peroxidase, and superoxide dismutase by 38.89%, 84.09%, 40.46%, and 27.37% in contrast with the optimal irrigation, respectively. The highest activity of catalase, guaiacol peroxidase, ascorbate peroxidase, and superoxide dismutase were obtained using different concentrations of Epibrassinolide under drought stress conditions. The application of 2, 4, and 6 μM of Epibrassinolide in drought stress conditions increased by 73.33%, 86.67%, and 113.33% in the catalase activity, increased by 56.36%, 71.82%, and 62.73% in the guaiacol peroxidase activity, increased by 12.82%, 46.15%, and 13.46% in the ascorbate peroxidase activity, and increased by 29.15%, 41.49%, and 47.11% in the superoxide dismutase activity in comparison with non-application of this hormone. It has been reported that the use of BRs significantly improves plant drought tolerance and reduces the accumulation of reactive oxygen species by increasing the activity of antioxidant enzymes. It has been reported that the application of BRs increased the antioxidant enzymes activity in maize, tomato, mustard, soybean, and barley. Also, nitrate reductase activity increased by using of Epibrassinolide, which can enhance plant tolerance to environmental stress. The highest activity of nitrate reductase was obtained by application of 4 μM of Epibrassinolide, which did not show any significant difference with other concentrations. Improvement in the activity of nitrate reductase can be attributed to the effect of BRs on translation or transcription of nitrate reductase, or nitrate absorption at the membrane surface. In optimal irrigation conditions, the use of different concentrations of Epibrassinolide has a slight increase in the contents of chlorophyll a, b, and total, and the use of 4 μM of this hormone resulted in the highest increase in the above traits compared to non-application of the hormone. However, under drought stress conditions, this increase was significant, and the use of 2 μM of this hormone resulted in the highest increase in the above traits compared to non-application of the hormone. In other words, the use of Epibrassinolide in drought stress conditions caused a higher increase in chlorophyll a, b, and total contents relative to optimal irrigation conditions. Application of Epibrassinolide increased the seed yield in both common bean genotypes by increasing the activity of antioxidant enzymes, nitrate reductase, and chlorophyll and carotenoid contents. The highest seed yield was obtained by application of 2 μM of Epibrassinolide with an average of 2068.2 kg/ha. Among the studied genotypes, the Kusha cultivar in optimal irrigation conditions (with an average of 3025.45 kg/ha) had the highest seed yield and the COS16 genotype in drought stress conditions (with an average of 980.89 kg/ha) had the lowest seed yield.

In general, the use of Epibrassinolide can be suggested as a solution to increase drought stress tolerance and enhance the growth and seed yield of common beans under optimal irrigation and drought stress conditions. In addition, the achievement of comprehensive information on the positive effects of Epibrassinolide requires a study of this hormone in different weather conditions and with other different bean genotypes.