The role of salicylic acid and proline treatment on induction of antioxidant enzyme activities and salt tolerance responses of soybean (Glycine max L.)

Introduction Salinity is of vital importance to present day agriculture, as rapid population growth especially in the developing world and consequently increased demand for agricultural products have made salinity oriented problems urgent. Salt stress reduces crop growth and yield in different ways. These primary effects of salinity stress causes secondary effects like reduced cell expansion, assimilate production and membrane function, as well as decreased cytosolic metabolism and production of reactive oxygen intermediates (ROS). Soybean is an important grain legume. The unique chemical composition of soybean has made it one of the most valuable agronomic crops worldwide. Its protein has great potential as a major source of dietary protein. In soybean, salinity stress inhibits seed germination and seedling growth, reduces nodulation, and decreases biomass accumulation and yield. Hormones are the biochemical language of living systems. Plant hormones have been defined as low molecular weight organic compounds governing physiological responses within plants, but distant from the sites of their synthesis. Exogenously applied, salicylic acid elicits several different physiological responses to stress and therefore increase plant resistance. For instance, SA was reported to improve resistance against salt strss in plant. Salinity stress reduced growth and protein content in Oryza sativa. This effect was, however, significantly reversed when proline was exogenously supplied. Proline (Pro) functions as compatible solutes are up-regulated in plants under abiotic stress. They play an osmoprotective role in physiological responses. Furthermore, the salinity-induced inhibition of the antioxidative enzymes catalase and peroxidase was significantly overcome in Oryza sativa when proline was supplied exogenously. Exogenous proline application, besides enhancing the activity of antioxidative enzymes (CAT, POX and SOD), is also known to enhance the activity of other enzymes. Materials and methods The present experiment was aimed to investigate the improvement of salt tolerance in soybean by exogenous application of proline and salicylic acid as split plot arrangement in a randomized completely design in box with four replication in College of Agriculture, Shahrekord University in 2016. Main plot included three levels irrigation by saline water; 0 (control), 50 and 100 mM NaCl) and sub plot in four level of foliar applied (10 mM proline, in combination with 10 mM proline + 3 mM salicylic acid, 3 mM salicylic acid and sprayed with water (control)). Data were analyzed using SAS program and treatment means were compared by using L. S. D at 5% level. Results and discussion The results revealed that proline content, leaf POX, CAT and APX activities, H2O2, MDA contents, height and dry matter were significantly affected by salt level, proline and salicylic acid applications and their interactions. The results revealed that effects exogenous application of proline and salicylic acid were significant increase in activities of antioxidant enzymes, height and dry matter of soybean under salt stress. Furthermore, 100 mM NaCl with proline and salicylic acid application decreased malondialdehyde and peroxide hydrogen 23 and 25% compared to water, respectively. So, interaction effects showed height and dry matter increased by proline and salicylic acid application 32 and 38% under 100 mM NaCl compared to water, respectively. Also, the activities of CAT, POX, and APX in most cases increased under salt stress and proline with salicylic acid application as compared to the 0 mM NaCl and without proline and salicylic acid. Therefore, these findings showed that the exogenous application of proline and salicylic acid has a vital role in the increase activation of antioxidant enzymes and resistance of soybean to salt stress. So in some works, here have been positive correlations between content of proline and salicylic acid application and increasing activity of CAT, APX and POX enzymes to tolerance of plant to salt stress. The present study, therefore, suggests that exogenous proline and salicylic acid improved tolerance to salt stress in soybean by increasing antioxidant defense system and decreasing membrane lipid peroxide.