Effect of Salt Stress on Physiological Traits and Antioxidant Enzymes Activity of Chickpea (Cicer arietinum L. cv. Azad)

Introduction Salinity is an important abiotic stress that reduces the crop production potential and the efficiency of plants in arid and semi-arid regions. Soil salinity can be increased by irrigation, inappropriate drainage, sea advancement to coastal regions and the accumulation of salts in desert and semi-desert regions. Salinity is a limiting factor for plant growth because it limits the feeding of the plants through reducing the uptake of P, K, nitrate and Ca and increasing inter-cellular ion concentration and osmotic stress. In addition to its adverse impacts on the yield and yield components of crops, salinity affects most processes involved in the growth and development of the plants too.Materials and Methods The present study was conducted in research greenhouse of Department of Agriculture, Birjand University in 2013 on the basis of a Randomized Complete Block Design with four replications. The soil salinity treatments included five levels of 1, 3, 5, 7 and 9 dSm-1. Soil texture was loam-sandy with the pH of 8.09 and EC of 1 dS m-1. The irrigation water was filtered with EC < 350 μS cm-1. The salinity was applied in accordance with soil saturation moisture percentage and field capacity moisture percentage. NaCl was used as the source of salinity. Irrigation was applied by daily weighing of pots in terms of field capacity moisture percentage. At early flowering stage, chlorophyll index (SPAD) of the leaves, leaf relative water content, electrolyte leakage and the activities of antioxidant enzymes catalase, superoxide dismutase, and ascorbate peroxidase were measured. After full yellowing the plants, grain yield per plant was recorded.Results and Discussion It was found that salinity level significantly influenced all measured traits. Salinity slightly increased SPAD index and adversely affected leaf relative water content (RWC), electrolytes leakage and grain yield, so that the highest level of salinity resulted in 17.7% lower RWC, 27.7% higher electrolytes leakage, and 59.8% lower grain yield per plant. Under salinity stress, plants reduce their leaf area to counteract the stress resulting in greater thickness of the leaves, the accumulation of more chloroplast per unit leaf area and the increase in leaf chlorophyll content. As a result, the leaves turn darker and SPAD index increases. Leaf relative water content is one of the photosynthesis-related indices in crops with is closely related to the yield. Membrane stability is a physiological trait that is influenced by environmental stresses. Higher accumulation of hydrogen peroxide and lipid peroxides due to salinity reduces membrane stability. Salinity stress causes the production of active oxygen species which is followed with the leakage of cellular membrane. One effect of salinity is on grain yield through changing 1000-grain weight. Lower 1000-grain weight can be associated with shorter grain filling period in salinity treatments and also with lower synthesis of assimilates. On the other hand, the changes in the pathway of assimilate partitioning to roots for counteracting the salinity can be another reason for lower dry weight of the grains. As well, salinity enhanced the activities of antioxidant enzymes in peas. It was revealed that the increase in salinity level from 1 to 7 dS m-1 increased the activity of enzyme catalase (CAT) by 63.79% and further increase to 9 dS m-1 resulted in the loss of its activity. The activity of enzyme superoxide dismutase (SOD) increased up to the salinity of 5 dS m-1 (by 57.22%), did not change up to 7 dS m-1 and decreased at 9 dS m-1. The highest salinity levels as compared to control increased the activity of enzyme ascorbate peroxidase (APX). The highest effects of salinity on the measured traits of peas were observed at the level of 7 dS m-1. As the first defense line, superoxide dismutase converts superoxide to hydrogen peroxide. This enzyme is capable of converting superoxide ions into oxygen and hydrogen peroxide and is active in Glyoxylate cycle. The detoxification of hydrogen superoxide is complemented with ascorbate peroxidase, thioredoxin peroxidase, glutathione peroxidase and catalase. Ascrobate peroxidase is an important enzymetic antioxidant in plants whose function is to detoxify H2O2 with ascorbic acid.Conclusions In total, the examination of the effect of different levels of salinity on the measured traits showed that salinity did not significantly affect SPAD index of peas. It had negative impacts on physiological traits and reduced grain yield. In addition, it increased the activity of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD) and ascorbate peroxidase (APX) as salinity was intensified. A look at the status of the activities of antioxidants revealed that they were increased with salinity. It seems that this increase played a role in counteracting the adverse effects of salinity so that their decrease deteriorates the effects of salinity stress of plants. In total, the application of these levels of salinity indicated that peas are moderately sensitive plants to salinity, particularly salinity stress level of >7 dS m-1.