Salicylic acid alleviates decreases in photosynthesis under salt stress by enhancing nitrogen and sulfur assimilation and antioxidant metabolism differentially in two mungbean cultivars
Introduction
Salt stress is a major abiotic stress that causes detrimental effects on plant growth and productivity mainly through changes at physiological, biochemical and molecular level (Tester and Devenport, 2003, Khan et al., 2009, Syeed et al., 2010). The physiological processes that are primarily adversely affected by salt stress include ion toxicity, osmotic stress, nutrient deficiency and oxidative stress (Flowers, 2004). Salt stress may negatively affect photosynthesis by causing excess accumulation of leaf Na+ and Cl−, stomatal closure and oxidative stress resulting in the formation of reactive oxygen species (ROS). Excessive amounts of ROS can enhance membrane lipid peroxidation and electrolyte leakage (Gunes et al., 2007) and damage chloroplast, inhibit photochemical reactions and decrease photosynthesis (Steduto et al., 2000). ROS are toxic by-products of stress metabolism, but also play an important role as important signal transduction molecules in stress signaling and regulation of acclimation responses (Miller et al., 2010). As an adaptation response, plants activate several mechanisms to counteract the adverse effects of salt-induced ROS in plants. These mechanisms include ion homeostasis, detoxification of ROS and the use of available resources which directly influence photosynthesis. One of the defense mechanisms of plants includes the up-regulation of reduced glutathione (GSH), a low molecular weight antioxidant. GSH plays essential roles within plant metabolism and stress tolerance to ROS (Szalai et al., 2009). The potential of GSH as antioxidant is related to the activity of glutathione reductase (GR), which catalyzes the regeneration of GSH from oxidized glutathione (GSSG). Nitrogen (N) and sulfur (S) are constituents of GSH. Therefore, the activity of nitrate reductase (NR) and ATP-sulfurylase (ATPS), the enzymes involved in N and S assimilation may influence GSH content in plant cell. Plants containing high activities of antioxidant enzymes have shown considerable resistance to oxidative damage caused by ROS (Khan et al., 2007, Gapinska et al., 2008, Frary et al., 2010, Syeed et al., 2010). These mechanisms of ion homeostasis, GSH synthesis and antioxidant enzymes may be influenced by plant growth regulators under salt stress.
Salicylic acid (SA) is a naturally occurring plant hormone, influences various physiological and biochemical functions in plants, acts as an important signaling molecule and has diverse effects on tolerance to biotic and abiotic stress (Arfan et al., 2007, Syeed et al., 2010, Wang et al., 2010). Its role in plant tolerance to abiotic stresses such as ozone, UV-B, heat, heavy metal and osmotic stress (El-Tayeb, 2005, Wang et al., 2010) has been reported. In contrast, it has also been shown that high concentration of SA increases oxidative damage generated by NaCl in Arabidopsis (Borsani et al., 2001) and decreases drought tolerance in Zea mays (Nemeth et al., 2002). These reports show that SA has contrasting effects in inducing stress tolerance that may depend upon the species or concentration of SA applied. Therefore, it is critical to identify the physiological processes of plants differing in salt sensitivity, and examine how much these processes are induced by SA application in tolerant and sensitive types to alleviate the decrease in photosynthesis under salt stress. One of the objectives of the reported research was to compare the physiological processes in Pusa Vishal (salt-tolerant) and T44 (salt-sensitive) cultivars of mungbean. The other objective was to study the effectiveness of SA application in modulating these processes and alleviating decrease in photosynthesis under salt stress. The cultivars, Pusa Vishal and T44 have been shown to differ in sensitivity to salt stress (Khan and Syeed, 2003).
Section snippets
Plant material and growth conditions
Healthy seeds of Pusa Vishal (salt-tolerant) and T44 (salt-sensitive) cultivars of mungbean (Vigna radiata L.) were surface sterilized and sown in 15-cm-diameter earthen pots filled with acid-washed sand purified according to Hewitt (1966). Plants grown in pots were kept in a greenhouse under natural day/night conditions with photosynthetically active radiation (PAR) ∼900 ± 28 μmol m−2 s−1 and average day/night temperature of 33/20 ± 2 °C. Plants (2 per pot) were subjected to either 0 (control) or 50 mM
Leaf and root Na+ and Cl− content
Salt treatment resulted in higher leaf Na+ and Cl− content in comparison to control in both the cultivars, but T44 exhibited greater accumulation than Pusa Vishal. Plants treated with 0.1 or 0.5 mM SA exhibited leaf Na+ and Cl− content lesser than the control in both the cultivars. Maximum reduction in the content of leaf Na+ and Cl− was noted with 0.5 mM SA in Pusa Vishal. Application of 0.5 mM SA resulted in reduction of leaf Na+ and Cl− content by 27.8% and 46.7% in Pusa Vishal and 10.0% and
Discussions
The reported research was undertaken to improve our understanding of physiological processes determining salt sensitivity and the induction of such processes by SA application for the alleviation of salt-induced decreases in photosynthesis. The salt-sensitive cultivar, T44 accumulated higher Na+ and Cl− in leaves and therefore, exhibited greater content of H2O2 and TBARS and electrolyte leakage than the salt-tolerant cultivar, Pusa Vishal. To counteract NaCl-induced oxidative stress, plants are
Conclusion
It is concluded that the low sensitivity of Pusa Vishal to salt stress was due to its capacity to accumulate less content of Na+ and Cl− in leaf. This cultivar also exhibited lower activity of SOD and higher activity of APX and GR and GSH content. These traits helped to remove salt-induced ROS in this cultivar more efficiently than T44. In contrast, Na+ and Cl− accumulation in leaf was higher than root in T44, and this cultivar exhibited higher SOD activity than Pusa Vishal showing greater
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