Mechanistic Elucidation of Salicylic Acid and Sulphur-Induced Defence Systems, Nitrogen Metabolism, Photosynthetic, and Growth Potential of Mungbean (Vigna radiata) Under Salt Stress

The potential of plant nutrients (such as sulphur, S) and phytohormones (such as salicylic acid, SA) has been explored in isolated studies by researchers in controlling the impact of abiotic stresses such as salinity in plants. However, information is scanty on the major mechanisms underlying the role of S and/or SA in modulation of enzymes involved in nitrogen (N) assimilation, GOGAT cycle, and antioxidant defence system; the cellular status of N-containing osmolyte proline, glucose, S-containing compounds; and their cumulative role in photosynthesis functions and growth in crop plants. The present study aimed to assess the role of cumulative effect of SA and S (SO42-) mediated induction of N assimilatory enzymes, GOGAT cycle, N-osmolyte proline and its metabolizing enzymes, glyoxylase enzymes, and antioxidant capacity in mungbean (Vigna radiata L.) exposed to NaCl with or without SO42- and SA. Salt-exposed V. radiate showed differential elevations in damage (O-2(.)-, H2O2, lipid peroxidation; glucose) and defence (ascorbate peroxidase, APX; glutathione reductase, GR; superoxide dismutase, SOD; reduced GSH; proline) and inhibitions in the activities of NR and NiR; N content, photosynthesis, photosynthetic N-use-efficiency (NUE), and growth. The separate supplementation of SA and SO42- to 50 mM NaCl almost equally strengthened the antioxidant machinery and diminished NaCl-accrued damages. However, combined supply of SA and SO42- to NaCl-exposed cultivars led to significant improvements in NR and NiR activities, the accumulation of N, GSH, proline, enhanced activity of APX, GR, and reduced activity of SOD, and also decreases in O-2(.)-, H2O2, lipid peroxidation and glucose. These observations were corroborated with SA, SO42- and NaCl-mediated changes in the traits of photosynthesis and growth, stomatal behaviour, and the polypeptide patterns of Rubisco in V. radiata. Overall, in V. radiata, SA-mediated higher enhancements in the activity of N assimilatory enzymes (NR, NiR, and GS), increase in the N and proline, and GSH; and decreases in the contents of Na+ and Cl- ions, and glucose (a photosynthesis repressor); maintenance of a fine tuning among SOD, APX, and GR enzymes; and higher minimization of ROS (O-2(.)-, H2O2) and lipid peroxidation finally led to a higher promotion in photosynthesis and growth.