Gibberellic Acid and Sulfur-Mediated Reversal of Cadmium-Inhibited Photosynthetic Performance in Mungbean (Vigna radiata L.) Involves Nitric Oxide

Gibberellic acid (GA) and sulfur (S) have been known to modulate physiological processes of plants in normal and stressful conditions. Cultivars of mungbean ( Vigna radiata L.), Pusa Vishal, PusaRatna, Pusa 9531, Ganga, and MH 318 were screened for photosynthetic S-use efficiency (p-SUE) and growth in presence of 200 mg Cd kg −1 soil. Pusa 9531 showed maximum p-SUE, growth, and Cd tolerance. The mechanism of 10 µM GA-induced alleviation of Cd stress in Cd tolerant cultivar Pusa 9531 grown with 100 mg S kg −1 soil (100S) or 200 mg S kg −1 soil (200S) was investigated. Plants receiving GA in presence of 100S maximally utilized available S and improved photosynthetic characteristics of plants through improvement in SUE and antioxidant metabolism and alleviated Cd stress; the addition of 200S was of no additional benefit. Such effects of GA and 100S in protection of photosynthetic performance and growth involved nitric oxide (NO), which was evidenced on the use of NO modulators: 100 µM SNP (sodium nitropruside; NO promoter) or 100 µM c-PTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; NO scavenger). The application of SNP improved the mechanisms of N- and S-assimilation and antioxidant metabolism which helped in tolerance of plants to Cd stress. Moreover, c-PTIO supplementation to GA plus S reversed the positive effects of GA and S on photosynthesis and growth, signifying the involvement of NO in mechanisms induced by GA + S under Cd stress. The study provides the evidence that GA and NO interact and mechanisms induced by GA in presence of S were mediated by NO. The inter-relation of GA and NO may be used for augmenting photosynthesis and growth through utilization of S in mungbean grown under Cd stress.