Role of ethylene in alleviation of cadmium-induced photosynthetic capacity inhibition by sulphur in mustard.
Sulphur (S) assimilation leads to the formation of glutathione (GSH) and alleviation of cadmium (Cd) stress. GSH is synthesized from its immediate metabolite cysteine, which also serves as a metabolite for ethylene formation through S-adenosyl methionine. To assess the role of ethylene in S-induced alleviation of Cd stress on photosynthesis, the effects of S or ethephon (ethylene source) on GSH and ethylene were examined in mustard (Brassica juncea L. cv. Varuna). Sufficient-S at 100 mg S kg(-1) soil alleviated Cd-induced photosynthetic inhibition more than excess-S (200 mg S kg(-1) soil) via ethylene by increased GSH. Under Cd stress, plants were less sensitive to ethylene, despite high ethylene evolution, and showed photosynthetic inhibition. Ethylene sensitivity of plants increased with ethephon or sufficient-S, triggering the induction of an antioxidant system, and leading to increased photosynthesis even under Cd stress. The effects of ethephon and S under Cd stress were similar. The effects of S were reversed by ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), suggesting that ethylene plays an important role in S-induced alleviation of Cd stress on photosynthesis.