Ethylene Supplementation Increases PSII Efficiency and Alleviates Chromium-Inhibited Photosynthesis Through Increased Nitrogen and Sulfur Assimilation in Mustard

The effect of chromium (Cr) toxicity on four cultivars of mustard (Brassica juncea), Varuna, Pusa Bold, Rohini and SS2, was studied to select the cultivar with the highest tolerance potential based on higher photosynthesis and growth and lower oxidative stress. Plants receiving Cr showed enhanced superoxide production, accumulation of H2O2 and lipid peroxidation and reductions in photosynthesis and growth. We also studied the potential of ethephon (ethylene source; 2-chloroethyl phosphonic acid) in the amelioration of Cr-induced oxidative stress in the Varuna cultivar which had the highest tolerance potential. Supplementation of plants with ethylene resulted in the alleviation of Cr-induced stress by enhancing proline accumulation and the activity of proline metabolism enzymes and glutamyl kinase, and reduction in proline oxidase activity. Ethylene also increased the activity of enzymes of sulfur assimilation, ATP-sulfurylase and serine acetyl transferase and the content of reduced glutathione (GSH). Furthermore, application of ethylene to plants under Cr stress resulted in ethylene levels in the optimal range and increased the activity of anti-oxidant enzymes such as glutathione reductase and ascorbate peroxidase, reduced oxidative stress and increased photosynthesis and growth. The key role of ethylene in the reversal of Cr-induced photosynthetic inhibition was clearly seen with the application of the ethylene action inhibitor, silver nitrate (AgNO3). The supplementation of AgNO3 resulted in decreased GSH and proline content and lowered ethylene production and photosynthetic and growth compared to ethylene-treated plants. This suggests that ethylene is involved in -reversal of Cr inhibited photosynthesis and growth in mustard.