Salicylic acid supplementation improves photosynthesis and growth in mustard through changes in proline accumulation and ethylene formation under drought stress

• Salicylic acid (SA) enhanced drought tolerance in mustard (Brassica juncea L.).
• SA improves photosynthesis and growth under drought stress.
• Exogenous SA alleviates drought stress by increasing proline production.
• SA application restricted the ethylene formation by inhibiting the 1-aminocyclopropane carboxylic acid synthase (ACS) activity.
• The inhibition of ethylene synthesis under drought stress by SA may result in influencing proline metabolism and photosynthesis.

Drought stress is becoming a major threat to plant productivity loss in agricultural system. The present study was carried out to evaluate the physiological and biochemical alterations induced by salicylic acid (SA) in mustard plant under moderate drought stress conditions. Therefore, a pot culture experiment was conducted to test whether SA application at concentration of 0.5 mM through foliar spray could protect the mustard (Brassica juncea L.) cultivar Pusa Jai Kisan subjected to drought stress on the basis of growth and photosynthesis. The treatments were as follows: (i) 100% FC + 0 mM SA, (ii) 50% FC + 0 mM SA, (iii) 100% FC + 0.5 mM SA and (iv) 50% FC + 0.5 mM SA. The control treatment received 100% field capacity (FC) irrigation, whereas moderate drought stress corresponded to 50% field capacity. Plants subjected to drought stress caused significant reduction in growth and photosynthetic parameters, activity of ribulose 1,5-bisphosphate carboxylase (Rubisco), nitrate reductase (NR), ATP-sulfurylase (ATPS) which accounted for decreased nitrogen (N) and sulfur (S) assimilation. Whereas, a pronounced increase was observed in proline metabolism. Exogenously applied 0.5 mM SA alleviated the stress by increasing the proline production through the increase in γ-glutamyl kinase (GK) and decrease in proline oxidase (PROX) activity. In addition SA application restricted the ethylene formation by inhibiting the 1-aminocyclopropane carboxylic acid synthase (ACS) activity more conspicuously under moderate drought stress than no stress. These findings reflect that SA application alleviates the drought-induced decrease in growth and photosynthesis through increased proline content. Higher proline content was a result of increased N and S assimilation and increased synthesis of proline synthesizing enzyme which lowers the oxidative stress in mustard.