Effect of melatonin or cobalt on growth, yield and physiological responses of cucumber (Cucumis sativus L.) plants under salt stress

The comparison between salt stress protectants in terms of their ability to increase growth and yield under salinity is of great importance in crop production. Melatonin (MT) and cobalt (Co) have been suggested to promote the plant's salt tolerance by mediating physiological mechanisms. To explore the difference between these protectants in increasing salt tolerance, cucumber plants (Cucumis sativus L., cv. Barracuda F1) were submitted to different levels of salt stress (control = 0.25, 2, 3 and 4 dS m−1) with pretreatment of MT (100 and 200 μM) or Co (15 and 20 mg L−1). The results indicated that the increasing level of salt reduced the growth and productivity of cucumber plants, which may result from ions toxicity (Na+ and Cl− accumulation), oxidative stress (hydrogen peroxide, H2O2 and malondialdehyde, MDA accumulation), and the reduction of essential nutrients content. The concentration used, whether from MT or Co, was instrumental in determining the level of salt tolerance in the cucumber plants. Foliar application with 15 mg L−1 Co and 100 μM MT mitigated, at a similar level, the NaCl- induced inhibition of cucumber growth, fruit number and weight. At the physiological levels, both treatments increased protein content, essential nutrients (N, P, K+, and Ca2+) content, and catalase (CAT) activity of stressed plants. The mitigation was also observed in the reduced levels of H2O2 and MDA. However, Co (15 mg L−1) showed superiority compared to MT in reducing the content of toxic ions (Na+ and Cl−) and increasing the total yield, especially under the high salt stress (4 dS m−1). These results suggest that Co and MT can improve the salt tolerance of the cucumber plants, but the level of improvement may differ in some aspects, and this may be due to the difference in salt tolerance mechanisms among those protectants.