Foliar-applied α-tocopherol enhances salt-tolerance in onion plants by improving antioxidant defence system

Abstract

α-Tocopherol (αTOC) is a vitamin and antioxidant compound that plays a crucial role in amelioration of biotic and abiotic stresses. It has been found that it improves salt-tolerance in some plant species. Two field experiments were conducted in 2013/14 and 2014/15 to evaluate the potential effects of foliar applications with 0.5 and 1.0 mM αTOC on growth, yield, plant water relations, osmoprotectants and the activity of antioxidant system of two onion varieties (i.e., Giza 20 and GizaRed) "classified as salt-sensitive" under saline soil condition (ECe = 7.94 - 8.81 dS m-1). Exogenous application of αTOC significantly improved salt stress tolerance in onion plants by reducing the endogenous H2O2 and lipid peroxidation, and increasing enzymatic (i.e., superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) and non-enzymatic (i.e., ascorbic acid and glutathione) antioxidant activity. However, Giza 20 was more sensitive to salinity, while it was more responsive to αTOC treatment. Moreover, αTOC application significantly affected photosynthesis efficiency and plant water status as evaluated by relative water content and membrane stability index. These results were positively reflected in plant growth, productivity and water use efficiency under salt stress conditions, indicating that αTOC may participate in the enzymatic and non-enzymatic antioxidants. Therefore, foliar application of αTOC could be used to induce salt-tolerance in onion plants.