Foliar application of 5-aminolevulinic acid (ALA) alleviates NaCl stress in cucumber (Cucumis sativus L.) seedlings through the enhancement of ascorbate-glutathione cycle

Abstract

The natural plant growth regulator 5-aminolevulinic acid (ALA) promotes plant growth. The present study assessed the effects of exogenously applied ALA (25 mg L−1) on the growth and H2O2 scavenging system (ascorbate-glutathione cycle, AsA-GSH cycle) under NaCl stress (50 mmol L−1) in cucumber (Cucumis sativus L.) seedlings. Growth of cucumber seedlings was significantly inhibited by NaCl stress. Moreover, NaCl caused accumulation of H2O2 and malonaldehyde (MDA) as well as dehydroascorbic acid (DHA) and oxidized glutathione (GSSG) in cucumber leaves. However, the application of ALA reversed the adverse effect of NaCl on growth of the cucumber seedlings by increasing the shoots and roots biomass. The application of ALA also increased the contents of AsA and GSH of the seedlings under moderate salt stress. The ratios of reduced/oxidized antioxidant, such as AsA/DHA and GSH/GSSG, in seedlings under NaCl treatment were also improved by ALA, indicating that the regeneration efficiency of antioxidants could be promoted by exogenous ALA. In addition, exogenous ALA also promoted the activities of enzymes involved in the AsA-GSH cycle, including ascorbic acid oxidase (AAO), ascorbate peroxidase (APX), monodehydroascorbic acid reductase (MDHAR), dehydroascorbic acid reductase (DHAR) and glutathione reductase (GR). These results suggested that ALA alleviate the damages caused by NaCl through the enhancement of the AsA-GSH pathway in cucumber seedlings.