5-Aminolevulinic acid pretreatment mitigates drought stress of cucumber leaves through altering antioxidant enzyme activity

To promote the application of 5-aminolevulinic acid (ALA) in agricultural production, a cucumber variety (Cucumis sativus L cv. Yuexiu no. 3) was pretreated with 3 mu M ALA for 2 d and then was exposed to drought stress induced by 10% polyethylene glycol (PEG) 6000. We investigated whether exogenous ALA could protect cucumbers from drought stress and whether the protective effect was associated with the regulation of antioxidant enzymes. After 2 d of ALA pretreatment, no changes were observed in plant growth indexes including plant heights and the fresh and dry weights of shoots and roots and the second leaves. Meanwhile, we obtained the increased activities of superoxide dismutase (SOD. EC 1.15.1.1), guaiacol peroxidase (GPX, EC 1.11.1.7), glutathione peroxidase (GSH-Px, EC 1.11.1.9), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), and monodehydroascorbate reductase (MDHAR. EC 1.6.5.4) in ALA-pretreated leaves and as well got the elevated content of reduced glutathione (GSH). When the ALA-pretreated plants were subjected to drought, the antioxidants changed further. Drought stress increased the activities of SOD, catalase (CAT. EC 1.11.1.6), GPX, GSH-Px, APX, GR, dehydroascorbate reductase (DHAR, EC 1.8.5.1) and MDHAR in leaves and also enhanced the contents of GSH and ascorbate. However, the combination of ALA pretreatment and drought enhanced the antioxidant activities even more. Moreover, drought decreased plant growth indexes, resulted in 83.3% withered leaves and increased the levels of superoxide radical (O-2(center dot-)) and hydrogen peroxide (H2O2) in leaves. When ALA pretreatment was combined with drought, we observed the improved growth inhibition and 48.2% withered leaves, and found the decreased levels of O-2(center dot-) and H2O2 in comparison to non-pretreated stressed plants. We propose that exogenous ALA can regulate antioxidant activities, which are altered further after ALA pretreatment is combined with PEG-induced drought stress; thus ALA pretreatment protects cucumbers from drought stress. (C) 2011 Elsevier B.V. All rights reserved.