Improvement of oxidation resistance and osmotic regulation of white clover seedlings by exogenous auxin under polyethylene glycol stress

This research was conducted to investigate the effect of auxin (IAA) on the drought tolerance of white clover (Trifolium repens cv. ‘Pixie’) under drought stress induced by polyethylene glycol-6000(PEG-6000) nutrient solution. The results showed that 1 μmol·L-1 IAA significantly improved the drought tolerance of white clover fed with a nutrient solution containing 15% PEG (-0.3 MPa). Exogenous IAA improved the activities of antioxidant enzymes including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase to varying degrees, increased the contents of ascorbate and glutathione, decreased electrolyte leakage, and decreased the contents of malondialdehyde, superoxide anions, and H2O2. These observations indicated that IAA effectively reduced oxidative damage and lipid modifications. Exogenous IAA also significantly increased the contents of total amino acids and proline, and efficiently decreased the osmotic potential, which was associated with an increased relative water content. Similarly, under drought conditions, the treatment with exogenous IAA (1 μmol·L-1) significantly improved the relative water content and alleviated lipid injury in detached leaves, but treatments with auxin biosynthesis inhibitors (60 μmol·L-1 naphthylphthalamic acid and 60 μmol·L-1 amino-oxyphenylpropionic acid) decreased the relative water content and lipid injury in detached leaves. In conclusion, the results indicated that IAA can effectively increase the capacities of white clover for antioxidant protection and osmoregulation, thereby leading to an improved drought tolerance.