5-Aminolevulinic Acid Activates Antioxidative Defence System and Seedling Growth in Brassica napus L. under Water-Deficit Stress

The present study assesses the effects of 5-aminolevulinic acid (ALA, 0, 0.1, 1 and 10 mg l⁻¹) on the growth of oilseed rape (Brassica napus L. cv. ZS758) seedlings under water-deficit stress induced by polyethylene glycol (PEG 6000, 0 and −0.3 MPa). Water-deficit stress imposed negative effects on seedling growth by reducing shoot biomass, cotyledon water potential, chlorophyll content and non-enzymatic antioxidants (glutathione and ascorbic acid) levels. On the other hand, water-deficit stress enhanced the malondialdehyde (MDA) content, reactive oxygen species (ROS) production, enzymatic antioxidants activities, reduced/oxidized glutathione ratio (GSH/GSSG) and reduced/oxidized ascorbic acid (ASA/DHA) ratio in seedlings. Application of ALA at lower dosages (0.1 and 1 mg l⁻¹) improved shoot weight and chlorophyll contents, and decreased MDA in rape seedlings, whereas moderately higher dosage of ALA (10 mg l⁻¹) hampered the growth. The study also indicated that 1 mg l⁻¹ ALA improved chlorophyll content, but reduced MDA content and ROS production significantly under water-deficit stress. Lower dosages of ALA (0.1 and 1 mg l⁻¹) also enhanced GSH/GSSG and ASA/DHA as compared to the seedlings under water-deficit stress. The antioxidant enzymes (ascorbate peroxidase, peroxidase, catalase, glutathione reductase and superoxide dismutase) enhanced their activities remarkably with 1 mg l⁻¹ ALA treatment under water-deficit stress. It was also revealed that 1 mg l⁻¹ ALA treatment alone induced the expression of APX, CAT and GR substantially and under water-deficit stress conditions ALA treatment could induce the expression of POD, CAT and GR to a certain degree. These results indicated that 0.1-1 mg l⁻¹ ALA could enhance the water-deficit stress tolerance of oilseed seedlings through improving the biomass accumulation, maintaining a relative high ratio of GSH/GSSG and ASA/DHA, enhancing the activities of the specific antioxidant enzymes and inducing the expression of the specific antioxidant enzyme genes.