Role of 5-aminolevulinic acid in the salinity stress response of the seeds and seedlings of the medicinal plant Cassia obtusifolia L.

Background: Soil salinity, one of the major abiotic stresses affecting germination, crop growth, and productivity, is a common adverse environmental factor. The possibility of enhancing the salinity stress tolerance of Cassia obtusifolia L. seeds and seedlings by the exogenous application of 5-aminolevulinic acid (ALA) was investigated. Result: To improve the salinity tolerance of seeds, ALA was applied in various concentrations (5, 10, 15, and 20 mg/L). To improve the salinity tolerance of seedlings, ALA was applied in various concentrations (10, 25, 50, and 100 mg/L). After 10 mg/L ALA treatment, physiological indices of seed germination (i.e., germination vigor, germination rate, germination index, and vigor index) significantly improved. At 25 mg/L ALA, there was a significant protection against salinity stress compared with non-ALA-treated seedlings. Chlorophyll content, total soluble sugars, free proline, and soluble protein contents were significantly enhanced. Increased thiobarbituric acid reactive species and membrane permeability levels were also inhibited with the ALA treatment. With the treatments of ALA, the levels of chlorophyll fluorescence parameters, i.e., the photochemical efficiency of photosystem II (F-v/F-m), photochemical efficiency (F-v'/F-m'), PSII actual photochemical efficiency ((sic)PSII), and photochemical quench coefficient (qP), all significantly increased. In contrast, the non-photochemical quenching coefficient (NPQ) decreased. ALA treatment also enhanced the activities of superoxide dismutase, peroxidase, and catalase in seedling leaves. The highest salinity tolerance was obtained at 25 mg/L ALA treatment. Conclusion: The plant growth regulator ALA could be effectively used to protect C. obtusifolia seeds and seedlings from the damaging effects of salinity stress without adversely affecting plant growth.