GA-K+-Priming Improves Physiochemical Responses in Thymus vulgaris during Seed Germination and Seedling Growth under Salinity Stress

Biological mechanisms of seed germination in response to salinity stress affect the successful establishment of plant populations. Therefore, this study was aimed to induce salt stress resistance in thyme seeds at the germination stage using different priming techniques. In order to perceive the most appropriate seed priming protocol, all priming treatments, including osmopriming (potassium nitrate), hormopriming (gibberellic acid), and hybrid-priming (potassium nitrate with gibberellin hormone), and non-priming treatment (control) were screened at different salinity levels (0, 3, 6, 9, and 12 dS/m). Hybrid priming technique (2% potassium nitrate with 0.6 mM gibberellin) is one of the most effective seed pretreatments to improve seed germination indices at all tested salinity levels. Our results showed an increase in hydrogen peroxide (H2O2) and malondialdehyde (MDA) content with increasing salinity concentrations. Therefore, in primed seeds, total antioxidant capacity and phenolic and flavonoid content were increased significantly as compared to non-primed seeds at all levels of salinity stress. This consequently decreased H2O2 and MDA content and improved seed germination capacity under salinity stress condition. Moreover, this favorable priming treatment could induce salinity resistance in the germination and seedling growth stages by promoting assimilates (soluble sugars, proline, and glycine betaine) accumulation and adjusting the mineral elements ratio (potassium to sodium ratio), resulting in an increase in seedling growth, germination percentage, and germination rate at different salinity levels. Overall, hybrid priming treatments could play a more effective role in increasing germination potential, seedling rate and biochemical indices of thyme seed under salt stress condition.