Pre-sowing seed treatment with kinetin and calcium mitigates salt induced inhibition of seed germination and seedling growth of choysum (Brassica rapa var. parachinensis)

Abstract

In recent years, improving plants' resistance towards abiotic stresses with exogenous application of plant growth regulators and nutrients has emerged as a matter of great interest. The present study assessed the potential roles of kinetin (Kn, 0.2 mM) and calcium (Ca, 2 mM) in mitigating the salt (200 mM NaCl) induced inhibitory effects on seed germination and growth of choysum seedlings. The results indicated that NaCl stress significantly reduced the seed germination percentage (42.6%), germination potential (42.0%), germination index (52.1%), seedling vigor index (65.2%), and declined the fresh weight (43.8%), dry weight (52.2%), radicle length (37.2%), and plumule length (41.2%) of germinated seeds, compared to control treatment. The delayed germination and decrease in seedling growth were positively correlated with salinity-induced hormonal imbalance, ion toxicity, and oxidative stress. However, Kn and Ca pretreatment partially mitigated the adverse effects of NaCl stress, evident by early germination and enhanced seedling growth. Kn and Ca effectively increased the accumulation of proline, soluble protein, and soluble sugars, and upregulated the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase that significantly reduced the production of malondialdehyde, hydrogen peroxide, and superoxide anions in germinating seeds, thereby minimizing the NaCl-induced oxidative damages. Moreover, Kn and Ca pretreatment counteracted the NaCl-induced ionic toxicity by decreasing Na+ and increasing K+ contents and maintained a balanced Na+/K+ ratio in radicles and plumules of choysum seeds. Additionally, Kn and Ca under NaCl stress enhanced hormonal regulation by decreasing the ABA levels with a concomitant increase of GAs (especially GA4) levels and promoted early germination. Remarkably, the co-application of Kn and Ca was most effective by completely counteracting the inhibitory effects of NaCl and maintaining seed germination kinetics, seedling growth, and biochemical parameters almost similar to that in the stress-free control treatment. These results demonstrate that supplementation of Kn and Ca on choysum seeds is an effective chemical strategy regulating the various physiological and biochemical responses that would result in better germination and growth of seeds under stress conditions.