The effects of salt preconditioning on physiological, biochemical, and molecular responses of Casuarina glauca seedlings grown under salt stress

Abstract

Salinity is one of the most harmful abiotic stresses with negative impacts on crop growth and development. Monitoring land salinization is, therefore, a major global issue. One of the suggested solutions to support rehabilitation programs is the use of fast-growing salt-tolerant woody species, like the tropical trees belonging to the Casuarinaceae family. Although salt preconditioning is a simple and practical method, its role in improving plant tolerance to salinity has still been scarcely applied. Thus, this study investigated the effect of salt preconditioning on different physiological, biochemical, and molecular mechanisms in Casuarina glauca seedlings grown in saline conditions. Preconditioned (PrS) and non-preconditioned (NPrS) seedlings were subjected to salt stress for 12 months. Gas exchange, electrolyte leakage, chlorophyll content, chlorophyll fluorescence, water relations parameters derived from pressure–volume curves, primary and secondary metabolism, and gene expression were assessed at the end of the experiment. Preconditioned plants promoted stronger tolerance to salt stress and improved gas exchange parameters, intrinsic water-use efficiency, and chlorophyll content compared to non-preconditioned plants. Also, membrane integrity was maintained in preconditioned plants but increased significantly in the absence of preconditioning. Salinity had no significant effect on the photochemical efficiency of PSII and resulted in a slight decrease in the initial fluorescence (Fo) in either treatment when compared to control. The results indicate an increase in secondary metabolism as shown by the enhancement of phenolic content as well as an increase in the expression of salt tolerance-related genes (GPX1, SOD1, SHD, APX, and GAPHD). Our findings also showed a considerable accumulation of soluble sugar and proline under salt stress, with a higher accumulation in preconditioned plants, confirming their salt tolerance and performance under saline conditions. Overall, these results indicate that C. glauca is highly suitable to be used on salinity-affected soils and can develop a higher tolerance to salt stress after preconditioning treatments.