Can saline preconditioning enhance plant survival in degraded soils? Physiological, biochemical, and molecular responses in Casuarina glauca saplings

Soil salinization has become a major environmental and socioeconomic issue. Excessive accumulation of salts usually goes beyond the tolerance of most plants, except for halophytes that can thrive in saline environments. Despite this, establishment is still a crucial stage, often hindering the use of these species in degraded soils. Preconditioning to stress seems a simple method to overcome this limitation, but its potential and the mechanisms involved are still scarcely known. This study investigated if saline preconditioning can indeed enhance tolerance, using Casuarina glauca saplings (preconditioned and non-preconditioned) subjected to salt stress for 12 months. We found that preconditioning promoted stronger tolerance to salt stress, improving gas exchange parameters, intrinsic water-use efficiency, and chlorophyll content. The decrease in leaf osmotic potentials was higher in preconditioned than in non-preconditioned saplings. The osmotic potential at full turgor (Ψπ 100 ) decreased in preconditioned plants by 47% and in non-preconditioned by 29%, and the osmotic potential at zero turgor (Ψπ 0 ) decreased by 17% in preconditioned plants and by 4% in non-preconditioned compared to the control, indicating that the preconditioning promotes cellular osmotic adjustment and protects leaf membrane cells against leakage induced by salinity. Results also showed an increase in secondary metabolism as shown by the enhancement of phenolic content and in the expression of salt tolerance-related genes, particularly in preconditioned plants. This suggests the potential for oxidative stress signaling and reactive oxygen species (ROS) detoxification, which are essential components of stress tolerance mechanisms. Overall, C. glauca is highly suitable to be used on salinity-affected soils and can develop a higher tolerance to salt stress after preconditioning treatments.