Comprehensive Morpho-Physiological Responses Underlying Salt Tolerance at Different Time Points in Brassica napus Seedlings

Soil salinization is a major environmental hazard, hindering rapeseed development due to sodium ion (Na⁺) toxicity and ionic imbalances in plant cells. Understanding tolerance mechanisms and categorizing reliable physiochemical indicators is vital for enhancing rapeseed tolerance. Herein, we aimed to enhance knowledge about the stress-responsive mechanism of ten rapeseed varieties (C71, C88, C91, C97, C123, C136, C196, C272, C280, and C320) exposed to five NaCl concentrations (0, 150, 200, 250, and 300 mM) through determining key factors related to salt tolerance at the seedling stage. Our results showed that salt stress significantly reduced seedling growth and biomass with increasing salt stress concentration in a similar pattern in all studied varieties, especially in sensitive seedlings. Furthermore, photosynthetic pigment, osmotic solutes, and MDA showed significant variations under salt treatment versus control in all studied varieties. Based on morpho-physiochemical trait analysis of ten rapeseed varieties, C71 and C272 were selected as tolerant and sensitive varieties to study stress responses during six weeks (weekly time points) in the leaf, petiole, stem, and root of seedlings under 250 mM NaCl. Current findings demonstrated superior osmotic adjustment of C71 through higher accumulation of total soluble sugars and protein, reflected in lower MDA levels, which contributed to maintaining cellular homeostasis and membrane integrity to improve resilience under salinity versus C272. Besides, total amino acid content was enhanced in C71 versus C272 seedlings, which was attributed to stress tolerance. In different tissues of C71 and C272, Na⁺ and K⁺ levels varied with increasing growing time, reaching the maximum increment at the 6th week under salt stress conditions. Moreover, Na⁺ initially accumulates in roots and enhances the K⁺ level in tolerant seedlings; besides, K⁺ was accumulated higher in the roots of tolerant seedlings, resulting in K⁺ homeostasis, thereby improving stress tolerance. Our results can be a great reference value for rapeseed plant breeders to develop salt-tolerant cultivars.