Brassinosteroid seed priming positively affects leaf anatomy and gas exchanges in a salt-sensitive sorghum genotype exposed to salt stress

Soil salinity is threatening the cultivation of major cereal crops worldwide. Sorghum is a promising alternative to other cereals for both human and animal nutrition. This is due to the nutritional characteristics of its caryopses and its natural ability to grow in marginal environments, characterized by various stresses, including salinity. However, stress tolerance varies among sorghum genotypes, highlighting the need for technologies to enhance salinity resilience. Phytohormone seed priming is a cost-effective and eco-friendly approach to improve horticultural crops environmental stress tolerance as well as plant growth and vigour. Among phytohormones, brassinosteroids (BRs), when used as primed seed agents, have been shown to enhance morpho-anatomical and physiological defences against abiotic stress in various crops. However, further research is needed to understand their role as seed priming agent in restoring photosynthesis affected by salt stress, especially in salt-sensitive sorghum genotypes. Thus, this study evaluated the effects of seed priming with 24-epibrassinolide (24-eBL), a bioactive BR precursor on Bianca and Tonkawa, salt-tolerant and salt-sensitive genotypes, respectively, grown for 44 days in pots under 150 mM NaCl. The results demonstrate that seed priming with 1 µM 24-eBL for 8 h restores growth in Tonkawa but not in Bianca, by inducing anatomical leaf adaptations, such as restoration of mesophyll and substomatal air spaces, regulation of bulliform cell area, and modifications in cuticle thickness without altering its chemical composition. Overall, 24-eBL seed priming mitigates salt stress effects in the salt-sensitive genotype by improving leaf anatomy, which in turn improves photosynthetic efficiency and ultimately promotes biomass recovery.