Comparative Physiological and Gene Expression Analyses Provide Insights into Ion Transports and Osmotic Adjustment of Sweet Sorghum under Salt Stress

Sweet sorghum is an important sugar crop and forage with a strong tolerance to soil salinity. We have previously analyzed the ion accumulation traits and transcriptome of a sweet sorghum cultivar under NaCl treatments. However, the mechanisms underlying Na+, K+, Cl-, and NO3- transports and the osmotic adjustment of sweet sorghum under salt stresses need further investigations. In this study, the growth, photosynthesis, inorganic ion and organic solute contents, and leaf osmotic adjustment ability of the sweet sorghum cultivars "Lvjuren" and "Fengtian" under NaCl treatments were determined; meanwhile, the expressions of key genes associated with the Na+, K+, Cl-, and NO3- transport were analyzed using the qRT-PCR method. The results showed that NaCl treatments more severely inhibited the growth and photosynthesis of "Lvjuren" than those of "Fengtian". After NaCl treatments, "Fengtian" could more efficiently restrict the overaccumulation of Na+ and Cl- in leaf blades than "Lvjuren" by withholding large amounts of Na+ in the roots or reserving high quantities of Cl- in the leaf sheaths, which could be attributed to the upregulated expressions of SbNHX2, SbHKT1;4, SbHKT1;5, SbCLCc, and SbCLCg or the downregulated expression of SbNPF6.4. "Fengtian" exhibited significantly lower leaf osmotic potential but higher leaf water potential and turgor pressure under NaCl treatments, suggesting that the former possessed a stronger osmotic ability than the latter. The contents of K+, NO3-, soluble sugar, and betaine in leaf blades, as well as the contributions of these osmolytes to the leaf osmotic potential, in "Fengtian" were significantly higher than those in "Lvjuren". In addition, the upregulated expressions of SbAKT1, SbHAK5, SbSKOR, SbNPF3.1, SbNPF6.3, and SbNPF7.3 should be responsible for maintaining K+ and NO3- homeostasis under NaCl treatment. These results lay a foundation for uncovering the salt tolerance mechanisms of sweet sorghum and large-scale cultivation of this species in saline areas.