Cotton (Gossypium hirsutum L.) genotypes with contrasting K+/Na+ ion homeostasis: implications for salinity tolerance

Cotton (Gossypium hirsutum L.) possesses notable inter- and intraspecific salt tolerance variation, but the mechanisms underlying this variation are, more or less, unclear. To explore salt stress responses, Na+/K+ transport and the transcriptional profiles of salinity tolerance candidate genes (e.g., GhSOS1, GhNHX1, and GhAKT1) were evaluated in four cotton cultivars (differing in salt sensitivities) using electrophysiological and qRT-PCR assays. Physiology and lipid peroxidation assays revealed the following pattern of salt tolerance, in decreasing order: CZ91 > CCRI44 > CCRI49 > Z571. Salinity caused ion imbalances in plants, but ion homeostasis was more pronounced in Z571, which accumulated more Na+ and less K+ in leaves. In contrast, salt-tolerant cultivars, especially CZ91, retained less Na+, but more K+ under salt stress. The non-invasive micro-test technique revealed that Z571 exhibited a higher capacity to transport Na+ from roots to shoots, but a lower capacity to extrude Na+ and retain K+ in leaves under salinity. Meanwhile, NaCl-induced changes in Na+ and K+ ion homeostasis were less pronounced in salt-tolerant cultivars, increasing leaf K+/Na+ ratios under salinity. Additionally, NaCl increased the GhSOS1 transcript abundance in hypocotyls, but this increase was more pronounced in salt-sensitive Z571. On the other hand, NaCl significantly enhanced the transcript abundances of GhSOS1 in leaves and/or GhAKT1 in hypocotyls of salt-tolerant cultivars. NaCl also increased the GhNHX1 transcript abundance, but there was no strong correlation between GhNHX1 expression level and salt resistance. The present findings provided an explanation for salt tolerance variation in cotton, illuminating the transcriptional regulation of K+/Na+ transport and the maintenance of the K+/Na+ ratio as underlying attributes.