Ability to Remove Na⁺ and Retain K⁺ Correlates with Salt Tolerance in Two Maize Inbred Lines Seedlings

Maize is moderately sensitive to salt stress; therefore, soil salinity is a serious threat to its production worldwide. Here, excellent salt-tolerant maize inbred line TL1317 and extremely salt-sensitive maize inbred line SL1303 were screened to understand the maize response to salt stress and its tolerance mechanisms. Relative water content, membrane stability index, stomatal conductance, chlorophyll content, maximum photochemical efficiency, photochemical efficiency, shoot and root fresh/dry weight, and proline and water soluble sugar content analyses were used to identify that the physiological effects of osmotic stress of salt stress were obvious and manifested at about 3 days after salt stress in maize. Moreover, the ion concentration of two maize inbred lines revealed that the salt-tolerant maize inbred line could maintain low Na⁺ concentration by accumulating Na⁺ in old leaves and gradually shedding them to exclude excessive Na⁺. Furthermore, the K⁺ uptake and retention abilities of roots were important in maintaining K⁺ homeostasis for salt tolerance in maize. RNA-seq and qPCR results revealed some Na⁺/H⁺ antiporter genes and Ca²⁺ transport genes were up-regulated faster and higher in TL1317 than those in SL1303. Some K⁺ transport genes were down-regulated in SL1303 but up-regulated in TL1317. RNA-seq results, along with the phenotype and physiological results, suggested that the salt-tolerant maize inbred line TL1317 possesses more rapidly and effectively responses to remove toxic Na⁺ ions and maintain K⁺ under salt stress than the salt-sensitive maize inbred line SL1303. This response should facilitate cell homoeostasis under salt stress and result in salt tolerance in TL1317.