Sodium Influx and Potassium Efflux Currents in Sunflower Root Cells Under High Salinity

Helianthus annuus L. is an important oilseed crop, which exhibits moderate salt tolerance and can be cultivated in areas affected by salinity. Using patch-clamp electrophysiology, we have characterized Na⁺ influx and K⁺ efflux conductances in protoplasts of salt-tolerant H. annuus L. hybrid KBSH-53 under high salinity. This work demonstrates that the plasma membrane of sunflower root cells has a classic set of ionic conductances dominated by K⁺ outwardly rectifying channels (KORs) and non-selective cation channels (NSCCs). KORs in sunflower show extreme Na⁺ sensitivity at high extracellular [Ca²⁺] that can potentially have a positive adaptive effect under salt stress (decreasing K⁺ loss). Na⁺ influx currents in sunflower roots demonstrate voltage-independent activation, lack time-dependent component, and are sensitive to Gd³⁺. Sunflower Na⁺-permeable NSCCs mediate a much weaker Na⁺ influx currents on the background of physiological levels of Ca²⁺ as compared to other species. This suggests that sunflower NSCCs have greater Ca²⁺ sensitivity. The responses of Na⁺ influx to Ca²⁺ correlates well with protection of sunflower growth by external Ca²⁺ in seedlings treated with NaCl. It can be, thus, hypothesized that NaCl tolerance in sunflower seedling roots is programmed at the ion channel level via their sensitivity to Ca²⁺ and Na⁺.