Ion transport in broad bean leaf mesophyll under saline conditions

Modulation in ion transport of broad bean (<i>Vicia faba</i> L.) mesophyll to light under increased apoplastic salinity stress was investigated using vibrating ion-selective microelectrodes (the MIFE technique). Increased apoplastic Na⁺ significantly affected mesophyll cells ability to respond to light by modulating ion transport across their membranes. Elevated apoplastic Na⁺ also induced a significant K⁺ efflux from mesophyll tissue. This efflux was mediated predominately by potassium outward rectifying channels (84 %) and the remainder of the efflux was through non-selective cation channels. NaCl treatment resulted in a reduction in photosystem II efficiency in a dose- and time-dependent manner. In particular, reductions in Fv′/Fm′ were linked to K⁺ homeostasis in the mesophyll tissue. Increased apoplastic Na⁺ concentrations induced vanadate-sensitive net H⁺ efflux, presumably mediated by the plasma membrane H⁺-ATPase. It is concluded that the observed pump’s activation is essential for the maintenance of membrane potential and ion homeostasis in the cytoplasm of mesophyll under salt stress.