Ion transport in broad bean leaf mesophyll under saline conditions

Main conclusion Salt stress reduces the ability of mesophyll tissue to respond to light. Potassium outward rectifying channels are responsible for 84% of Na1 induced potassium efflux from mesophyll cells. Modulation in ion transport of broad bean (Vicia faba 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 nonselective cation channels. NaCl treatment resulted in a reduction in photosystem II efficiency in a dose- and timedependent manner. In particular, reductions in Fv0/Fm0 were linked to K+ homeostasis in the mesophyll tissue. Increased apoplastic Na+ concentrations induced vanadatesensitive 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.