Differential ion accumulation and ion fluxes in the mesophyll and epidermis of barley

In barley (Hordeum vulgare L.) leaves, differential ion accumulation common ly results in inorganic phosphate (Pi) being confined to the mesophyll and Ca2+ to the epidermis, with preferential epidermal accumulation of Cl-, Na, and some other ions. The pattern was confirmed in this study for major in organic anions and cations by analysis of barley leaf protoplasts. The work focused on the extent to which differences in plasma membrane ion transpor t processes underlie these observations. Ion transport across the plasma me mbrane of barley epidermal and mesophyll protoplasts was investigated elect rophysiologically (by microelectrode impalement and patch clamping) and rad iometrically. Data from both approaches suggested that similar types of ion -selective channels and membrane transporters, which catalyze the transport of Ca2+, K+, Na+, and Pi, exist in the plasma membrane of the two cell typ es. In general, the simple presence or absence of ion transporters could no t explain cell-type-specific differences in ion accumulation. However, patc h-clamp data suggested that differential regulation of instantaneously acti vating ion channels in the plasma membrane could explain the preferential a ccumulation of Na+ in the epidermis.