Effects of transpiration on potassium and sodium fluxes in root cells and the regulation of ion distribution between roots and shoots of barley seedlings.

The effects of transpiration on uptake and xylem transport of K and Na and on the individual fluxes of these ions in root cells of barley seedlings (Hordeum distichon L., age 4.5 d) were studied. Transpiration affected the fluxes of K even at low K concentrations (0.02 and 0.2 mM), provided that seedlings were used in which only one seminal root was left. In these seedlings transpiration induced substantially higher rates of water flow relative to the root weight than in seedlings with all (5-7) roots. Na fluxes were affected even in seedlings with all roots. Accelerated water flow promoted uptake and xylem transport of K to the shoot, but decreased the K concentration in the roots. Compartmental analysis showed that this was due to a decreased vacuolar K content. The cytoplasmic K content was unaffected or slightly increased. Many of the effects of transpiration on Na were similar, though more pronounced than those observed with K. As a consequence transpiration shifted the K/Na selectivity in the roots and shoots in favour of Na. Na differed from K regarding effects on contents in the vacuole; transpiration either lowered or raised the vacuolar Na content, depending on the presence and concentration of K. Enhanced water flow promoted the plasmalemma influx of Na more than its xylem transport, and it led to an increase in the cytoplasmic Na content and in the plasmalemma efflux. These data suggest that increased water flow interacts with ion flow across membranes of root cells at more than one site. Moreover water flow interferes with the processes which regulate the balance between accumulation of ions in root cell vacuoles and transport to the shoot. The data are compatible with a cell to cell transport of water during its radial passage across the root.