Maintenance of Low Cl− Concentrations in Mesophyll Cells of Leaf Blades of Barley Seedlings Exposed to Salt Stress

The concentrations of vacuolar Na(+) and Cl(-) in the epidermal and mesophyll cells of the leaf blade and sheath of Hordeum vulgare seedlings (cv California Mariout and Clipper) were measured by means of quantitative electron probe x-ray microanalysis. A preferential accumulation of Cl(-) in vacuoles of epidermal cells in both blade and sheath and a low level in mesophyll cells of the blade were evident in plants grown in full strength Johnson solution. The concentration of Cl(-) in the mesophyll cells of the blade remained at a low level after exposure to 50 or 100 millimolar NaCl for 1 day or to 50 millimolar for 4 days, while at the same time the concentration of Cl(-) in the epidermis and mesophyll of the sheath showed a dramatic increase. Clipper generally contained more Cl(-) in the mesophyll cells of the blade than California Mariout. A greater accumulation of Na(+) in the mesophyll of the sheath relative to that of the blade was only apparent after treatment with 100 millimolar NaCl for 1 day or 50 millimolar for 4 days. These results confirm the suggestion that sheath tissue is capable of accumulating excess Cl(-) (and to a lesser extent Na(+)) and suggest that the site of regulation of Cl(-) concentration in the barley leaf is located in the mesophyll cells of the blade.