The intercellular distribution of vacuolar solutes in the epidermis and mesophyll of barley leaves changes in response to NaCl

Concentrations of inorganic and organic solutes were measured in sap extracted from individual mesophyll and epidermal cells of the third leaf of barley. During the development of the third leaf plants were grown in various salt solutions (NaCl; 2, 50, 100, and 150 mM, KCI; 100 mM or KNO3; 100 mM). Leaves were analysed 2–4 d after full expansion. Cell-sap was extracted using a modified pressure probe and analysed for its osmolality, concentrations of P, Na+ K+ Ca2+, and Cl− and, in some cases, of nitrate, hexoses and total amino acids. Salt treatment caused differential changes in the concentrations of solutes in mesophyll and epidermal cells, but did not affect the basic pattern of solute compartmentation between these tissues. Calcium was found at osmotically significant concentrations only in the epidermis, whereas P and organic solutes were almost exclusively found in the mesophyll. Chloride and Na+ accumulated preferentially in the epidermis, although mesophyll concentrations also increased considerably. At 150 mM external NaCl, mesophyll cells contained 302 mM Na and 167 mM Cl−, compared to 29 mM Na+ and 16 mM Cl− in the control. Mesophyll Cl− levels were even higher in the 100 mM KCl treatment (216 mM) where mesophyll and epidermal K+ accumulated to 424 and 491 mM, respectively. These huge increases in mesophyll Na+ Cl− and K+ were not associated with a breakdown in leaf performance since net rates of photosynthesis decreased only by less than 20%. Under control (2 mM NaCl) conditions, solutes followed patterned gradients between the various epidermal cell types. The extent of these gradients changed with leaf age. During 50 mM NaCl treatment, gradients in Cl−, nitrate and malate concentrations progressively disappeared, with malate concentrations approaching zero. Potassium and Na+ exhibited altered distribution profiles, whereas Ca2+ distribution was unaffected. NaCl-dependent increases in osmolalities differed between cells. Exposure of plants to 150 mM NaCl caused qualitatively similar changes in both epidermal solute and osmolality profiles, although absolute values differed from those at 50 mM NaCl. In particular, epidermal Cl− and Na+ increased to about 500 mM and K+ disappeared (≪5 mM) from the vacuole of certain epidermal cell types completely.