Nitrate-selective microelectrodes were used to measure intracellular nitrate concentrations (as activities) in epidermal and cortical cells of roots of 5-d-old barley (Hordeum vulgare L.) seedlings grown in nutrient solution containing 10 mol · m(-3) nitrate. Measurements in each cell type grouped into two populations with mean (±SE) values of 5.4 ± 0.5 mol · m(-3) (n=19) and 41.8 ± 2.6 mol · m(-3) (n = 35) in epidermal cells, and 3.2 ± 1.2 mol · m(-3) (n = 4) and 72.8 ± 8.4 mol · m(-3) (n = 13) in cortical cells. These could represent the cytoplasmic and vacuolar nitrate concentrations, respectively, in each cell type. To test this hypothesis, a single-cell sampling procedure was used to withdraw a vacuolar sap sample from individual epidermal and cortical cells. Measurement of the nitrate concentration in these samples by a fluorometric nitrate-reductase assay confirmed a mean vacuolar nitrate concentration of 52.6 ± 5.3 mol · m(-3) (n = 10) in epidermal cells and 101.2 ± 4.8 mol · m(-3) (n = 44) in cortical cells. The nitrate-reductase assay gave only a single population of measurements in each cell type, supporting the hypothesis that the higher of the two populations of electrode measurements in each cell type are vacuolar in origin. Differences in the absolute values obtained by these methods are probably related to the fact that the nitrate electrodes were calibrated against nitrate activity but the enzymic assay against concentration. Furthermore, a 28-h time course for the accumulation of nitrate measured with electrodes in epidermal cells showed the apparent cytoplasmic measurements remained constant at 5.0 ± 0.7 mol · m(-3), while the vacuole accumulated nitrate to 30-50 mol · m(-3). The implications of the data for mechanisms of nitrate transport at the plasma membrane and tonoplast are discussed.

Plantlets of Aeluropus lagopoides (Linn.) Trin. Ex Thw. were grown at different NaCl concentrations (26, 167, 373 and 747 mM) for 3, 7 and 15 days; their growth, osmotic adjustment, gas exchange, ion compartmentalisation and expression of various genes related to Na + flux was studied. Plantlets showed optimal growth in non-saline (control; 26 mM NaCl) solutions, whereas CO 2 /H 2 O gas exchange, leaf water concentration and water use efficiency decreased under all salinity treatments, accompanied by increased leaf senescence, root ash, sodium content and leaf osmolality. A decrease in malondialdehyde (MDA) content with time was correlated with Na + accumulation in the leaf apoplast and a concomitant increase in Na + secretion rate. A. lagopoides accumulated a higher concentration of Na + in root than in leaf vacuoles, corresponding with higher expression of V-NHX and lower expression of PM-NHX in root than leaf tissue. It appears that V-ATPase plays a vital role during Na + transport by producing an electromotive force, driving ion transport. Leaf calcium increased with increasing salinity, with more rapid accumulation at high salinity than at low salinity, indicating a possible involvement of Ca 2+ in maintaining K + : Na + ratio. Our results suggest that A. lagopoides successfully compartmentalised Na + at salinities up to 373 mM NaCl by upregulating the gene expression of membrane linked transport proteins (V-NHX and PM-NHX). At higher salinity (747 mM NaCl), a reduction in the expression of V-NHX and PM-NHX in leaves without any change in the rate of salt secretion, is a possible cause of the toxicity of NaCl.