Regulation of Apoplastic NH(4)(+) Concentration in Leaves of Oilseed Rape

Regulation of apoplastic NH(4)(+) concentration in leaves of oilseed rape (Brassica napus L.) was studied using a vacuum-infiltration technique that allowed controlled manipulations of the apoplastic solution. In leaves infiltrated with NH(4)(+)-free solution, the apoplastic NH(4)(+) concentration returned in less than 1.5 min to the preinfiltration level of 0.8 mm. Infiltrated (15)NH(4)(+) was rapidly diluted by (14)NH(4)(+)/(14)NH(3) effluxed from the cell. The exchange rate of (15)N/(14)N over the apoplast due to combined (14)N efflux from the symplast and (15)N influx from the apoplastic solution was 29.4 μmol g(−1) fresh weight h(−1) between 0 and 5 min after infiltration. The net uptake of NH(4)(+) into the leaf cells increased linearly with apoplastic NH(4)(+) concentrations between 2 and 10 mm and could be partially inhibited by the channel inhibitors La(3+) and tetraethylammonium and by Na(+) and K(+). When apoplastic pH increased from 5.0 to 8.0, the steady-state apoplastic NH(4)(+) concentration decreased from 1.0 to 0.3 mm. Increasing temperature increased the rate of NH(4)(+) net uptake and reduced the apoplastic steady-state NH(4)(+) concentration. We conclude that the apoplastic solution in leaves of oilseed rape constitutes a highly dynamic NH(4)(+) pool.