Nielsen KH, Schjoerring JK
Plant Nutrition Laboratory, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark.
Plant Physiol. 1998 Dec;118(4):1361-8. doi: 10.1104/pp.118.4.1361.
Regulation of apoplastic NH4+ 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 NH4+-free solution, the apoplastic NH4+ concentration returned in less than 1.5 min to the preinfiltration level of 0.8 mM. Infiltrated 15NH4+ was rapidly diluted by 14NH4+/14NH3 effluxed from the cell. The exchange rate of 15N/14N over the apoplast due to combined 14N efflux from the symplast and 15N influx from the apoplastic solution was 29.4 &mgr;mol g-1 fresh weight h-1 between 0 and 5 min after infiltration. The net uptake of NH4+ into the leaf cells increased linearly with apoplastic NH4+ concentrations between 2 and 10 mM and could be partially inhibited by the channel inhibitors La3+ and tetraethylammonium and by Na+ and K+. When apoplastic pH increased from 5.0 to 8.0, the steady-state apoplastic NH4+ concentration decreased from 1.0 to 0.3 mM. Increasing temperature increased the rate of NH4+ net uptake and reduced the apoplastic steady-state NH4+ concentration. We conclude that the apoplastic solution in leaves of oilseed rape constitutes a highly dynamic NH4+ pool.
利用一种可对质外体溶液进行可控操作的真空渗透技术，研究了油菜（Brassica napus L.）叶片中质外体NH₄⁺浓度的调节。在用无NH₄⁺溶液渗透的叶片中，质外体NH₄⁺浓度在不到1.5分钟内恢复到渗透前0.8 mM的水平。渗透的¹⁵NH₄⁺被从细胞中流出的¹⁴NH₄⁺/¹⁴NH₃迅速稀释。在渗透后0至5分钟之间，由于共质体中¹⁴N的流出和质外体溶液中¹⁵N的流入，质外体上¹⁵N/¹⁴N的交换率为29.4 μmol g⁻¹鲜重 h⁻¹。叶片细胞对NH₄⁺的净吸收随质外体NH₄⁺浓度在2至10 mM之间呈线性增加，并且可被通道抑制剂La³⁺和四乙铵以及Na⁺和K⁺部分抑制。当质外体pH从5.0增加到8.0时，质外体NH₄⁺的稳态浓度从1.0 mM降至0.3 mM。温度升高增加了NH₄⁺净吸收速率并降低了质外体NH₄⁺的稳态浓度。我们得出结论，油菜叶片中的质外体溶液构成了一个高度动态的NH₄⁺库。
