Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots
1997
Kreuzwieser, J. | Herschbach, C. | Stulen, I. | Wiersema, P. | Vaalburg, W. | Rennenberg, H.
The processes of NO3(-) uptake and transport and the effects of NH4(+) or L-glutamate on these processes were investigated with excised non-mycorrhizal beech (Fagus sylvatica L.) roots. NO3(-) net uptake followed uniphasic Michaelis-Menten kinetics in a concentration range of 10 micromolar to 1 mM with an apparent Km of 9.2 micromolar and a Vmax of 366 nmol g-1 FW h-1, NH4(+), when present in excess to NO3(-), or 10 mM L-glutamate inhibited the net uptake of NO3(-). Apparently, part of NO3(-) taken up was loaded into the xylem. Relative xylem loading of NO3(-) ranged from 3.2 +/- 1.6 to 6.4 +/- 5.1% of NO3(-) net uptake. It was not affected by treatment with NH4(+) or L-glutamate. 15N/13N double labelling experiments showed that NO3(-) efflux from roots increased with increasing influx of NO3(-) and, therefore, declined if influx was reduced by NH4(+) or L-glutamate exposure. From these results it is concluded that NO3(-) net uptake by non-mycorrhizal beech roots is reduced by NH4(+) or L-glutamate at the level of influx and not at the level of efflux.
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