Abstract
The contribution of influx and efflux of NO3 - on NO3 - net uptake has been studied in excised mycorrhizae of 18–20 week old beech (Fagus sylvatica L.) trees. Net uptake rates of NO3 - followed uniphasic Michaelis-Menten kinetics in the concentration range between 10 μM and 1.0 mM external NO3 -, with an apparent Km of 88±7 μM, and a Vmax of 110±7 nmol g-1 root f.wt. h-1. The relative xylem loading of N, i.e. the portion of NO3 - taken up that was loaded into the xylem vessels as NO3 - plus reduced N, was constant over the concentration range tested (4.6–7.7%). NO3 - influx proceeded linearly with increasing external NO3 - supply. When the assumed regulators of net NO3 - uptake, i.e. NH4 + or L-glutamate, were applied together with NO3 -, net uptake rates of NO3 - decreased. This inhibitory effect was caused by a reduction of NO3 - influx rather than an enhanced efflux. The comparison of the present data with a recent study with non-mycorrhizal beech roots (Kreuzwieser et al., 1997; J. Exp. Bot. 48, 1431–1438) revealed that mycorrhization leads to reduced rates of NO3 - net uptake. This effect is caused by reduced influx plus enhanced efflux of NO3 - as compared with non-mycorrhizal beech roots.
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