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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 mu M and 1.0 mM external NO3-, with an apparent K-m of 88 +/- 7 mu M, and a V-max 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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