Nitrate transport processes in Fagus-Laccaria-mycorrhizae

The contribution of influx and efflux of NO3- on NO3- net uptake has been s tudied in excised mycorrhizae of 18-20 week old beech (Fagus sylvatica L.) trees. Net uptake rates of NO3- followed uniphasic Michaelis-Menten kinetic s 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- tak en up that was loaded into the xylem vessels as NO3- plus reduced N, was co nstant over the concentration range tested (4.6-7.7%). NO3- influx proceede d linearly with increasing external NO3- supply. When the assumed regulator s 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 comparis on 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 mycor rhization leads to reduced rates of NO3- net uptake. This effect is caused by reduced influx plus enhanced efflux of NO3- as compared with non-mycorrh izal beech roots.