Interactions of NH4+and L-glutamate with NO3−transport processes of non-mycorrhizalFagus sylvaticaroots

Abstract

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 10μM to 1 mM with an apparent Km of 9.2 μM 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. 16N/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.