ABSTRACT
The mechanisms of regulation of NH£ absorption by plants are still not clearly understood even though the nitrogen status of the tissue is considered a major element in its regulation. Therefore, nitrogen deficiency greatly increases the uptake of NH£ whereas availability of nitrogen induces a decrease in uptake (Ryan and Walker, 1994). The rate of absorption generally does not correlate with ion concentrations in tissues, which hardly favours the possibility of a control by intracellular ammonium over entry of fresh NH ^, but suggests the involvement of a product of ammonium assimilation, such as amino acids for instance. Unfortunately, even though there are many arguments attributing a central role to glutamine in controlling transport of ammonium in algae and fungi, there is no unequivocal demonstration of such a mechanism in higher plants. Other compounds such as glutamate, asparagine or aspartate may also play a role together
with glutamine and NH + . Nevertheless it is clear that prior supply of NH + significantly decreases the subsequent speed of functioning of HATS by a mechanism of reverse inhibition which concomitantly affects Km and Vmax whereas LATS does not seem to be susceptible to such inhibition (Wang et al., 1993a).
