ABSTRACT
Absorption of NO 3 by roots of higher plants is a complex process. The complexity is expressed at many levels: in the structure of the system itself by coexistence of influx, efflux and probable multiplicity of protein systems of transport; in the energisation through interactions with transmembrane electric potential and pH gradient; and finally in the control of both influx and efflux by a multiplicity of regulatory molecules whose actions are sometimes antagonistic. A major functional consequence of this complexity is that NO 3 uptake by roots shows direct sensitivity to a multitude of external factors. For example, energisation of uptake by transmembrane gradient of H+ concentration results in a strong effect of extracellular pH on NO 3 transport rate. Similarly, the existence of several systems of transport with varying affinity to NO 3 results in extreme dependence of influx on external concentration of the anion.
