ABSTRACT

The main pathway of nitrogen entry into food chains is by absorption of NO 3 by roots of terrestrial plants. Use of nitrogenous fertilisers has gradually increased since the middle of the nineteenth century. Improved yield as a result of increased nitrogenous fertiliser usage suggests that the availability of nitrate is often a limiting factor in plant growth. However, physiological studies show that absorption systems have high affinity for N O 3 , indicating that the transport capacity of the root can be saturated at concentrations of NO 3 much lower than that normally occurring in field soil. Concomitantly, physiological approaches demonstrate the power of the regulatory mechanisms which signal the requirements of the aerial parts of plants to NO 3 transport systems of the roots. These regulatory mechanisms show that NO 3 absorption by roots is determined by the nitrogen demand of the plant and not by its availability in the soil environment. Observations from field studies and conclusions of laboratory physiological approaches are contradictory. This could be attributed to interactions in nature between development of the root system and absorption of NO 3, together with the structural and functional complexities of the rhizosphere. These interactions are not really well understood despite the fact of being quite obvious.