*3.4.2.2. Nitrogen donor ligands*

This group consists of amino acids and their derivatives which have relatively high affinity for specific metals. Krämer *et al.,* (1996) revealed histidine to be involved in the Ni tolerance and translocation of the hyperaccumulator plant *Alyssum lesbiacum.* The majority of Zn in roots of the Zn hyperaccumulator, *T. caerulescens*, was complexed with histidine (Salt *et al.,* 1999). Studies have also shown histidine to be involved in the restriction of metal uptake. For example, plants chelate Ni with histidine in the rhizosphere which prevents the uptake of this metal (Wenzel *et al.,* 2003).
