THE NICOTIANAMINE MOLECULE IS MADE-TO-MEASURE FOR COMPLEXATION OF METAL MICRONUTRIENTS IN PLANTS

The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous amon g plants. In meristematic tissues it reaches concentrations of about 4 00 mu mol (g fresh weight)(-1). NA forms complexes, among others, with the metal micronutrients (MN) copper, zinc, iron and manganese (log K -MeNa- 18.6-8.8). Calculations of the dissociation curves of the metal -NA complexes based on the complex formation constants and on the acid dissociation constants of NA revealed their stability at the neutral or weak alkaline pH of cytoplasm and sieve tube sap, For the Mn-NA com plex, dissociation begins at about pH 6.5, for all others dissociation occurs at more acid pHs, Thus, metal-NA complexes could theoretically persist also in the apoplasm and in xylem sap, The octanol water part ition coefficient of NA is about 1 and those of its metal complexes ar e in the range of 0.3-0.4. The reason for this shift is perhaps the ne gative charge of the complexes, The higher lipophilicity of the free N A indicates that the NA supply to sites of requirement is faster than the removal of the complexes as long as membranes are an integral part of the transport paths. Changing phloem transport rates of MN-NA comp lexes by manipulation of the cotyledon apoplasm of Ricinus communis L. suggest a competition of MN for NA at the site(s) of phloem loading, Thus, NA could control MN transport via phloem including recirculation .