ACTION DES METAUX [Cd(II), Cu(II), Pb(II), Zn(II)] SUR LA PRODUCTION D'EXSUDATS RACINAIRES SOLUBLES CHEZ LE MAIS (ZEA MAYS L.)

Abstract

The effect of metals on soluble root exudation was studied by growing maize plantlets under hydroponic and sterile conditions. Cadmium, copper, lead or zinc was added to the Hoagland medium to give a final concentration of: Cd 10-6 M, Cd 10-5 M, Cu 10-4 M, Cu 10-3 M, Pb 10-6 M, Pb 10-5 M, Zn 10-4 M, Zn 10-3 M. After 15 days of growth, the nutrient solutions were filtered and analysed for total soluble carbon, reducing sugars, proteins and amino-acids. Both root and leaf biomass were significantly modified by the presence of cadmium (Cd 10-5 M) and copper (Cu 10-4 M, Cu 10-3 M) while no difference was observed in the other treatments. In addition, an increase in total exudation and a change in root exudate composition were detected. The effect of metal ion supply on root exudation can be ranked as follows; control, Pb 10-6 M, Pb 10-5 M, Cd 10-6 M < Zn 10-4 M, Cd 10-5 M < Cu 10-4 M < Cu 10-3 M. Consequences of high metal concentration (i.e. Cu, Cd) are discussed from two points of view: inhibition of root growth as a result of metabolic disorder, induction of both a qualitative and a quantitative change of the root exudates, especially complexing compounds, probably giving rise to modifications of rhizosphere activity.