The root-soil interface is the site in the soil matrix where chemical and biological characteristics differ from those of bulk soil. The risk of accumulation of heavy metals like cadmium, lead, nickel and zinc in plant may contaminate the food chain affecting health and ecosystem, though nickel and zinc are essential elements for crop nutrition. A study was conducted with the objective to investigate the important role played by rhizodeposits of maize and wheat, the two important staple crops, due to their metal binding properties. Rhizodeposits were collected by growing maize and wheat in sand culture. Composition of root exudates were determined by chemical methods and total carbon, total nitrogen, polysaccharides, proteins and uronic acids were determined as percentage based on dry matter. These groups of constituents/ compounds were found to be 41.1, 1.2, 96.4, 12.1 and 20.6 for maize root exudates and 36.7, 1.8, 93.2, 9.8 and 17.8 for wheat exudates, respectively. To study biding of root exudates with metal ions, an equilibrium dialysis procedure given by Zunino and Martin (1977) was followed. Rhizodeposits were taken in dialysis bags (20 ml of exudates at the concentration of 250 mg l^-1). The bags were introduced in metallic solution (20 ml) ranging from 5 to 50 mM. After equilibrium, the concentrations of the respective metallic ions in the outer solution were estimated using atomic absorption spectrophotometer which gives free metal concentration (Mf). The difference between the total concentrations (Mt) and free metal concentrations gives the bound metal concentrations (Mb). The maximum binding ability (MBA) was calculated and found to be 150, 205, 213 and 223 meq 100 g^-1 for Pb, Ni, Cd and Zn, respectively, for root exudates of maize; whereas the corresponding values were 110, 168, 178 and 201 meq 100 g^-1 for root exudates of wheat. Overall stability constant (K) was calculated and logK values were found to be 4.402, 4.334, 4.416 and 4.367 in respect of root exudates of maize for the corresponding metals. The corresponding values were 4.548, 4.423, 4.403 and 4.319 for rhizodeposits of wheat. Because of binding reactions the mobility of high affinity metals for root exudates like zinc in maize would decrease in the rhizosphere, while for Cd for root exudates in wheat implicating plant nutrition and environmental concern.

Zunino, H and Martin, J.P. (1997). Soil Science 123: 188-202

Key words: Root exudates, lead, nickel, cadmium, zinc, dialysis, maize, wheat.