|Van Genuchten, Martinus|
Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2006
Publication Date: 11/15/2006
Citation: Wang, H., Shan, X., Liu, T., Xie, Y., Wen, B., Zhang, S., Han, F., Van Genuchten, M.T. 2006. Organic acids enhance the uptake of lead by wheat roots. Planta. Vol.225:1483-1494 Interpretive Summary: Lead (Pb) is one of the most frequently encountered metal contaminants in agricultural and industrial areas. Many attempts have been made to clean up Pb-polluted areas using phytoremediation, i.e., the use of green plants to remove pollutants from contaminated areas. Because of its limited solubility, bioavailable levels of Pb in soils are generally low compared to other heavy metals, thus making it difficult to apply phytoremediation techniques to Pb-contaminated soils. Recent studies have shown that the addition of synthetic organic chemicals (chelates) to contaminated soil can dramatically increase Pb release into the soil solution, thus promoting Pb uptake by plants. Compared to other metals, limited information exists on the interactions between organic acids and Pb at or near the soil-root interface, and the resultant processes of Pb uptake by plant roots. The objective of this study was to gain insight into the effects of organic acids on the uptake of Pb by wheat seedlings grown under hydroponic conditions. Relevant mechanisms were investigated by using time- and concentration-dependent Pb uptake experiments using two organic acids (acetic and especially malic acid). Acetic acid and especially malic acid were found to significantly increase Pb uptake, with the net effect depending upon the organic:acid ratio in the nutrient solution. Uptake was found to be initially fast and then became more or less constant in time. Enhanced Pb uptake was attributed, in part, to enhanced diffusion of Pb towards the roots in the presence of the organic acids. Results of this study are important for understanding the complex role of organic acids in promoting Pb uptake by plant roots, and to design and implement more effective phytoremediation cleanup technologies.
Technical Abstract: The uptake and bioavailability of lead (Pb) in soil-plant systems remains poorly understood. This study indicates that acetic and malic acids enhance the uptake of Pb by wheat (Triticum aestivum L.) roots under hydroponic donditions. The net concentration- dependent uptake influx of Pb in the presence and absence of organic acids was characterized by Michaelis–Menten type nonsaturating kinetic curves that could be resolved into linear and saturable components. Fitted maximum uptake rates (Vmax) of the Michaelis–Menton saturable component in the presence of acetic and malic acids were, respectively, 2.45 and 1.63 times those of the control, while the Michaelis-Menten Km values of 5.5, 3.7 and 2.2 microM, respectively, remained unchanged. Enhanced Pb uptake by organic acids was partially mediated by Ca and K channels, and also depended upon the physiological function of the plasma membrane P-type ATPase. Uptake may have been further enhanced by an effectively thinner unstirred layer of Pb adjacent to the roots, leading to more rapid diffusion towards roots. X-ray absorption spectroscopic studies provided evidence that the coordination environment of Pb in wheat roots was similar to that of Pb(CH3COO)2.3H2O in that one Pb atom was coordinated to four oxygen atoms via the carboxylate group.