Abstract
Maggioni, A., Varanini, R., Pinton and M.G. De Biasi, 1991. Humic substances affect transport properties of root membranes. The effect of low molecular size (< 5 KDa) humic substances on plasmalemma and tonoplast H+-ATPases was studied by utilizing membrane vesicles isolated from oat roots. Humic molecules at concentrations up to 0.50 μg org C × ml−1 stimulated both plasma membrane and tonoplast ATPase activity. When the assay was run by clamping to zero the electrical gradient, the activity of plasma membrane ATPase was still stimulated, indicating a direct action of humic substances only on this enzyme. ATP-dependent intra-vesicular acidification of both types of membrane vesicles was stimulated (35–40%) by humic substances at the concentration of 0.50 μg org C × ml−1, as well as the transport of No3- in plasma membrane vesicles. Results show that transport properties of plasma membrane and tonoplast regulating cell metabolism and mineral nutrition can be modulated by different modes of action by low molecular size humic substances.
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