Maize root tonoplasts are able to accumulate Ca2+ using the energy derived from the H+ gradient formed during PPi hydrolysis. Oxalate increases 6- to 10-fold the amount of Ca2+ accumulated by tonoplast. Two apparently different K-s values for Ca2+ with values of 0.36 and 4.70 muM were detected when oxalate was included in the medium and the free Ca2+ concentration in the medium was buffered with the use of EGTA. Binding of Ca2+ to the outer surface of tonoplasts inhibits the outflow of Ca2+ previously accumulated by the tonoplast, half-maximal inhibition being observed in presence of 1 muM Ca2+. Thapsigargin, a specific inhibitor of Ca2+-ATPase, inhibits the Ca2+ uptake driven by H+ gradient but does not inhibit the hydrolysis of PPi nor the formation of a H+ gradient. (C) 2003 Elsevier Inc. All rights reserved.