Metals and oxalate in Tithonia diversifolia (Asteraceae): concentrations in plants growing in contrasting soils, and Al induction of oxalate exudation by roots

Oxalate has been associated with Al tolerance in higher plants. We found higher concentrations of free oxalate and Al in roots as compared to leaves of Tithonia diversifolia sampled from either a cultivated field or a native undisturbed soil in an ecological reserve located in the coastal range in northern Venezuela. Both soils showed similar concentrations of Ca, Mn and Ni whereas the concentrations of Al, Fe, Cu and Zn of the cultivated field were higher than those of the native soil. Plants from the cultivated field had leaves with higher concentrations of Al, Ca, Fe, and P than those from the native soil. The same was observed in roots, but only for Fe and P. The highest values of free oxalate, measured by ion chromatography, were found in plants growing on native soil. In leaves, higher levels of free oxalate were correlated with lower total Ca concentrations. Roots of young plants submersed in Al containing nutrient solutions were induced to exudate oxalate, in proportion to the Al concentration, and exudation increased over a period of 8 days in plants maintained in solutions with 9 and 90 mmol Al L−1. We conclude that oxalate is acting in T. diversifolia as an Al chelator, thus preventing the toxic effect of this metal.