Aluminum (Al)-induced organic acid exudation in an Al-accumulating species from the Brazilian savanna

Abstract

Styrax camporum, an Al-accumulating species from the Cerrado, seems to rely on the exudation of citric and oxalic acids to avoid excessive Al. Organic acid (OA) exudation by the roots of plants to chelate aluminum (Al) and forming non-toxic complexes is a known mechanism of Al exclusion in some plants, including some Al-accumulating species. The Cerrado vegetation in South America is composed of Al non-accumulating species and some Al-accumulating species from few families, all growing healthy on acidic soils with high Al saturation but never tested for OA exudation. We elected Styrax camporum (Styracaceae), a Cerrado woody species that accumulates in their leaves approximately 1500 mg Al kg−1 dry mass, to examine whether plantlets of this species exude OAs in response to changes in Al concentrations in a nutrient solution, for 30 days. Citric, malic and oxalic acids exuded by the roots of this species were cumulatively measured in nutrient solutions containing 0, 740 and 1480 μM Al. Also, we measured the Al concentration of whole plantlets at 0 and 30 days. Malic acid was not exuded by the plantlets, but it was detected inside root tips of plantlets exposed to Al. Plantlets exposed to 740 μM Al released more oxalic and citric acid in the nutrient solution than those exposed to 0 and 1480 μM Al after 30 days. On the other hand, between 0 and 30 days, plantlets exposed to 740 μM Al increased the Al concentration (in whole plantlet) by three times while those exposed to 1480 μM Al, by seven times. This higher OA exudation associated with lower Al uptake at 740 μM Al suggests an Al exclusion mechanism that is impaired at higher Al concentrations. This is the first report showing that an Al-accumulating species from the Cerrado exudes OAs in response to the Al concentration in the root environment.