Effect of aluminium and pH on the mobility of phosphate through a soil-root interface model

The aim of this work was to investigate the effect of aluminium (Al), a toxic metal for plant growth, as well as pH on the mobility of phosphate across a calcium-polygalacturonate (Ca-PG) network used as a soil–root interface model. The Ca-PG fibrils, having acidic properties, are able to complex ions selectively: thus a Ca-PG model could be useful to study the ion uptake by plants. Ca-PG networks were exposed to Al solutions at different concentrations (25, 100 and 200lM) at pH 3.50, 4.00 and 4.50. These Ca-PG and Ca–Al-PG networks were subsequently used to measure the phosphate flux at pH 3.50, 4.00 and 4.50. The results showed that the phosphate’s mobility across the soil–root interface is strongly influenced by pH and aluminium: its mobility is much greater at a low pH. The presence of Al slowed down the phosphate even more leading to a complete flux impedance in the first 3–5 h at pH 4.00 and 4.50. This impedance is probably not only due to interactions between phosphate and Al but it is also due to structural changes: the interaction of Al (hydrolytic and/or polymeric species) at pH 4.00 and 4.50 with the polygalacturonic chains could lead to a collapse of the porous structure. These results suggest that the apoplastic-bound Al hinders, especially at pH 4.00 and 4.50, the phosphate uptake by plants.