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-poly-alacturonate (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 200 mu M) 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.