KINETICS OF ALUMINUM UPTAKE IN TRITICUM-AESTIVUM L - IDENTITY OF THE LINEAR-PHASE OF ALUMINUM UPTAKE BY EXCISED ROOTS OF ALUMINUM-TOLERANT AND ALUMINUM-SENSITIVE CULTIVARS

The identity of a linear phase of aluminum (Al) uptake in Triticum aestivum was investigated by analysis of the kinetics of Al uptake by excised roots and purified cell wall fractions. Classical interpretation of kinetic data suggests that a linear phase of uptake with time reflects uptake across the plasma membrane; however, in studies with Al the possibility that the linear phase of uptake includes accumulation of Al in both the symplasm and the apoplasm has not been discounted. In our experiments, we observed a linear phase of Al uptake at both ambient and low temperatures, although the rate of uptake at 0°C was 53 to 72% less than at 23°C, depending on cultivars. This nonsaturable phase of uptake at low temperature suggests that a portion of the linear phase of Al uptake is nonmetabolic. Furthermore, analysis of Al in cell wall fractions isolated from excised roots pretreated with Al suggests that the linear phase of uptake includes a cell wall component. When excised roots were pretreated with Al, accumulation of Al in purified cell wall material included a linear phase that could not be desorbed with a 30 minute wash in citrate. The rates of linear-phase accumulation of Al by cell wall material and cell contents were similar. In contrast, the linear phase of in vitro uptake of Al by purified cell wall material was completely desorbed by a 30 minute wash with citrate. These results suggest that the linear phase of Al uptake observed in excised roots of T. aestivum included metabolism-dependent binding of Al in apoplasm.