Kinetics of Aluminum Uptake by Cell Suspensions of Phaseolus vulgaris L.

Abstract

Summary The Kinetics of aluminum (Al) uptake by cell suspensions derived from an Al-resistant (Dade) an an Al-sensitive (Romano) cultivar of Phaseolus vulgaris L. were investigated. Uptake of Al from low-volume (3-mL) uptake solutions containing 75 μM AlCl 3 was rapid during the first 20 min with little additional absorption occurring over the remainder of the 180 min experimental period. In contrast studies with excised roots showed a longer rapid phase (30 min) which was followed by a linear phase of uptake. Differences in Al uptake between excised roots and cell suspensions appear to reflect the unique characteristics of our low-volume cell system rather than real differences in uptake between the two systems. The rate, extent, and saturable nature of uptake in this cell system suggested that depletion of Al from uptake solutions may have been responsible for the lacf of an observable linear phase of uptake. However, when the concentration of Al in uptake solutions was increased to 500 and 1000 μM, total accumulation of Al increased, while the general pattern of uptake was not affected. Moreover, mock-uptake experiments at these high concentrations ruled out the possibility of artifacts arising from formation of solid phase phase Al. While increasing the concentration of Al in uptake solutions provided a means of increasing Al available for uptake, Pyrocatechol violet analysis of monomeric Al indicate incomplete recovery of added Al particularly at concentrations above 300 μM. To overcome potential problems associate with the formation of complex Al species in solution, the total amount of Al in solution was increased using a high-volume (100-mL), low-concentration (75-μM AlC1 3 ) system. Under these conditions, a biphasic pattern of uptake was observed, with a rapid phase of uptake during the first 20 min, and a linear phase of uptake over the remainder of the 180 min uptake period. We were also able to isolate the linear phase of uptake after desorption in 9.0 mM citric acid. These results suggest that the basic pattern of Al uptake by cell suspensions (saturated vs. linear) is strongly affected by the speciation of Al, which is indirectly related to the volume of uptake solutions. The biphasic pattern of uptake observed in our high-volume system also supports the hypothesis that the kinetics of short-term Al uptake observed in excise roots reflect transport events occurring at the cellular level. We believe further investigation of the kinetics of Al uptake at the cellular level could provide a more direct means of measuring the uptake of Al across the plasma membrane in plants.