KINETICS OF ALUMINUM UPTAKE BY CELL-SUSPENSIONS OF PHASEOLUS-VULGARISL

The kinetics of aluminum (Al) uptake by cell suspensions derived from an Al-resistant (Dade) and an Al-sensitive (Romano) cultivar of Phaseo lus vulgaris L. were investigated. Uptake of Al from low-volume (3-mL) uptake solutions containing 75 mu M AlCl3 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 lin ear phase of uptake. Differences in Al uptake between excised roots an d 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 lack of an observable linear phase o f uptake. However, when the concentration of Al in uptake solutions wa s increased to 500 and 1000 mu M, total accumulation of Al increased, while the general pattern of uptake was not affected. Moreover, mock-u ptake experiments at these high concentrations ruled out the possibili ty of artifacts arising from formation of solid phase Al. While increa sing the concentration of Al in uptake solutions provided a means of i ncreasing Al available for uptake, pyrocatechol violet analysis of mon omeric Al indicated incomplete recovery of added Al, particularly at c oncentrations above 300 mu M. To overcome potential problems associate d with the formation of complex Al species in solution, the total amou nt of Al in solution was increased using a high-volume (100-mL), low-c oncentration (75-mu M AlCl3) system. Under these conditions, a biphasi c 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 t he 180 min uptake period. We were also able to isolate the linear phas e of uptake after desorption in 9.0 mM citric acid. These results sugg est that the basic pattern of Al uptake by cell suspensions (saturated vs. linear) is strongly affected by the speciation of Al, which is in directly related to the volume of uptake solutions. The biphasic patte rn of uptake observed in our high-volume system also supports the hypo thesis that the kinetics of short-term Al uptake observed in excised r oots reflect transport events occurring at the cellular level. We beli eve 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.