A COMPARISON OF THE KINETICS OF ALUMINUM (AL) UPTAKE AND DISTRIBUTIONIN ROOTS OF WHEAT (TRITICUM-AESTIVUM) USING DIFFERENT ALUMINUM SOURCES - A REVISION OF THE OPERATIONAL DEFINITION OF SYMPLASTIC AL

The lack of information about the movement of aluminum (Al) across the plasma membrane presents a significant barrier to the elucidation of resistance mechanisms which may involve exclusion of Al from the sympl ast. An understanding of mechanistic aspects of exclusion requires the estimation of symplastic Al levels. Such measurements may be achievab le through the use of a kinetic approach. A kinetic protocol was devel oped to characterize the accumulation and distribution of Al in variou s cellular compartments in roots of wheat (Triticum aestivum L.). The kinetics of uptake and desorption were similar when Al was supplied as AlK(SO4)(2) or as AlCl3. When both salts were supplied at low concent ration (50 mu M), Al bound to a purified cell wall fraction could be r educed to less than 10-20% of non-exchangeable Al, if roots were washe d for 30 min in citric acid following exposure. In contrast, when AlK( SO4)(2) was supplied at a high concentration (200 mu M), a strong line ar phase of uptake into cell wall material was observed, which account ed for approximately 48% of nonexchangeable Al in roots. These results suggest that the use of low concentrations of Al in simple salt solut ions is required to minimize accumulation of non-exchangeable Al in th e apoplasm. A series of multiple-desorption experiments confirmed that citric acid was effective in removing Al from the cell wall compartme nt of roots exposed to Al for short periods (3 h). However, long expos ures (48 h) appeared to create conditions conducive to the accumulatio n of non-exchangeable Al in the cell wall. in experiments where uptake from solutions containing 50 mu M AlCl3 was followed by desorption in citric acid, non-exchangeable Al in microsomal membrane fractions rep resented less than 4% of total non-exchangeable Al. Thus, we can exclu de the plasma membrane and cell wall as major sites for accumulation o f non-exchangeable Al in short exposure studies. Although we cannot pr ovide unequivocal evidence for the localization of Al within the sympl ast, use of simple salt solutions followed by desorption in citric aci d provides the best kinetic technique currently available for the quan titation of Al in the symplasm.