RAPID UPTAKE OF ALUMINUM INTO CELLS OF INTACT SOYBEAN ROOT-TIPS - A MICROANALYTICAL STUDY USING SECONDARY-ION MASS-SPECTROMETRY

A wide range of physiological disorders has been reported within the f irst few hours of exposing intact plant roots to moderate levels of Al 3+. Past microanalytic studies, largely limited to electron probe x-ra y microanalysis, have been unable to detect intracellular Al in this t ime frame. This has led to the suggestion that Al exerts its effect so lely from extracellular or remote tissue sites. Here, freezedried cryo sections (10 mu m thick) collected from the soybean (Glycine max) prim ary root tip (0.3-0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for Al perm itted the first direct evidence of early entry of Al into root cells. Al was found in cells of the root tip after a 30-min exposure of intac t roots to 38 mu M Al3+. The accumulation of Al was greatest in the fi rst 30 mu m, i.e. two to three cell layers, but elevated Al levels ext ended at least 150 mu m inward from the root edge. Intracellular Al co ncentrations at the root periphery were estimated to be about 70 nmol g(-1) fresh weight. After 18 h of exposure, Al was evident throughout the root cross-section, although the rate of accumulation had slowed c onsiderably from that during the initial 30 min. These results are con sistent with the hypothesis that early effects of Al toxicity at the r oot apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of Al on cell function.