A wide range of physiological disorders has been reported within the first few hours of exposing intact plant roots to moderate levels of AI3+. Past microanalytic studies, largely limited to electron probe x-ray microanalysis, have been unable to detect intracellular AI in this time frame. This has led to the suggestion that AI exerts its effect solely from extracellular or remote tissue sites. Here, freezedried cryosections (10 pm thick) collected from the soybean (Glycine m a ) primary root tip (0.3-0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for AI permitted the first direct evidence of early entry of AI into root cells. AI was found in cells of the root tip after a 30-min exposure of intact roots to 38 p~ AI3+. The accumulation of AI was greatest in the first 30 fim, i.e. two to three cell layers, but elevated AI levels extended at least 150 pm inward from the root edge. Intracellular AI concentrations at the root periphery were estimated to be about 70 nmol g-' fresh weight. After 18 h of exposure, AI was evident throughout the root cross-section, although the rate of accumulation had slowed considerably from that during the initial 30 min. These results are consistent with the hypothesis that early effects of AI toxicity at the root apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of AI on cell function. ~~

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The absorption of N03-was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A x W182E) growing in a complete nutrient solution. Based on changing rates of '5N accumulation during 15-min time courses, translocation of the concurrently absorbed N through each region of the intact root was calculated and distinguished from direct absorption from the medium. Of the 15N accumulated in the 5-mm root tip after 15 min, less than 15 and 35% had been absorbed directly from the external solution at 0.1 and 10 mm N03-concentration of the external solution, respectively. The characterization of the apical portion of the primary root as a sink for concurrently absorbed N was conconfirmed in a pulse-chase experiment that showed an 81% increase of 15N in the 5-mm root tip during a 12-min chase (subsequent to a 6-min labeling period). The lateral roots alone accounted for 60% of root influx and 70% of 15-min whole root "5N accumulation at either 0.1 or 10 mm. NO3-concentration of the external solution. Because relatively steady rates of 15N accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying 15N to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in 15N during the chase experiment, confirming their role in rapid translocation.The plant root, as a well-organized collection of cells differing in both developmental stage and cell type, has long been known to exhibit regional differences in nutrient absorption, translocation, and assimilation processes. Accordingly, the description of root function as an average of the entire root is likely to lead to erroneous interpretations of regional transport and assimilation characteristics (1). In studies that have attempted to define transport characteristics within distinct root regions, however, difficulties are encountered in interpreting results quantitatively.Two general methodological approaches have allowed some assessment of absorption and translocation capacities of defined root region: (a) Use of a chambered system to isolate a single root zone, which can be bathed in labeled nutrient solution (25), and (b) exposure of an entire root system to labeled solution, followed by rapid separation of the regions (2). The first approach has been employed using perspex chambers, latex diaphragms, or plastic tubing for isolation of regions (e.g. 5, 7, 14, 20, 25 There have been two studies of N03-absorption along the root (5, 26). In the one study of NO3-absorption by root regions using the method of isolating regions after labeling, Yoneyama et al. (26) reported a high rate of [15N]NO3-absorption in two regions of intact low-salt corn roots, located 6 to 8.5 and 59 to 61 mm basal to the tip. The first time point in their study was 1 h. Neither of the two previous studies of NO3 uptake in root regions attempted to estimate unidirectional influx or translocation through regions.In this study, we attempt to define the N transport capacities...

The interaction between proton and aluminium toxicities was examined in soybean (Glycine max L.), pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.). Accurate characterization of genotype Al-sensitivity in solution culture experiments, employing a continuous Al-exposure, required response definition at both growth-enhancing and growth inhibiting activities. Essentially, this involved multiple controls to account for the interactive effects. Experiments measuring growth during a recovery period in Al-free solution were more successful in distinguishing genotypic response to Al, especially in species demonstrating high H+ -sensitivity. The time, concentration and basal solution dependencies of the characteristic genotype response for the three species examined were consistent with the hypothesis that responses of genotypes may be predicated upon conditions emphasizing irreversible (cell division) effects. Cell extension effects, however, appear to be rapid, largely-reversible and often not different between differentially Al-sensitive genotypes growing in a complete nutrient solution.