Knowledge of the mechanistic basis of differential aluminum (Al) tolerance depends, in part, on an improved ability to quantify Al located in the apoplastic and symplastic compartments of the root apex. Using root tips excised from seedlings of an Al-tolerant wheat cultivar (Triticum aestivum L. cv Yecora Rojo) grown in Al solutions for 2 d, we established an operationally defined apoplastic Al fraction determined with six sequential 30-min washes using 5 mM CaC12 (pH 4.3). Soluble symplastic Al was eluted by freezing root tips to rupture cell membranes and performing four additional 30-min CaC12 washes, and a residual fraction was determined via digestion of root tips with HNO3. The three fractions were then determined in Yecora Rojo and a sensitive wheat cultivar (Tyler) grown at 18, 55, or 140 Mm total solution Al (AIT). When grown at equal AlT, Tyler contained more Al than Yecora Rojo in all fractions, but both total Al and fractional distribution were similar in the two cultivars grown at AIT levels effecting a 50% reduction in root growth. Residual Al was consistently 50 to 70% of the total, and its location was elucidated by staining root tips with the fluorophore morin and examining them using fluorescence and confocal laser scanning microscopy. Wall-associated Al was only observed in tips prior to any washing, and the residual fraction was manifested as distinct staining of the cytoplasm and nucleus but not of the apoplastic space. Accordingly, the residual fraction was allocated to the symplastic compartment for both cultivars, and recalculated apoplastic Al was consistently approximately 30 to 40% of the total. Distributions of Al in the two cultivars did not support a symplastic detoxification hypothesis, but the role of cytoplasmic exclusion remains unsettled.Root lesions caused by Al toxicity can cause disruption of membrane structure and function, disruption of DNA synthesis and mitosis, cell wall rigidification and reductions in cell elongation, and/or disturbance in mineral assimilation and metabolism, and these postulated toxicity mechanisms have recently been reviewed by Taylor (29). Despite vast quantities of published research, however, the principal physiological mechanism(s) of Al rhizotoxicity remains unresolved, and it remains unclear which are primary dysfunctions and which should more properly be considered secondary effects. There are broad, genetically determined differences in Al tolerance between plant species and genotypes (8). Intraspecfic differences in responses to Al may provide clues to mechanisms of toxicity and aid in plant breeding for superior Al tolerance (8).In general, theories concerning mechanisms of differential tolerance may be divided into three categories (3, 30): (a) the primary lesion of Al rhizotoxicity is cytoplasmic, and differential tolerance is the result of variation in the ability of a plant to detoxify or tolerate Al within the symplast; (b) the primary lesion is cytoplasmic, and differential tolerance is a result of genotypic variation in abili...

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Granitic bedrock, generally weathered several meters deep, is widespread in upland areas of California. Sound management of these areas requires knowledge of the hydraulic properties of both soils and weathered bedrock. The objectives of this research were to determine the saturated hydraulic conductivity (Ksat) of weathered granitic bedrock and overlying soils and to relate the measured Ksat to the regolith morphology and porosity. The study sites in the foothills of the San Jacinto Mountains represent common regolith conditions in granitic terrain of southern California: a coarse‐loamy Entisol <50 cm thick and a fine‐loamy Alfisol ≈ 100 cm thick, both over weathered granitic rock. The underlying bedrock (Cr horizon) at each site has a low clay content (<6%) and is weathered such that chunks of it can be excavated by chopping with a spade and crushed into grains using bare hands. In situ Ksat, measured using constant‐head permeameters, was highest (29 cm h−1) in the Entisol AC horizon, which also had the greatest macrovoid (>0.1 mm diam.) space (24%), a low clay content (6%), and abundant roots to provide pore continuity. A pedogenic horizon also had the lowest Ksat, with the Alfisol Bt horizon averaging 0.9 cm h−1. This horizon had 9% macrovoid space and 21% clay. The Cr horizons had intermediate Ksat values (1.4–3.7 cm h−1). Low clay contents, joint traces, and macrovoids (7–9%) largely unimpeded by illuvial clay help make weathered granitic bedrock an effective conductor of water under saturated conditions. On a landscape scale, water should percolate readily into regolith on Entisol sites, whereas runoff is more likely on Alfisol sites due to the low Ksat of the Bt horizon.

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