Aluminum Tolerance in Wheat (Triticum aestivum L.) (I. Uptake and Distribution of Aluminum in Root Apices)

Abstract

We investigated the uptake and distribution of AI in root apices of near-isogenic wheat (Triticum aesfivum L.) lines differing in AI tolerance at a single locus (Altl: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 p~ AI, and the roots were subsequently stained with hematoxylin, a compound that binds AI in vitro to form a colored complex. Root apices of Alsensitive genotypes stained after short exposures to AI (10 min and 1 h), whereas apices of AI-tolerant seedlings showed less intense staining after equivalent exposures. Differential staining preceded differences observed in either root elongation or total AI concentrations of root apices (terminal 2-3 mm of root). After 4 h of exposure to 100 p~ AI in nutrient solution, AI-sensitive genotypes accumulated more total AI in root apices than AI-tolerant genotypes, and the differences became more marked with time. Analysis of freeze-dried root apices by x-ray microanalysis showed that AI entered root apices of AI-sensitive plants and accumulated in the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to AI (24 h) resulted in a distribution of AI coinciding with the absence of K. Quantitation of AI in the cortical layer showed that sensitive apices accumulated 5-to 10-fold more AI than tolerant apices exposed to AI solutions for equivalent times. These data are consistent with the hypothesis that Altl encodes a mechanism that excludes AI from root apices.A1 toxicity is one of the major factors that limit plant growth in many acid soils (Wright, 1989). The primary effect of A1 is to inhibit root growth in Al-sensitive genotypes with subsequent effects on nutrient and water uptake (Foy, 1983). Root elongation is affected within hours of A1 exposure (Wallace et al., 1982), and, as in many plant species, tlie primary site of A1 toxicity in wheat (Triticum aestivum L.) appears to be the root apex (Bennet and Breen, 1991). have shown that in wheat and maize, root elongation is inhibited only when apices are exposed to Al, whereas selectively exposing the remainder of the root does not inhibit elongation. Hematoxylin, a stain for Al, stains root apices of Al-sensitive wheat genotypes more intensely than root apices of Al-tolerant genotypes, but the remainder of the root shows the same degree of staining in different genotypes (Polle et al., 1978;Wallace et al., 1982), indicating that tolerance might be a property of the root apex.Differential uptake of A1 into roots could account for differences in tolerance between genotypes, but conflicting results have been reported regarding differences in A1 uptake in roots of different wheat genotypes. Some of these conflicting results appear to be due to the size of the root portion analyzed and the time of exposure to Al. Recently RincÓn and Gonzales (1992) showed that an Al-sensitive wheat cultivar accumulated more A1 in its root apices (2 mm terminus of root) than an Al-tolerant cultivar, which is consistent with the above discus...

Direct Evaluation of the Ca2+-Displacement Hypothesis for Al Toxicity

“…Third, we have demonstrated that the accumulation of A1 in the apices of AI-tolerant roots was significantly less than in AI-sensitive genotypes (Table 111). This agrees with previous studies by Polle et al (1978), Tice et al (1992), Rincón and Gonzales (1992), and Delhaize et al (1993a), but in the present study we included severa1 improvements. First, we used near-isogenic lines of wheat, which allowed us to be confident that any differences observed between the tolerant and sensitive genotypes were related to the Alfl locus, which confers AI tolerance in these lines (Delhaize et al, 1993a).…”

“…This agrees with previous studies by Polle et al (1978), Tice et al (1992), Rincón and Gonzales (1992), and Delhaize et al (1993a), but in the present study we included severa1 improvements. First, we used near-isogenic lines of wheat, which allowed us to be confident that any differences observed between the tolerant and sensitive genotypes were related to the Alfl locus, which confers AI tolerance in these lines (Delhaize et al, 1993a). Second, we chose concentrations of AI that were at least 20-fold lower than those used previously and that inhibited growth in the Al-sensitive genotype but not the Al-tolerant genotype.…”

“…The genotypes ET8 and ES8 are nearisogenic lines that differ in A1 tolerance at a single locus designated Altl (Delhaize et al, 1993a). Seeds were surfacesterilized with 5% hypochlorite for 2 min, washed severa1 times, and then soaked for 24 h in aerated, deionized water.…”

The use of hematoxylin in screening perennial ryegrass (Lolium perenne) for aluminium tolerance

“…Differential staining of root apices with hematoxylin preceded detectable differences in total AI concentrations found in the root apices of wheat (Triticum aestivum) plants differing in Al tolerance (Delhaize et at., 1993). It is, therefore, presently not possible to account for, with any certainty, the differences observed in the staining intensity of plant roots treated with Al and stained in hematoxylin.…”

Macronutrients accumulation and growth of pineapple cultivars submitted to aluminum stress

“…Rincón & Gonzalez (1992) observed that the level of sensitivity to Al was correlated with its concentration in the meristems, suggesting that the metabolic exclusion in the meristems was an important mechanism of tolerance. Delhaize et al (1993) investigated the function of the organic acids and observed that the presence of Al in the nutrient solution stimulated the secretion of malic acid. These authors also found that the final 3 to 5 mm of the roots formed a primary source of the secreted organic acid; thus, there was a correlation between tolerance and high secretion of root exudates.…”