Mechanisms of Aluminum Tolerance in Triticum Aestivum L. (Wheat). I. Differential Ph Induced by Winter Cultivars in Nutrient Solutions

Abstract

Twenty winter cultivars of Triticum aestivum L. (wheat) were grown in solution culture with and without aluminum (Al) (74 μM, 2.0 mg L‐1) for 14 days. Exposure to Al increased root growth of the most tolerant cultivar, while both root and shoot growth were depressed in all other cultivars. On the basis of a root tolerance index (RTI = weight of roots grown with Al/weight of roots grown without Al), cultivar tolerance to Al ranged 9‐fold, from 0.13 ± 0.01 to 1.16 ± 0.10. Symptoms of Al toxicity were most evident on roots. Aluminum‐affected roots were relatively short and thick and had numerous undeveloped laterals. Leaves of some cultivars showed chlorosis resembling iron deficiency, and others showed purple stems typical of phosphate deficiency. Plants of all cultivars grown with and without Al depressed the pH of nutrient solutions, presumably until NH4+ was depleted, at which point the pH increased. Cultivar tolerance, expressed both as the root tolerance index and a shoot tolerance index, was negatively correlated with the negative log of the mean hydrogen ion (H+) concentration, the minimum pH, and the slope of the pH decline, each calculated from pH data collected during the first 9 days of the experimental period before any sharp rises in pH occurred. These results are consistent with the hypothesis that the Al tolerance of a given cultivar is a function of its ability to resist acidification of the nutrient solution and hence to limit the solubility and toxicity of Al.

Measurements of H+ Fluxes and Concentrations in the Rhizosphere

“…Several authors [13,[143][144][145][146][147] have shown that the tolerance to soil acidity and Al toxicity in certain species or genotypes was related to their ability to take up H ϩ and thereby to increase rhizosphere pH. Indeed, aluminum is a toxic element that severely limits growth at pH below 5.0 as a consequence of its pH-dependent speciation and the considerable increase in the solubility of aluminum-bearing minerals with decreasing pH.…”

Coadaptation of Plants to Multiple Stresses in Acidic Soils

“…Subsequently, however, Taylor (1988) demonstrated that the superior Al tolerance of Atlas-66 was not related to its ability to maintain a high solution pH by NO 3 − uptake. Glutamine synthetase Taylor and Foy, 1985a, 1985b, 1985c *Decrease for some genotypes but not for other genotypes. (The relationship between Al tolerance and medium pH is discussed in detail later.)…”

“…Thus, it is logical to expect that plants will be more Al tolerant growing on NO 3 − -containing medium than on NH 4 + -containing medium (Taylor and Foy, 1985a, 1985b, 1985c. In general, NH 4 + predominates in acidic soils because of their low nitrification rates, whereas NO 3 − is the main form of N in neutral and calcareous soils.…”

“…The intensification of Al toxicity by NH 4 + (Fleming, 1983) can be attributed to NH 4 + toxicity because wheat is very sensitive to NH 4 + . In addition, early research by Taylor and Foy (1985a, 1985b, 1985c proposed that the Al tolerance of wheat was negatively correlated with NH 4 + uptake and positively correlated with NO 3 − uptake because the medium pH was decreased by NH 4 + and increased by NO 3 − . When NH 4 + toxicity prevails over Al toxicity, NH 4 + addition, especially at higher concentrations, could aggravate Al toxicity of NH 4 +sensitive plants.…”

An investigation into the use of Australian native plants and designer metal-binding particles for remediation of metal-contaminated landscapes

“…Symptoms of Al toxicity in wheat (Triticum aestivum L.) were relatively short and thick roots with numerous undeveloped laterals (Taylor and Foy, 1985). Aluminium toxicity symptoms were also observed in tropical leguminous plants, for example darkening, shrinking and inhibition of lateral root growth two days after transplanting (Meda and Furlani, 2005).…”