C. D. Foy

Aluminum toxicity is discussed, including general effects (symptoms and physiological effects), differential aluminum tolerance in plants, beneficial effects of aluminum, and the genetic control of aluminum tolerance. Manganese and iron toxicity are discussed in the same framework. The toxicity of other metals (Zn, Cu, Ni, Cd, and Pb) is also discussed, though not as extensively as aluminum, manganese, and iron. 476 references.

Soil acidity is a major growth-limiting factor for plants in many parts of the world. Soils are acid because their parent materials were acid and initially low in the basic cations or because these elements have been removed from the soil profile by normal rainfall leaching or the harvesting of crops. The direct effects of the hydrogen ion on plant growth are difficult to determine in acid soils, because at soil pH levels where it is considered harmful, aluminum, manganese, and other mineral elements may also be soluble in toxic concentrations, and the availabilities of essential elements, particularly calcium, magnesium, phosphorus, and molybdenum, may be suboptimal. Stress-tolerant and stress-sensitive genotypes are also valuable indicators of present and potential problems of mineral stress in soils, particularly when used in conjunction with conventional soil testing procedures. Vose emphasized the potential value of plant mutants in studying physiological processes.

Abstract Aluminum (Al) toxicity is an important growth‐limiting factor for plants in many acid soils. The problem is not always economically correctable with conventional liming practices. But plant species and genotypes within species differ widely in tolerance to excess Al, and some of these differences are genetically controlled. Hence, an alternative or supplemental approach to the problem is to select or breed plant genotypes having greater tolerance to Al. A plant genetic approach has great potential for solving difficult soil fertility problems such as Al toxicity in acid subsoils. An important part of this approach is the determination of plant genetic, physiological and biochemical mechanisms by which plants tolerate mineral stress. Better understanding of stress tolerance mechanisms could lead to the development of more tolerant plants and more effective liming and fertilization practices for plants already in use. The objective of this presentation is to discuss our state of knowledge concerning the range of plant tolerance to excess Al, genetic control of tolerance and release of Al‐tolerant germplasm, and plant physiological and biochemical characteristics associated with differential Al tolerances among genotypes within species.

One proposed mechanism of aluminum (Al) tolerance in plants is the release of an Al-chelating compound into the rhizosphere. In this experiment, two cultivars of snapbeans (Phaseolus vulgaris L. "Romano" and "Dade") that differ in Al tolerance were grown hydroponically with and without Al under aseptic conditions. After growth in nutrient solutions for 8 days, aliphatic and phenolic organic acids were analyzed in the culture solutions with an ion chromatograph and a high pressure liquid chromatograph. The tolerant snapbean, "Dade", when exposed to Al, exuded citric acid into the rhizosphere in a concentration that was 70 times as great as that of "Dade" grown without Al, and 10 times as great as that of "Romano" grown with or without Al. The sensitive cultivar, "Romano", exuded only slightly more citric acid into the growing medium under Al-stress, compared to nonstressed conditions. Citric acid is known to chelate Al strongly and to reverse its phytotoxic effects. Also, citric acid has been shown previously to enhance the availability of phosphorus (P) from insoluble Al phosphates. Thus, one mechanism of Al-tolerance in snapbeans appears to be the exudation of citric acid into the rhizosphere, induced either by toxic levels of Al or by low P due to the precipitation of insoluble Al phosphates. Our experiment was not able to distinguish between these two factors; however, tolerance to both primary and secondary Al-stress injuries are important for plants growing in Al-toxic soils.