The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil P for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive 32P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.

A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants

High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed.

Effect of heavy metal pollution on mycorrhizal colonization and function: physiological, ecological and applied aspects

The essential micronutrients for field crops are B, Cu, Fe, Mn, Mo, and Zn. Other mineral nutrients at low concentrations considered essential to growth of some plants are Ni and Co. The incidence of micronutrient deficiencies in crops has increased markedly in recent years due to intensive cropping, loss of top soil by erosion, losses of micronutrients through leaching, liming of acid soils, decreased proportions of farmyard manure compared to chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production. Micronutrient deficiency problems are also aggravated by the high demand of modern crop cultivars. Increases in crop yields from application of micronutrients have been reported in many parts of the world. Factors such as pH, redox potential, biological activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root hair morphology (length, density, surface area), root-induced changes (secretion of H + , OH − , HCO 3 − ), root exudation of organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial associations (enhanced CO 2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant ability to absorb and utilize micronutrients from soil. The objectives of this article are to report advances in research on the micronutrient availability and requirements for crops, in correcting deficiencies and toxicities in soils and plants, and in increasing the ability of plants to acquire needed amounts of micronutrient elements.

Micronutrients in Crop Production

Arbuscular mycorrhizal fungi and organic farming

Arbuscular mycorrhizal fungi (AMF) benefit plants by allowing them to grow and produce in relatively harsh mineral stress environments. This has been attributed extensively to ability of AMF to expand the volume of soil for which mineral nutrients are made available to plants compared to what roots themselves would contact. This article reviews the effects of AMF on enhancing/reducing acquisition of phosphorus (P), nitrogen (N), sulfur (S), boron (B), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), zinc (Zn), copper (Cu), manganese (Mn), iron (Fe), aluminum (Al), silicon (Si), and some trace elements in plants. The nutrients enhanced most in host plants grown in many soils (e.g., high and low soil pH) are P, N, Zn, and Cu, but K, Ca, and Mg are enhanced when plants are grown in acidic soils. Many AMF have also the ability to ameliorate Al and Mn toxicities for plants are grown in acidic soil.

D. H. Lambert

Papers

The Role of Mycorrhizae in the Interactions of Phosphorus with Zinc, Copper, and Other Elements1