The results of analyses of the distribution, structure and function of ericoid, ecto and vesicular-arbuscular mycorrhizas are used to challenge the conventional view, which was based largely upon studies of isolated plants and excised plant roots under controlled conditions, that the symbiosis is primarily involved in the capture of phosphate ions. In nature, each mycorrhizal type is associated with an ecosystem and soil environment with distinctive characteristics in which selection has favoured the development of a particular range of attributes. These attributes are evaluated and their importance for the individual plant and for the ecosystems in which they occur is assessed. It is concluded that knowledge of the full range of functions of each mycorrhizal type is essential for an understanding of the distribution and dynamics of the ecosystem in which it predominates.

Mycorrhizas in ecosystems

summary
One of the most dramatic effects of infection by vesicular-arbuscular mycorrhizal fungi on the physiology of the host plant is an increase in phosphorus absorption. When phosphorus is limiting, the maximum extent to which mycorrhizal infection can improve plant performance is thus predicted to be a function of the phosphorus deficit of the plant, the difference between phosphorus demand and phosphorus supply. Phosphorus demand is defined as the rate of phosphorus absorption that would result in optimum performance of the plant as measured by growth rate, reproduction or fitness. The phosphorus supply is defined as the actual rate of phosphorus absorption under the prevailing conditions. Variation among plant taxa in morphological, physiological or phenological traits which affect either phosphorus demand or phosphorus supply (and thus phosphorus deficit) is predicted to lead to variation in potential response to mycorrhizal infection. The actual response to mycorrhizal infection is predicted to be a function of the increase in phosphorus uptake due to mycorrhizal infection and the phosphorus utilization efficiency of the plant. Demonstrated variability in responsiveness to mycorrhizal infection among plant taxa suggests that mycorrhizal fungi may play an important role in determining the structure of plant communities.

Mycorrhizal infection may alter the phosphorus deficit or phosphorus utilization efficiency independently from its direct effect on phosphorus uptake, making the prediction of response to mycorrhizal infection based on the traits of non-mycorrhizal plants quite difficult. For example, infection may at times increase the rate of phosphorus accumulation beyond that which can be currently utilized in growth, reducing the current phosphorus utilization efficiency. Such momentary ‘luxury consumption’ of phosphorus may, however, serve a storage function and be utilized subsequently, allowing mycorrhizal plants ultimately to outperform non-mycorrhizal plants.

Nutrient supply, nutrient demand and plant response to mycorrhizal infection.

Arbuscular mycorrhizal fungi influence growth, osmotic adjustment and photosynthesis of citrus under well-watered and water stress conditions.

The enhanced growth of plants infected by vesicular–arbuscular (VA) mycorrhizal fungi results primarily from improved uptake of soil phosphate. Extra phosphate reaches the root through the fungal h...

The physiology of vesicular-arbuscular endomycorrhizal symbiosis

Publisher Summary The chapter discusses mycorrhiza, a symbiotic, non-pathogenic association between a plant root and a fungus. This paper first summarizes evidence that mycorrhizal links between plants that occur. It then reviews evidence on various possible functions of these links, and finally considers if there is any evidence that the links influence the species composition and structure of plant communities, and ecosystem processes such as nutrient cycling. Evidence on almost every aspect of mycorrhizal links and their possible roles is inadequate; any conclusions must therefore be preliminary and tentative. There is evidence that seedlings can become infected by forming mycorrhizal links with established plants, and growth of seedlings of some species is faster under these conditions. Transfer of I4C from one plant to another via mycorrhizal links can occur, but it is not clear that net transfer of organic compounds is ever great enough to increase significantly growth or survival of the receiver plant. There is no clear evidence that mycorrhizal links prevent these relationships from occurring or introduce fundamentally new interactions between plants. The evidence so far available suggests that mycorrhizal links can alter the relationships between plants, but that they do so by modifying competition and nutrient cycling rather than replacing them.

Mycorrhizal Links Between Plants: Their Functioning and Ecological Significance

Onion plants (Allium cepa L, cv. Downing Yellow Globe) grown in pots and infected by the mycorrhizal fungusGlomus etunicatus Becker and Gerdemann were more drought tolerant than were non-mycorrhizal individials when exposed to several periods of soil water stress separated by periods of high water supply, as shown by greater fresh and dry weights and higher tissue phosphorus levels in the mycorrhizal plants. The tissues of stressed, non-mycorrhizal plants were deficient in P, despite the fact that only non-mycorrhizal plants were fertilized with high levels of P (26 mg P per 440 g soil). Differences in plant water relations (leaf water potentials or transpiration rates) and changes in soil P levels which may have affected plant growth were investigated, and discounted as factors important for the results. The P nutrition of plants has been implicated in the ability of plants to tolerate drought and it was concluded that the ability of the mycorrhizal fungus to maintain adequate P nutrition in the onions during soil water stress was a major factor in the improved drought tolerance. Infection of the root by the fungus was found not to be affected by water stress or P fertilization but fungal reproduction, as determined by spore numbers in the soil, was decreased by water stress and by P fertilization.

C. E. Nelsen

Papers

Increased drought tolerance of mycorrhizal onion plants caused by improved phosphorus nutrition.