External hyphae of vesicular arbuscular mycorrhizal fungi associated with trifolium subterraneum l. 1. spread of hyphae and phosphorus inflow into roots

Publisher Summary This chapter discusses the ecological implications of mycorrhizal associations in natural ecosystems and the role of soil or environmental factors, mycorrhizal fungus characteristics or host plant properties. Mycorrhizal associations are regulated by features of the host plant and mycorrhizal fungus, as well as by soil conditions and environmental factors. Factors which can influence the occurrence and effectiveness of mycorrhizal associations include (1) root properties (2) edaphic or climatic factors (3) soil organisms, (4) soil disturbance, and (5) host-fungus compatibility. Several other complex ecological topics also discussed in the chapter include (1) mycorrhizal phenology , (2) factors responsible for varying degrees of mycorrhizal dependency in host plants, (3) the role of mycorrhizal hyphae in soil (4) nutrient competition involving mycorrhizal and non-mycorrhizal plants, and (5) mycorrhizal interactions involving pollution and other stresses, the rhizosphere, soil properties and allelopathy. Finally, the chapter also discusses the population ecology of mycorrhizal fungi and the influence of their associations on plant population ecology.

Mycorrhizas in Natural Ecosystems

Publisher Summary The chapter presents a discussion on both rhizosphere and rhizoplane. Rhizosphere is defined as the soil adjacent to roots with a different physical, chemical, and biological environment from the bulk soil. The rhizoplane is defined as the actual root surface or soil interface. The chapter describes plant responses, the nature and dynamics of rhizosphere populations and their determinants, the management of the plant, and management of microorganisms. The chapter also explores the impact of soil conditions and agronomic practices on the rhizoshere microflora and suggests areas that require more study in order to achieve effective management. The chapter also illustrates different methods that are used by agronomists, plant pathologists, and microbiologists in order to modify the rhizosphere. In most cases, when microbiologists and plant pathologists needs to change the rhizosphere in order to improve plant growth they often think in terms of introducing microorganisms that possess the desired characteristics, be they antibiotic or siderophore production, or the ability to stimulate plant growth.

The rhizosphere and its management to improve plant growth

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were quantified over a growing season in a reconstructed tallgrass prairie and an ungrazed cool-season pasture. In both sites, hyphal lengths increased throughout the growing season. Peak external hyphal lengths were 111 m cm−3 of soil in the prairie and 81 m cm−3 of soil in the pasture. These hyphal lengths calculate to external hyphal dry weights of 457 μg cm−3 and 339 μg cm−3 of soil for prairie and pasture communities, respectively. The relationships among external hyphal length, root characteristics, soil P and soil moisture were also determined. Measures of gross root morphology [e.g., specific root length (SRL) and root mass] have a strong association with external hyphal length. Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured reduction in external hyphal length. Recovery for both the pasture and prairie occurred not by increasing SRL, but rather by increasing external hyphal length. This study suggests that growth is coordinated between VAM hyphae and root morphology, which in turn, are constrained by plant community composition and soil nutrient and moisture conditions.

External hyphal production of vesicular-arbuscular mycorrhizal fungi in pasture and tallgrass prairie communities.

Leek plants (Allium porrum L.) were grown in a controlled environment on a mixture (2: 1 w/w) γ-irradiated (1.0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble phosphorus (P) ranging from 22 to 344 mg P kg-1 (soil basis). Inoculum of the vesicular-arbuscular (VA) mycorrhizal fungus Glomus mosseae (Nicolson & Gerdemann) Gerdemann and Trappe was placed (M) or not (NM) in a layer 3 cm below the soil surface. At intervals of 10 d, lengths of main axes of roots and their lateral branches, and of the segments of infection within them, were measured. From these data we calculated the mean (harvest interval method} rates of linear extension of root tips and of infection fronts for each member of root, averaged over the whole root system. The mean delay, d, between a root encountering the layer of inoculum and the subsequent formation of internal infection, was also derived. Addition of P to soil did not affect rates of extension of roots, but increased the initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization. When bicarbonate-soluble P exceeded 140 mg kg-1 , the rates of extension of infection fronts in both main axes and laterals were approximately halved, and d was considerably increased. The density (the area ratio of fungal to host tissue in a longitudinal squash) of the hyphae and arbuscules respectively, and the number of entry points per unit length of root, were greatly reduced by added P. However, the ratios of numbers of entry points/hyphal density and of arbuscule density/hyphal density were unaltered, and the morphology of the fungus was not noticeably affected. It is probable that formation of entry points was the rate-limiting step for colonization, and that this rate was reduced by added P.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1982.tb03346.x

The development of endomycorrhizal root systems: VII. A detailed study of effects of soil phosphorus on colonization.

Many crop plants carry VA mycorrhizal infection. The fungi benefit their host principally by increasing the rate of phosphorus uptake from soils low in available phosphorus.

The development of vesicular-arbuscular mycorrhizal infection in plant root systems

Possibilities for modelling developing ectomycorrhizal root systems are discussed in the light of recent experience with vesicular-arbuscular mycorrhizal systems. Emphasis is given to experimental techniques and measurements which can provide suitable data on which to base models.

A note on modelling methods for studies of ectomycorrhizal systems

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The development of vesicular-arbuscular mycorrhizal infection in plant root systems

A note on modelling methods for studies of ectomycorrhizal systems