The rhizosphere

The rhizosphere is a zone in the soil that is influenced by roots. Living roots change the atmosphere around them by using up oxygen and producing carbon dioxide (see respiration p118). Roots exude a variety of organic compounds that hold water and form a coating that bridges the gap between root and nearby soil particles. Micro-organisms occur in greatly increased numbers and are more active in proximity to roots. Some actually invade the root cells where they live as symbionts. The Rhizobium spp. of bacteria lives symbiotically with many legumes (see nitrogen cycle p366).

Symbiotic associations involving plant roots and fungi are known as mycorrhizae (see Figure 18.3). There is considerable interest in

Absorptive hyphae

Distributive hyphae

Spores

Soil

External hyphae Hartig net hyphae

Plant epidermis Plant cortex

Figure 18.3 Mycorrhizal structures. Ectomycorrhizae, top found mainly around tree roots have most of their structure on the outside whereas the Endomycorrhiza have most of their hyphae on the inside of the very wide range of plants with which they are symbiotic

Plant cortex Plant epidermis Intracellular hyphae

Absorptive hyphae

Distributive hyphae

Spores

Soil

External hyphae Hartig net hyphae

Plant epidermis Plant cortex

Figure 18.3 Mycorrhizal structures. Ectomycorrhizae, top found mainly around tree roots have most of their structure on the outside whereas the Endomycorrhiza have most of their hyphae on the inside of the very wide range of plants with which they are symbiotic exploiting the potential of mycorrhizae, which appear to be associated with a high proportion of plants especially in less fertile soils. In this symbiotic relationship the fungus obtains its carbohydrate requirements from the plant. In turn, the plant gains greater access to nutrients in the soil, especially phosphates, through the increased surface area for absorption and because the fungus appears to utilize sources not available to higher plants. Most woodland trees have fungi covering their roots and penetrating the epidermis. Orchids and heathers have an even closer association in which the fungi invade the root and coil up within the cells. The association appears to be necessary for the successful development of the seedlings. Mycorrhizal plants generally appear to be more tolerant of transplantation and this is thought to be an important factor for orchard and container grown ornamentals.

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