ABSTRACT
Fig. 5.1. Schematized transversal cutting of two most widespread types of mycorrhizae: ectomycorrhiza (left) and arbuscular mycorrhiza (right). Ectomycorrhiza is characterized in most cases by a hyphal mantle covering the surface of mycorrhizal root and Hartig net - a net of fungal hyphae in intracellular spaces of the root cortex. Unlike arbuscular mycorrhiza, ectomycorrhiza is morphologically distinct from nonmycorrhizal root. In arbuscular mycorrhiza, the fungus does not form a hyphal mantle and Hartig net, nor penetrates into the internal spaces of cortical cells. Instead, it forms highly branched structures called arbuscules, ensuring most intimate contact between both partners in symbiosis
described as compact tangles of hyphae inside the plant cells (Hadley and Williamson, 1972). These structures are typical for major sub-type of orchideoid mycorrhizal symbiosis, the so-called tolypophagous sub-type. Dichotomically branched intracellular structures similar to arbuscules are not formed. Intracellular pelotons in orchideoid mycorrhizae undergo lysis in a manner similar to arbuscules in arbuscular mycorrhizae. Once the peloton is destroyed, the fungus may colonize the host cells again (Hadley, 1982). This cycle may be repeated several times. Such colonization occurs not only in the root cortex, but also in tissues of tiny germinating seeds of orchids, called protocorms. These almost imperceptible seeds do not have sufficient reserves of energy and usually cannot survive the first stages of germination without the intervention of mycorrhizal fungi (Arditti and Ghani, 2000). In fact, protocorms receive carbon nutrition from the mycorrhizal fungi. Generative reproduction of orchids in nature thus depends on symbiosis with mycorrhizal fungi and this is also reflected in artificial techniques of seed germination and seedlings cultivation (Stewart and Zettler, 2002; Sharma et al., 2003). Similarly, adult plants in some species of Orchidaceae lack active photosynthetic apparatus and thus have to rely on carbon nutrition gained from mycorrhizal fungus. This life strategy, where the plant is
Fig. 5.2. Fine mycelia of mycorrhizal fungi reach a much larger soil volume than roots and may, therefore, more effectively absorb mineral nutrients than the root itself
present in the soil in low amounts as mobile phosphates, which are inaccessible to plants. Mycorrhizal fungi take up phosphate ions and make them easily available for plants. This effect mainly translates into an increase of soil volume where the plant can obtain its mineral nutrition (Paszkowski and Boiler, 2002).
