From the earliest origins of a land flora, more than 400 million years ago, there has been a structural-functional interaction between plant roots and arbuscular mycorrhizae (AM) [Fig. 2.4(a)] as shown in the fossil record (Pirozynski and Malloch, 1975; Malloch et al., 1980). Probably the earliest land plants arose during the Ordovician period and were similar to present-day hornworts and liverworts (Redeker, 2002). Today's hornworts and liverworts do form associations with AM fungi, but because they do not have true roots, they do not meet all the criteria of AM (Read et al., 2000). Most families of terrestrial plants have mycorrhizal symbionts; however, two families—the Cruciferae and the Chenopodiaceae are conspicuous exceptions (Allen, 1991). Indeed, recent analyses have shown that zygomycetous fungi colonize a wide range of lower land plants (hornworts, many hepatics, lycopods, Ophioglossales, Psilotales [horsetails], and Gleicheniaceae) (Read et al., 2000). These associations are structurally analogous to mycorrhizas, but their functions remain to be determined (Read et al., 2000).
The ectomycorrhizae, or ECM [Fig. 2.4(b)], are prevalent in several tree families such as the Fagaceae (including the beeches and oaks), and also in the Pinaceae within the conifers. ECM arose relatively recently, only 160 million years ago, in the Cretaceous period (St. John and Coleman, 1983).
After examining the structures of the principal types of mycorrhizae, we will consider information on carbon costs to the plant as well.
Symbiotic Associates of Roots
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