Saprotrophic parasitic and hemiparasitic plants

A number of plants have lost their autotrophic status by losing their ability to photosynthesize, and live instead as heterotrophs, either as saprotrophs or parasites. The saprotrophs essentially live by extracting food from decomposing leaf litter using fungi as an intermediary and so should be called myco-heterotrophs (living off fungi), as outlined in Box 3.1. It is often difficult to tell what the fungus receives in return from the plant; perhaps the plant is primarily parasitic on the fungus? The parasitic plants are simpler in that they have a direct root connection such that the parasite draws all its needs (water, minerals, sugars and other compounds) directly from the host. To do so the parasite has to overcome a good deal of host resistance which requires a high degree of biochemical specialization, so parasites tend to be specific to a small range of hosts.

One solution to host resistance is to be a hemiparasite, a plant that takes water and minerals from the host but remains green and capable of photosynthesis. Water is extracted via short modified roots that form haustoria in the host. Because these are extracting liquid from dead tissue (the xylem) the host resistance is more easily overcome and a wider range of hosts are available. (The term semiparasitic is avoided since this implies it is partly parasitic, i.e. gets some of its carbon from the host.) Hemiparasites include many of the mistletoes living on trees, including members of the family Viscaceae (notably the European mistletoe Viscum album and the broadleaf mistletoes Phoradendron spp. of the New World -see Fig. 5.11a) and members of the Loranthaceae in the southern hemisphere. These are obviously green and photosynthetic, and gain advantage by being able to grow high up in a tree canopy in the light while stealing water from the host. On the whole they do little damage although heavily infected trees can lose vigour. Just to add confusion, it appears that some mistletoes can also steal sugars, and it should be noted that species of Arceuthobium (Viscaceae), the dwarf mistletoes (Fig. 5.11b), are small, often yellow and are completely parasitic! They are notoriously destructive pathogens of conifers in western North America and Asia.

Other forest hemiparasites are 'rooted' in the ground and look like normal green plants. Many are members of the figwort family (Scrophulariaceae)

David Morse Florida Forest Service

Figure 5.11 (a) Broadleaf mistletoe Phoradendron sp. on oak in eastern USA; (b) American dwarf mistletoe Arceuthobium americanum on pine in western USA (a: Photograph by Edward L. Barnard, Florida Department of Agriculture and Consumer Services, www.forestryimages.org; b: David W. Johnson, USDA Forest Service, www.forestryimages.org.)

Figure 5.11 (a) Broadleaf mistletoe Phoradendron sp. on oak in eastern USA; (b) American dwarf mistletoe Arceuthobium americanum on pine in western USA (a: Photograph by Edward L. Barnard, Florida Department of Agriculture and Consumer Services, www.forestryimages.org; b: David W. Johnson, USDA Forest Service, www.forestryimages.org.)

including the Indian paintbrushes Castilleja of North America and the circumpolar cow-wheats Melampyrum. They gain a competitive advantage by not having to grow roots and can invest more in above-ground growth.

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Responses

  • damian
    Is mistletoe a saprotrophs?
    6 years ago

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