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Orchid variations

Posted: July 7, 2011 - 3:26pm
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The yellow coralroot cannot synthesize carbohydrates and is totally dependent on its fungal associate.    Photoo by Bob Armstrong
Photoo by Bob Armstrong
The yellow coralroot cannot synthesize carbohydrates and is totally dependent on its fungal associate.

Southeast Alaska has several species of orchid, which are not as gaudily showy as the types cultivated by orchid fanciers, but they have their own allure.

Many local folks are familiar with the white bog orchid, whose tall inflorescences send out a lovely aroma. This species is probably pollinated by moths that come to collect nectar. The calypso or fairy slipper orchid draws in bumblebee pollinators by looking lovely and smelling sweet, as if it offers nectar, but it has none. Visiting bees learn quickly that these flowers offer no food reward, so successful pollination depends on a supply of inexperienced bees. (Calypsos, and perhaps other orchids, should not be picked, because that tweaks the delicate root system and is likely to kill the plant.)

In addition to a wide variety of relationships with pollinators, orchids and many other flowering plants have an intimate relationship with fungi that connect with their roots. This mycorrhizal (fungus-root) relationship is classically thought to be mutualistic: both partners get something from it. The fungus gets carbohydrates from the plant, which typically has chlorophyll (green pigment) and synthesizes sugars that the fungus cannot make for itself. The flowering plant gets nutrients that the fungus gleans from the soil or decaying organic material. Some researchers suggest that mycorrhizal associations were probably essential when plants began to colonize land, millions of years ago.

Orchids, however, take mycorrhizal relationships to new levels of complexity. All orchids produce minute, dust-like seeds. The seeds are so tiny that they contain almost no stored carbohydrates or other nutrients that are needed for germination and growth. They rely on mycorrhizal associations to provide the nutrition needed for germination and initial growth. Thus, all orchids begin their lives as parasites, not mutualists, of fungi (the fungus gets nothing from the seed).

Now the fun begins! Some orchids have no green pigment, so they can’t photosynthesize carbohydrates to give to the fungus. These species remain parasitic on their fungi throughout their lives. The fungus may extract nutrients from the soil and decaying vegetation. However, in many cases, the root-associated fungus acts as a conduit for carbohydrates and other nutrients from a tree (which does have green pigment and can synthesize carbohydrates). So the orchid then is also indirectly parasitic on the tree to which it is connected. For example, in the yellow coralroot orchid (Corallorhiza trifida), which grows here, the fungal associate connects the roots of several species of tree to the orchid, and the orchid thus pirates nutrients from the trees. Even orchids with green pigment and photosynthetic ability may extract carbohydrates from the associated fungus (and a connected tree) without giving anything back. So they too, are at least semi-parasitic, in many circumstances.

In a further evolutionary complexity, many orchids “eat” their fungal associates, digesting the ends of fungal filaments that connect to the orchid. If the orchid does no photosynthesis, it thus seems to be destroying at least part of its essential source of nutrition. Even if the orchid can photosynthesize carbohydrates, digestion of filaments would interrupt the derivation of materials by the orchid from the fungus or a connected tree, at least partially.

The digestion of fungal filaments opens up many questions, to which I’ve found no concrete answers in the literature (although this fact has been known for over a hundred years): Why would the orchid destroy a major source of nutrients? Is it not needed any more? Or are only certain filaments eaten? What is the contribution of the digested filament itself to orchid nutrition, compared to what the filament delivers from a tree? What is the effect of filament digestion on the fungal organism? Does destruction of the orchid-connected filament tips affect the growth and reproduction of the fungus, as well as limiting its expansion in the orchid roots?

Whether parasite or mutualist, some orchids keep their mycorrhizal associations all their lives, some change their fungal associates as they grow, and some apparently become independent of fungi as they mature (especially if they grow in rich soil with good sunlight).

Things get still more complex. Within some orchid species, genetically different individuals have their own, specific mycorrhizal associates. For example, different genetic types of the spotted coralroot (Corallorhiza maculata), another local species, are reported to have different mycorrhizal associates, accompanied by subtle differences in floral shape. A given population of this coralroot orchid may contain several genetic types (or races) of the orchid, each with its own floral features and fungal associate, potentially deriving nutrients from a variety of trees.

Worldwide, the orchid family encompasses many thousands of species, hugely diverse in floral structure, as well as habitat, leaf shape, life history, and so on. The traditional explanation for the great diversity is adaptation to an equivalent diversity of pollinators. For instance, both of our local species of coralroot orchids are pollinated by small insects such as dance flies, in contrast to the bee-pollinated calypsos and the moth-pollinated bog orchids. However, it has recently been suggested that some of the great diversification of orchids may be related to adaptations to different fungal partners. The variety of floral design and fungal association within the single species of spotted coralroot suggests that this may be a step toward the origin of several new species.

• Mary F. Willson is a retired professor of ecology.

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