If you were to work out what a plant is, then near the top of the list would be things like “they grow in the soil”, “they’re green”, or “they make their own food”. Orchids tend to treat these as guidelines rather than rules, and research in New Phytologist examines how some orchids have learned to feed themselves through fungi, instead of making all of their own carbohydrates. The paper by Félix Lallemand and colleagues looks at these mixtrophic orchids. They’re mixtrophic because they get some carbon from fungi and some from the air through photosynthesis. The reason we know is they’re getting it this way is because not all carbon is the same.
Most carbon is the same, with six protons, six neutrons and six electrons. However, some carbon 13C has an extra neutron, making it a little bit heavier. This difference means that the carbon plants pull from the air is a little bit less likely to have 13C in it compared to the carbon fungi get. If you examine plant tissue and find it has more 13C in it than you’d expect, then you know it’s been getting it from fungi. Lallemand’s team looked at Cephalanthera damasonium to see what the orchid was doing with the mix of carbon.
C. damasonium, White Helleborine, is a good plant to test as it can act in a few ways. An albino form doesn’t photosynthesize, so it’s easy to compare it with the green form and see what differences the varied diets make. What they found is that different parts of the plant have carbon from different sources. The underground organs are built from fungal carbon. For green plants, it’s photosynthesis that supplies the carbon for the part of the plant above the soil.
Taking this information, the authors are able to put forward a few ideas about why the orchid behaves this way. For a start building a body takes energy and material. You lose this if you’re in the shade, but being able to take what you need from fungi opens up a lot more habitats. If fungi are taking care of the rhizomes then you can put the limited photosynthetic carbon into fruits.
The downside is that the fungal carbon isn’t really enough to support building seeds, and so there are fewer seeds to set. Lallemand and colleagues look at other solutions for reproduction. They say: “[M]ixotrophy predisposes to an evolution of heterotrophy since underground survival is already largely independent of photosynthesis.” Examples they give are orchids sending out stolons (a runner that can take root), or roots that can sprout themselves. These extra methods of reproduction could compensate for reduced seed set, they say. This explains how – once an ability to take food from fungi evolved – full heterotrophy evolved multiple times in related species, eliminating the need to rely directly on sunlight.
Lallemand, F., Figura, T., Damesin, C., Fresneau, C., Griveau, C., Fontaine, N., … Selosse, M.-A. (2018). Mixotrophic orchids do not use photosynthates for perennial underground organs. New Phytologist. https://doi.org/10.1111/nph.15443