Tuesday, February 12, 2019

My Year of Orchids: Mycorrhizae-the plant fungus symbiosis in orchids

I knew from what I’ve read that fungi played a role in orchid germination. In that situation the orchid seeds, which lack nutrients of their own, depend on the fungus to supply them with what they need to germinate. I didn’t know that adult orchids also partner with fungi. And I’ve never had the opportunity to observe this in nature. Until today.

My friend Calvin gave me a big mount of Encyclia tampensis. The orchid had been growing on a dead oak branch, which now might be why it started to fail. Oak is good. But the orchids want live wood. The reasons are not clear to me, nor can I imagine how the orchid “knows” it’s on something dead. But nothing these creatures do comes as a surprise.

OK. Lunch at Calvin’s. Big bag with big bunch of orchids and dead wood. Next morning I went to work. The orchid hadn’t been thriving, at least recently, but it occupied a large area of the wood. Enough I’d say for several jumbo handfuls, if you could measure orchids that way. Because it was a big colony I decided to separate it however it would come apart and do a little experiment to see how the different clumps would grow in different parts of the garden. By the way Calvin came over about a week later and did a quick inspection. I think he was satisfied.

The clumps didn’t look too healthy but they weren’t dead. Some good pseudobulbs and stiff leaves and a lot of roots. They went up into the trees on a cold damp week and even as it has warmed up the mornings are foggy and dew is everywhere. So my daily misting is more a “want” than a “need” for the new orchids.

Today I went up close and saw roots on one clump that looked like typical mycorrhizal roots in terrestrial plants. The root tips were blunt, the roots themselves stubby and thicker than neighboring roots. Some were blackish, some whitish, but notably, none of them had the green scum you usually see on older roots.

If these we’re mycorrhizae it’s possible that the fungus was fending off or even digesting whatever green stuff (algae or photosynthetic bacteria) might have landed on the root. And perhaps transferred some of those nutrients to the plant.

Mycorrhizal fungi provide extra nutrients for their plant hosts. Usually the nutrients are transferred from the soil to the plant via the fungus. But what about in a situation where the plant roots are not in the soil, like orchids? The fungus must be making micronutrients available to the orchid in exchange for carbohydrates the orchid produces during photosynthesis. It makes sense that both organisms benefit from this partnership.

Mycorrhizal fungi in the soil go a step further and improve the delivery of water to their hosts’ roots. Partly this is strictly mechanical. The fungal cells (hyphae) are thinner and have more surface area than the plant roots. This way they can penetrate the soil more effectively than the plants or simply penetrate tinier spaces where water molecules are hiding.

What about in epiphytes? The crowning observation I made, the thing that got me really excited, was that fungal-covered roots retained water for longer. A lasting drop of water developed at the tip of every root infected with fungi. It stuck around longer than the water on uninfected root tips. Again, possibly a mechanical explanation. More (fungal) surface area, more water adhesion and cohesion at work.

Another thought. We know that fungal species have lots of different lifestyles, parasitic, saprobic, mycorrhizal, and lots and lots of in between. We know as well that fungi growing on wood have different molecular microhabitats. One wood rotter goes after nitrogen, another digests cellulose, and so on. Mycorrhizal fungi are some of the same wood inhabiting species. What if Calvin’s oak mount was just too old, too depleted to support a healthy fungal community. As the fungal ecosystem degraded and possibly simplified (less species on the log because of less nutrition) the orchid also started losing ground. We’ll have to keep a close eye on these babies and see how they develop over the months now that they’re not on their log. Sorry for the dad humor but LOTS of food for thought here.

Wonderful to observe these details that are actually vital to orchid growth in the wild. What an exciting day.

1 comment:

  1. I really enjoyed this article. It simplified the mycorrhizal fungi discussion better than the John Whitfield paper, which was great but a little too complex for someone like me who is just getting introduced to Botany.

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