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Achlorophyllous plants

Achlorophyllous plants are puzzling! They are always characterized by remarkable reductions concerning root, shoot and leaf structure and sometimes even hardly resemble a flowering plant. It is no wonder that they are often collected by mycologists! Here some basic information:

Since assimilation of carbohydrates through photosynthesis without chlorophyll is impossible (as far as we know) and the direct metabolization of dead organic material has never been detected in flowering plants, achlorophyllous plants must have another source of carbon. They split up in two distinct groups:

Parasites

These plants develop special organs (haustoria) penetrating foreign plant tissue in order to participate at least to some extend from their host’s assimilates (carbon compounds), water or nutrient uptake (e.g. Dodder, Broomrapes).

Mycoheterotrophic Plants (‘Saprophytes’)

In these plants a fungus (or several?) lives inside their roots (‘mycorrhiza’) providing all requirements for plants growth. This is the plant group that I investigate.

Voyria truncata just emerged upon the soil surface. The pencil-sharpener serves as a scale

More than 400 species, in 87 genera and 11 families, of mycoheterotrophic plants have been described. Of those, only orchids and members of the Monotropaceae (Indian-Pipe Family) are fairly well investigated. Information about the other families are scarce. The most recent work on the neglected genera has been done by Hiltje and Paul Maas and co-workers in Utrecht/Netherlands. Nevertheless, root structures (morphology, anatomy, mycorrhiza) are often entirely unknown. Most probably this is due to the remote and hardly accessible habitat of these plants, the deep shaded tropical rainforest, and the fact that they are easily overlooked in the field (see the picture to your left!). Hence, they get found by mycologists!

Lately, I focused on the genus Voyria of the Gentianaceae (Gentian Family) where 19 species have been distinguished so far, all except one living in tropical America. Then I looked after Triuridaceae and Burmanniaceae (TriurisSciaphilaBurmanniaDictyostega). Momentarily, I’m working on Burmanniaceae and Polygalaceae (AfrothismiaEpirixanthes). All of these plants share some morphological characters with Voyria but are not at all related to them. I could show that their mycorrhiza is an arbuscular mycorrhiza (AM), a form of fugus-plant-symbiosis which is very well known for more than hundred years. However, the achlorophyllous species so far investigated revealed some very unique features, yet unknown despite the long and intensive research on AM.

My approach was led by the following questions. Answers I found so far are indicated shortly:

What kind of morphological/anatomical adaptations have evolved in connection to its special life form?
At least one of those adaptations is a ‘condensation’ of the root system (becoming short and thick).
Are mycorrhizas in myco-heterotrophic species different from mycorrhizas in autotrophic species?
Yes, definitely in Voyria tenella, V. obconica, V. aphylla, Triuris hyalina, and Afrothismia winkleri, less pronounced but still different in Voyira truncata, Burmannia tenella, and Dictyostega orobanchoides. More strange mycorrhizal patterns may be anticipated.
Do the mycorrhizas between various myco-heterotrophic species differ?
Yes they do, only in Voyria tenella and V. obconica I found the same ‘intraradical fungus garden’.
What do mycorrhizal structures tell us about taxonomy and systematics?
The closely related Voyria tenella and V. obconica do have the same mycorrhiza whereas V. aphylla shows an intermediate pattern, linking to the mycorrhiza of V. truncata and the autotrophic gentians. The two Burmanniaceae Burmannia tenella and Dictyostega orobanchoides show at least in the root cortices the same intracellular hyphal pattern. Afrothismia winkleri, a Burmanniaceae from Africa, however, has an entirely deviating mycorrhizal pattern (although it is an AM!).
How do these plants use their root fungus?
Very sophisticated!! Please read the abstracts e.g. on Afrothismia winkleri and Voyria tenella .
What is the actual carbon source?
From the observed direct hyphal bridges between roots of neighboring plants and the achlorophyllous plants we must infer the carbon (and most probably everything else too) must come from the neighboring plant.

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