The orchid study blog : A little bit of orchid science

Orchids are very complex plants, I am sure some of you already know that some grow without soil and some grow with but before it gets to that stage, there is something about their germination that you need to know.

Orchids and fungi have a symbiotic relationship, this means that they each benefit from each other. And you ask yourself the question, “how do orchids benefit from fungi?” well here is the explanation.

From the moment the Zygot* is formed after pollination, it has to undergo a number of divisions to create the embryo. This first division happens in the whole seed creating two poles, one where growth happens, usually at the top of the seed and one that has a stalk like shape, usually this is at the bottom. This part of the seed is called the suspensor, this allows the seed to absorb and manufacture nutrients from the embryo. The only problem is, Orchid seeds have no embryo (see plate 1). So what happens now?

Orchid seeds still have suspensors in order to help the embryo absorb the necessary nutrients. But the fact that they don’t have an endosperm to hold the nutrients and necessary food sources like starch and protein like in other plants e.g. Barley (see plate 2), they rely on the fungi to get those missing food sources.

The fungi that help terrestrial orchids are called Hyphae, these are long filamented branching structures. Once they break into the Testa or seed coat, they do not cause any damage inside; instead they allow the seed to absorb the necessary nutrients to feed the embryo. The fungus penetrates the embryo through the suspensor cell, growing its hyphae in the inner embryonic cells in little coils that are called peletons. The Orchid then simply absorbs the peletons thus creating a nutrient source. When this has happened, the embryos mass increases to for a protocorm. Even when the protcorm is developing the Hyphae continue to enter the seed, though it is in larger numbers and continue to feed the plant until it produces a shoot with absorption hairs to increase root, soil and fungus contact.

It is still unknown what the fungus benefits from this relationship and it is said in some cases that the orchid is a parasite to the fungus. However the relationship between orchid and fungus continues. The orchid consumes the peletons as they are a main carbon source until chlorophyll organelles are produced, when this has happened the orchid no longer needs the peletons as it can get its carbon source from the process of photosynthesis, this can however depend on the orchid as some species only produce the organelles at a certain maturity and other species don’t produce any at all and depend entirely on the fungus, this is why in some cases they can be considered parasitic.

So, what happens when you want to grow orchids in a lab? This is where micro propagation or in vitro cultures come in and you can find the explanation for this on the next page. (The culture sheet project, 2008).