CAM Photosynthesis

 

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I was in a lecture the other day and I heard something that made the plant nut inside of me chuckle. The professor was trying to make the point that C3 photosynthesis is the most common photosynthetic pathway on the planet. To do this he said "it is the vanilla pathway." In this context, he was using vanilla as an adjective meaning "plain or ordinary." Of course, this was all very facetious, however, I thought it interesting and funny how, if taken literally, that statement was just plain wrong. 

I have written before about the reproductive ecology of Vanilla orchids (http://bit.ly/1LcC857). They are anything but vanilla the adjective. The other part of the statement that was wrong (again, if taken literally) is that C3 is the photosynthetic pathway of the vanilla orchid. In reality, vanillas are CAM photosynthesizers.

Last week I wrote about the C4 pathway and how it has helped plants in hot, dry places, but the CAM pathway is yet another adaptation to such climates. The interesting thing about CAM photosynthesis is that it separates out the different reactions in the photosynthetic pathway on a temporal basis. 

CAM is short for Crassulacean acid metabolism. It was first described in succulents in the family Crassulaceae. Hence the name. Similar to the C4 pathway, CO2 is taken into the leaves of the plant and stored as an organic acid. This is where the process differs. For starters, having acid hanging around inside your leaves is not necessarily a good thing. CAM plants deal with this by storing it in large vacuoles. That is one reason for the succulent appearance of many CAM species. 

Because these plants so often grow in hot, dry climates, they need to minimize water loss. Water evaporates from holes in the leaves called stomata so to avoid this, these holes must be closed. However, closing the stomata means not letting in any CO2 either. Whereas C4 plants get around this by only opening their stomata during the cooler hours of the day, CAM plants forgo opening their stomata entirely when the sun is up. 

Instead, CAM plants open their stomata at night when the vapor pressure is minimal. This ensures that water loss is also minimal.  Like camels storing water for lean times, CAM plants store CO2 as organic acid to use when the sun rises the next day. In this way, CAM plants can close their stomata all the while the hot sun is baking the surrounding landscape yet still undergo ample photosynthesis for survival. 

Not all orchids do this. In fact, some can switch photosynthetic pathways in different tissues. However, there are many other CAM plants out there including some very familiar species like pineapples, cycads, peperomias, and cacti. If you're like me and prone to talking to your plants, it is probably best to talk to your CAM plants after the sun has set. Not only does it confuse neighbors and friends, it provides them with CO2 when they are actively absorbing it. 

Photo Credit: C T Johansson (Wikimedia Commons)

Further Viewing: https://www.khanacademy.org/science/biology/cellular-molecular-biology/photosynthesis/v/cam-plants

Vanilla Is Anything But Vanilla

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Vanilla companies seem to be lacking in their plant ID skills. You so rarely see any vanilla products with the correct flower on the label. While I can't speak for everyone, I think I may have a hunch as to why most companies slap a white Phalaenopsis, Dendrobium, or any of the other orchid flowers that could remotely pass for being Vanilla on their products. At the same time, it also explains the rather pricey nature of Vanilla "beans."

The answer, I believe, lies in the flowers themselves. Vanilla is a genus of orchids that contains roughly 110 species that span the tropical regions of the globe. They are vining orchids, climbing trunks of trees in an attempt to make their bid for the canopy. Some Vanilla orchids have lost their leaves entirely, relying solely on their green, photosynthetic stems and roots. The species that gives us the highly coveted vanilla flavor is Vanilla planifolia from Central America.
 

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Vanilla planifolia, like most other species of Vanilla, produce very short-lived, non-selfing flowers. They open up as the sun begins to rise and are mostly closed by afternoon. Vanilla are not self-fertile so if the flower has not been fertilized by afternoon, it will simply wither and fall off. Because of their ephemeral nature, it is probably hard for most vanilla companies to do the kind of photo shoot they would need to do their marketing. It is likely that they just fall back on orchids that kind of look like Vanilla and I am sure that outside of us botanical enthusiasts, no one really faults them for it.

The Vanilla reproductive strategy also lends to the pricey nature of real Vanilla "beans." In the wild, Vanilla relies on stingless bees for pollination. In most cases, Vanilla growers do not rely on the bees because, if they are present, fertilization rates are often extremely low. And, if the bees are not present, the plants will not reproduce on their own. Because of this, Vanilla growers must hand pollinate all of the flowers individually.

This is a labor intensive process that must be done at just the right time if it is to work. The resulting "bean" is not a bean at all but rather a large capsule filled with millions of dust-like seeds. The capsules themselves require about 6 weeks to fully mature and then sometimes as long as 9 months to properly cure and produce their characteristic vanilla flavor. So yes, I think it is safe to say that Vanilla is anything but vanilla.

Photo Credit: [1] [2]

Further Reading: [1]