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