CAM Photosynthesis

(From https://www.indefenseofplants.com/blog/2015/9/18/cam-photosynthesis)

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.