Cool Cacti Facts- Crassulacean Acid Metabolism (CAM)

Cacti, Agave and Yucca are highly adapted to the arid climates they inhabit. They have extensive root systems and even develop “rain roots,” where roots are expanded during rain events to capture as much water as possible. Since there are few opportunities to capture water in the desert ( or other arid climates) it’s important that cacti and other species don’t lose water once it’s captured.

Example of a Silver Cholla, which is covered in silvery white spines.

The spines on cacti, which are actually modified leaves, perform several functions. They provide shade to the main stem, reflect light and heat and form a protective barrier from wildlife, curious people, etc. Cacti also don’t have the typical stomata of a leaf for transpiration of water vapor, which in turn reduces water loss. In lieu of leaves, their stems are used for photosynthesis.

Transpiration, or process where water evaporation is released from plant leaves, is a natural occurrence in plant life. But cacti only have a limited amount of water to store for long periods so it’s crucial to limit evaporation as much as possible.

In order to reduce water loss, Cacti, Agave and Yuccas (most but not all) have developed Crassulacean acid metabolism or CAM. (Joshua tree for example, does not use CAM, but Banana Yuccas do use CAM. Other plants such as Tilladansia use CAM)

Essentially, plants using CAM close their stomata during the day and only open them for gas exchange at night. Thus reducing the possibility of additional water loss during the hot and dry parts of the day. The CO2 captured at night is stored as an acid (malic acid). This acid is then used during photosynthesis during the day, but the plants stomata remain closed. If there is an adequate supply of water, CAM plants may open their stomata and intake Co2.

This is a diagram from the UST Greenhouse project showing the C4 CAM pathway, which differs from the C3 photosynthesis pathway.

Stay tuned for my next post on the plants of Joshua Tree National Park!

Sources:

Cacti, Agaves & Yucca of California and Nevada, by Stephen Ingram. Cachuma Press, 2008.

UST Greenhouse project - https://ustgreenhouse.wordpress.com/tillandsia-air-plants/

Ask a Biologist - https://askabiologist.asu.edu/cam-plants