CAM Plants: Survival in the Desert

Pineapple plantation
The pineapple is a CAM plant. Daisuke Kishi / Getty Images

Say you had two plants in your windowsill—one a cactus, and the other a peace lily. You forget to water them for a few days, and the peace lily wilts. (Don't worry, just add water as soon as you see that happen and it snaps right back to life, most of the time.) However, your cactus looks just as fresh and healthy as it did a few days ago. Why are some plants more tolerant to drought than others?

What Is a CAM Plant?

There are several mechanisms at work behind drought tolerance in plants, but one group of plants possesses a way to utilize that allows it to live in low-water conditions and even in arid regions of the world such as the desert.

These plants are called Crassulacean acid metabolism plants, or CAM plants. Surprisingly, over 5% of all vascular plant species use CAM as their photosynthetic pathway, and others may exhibit CAM activity when needed. CAM is not an alternative biochemical variant but rather a mechanism enabling certain plants to survive in droughty areas. It may, in fact, be an ecological adaptation.

Examples of CAM plants, besides the aforementioned cactus (family Cactaceae) are pineapple (family Bromeliaceae), agave (family Agavaceae), and even some species of Pelargonium (the geraniums). Many orchids are epiphytes and also CAM plants, as they rely on their aerial roots for water absorption.

History and Discovery of CAM plants

The discovery of CAM plants was begun in a rather unusual manner, when Roman people discovered that some plant leaves used in their diets tasted bitter if harvested in the morning, but were not so bitter if harvested later in the day.

A scientist named Benjamin Heyne noticed the same thing in 1815 while tasting Bryophyllum calycinum, a plant in the Crassulaceae family (hence, the name "Crassulacean acid metabolism" for this process). Why he was eating the plant is unclear, since it can be poisonous, but he apparently survived and stimulated research as to why this was happening.

A few years before, however, a Swiss scientist named Nicholas-Theodore de Saussure wrote a book called Recherches Chimiques sur la Vegetation (Chemical Research of Plants). He is considered as the first scientist to document the presence of CAM, as he wrote in 1804 that the physiology of gas exchange in plants such as the cactus differed from that in thin-leaved plants.

How Do CAM Plants Work?

CAM plants differ from "regular" plants (called C3 plants) in how they photosynthesize. In normal photosynthesis, glucose is formed when carbon dioxide (CO2), water (H2O), light, and an enzyme called Rubisco work together to create oxygen, water, and two carbon molecules containing three carbons each (hence, the name C3). This is actually an inefficient process for two reasons: low levels of carbon in the atmosphere and the low affinity Rubisco has for CO2. Therefore, plants must produce high levels of Rubisco to "grab" as much CO2 as it can. Oxygen gas (O2) also affects this process, because any unused Rubisco is oxidized by O2. The higher the oxygen gas levels are in the plant, the less Rubisco there is; therefore, the less carbon is assimilated and made into glucose. C3 plants deal with this by keeping their stomata open during the day in order to gather as much carbon as possible, even though they can lose a lot of water (via transpiration) in the process.

Plants in the desert can't leave their stomata open during the day because they will lose too much valuable water. A plant in an arid environment has to hold onto all the water that it can! So, it must deal with photosynthesis in a different way. CAM plants need to open the stomata at night, when there is less of a chance of water loss via transpiration. The plant can still take in CO2 at night. In the morning, malic acid is formed from the CO2 (remember the bitter taste Heyne mentioned?), and the acid is decarboxylated (broken down) to CO2 during the day under closed stomata conditions. The CO2 is then made into the necessary carbohydrates via the Calvin cycle.

Current Research

Research is still being performed on the fine details of CAM, including its evolutionary history and genetic foundation.

In August 2013, a symposium on C4 and CAM plant biology was held at the University of Illinois at Urbana-Champaign, addressing the possibility of the use of CAM plants for biofuel production feedstocks and to further elucidate the process and evolution of CAM.