A research world under glass: plants, survival, and a changing climate PART 2 of 3 – Photosynthesis has adapted…

Another much less visible difference between desert and rainforest plants is in their photosynthetic strategies. Most plants on earth employ a photosynthetic pathway known as C3, so called because the first recognizable compound produced through it contains three carbon atoms. Most rainforest plants use the C3 pathway and they are pretty efficient in carrying out photosynthesis. In hot, sunny regions, however, C3 plants can become less efficient because Rubisco, the enzyme that fixes carbon, also has an affinity for oxygen and attempts to fix it instead. This produces a situation known as photorespiration, which causes the plant to waste energy that could otherwise be used to conduct photosynthesis. In cooler regions or areas with plenty of water, this is perhaps not so much an issue, but in the desert, it can inhibit or even prevent plant growth.

To avoid photorespiration, plants in warm climates have evolved another photosynthetic pathway,  known as C4 photosynthesis, because the first product is a four-carbon molecule. In effect, C4 plants conduct carbon fixing reactions in a different location of the leaf than C3 plants. This protects Rubisco from interacting with oxygen, which prevents photorespiration from occurring, and saves the plant energy that can be used on plant growth. C4 plants also have greater water use efficiency. Although only a small percentage of plants on earth are C4 plants, these include many weeds and grasses that are very successful in the desert – including invasive species like buffel grass. 

A third photosynthetic pathway, known as CAM photosynthesis (short for Crassulacean Acid Metabolism because it was first observed in the Crassulaceae or stonecrop family) can be found in many succulents that inhabit hot, dry climates. Plants that use this pathway open their stomata during the night to take up carbon dioxide and store it as an acid until morning, when the stomata close and photosynthesis takes place. Closing their stomata during the day allows plants to greatly reduce water loss. CAM plants are extremely efficient at using water, but their growth rate is considerably slower due to the added steps in carrying out photosynthesis. Cactus, agaves, and other desert plants use CAM photosynthesis.