|Stonecrop sedum in bloom on the green roof, June 25, 2013|
Note the round, spongy leaves of stonecrop (shown below)—an adaptation for survival in extremely dry environments. In the wild, you may find stonecrop growing in very thin soils on rocks, where the only moisture is from recent precipitation, quickly lost in the hot sun. Stonecrop is a succulent, a plant that holds lots of water in reserve within its own tissues—like a cactus, another succulent. Stonecrop has another adaptation that further enhances its ability to succeed where normal plants would shrivel, which involves its ability to retain its precious water reserve.
|The leaves of the Stonecrop sedum|
All plants must open their stomata (small pores on the leaf surface) to allow CO2 to enter, in order for photosynthesis to occur. But, open stomata also allow water vapor to escape. Some plants have evolved a more efficient method to fix the CO2 (called C4 photosynthesis) that doesn’t require keeping stomata open all the time (see grasses blog), thus reducing water loss. Stonecrop, and its relatives in the sedum family, Crassulaceae, have taken C4 photosynthesis one step further—not only are they efficient so that stomata are not open much, but the stomata are open only at night.
Most plants are just the opposite! Since photosynthesis can occur only when the sun is up, that’s when they take in CO2 to make sugars. At night, when sugar can’t be made, they shut their stomata to conserve water, even though the lower temperature and higher humidity of night mean less water would be lost then. But these clever succulents have evolved a method to store nighttime-collected CO2 until it can be used the following day. The CO2 is incorporated into organic acid molecules and stored in vacuoles during the night, then converted into a usable molecule for photosynthesis during the day.
This process is called Crassulacean Acid Metabolism, or CAM, named for the sedum family. But it’s not restricted to one family—a variety of very drought tolerant plants use it, including yuccas, agaves, cacti, some orchids, even some unusually drought tolerant ferns. We have several CAM plants on the Berry Prairie, and you may have them where you live, too.
Written by Dorothy Tuthill, Biodiversity Institute