Ill.: www.sublimesucculents.com & www.clipart.email, montage: laidbackgardener.blog
Do you remember how photosynthesis works from your biology class in high school? If not, here’s a quick revision.
In their green cells (chloroplasts), plants use the energy from sunlight, carbon dioxide (CO2) captured from the air and water absorbed from their roots to produce glucose they can later use for their growth and share, via their sap, with all their parts. Photosynthesis occurs during the day, while the sun shines. So, plants breathe in CO2 during the day through pores in the leaves called stomata and do all their photosynthesizing. They then carry out respiration at night, using the stored energy for growth and giving off oxygen and water vapor. Story closed.
Or is it?
Actually, some plants do their breathing at night, absorbing CO2 in the dark. Then they essentially “hold their breath” all day long, keeping their stomata closed. The CO2 they stored (in the form of malic acid) is then released for photosynthesis during the day, in the presence of sunlight. It would be the equivalent of a human being holding its breath all day and only breathing in the dark.
This remarkable aberration has been known for over 200 years, although it was only named in 1940. It’s called CAM: crassulacean acid metabolism or CAM photosynthesis. It was first discovered in plants from the crassula family, whence the name crassulacean.
Designed for Drought
CAM is essentially an adaptation to drought conditions. The typical green plant, with its stomata open during the day, releases about 97% of the water it absorbs into the air through evapotranspiration. Yes, all that moisture you so carefully applied through watering is lost to the air and contributes nothing to the plant’s growth. What a shocking waste!
CAM plants are much more efficient at water use. Because their stomata are completely closed during the heat of the day and only open during cool night hours, they lose much moisture to transpiration. They can therefore get along with much less water.
Desert and arid-climate plants are therefore often CAM plants, but so are epiphytic plants: those that grow on tree branches. Constantly exposed to moving, drying air, they need to hang on to their moisture to survive and CAM is a good way of doing so.
There is a price to pay for this, though: CAM photosynthesis is much less efficient than regular photosynthesis and plants that use it grow more slowly than normal plants. But at least it allows them to survive difficult conditions.
Although in some plant families, all the plants carry out CAM photosynthesis (the crassula family is one), CAM actually evolved independently many, many times in many different plant families. When plants grow under arid or epiphytic conditions, they develop different adaptations to that situation (a waxy coating, improved water storage capacity, more efficient roots, etc.) and many “learn” to do CAM photosynthesis.
Some 7% of all the world’s plants carry out CAM photosynthesis, including plants in over 300 genera and 40 plant families. Some do so exclusively. Others use regular photosynthesis under humid conditions and switch to CAM during periods of drought.
Among plant families where CAM photosynthesis is the rule is, as mentioned, the crassula family, but also the cactus family, both of which contain only CAM species. Both the bromeliad family and the orchid family have over 50% CAM species, which is not surprising, since most are epiphytes. But you even find CAM plants in such unlikely families as ferns (usually epiphytic ones) and cucurbits (the squash family).
Curiously, there are a few aquatic plants, like Sagittaria and Littorella, that have also adopted CAM. There’s obviously no problem about losing moisture through open stomata with these plants, but CO2 is less available in water (it diffuses 10,000 times more slowly than in air) and during the day, when most aquatic plants are photosynthesizing, competition for it is fierce. So, they switch to CAM photosynthesis to get a better share of a rare product.
Crassula acid metabolism: it’s not the photosynthesis they told you about in school, but some of the plants you grow probably use it.