To function effectively, stomata must open and close in response to both CO2 demand and water loss. Stomata are thus key sites for the processing of physiological information.
Light stimulates stomatal opening, while high levels of CO2 inside the leaf (a signal that CO2 is being supplied faster than photosynthesis can take it up) cause stomata to close.
When the soil can't supply enough water to keep up with evaporation from the leaf, guard cells become dehydrated and shrink in volume, closing the stomata.
Guard cell volume also changes in response to signaling molecules. For example, abscisic acid, a hormone produced during drought, causes stomata to close. This signal allows leaves to close their stomata early enough to prevent dehydration, rather than responding only when water has already been lost.
Clearly, then, stomata are a key innovation in the evolution of land plants. Not surprisingly, fossils indicate that stomata evolved early in the colonization of land.
CAM occurs in all four groups of vascular plants. It is most widespread in the angiosperms, occurring in 5% to 10% of species. Vanilla orchids and Spanish moss, two well-known epiphytes, as well as many cacti, exhibit CAM.
A plant opens its stomata to capture CO2 at night, when cool air limits rates of evaporation. It provides a system for overnight storage, converting CO2 into a form that will not diffuse away. The storage form of CO2 is produced by the activity of the enzyme PEP carboxylase, which combines a dissolved form of CO2 (bicarbonate ion, HCO3-) with a 3-carbon compound called phosphoenol pyruvate (PEP). The resulting product is a 4-carbon organic acid that is stored in the cell's vacuole.
However, CAM has a drawback. The rates of photosynthetic carbohydrate production by CAM plants tend to be low because ATP is needed to drive the uptake of organic acids into the vacuole and because only so much organic acid can accumulate in the vacuole.
CAM is therefore most common in habitats such as deserts, where water conservation is crucial, and among epiphytes, plants that grow high in the canopy of other plants, without contact with the soil.