Crassulacae Acid Metabolism in Cactus and Succulents ... 3/3

from previous page

CAM photosynthesis is less energy efficient and results in slower growth.
There is only a limited amount of Carbon dioxide than be fixed the night before, and this may be used up by midday the next day so there is no more photosynthesing the rest of the day.
Another reason it is less efficient is that the process of losing water through the stomata forms a "vacuum" sucking up water through the plant, then roots and ultimately from the surrounding soil. But in CAM plants this does not occur to such a degree so the "sucking" power, to draw water out of the soil, is not present (some C&S make up for this with their special root structure and ability to form new roots very quickly after rains).

However some species (eg Sedums, Mesembryanthemum and Kalanchoes are a few) are able to use normal photosynthesis to maximize growth at times of sufficient water supply eg winter, but switch to CAM during periods of limited water supply eg summer to survive. This gives them a huge advantage over their non-switching neighbour plants.
Other plants have C3 in the leaves and CAM on the stems (some Euphorbia species).
Still other mesembs start out as seedlings with C3 then later in adult life switch to the CAM pathway, and the age at which they do switch depends on the water stress the plant suffers during its lifetime.

Some other forms of CAM are:
CAM idling, or "strong" CAM, during excessive drought, where stomata remain closed both day AND night and the organic acid cycle is fed by internal recycling of nocturnally refixed respiratory CO2.
This means the plant is effectively sealed from the outside world and is doing just enough to maintain basic metabolism and not die. It also means that when the rains do come it can jump straight back into full (CAM) metabolism.

CAM cycling, occuring during periods of good water supply, means stomata are closed at night (but with some synthesis of organic acid fed by respiratory CO2) and the stomata are open during the day with intake of atmospheric CO2 and hence "normal" C3-photosynthesis (plus the use of CO2 from nocturnally stored acid). This "weak" CAM may have been a prelude to "real" CAM in evolution.

Another method used by some arid region plants is the C4 pathway, also used by plants growing in areas of high temperature, high light levels and high rainfall in summer, like sugar cane and corn. In this process a 4 carbon sugar (malate) is formed (instead of the 3 carbon in "normal" C3 photosynthesis), Carbon dioxide is concentrated within the cell but the whole process is once again less efficient. C4 and CAM does not occur in the same plant, switching according to conditions, unlike C3 and CAM.

It is possible that the ecological, economical and agricultural importance of CAM species to humans (and other animals) will increase due to global warming, changing rainfull patterns, and expansion of semi-arid regions around the world.