Re: How the stomatal pore in desert plants work

Plants which live in the desert must deal with severe conditions of light, heat, and water stress. The most damaging of these is probably the scarcity of water and plants have come up with many adaptations to reduce water loss. In desert plants, cuticles on leaves are thicker and leaves are smaller (even reduced to spines in cacti).

One of the major sources of water loss is by transpiration through the stomata. Mesophyll cell surfaces must be kept wet to absorb the carbon dioxide from the air. To decrease the evaporation, stomata in desert plants may be sunken or covered with hairs. Of course, there is a cost to this. Less carbon dioxide can get in under these conditions, so growth is slower.

One group of plants has taken a different path. These plants use what is called "Crassulacean Acid Metabolism" (CAM) photosynthesis, named after the family of succulent plants where it was first found. These plants reverse the pattern of stomatal opening, opening them only in the dark rather than in the light. During the night, temperatures are lower and humidity is higher, reducing the transpiration. The carbon dioxide is stored as part of an acid (malic acid) and then released inside the leaf in the morning when the light reactions of photosynthesis can occur using the energy trapped from sunlight. Early researchers found that these plant leaves tasted sour at night and in the early morning but the sour taste disappeared during the day.

CAM plants also have about 1/10 as many stomata as other plants and although they grow slowly, they use only about 1/10 as much water per unit of carbon dioxide taken up as other plants do.

To find out more about adaptations to dry environments go to http://www.desertusa.com/ du_plantsurv.html (for whole plant adaptations) or http:// www.bl.rhbnc.ac.uk/plant/xerophytic.html (for leaf adaptations).

For an explanation of how stomata open and close try

http:// www.esf.edu/course/lbsmart/efb530/18Stomata.htm.

Search the net under "CAM plants" for more information on this group.