Life of a Cactus Part 5: CAM Photosynthesis

This is a microscopic image of the surface of a cactus stem.  The oval shaped objects with dark areas in the middle are stomata, the pores through which cacti and other plants breath.

Once water is stored inside of the cactus water conservation does not stop.  Microscopic pores called stomata cover the green tissue of all plants, allowing them to “breathe” and carry out photosynthesis.  Through these stomata, all plants “exhale” water vapor and oxygen and “inhale” carbon dioxide.  The exhaled oxygen is a waste product of photosynthesis.  Inhaled carbon dioxide, along with water stored in the plant, are converted into sugar and starches by use the sun’s rays through photosynthesis.  Water vapor is passively lost through the stomata whenever they are open to inhale and exhale carbon dioxide and oxygen.  Even when the stomata are closed a small amount of water vapor is lost.  Nearly all plants open their stomata to carry out this breathing process during the day.  As a result, most plants are opening their stomata when it is hottest and are therefore releasing huge amounts of water through evaporation.  In areas where there is plenty of water this really isn’t much of a problem, if you have water to spare you have water to waste and can afford to use it quite liberally.

Crested Saguaro Cactus.

 In the desert, there is no water to spare and to waste.  The cactus therefore does the exact opposite of what almost all plants do; it opens its stomata to “breath” during the night and closes them during the day.  By closing stomata when it is hottest and opening them when it is cooler the cactus conserves huge amounts of water.  The problem with this is cacti still need a constant input of carbon dioxide in order to carry out photosynthesis during the day, and without the stomata open there is no direct supply.  To overcome this, cacti absorb huge amounts of carbon dioxide at night time when their stomata are open.  They do not absorb this carbon dioxide like a balloon would though.  Instead, they convert it to a chemical called malic acid, in which form the carbon dioxide can be stored until day light.  Once daylight appears the, malic acid is then converted back to carbon dioxide needed to carry out photosynthesis.  Plants only need to breathe when it is daylight and photosynthesis is being carried out.  So in the dark no photosynthesis is being carried out and there is no reason for the plant to breathe.  This is why most plants only open their stomata and breathe during the day, and because day temperatures are warmer, more water is lost through the stomata.  As said before, cacti open their stomata at night to breath, storing the carbon dioxide as malic acid for later use when daylight appears.  Once daylight appears, the cactus closes its stomata and sort of holds its breath, converting the malic acid back into carbon dioxide for use in photosynthesis.  By only opening their stomata in the cool of night, far less water is lost.

Cacti stomata not only open and close in an ideal fashion to conserve water, they are also specifically designed to conserve water.  Most plants have large amounts of relatively small stomata all over their green tissues.  Relatively small cells also open and close these stomata.  Cacti however have far fewer stomata but the stomata they do have are much larger.  Overall though, this decreases the amount of water that can evaporate through the stomata.  Cacti also locate their stomata is a shallow pit as opposed to directly on the surface of green tissue.  This protects the stomata from drying winds.  Lastly, the cells that open and close the stomata are huge in comparison to the typical plant.  This allows the cacti to firmly close their stomata so water vapor does not accidentally leak out.