CAM (Crassulacean Acid Metabolism) plants have the highest wateruse efficiency because an additional enzyme allows a temporal variation in C02 uptake, fixing C02 from the air at night but reducing it to carbohydrates during the day when the stomata are closed. This group comprises succulent xerophytes most of which have no economic value, with one exception - the humble pineapple.
So much for the scientific explanations, but what does this mean in practice to the agriculturist who would like to put this knowledge to good use?
In general terms, it is possible that the selection of crop species most suited to changing conditions may be one of the first elements in any planned contingency program of adaptation to a high C02 atmosphere of the future. However, the choice of future crops will depend as much upon diurnal and seasonal patterns of temperature and rainfall as it will upon C02 levels. The kind of climate scenario being projected for the middle of the 21st century could lead to major shifts away from crops such as wheat, barley, potatoes and sugar-beet, to more water efficient crops like rice, cassava, sweet potato, maize, sorghum, pearl millet and sugarcane.
A side benefit of an enriched C02 atmosphere is increased water use efficiency in many plants. Under normal C02 atmospheric conditions, C02 diffuses into the leaf while water travels up the root system and transpires through the stomata. But in a C02 enriched environment the stomata shrink, with the result that plants transpire less water. In essence, this means plants become more water efficient.
CAM
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