EA Practice Test 10

Photosynthesis on the Night Shift

¹To take in the carbon dioxide they need for photosynthesis, plants open small pores in their leaves called stomata. Open stomata, however, also let water vapor escape, and in hot, dry air a plant can lose large amounts of water while it gathers carbon. Most plants accept this trade-off, opening their stomata during the day, when sunlight drives photosynthesis. A minority, including many cacti and other succulents as well as the pineapple, follow a different schedule known as crassulacean acid metabolism, or CAM.

²CAM plants open their stomata mainly at night, when cooler, more humid air sharply reduces water loss. The carbon dioxide they absorb in darkness cannot be used at once, because the reactions that build sugars require light. Instead, the plant binds the gas into an organic acid, which it stores in large vacuoles within its cells. When daylight returns, the stomata close, the stored acid is broken down to release carbon dioxide inside the leaf, and sugar production proceeds with little further loss of water. A CAM plant may lose only a fraction of the water that a conventional plant loses for each unit of carbon it gains.

³The strategy has a cost. Because a plant can use only as much carbon in a day as it was able to store the night before, its growth is limited by the capacity of its vacuoles, and CAM plants generally grow more slowly than conventional plants when water is plentiful. This helps explain why CAM is common in deserts and on the branches of trees in tropical forests, where water is scarce or arrives only intermittently, but rare among crops grown in well-watered fields. Some species avoid the cost by switching modes: they photosynthesize conventionally when water is available and shift to CAM only during drought. Such flexibility has drawn interest from plant breeders, who hope that crops able to make the same switch could keep yields high in wet years while surviving dry ones.