Nelson-Denny Practice Test 3

How Plants Manage Water

¹A plant faces a constant trade-off. To make food by photosynthesis, its leaves must take in carbon dioxide from the air, which enters through tiny pores called stomata. In many plants, most stomata lie on the underside of the leaf. Yet the same openings that let carbon dioxide in also let water vapor out. In fact, the great majority of the water a plant absorbs through its roots is eventually lost through its stomata, in a process called transpiration.

²Each stoma is bordered by a pair of curved guard cells that control its size. When the guard cells take up water and swell, they bow apart and the pore opens; when they lose water and go limp, they sag together and the pore closes. In many plants, stomata open in the morning light and close at night, when photosynthesis cannot occur and there is no benefit to keeping them open.

³Drought upsets this daily rhythm. When the soil dries, the plant produces more of a hormone called abscisic acid, which signals the guard cells to lose water. The stomata close, even in bright sunlight, and the plant conserves its remaining moisture. The price is high. With its pores shut, the leaf takes in little carbon dioxide, so photosynthesis and growth slow. A plant that keeps its stomata closed for too long may also overheat, because the evaporation of water through the stomata normally helps cool the leaf.

⁴Plants native to deserts have evolved ways to ease the trade-off. Many cacti, for example, open their stomata only at night, when the air is cooler and more humid, and store the carbon dioxide they absorb in the form of an acid. During the day, with their stomata closed, they release the stored carbon dioxide inside their tissues and use it in photosynthesis.