Questions and Answers about BIOLOGY by Ken Miller and Joe Levine

 

QUESTION: How do plants balance their need for carbon dioxide with their need to prevent dehydration?

This is a great question, because it points out a problem that nearly all plants have to deal with. Most plants get carbon dioxide, of course, from the air, which enters the leaves through little openings on their undersides known as stomata (shown on pages 596 and 597 of the Dragonfly Book). So, when the sun is shining and conditions for photosynthesis are good, plants should keep those stomata open, right?

Well, not always. The interior of a leaf is moist, so on a hot day a plant will lose water — sometimes lots of it — due to evaporation through open stomata. So, how does a plant "decide" between the need to conserve water and the need to admit carbon dioxide for photosynthesis?

The answer seems to be built into to the mechanisms that open or close stomata. As I wrote on page 597, when water pressure drops in the guard cells on either side of the stomata, the opening closes, preventing water vapor from leaving. When water pressure is high, the stomata open. So, if the plant has plenty of water, it automatically tends to keep its stomata open. If it's short of water, it automatically closes them. Yes, that slows down photosynthesis, but it also prevents the plant from wilting and dying due to dehydration.

Most well-watered plants, by the way, tend to have their stomata open in the daytime and closed at nighttime. That's partly because most plants have an internal "clock" in their guard cells that opens and closes them on a daily basis, and also because photosynthesis in the guard cells during the daytime gives these cells the ATP they need to pump ions and keep their stomata open.

Ken Miller (2/7/02)

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