Why Do Plants Close Their Stomata at Night?

The stomata on these leaves will close when the sun goes down, ensuring the plant's good health.
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A plant's stomata are easy to see -- paint the underside of the leaf with clear nail polish, peel it off when dried and examine the clear leaf print under a microscope. But as fascinating as these stomata look, their function is even more exciting. Responsible for the plant's carbon dioxide and water regulation, stomata are often closed at night and reopen with the sunrise, an adaptation that is key to these plants' survival.

What Are Stomata

Stomata are small holes on the back of plant leaves that help a plant regulate carbon dioxide and water. Small but plentiful, stomata can number as high as 1,000 per leaf, depending on plant species, light exposure, carbon dioxide concentrations and moisture content of the air.

How Stomata Work

During the process of photosynthesis, the leaves' stomata open to allow the plant to absorb the carbon dioxide needed for plant respiration. It is this respiration that is responsible for a plant's ability to feed itself. Animals that depend on plants for food and oxygen are unwittingly dependent on these hundreds of tiny holes in each leaf. Stomata also help the plant to regulate water by releasing excess water from the leaf via transpiration. In effect, the stomata help the plant to "breathe" carbon dioxide in and water out.

Closed for the Night

Because carbon dioxide and water are exchanged through the same holes in plant leaves, a plant cannot absorb carbon dioxide without allowing water vapor to escape. In order to minimize excessive water loss, stomata tend to close at night, when photosynthesis is not occurring and there is less benefit to taking in carbon dioxide.

An Exception to the Rule

While many plants do close their stomata at night, this is not always the case. Plants in deserts and alpine landscapes risk losing dangerously high levels of water if they open their stomata during the day, when heat from the sun is more intense and the air is extremely dry. To avoid drying out in these conditions, many of these plants have adapted to open their stomata at night, taking in lower amounts of carbon dioxide but also lowering the risk of drying out.