Why Do Some Plants Open Their Leaf Pores at Night?

1. What Are Stomata, and Why Does a Leaf Need Them?

A leaf is covered with tiny openings called stomata (one opening is a stoma). Each one can open and close. Through them a plant takes in carbon dioxide, the gas it needs to build sugars by photosynthesis. The same open pore also lets water vapor leak out of the leaf, which is called transpiration [1].

So every plant faces a trade. Open the pores and you can feed the leaf, but you lose water. Shut them and you save water, but the leaf cannot get carbon dioxide. A plant that grows where rain is rare needs a way to manage this trade.

2. How Can a Plant Open Its Pores at Night When Photosynthesis Needs Light?

This is the puzzle. Photosynthesis runs on sunlight, so it seems silly to collect carbon dioxide in the dark. Plants that use a method called CAM (crassulacean acid metabolism) solve it by splitting the job into two shifts [1].

Night shift: the stomata open and carbon dioxide flows in. The leaf cells do not turn it into sugar yet. They lock it into an organic acid with four carbons, called malic acid, and store it in a large bag inside each cell, the vacuole [1]. The sugar-making steps cannot run in the dark, because they depend on energy captured from light, so the acid simply waits [1].

Day shift: the stomata close to hold water in. The stored acid is released from the vacuoles, and it gives back carbon dioxide inside the leaf. That gas goes into the usual sugar-making cycle, powered by sunlight [1]. A CAM plant collects its raw material at night and uses it in daylight, behind closed pores.

3. Why Does Opening the Pores at Night Save Water?

The most important benefit of CAM is being able to leave most stomata closed during the day, the hottest and driest part of the day, which cuts the water lost through transpiration [1]. A summary of a review of CAM stomata describes the result as less water loss, better water-use efficiency, and greater drought tolerance [3].

A simple way to picture it, though it is my own comparison and not a claim from these sources, is a window in a desert house. Opening it at noon lets in a blast of dry heat. Opening it after dark lets in cooler air, so less of the house’s moisture is carried out.

The savings can be large. A scientific review notes that some CAM species grown on poor, dry land reach yields comparable to the most water-efficient ordinary crops while needing only about 20 percent of the water [2]. That figure comes from the review’s abstract and applies to certain species grown for farming, not to every CAM plant.

4. Why Do Some Leaves Taste Sour in the Morning?

Here is where taste enters the story. Because malic acid piles up in the leaf all night and is used up during the day, the leaves of some of these plants can grow more sour through the night and taste sweeter during the day [1].

Botanists noticed this long before anyone knew why. Wikipedia says that Benjamin Heyne noted in 1812 that the leaves of a plant called Bryophyllum in India were acidic in the morning and tasteless by afternoon [1]. I could not read his original text, so treat 1812 as approximate. The same article says the idea that some plants take in gas at night goes back even earlier, to a 1804 book by the Swiss scientist Nicolas-Théodore de Saussure [1].

The name “crassulacean acid metabolism” comes from the stonecrop family, Crassulaceae, where the pattern was studied. That family includes jade plants and sedums. There is no chemical called “crassulacean acid” [1]. Please do not go tasting leaves to check this. Many garden and house plants are poisonous.

5. Which Plants Use This Night-Shift Method?

More than you might expect. CAM is found in over 99 percent of the roughly 1,700 known species of cacti, and the pineapple is a well-known example [1]. It also shows up in plants that live on trees or rocks, where water comes and goes unpredictably [1].

The method did not come from one ancestor. Reviewers describe CAM as a trait that evolved independently many times in different plant families [3]. When very different plants end up with the same solution to the same problem, it suggests the solution is a good one.

Not every CAM plant is all-or-nothing. Some use CAM all the time, while others can switch between ordinary daytime photosynthesis and CAM when conditions change, such as during a dry spell. A third group, called “CAM-cycling” plants, keeps the stomata shut at night and instead recycles carbon dioxide made by the plant’s own respiration [1]. Even typical CAM plants are not on a rigid night timer. Their stomata show a lot of flexibility during the day [3].

There are even underwater CAM plants. Some aquatic quillworts (Isoetes) and other water plants follow the same night-acid pattern [1].

6. If It Saves So Much Water, Why Don’t All Plants Do It?

Because there is a cost. The same review that praises CAM for water savings points out that the thick, water-filled tissues of CAM plants make it harder for carbon dioxide to reach the cells that use it [2]. Wikipedia adds that a related method, C4 photosynthesis, is more efficient at one step of sugar-making, because CAM needs the waiting step (that section of the article says it needs more citations) [1]. So CAM may be a trade of speed for water, which can pay off in dry places.

Scientists are now asking whether this trick can be moved into crops. One aim of current research is to study plants that can switch to CAM and see whether crops could be given a similar ability to survive droughts [3]. That is a research goal, not something farmers can do today.

7. How Can You Look for Night-Opening Plants Yourself?

You do not need a lab. If you see a cactus, a jade plant, or a pineapple plant at home, in a store, or in a botanical garden greenhouse, read its care label for watering advice and compare it with another houseplant if you have one. A label cannot tell you whether a plant uses CAM, but it is a fun way to start wondering how each plant handles water. Look, but do not taste: the sour-leaf story is a good reason to be curious, not a reason to chew on a houseplant.

If you want to read more, start with the abstract of the bioenergy review [2] and the short summary of the stomata review [3]. They are written for scientists, but the first paragraphs show how researchers describe the same puzzle.

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