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Why do C4 plants lose less water than C3 plants during photosynthesis?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

C4 plants lose less water than C3 plants because of the efficiency of their primary carbon-fixing enzyme, phosphoenolpyruvate (PEP) carboxylase.1

In hot and dry environments, plants must partially close their stomata to prevent excessive water loss, which reduces the internal concentration of carbon dioxide. In C3 plants, the primary enzyme Rubisco has a lower affinity for carbon dioxide and can mistakenly bind with oxygen, leading to photorespiration and reduced efficiency.12

In contrast, PEP carboxylase does not react with oxygen and has a much higher affinity for carbon dioxide. Because this enzyme can effectively fix carbon even when internal carbon dioxide levels are low, C4 plants do not need to keep their stomata open as wide as C3 plants to maintain photosynthesis. By requiring less stomatal opening, C4 plants significantly reduce the amount of water lost through transpiration while still maintaining high rates of carbon fixation.12