Plants release oxygen as a direct by-product of water-splitting during oxygenic photosynthesis, making this fundamental biological fact common knowledge that requires no citation.
The claim describes the foundational and universally accepted biochemical mechanism of oxygenic photosynthesis, wherein water molecules are split to provide electrons, releasing molecular oxygen as a byproduct. Because this is a well-established textbook fact and a matter of direct scientific observation, it is classified as common knowledge.