Photosystem I is not solely responsible for oxygen reduction in isolated thylakoids.

It was found that the contribution of segments of photosynthetic electron transport chain (PETC) besides Photosystem I (PSI) to oxygen reduction increased with increase in light intensity, and at high intensities achieved 50% at pH 5.0, and was higher than 60% at pH 6.5 and pH 7.8. The data are explained as the result of O2 reduction in plastoquinone (PQ) pool as well as in PSI followed by reduction of superoxide radicals generated in both processes by plastohydroquinone.