Involved in the balancing of excitation energy between the two photosystems I (PSI) and II (PSII). Component of the photosystem I (PSI) complex. Interacts directly with PSAL. Prevalent in stroma lamellae with low amounts in grana. Reduction by 50 percent in state transitions due to a disturbed balancing of excitation energy between the two photosystems. Short flowering delay. Belongs to the PSAO family.