Water undergo hydrolysis in presence of sunlight to form proton(H+) and hydroxyl ion(OH-) ion.The OH- ion liberates electron to reaction center of photosystem 1 thereby exciting the later .The excited photosystem then donates that extra electron to the primary electron acceptor and from the primary electron acceptor the electron moves through various electron carriers to generate ATP .
Basically water act as reducing agent in photosynthesis process.
