Best defined function of manganese in green plants is

Photolysis of water requires light energy, an Oxygen Evolving Complex (OEC) and electron carrier 1/2. OEC complex was formerly called Z-enzyme. It has four Mn ions. Light energy brings about changes in $\mathrm{Mn}\left(\mathrm{Mn}^{+2}, \mathrm{Mn}^{3+}, \mathrm{Mn}^{+4}\right)$ which helps in removing electrons from $\mathrm{OH}^{-}$component of water forming oxygen. Liberation of oxygen also requires $\mathrm{Cl}^{-}$and $\mathrm{Ca}^{+2}$.