The significance of oxidation states of Mn in oxygen evloving complex and its possible transition states

Mn, a transition element found in different variable oxidation states, is highly reactive, one of the essential elements among 17 for all living creatures, including humans and plants. It is as essential as Fe and required by plants in the subsequent highest quantity over Fe for sustainable growth of plants. Mn, having critical roles in plant metabolism, is an indispensable cofactor for the oxygen-evolving complex (OEC) in photosystem II (PSII). It catalyzes the water-splitting process to evolve oxygen from leaves, thereby protecting the plant from oxidative stress. When chlorophyll absorbs light, excitation takes place with the release of energy. PSII accomplishes a series of electron transfer reactions by this energy that involves the transition of the OEC in several singlet states viz S_o – S₁, S₁→S₂, S₂→S₃ and S_o →S₄ during water splitting studied through quantum /molecular mechanisms using chemical sources of Mn in different oxidation states. Although required in low quantity, deficiency causes adverse effects on the growth of the plants. Its bioavailability to the plants is related to the soil pH and high pressure of oxygen as it is rapidly converted into oxides where oxidation states are altered. It significantly contributes to plant growth regulatory systems like photosynthesis, respiration and nitrogen accumulation, pollen tube, root cell germination, and resistance against bacteria. This review article highlights the oxidation and transition states of Mn in OEC linked to photosystem II.