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On the Role of Manganese in Photosynthesis

Kinetics of Photoinhibition in Manganese-deficent and 3-(4-Chlorophenyl)-1, 1-dimethylurea-inhibited Euglena gracilis¹

^a Department of Biochemistry and Microbiology, Rutgers University, New Brunswick, New Jersey 08903.

Euglena gracilis (Klebs) cultures were grown under conditions where limitation in supply of manganese limited chlorophyll content much more than growth. Although the initial rates of photosynthetic oxygen evolution were not affected by the level of manganese, photoinhibition in high intensity light was markedly influenced. All cultures showed first order kinetics for photoinhibition, with the half-time exponentially related to the Mn concentration in the medium. Treatment with 3-(4-chlorophenyl)-1, 1-dimethylurea (CMU) also increased the rate of photoinhibition. Manganese-deficient cells were also more sensitive to CMU inhibition of photosynthesis. The similar effects on photoinhibition of manganese deficiency and of CMU treatment and the protective action of manganese against photoinhibition and CMU poisoning are interpreted to indicate a site of action of manganese on the reducing side of photosystem II, close to the CMU-sensitive site. This manganese-affected site may represent a secondary structural or metabolic consequence of manganese deficiency, not necessarily involved in quantum yields of oxygen.

¹ This work was supported in part by a fellowship to N. A. Gavalas from the State Scholarship Foundation of Greece, by a Walter C. Russell Scholarship from Rutgers University, and by a United States Public Health Service Biomedical Sciences Support Grant to H. E. Clark. Journal Series paper, New Jersey Agricultural Experiment Station.