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Inhibition of Photosynthesis by Azide and Cyanide and the Role of Oxygen in Photosynthesis ¹

^a Istituto di Scienze Botaniche, Università degli Studi di Milano, Via Giuseppe Colombo, 60-20133 Milano, Italy

Cyanide and azide inhibit photosynthesis and catalase activity of isolated, intact spinach (Spinacia oleracea) chloroplasts. When chloroplasts are illuminated in the presence of CN^– or N₃^–, accumulation of H₂O₂ is observed, parallel to inhibition of photosynthesis. Photosynthetic O₂ evolution is inhibited to the same extent, under saturating light, whether CO₂ or phosphoglycerate is present as electron acceptor.

The illumination of chloroplasts with CN^– or N₃^– inactivates the NADPH- and ATP-dependent phosphoglycerate reduction. This enzyme system can be reactivated by dithiothreitol. In reconstituted, envelope-less chloroplasts, the phosphoglycerate-dependent and the ribose 5-phosphate-dependent O₂ evolution are inhibited to the same extent, while electron transport to NADP is unaffected.

It is concluded that the inhibition of photosynthesis by CN^– and N₃^– is due to H₂O₂ accumulation, which is a consequence of catalase inhibition.

The inhibition of phosphoglycerate reduction, but not of CO₂ reduction, is abolished under conditions where ATP is available in excess of NADPH (low light, supply of ATP). This is taken as an indication that electron flow from photosystem I is diverted to O₂ (Mehler reaction, which produces H₂O₂) when the unavailability of ATP is limiting the rate of reoxidation of NADPH. The Mehler reaction is considered a physiological process supplying ATP for photosynthesis.

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