Photoreduction of Oxygen in Mesophyll Chloroplasts of C4 Plants

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Abstract
Mesophyll chloroplasts of three C4 sub types, Panicum miliaceum (NAD-malic enzyme), Panicum maximum (PCK), and Zea mays (NADP-malic enzyme), were prepared from protoplast extracts and used to study the photoreduction of O2. The processes of O2 uptake and evolution in these preparations, which lack ribulose 1,5-bisphosphate carboxylase/oxygenase, were studied simultaneously using stable isotopes of O2 and mass spectrometry. The responses of O2 uptake to O2 tension and addition of various substrates (3-phosphoglycerate, pyruvate, and oxaloacetate) were studied in detail. The addition of photosynthetic substrates differing in ATP to NADPH demands indicated that photoreduction of O2 in these chloroplast preparations is linked to ATP production and strongly regulated by NADP+ levels. The results clearly indicate that photoreduction of O2 could be of physiological relevance in balancing the ATP to NADPH requirements of C4 mesophyll chloroplasts.