Photoreduction of Oxygen in Mesophyll Chloroplasts of C₄ Plants

Mesophyll chloroplasts of three C(4) 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 O(2). The processes of O(2) uptake and evolution in these preparations, which lack ribulose 1,5-bisphosphate carboxylase/oxygenase, were studied simultaneously using stable isotopes of O(2) and mass spectrometry. The responses of O(2) uptake to O(2) 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 O(2) in these chloroplast preparations is linked to ATP production and strongly regulated by NADP(+) levels. The results clearly indicate that photoreduction of O(2) could be of physiological relevance in balancing the ATP to NADPH requirements of C(4) mesophyll chloroplasts.