Regulation of Steady-State Photosynthesis in Isolated Intact Chloroplasts under Constant Light: Responses of Carbon Fluxes, Metabolite Pools and Enzyme-Activation States to Changes of Electron Pressure

Abstract

2Lehrstuhl fur Pflanzenphysiologie, Humboldt-Universitdt, D-10099 Berlin, Germany The reactions of isolated intact spinach chloroplasts at saturating light and CO2 to changes in steady-state electron flow were followed at the various stages of photosynthesis. Alterations in the rate of electron flow were induced by the addition of oxaloacetate (OAA), nitrite or methyl viologen (MV). Two types of effect can be distinguished: (1) When a small fraction of the electrons produced are accepted by OAA or nitrite (up to 20% of the electrons produced in the light), the activation state of the NADP + -dependent malate dehydrogenase (NADP-MDH) was strongly decreased, whereas qP and the rate of O2-production were increased. qN, the stromal metabolite pools and the [ 14 C]CO2-fixation rate were only marginally influenced. (2) Higher amounts of nitrite or MV decreased O2 production and strongly inhibited [ 14 C]CO2 fixation. This treatment further increased the ATP/ADP ratio, but had little effect on the NADPH + H + /NADP + ratio. The stromal concentrations of 3PGA, DHAP and FBP, and the rates of 3PGA and DHAP export were drastically changed. In particular, the DHAP/3PGA ratio increased, and the rate of 3PGA export was decreased by minor changes in the rate of electron flow. Addition of high amounts of nitrite or MV, but not of OAA decreased the activation states of NADP-MDH and fructose 1,6-bisphosphatase (FBPase), while the activation states of NADP + -dependent glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK) remained unchanged under all conditions.