Limitation of Photosynthesis by Carbon Metabolism

The occurrence of O(2)-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves. In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions. Upon switching to low O(2) in O(2)-insensitive conditions the ATP/ADP ratio fell dramatically within one minute. The P-glycerate pool increased over the same time. Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air. The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O(2) of 21 millibars than at 210 millibars. These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O(2)-insensitive conditions.