Net photosynthetic CO assimilation: more than just CO and O reduction cycles.

Net photosynthetic assimilation in C plants is mostly viewed as a simple balance between CO fixation by Rubisco-catalyzed carboxylation and CO production by photorespiration (and to a lower extent, by day respiration) that can be easily manipulated during gas exchange experiments using the CO  : O ratio of the environment. However, it now becomes clear that it is not so simple, because the photosynthetic response to gaseous conditions involves 'ancillary' metabolisms, even in the short-term. That is, carbon and nitrogen utilization by pathways other than the Calvin cycle and the photorespiratory cycle, as well as rapid signaling events, can influence the observed rate of net photosynthesis. The potential impact of such ancillary metabolisms is assessed as well as how it must be taken into account to avoid misinterpretation of photosynthetic CO response curves or low O effects in C leaves.