Carbon allocation to major metabolites in illuminated leaves is not just proportional to photosynthesis when gaseous conditions (CO₂ and O₂) vary

In gas-exchange experiments, manipulating CO₂ and O₂ is commonly used to change the balance between carboxylation and oxygenation. Downstream metabolism (utilization of photosynthetic and photorespiratory products) may also be affected by gaseous conditions but this is not well documented. Here, we took advantage of sunflower as a model species, which accumulates chlorogenate in addition to sugars and amino acids (glutamate, alanine, glycine and serine). We performed isotopic labelling with ¹³CO₂ under different CO₂/O₂ conditions, and determined ¹³C contents to compute ¹³C-allocation patterns and build-up rates. The ¹³C content in major metabolites was not found to be a constant proportion of net fixed carbon but, rather, changed dramatically with CO₂ and O₂. Alanine typically accumulated at low O₂ (hypoxic response) while photorespiratory intermediates accumulated under ambient conditions and at high photorespiration, glycerate accumulation exceeding serine and glycine build-up. Chlorogenate synthesis was relatively more important under normal conditions and at high CO₂ and its synthesis was driven by phosphoenolpyruvate de novo synthesis. These findings demonstrate that carbon allocation to metabolites other than photosynthetic end products is affected by gaseous conditions and therefore the photosynthetic yield of net nitrogen assimilation varies, being minimal at high CO₂ and maximal at high O₂.