Plants exposed to elevated atmospheric CO2 concentrations show an increased photosynthetic activity. However, after prolonged exposure, the activity declines. This acclimation to elevated CO2 is accompanied by a rise in the carbon‐to‐nitrogen ratio of the biomass. Hence, increased sugar accumulation and sequential downregulation of photosynthetic genes, as well as nitrogen depletion and reduced protein content, have been hypothesized as the cause of low photosynthetic performance. However, the reason for reduced nitrogen content in plants at high CO2 is unclear. Here, we show that reduced photorespiration at increased CO2‐to‐O2 ratio leads to reduced de novo assimilation of nitrate, thus shifting the C/N balance. Metabolic modeling of acclimated and non‐acclimated plants revealed the photorespiratory pathway to function as a sink for already assimilated nitrogen during the light period, providing carbon skeletons for de novo assimilation. At high CO2, low photorespiratory activity resulted in diminished nitrogen assimilation and eventually resulted in reduced carbon assimilation. For the hpr1‐1 mutant, defective in reduction of hydroxy‐pyruvate, metabolic simulations show that turnover of photorespiratory metabolites is expanded into the night. Comparison of simulations for hpr1‐1 with those for the wild type allowed investigating the effect of a perturbed photorespiration on N‐assimilation.

A methodology for the synthesis of sulfonyl hydrazides mediated by hypervalent iodine is described. Taking advantage of the umpolung properties of hypervalent iodine reagents, the polarity of sodium sulfinate salts...

It has been shown repeatedly that exposure to elevated atmospheric CO2 causes an increased C/N ratio of plant biomass that could result from either increased carbon or – in relation to C acquisition - reduced nitrogen assimilation. Possible reasons for diminished nitrogen assimilation are controversial, but an impact of reduced photorespiration at elevated CO2 has frequently been implied. Using a mutant defective in peroxisomal hydroxy-pyruvate reductase (hpr1-1) that is hampered in photorespiratory turnover, we show that indeed, photorespiration stimulates the glutamine-synthetase 2 (GS) / glutamine-oxoglutarate-aminotransferase (GOGAT) cycle, which channels ammonia into amino acid synthesis. However, mathematical flux simulations demonstrated that nitrate assimilation was not reduced at elevated CO2, pointing to a dilution of nitrogen containing compounds by assimilated carbon at elevated CO2. The massive growth reduction in the hpr1-1 mutant does not appear to result from nitrogen starvation. Model simulations yield evidence for a loss of cellular energy that is consumed in supporting high flux through the GS/GOGAT cycle that results from inefficient removal of photorespiratory intermediates. This causes a futile cycling of glycolate and hydroxy-pyruvate. In addition to that, accumulation of serine and glycine as well as carboxylates in the mutant creates a metabolic imbalance that could contribute to growth reduction.