Interaction of Nitrate Assimilation and Photorespiration at Elevated CO2

Abstract

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.

Elevated carbon‐dioxide effects on wheat grain quality differed under contrasting nitrogen and phosphorus fertiliser supply

“…In these cases, total plant dry weight (DW) increased more under eCO 2 than N uptake causing tissue N concentration per unit DW to be lower. This effect of eCO 2 has been termed N ‘dilution’ and has been linked, in part, to increased accumulation of non‐structural carbohydrates (e.g., soluble sugars and starch) and plant secondary compounds (e.g., structural carbohydrates or organic acids or lignin; Andrews et al, 2020; Krämer et al, 2022; Taub & Wang, 2008).…”