Nitrate and ammonium differ in their impact on δ13C of plant metabolites and respired CO2 from tobacco leaves.

The carbon isotopic composition (δ13C) of foliage is often used as proxy for plant performance. However, the effect of NO3- vs. NH4+ supply on δ13C of leaf metabolites and respired CO2 is largely unknown. We supplied tobacco plants with a gradient of NO3- to NH4+ concentration ratios and determined gas exchange variables, concentrations and δ13C of tricarboxylic acid (TCA) cycle intermediates, δ13C of dark-respired CO2, and activities of key enzymes nitrate reductase, malic enzyme and phosphoenolpyruvate carboxylase. Net assimilation rate, dry biomass and concentrations of organic acids and starch decreased along the gradient. In contrast, respiration rates, concentrations of intercellular CO2, soluble sugars and amino acids increased. As NO3- decreased, activities of all measured enzymes decreased. δ13C of CO2 and organic acids closely co-varied and were more positive under NO3- supply, suggesting organic acids as potential substrates for respiration. Together with estimates of intra-molecular 13C enrichment in malate, we conclude that a change in the anaplerotic reaction of the TCA cycle possibly contributes to 13C enrichment in organic acids and respired CO2 under NO3- supply. Thus, the effect of NO3- vs. NH4+ on δ13C is highly relevant, particularly if δ13C of leaf metabolites or respiration is used as proxy for plant performance.