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

Abstract

The carbon isotopic composition (δ¹³C) of foliage is often used as proxy for plant performance. However, the effect of $N{O}_3^{\text{-}}$ vs. $N{H}_4^{+}$ supply on δ¹³C of leaf metabolites and respired CO₂ is largely unknown. We supplied tobacco plants with a gradient of $N{O}_3^{\text{-}}$ to $N{H}_4^{+}$ 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 $N{O}_3^{\text{-}}$ decreased, activities of all measured enzymes decreased. δ¹³C of CO₂ and organic acids closely co-varied and were more positive under $N{O}_3^{\text{-}}$ 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 $N{O}_3^{\text{-}}$ supply. Thus, the effect of $N{O}_3^{\text{-}}$ vs. $N{H}_4^{+}$ on δ¹³C is highly relevant, particularly if δ¹³C of leaf metabolites or respiration is used as proxy for plant performance.

Keywords: Ammonium; carbon-13; carboxylase; compound specific carbon isotope analysis; isotope ecology; nitrate; nitrate reductase; phosphoenolpyruvate; tobacco plants.