The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

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Abstract
SUMMARY: This paper relates the13C/12C ratio of C3plant material relative to that of source CO2to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The13C/12C ratio is quantified as Δ, defined as (δ13C substrate –δ13C product)/(1+δ13C product), where δ13C values of substrate or product (i.e. the samples) are defined as [13C/12C]sample]/[(13C/12C)standard]−1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO2). Application of this approach to a ‘typical’ C3land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0–24 %, for NH4+assimilation (which always occurs in the roots) to 2–80%, for NO3−assimilation in shoots with the organic acid salt which results from acid‐base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N2, or where NO3−assimilation in root or shoot is accompanied by some acid‐base regulation via OH‐ loss to the root medium.Comparison with published data on the difference in Δ ofRicinus communiscultured with NH4+or NO3−shows that the measured influence of nitrogen source is in the right direction (NO3−grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non‐Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ13C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH4+‐grown plants.The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C3terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.