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

SUMMARY This paper relates the 13 C/ 12 C ratio of C 3 plant material relative to that of source CO 2 to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The 13 C/ 12 C ratio is quantified as Δ, defined as (δ 13 C substrate –δ 13 C product)/(1+δ 13 C product), where δ 13 C values of substrate or product (i.e. the samples) are defined as [ 13 C/ 12 C] sample ]/[( 13 C/ 12 C) 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 CO 2 ). Application of this approach to a ‘typical’ C 3 land 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 NH 4 + assimilation (which always occurs in the roots) to 2–80%, for NO 3 − 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 N 2 , or where NO 3 − 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 of Ricinus communis cultured with NH 4 + or NO 3 − shows that the measured influence of nitrogen source is in the right direction (NO 3 − 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 δ 13 C 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 NH 4 + ‐grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C 3 terrestrial 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.