This work explored whether the natural abundances of carbon and nitrogen isotopes could be used to describe the movement of C and N within wheat plants; we also considered whether isotopic analyses of aphids or their honeydew would substitute for direct analysis of phloem exudate.

The δ13C of ears and roots (sinks) most closely matched those of the sugars+organic acids fraction (sources) in both growth stages; phloem δ13C matched that of leaf blade sugars. Xylem exudate δ13C matched no other putative (and measured) source in the ear-forming stage and matched that of whole roots and ears in the grain-filling stage. The δ15N of grain and roots (sinks) resembled that of leaf amino acids (sources) in the ear-forming stage. In the grain-filling stage, ear δ15N continued to resemble that of leaf amino acids, and δ15N of roots most closely resembled that of whole leaves. In the grain-filling stage, phloem δ15N fell between that of leaf blade amino acids and that of whole leaves and was 15N-depleted relative to internal and external NO3−-N. In both growth stages, xylem exudate δ15N was less than that of soil NO3−-N and and more than that of residual soil N after mineral N extraction. The isotopic values are generally in agreement with data from other approaches, such as isotope labelling; they show NO3−-N reduction in both shoots and roots of wheat and significant N recycling (root-shoot-phloem-root) and C movement.

Aphids might serve as a substitute for isotopic analysis of phloem δ15N, having the same value as their food source. Their excreta was 15N-enriched relative to phloem.