Variations of the natural abundances of nitrogen and carbon isotopes in Triticum aestivum, with special reference to phloem and xylem exudates

summary This work explored whether the natural abundances of carbon and nitrogen isotopes could be used to describe the movement of C and X within wheat plants; we also considered whether isotopic analyses of aphids or their honeydew would substitute for direct analysis of phloem exudate. The δ 13 C of ears and roots (sinks) most closely matched those of the sugars + organic acids fraction (sources) in both growth stages; phloem δ 13 C matched that of leaf blade sugars. Xylem exudate δ 13 C 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 δ 15 N of grain and roots (sinks) resembled that of leaf amino acids (sources) in the ear‐forming stage. In the gram‐filling stage, ear δ 15 N continued to resemble that of leaf amino acids, and δ 15 N of roots most closely resembled that of whole leaves. In the grain‐filling stage, phloem δ 15 N fell between that of leaf blade amino acids and that of whole leaves and was 15 X‐depleted relative to internal and external NO, ‐N. In both growth stages, xylem exudate δ 15 N was less than that of soil NO 3 − ‐N 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 NO 3 − ‐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 δ 15 N. having the same value as their food source. Their excreta was 15 N‐enriched relative to phloem.