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.