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

Abstract

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.