VARIATIONS OF THE NATURAL ABUNDANCES OF NITROGEN AND CARBON ISOTOPES IN TRITICUM-AESTIVUM, WITH SPECIAL REFERENCE TO PHLOEM AND XYLEM EXUDATES

This work explored whether the natural abundances of carbon and nitrog en isotopes could be used to describe the movement of C and N within w heat plants; we also considered whether isotopic analyses of aphids or their honeydew would substitute for direct analysis of phloem exudate . The delta(13)C of ears and roots (sinks) most closely matched those of the sugars + organic acids fraction (sources) in both growth stages ; phloem delta(13)C matched that of leaf blade sugars. Xylem exudate d elta(13)C matched no other putative (and measured) source in the ear-f orming stage and matched that of whole roots and ears in the grain-fil ling stage. The delta(15)N of grain and roots (sinks) resembled that o f leaf amino acids (sources) in the ear-forming stage. In the grain-fi lling stage., ear delta(15)N continued to resemble that of leaf amino acids, and delta(15)N of roots most closely resembled that of whole le aves. In the grain-filling stage, phloem delta(15)N fell between that of leaf blade amino acids and that of whole leaves and was N-15-deplet ed relative to internal and external NO3--N. In both growth stages, xy lem exudate delta(15)N 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, suc h as isotope labelling; they show NO3--N reduction in both shoots and roots of wheat and significant N recycling (root-shoot-phloem-root) an d C movement. Aphids might serve as a substitute for isotopic analysis of phloem delta(15)N, having the same Value as their food source. The ir excreta was N-15-enriched relative to phloem.