In situ ¹⁵N labelling of lupin below-ground biomass

Abstract

This paper describes the use of a cotton-wick method to enrich lupin plants with 15N. The method involved the insertion of a cotton thread through the stem and the submergence of the ends of the cotton thread in a solution of highly enriched 15N urea. The distribution of 15N in lupin plant components during pre-reproductive growth and pod filling. and in relation to the frequency of labelling, was determined. The recovery of applied 15N within plant tissues was close to 100% for lupins grown in solution culture, but 15N was not distributed between plant components in the proportions observed for total plant N. Stems and leaves were preferentially labelled with 15N irrespective of the phase of lupin growth when the 15N was applied. Pre-reproductive and mature lupin root biomass was depleted in 15N because of the poor assimilation of 15N within lupin nodules. More applied 15N was found in the root biomass of lupin plants that received fortnightly, compared with weekly, applications of 15N. The distribution of 15N between lupin components was reproducible when 15N-urea was wick-applied to plants of the same age. Recovery of 15N was incomplete when urea was fed to lupins grown in sand culture. Incomplete recovery of root material and loss of 15N associated with root exudates probably contributed to the lower recoveries of 15N in root material in sand compared with solution culture. The ability to manipulate the 15N solution concentration, the volume of solution fed to plants, time of application, and frequency of 15N application underscore the usefulness of the wick technique to label woody legumes with 15N.