Leaf Feeding of [15N]Urea for Labelling Wheat With Nitrogen

A leaf feeding technique that uses [15N]urea was examined to determine its suitability for labelling plant material in order to study the long-term storage and remobilisation of nitrogen in wheat. Leaves were fed continuously for intervals up to 4 days by dipping their cut tips into [15N]urea solutions (99 atom %) at tillering, stem elongation and flowering. The [15N]urea uptake closely followed the transpiration rate of the leaf being fed, with higher rates at midday when the transpiration rate averaged 4 mmol H2O m-2 s-1 and lower rates at early morning or late afternoon when the transpiration rate averaged 0.1 mmol H2O m-2 s-1. Uptake of [15N]urea was higher by mainstem leaves than by leaves on subtending tillers on the same plant. The simultaneous feeding of more than one leaf per shoot had little effect on the overall 15N uptake per plant. The overall uptake of 15N by mainstem leaves was reduced as soil water potentials (Ψsoil) decreased below -0.2 MPa. The necrotic area of the leaf that was fed increased as the concentration of the urea solution increased, and was critically burned when the solution concentration exceeded 2.5%. Two days after leaf feeding, 76% of the total 15N taken up was in the fed leaf and was continuously exported to the remainder of the plant throughout the period of measurements (up to 10 days after leaf feeding). The feeding of a mainstem leaf for 4 days at tillering using up to 1.5% [15N]urea solutions labelled the plant parts by about 2 stom % 15N excess. Such enrichments are required when labelling plant material to study the long-term storage and remobilisation of nitrogen.