[15N]urea foliar application effect on allocation of overwinter reserves for Pinusradiata seedling growth

Abstract

Urea labelled with 15N was applied to the foliage of Pinusradiata D. Don seedlings growing outdoors in a nursery during winter. Thereafter seedlings were either left outdoors or kept under controlled-environment conditions simulating winter (6 weeks) followed by spring (6 weeks). The fate of urea N was followed by analysis of 15N. All 15N was taken up within 6 h and existed initially in the aqueous acetone soluble fraction. Nitrogen enrichment was highest in glutamine, arginine, proline, 4-aminobutyric acid, glutamic acid, and pipecolic acid. Within 1 week, amino acid and soluble-15N enrichment declined in the needles. This was more rapid in the faster growing seedlings kept outdoors. Concurrent increases of 15N in the insoluble fraction were observed in all seedlings. Subsequently, a decline in 15N enrichment predominantly from the amino acids (glutamine and arginine) was apparent. Nitrogen extracted from the insoluble-N fraction was used to sustain seedling growth. Foliar N concentration, which in this experiment was almost entirely dependent upon stored reserves, declined from 2.1 to 1.1% N over the 12-week period. Trends in foliar starch concentrations were not affected by growth conditions. Starch concentrations in needles declined during the 1st week after [15N]urea application but thereafter increased during winter and declined in spring. These results confirmed the importance of reserve N in juvenile needles for shoot expansion during spring.