Experimental evidence for diel δ15 N-patterns in different tissues, xylem and phloem saps of castor bean ( Ricinus communis L.).

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ¹⁵ N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ¹⁵ N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ¹⁵ N was correlated fairly well between phloem and xylem saps. These variations in δ¹⁵ N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to ¹⁵ N-enriched glutamine acting as transport form of N. δ¹⁵ N of the water soluble fraction of roots and leaves partially affected δ¹⁵ N of phloem and xylems saps. δ¹⁵ N patterns are likely the result of a complex set of interactions and N-fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course - a fact that needs to be taken into account when sampling for isotopic studies. [ABSTRACT FROM AUTHOR]