δ¹⁵N values in plants are determined by both nitrate assimilation and circulation

Abstract

2 UPR34 Performance des syst emes de culture des plantes p erennes, D epartement PERSYST, Centre de Coop eration Internationale en Recherche Agronomique pour le D eveloppement (CIRAD), SummaryNitrogen (N) assimilation is associated with 14 N/ 15 N fractionation such that plant tissues are generally 15 N-depleted compared to source nitrate. In addition to nitrate concentration, the d 15 N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of d 15 N values in nitrate is limited by the relatively small number of compound-specific data.We extensively measured d 15 N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging.There were strong interorgan d 15 N differences for contrasting situations between the two species, and a high 15 N-enrichment in root nitrate. Modelling shows that this 15 N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30& difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference.Our study thus suggests that plant d 15 N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of d 15 N of circulating phloem nitrate.

Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady‐state hydroponics

“…In the present study, the averaged whole plant Δ 15 N was 1.66‰, indicating that 14 NO 3 − was preferentially assimilated in the roots and a greater proportion of the 15 NO 3 − was returned to the hydroponic medium. Similar to the results of Kalcsits and Guy (2013a) for the closely related Populus balsamifera L. (balsam poplar), and many other species (Cui et al, 2020), we found leaves had higher δ 15 N values (i.e., lower Δ 15 N; Figure 4) than roots. This is the expected pattern if residual unassimilated NO 3 − (enriched in 15 N) is transported from roots to shoots.…”

“…Although this is a high proportion relative to many species (Cui et al, 2020;Oh, Kato, & Xu, 2008), it is not unusual for Populus. Dluzniewska et al (2006) obtained a P root of 96% and Siebrecht and Tischner (1999) estimated P root to be 40 60%, both in hybrid aspen (Populus tremula × P. alba).…”

“…In essence, and unlike Cui et al (2020) where isotope discrimination associated with efflux is considered fixed, Equation 11rearranged yields whole plant isotope discrimination as a function of the exchange of cellular nitrate with the external source. This is similar to the accepted model for carbon isotope discrimination during CO 2 fixation in photosynthesis (Farquhar, Ehleringer, & Hubick, 1989).…”

“…Similarly, under these specific conditions, the return of nitrogen to the roots in either inorganic or organic form is of little consequence to the plant or organ level N budget and is ignored. However, Cui, Lamade, Fourel, and Tcherkez (2020) have recently argued that the return of residual, unassimilated 15 N‐enriched nitrate back to the roots via phloem transport can be isotopically significant in oil palm and sunflower, two species where N assimilation is primarily in the leaves. Because there is no evidence for significant isotope discrimination associated with membrane transport (Evans, 2001; Handley & Raven, 1992), the IMB model considers the cytosolic and vacuolar nitrate pools within root cells to be in isotopic equilibrium, and, similarly, that there is no discrimination during xylem loading.…”

Metabolic Responses to Waterlogging Differ between Roots and Shoots and Reflect Phloem Transport Alteration in Medicago truncatula

“…In leaves, waterlogging leads to a strong increase in sugars (hexoses and polyols derived therefrom) and a decline in several amino acids (sometimes including alanine), suggesting an inhibition of sugar export and N assimilation [ 26 ]. In addition, there is a decline in nitrate content in leaves, much less pronounced in roots, suggesting an inhibition of xylem nitrate transport and in fact, computations based on 15 N natural abundance have shown a strong effect of waterlogging on nitrate circulation [ 28 ]. Additionally, in cotton, waterlogging causes a decline in the expression of nitrite reductase and an increase in nitrite concentration in leaves [ 29 ].…”