δ¹⁵ N values in plants are determined by both nitrate assimilation and circulation

Area of Science:

• Plant physiology
• Isotope geochemistry
• Biogeochemical cycling

Background:

• Nitrogen (N) assimilation in plants results in ¹⁴N/¹⁵N fractionation, typically making plant tissues ¹⁵N-depleted relative to the source nitrate.
• Understanding the δ¹⁵N values in plant nitrate is crucial but limited by scarce compound-specific data.
• Isotopic heterogeneity among plant organs and metabolites also influences overall plant δ¹⁵N.

Purpose of the Study:

• To extensively measure δ¹⁵N in nitrate across different plant organs and conditions.
• To investigate the impact of nitrate circulation, modulated by potassium (K) levels and waterlogging, on plant δ¹⁵N.
• To elucidate the mechanisms driving interorgan isotopic heterogeneity in nitrate.

Main Methods:

• Measured δ¹⁵N values in nitrate from sunflower and oil palm under controlled nitrate concentrations.
• Manipulated nitrate circulation by varying potassium (K) conditions and inducing waterlogging.
• Employed isotopic modeling to explain observed ¹⁵N enrichment patterns.

Main Results:

• Observed significant interorgan δ¹⁵N differences between sunflower and oil palm, with notable ¹⁵N-enrichment in root nitrate.
• Modeled results indicate that nitrate circulation and compartmentalization, particularly phloem nitrate backflow to roots, can cause substantial leaf-to-root isotopic differences (approx. 30‰).
• Waterlogging and low K conditions reduced sap circulation, consequently decreasing the leaf-to-root isotopic difference.

Conclusions:

• Plant δ¹⁵N values serve as a valuable natural tracer for nitrogen (N) fluxes between plant organs.
• The δ¹⁵N of circulating phloem nitrate is a potentially important factor influencing whole-plant N isotope signatures.
• Nitrate circulation dynamics significantly contribute to isotopic heterogeneity within plants.