Use of ¹³C and ¹⁵N Plant Label Near Natural Abundance for Monitoring Carbon and Nitrogen Partitioning

Abstract

Tracing with stable isotopes by using naturally or weakly labelled compounds has become a reliable approach in metabolic studies due to the high precision of isotope measurement by mass spectrometers fitted for natural range. Rapid and numerous isotope ratio determinations are now possible due to the recent automation of analyses. Three methods of analysis of carbon and nitrogen partitioning are reviewed from experiments on maize plants: (a) use of natural differences in organ isotope composition; (b) labelling with industrial CO₂ naturally depleted in ¹³C; (c) double C and N labelling with CO₂ and NO₃ slightly enriched in ¹³C and ¹⁵N. For method (c) which is the most precise, the obtaining of plant matter with ¹³C and ¹⁵N label near their natural isotope abundance (1.1% for C and 0.36% for N) as well the principles of exposure and apparatus for feeding plants are described. Calculations of distribution parameters (relative specific allocation, RSA, and partitioning, %P) are presented and compared with their use in high-enrichment experiments. The precision of parameters and the theoretical or practical limitations of the methods are discussed. We show that the use of stable isotopes near their naturally occurring concentrations allows the tracing of new C or N input with precision, and is suitable for monitoring long-term partitioning. The significant advantages of this method with respect to precision, security and cost of handling compared with high abundance or radioactive tracing are discussed.