Nitrogen control of 13C enrichment in heterotrophic organs relative to leaves in a landscape-building desert plant species

[摘要] A longstanding puzzle in isotope studies of C₃ plant species is thatheterotrophic plant organs (e.g., stems, roots, seeds, and fruits) tend to beenriched in ¹³C compared to the autotrophic organ (leaves) that providesthem with photosynthate. Our inability to explain this puzzle suggests keydeficiencies in understanding post-photosynthetic metabolic processes. Italso limits the effectiveness of applications of stable carbon isotopeanalyses in a variety of scientific disciplines ranging from plant physiologyto global carbon cycle studies. To gain insight into this puzzle, weexcavated whole plant architectures of Nitraria tangutorum Bobrov,a C₃ species that has an exceptional capability of fixing sands andbuilding sand dunes, in two deserts in northwestern China. We systematicallyand simultaneously measured carbon isotope ratios and nitrogen andphosphorous contents of different parts of the excavated plants. We alsodetermined the seasonal variations in leaf carbon isotope ratios on nearbyintact plants of N. tangutorum. We found, for the first time, thathigher nitrogen contents in heterotrophic organs were significantlycorrelated with increased heterotrophic ¹³C enrichment compared toleaves. However, phosphorous contents had no effect on the enrichment. Inaddition, new leaves had carbon isotope ratios similar to roots but wereprogressively depleted in ¹³C as they matured. We concluded that anitrogen-mediated process, hypothesized to be the refixation of respiratoryCO₂ by phosphoenolpyruvate (PEP) carboxylase, was responsible for thedifferences in ¹³C enrichment among different heterotrophic organs, whileprocesses such as fractionating foliar metabolism and preferentially loadinginto phloem of ¹³C-enriched sugars may contribute to the overallautotrophic–heterotrophic difference in carbon isotope compositions.

[学科分类] 地球化学与岩石