Diel variations in carbon isotopic composition and concentration of organic acids and their impact on plant dark respiration in different species

Leaf respiration in the dark and its C isotopic composition (C-13(R)) contain information about internal metabolic processes and respiratory substrates. C-13(R) is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated C-13-enriched organic acids, however, studies simultaneously measuring C-13(R) during LEDR and potential respiratory substrates are rare. We determined C-13(R) and respiration rates (R) during LEDR, as well as C-13 and concentrations of potential respiratory substrates using compound-specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. C-13(R) and R patterns during LEDR were strongly species-specific and showed an initial peak, which was followed by a progressive decrease in both values. The species-specific differences in C-13(R) and R during LEDR may be partially explained by the isotopic composition of organic acids (e.g., oxalate, isocitrate, quinate, shikimate, malate), which were C-13-enriched compared to other respiratory substrates (e.g., sugars and amino acids). However, the diel variations in both C-13 and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong C-13 enrichment in leaf dark-respired CO2.