Short‐term dynamics of isotopic composition of leaf‐respired CO2 upon darkening: measurements and implications

Recent advances in understanding the metabolic origin and the temporal dynamics in δ¹³C of dark‐respired CO₂ (δ¹³C_res) have led to an increasing awareness of the importance of plant isotopic fractionation in respiratory processes. Pronounced dynamics in δ¹³C_res have been observed in a number of species and three main hypotheses have been proposed: first, diurnal changes in δ¹³C of respiratory substrates; second, post‐photosynthetic discrimination in respiratory pathways; and third, dynamic decarboxylation of enriched carbon pools during the post‐illumination respiration period. Since different functional groups exhibit distinct diurnal patterns in δ¹³C_res (ranging from 0 to 10‰ diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e. growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates on diurnal changes in δ¹³C_res was negligible in all investigated species. Further, rapid post‐illumination changes in δ¹³C_res (30 min), which increased from 2.6‰ to 5‰ over the course of the day, were examined by positional ¹³C‐labelling to quantify changes in pyruvate dehydrogenase (PDH) and Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance calculations based on theoretical assumptions on fractionation processes. Neither the estimated changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed pattern in δ¹³C_res. However, a Rayleigh fractionation of ¹²C‐discriminating enzymes and/or a rapid decline in the decarboxylation rate of an enriched substrate pool may explain the post‐illumination peak in δ¹³C_res. These results are highly relevant since δ¹³C_res is used in large‐scale carbon cycle studies. Copyright © 2009 John Wiley & Sons, Ltd.