A new simple method for labeling field crops with stable isotope tracers

Numerous systems have been used to label plants with ¹³C, which differ in design and complexity depending upon the desired experimental goals. However, most of these systems have generally been applied to greenhouse grown plants. Here, we report on a relatively simple ¹³C labeling system designed to label crops such as switchgrass (Panicum virgatum L.) grown in the greenhouse or small field plots. The main goals of this study were to validate the system and establish performance benchmarks. We constructed and field tested a simple design plexiglass chamber that was sealed at the soil level with a buried rubber apron. Chamber air was circulated through an infrared gas analyzer to monitor CO₂ levels within the chamber. Provisions were made to control temperatures and minimize condensation inside the chamber during labeling. Control experiments using the empty chamber both under greenhouse and field settings showed that decline in CO₂ levels was only due to plant CO₂ absorption and not due to leakage. Results indicated that the system had generally suitable performance characteristics in both greenhouse and field settings. Isotope enrichment data from our studies revealed that switchgrass fixed ¹³CO₂ that was injected into the chamber within 15-20 min labeling periods. The mean and standard deviation of leaf δ¹³C values across nine plants enclosed in the chamber was 34 ± 8.9 and 96.1 ± 23.6 for the single and double labeling experiments, respectively. Results indicate that a chamber of this construction type can be effectively used also for labeling other crop plants.