Respiratory Turn-Over and Metabolic Compartments: From the Design of Tracer Experiments to the Characterization of Respiratory Substrate-Supply Systems

Maintenance, defense and growth in plants – and hence their ability to survive and propagate despite stress and competition – are strictly dependent on the availability of respiratory substrate as an energy source. Quantitative labeling of photosynthetic products, in conjunction with monitoring of tracer appearance in respiratory CO2 and compartmental modeling of tracer kinetics, are powerful tools to assess key features of the metabolic system supplying substrate for respiration. Such features include the number, the size and the turn-over of kinetically distinct pools that compose the system. Biological knowledge is essential for deriving a meaningful topology/architecture of respiratory substrate pools. Here, we describe basic characteristics and requirements of quantitative labeling techniques and principles of compartmental modeling for the study of the respiratory substrate supply system at both ecosystem and plant levels. Dynamic labeling associated with compartmental analysis has been used successfully to partition autotrophic and heterotrophic components of grassland ecosystem respiration. This combination of methodologies has also shown that the substrates feeding root and shoot respiration of a perennial grass are located in the shoot and sustain most of the respiratory activity of shoots and roots even during undisturbed growth. And it has provided strong support for the fructan pool in the shoot being the main storage compartment supporting respiration in this grass species. Finally, we show how a compartmental analysis of the respiratory substrate supply system can be combined with a compartmental analysis of carbohydrate metabolism in the same plant to investigate the potential identity of pools sustaining respiration.