Leaf carbon monoxide emission under field conditions: a potential stress indicator?

Carbon monoxide (CO) is produced in living plants and can act as a stress-signaling molecule in both animals and plants. While CO emissions from soil, litter decomposition, and incomplete combustion have been extensively studied, there is a scarcity of research on CO flux from living vegetation, particularly under field conditions. We present the results of continuous CO fluxes measurements (together with those of water, CO₂, and COS) using twig chambers in summer-droughted and in non-droughted (irrigated) Pinus halepensis trees across the seasonal cycle. We found significant CO emissions from leaves, which were correlated with environmental parameters (radiation, leaf temperature, and VPD). It peaked under the stressful summer conditions at the study site, when CO₂ exchange and leaf conductance were at a minimum.  The CO fluxes were strongly correlated to twig transpiration and were enhanced under irrigated treatment. It is speculated that leaf CO emission is related to biotic reactions, such as heme degradation, which is enhanced under stress conditions and is possibly associated with photorespiration. Our results provide a rare, high-resolution, annual scale study of the environmental factors controlling leaf CO emissions under field conditions and indicate that including it in plant gas exchange studies may provide additional means to interpret their response to stress.