Carbon monoxide (CO) is known primarily as a globally emitted by-product of incomplete combustion from the industry and biomass burning. However, CO is also produced in living plants and acts as a stress-signalling molecule in animals and plants. While CO emissions from soil and litter decomposition have been studied, research on the CO flux from living vegetation is scarce, particularly under field conditions. Here, we present a year-long field study on the effects of light, heat, and seasonal drought on leaf CO production and flux using automated twig chambers on mature Pinus halepensis trees grown under summer-droughted and nondroughted (irrigated) conditions. We found CO buildup in drought-stressed tree leaves, with emissions linked to the heat-controlled biogenic production of CO rather than to photodegradation. In irrigated trees, CO fluxes occurred through open stomata, whereas in droughted trees, CO buildup overcame stomatal closure to result in a flux. The results support the role of CO in heat stress response and the likely mitigation of damage induced by reactive oxygen species. We highlight the need for further research into the mechanistic basis for CO flux from living plants.

Most terrestrial plants are sensitive to prolonged flooding or soil salinity, and exposure to the combination of these factors generally compounds the negative effects of each one considered separately. Achachairu (Garcinia humilis, fam. Clusiaceae), a tropical fruit tree from the Bolivian Amazon, is tolerant to flooding and moderately tolerant to soil salinity, but its physiological and biochemical responses to the combined effects of flooding and salinity have not been reported. This study assessed the physiological and biochemical responses of G. humilis to the combined effects of 30 d flooding and salinity levels of 4 dS m-1. Physiological variables measured included leaf gas exchange [net CO2 assimilation (A), stomatal conductance of H2O (gs), and intercellular CO2 concentration (Ci)], leaf chlorophyll index (LCI), and the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm). Leaf and root nutrient analyses were performed to assess nutrient imbalances and the accumulation of toxic ions. Antioxidant responses, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and glutathione (GSH); reactive oxygen species (ROS); and lipid peroxidation (MDA) were also measured. The results indicate that G. humilis can tolerate the combined effects of prolonged flooding of 30 d and soil salinity of at least 4 dS m-1, maintaining basal A and gs levels of approximately 30%, with no evidence of physiological damage to LCI, Fv/Fm, or visible stress symptoms. While Na and Cl concentrations increased in leaf and root tissues, trees were able to maintain nutrient homeostasis within non-toxic levels. A robust antioxidant response was observed and possibly countered the potentially noxious effects of flooding and salinity.