Drought effects on the leaf uptake of carbonyl sulfide and CO₂ in Pinus sylvestris and Juniperus communis

• The LRU in Pinus sylvestris was stable across the soil water content (SWC) gradient • This reflected opposing effects of COS conductance and intercellular-to-ambient CO₂ ratio. • In Juniperus communis, LRU was constant above 17% SWC but rose at lower SWC. • This rise in LRU was due to reduced stomatal-to-internal COS conductance. • LRU was variabile among species and water availability levels, supporting its species-specific and drought-dependent nature. Gross primary productivity (GPP) drives the land carbon sink, but its response to climate change and extreme weather events like drought remains uncertain. However, GPP cannot be measured directly but must be inferred through proxies, which introduces uncertainties that limit predictions. One promising approach is to measure carbonyl sulfide (COS) fluxes, supported by a thorough understanding of the relative uptake ratio between COS and CO₂, the leaf relative uptake (LRU). We derived plant-scale COS and CO 2 fluxes and calculated the LRU of Pinus sylvestris (pine) and Juniperus communis (juniper), under controlled drought conditions. The LRU remained constant (median daytime value of 1.47) in pine across the whole drought gradient due to opposing physiological processes: adjustment of conductances to COS and changes in the ratio of intercellular-to-ambient CO₂ concentration. In juniper, the LRU also had a constant value (daytime median of 1.41) for soil water content (SWC) above 17 % and increased with decreasing SWC below this threshold, driven by a decline in the stomatal to internal conductance to COS. Under drought stress, both COS and CO 2 uptake declined more in pine than in juniper. This study highlights LRU variability among species and water availability levels, providing insights into the underlying ecophysiological processes.