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 CO2 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 CO2 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.