Effects of drought on carbonyl sulfide exchange in four plant species

• Bi-directional carbonyl sulfide fluxes in rapeseed and soybean . • Drought stress decreases LRU in soybean and tobacco . • Drought stress increases LRU in amaranth . The trace gas carbonyl sulfide (COS) is used for estimating gross primary productivity at ecosystem level (GPP), as the net CO₂ flux is confounded by ecosystem respiration. Laboratory measurements studying the ratio of the deposition velocities of COS and CO 2 at leaf level, i.e. leaf relative uptake rate (LRU), are required for calculating GPP. Under optimal conditions, the LRU has been suggested to be relatively constant. However, stress factors may affect the LRU and even lead to COS emission, which contradicts the prevailing scientific consensus. This study investigated the effect of drought on LRU in three C 3 species, rapeseed, soybean and tobacco, and the C 4 plant, amaranth. Our results revealed species-specific responses, with the LRU decreasing in C 3 plants and increasing primarily in the C 4 species under drought. We observed net COS emissions in soybean and rapeseed during drought and for the latter also under unstressed conditions. These emissions suggest bidirectional COS exchange, likely interfering with the unidirectional COS uptake concept underlying LRU even during net COS uptake. In all C 3 species, drought induced an increase in leaf cysteine, supporting a cysteine-related COS emission pathway. However, in amaranth cysteine levels decreased in contrast to the COS flux, and were not the highest in rapeseed despite elevated COS emission, altogether showing that factors involved in COS flux require further investigation. Overall, our findings challenge the use of COS as a universal tracer for GPP and underscore the need for further research into COS emissions and LRU variability across species, particularly under environmental stress.