One-way or return? – Carbonyl sulfide exchange of drought stressed plants

Abstract:

The trace gas carbonyl sulfide (COS) is used as a tracer for canopy gross primary production (GPP). COS enters the plant leaf through the stomata and diffuses through the intercellular space, the cell wall, the plasma membrane and the cytosol like carbon dioxide (CO2). Compared to CO2, which is rereleased in respiration processes, COS is metabolized by the enzyme carbonic anhydrase (CA) within the cytosol and the chloroplast in a one-way reaction. Studies show, that the leaf relative uptake (LRU), which is the ratio of the deposition velocities of COS to CO2 on leaf level, is fairly constant across multiple species with a median of about 1.68. However these chamber measurements were all conducted under optimal conditions for plants. Recent studies revealed that severely stressed plants (e.g. by fungal infection) might become emitters for COS. These stress induced COS emissions would alter the LRU and in turn distort the calculation of GPP.
To determine the influence of drought on the LRU, we measured the CO2 and COS gas exchange of Nicotiana tabacum, Amaranthus hypochondriacus x powelli, Brassica napus and Glycine max in a plant chamber experiment. Along with a well-watered control treatment, we exposed the plants to an intermediate and a strong drought, with predawn leaf water potentials of about 4 MPa, 20 MPa and 38 MPa respectively. We observed a decline in CO2 as well as COS uptake across all plant species, with the severe drought treatment having a stronger effect. Compared to the other species, the severe drought turned Brassica napus plants into net emitters of COS, which is possibly related to the high sulfur content of rapeseed. We will also compare the LRU of the 3 treatments and link the COS fluxes to the leaf content of low weight thiols like cysteine and glutathione in our search for potential precursors of COS within plants.