Understanding the mechanism of plant carbonyl sulfide uptake

The Earths climate is currently changing due to in the human-induced increase in atmospheric carbon dioxide (CO2). The biosphere is the largest sink of CO2, and it is not completely clear how the magnitude of this sink will change in reaction to climate change. Therefore, it is important to quantify the separate aspects of the biospheres CO2 exchange, being gross primary production (GPP or photosynthesis) and respiration (Re). Methods that are currently used to measure GPP have their limitations and therefore carbonyl sulfide (COS) has been proposed as a proxy for GPP. COS is an atmospheric trace gas with a concentration of 500 parts per trillion that enters plants through a similar diffusion pathway as CO2. In contrast to CO2, COS does not exit the plant anymore. Through the leaf relative uptake ratio (LRU) of COS versus CO2 uptake, GPP can be estimated. However, recent research suggests that the LRU varies between species and with environmental conditions. Therefore, more research is needed on the mechanisms that cause these variations in LRU. When COS and CO2 enter a leaf, they pass a series of resistances before they enter the location where they are fixed. These are called the stomatal conductance, internal conductance and the mesophyll conductance. The rate at which the gases are fixated by the plant depends on the activities of the enzymes inside of the leaf, which can also affect the LRU. This research will consist of a set of lab and field experiments that will target the different factors controlling LRU, with the ultimate goal of improving the use of COS to derive photosynthesis. Different types of plants will be used that have known differences in their resistance path for COS and CO2. The plants will also be exposed to various environmental conditions, such as the amount of light, air moisture content and drought. The field measurements will be conducted in an alpine forest field site in Mieming. Next to flux measurements of COS and CO2, we will also measure the isotopes of COS and CO2. Isotopes are molecules of the same chemical that have a different mass, and therefore have different phyisical and chemical properties. They can give additional useful information on chemical reactions and physical processes such as diffusion across resistances. Whereas CO2 isotope measurements are already common practice in the field of ecology, COS isotope measurements are a highly novel, but also challenging tool to better understand COS uptake by plants.