During photosynthesis, light and water are used by plants to assimilate CO2 and produce organic compounds, while oxygen (O2) is released. Without photosynthesis, life on earth would be very different. While the produced biomass serves as food, fibre and energy source, the exchange of carbon, oxygen and water affects the composition of the atmosphere, as well as the climate. The carbon assimilated during photosynthesis over a certain time and space is referred to as gross primary production (GPP). The quantification of GPP and its spatio-temporal description from field to global scale is therefore of fundamental importance, not only in terms of climate change research, but also in respect to food security.
Space- and airborne-based spectroscopy can be considered the only technology that continuously observes vegetation status and functioning at field to global scale, allowing to derive GPP. Because it is directly related to the photosynthetic process, SIF (sun-induced chlorophyll fluorescence) is a promising signal that might allow overcoming the limitations of traditional reflectance-based methods (e.g. surface greenness) for estimating GPP. Advances in optical sensor technology and methods allow reliable measurements of SIF using ground, airborne, and satellite sensors and have shown good relationships between SIF GPP. However, it is still unclear if this relationship is primarily driven by their common dependence on the absorbed incoming light (APAR), rather than being directly related. Recent studies have shown that the SIF fractior per incoming light (SIF/APAR = Fyield) might be related to light-use efficiency (LUE), a measure of the fraction of photons used in the photosynthesis reactions.
Nevertheless, there are two major factors which complicate the predictability of LUE by Fyield: i) SIF and Fyield are related to the photosynthetic light reactions, while the LUE concept includes the dark reactions, and the stomatal conductance; ii) only 1-2 % of APAR is re-emitted as SIF, while heat dissipation (non-photochemical quenching, or NPQ) can contribute by 17.5 to 98 %. Therefore, the relation between Fyield and LUE will always be indirect and poor. To overcome this problem, the SIF signal has to be linked to the light reactions and the signal of NPQ needs to be accessed. It is currently being discussed if the photochemical reflectance index (PRI), which is sensitive to changes in xanthophyll activity (a pigment used for protection from photodamage), could be used to estimate NPQ on plant and ecosystem level. However, the application of the PRI at plant and ecosystem level is complicated by factors like canopy structure, leaf angle distribution and changes in leaf pigment pools. Furthermore, plant or ecosystem-derived PRI can only be compared to leaf level NPQ measurements, further complicating the use of PRI to estimate NPQ.
The main objective of ReSPEc “Towards a Remotely Sensed estimation of the Photosynthetic Energy balance” is to overcome the named problems by developing a semi-mechanistic model that allows an improved estimation of plant and ecosystem photosynthesis by traditional and novel remote-sensing applications.
