Spatio-temporal variations of photosynthesis: the potential of optical remote sensing to better understand and scale light use efficiency and stresses of plant ecosystems
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The light use efficiency (LUE) of photosynthesis dynamically adapts to environmental factors, and this leads to complex spatio-temporal variations of photosynthesis on various scales from the leaf to the canopy level. These spatio-temporal pattern formations not only help to understand the regulatory properties of photosynthesis, but may also have a constructive role in maintaining stability in metabolic pathways and during development. Optical remote sensing techniques have the potential to detect physiological and biochemical changes in plant ecosystems, and non-invasive detection of changes in photosynthetic energy conversion may be of great potential for managing agricultural production in a future bio-based economy. Here we review the results from selected remote sensing projects for their potential to quantify LUE from the level of single leaves to the canopy scale. In a case study with soybean grown under elevated CO2 conditions at the SoyFACE facility, we tested the photochemical reflectance index (PRI) for its capacity to quantify higher photosynthetic efficiency. In this study the PRI failed to detect differences in photosynthetic light conversion, most likely because of the variable canopy structure of the soybean canopy. We thus conclude that at the current state of the art the PRI cannot serve as an easy remote sensing approach to detect changes in photosynthetic energy conversion in agriculture. As an alternative we present approaches that aim to quantify the fluorescence signal of chlorophyll and thus estimate photosynthetic efficiency. In a second case study, using avocado as a model species, an active laser induced fluorescence transient (LIFT) method was applied to deliver maps of different photosynthetic efficiency within the canopy. Cold-induced down-regulation of photosynthesis in the upper canopy was detected, so active fluorescence may prove its potential for non-invasive monitoring of crops. With a view to the future, we present a method for large scale managing of agricultural practices within the framework of the FLuorescence EXplorer (FLEX) mission, which proposed launching a satellite for the global monitoring of steady-state chlorophyll fluorescence in terrestrial vegetation. This mission was selected for inclusion in pre-phase A by the European Space Agency.