HYPERSPECTRAL IMAGING OF PHOTOSYNTHESIS FROM THE SINGLE LEAF TO THE COMPLEX CANOPY - UNDERSTANDING THE SPATIO- TEMPORAL VARIATIONS OF PHOTOSYNTHESIS WITHIN A DROUGHT-STRESSED TROPICAL CANOPY

Light use efficiency of photosynthesis adapts dynamically to environmental factors and is affected by internal and external stress factors, all of which lead to complex spatio-temporal variations of photosynthesis on various scales from the leaf to the canopy level. We tested a new, field portable hyperspectral imaging system (SOC-700), which produces 12-bit reflectance images between 440 and 880 nm with a 4nm spectral resolution. Data were filtered by smoothing the low order principal components reducing the noise of the instrument substantially. Leaf level scans of reflectance were used to detect differences in the non-photochemical energy dissipation of genetically modified Arabidopsis thaliana (L.) Heynh. mutants, which were deficient or over-expressed psbS, an intrinsic pigment-binding photosystem II subunit (also known as CP22). In a second experiment, the Photochemical Reflectance Index (PRI) was used to track drought stress induced inactivation of photosynthesis in leaves of four tropical tree species. Based on these leaf-level measurements a canopy element of the tropical rainforest in Biosphere 2 Centre was monitored using the same approach in the early morning after sunrise and two hours later when exposed to full sun. PRI developed a functional dependency only after activation of the biochemical non-photochemical energy dissipation processes were initiated after high light exposure. This study indicates the potential of hyperspectral reflectance measurements to quantify physiological adaptation of the photosynthetic apparatus and highlights the prerequisite to take structural and physiological heterogeneity of natural canopies into account.