To reach the main goal of the project, an important methodological component planned in PHOTOFLUX received major attention in the project so far. The development of an unmixing strategy to spectrally decouple photosynthetic and non-photosynthetic fluxes of absorbed light for leaf, canopy and complex scenes was proposed as one of the sub-objectives of PHOTOFLUX.
To achieve this objective, we developed a core algorithm based on a non-negative least square (NNLS) spectral unmixing algorithm for reflectance (500–780 nm) data. The version implemented can retrieve the effective absorbance of individual pigments, i.e. Chlorophyll (Chl) a and b, beta-Carotene and (overall) xanthophylls. The NNLS fitting was successfully applied to fit the total effective absorbance at leaf level, linearly composed of pigment and background absorption coefficient spectra. The novel aspect of this spectral unmixing strategy is the implementation of a spectral endmember characterizing the absorption behaviour of xanthophylls, affecting the leaf absorbance only by a few percent. This subtle absorbance feature, linked to the xanthophyll-based energy dissipation mechanism, could be meaningfully retrieved from the spectral data at leaf scale. We applied the leaf-based algorithm to simulated images of the Fluorescence Explorer, the foreseen satellite mission of the European Space Agency devoted to the estimation of vegetation fluorescence and actual photosynthesis.
Further it was tested if the proposed absorption unmixing protocol could (1) detect the activation of the fast regulated heat dissipation, and (2) tackle the ambiguity in the fluorescence-photosynthesis relationship, currently an issue in remote sensing applications. To accomplish this, we achieved to test our algorithm using an inter-disciplinary experiment based on in vivo spectroscopy (absorption and fluorescence), combined with common plant physiology measurements, i.e. active fluorescence and gas exchange measurements. Our results so far underscore the potential of complementary in vivo quantitative spectroscopy-based products in the early and non-destructive stress diagnosis of plants, marking the path for further applications across various monitoring scales.
