Relationships between net photosynthesis and steady-state chlorophyll fluorescence retrieved from airborne hyperspectral imagery

Previous studies have demonstrated the link between leaf chlorophyll fluorescence and photosynthesis, mainly at the leaf level and under controlled laboratory conditions. The present study makes progress in demonstrating the relationship between steady-state fluorescence and net photosynthesis measured under natural light field conditions both at the leaf and image levels. Ground measurements and airborne campaigns were conducted over two summers to acquire hyperspectral imagery at 40 cm resolution and 260 spectral bands in the 400-885 nm spectral region. This enabled the identification of pure vegetation pixels to extract their radiance spectra. The datasets were collected in August 2010 and 2011 in the western part of the area included in the Ribera del Duero Designation of Origin (Denominación de Origen), in northern Spain. The experiments were conducted in twelve full production vineyards where two study plots per field were selected to ensure adequate variability in leaf biochemistry and physiological condition. The vineyard fields were selected on the basis of their gradient in leaf nutrition and plant water status and showed variability in leaf pigment values and stomatal conductance. Leaves were collected for destructive sampling and biochemical determination of chlorophyll a + b, carotenoids and anthocyanins in the laboratory. Leaf steady-state and dark-adapted fluorescence parameters, net photosynthesis (Pn) and stomatal conductance (Gs) were measured in the field under natural light conditions. Such data were used as a validation dataset to assess fluorescence-photosynthesis relationships both at the leaf and the image level. The Fraunhofer Line Depth (FLD) principle based on three spectral bands (FLD3) was the method used to quantify fluorescence emission from radiance spectra extracted from pure vegetation pixels identified in the hyperspectral imagery. Fluorescence retrievals conducted using the FLD3 method yielded significant results when compared to ground-measured steady-state Fs (r2 = 0.48; p < 0.01) and Fv′/Fm′ (r2 = 0.53; p < 0.01). The two-year assessment yielded consistent results on the relationship between Pn and Fs both at the leaf level and based on the airborne hyperspectral imagery. At the leaf level, significant relationships were found between leaf Fs and Pn (r2 = 0.55; p < 0.001 for 2010; r2 = 0.59; p < 0.001 for 2011). At the hyperspectral image level, the agreement between leaf Pn and airborne F was consistent for both years separately, yielding significant relationships at p < 0.01 for 2010 (r2 = 0.54) and 2011 (r2 = 0.41) and a significant relationship at p < 0.001 for the aggregated years (r2 = 0.52). Results show the link between net photosynthesis and steady-state fluorescence obtained under natural sunlight conditions at both leaf and airborne hyperspectral imagery levels.