Seasonal stability of chlorophyll fluorescence quantified from airborne hyperspectral imagery as an indicator of net photosynthesis in the context of precision agriculture

The seasonal stability of solar-induced chlorophyll fluorescence (SIF) vs field-measured leaf CO2 assimilation (A) was assessed over a period of 2years by means of airborne flights performed at midday and diurnally over a citrus (evergreen) crop canopy. The orchard was cultivated under a control treatment (ET) that received 100% of its water requirements and two regulated deficit irrigation (RDI) treatments with water supply reduced to 37% and 50% of the control level during the summer. Field measurements consisted of assimilation rate, stomatal conductance, stem water potential, leaf fluorescence and leaf reflectance. The airborne campaigns took place in 2012 and 2013, and were flown on the solar plane in order to acquire hyperspectral imagery at 40cm resolution, 260 spectral bands and 1.85nm/pixels in the 400–885nm spectral region. A thermal camera was installed in tandem in all flights, acquiring imagery in the 7.5–13μm spectral region at 640×480 pixel resolution, yielding a 50cm pixel size. The robustness of the SIF quantification through the Fraunhofer Line Depth (FLD) principle based on three spectral bands (FLD3), as well as the performance of physiological and structural hyperspectral indices, was evaluated in order to understand their ability to track photosynthesis at different phenological and stress stages throughout the season. Solar induced fluorescence quantified as FLD3 was the most robust indicator of photosynthesis in all the airborne campaigns performed in the course of the two-year experiment, which comprised seven midday flights and two diurnals. The relationships between fluorescence (FLD3) and assimilation rates yielded correlation coefficients (R) between 0.64 and 0.82 across all dates, these being statistically significant with p-values between p