1 Plant Fluorescence Measured under Two Extreme 2 Lighting Conditions Using a Passive Spectroradiometer 3

In this study, we evaluated chlorophyll fluorescence (CF) under two extreme 13 illumination conditions at plant scale with a passive spectroradiometer. Fluorescence (F) was 14 estimated by reading directly from radiance spectra of a variety of plants illuminated with 15 light-emitting diode (LED) grow lights in the laboratory. Solar-induced fluorescence (SIF) was 16 estimated from spectral measurements of the same plants under sunlight using the Fraunhofer 17 Line Depth (FLD) method. Chlorophyll fluorescence yield (Fyield) and solar-induced fluorescence 18 yield (SIFyield) were calculated by normalizing F and SIF with absorbed photosynthetically active 19 radiation (APAR). Two approaches to estimating APAR were compared: utilizing white reference 20 spectra and reflected spectra versus white reference spectra combined with the fraction of absorbed 21 photosynthetically active radiation (fPAR) derived from literature. Average F and SIF were 22 different by a factor of approximately twenty-four (F = 0.110 ± 0.038 Wm−2μm−1sr−1 versus SIF = 2.60 23 ± 1.87 Wm−2μm−1sr−1). In contrast, the average normalized values Fyield and SIFyield were within the 24 margin of error of one another (Fyield = 0.022 ± 0.008 μm−1sr−1 and SIFyield = 0.030 ± 0.020 μm−1sr−1). This 25 study highlights the influence of APAR on CF and the importance of properly accounting for it 26 when estimating yield and demonstrates the ability of two simple and portable experimental 27 setups with a passive instrument to obtain fluorescence metrics. 28