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Coupled spectrometers using optical shutters offer insight into plant stress, SIF, reflectance and quality of measurements under changing sky conditions
  • Arthur Zygielbaum,
  • Timothy Arkebauer,
  • Elizabeth Walter-Shea
Arthur Zygielbaum
University of Nebraska Lincoln

Corresponding Author:aiz@unl.edu

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Timothy Arkebauer
Univ Nebraska Lincoln
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Elizabeth Walter-Shea
University of Nebraska Lincoln
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Light absorption in the photosynthetically active (400 – 700 nm) spectral region is necessary for plant CO2 fixation via photosynthesis. Light absorption in excess of that which can be used for photosynthesis may result in photoinhibition and/or other processes detrimental to normal plant function. Plants have evolved several photoprotective mechanisms to reduce light absorption under stressful conditions. For example, leaf-level reflectance and transmittance increased as a result of chloroplast movement within leaf cells in response to water stress in greenhouse-grown maize and soybean. This has implications for detecting (as a signal and noise) diurnal and stress-related changes in canopy reflectance in field-grown crops. These changes were recently investigated in the field using newly developed instrumentation systems and software. Two hyperspectral spectrometers, an Ocean Optics QE Pro (0.3 nm resolution in the 650 - 813 nm range) and a Flame (2.0 nm resolution in the 340 - 1028 nm range) are coupled through optical shutters to a downward looking fiber (25° field of view) and an upward looking fiber with cosine corrector. The spectrometers can be configured to see sky or surface targets concurrently or separately. This new configuration offers concurrent measures of derived solar induced fluorescence (SIF), and visible and near infrared reflectance on a mobile platform, acquiring spatially averaged responses. Our goal is to use SIF as an indicator of the level of photosynthetic activity in comparison to reflectance-derived indication of photoprotective response. In conducting data acquisition, several technical issues arose. Different spectrometer integration times, due to differing radiometric sensitivities and changing sky conditions, causes differences in measured reflectance between the two spectrometers. Also the approach highlighted the difficulty of obtaining reliable system calibration under varying sky conditions when using near-Lambertian reference panels. While results are promising in detecting SIF along with more conventional remote sensing spectral resolution, further research is needed to refine data acquisition to ensure quality reflectance measurements. We report on technical issues and on our success in tying photoprotection to changes in photosythentic activity.