Coupled spectrometers using optical shutters offer insight into plant
stress, SIF, reflectance and quality of measurements under changing sky
conditions
Abstract
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.