ARS | Publication request: Diurnal and directional responses of chlorophyll fluorescence and pri in a cornfield

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: January 15, 2011
Publication Date: June 1, 2011
Citation: Middleton, E.M., Cheng, Y., Corp, L.A., Huemmrich, K.F., Zhang, Q., Campbell, P.K., Kustas, W.P. 2011. Diurnal and directional responses of chlorophyll fluorescence and pri in a cornfield. In: Proceedings of the 4th International Workshop on Remote Sensing of Vegetation Fluorescence, November 15-17, 2010, Valencia, Spain. 2011 CDROM.

Technical Abstract: Two high spectral resolution reflectance-based indices were used to assess whether vegetation in a cornfield was performing near-optimally or exhibiting symptoms of environmental stress. These were the Photochemical Reflectance Index (PRI) and solar-induced Chlorophyll Fluorescence (SIF). This study continues investigations of the spectral responses of sunlit and shaded foliage associated with the Bidirectional Reflectance Function (BRF). Experiments were conducted in 2007/2008 and 2010 along transects in a USDA-ARS experimental field site located in Beltsville, MD, USA. In 2008 and 2010, measurements at subplots were additionally acquired from nadir and fixed off-nadir view angles (30, 45, 60 degrees) at 45 degrees azimuth increments in polar sweeps, which were compared to simulations obtained from a radiative transfer model, the Markov-Chain Canopy Reflectance Model (MCRM). In 2007 and 2008, the measurements were made above stands with an ASD FieldSpec Pro (Analytical Spectral Devices, Inc., Boulder, CO, USA) to obtain the PRI. In 2010, measurements were acquired with a USB4000 Miniature Fiber Optic Spectrometer (Ocean Optics Inc., Dunedin, Florida, USA) to obtain both the PRI and SIF responses as influenced by view zenith and azimuth geometries at different times of day at three growth stages (vegetative, reproductive, early senescence). In both years, we found that the PRI consistently had higher values, indicating lower stress, in the BRF darkspot associated with shaded foliage than in the hotspot associated with sunlit foliage. We also found that SIF exhibited differences associated with sunlit and shaded canopy sectors, which were most pronounced for the red/far-red SIF ratio. Values indicated greater physiological stress in afternoons compared to mornings, and in the early senescent canopy as compared to the vegetative growth stage. BRFs for both the PRI and the red/far-red SIF ratio were bowl-shaped for the full azimuth sweep of the canopy.