Wednesday, November 15, 2006
286-2
Resolving Crop Nitrogen Requirements with a Depolarized Reflectance Sensor.
Louise Egerton-Warburton1, Paul C. Nordine2, Steven Finkelman2, Margaret Ingalls1, and Jonathan Podbielski1. (1) Chicago Botanic Garden, 1000 Lake Cook Rd, Glencoe, IL 60022, United States of America, (2) Containerless Research Inc., 906 University Place, Evanston, IL 60201
The over-fertilization of agricultural fields is a causal factor in nitrogen (N) pollution of surface and ground waters. The ability to rapidly identify crop N requirements is therefore key to N fertilizer management. Here, we report on the results of a portable broadband depolarized reflectance spectrometer that differentiates among N-fertilization levels in corn. Depolarized reflectance eliminates surface reflection and gives a sensitive and reproducible measurement of reflectance that occurs by the scattering and absorption processes in the leaves. We made measurements on the leaves of corn plants growing in plots receiving 0 to 240 lb N fertilizer per acre (0 to 216 kg N ha-1). The results demonstrated a strong influence of N fertilizer on reflectance values at wavelengths below 650 nm, i.e., the region incorporating chlorophyll b and associated Light Harvesting Complex (LHC) proteins, and a small influence on values in the 680 nm range (chlorophyll a and associated LHC). These same trends-- large differences below 650 nm and minor differences above-- were consistently detected in the depolarized reflectance spectra of different plant species. Such results infer that the chlorophyll b-LHC assemblage is a far more variable component of vegetation than chlorophyll a-LHC assemblage, since the variations occur primarily in the region where chlorophyll b absorbs light. However, alternative explanations have not been exhausted. For example, the light intensity delivered by the flash lamp driven photopolarimeter may be sufficient to produce excited state absorption that would contribute to the observed spectral variations. Experiments are in progress to resolve this question.