Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 31, 2003
Publication Date: July 1, 2004
Citation: Hunt, E.R., McMurtrey, J.E., Parker-Williams, A.E., Corp, L.A. 2004. Spectral characteristics of leafy spurge leaves and flower bracts. Weed Science. 52(4):492-497.
Interpretive Summary: Leafy spurge is a noxious invasive weed that causes substantial economic loses in the upper Great Plains of the United States. The flowering shoot is readily distinguishable from co-occurring vegetation due to the yellow-green color of the flower bracts. The overall goal of this research is to use remote sensing to locate populations of leafy spurge for control by integrated pest management and monitor populations to determine the effectiveness of the control measures. Plants reflect, transmit or absorb light in different proportions at different wavelengths depending on the biochemical composition. The class of yellow pigments called carotenoids absorb blue light, thereby allowing other wavelengths to be either transmitted or reflected. Chlorophylls are green pigments that absorb blue and red light. Higher concentrations of pigments absorb more light, causing reflectance to be lower. These pigments also emit light of certain wavelengths by fluorescence, after absorbing light at shorter wavelengths. The reflectance and fluorescence spectra were measured for leaves and flower bracts of leafy spurge, and compared to the pigment concentrations. The evidence indicates the yellow-green signature is due to a one-to-one ratio of chlorophyll and carotenoids. The types of carotenoids are those found normally in green plants; there was no indication of a new type of yellow pigment. By understanding the physiological basis of the yellow-green signature of leafy spurge flower bracts, we hope to extrapolate remote sensing procedures developed for leafy spurge to other noxious weeds.
Leafy spurge (Euphorbia esula L.) is readily detectable from remote sensing by the distinctive yellow-green color of the flower bracts. The spectral characteristics of bracts and leaves were analyzed to determine the physiological basis of the remote sensing signature. Compared to leaves of leafy spurge, flower bracts had lower reflectance at blue wavelengths, more reflectance at green and yellow wavelengths, and approximately equal reflectances at 680 nm (red) and at near-infrared wavelengths. Flower bracts had less chlorophyll a, chlorophyll b, and total carotenoids than leaves, thus absorption should be less and the reflection should be greater at blue and red wavelengths. The bract carotenoid to chlorophyll ratio was approximately one to one, whereas this ratio was approximately one to four for leaves. The primary carotenoids for both leaves and flower bracts were lutein, ß-Carotene, and ß-Cryptoxanthin, with some phytofluene present in the flower bracts only. The fluorescent emission spectrum indicated high blue, red and far-red emission for leaves compared to flower bracts. Fluorescent emissions from leaves may contribute to the higher leaf reflectance in the blue and red wavelength regions. There was no spectral reflectance or fluorescence evidence found to support the presence of a unique pigment in the flower bracts of leafy spurge, which contributes to its distinctive color.