FLUORESCENCE. 10 photons/s 
similar results fear the fluorescence spectra of various crop residue weathered in the field. 
Because the emission spectra indicate a broad-band phenomenon, the spectra were averaged over the 420 
to 550 nm interval. In Figure 3, the emission in the 420-550 nm band is plotted versus fraction of the original dry 
weight remaining at the various times of incubation. The relationship, in Figure 3, is curvilinear with a 50% decrease 
in fluorescence occurring during the time the plant residue lost 25% of its original weight Previously we showed 
that as crop residues weathered and decomposed, their fluorescence decreased and approached the fluorescence of 
soils (Daughtry et al., 1993; McMurtrey et al., 1993). Nevertheless, we believe that fluorescence techniques, such 
as described here, can provide the Soil Conservation Service with a rapid and objective method of assessing crop 
residue cover in farmers' fields and assist them in minimizing the loss of arable soils due to erosion. 
In conclusion, while components responsible for the blue-green fluorescence of plants have not yet been 
rigorously identified, two applications of this phenomenon are evident.. The first is the possible use of the blue 
fluorescence to estimate photosynthetic rates (Chappelle, et al., 1991) and the second is its use to assess crop residue 
cover (McMurtrey et al., 1993). We believe that, with the use of techniques such as time resolved fluorescence 
measurements, asynchronous fluorometry, and improved deconvolution algorithms, it will be possible to define the 
nature of many of the compounds involved in the blue-green fluorescence of vegetation. Undoubtedly, several of 
these compounds will be involved in cell structure and will be relatively inert It can be speculated that these are 
the compounds primarily seen in crop residues. It is felt, however, that many of the highly conjugated compounds, 
both heterocyclic and straight-chained, involved in the dynamics of the biochemistry and photosynthesis of the plant 
provide a significant percentage of the blue fluorescence. It may well be that defining of the constituents of the blue 
fluorescence band will allow the improved remote assessment of photosynthetic and biochemical changes occurring 
in plants , as well as providing a unique means of differentiating between live, dormant, and dead vegetation. 
Figure 1. Fraction of original dry weight of 
wheat residue remaining after various times 
of incubation. 
WAVELENGTH, NM 
Figure 2. Changes in the fluorescence spectra of wheat 
residue after various times of incubation. The sample 
were excited by 340 nm radiation.