experiments measuring the reflectance of particular plant species stressed by 
various heavy metals ; 2) field measurements of reflectance and reflected ra- 
diance of vegetation growing in situ in various mineralized zones ; 3) Airborne 
multispectral scanner measurements of various vegetation types growing in situ 
on various types of ore deposits ; 4) and Landsat satellite studies. 
A typical reflected radiance curve of vegetation is shown in Figure 1. The 
relatively low reflected radiance in the visible part of the spectrum 
(430-700 nm) is due to strong absorption by chlorophyll centered in two bands 
at 500 and 690 nm. For a detailed discussion of the visible and near infrared 
reflectance properties of vegetation, the reader is referred to Knipling (1970). 
The narrow absorption band centered at 760 nm is due to absorption by oxygen 
in the atmosphere ; and the absorption bands centered at 720, 820 and 940 nm 
are due to absorption by water vapor in the atmosphere. While Figure 1 ends at 
1000 nm, the near infrared reflected radiance stays high out to about 1300 nm 
but then starts to fall off due to the presence of water in vegetation. 
LAB AND GREENHOUSE STUDIES 
  
Early greenhouse experiments were reported by Press and Norman (1972) who grew 
bean plants in a nutrient medium spiked with either Pb or Zn. They noted a 
distinct increase in the reflectance of stressed plants in the 550 nm region 
of the spectrum but little or no change in the near infrared region (700-990nm) 
The changes were more pronounced with Pb than Zn.and were interpreted as a 
chlorophyll deficiency in the leaves which reduces the absorption of visible 
radiation. 
Horler et al. (1980 a and 1980 b) undertook greenhouse experiments with pea, 
sunflower, and soybean plants whose nutrient solutions were spiked with Cd, Cu, 
Pb or Zn. For the pea plants, they noted a progressive increase in leaf reflec- 
tance in the visible wavelengths (475, 550 and 660 nm) and a progressive 
decrease at near infrared wavelengths (850, 1600 and 2200 nm). These. wavelenghts 
conform to the wavebands in Landsat-D's thematic mapper. Horler et al. note 
that their results in the visible part of the spectrum are consistent with a 
decrease in chlorophyll concentration in the stressed leaves, whereas the near 
IR results support a change in leaf structure. Zn treated soybeans behaye 
similarly to the peas in the infrared, but the soybeans showed a decreased 
reflectance- in the visible (660 nm), while Cu treatment of sunflower showed 
little effect. 
  
Chang and Collins (1980) have studied the toxic effects of a variety of heayy 
metals on greenhouse grown varieties of sorghum and mustard. They noted a 
strong correlation of increased reflected radiance in the visible (550-760 nm) 
and decreased total chlorophyll in the leaves of plants grown in the presence 
of Cu, Zn and Ni. First and second derivative spectra of Chang and Collins' 
data lead them to propose that the long wavelength end of both the 500 and 
690 nm chlorophyll absorption bands shifts about 10 to 40 nm toward the.shorter 
wavelengths when the plants suffer heavy metal stress. Howeyer, Horler et àl. 
(1980 a and 1980 b) report no shift in absorption spectra. Chang and Collins 
(1980) also note a small decrease in the near infrared reflectance spectra and 
report that subtle spectral differences in the 400-500 nm spectral region may 
be related to an increase in chlorophyll a/b ratio. Horler et al. (1980 b) 
report decreased chlorophyll a/b ratios in Cd and Cu treatments of pea plants 
and no change with Pb and Zn, but they conclude that such changes are less 
reliable indications of heavy metal stress than a decrease in chlorophyll 
concentration. However, they maintain that their results are not universal and 
may be reversed. 
  
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