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leaves 
over the 0.5- to 2.5-um waveband are plotted in Fig. 4. Refelec- 
tance for the 1,500-ppm Pb treatment was. affected by leaf size. 
The leaves were very small (average leaf area was 2.4 cm2) with 
closely netted veination and interveinal "pimpling". Some leaves 
were so small that the spectrophotometer's light beam impinged 
on veins, rather than on an interveinal area, which resulted in 
an increased near-infrared reflectance (0.75 to 1.35 um) as com- 
pared with interveinal reflectance. Therefore, we will not dis- 
cuss leaf spectral characteristics for the 1,500 ppm Pb treat- 
ment. 
At the 0.55- and 0.65-pm visible light wavelengths, Pb-treated 
leaves had higher reflectance than did control leaves because 
they had less chlorophyll /7.8 and 8.3 mg/g for 100- and 500-ppm 
Pb treatments, respectively) than the control leaves (10.5) mg/g). 
Mean reflectances were significantly different at the 0.55-um 
wavelength: 12.8, 17.3, and 15,2% for the control and 100-, and 
500-ppm Pb treatments, respectively. Reflectance of the control 
leaves (6.2%) at the 0.65-um wavelength was significantly lower 
than that of the 100-ppm (7.7%) and 500-ppm /7.2$) Pb-treatments, 
which were alike. Leaf treatments did not significantly affect 
near-infrared reflectance over the 0.75- to 1.35-pm waveband 
(Fig. 4). 
In the near-infrared water absorption waveband (1.35 to 2.5 um), 
reflectances of the control leaves at the 1.45-, 1.65-, and 2.2-um 
wavelengths were significantly higher than that of the 100- and 
500-ppm Pb-treated leaves, which were alike. The lower reflectance 
of Pb-treated leaves at these wavelengths may not have been caused 
by water absorption of radiation, because among treatments the 
leaf water contents were essentially alike. In addition, the 
mean reflectances for the 1.95-um water-absorption band for all 
treatments were statistically alike. 
Unfortunately, the effects of Pb on squash-leaf reflectance in the 
visible region were essentially the same as those caused by nutrient 
deficiencies (Gausman et al., 1973e). Press (1974) also recognized 
that heavy metal-stressed plants would be difficult to distinguish 
from plants with other stresses. He concluded that a variety of 
integrating imaging sensors might be helpful to detect metal-stressed 
plants by simultaneously monitoring variable factors, like soil 
type, climate, and the ratio of total leaf area to exposed back- 
ground. 
Detecting Pb-stressed crops may be feasible by measuring plant 
canopy reflectance if further research is conducted to distinguish 
the reflectance spectrum of Pb-stressed plants from that caused 
by other plant stresses, particularly nutrient deficiencies. Possib- 
ly, this could be achieved by using multispectral scanners with 
narrower wavebands, ratioing of wavebands, conducting multispec- 
tral photography with appropriate filter combinations, or by 
using a variety of integrating imaging sensors. In addition, 
further research should be conducted with other heavy metals 
(other than Pb) to determine if their possible stress to plants 
causes any other leaf reflectance patterns.