There was a statistically significant (P - 0.01) difference be-
tween mean total chlorophyll concentrations of :chlorotic and
apparently normal green sorghum plants. Chlorophyll concentrations
in leaves were 9.4+1,5 (standard deviation) and 0.4+0.15 mg/g of
plant tissue on a dry weight basis for normal and chlorotic plants,
respectively.
Chlorophyll concentrations of chlorotic and normal plants signifi-
cantly (P - 0.01) affected reflectance measurements made in the
field with a spectroradiometer (Fig. 3). Reflectance was 9.4,
27.7, and 26.3 $ greater for chlorotic than for normal plant
canopies at the 0.45-um (chlorophyll absorption band), 0.55-pm
(green reflectance peak), and 0.65-um (chlorophyll absorption
band) wavelenghts, respectively. These reflectances differences
were caused by the unequal chlorophyll concentrations because
chlorotic and normal plants had the same soil background, and
their size and geometry were essentially the same.
Band 5 (0.6 to 0.7 um) data were selected to represent the chloro-
phyll absorption band at the 0.65-um wavelenght. Chlorotic sorghum
areas 1.1 ha (2.8 acres) or larger were identified on a computer
printout of band 5 data. This resolution was sufficient for prac-
tical applications in detecting chlorotic areas in otherwise homo-
geneous grain sorghum fields.
Lead toxicity (Escobar, D.E. and H.W. Gausman, 1976)
We conducted this study to ascertain if leaf reflectances of Mexi-
can squash plants (Cucurbita pepo L., cv Tatume) grown with varied
lead (Pb) concentrations differed enough for their possible use
in spectrally detecting Pb-contaminated vegetation.
The 500- and 1,500-ppm Pb treatments stunted the squash plants
and caused them to be lighter green (less chlorophyll as will be
shown later) than were the control plants. The 1,500-ppm Pb-treated
plants did not have enough leaves for statistical comparisons
with the other treatments. The average area per leaf for the
500-ppm Pb treatment was significantly smaller (10.2 cm2) than
that of the control and the 100-ppm Pb treatments (28.8 and 24.4
cm“, respectively), which were not statistically different. Leaf
thicknesses of 0.111, 0.110, and 0.112 mm, and water contents of
91.2, 92.0, and 91.0% for the control, 100-, and 500-ppm Pb treat-
ments, respectively, did not differ statistically.
Plant Pb concentrations were 30, 87, 84, and 77 ppm for the control
and 100-, 500-, and 1,500 ppm Pb treatments, respectively. The
lower values for the 500- and 1,500-ppm Pb treatments, as com-
pared with the 100 ppm Pb treatment, may have been caused by
effects on Pb absorption because we observed root growth reduction.
This agreed with plant growth reduction in sand culture (Miller
and Koeppe, 1970) and seed germination inhibition (Dilling, 1926)
with high Pb concentrations. The high Pb concentration of the
control plants (30 ppm) may have been caused by atmospheric Pb
pollution (Ganje and Page, 1972), because we conducted the experi-
ment only 100 m from a heavily traveled highway.
Reflectance spectra for the control and Pb-treatment squash leaves