= 722 =
tance attachment, was used to measure total diffuse reflectance on
upper (adaxial) surfaces of single leaves over the 6,5- to 2.5-un
waveband. Data were corrected for decay of the barium sulfate stan-
dard (Allen and Richardson, 1971) to give absolute radiometric
data. Laboratory measurements of leaf reflectance and fixations
of tissue were completed within about 6 h after leaves were collec-
ted. Leaf thickness was measured with a linear-displacement trans-
ducer and digital voltmeter (Heilman et al., 1968). Leaf area was
determined with a planimeter. Water content of leaves was deter-
mined on a dry-weight basis; leaves were oven-dried at 68C for 72 h
and cooled in a desiccator before weighing. Total chlorophyll was
determined by a routine method (Horwitz, 1965) on leaf samples
stored at -15C€* 5 C.
Tissue pieces sampled from the center of leaves were fixed in for-
malin-acetic acid-alcohol, dehydrated with a tertiary butanol
series, embedded in paraffin, stained with the safranin fast-green
combination, and transversally microtomed at 12-um thickness
(Jensen, 1962). Photomicrographs were obtained with a Zeiss Stan-
dard Universal Photomicroscope.
To reduce the enormous amount of spectrophotometrically generated
data and facilitate interpretation, seven wavelengths, were selected
£rom the 41 wavelengths measured at o,o5-um increments over the
o.50-to 2.50 - um waveband. Wavelengths selected were 0,55, o.65,
0,85,1.45,1.65, 1.95, and 2.20 um; representing, respectively, the
green reflectance peak, chlorophyll absorption band, a wavelength
on tke near-infrared plateau, the 1.45-um water-absorption band,
the 1.65-um water-absorption band, the 1.95-um water-absorption
band, and the 2.2-um peak.
An Exotech Model 20 spectroradiometer (Leamer et al., 1973) was
used to measure reflected radiation from nonstressed and stressed
single plant canopies over the o.5-to 2.5-um waveband. Measurements
were usually made with sensors with a 15-degree field of view
(0.2 m^) placed 1.5 m above each plant canopy.
Standard statistical techniques were used (Steel and Torrie, 1960)
and Duncan's multiple range test (Duncan, 1955) was usually used
to test differences among means. Randomized complete block experi-
ments were generally used. At least 1o leaves were randomly collec-
ted from each treatment for laboratory relfectance measurements on
singe leaves; field measurements were replicated five times.
Results and Discussion Stressed vs nonstressed
Nematodes on cotton (Gausman et al., 1975 a)
We conducted this research to determine if there were differences
between the light reflectance for leaves of cotton (Gossypium hirsu-
tum L.) plants grown with a low- or no-nematode population (non-
stressed) and plant leaves grown with a high nematode population
(stressed).
Stressed cotton plants were stunted with fewer, smaller, and darker-
green leaves than nonstressed plants. Differences between non-
stressed and stressed plants in leaf water content and leaf area
were highly significant (P = 0.01). Nonstressed leaves were larger
in area (25.5 cm“) than stressed leaves (17.3 cm ) with lower wa-
ter contents (77.5%) than stressed leaves (78.3%). Leaf thicknesses
of nonstressed (.16 mm) and stressed (.15 mm) leaves were statis-
tically alike.