348
prevented by predators.
e. Unless stresses are extreme, cumulative
seasonal light interception relates closely to the
phytomass achieved (Monteith, 1981; Steven et al.,
1983; Daughtry, et al., 1983; Gallo, et al., 1985;
Wanjura and Hatfield, 1985). Cumulative seasonal
light interception is, in turn, a function of the
seasonal amount and duration of photosynthetically
active tissue.
f. Seasonal integral VI are. essentially estimates
of integral intercepted solar radiation (Wiegand and
Richardson, 1984) which Daughtry et al. (1983) and
Hatfield, et al. (1984) have shown can be estimated
spectrally. Consequently, the VI relate to both
APAR and YIELD as shown by the right hand sides of
equations [1] and [2]. Likewise, cumulative APAR
for the season, or for the reproductive portion of
it, and YIELD are related since they each relate to
the common variable, VI.
3 MODIFICATIONS IN EDS. [1] and [2]
Hie spectral reflectance observations, used to
calculate VI, and the PAR transmission and
reflectance observations, used to calculate APAR,
are, themselves, seasonally and diurnally Sun angle
dependent (Anderson, 1971; Colwell, 1974). The path
length through the canopy is inversely proportional
to the cosine of the solar zenith angle (cos Z)
giving light entering the canopy from off nadir
angles a better chance of being intercepted. When
the same workforce makes both sets of observations,
first one set is made on a given day and then the
other so that Sun position differs in the data
paired for analysis. Differences in solar zenith
angle during a growing season are even greater.
Consequently, we currently express equations [1] and
[2] as
/LAI n
IcosZ-^
VI
1 X /cosZ^\ APAR72 = APARp^cosZ? [ 1* ]
Z1
LAI ) T
VI y *j cosZ
LAI
cosZi
3 ZV
VI
Z1
^ LAI
YIELD = YTET.D
1 vi z1
[2 ]
wherein Z-j is the solar zenith angle at the time
the reflectance factor observations were made, and
Z? is the solar zenith angle at the time of the
light interception measurements.
4 IMPLEMENTING SPECTRAL COMPONENTS ANALYSIS
To determine the functional relation between
numerator and denominator variables for any term in
equation [1 ], the seasonal observations for the
involved variables are paired within treatment
groups that respond similarly in the experiment and
the data are fit by least squares.
For equation [2' ], the LAI versus VI data are the
same as in equation [1' ]. However, the YIELD/LAI
and YIELD/VI terms are best applied to canopies
during a time interval after LAI has reached its
seasonal plateau when the sinks for the assimilates
of photosynthesis are dominated by the plant parts
that constitute YIELD. For example, Pinter (1981)
used the integral area under the ND versus time
curve from 50 percent heading to full senescence for
wheat and barley subjected to various irrigation
treatments and found a coefficient of determination
of 0.88 with grain yield.
5 METHODS
Replicated small plot studies were conducted near
Weslaco, TX, with cotton cv. 'McNair 220' in 1983,
with hard red spring wheat cv. 'Aim' , 'Nadadores',
and 'Yavaros' in the fall 1983 to spring 1984, and
with a white field corn cv. 'Asgro 405' in the
summer of 1985. Cotton and wheat were planted at
optimum dates and recommended configurations and
populations, but corn was planted three months late
(23 May) to insure water stress in the nonirrigated
treatment during the midsimmer period (15 June to 15
August) of normally low rainfall. Also, the corn
was not fertilized although N fertilization at 200
kg/ha is recommended for cammerical production.
The cotton and wheat experiments were conducted at
the South Research Ftarm (lat. 26.16°N and long.
97.96°W) on Raymondville clay loam, a Vertic
Calciustolls and the corn was grown on the North
Research Farm (lat. 26.22°N, long. 97.99°W) on a
Hidalgo sandy clay loam, a Typic Calciustolls. Both
soils are inherently fertile.
Three cotton treatments were 99,000 plants/ha in
north-south rows 1.02 m apart while a fourth was
thinned to 52,000 plants/ha. Che densely planted
treatment (MC) received a single application of a
growth regulator, mepiquat choloride, at the rate of
74 g/ha active ingredient (a.i.) at pinhead-sized
squares (21 April). Another of the 99,000 plant/ha
treatments received split applications of mepiquat
chloride (MC2) at rates of 49 g/ha a.i. and 25 g/ha
a.i. at pinhead sized square (21 April) and at first
bloom (19 May), respectively. The remaining densely
planted treatment (non-thinned, NT) and the
treatment thinned to 52,000 plants/ha (
receive the growth regulator and were maintained as
controls. We designated the four combinations of
treatments as
The spring wheats were planted 17 Nov. 1983 at the
rate of 80 kg/ha with a commerical drill in rows
spaced 0.2 m apart. The populations achieved two
weeks after emergence were 302, 313, and 197
plants/m^ for Aim (CV1), Nadadores (CV2), and
Yavaros (CV3), respectively.
Corn was planted in east-west rows 0.66 m apart.
Populations achieved averaged 8.2 plants/nr/.
Three irrigated (I) and three dryland (D) plots each
30 m x 30 m in size were established. Twelve
irrigations were sufficient to insure that the
irrigated treatment did not experience significant
water stress any time during the season. Rainfall
in six events that totaled approximately 100 mm all
occurred prior to anthesis (16 July).
yhe measurements needed to determine each term in
[1 ] and [2 ] were made for each of the three
crops. For LAI determinations 1-m row segments of
cotton were sampled whereas 0.24 m2 (0.4 x 0.6 m)
areas were sampled for wheat, and representative
plants from the wet and dry treatments of corn were
harvested. LAI was determined at one site per
replicate in each crop on the dates listed in Table
1.
The vegetation indices were calculated frcm
reflectance factor (Robinson and Biehl, 1982)
observations made approximately weekly (Table 1)
using a Mark II radiometer (Tucker et al., 1981)
which has a visible red (RED) band in the 630 to 690
nm wavelength interval and a reflective infrared
(RIR) band in the 760 to 900 nm wavelength interval.
The radiometer has a 24 degree field of view and was
held vertically 1 m above the canopies centered over
the crop rows. The spectral band responses plus
incident solar radiation and time of observations
were electronically logged concurrently for each of
four observations per replicate and reduced by the
procedures described by Richardson (1981).
The vegetation indices employed were the
normalized difference (ND) (Tucker, 1979) and
perpendicular vegetation index (FVI) (Richardson
and Wiegand, 1977). The equations for these
respective VI are:
ND = (RIR-RED)/(RIR+RED), and
PVI = 0.580(RIR) - 0.815(RED) - 0.410 based on the
soil line
RED = -1.92 + 0.789(RIR) for the Raymondville clay
loam, and
Photo
on (Io)
from th<
the ban
PAR, te:
(1-T'-R
et al.
Io =
T =
<
R =
*s
For the
measurec
sensors
quantum
was ins(
rows (wi
(corn)
measurii
30 cm a)
below t)
line qu<
area in
soon afi
from pi
leveled
and I,
■^o
electrc
taken b
the sen
The dat,
relatioi
wherein
extinct
the pat!
the APA]
At th<
require«
sensor (
reflect;
impossil
the sam<
the var
experim«
merged,
the PAR
measure
data we
estimat
measure
The L
abovegr
samplirx
determi
Solar
and lorn
of year
ephimer
Equat
the dat
differe
zenith
terms o
Menti
prefere
Departan
availab
