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 (<NT) did not 
receive the growth regulator and were maintained as 
controls. We designated the four combinations of 
treatments as <NT, NT, MC, and MC2. 
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 
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