351 
spectral 
[1 ] 
VI and 
Goeff. of 
Deter., r^ 
.930 
.887 
side] 
20 
30 
10 
.867 
.857 
.969 
.942 
.581 
.469 
.835 
.825 
.878 
.751 
.986 
.969 
rent in the 
riable (y) 
esented for 
4 
2 
10 20 30 40 
PVIZ1 
Figure 1. Hie first term 
of equation [ 1' ], FVI 
versus LAI, for each of 
the crops cotton, wheat, 
and corn (maize) and the 
exponential and power 
expression (Table 2) fits 
for each. 
0 2 4 6 
LA1/C0SZ2 
Figure 2. Hie second term 
of equation [1'], LAI 
versus APAR, for three 
crops and the exponential 
and power expression 
(Table 2) fits for each. 
PVI*C0SZ 1 /C0SZ2 
Figure 3. Right hand side 
of equation [1'], PVI 
versus APAR, for three 
crops and the linear and 
power expression (Table 2) 
fits for each. 
each term in equations [1'] for snail plot 
experiments using cotton, wheat, and maize. PAR 
light absorption could be estimated almost as well 
for cotton and corn from the perpendicular 
vegetation index, PVI, (right side equation [1 ]) 
as from the leaf area index (2nd term on the left). 
(Results for all relationships were somewhat poorer 
for wheat (Table 2)). Hiis finding implies that the 
vegetation indices are a good measure of the 
photosynthetically active tissue in the canopy. 
Hi us APAR can be estimated directly from VI. 
Alternatively, LAI expressed in terms of VI can be 
inserted into APAR versus LAI expressions in the 
literature to estimate APAR. Hie ability to 
estimate APAR remotely is important because of the 
close association between cumulative APAR and above 
ground dry matter of crops. 
VI and APAR became asymptotic to the LAI axis at 
large LAI (Figures 1 and 2), respectively, whereas 
APAR was nearly a linear function of VI for each 
crop (Figure 3). In general, these interrelation 
ships demonstrate the unity among LAI, light 
absorption, and yield that is consistent wnth 
growing condition and stress influences on canopy 
size and field observed yields. Therefore, direct 
spectral observations expressed as vegetation 
indices can provide valuable current information on 
crop prospects. In addition the "pure spectral" 
observations of SCA can provide useful independent 
estimates of LAI and APAR for use in conjunction 
with the "pure nonspectral" plant process models of 
crop development and yield. 
REFERENCES 
Anderson, M.C. 1971. Radiation and crop structure, 
pp. 412-466, in Plant Photosynthetic Production 
(Z. Sestak, J. Catsky, and P.J. Jarvis, eds.) 
Junk. The Hague. 
Asrar, G., M. Fuchs, E.T. Kanamasu & J.L. Hatfield. 
1984. Estimating absorbed photosynthetic radiation 
and leaf area index from spectral reflectance in 
wheat. Agron. J. 76:300-306. 
Black, A.L. & J.K. Aase 1982. Yield component 
comparisons between US and USSR winter wheat 
cultivars. Agron. J. 74:436-441. 
Colwell, J.E. 1974. Grass canopy bidirectional 
spectral reflectance, Proc. 9th Int'l. Syrnpos. 
Remote Sens. Environ. Vol. II, pp. 1061-1086. 
Univ. Michigan, Ann Arbor, USA. 
Charles-Edwards, D.A. 1982. Physiological 
determinants of crop growth. 161p. Academic 
Press, New York, NY.