The absorption coefficient (k), infinite reflectance (R), and scattering coefficient (s) were tabulated for five wavelengths and analyzed for statistical differences for seven weed species. The wavelengths were: 0.55-micrometer, 0.65-micrometers, 0.85-micrometer, 1.65-micrometers, and 2.20-micrometer. The R of common lambsquarters (Chenopodium album L.), Johnsongrass (Sorghum halepense (L.) Pers.), and annual sowthistle (Sonchus oleraceus L.) leaves at the 0.85-micrometer wavelength were significantly (p=0.05) higher than for sunflower (Heliantus annus L.), ragweed parthenium (Parthenium hysterophorus L.), or London rocket (Sisymbrium irio L.). Annual sowthistle had the largest k value, and Plamer amaranth (Amaranthus palmer S. Wats.) had the smallest k value at the 0.65 approximately chlorophyll absorption wavelength. In general, john-songress, ragweed parthenium, or London rocket had the largest s values among the five wavelengths, wereas annual sowthistle and plamar amaranth were usually lowest

Gausman, H. W.

Menges, R. M.

Richardson, A. J.

Rodriguez, R. R.

Tamez, S.

Walter, H.

English

NASA Technical Reports Server

Ae ros pace 
Remote Sensing Early Warning and Crop" . 
Conditiori Assessment Ir JUNE 1982 
OPTICAL PAW,ETERS 9F LEAVES OF SEVEN !%ED SPECIES J 
3, H, W 8  GAUSMAd R8 M I  A 8  Jt  RICHARDSON, 
H, WALTER# R ,  R ,  AND SmTAMEZ 
- 
(E83-1C010) O P T I C A L  F A i l J H E T E F Z  CF L E A B E S  CF b d 3 -  12469 2 E V E 6  EEED E F E C I E S  ( C e p a r t ~ e n t  of 
Aqriculture) S p BC A O Z / I I E  A01 C S C L  02c 
U A d  as 
63/43 v3010 
v b. 'r 
i 
-4 U,S8  DEPARTMENT OF AGRICULTURE 
1050 BAY AREA BOULEVARD 
HOUSTON, TEXAS 77058 
Lyn-6. JohmonSu8aecmta 
Housion. Texas 77058 
https://ntrs.nasa.gov/search.jsp?R=19830004219 2020-03-21T05:15:07+00:00Z
1. R.pM Na 2. GaVerr.mt AccuSon . N o .  3. Aeeicipirnt's Catalog No. 
Bw-U2-04328, 55018259 
9. w. (afmbng0rtJ 
Unclassified 
I 
&rnhndsu#itk 
OPTICAL PARMTERS OF LEAVES OF SNEN Wm, SPECIES 
20. SICuritv olrrif. (of M a  prprl 21. NO. of P- t2 ma* 
Uncl assi f i ed 7 
7. knhor(t) 
H. Y. 6ausman, R. H. knges, A. 3. Richardson, H. Walter, 
R. R. Rodrfguez, and S. Tamer 
USM-ARS 
P.O. Box 267 
Weslaco, TX 78596 
9.prtanrinsor9miz8tian9J&nemdAddrrr 
' 2 - e -  
1050 Bay Area Blvd. 
Houston, TX 77058 
I 11. bnmct of cnm No. 
la Absrr 
Absorption eoefficient (k] , i n f i n i t e  reflectance (R-) , and scattering coeff ic ient  (s) 
were tabulated f o r  f ive wavelengths and analyzed for s t a t i s t i c a l  differences f o r  seven 
weed species. The wavelengths were: 0 . 5 5 9 ,  0.65-w, 0.85g11, 1.65-pm, and 2.201~13. 
The H Q D  of cOmzlDn ladsquarters (Chenopodim albun L.:, Johnsongrass (Sorghun halepense 
(L.) Pers.), and annual sowthistle (Sonchus oleraceus L.) leaves a t  the 0.85-pm wavelenqth 
were signif icantly (p= 0.05) higher than f o r  sunflower (Helianthus annus L.), ragweed 
parthenim (Parthenium hysterophorus L.), o r  London rocket (Sisymbrium i r i o  L.). Annual 
sowthistle had the largest k value, and Palmer amaranth (Amaranthus palmer4 S. Wats.) had 
the smallest k value a t  the 0.65ym chlorophyll absorption wavelength. I n  general, john- 
songrass, ragweed parthenium, or  London rocket had the largest s values among the f i v e  
wavelengths, wereas annual sowthistle and Palmer amaranth were usually lawest. 
7. Ic.v worb (suoo1rad bv AuthaW)  
Absorp t i on coef f i c i  en t 
i n i  f i n i  t e  reflectance 
scattering coefficient 
la Distribution satnna 
OPTICAL PABAMETBRS OF LEAVES (IF SEVEN WBBD SPECIES 
E. W. Gausman, It. M. Menges, A. J. Richardson, E. Walter, 
R. R. Rodriguez, and S. Tam- 
_--__- - ---- - - __ . - - - Abstract 
Absorption coefficient (k), infinite reflectance ( P I ,  and scattering 
statistical differences for seven weed species. The wavelengths were: 
0 .55-p ,  0.65-p1, 0 . 8 5 7 ,  1.65-p, and 2.20-pm. The R =  of common 
lambsquarters (Chenopodium album L. 1, Johnsongrass (Sorghum halepense (L. 
Pers.), and annual sowthistle (Sonchus oleraceus L.) leaves at the 0.85-LIUI 
wavelength were significantly (p = 0.05) higher than for sunflower 
(Eelianthus annuus L. 1, ragweed parthenium (Parthenium hysterophorus L. 1, 
or London rocket (Sisymbrium irio L.). Annual sowthistle had the largest 1.. 
value, a d  Palmer aRaEanth (Amaranthus palmeri S. Wats.) had the smallest k 
value -t the 0.65-pn chlorophyll absorption uavelength. In general, john- 
songrass, ragweed parthenium, or London rocket had the largest s values 
among the five wavelengths, whereas annual sowthistle and Paher amaranth 
were ueually lowest. 
. .  
-- coe- sl-uere-tabulated for five wavelengths and analyzed for 
- 
1 
The optical parameters k, ROD, and s have been tabulated for seven wave- 
lengths, and analyzed for statistical differences for 30 plant species (6). 
The wavelengths were: 
tion band), 0.85-vm (infrared reflectance plateau) , 1.45-Pm (water absorg - 
tion band), 1.65-p (reflectance peak following water absorption band at 
1.45- m), 1.95-pm (water absorption band)., and 2.2-Pm (reflectance peak 
following water absorption band at 1.95-b). 
0.55-pm (green peak), 0.65-~m (chlorophyll absorp- 
Our objective was to present significant differences among the three opti- 
cal parameters for seven weed species at the 0.557, 0.65-pn, 0.85-p, 
1.65-p, and 2 . 2 - p  wavelengths. The optical parameters can be used to 
predict the response of a weed leaf to insolation. The rate of photosyn- 
thesis vi11 be affected by changes in the amount of insolation in the PAR 
(0.4 to 0.7-p)  that is absorb&, reflected, or scattered by a single weed 
leaf (3, 5) .  
the amount of insolation absorbed by a weed leaf. 
weed leaf is energy lost to the photosynthetic activity of useful crops, 
and will have a corresponding effect on yield. Thus, data presented in 
this paper should be of interest to investigators developing various crop 
yield models (4). 
Optical parameters could be especially useful for determining 
Insolation absorbed by a 
These data should also be of interest to the crop discrimination problem 
in remote sensing. 
using Suit's (14) plant canopy reflectance models and Smith and Oliver's 
(12) stochastic plant reflection model. 
infer crop and weed reflectance from LANDSAT altitudes using atmospheric 
radiative transfer correction procedures as presented by Richardson et al. 
(11). These cro , reflectance modeling studies would have application to 
remote sensing in weed science for the detection of weeds in various crops 
and for studying the associated crop yield reductions ( 9 ) .  
The data presented could be used for investigation work 
These models could be used to 
MATERIALS AND METBODS 
Ten fully expanded and healthy appearing leaves were collected from each of 
the seven field-grown weed species: ragweed parthenium, lambsquarters, 
sunflower, annual sowthistle, Palmer amaranth, johnsongrass, and London 
rocket. The adaxial leaf surfaces of the weed species were essentially 
glabrous, except that sunflower leaves were sparsely pubescent. Johneon- 
grass leaf venation was parallel, whereas venation laas netted in the other 
species. Leaves were positioned on the spectrophotometer so that veins or 
hairs did not interfere with the impinging light beam. 
excision, leaves were wrapped in Saran or Glad-Wrap to minimize moisture 
loss. Leaves were wiped with a slightly dampened cloth preceding spectro- 
photometric measurements to remove surface contaminants. 
Immediately after 
2 
The E m ,  k, a d  8 coefficients were calculated by the equations of Allen 
and Richardson (1): 
Bm = l/a, 
k = ((a - li/(a + 1))log b, 
s = ((2a/(q - $1) log b, 
a = (1 + r - t + )/2r, and 
b = (1 - r2 + t2 + )/2t, where 
a = optical constant, b 
transmittance. The quantity is defined by the relation 
optical constant, r = reflectance, and t = 
= (1 + r + t)(l + r - t)(l - r + t)(l - r - t) (VI) 
The quantities a and b (equations IV and V) are constants at a given 
wa5slength. 
are also functions of wavelength. 
as being absorbed and scattered in direct proportion to a differential 
distance, dn, traversed through the leaf and in direct proportion to the 
amplitude of the light at that point in the leaf. The quantity n is the 
cumulative leaf area index. Absorbed radiation disappears from the model. 
For the case of a single leaf (n = 1) Allen and Richardson's (1) equations 
reduce to the form shown above. 
are stacked, and the formulation becomes more complex (1). Scattered 
radiation is merely changed in direction. Because the model is one- 
dimensional, the scattering must be either forward or backward. The 
forward-scattered component is indistinguishable from the incident light, 
but the backward-scattered component adjoins the light moving in the 
opposite direction. The absorption coefficient k (equation 11) and the 
scattering coefficient s (equation 111) are coefficients that result from 
modeling light interaction with leaves (1). 
correspond to fractions of light that are scattered and absorbed respect- 
ively per unit of leaf area index. 
Because r and t vary with wavelength, the quantities a and b 
Light passing through a leaf is modeled 
For the case of n >1, two or more leaves 
The coefficients s and k 
Leaf thickness was measured with a linear-displacement transducer and 
digital voltmeter (10). 
weight basis; leaves were oven-dried at 68 C for 48 h, and cooled in a 
desiccator before weighing. 
Water content of leaves was determined on a dry- 
Variance analysis and Duncan's multiple range test (13) were used on the 
spectrophotometric data for the selected wavelengths at 0 .55 ,  0.65, and 
0 . 8 5 ,  1 . 6 5 ,  and 2.2-m. 
3 
Leaf characteristrics. 
leaf thickness (0.122 to 0.235 yd, leaf water corrent (74.2 to 83.8%), and 
area per leaf (13.4 to 105.0 cm 
1). 
optical parameters represented the optical differences among leaves of the 
weed species. 
These data are included to show the wide range of 
represented by the weed species (Table 
These data are used for descriptive and correlative purposes. The 
Infinite reflectance R m .  
species differences were obtained at the 0.85q.m wavelength on the near- 
infrared reflectance plateau. The reflectances of common lambsquarters, 
johnsongrass, annual sowthistle, and Palmer amaranth were significantly 
larger (p = 0.05) than sunflower, ragweed parthenium, or London rocket 
(Table 2). These results were not consistent with leaf thickness and water 
content measurements (Table 1). High Ftm was associated with more finely 
divided mesophyll structure, which was conducive to short path lengths of 
light and subsequently less light absorptance (2).  
The highest reflectaaces and the largest inter- 
Absorption coefficient It. 
.-* wavelength, which represent the chlorophyll absorption band in the red 
l?ght region (Table 3 ) .  Annual sowthistle Lad the largest k value, and 
Palmer amaranth had the smallest k value at the 0.65-vm wavelength. 
annual sowthistle would probably cause the largest loss of insolation for 
photosynthetic activity of agriculturally useful plants. 
measurements showed that annual sowthistle had a higher chlorophyll 
concentration than did Palmer amaranth (7, 8). 
the 1.65- and 2.20-w wavelengths are predominantly affected by the amount 
of water over a spectrophotometer's port (2).  
that annual sowthistle leaves had a significantly higher water content, and 
were thicker than, Palmer mnaranth leaves. These factors contributed to 
high k values for annual sowthistle in relation to those for Palmer 
amaranth at 1.65 and 2.20 pm. 
London rocket (1.65 p m) and c m o n  lambsquarters (2.20 p m) than for annual 
sowthistle are not known; London rocket and lambsquarter both had a lower 
water content that did annual sovthistle. 
The largest b values were obtained at the 0.65-l.m 
Thus, 
Reflectance 
Note that the k values at 
Reference to Table 1 shows 
The reason for the larger b values for 
Scattering coefficient. In general, johnsongrass, ragweed parthenium, or 
London rocket had the larger 8 values among the five wavelengths, whereas 
annual sowthistle and Palmer amaranth were usually lowest (Table 4). The 8 
values were not clearly associated with the leaf thickness and water 
content measurements in Table 2. It is known, however, that leaf structure 
causes light scattering, especially at the 0 . 8 5 - b  wavelength in the near- 
infrared reflectance plateau region (1); on the average, the scattering 
coefficieats at 0.85 pm (Table 4) were higher than at the other wave- 
lengths. The scattering coefficient s is a function of leaf structure. 
the leaves of all seven weed species had essentially the same internal 
structure, s would have been strongly correlated with leaf thickness. 
was not true, so structure was important in light scattering. 
If 
This 
4 
Table 1. 
w e d  species. Each mean is based on 10 replications . b a n  leaf thickness, water content, and sr$a per leaf for seven 
Species Thicknesa Water Conteat k e a  per leaf 
Ragweed parthenium 0.236a 
Comnon lambsquarters 0.235a 
Annual sowthistle 0.218b 
Palmer amaranth 0.166~ 
Johnsongrass 0.145d 
London rocket 0.122e 
Wild campon sunflower 0.2348 
74.2d 56.6b 
77. 5c 13.4d 
82.3ab 105. Oa 
S3.8c 46.6bc 
80. Obc 44.3c 
76.9cd 98. Sa 
78.8c 16.7d 
s a n s  within each column followed by a c o m m  letter are not significantly 
different, p = 0.05, according to Duncan's multiple range test. 
Table 2. 
wavelengths. 
Infinite reflectance for leaves of seven weed species at five 
Each coef f icient is based on 10 replicat ionsa. 
Wavelength 
(vn) 
Species 0.55 0.65 0.85 1.65 2.20 
Ragweed parthenium 13.28a 07.32b 68.94~ 46.50a 
Cmmon lambsquarters 12.33b 07.39b 93.32a 36.17cd 
Wild common sunflower 11.20~ 07.70b 81.96b 42.14b 
Annual sowthistle 10.26d 05.36~ 90.85a 34.30d 
Palmer amaranth 12.14b 09.05a 90.75a 41.76b 
Johnsongrass 13.84a 08.59a 92.48a 46.57a 
London rocket 13.18a 07.37b 56.64d 38.54~ 
25.308 
16.30bc 
19.83b 
14.90~ 
18.52b 
23.54a 
20.99a 
%ana within each wavelength followed by a common letter are cot 
significantly different, p = 0.05, according to Duncan's multiple range 
test. 
5 
Table 3. 
wavelengths. 
Absorption coefficients for leaves of seven weed species at five 
Each coefficient is based on 10 replicationsa. 
Wavelength 
( m) 
Species 0.55 0.65 0.85 1.65 2.20 
Ragweed parthenium 1.92a 4.45c 0.08b 0.25cd 0.66~ 
Comnon lambsquarters 1.71b 4.80bc c. uoc 0.34b 0.92a 
Wild cammon sunflower 2.03a 4.2% - 0.02~ 0.29~ 0.84ab 
Annual sowthistle 1.87b 6.20a 0.01c 0.35ab 0.92a 
Palmer amaranth 1.86ab 3.21d 0.01c 0.24e 0.69~ 
Johnsongrass 1.95a 5.56ab 0.03~ 0.2le 0.62~ 
London rocket 1.95a 5. 56ab 0.17a 0.38a 0.82b 
%leans within each wavelength followed by a common letter are not 
significantly different, p = 0.05, according to Duncan's multiple range 
test. 
Table 4. 
wavelengths. 
Scattering coefficients for leaves of seven weed species at five 
Each coefficient is based on 10 replicationsa. 
Wavelength 
( m) 
Species 0.55 0.65 0.85 1.65 2.20 
Ragweed parthenium 0.67a 0.75~ 
C m o n  lambsquarters 0.55bc 0.83bc 
Wild couunon sunflower 0.58b 0.77~ 
Annual sowthistle 0.48~ 0.74~ 
Palmer amaranth 3.58b 0.70~ 
Johnsongrass 0.73a 1.15a 
London rocket 0.68a 0.95b 
1.03a 0.81a 0.58a 
0.85bc 0.60d 0.43~ 
0.93b 0.73b 0.52b 
0.63~ 0.55d 0.38d 
0.82c 0.59d 0.39cd 
0.88bc 0.68~ 0.49b 
1.02a 0.77ab 0.52b 
%leans within each wavelength followed by a common letter are not 
significantly different, p = 0.05, according to Duncan's multiple range 
test . 
6 
LITERATURE CITED 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
11. 
12. 
13. 
14. 
Allen, W. A. and A. J. Richardson. 1968. Interaction of light with a 
plant canopy. J. Opt. SOC. Am. 58:1023-1028. 
Allen, W. A., H. W. Gausman, A. J.  Richardson, and C. L. Wiegand. 
1970. 
leaves. Appl. Opt. 9:2573-2577. 
Mean effective optical constants of thirteen kinds of plant 
Arkin, G. F., J.  T. Ritchie, and S. J. Maas. 1978. A model for 
calculating light interrelation by a grain sorghum canopy. 
ASAE. 21:303-308. 
Arkin, G. F., R. L. Vanderlip, and J.  T. Ritchie. 1976. A dynamic 
grain sorghum growth model. Trans. ASH. 19:622-630. 
Trans. 
Chance, J. E. and E. W. Le-ster. 1978. Plant canopy light 
absorption model with application to wheat. Appl. Opt. 17:2629-2636. 
Gausman, E. W. and W. A. Allen. 1973. Optical parameters of leaves 
of 30 plant species. Plant Physiol. 32:S7-62. 
Gausman, 8. W., W. A. Allen, M. L. Schupp, C. L. Wiegand, D. E. 
Escobar, and R. R. Rodriguez. 1970. Reflectance, transmittance, and 
absorptance of light of leaves for 11 plant genera with differcat leaf 
mesophyll arrangements. Texas A& Univ. Tech. Monograph No. 7. 38pp. 
Gausman, E. W., W. A. Allen, C. L. Wiegand, D. E. Escobar, R. R. 
Rodriguez, and A. J. Richardson. 
crops, their light spectra, and optical and geometrical parameters. 
“.S. Dep. Agric. Tech. Bull. 1465. 59 pp. 
1973. The leaf mesophyll of twenty 
Gausmcn, H. W., 8. M. Menges, D. E. Escobar, J. E. Everitt, and R. L. 
Bowen. 1977. Pubescence affects spectra and imagery of silverleaf 
sunflower (Eelianthua argophyllus). 3eed Sci. 25:437-440. 
Heilman, M. D., C. L. Gonzalez, W. A. Swanson, and W. J. Rippert. 
1968. 
Agron. J. 60:578-579. 
Adaptation of a linear transducer for measuring leaf thickness. 
Richardson, A. J., D. E. Escobar, H. W. Gausman, and J. E. Everitt. 
1980. 
signatures of south Texas rangeland communities. Symp. on Machine 
Processing of Remotely Sensed Data. 
Comparison of LANDSAT-2 and field spectrometer reflectance 
June 2-6. 
Smith, J. A. and R. E. Oliver. 1972. Plant canopy models for 
simulating compDsite scene spectroradiance in the 0.4 to 1.05 
micrometer region. Proc. Eighth Int. Symp. on Remote Sensing of 
Environ. II:1333-1353. 
Steel, R .  G. D. and J. H. Torrie. 1960. Principles and Procedures of 
Statistics. McGraw-Hill Book Co., Inc., New York. 481 pp. 
Suits, G. H. 1972. The calculation of the directional reflectancc of 
a vegetative canopy. Remote Senaing Envirot. 2:117-125. 


Optical parameters of leaves of seven weed species

Absorption coefficient (k), infinite reflectance (R inf.) and scattering coefficient (s) were tabulated for five wavelengths and analyzed for statistical differences for seven weed species. The wavelengths were: 0.55, 0.65, 0.85, 1.65, and 2.20 microns. The R inf. of common lambsquarters (Chenopodium album L.), johnsongrass (Sorghum halepense L. Pers.), and annual sowthistle (Sonchus oleraceus L.) leaves at the 0.85 micron wavelength were significantly (p = 0.05) higher than for sunflower (Helianthus annuus L.), ragweed parthenium (parthenium hysterophorus L.), or London rocket (Sisymbrium irio L.). Annual sowthistle had the largest k value, and Palmer amaranth (Amaranthus palmeri S. Wats.) had the smallest k value at the 0.65 micron chlorophyll absorption wavelength. In general, johnsongrass, ragweed parthenium, and London rocket had the largest s values among the five wavelengths, whereas annual sowthistle and Palmer amaranth were usually lowest

  
 
 
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	 Surveys Through
Earlyrl Warning nd Crop Aerospace9
	
Remote Sensing
Condition Assessment	 KARCH 19082
OPTICAL PARAMETERS OF LEAVES
OF SEVEN WEED SPECIES
He We GAUSMAN, Re Me MENGES, As J. RICHARDSON,
He WALTER, Re Re RODRIGUEZ, AND Se TAMEZ
(E8i-10168) UPTiCAL FARAMEIIES OF LLAVES OF
SEVEN WEED SPECIES (Ayricuiturai fiesearch
Service) b p HC AU21ilk A01	 CSCL 02C
N82-24546
Urcici S
63/43 U0z68
}	 U,S• DEPARTMENT OF AGRICULTURE
`	 AGRICULTURE RESEARCH SERVICE
WESLACO, TEXAS
U SA
Q' 4oENT OF COMy^
TA
4i	 9o	 ^
ifo	 1,.^^
Sr4rES OF
p
O wno staffs of wWA
^q^N_A Sf
Lyndon B. Johnson Space Center
Houston. Texas 77058
APPROVED BY
Glennis U. 6oatwrig , Manager
Early Warning /Crop Condition ssessment Project
AgRISTARS Program
Houston, Texas
February 1982
Y^
ORIGINAL PAGE IS
OF POOR QUALITY
Optical Parameters of Leaves of Seven Weed Species'
H. W. GAUSMAN, R. M. MENGES. A. J. RICHARDSON. H. WALTER, R. R. RODRIGUEZ. and S. TA11EZ3
:i•
Abstract. Absorption coefficient (k). infinite reflectance (R,.), and
scattering eodficietrc (a) were tabulated for five wavekngtha and
analysed for statistical differences for seven weed species. The wave-
kagths wen: 0.55 pm. 0.63 son. 0.85 pm, 1.65 pm, and 2.20 pm. The
RM of common lambequarters (Cbwmpedium dkon L), johtaongrass
(Sorgban bdep"m (L) Pers 1. and annual sowthisdR (Sonebas
oleracent L) leaves at the 0.35-pm wavelength were siptifieandy(p . o.os) higher than for sunflower Ufdaastbat sameas L.), ragweed
pacthetaum (Phrdmaium bystempboms L). or London rocket (S4w-
brium irio L.). Annual sowthistle had the largest Is value, and Palmer
amaranth (.4man:atbus pdmM S. Wats.) had the smallest k value at the
0.65-pm chlorophyll absorption wavelength. In general, johmangrass,
ragweed parrbenium, or London rocket bad the largest a values among
the five wavelengths. whereas annual sowthistle and Palmer amaranth
were usually lowest.
Addirioad Wex words. Absorption coefficient, infinite reflectance.
scattering coefficient.
INTROD=ION
The optical parameters k, R,,, and s have been tabulated
for seven wavelengths, and analyzed for statistical differences
for 30 plant species (6). The wavelengths were: 0.55 µm
(green peak), 0.65 Wn (chlorophyll absorption band), 0.85
;Lm (infrared reflectance plateau), 1.45 {nn (water absorption
band), 1.65 µm (reflectance peak following water absorption
band at 1.45 )tm), 1.95 lun (water absorption band), and 2.2
;an (reflectance peak following water absorption band at 1.95
AM).
Our objective was to present significant differences among
the three optical parameters for seven weed species at the
0.55-µm, 0.65-µm, 0.85-µm, 1.65 -AM. and 2.2-µm wave-
lengths. The optical parameters can be used to predict the
response of a weed leaf to insolation. The rate of photo-
synthesis will be affected by changes in the amount of insola-
tion in the PAR (0.4 to 0.7 ism) that is absorbed. reflected,
or nattered by a single weed leaf (3, 5). Optical parameters
could be especially useful for determining the amount cf
insolation absorbed by a weed leaf. Insolation absorbed by
a weed leaf is energy lost to the photosynthetic activity of
useful crops, and will have a corresponding effect on yield.
Thus, data presented in this paper should be of interest to
investigators developing various crop yield models (4).
These data should also be of interest to the crop discrimina-
tion problem in remote sensing. The data presented could be
' Received for publication December 15. 1979. Contribution of the
Sci. Ed. Admin., U.S. Dep. Agric.. Weslaco, TX. This study was sup-
ported in part by the Nat. Aaron. Space Admin. under Contract No.
S-31876 -AG.
100.rvry. Plant Physiol.; )Iortic.; Physicist; Visiting Sci. from Univ.
of Hohenheim, west Germany. Sponsored by Deutsche Forschungsge-
meinschaft; Siol. Tech.; and Agric. Res. Tech.: respectively, Sci. Ed.
Admin., U.S. Dep. Agric.. Weslaco, TX 785%.
'Trade and company noun me for the convenience of the reader
and do not imply endorsement or preferential treatment by the U.S.
Dep. Agric.
used for investigation work using Suit's (14) plant canopy
reflectance models and Smi-h and Oliver's (12) stochastic
plant reflection model. These models could be used to infer
crop and weed reflectance from LANDSAT altitudes using
atmospheric radiative transfer correction procedures as pre-
sented by Richardson et al. (11). These crop reflectance
modeling studies would have application to remote sensing
in weed science for the detection of weeds in various crops
and for studying the associated crop yield reductions (9).
MATERIALS AND METHODS
Ten fully expanded acd healthy appearing leaves were
collected from each of the seven field-grown weed species:
ragweed parthenium, lambsgaarters. sunflower, annual sow-
thistle, Palmer amaranth, johnsongraw and London rocket.
The adaxial leaf surfaces of the weed species were essentially
glabrous, except that sunflower leaves were sparsely pubescent.
Johnsongrass leaf venation was parallel, whereas venation
was netted in the other species. Leaves were positioned on the
spectrophotometer so that veins or hairs did not interfere with
the impinging light beam. Immediatl. after excision, leaves
were wrapped in Saran 3 or Glad-Wrap]
 to minimize moisture
loss. Leaves were wiped with a slightly dampened cloth pre-
ceding spectrophotometric measurements to remove surface
contaminants.
The R,,, k, and s coefficients were calculated by the
equations of Allen and Richardson (1):
R. - 1/a,	 ( 1)
k - I(a - 1)/(a + 1)I log b,	 ( )I)
s	 - 12a/(a2 - 1)j log b,	 (1II)
a - 0 +r 2  - t2 +,&)/2r, and	 (1V)
b - (1-r 2 + t2 + A)/2t, where	 ( V)
a - optical constant, b - optical constant, r - reflectance,
and t = transmittance. The quantity A is defined by the
relation
A2=(1+r+t)(l+r-txl-r+t)(1-r-t)
	
(VI)
The quantities a and b (equations IV and V) are constants at
a given wavelength. Because r and t vary with 1',avelength, the
quantities a and b ate also functions of wavelength. Light
passing through a leaf is modeled as being absorbed and scat-
tered in direct proportion to a differential distance, dn,
traversed through the leaf and in direct proportion to the
amplitude of the light at that point in the leaf. The quantity
n is the cumulative leaf area index. Absorbed radiation disap-
pears from the model. For the case of a single leaf (n - 1)
Allen and Richardson's (1) equations reduce to the form
shown above. For the case of n > 1, two or more leaves are
stacked, and the formulation becomes more complex (1).
Scattered radiation is merely changed in direction. Because
the model is one-dimensional, the scattering must be either
forward or backward. The forward-scattered component is
1
ORIGINAL PAGE IS
OF POOR QUALITY
k
indistinguishable from the incident light, but the backward-
scattered component adjoins the light moving in the opposite
direction. The absorption coefficient It (equation 11) and the
scattering coefficient s (equation 111) are coefficients that
result from modeling light interaction with leaves ( 1). The
coefficients s and k correspond to fractions of light that
are scattered and absorbed respectively per unit of leaf area
index.
Leaf thickness was measured with a linear -displacement
transducer and digital voltmeter ( 10). Water content of leaves
was drterminel on a dry - weight basis; leaves were oven-dried
at 68 C for 48 h, and cooled in a desiccator before weighing.
Variance analysis and Duncan 's multiple range test (13)
were used on the spectrophotometric data for the sekcted
wavelengths at 0.55, 0.65, 0 . 85, 1.65, and 2 . 2 Inn.
RESULTS AND DISCUSSION
Leaf ebaracteristics. These data are included to show the
wide range of leaf thickness (0.122 to 0.235 mm), leaf water
content (74.2 to 83.8%). and area per leaf (13.4 to 105.0
cm a ) represented by the weed species (Table 1). These data
are used for descriptive and correlative purposes. The optical
parameters represented the optical differences among leaves
of the weed species.
Infinite reflectance R,,. The highest reflectances and the
largest inter-species differences were obtained at the 0.85-
tan wavelength on the near - infrared reflectance plateau. The
reflectances of common lambsquarters, johnsongrass, annual
sowthistle. and Palmer %trlaranth were significantly larger
(p = 0.05) than sunflower, ragweed parthenium, or London
rocket (Table 2). These results were not consistent with leaf
thickness and water content measurements (Table 1). High
R„ was associated with more finely divided mesophyll struc-
ture, which was conducive to short path lengths of light and
subsequently less tight absorptance (2).
Absorption coefficient k. The largest k values were ob-
tained at the 0 .65-gw. wavelength, which represent the chloro-
phyll absorption band in the red light region (Table 3). Annual
sowthistle had the largest k value. and Palmer amaranth had
the smallest k value at the 0.65 -gm wavelength. Thus, annual
r
Table I. Mean leaf thickness, water content, and area per leaf for awn
weed species. Each mean is based on 10 replications
Species	 Thickness Water content Area per leaf
T1bk 2. Infinite reflectance for leaves of seven weed species at five
%avelengths. Each coefficient is based on 10 replications&.
Wavelength
(wn)
species 0.55	 0.65	 0.85 1.65 2.20
(K-)
Ragweed p cthenium 13.28a 07.32b 68.94c 46.Soa 25.30a
Common lambsquarters 12.33b 07.39b 93.32* 36.17cd 16.3Wic
Wild common sunflower 11.20c 07.7ub 81.%b 42.14b 19.83b
Annual sowthistle 10.26d 05.36c 90.85a 34.304 14.90c
Palmer amaranth 12.14b 09.05a	 90.75s 41.76b 18.52b
Johnsongrass 13.84a 08.59a	 92.48a 46.57a 23.548
London rocket 13.18a 07.37b 56.64d 38.54c 20.998
a Means within each wave. ^,th followed by a common letter are
not significantly different, p - 0.05, according to Duncan's multiple
range test.
sowthistle would probably cause the largest loss of insoiation
for photosynthetic activity of agriculturally useful plants.
Reflectance measurements showed that annual sowthistle had
a higher chlorophyll concentration than did Palmer amaranth
(7, 8). Note that the k values at the 1.65- and 2 . 20-gym wave-
lengths are ptedominantly affected by the amount of water
over a spectrophotometer's port (2). Reference to Table 1
shows that annual sowthistle leaves had a significantly higher
water content, and were thicker than, Palmer amaranth
leaves. These factors contributed to high k values for annual
sowthistle in relation to those for Palmer amaranth at 1.65
and 2 .20 µm. The reason for the larger k values for London
rocket ( 1.65 µm) and common lambsquarters (2.20 µm)
than for annual sowthisde are not known; London rocket and
lambsquarter both had a lower water content than did annual
sowthistle.
Scattering eoef^cient. in general, johnsongrass, ragweed
parthenium, or London rocket had the larger s values among
the five wavelengths, whereas annual sowthistle and Palmer
amaranth were usually lowest (Table 4). The s values were not
Table 3. Absorption coefficients fot !ryes of seven weed species at five
wavelengths. Each coefficient is based on 10 replications&.
Wavelength(µm)
species	 0.55	 0.65	 0.85	 1.65	 2.:0
RagaYed parthenium
Common lambsquarters
Wild common sunflower
'i	 Annual sowthistley,	 Palmer smmnth
F Johnsongrass
London rocket
(mm) M (crn') (k)
0.236a 74.2d 56 .6b Ragweed parthenium 1.928 4.45c 0.08b 0.25cd 0.66c
0.235s, 77.5c 13.4d Common lambsquarters 1.71b 4.80be 0.00c 0.34b 0.928
0.234s 82.3ab 105.08 Wild common sunflower 2.03a 4.25c 0.02c 0.29c 0.84ab
0.218b 83.8c 46.6bc Annual sowthistle 1.87b 6.20s O.Olc 0.35ab 0.92a
0.166c 80.0bc 44.3c Palmer amaranth 1.86ab 3.21d 0.01c 0.24e 0.69c
0.145d 76.9ed 98.52 Johnsongrass 1.95a 5.56ab 0.03c 0.21e 0.62c
0.122e 78.8c 16 . 7d London rocket 1.95a 5.56ab 0.17a 0. 38a 0.82b
i aMeans within each column followed by a common letter are not
significantly different, p - 0.03, according to Duncan 's multiple range
test.
&clans within each wavelength followed by a common letter are
not significantly different, p - 0.05, according to Duncan'. multiple
range test.
ORIGINAL PAGE IS
OF POOR QUALITY
U640 4, Ecattedrg coeffic'Wou for leaves of seven weed species at five
wavelengths. Each eoefficieat is bared on 10 replicatioaO.
Waveleapth
(0ra)
Species	 0.33	 663	 0.05	 1.63	 2.20
(s)
Ragweed parthenium 6674 0,73c 1.016 atla a3$,
Common lambagwttan 0,113bc 0.43bc 0.113bc 0.60d 0,43c
wild common sunflower a311b 0.77c &93b 0.73b 0,32b
Annual sowthistle a411c 0.74c 0.83c 0.594 0.3114
pliant amaranth af11b 0.70c 0.E2c 0.39d 0.39cd
Johnsongrass x736 1.15a 0,114bc a6Be 0.49b
London rocket 0.6Ea 0.93b 1.02a 0.77ab 032b
*Means within each wavelength followed by a common letter are
not significantly different, p a 0.03, aecot ling to Duncan's multiple
sage test.
clearly associated with the leaf thickness and water content
measurements in Table 2. It is known, however, that leaf
structure causes light scattering, especially at the 0.85-ftm
wavelength in the near-infrared reflectance plateau region
(1) i on the average, the scattering coefficients at 0.83 µm
(Table 4) were higher than at the other wavelengths. The
scattering coefficient s is a function of leaf structure. If the
leaves of all seven weed species had essentially the same
internal structure, s would have been strongly correlated
with leaf thickness. This was not true, so structure was im-
portant in light scattering.
ACKNOWLEDGMENTS
We thank Maricela Garza for making the leaf transections
and Maria Rodrigues for assistance in data analyses.
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Optical parameters of leaves of seven weed species

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