THE EFFECT OF LlfflHT INTENSITY ON THE PHOTOSYNTHETIC
EFFICIENCY OF TOMATO PLANTS

By
ALTON MILLETT POHTEH
inMii

A THESIS

Submitted to the fa c u lty o f the Michigan S ta te
C ollege of A griculture and Applied Science in p a r t ia l
f u lf illm e n t o f th e requirements fo r the degree o f
Doctor of Philosophy

H orticu ltu re Department
East Lansing*
1936

Michigan

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ACKN0WLED05MENTS
The w r ite r i s g r a te fu l to Dr* J* W* C rist fo r
guidance and a id in conducting t h is study and re­
view ing th e manuscript; to P rofessor V* R*Gardner
fo r su g g estio n s in th e preparation of the manuscript;
to P rofessor P* C. Bradford fo r ad vice in review ing
the m anuscript; to Dr* W illiam W* Smith fo r a s s is ta n c e
in c o lle c t in g data; and to P ro fesso r £• Safford Torrey
and Dr* H. E» H ill fo r h elp w ith the photographic
work.

Introduction
She tomato stands forem ost among the sev era l v eg eta b le
p la n ts which are c u ltiv a te d as greenhouse crops*

In i t s

cu ltu r e under gla ss* e s p e c ia lly in the northern sta tes* the
q u estion o f a s u ffic ie n c y o f lig h t fo r i t s b est development
and h i p e s t p r o d u c tiv ity a r is e s and becomes acute*

The lig^ it

o f the n atural day* during the w inter months* appears to be
inadequate w ith resp ect to i t s duration and a ls o i t s ordinary
in te n s ity *
The p o s s i b i l i t y o f u sin g a r t i f i c i a l lig h t* to r e in fo r c e
daylight* e x is ts *

As m atter o f fact* t h is has already been

t r ie d in not a few in stances*
C ertain ly p h otosyn th esis i s one of the most fundamental
p ro cesses which con d itio n p lan t behavior and production* and
lig h t i s a major fa c to r in i t s dynamic complex*

N eg lectin g

the c h a r a c te r is tic s of lig h t* other than i t s in ten sity * how
i s i t s in te n s ity r e la te d to the rate* the s o -c a lle d e ffic ie n c y *
o f p hotosynthesis?

More p a r tic u la r ly , what i s th is r e la tio n s h ip

r e sp ectin g the tomato plant* when grown under greenhouse
conditions?

A study o f th is* - induced by the d esir e to extend

th e knowledge d isc lo se d by in v e s tig a tio n s already made and
reported* - was made and i s h erein presented*

-

2-

Beview of L itera tu re
The e f f e c t s of stro n g , d iffu se d lig h t on p h otosyn th esis
were e x te n s iv e ly stu d ied by Muntz ( l l ) in 1913*

He found

from f i e l d ob servation s th a t a lf a lf a produced l e s s dry m atter
per square centim eter o f le a f area in the summer o f 1911* - a
summer u n u su ally fr e e from clouds - than in 1910, when cloudy
s k ie s p r e v a ile d much o f th e time*

A dditional observations

were made in the lab oratory where i t was p o s sib le to eq u a lize
th e amounts o f w ater receiv ed by th e l o t s o f p la n ts grown under
d iffe r e n t lig h t in t e n s it ie s *

The r e s u lt s o f the lab oratory

experiments accorded w ith those obtained from the work in the
f ie ld *

He concluded th at carbon a ssim ila tio n i s governed and

lim ite d by th e in te n s ity of the lig h t*
Lubimenko (9 ) &nd ?opp (3-7) found that in h e lio p h ilo u s
p la n ts the ra te of the accumulation o f elaborated m a teria ls
was in creased w ith in crea se in the lig h t in t e n s it y , up to an
optimum p o in t, and th a t any in crease beyond t h is optimum r e su lte d
in a decrease in the rate*

Heliophobous p la n ts behaved in the

same manner, the optimum, however, being a t a much lower p oin t
than th a t fo r the h e lio p h ilo u s types*
Arthur, Guthrie and Newell ( l ) working w ith JO d iffe r e n t
sp ec ies o f p la n ts found the tomato to be the most s e n s it iv e to
lig h t*

Light in t e n s it ie s o f 350, U50, 760, 800, 1200 and 1*400

fo o t-c a n d le s were used c o n jo in tly w ith len g th s of day which
ranged from f i v e to 2*4 hours*

This rev ea led the fa c t th at

-

3-

th e "time f a c t o r 0 was o f importance*

The peak o f in crease

in carbohydrate production was reached a t higher lig h t in ­
t e n s i t i e s w ith the 12-hour day*

In ju rious e f f e c t s r e s u lte d

when the day was lengthened to 17 and 19 hours#

The maximum

carbohydrate in crease was reached w ith th e 17 and 19-hour day
when lower lig h t in t e n s it i e s were used* or at the p oin t of
injury fo r th e higher in te n s itie s #
Combes ( 2 ) working w ith potatoes* and other tuber-forraing
species* found that the higher the lig h t in te n sity * the
greater the accumulation o f elaborated organic compounds in
th e storage p a rts o f th e p lan ts#

Apparently* a t lower in ten ­

s i t i e s the sto ra g e fu n ctio n caased and th e e n tir e amount o f
th e products o f p h otosyn th esis was consumed in the growth o f
the a e r ia l p a rts of the plant*
D eB esteriro and Durand (3 ) obtained very d e f in ite
r e s u lts experim enting w ith the garden pea#

The p la n t s 9

dry-weight in crea se was in d ir e c t proportion to the in te n s ity
o f the lig h t employed fo r i t s irra d ia tio n #
Folmer (3 ) and T o sh ii ( 2*0 experimented w ith sev eral
o f the d iffe r e n t environmental factors* and of th ese se v era l
factors* lig h t in te n s ity had th e g r e a te st e f f e c t on th e pro—
duct ion o f carbohydrates in c e r e a ls and peas#

Their data show

a g rea ter production o f carbohydrates under the co n d ition o f
short days w ith b righ t su n lig h t than th at o f lo n g days w ith
reduced su n lig h t, although the product o f th e in te n s ity and

-Uth e duration of lig h t was higher in th e l a t t e r case#
Kostytschew and Kardo—Sys-Soiewa (S) found th a t d esert
p la n te in crea sed in carbon a s s im ila tio n up to an optimum
lig h t in t e n s it y and decreased as the in t e n s it y went above
t h is point#

Later in th e day* as th e lig h t in te n s ity f e l l

to the optimum point* th e carbon a s s im ila tio n again reached
a maximum causing the d a ily curve o f p h otosyn th esis to show
two peaks in i t s ou tlin e#
The lit e r a t u r e which bears d ir e c t ly on the q u estion o f
the response o f th e photo sy n th e tic fu n ctio n to the fa c to r o f
lig h t in te n s ity i s not so p le n tifu l#

The foregoing re feren ces

are not a ll* but are re p r ese n ta tiv e o f those o f greater
importance, and a ls o , are s u ff ic ie n t to show the e x iste n c e o f
a q u a n tita tiv e r e la tio n s h ip between th e se two phenomena#
B esid es a d ir e c t e f f e c t o f l i g h t , w ith resp ect to i t s
in te n s ity , upon the behavior o f the p h otosyn th etic p ro cess,
a c tin g a s a c a t a ly t ic and en ergizin g agen t, i t appears to
a f f e c t c e r ta in other f a c t o r s , which are e s s e n t ia l in th e process#
Among th e se are the ch lo ro p h y ll content of th e le a f , and i t s
anatom ical structure* - the l a t t e r being important w ith r e fe r ­
ence to the r a t e o f the d iffu s io n o f g a ses w ith in the l e a f ' s
in te rio r *
^ i l l s t a t t e r and S t o ll (23) observed that th e r a te o f
p h o to sy n th esis in creased w ith the ch lorop h yll con ten t, but
were unable to e s ta b lis h a d e f in it e q u a n tita tiv e r e la tio n s h ip

-

5-

between the two, - the fu n ctio n and th e independent v a ria b le*
P a lla d in (lU ) and Lubimenko (9) s t a t e on the b a s is o f
th e ir experim ents that heliophobous p la n ts are r e la t iv e ly
higher in ch lo ro p h y ll content than h e lio p h ilo u s p lants*

The

la t t e r in v e stig a to r was a b le to e s ta b lis h the fa c t th at th e
optimum lig h t in te n s ity fo r p h o to sy n th esis i s lower in c o r r e sc
pondence w ith red u ced ^ n ten t o f chlorophyll* Shade p la n ts ,
a t the lower lig h t in t e n s it i e s were aB e f f ic ie n t in photo*
sy n th e tic a c t iv it y as nonshade p la n ts a t th e se same lig h t
in te n s itie s *
A number o f more recen t in v e s tig a to r s , Johnson ( 7 ) ,
UacDougal ( 10) , Spoehr (19) and Ifiesner (21) , working w ith
long day p la n ts , report that the amount o f ch lorop h yll in
th e le a v e s o f p la n ts in creased in d ir e c t proportion to the
average quantity o f lig h t receiv ed by them*
Sprague and Shive (20) demonstrated that there was
a degree o f r e la tio n s h ip between the t o t a l ch lorop h yll con­
te n t and the dry w eigh ts o f tops in corn*

The t o t a l q u an tity

o f ch lo ro p h y ll contained in th e le a v es of the various s tr a in s
o f maize co r re la te d c lo s e ly w ith th e ir dry w eigh ts a t su c c e ssiv e
harvests*

S tra in s that showed a high ch lorop h yll con centration

per u n it o f l e a f area a lso had high average r a te s o f in crea se
in dry w eights o f top s, and v ic e versa*

This r a t io between

th e t o t a l ch lorop h yll and dry weight of tops was p r a c t ic a lly
id e n tic a l w ith a l l th ree str a in s of com * t e s t e d *

-

6-

finerson ( 4 ) , working w ith C h lo rella , observed th a t p lan t
c e l l s low in ch lorop h yll reached th e ir maximum r a te o f photo­
sy n th esis a t approximately the same lig h t in te n s ity as normal
c e lls #

In working w ith d iffe r e n t ch lorop h yll con cen tration s

in p la n ts th a t w ere kept constant in th e se v a r ia tio n s, w ith the
same lig h t in te n s ity , he found that the r a te o f p h otosyn th esis
in creased a t th e same speed reg a rd less of the ch loroph yll con­
centration#

The con clusion was that ch lorop h yll i s probably

a chemical reactan t in p h otosyn th esis as w e ll as b ein g th e
p h o to s e n s itiz e r which absorbs th e radiant energy necessary in
th e process*
Hayden (6) and P oole (15) found the spongy parenchyma
c e l l s (m esophyll) of th e le a v e s were poorly developed in sun
p la n ts , but in shade p la n ts th e se c e l l s replaced the p a lisa d e
c e lls #
Shibata (lS ) observed that lig h t in te n s ity had a d e f in it e
e f f e c t on th e anatomy o f the le a v e s , in th a t the epidermal c e l l s
are sm aller in short day p lan ts*

Osterhout (1 3 ), and N igh tin gale

and H itc h e ll (12) observed th at le a v es were th ick er and had more
elongated, more d en sely packed, p a lisa d e c e l l s , a s the average
lig h t in t e n s it y was m aintained at a higher point#
The lit e r a t u r e le a v e s no doubt concerning the d ir e c t , and
a ls o in d ir e c t, importance o f lig h t in te n s ity in the p l a n t s
photo sy n th etic behavior#

The r e s u lts c ite d from the work o f

Arthur, G uthrie, and Newell ( l ) are e s p e c ia lly s ig n if ic a n t and

-

7-

h elp fu l* s in c e th e tomato p la n t i t s e l f was among th ose used in
th e ir experiments*

However* a d d itio n a l co n trib u tio n s from

co n tr o lle d experim entation are d esirable* and necessary* b efore
th e m atter o f the u se o f a r t i f i c i a l l l ^ i t in fo rcin g houses*
devoted to tomato growing and production* can be c e r ta in ly
and soundly determined*
General Procedure
The tomato p la n ts used in the experiment were o f th e
Grand Bapids Forcing v a riety *
f l a t s on January 26, 1933*

The seeds were sown in greenhouse

Gn February 3, a la r g e number o f

se e d lin g s were s e le c te d and pricked o f f in to two-inch pots*
These were tran sferred on February 12 in to fou r-in ch p o ts and
l e f t th erein u n t il March 1* or u n t il t h e ir development was such
th at they were ready fo r f in a l tran sp lan tation *

On th at d ate,

36 o f th e p la n ts were s e le c te d from th e remaining 108* and
tran sferred to lU -inch pots* in which th ey were grown s in g ly
and to f u l l maturity*
The s o i l was a f a i r l y r ic h orchard loam* which had been
p rev io u sly screened and thoroughly mixed* by having been
shoveled over* in bulk*

I t s uniform ity was as good as could

be expected and secured*
The 36 p la n ts were d ivid ed in to three l o t s of 12 each*
and each o f th e l o t s placed* w ith wide sp acin g, on a separate
greenhouse bench where the p o ts were surrounded by m oist sand.

-s-

(afterw ards kept m oistened) to a depth o f f i v e inches*
During th e c o a r s e of th e experiment* the in d iv id u a l p o ts
w ith in each l o t were s y ste m a tic a lly sh ifte d * tw ice each week*
in th e ir p o sitio n s*

This insured g rea ter uniform ity in t h e ir

exposure to th e environment* p a r tic u la r ly the fa c to r o f lig h t*
The number o f c lu s te r s o f f r u it per p lan t was r e s t r ic te d to
fiv e *
A lU—hour day was m aintained over th e p la n ts o f each lo t*
Extension o f th e regular d a y lig h t p eriod was accomplished by
means o f a 1000-watt e l e c t r ic lamp* w ith dome r e fle c t o r and
ad ju sta b le in height* suspended c e n tr a lly above each group of
plants*

A wooden frame was con stru cted above each of two o f

the benches* under each lig h t* and made to be v e r t ic a lly moveable*
One of th e se frames was covered w ith one la y e r o f w hite cheese­
cloth* th e other w ith two la y ers*

This e ffe c te d three r e sp e c tiv e

I n t e n s it ie s o f the lig h t* both natural and a r t i f i c i a l , fo r the
p la n ts: no. shade or f a l l in te n s ity - o n e-h a lf in te n s ity (*>0**1#) and a l i t t l e l e s s than one-fourth in te n s ity (2 2 * ^ ) * The daades
were kept a d ju sted in th e ir h e ig h ts so a s always to be approxi­
m ately 2 k inches above the tops o f the growing p lants*
Zt was aimed* o f course* to keep th e co n d itio n s o f the
environment* a s id e from th e c o n tr o lle d v a r ia tio n s in lig h t
In ten sity* th e same fo r the three benches*

Data were recorded

fo r r e la t iv e humidity* a ir temperature* and s o i l temperature,
under each o f th e th ree lig h t conditions* from March to July*

-

9-

by the uee o f hygrothermographs and s o i l thermographs*
A d d ition al inform ation regarding methods - th ose more
p a r tic u la r ly te c h n ic a l - i s given* where appropriate, in
the fo llo w in g s e c tio n , w ith i t s p resen ta tio n o f the d ata
obtained*

EXPERIMENTAL RESULTS

Growth Response - Leaf Area:
P o s s ib le r e la tio n s h ip s between each o f several
d iffe r e n t lin e a r measurements o f the tomato le a f and i t s
t o t a l area were examined in a previous experiment (l6 )*
The len g th o f t h e l e a f from th e base o f i t s f i r s t l e a f l e t s
to the t i p o f the midrib proved to be the most accurate index*
The type o f a ss o c ia tio n was c le a r ly c u r v ilin e a r , and s p e c if i c a ll y ,
p arab olic in the second degree*

The derived equation was

y (area) - 3 .1 6 + 0.1*17 x + O.307 x2 .*
A ll o f the le a v e s on each p lan t in each o f the th ree
l o t s were measured, in th e manner in d ica ted above, and a t
in te r v a ls o f th ree to seven days during the cou rse o f the
present eaqperiment, and t h e ir areas c a lc u la te d through the
given equation*

The data are presented in Table 1 *

* This l e a f area equation was te s te d out on about 20
le a v e s from each s e t o f p la n ts and i t was found th a t
the formula ap plied eq u ally w e ll to a l l three types
o f illum in ation*

-

Table lz

Sate of
Measurement

10-

Growth Response - In Terms o f Leaf Area

Average le a f area
per p la n t
_ __ in so* cm*
No Shade 1 Layer 2 Layers
Cheese­ Cheese­
c lo th
c lo th

Average d a ily in crease
in le a f area per p lant
in
No Shade 1 Layer 2 Layers
Cheese­ Cheese­
c lo th
c lo th

March 28

1350

1503

1652

A pril U

10*9

2253

2*173

1*2.7

107*0

116.0

A pril 8

1888

2687

3122

5 9 .7

105.S

162.1

A pril 11

2088

2891*

359*1

6 6 .6

100. 1*

157.1

A pril 15

235S

3>*0S

1*202

6 7 .5

107.5

152.0

A p ril 18

2668

361*6

1*69>*

102.8

77.7

158.5

A pril 22

3188

3901

529*1

130.0

6 3 .6

1*17.5

A pril 29

375*1

1*303

6259

80. 7

57.3

139.0

May 6

1*16^

1*680

661*9

5 8 .5

56.1

55-6

May 13

>*325

5033

6761*

2 3 .0

1*8 .0

l6.1*

May 20

1*1*85

5*133

6868

13.3

5 7 .0

ll*.8

June 5

1*563

5559

7072

6 .5

7 .9

12 .7

July 3

1*563

5559

7072

-

11-

As shown in Table 1, expansion in l e a f area was both
con tinu ou sly and f i n a l l y the g rea test fo r the p la n ts under the
low est lig h t in te n s ity ; next g r e a te st where medium in te n s ity
prevailed* and le a s t in the unshaded condition*

The o r d e r lin e ss

o f the change in the d a ily r a te o f th e in crease in the f o l i a r
surface o f the unshaded p la n ts i s outstanding*

This rose over

gradual step s to a d is t in c t maximum (A p ril 2 2 ), and th e r e a fte r
f e l l o f f c o n s is te n tly to zero a t the end*

D iffe r in g from th is*

th e two other maxima were reached more q u ick ly and much e a r lie r
in th e l i f e o f the plants* and were m aintained over lon ger
p eriods o f time*
Thus* the u sual r e s u lt was obtained*

Growth* when measured

in terms o f l e a f area* augments under reduced lig h t in te n s ity *
The le a v e s a t ta in grea ter size* hut commonly are thinner and may
have even l e s s t o t a l mass*

The g rea ter spread* o f l e a f su rface

g iv e s in creased exposure to the lig h t* such as i t is* and tends
in some degree to condensate on th e whole fo r the le s s e r quantity
o f lig h t re ce iv ed per u n it o f eaposed surface*
Growth Besponse - Stem B longationl
The measurements taken fo r le a f area were accompanied by
determ inations which gave the growth r a te o f th e main a x is o f the
plants* under each lig h t treefcment*
th at o f the stem axis*

The d ista n ce measured was

The p eriod of th ese in term itten t measure­

ments was A p ril 4 to June 5* when the p la n ts were pinched out at
th e top* and thus r e s t r ic te d to the production of but f i v e f r u it
c lu s te r s per plant*

Table 2 g iv e s the data*

12-

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The responses in stem elon gation shown in Table 2
were akin to th ose shown far in crea ses in le a f area*

Growth

in h e ig h t, a s r e fle c t e d in more atten u ated internodes* was
more rap id as shading was heavier* and th e p la n ts t a lle r a t
th e time o f being topped*

The maxima fo r th e r a te s o f elon­

g a tio n were in th e same order as th ose follow ed by th e leaves*
and th e ir occurrences in time p r a c tic a lly id e n tic a l w ith th ose
which obtained fo r the leaves*
Growth fiesponse - F ruit Productions
The records taken on f r u it production show a d e f in it e
f r u it s e t and production in d ir e c t r e la tio n w ith the ligjht
receiv ed by the plants*

These measurements were made by

tagging the f r u it w ith th e date s e t on each c lu s te r and
recording the number o f days n ecessary fo r each f r u it to
rip en and i t s weight*

This inform ation was assembled and

to ta le d g iv in g comparative fig u r e s to i l l u s t r a t e the valu e of
lig h t in t e n s it y on f r u i t production and on the e f f ic ie n c y o f
the p lan t le a f area*

These data are presen ted in Table 3 *

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The amount o f f r u it s e t under reduced lig h t in te n s ity
i s much l e s s than under normal l i g h t , as can he seen in
Table 3*

ln

ea rly part of the season when the p la n ts

were r e c e iv in g a r e la t iv e ly sn a il amount o f lig h t the f r u it
s e t was in in v erse proportion to a l l amounts of shading, hut
as the season progressed and lig h t in te n s ity became h igh er,
th e fr u it s e t co r re la te d b e st w ith th e h e a v ie st shading as
would be expected#

This may be accounted fo r by the fa c t

th a t th e lig h t in te n s ity receiv ed by the h e a v ie st shaded
p la n ts was o r ig in a lly a t or near the minimum fo r f r u it set*
Then as th e season progressed and lig h t in creased beyond the
minimum, other en v iro m en ta l fa c to r s entered, causing a
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was o r ig in a lly the case*

The amount o f f r u i t when ripened,

however, c o r r e la te s c lo s e ly w ith the fo o t candle hours o f
lig h t , (Table 7)* to which the p la n ts were exposed*
F ru it production req u ires a g rea ter area o f le a v e s in
proportion to the amount o f shading the p la n ts receive*
The f r u i t s a tta in greater w eigh ts and ripen sooner when the
lig h t I n te n s ity i s not reduced*

D oubtless th e great a s s im ila tio n

o f p lant food by the le a v e s tends to speed up f r u it growth
and ripening#

—

16-

Growth Response - T otal P lan t Production:
The data fo r t o t a l p la n t production determined on
tw elve in d iv id u a ls under each o f th e th ree d iffe r e n t lig h t
in t e n s it i e s are shown in Tables 5* 6 an& 7*

A cursory glan ce

a t th e se data shows th a t in d iv id u a l tomato p la n ts vary w ith in
wide lim its *

The w eigh ts taken fo r t o t a l p lan t production

show a v a r ia tio n in p lant food under each lig h t treatment that
i s le s s than the d iffe r e n c e s in lig h t in te n sity *

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Growth fiesponse - In Terms of Total Plant Production by Plants Shaded with 2 Layers of Cheesecloth

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-

20-

In accordance w ith expectations* in crea ses in lig h t
a v a ila b le fo r carbohydrate form ation showed a greater
quan tity of fresh* dry,.ash and plant food w eights in the
average case*

The r a te o f p h otosyn th esis was slowed up

according to th e amount o f lig h t redu ction in each block*
The d iffe r e n c e s in lig h t in te n s ity appeared to have l e s s
e f f e c t on th e ash content o f the plants* but are in a rather
d e f in it e r e la tio n w ith th e average r e s u lt s on the fresh* dry
and p la n t food w eights in a l l p arts o f the plants*

The

p lant food manufactured per u n it le a f area i s g r e a te st under
th e no shade con d ition and i s reduced according to the amount
o f shade the p la n ts receiv e*

This e f f ic ie n c y in food manu­

fa ctu re seems to have a d e f in ite e f f e c t on th e plant m a terial
used in f r u it production and i s in approximately the same
r a tio a s the average decreased p la n t e f f ic ie n c y where shaded*
I t would seem from th is inform ation that d iffe r e n c e s in lig h t
in te n s ity during the seasons o f th e year are the d ir e c t causes
fo r v a r ia tio n s in p lan t e f f ic ie n c y in growth and f r u it
production* but Plant 10 in Table U, P lant 11 in Table

and.

P lan t 11 in Table 6 are p r a c t ic a lly equal in th e ir e f f ic ie n c y
under each r e sp e c tiv e condition*

This appears to in d ica te

that p lan t v a r ia tio n i s re sp o n sib le for some of the d iffe r e n c e s
in photo sy n th etic a c t iv it y under the d iffe r e n t lig h t
in te n s itie s *

-

21-

Environmental C ond ition s.
The experim ental aim, as s ta te d e a r lie r , was to
have the same len g th of day ( lh hoars) fo r the three l o t s
o f p la n t8, w h ile having them exposed to th ree d iffe r e n t
lig h t in t e n s it ie s *

Light measurements were made d a ily , a t

two hour in te r v a ls , throughout the p eriod o f growth, by
means o f a Clement^ Photometer*

The data fo r th ese

measurements are presented in Table 7*

-

22 -

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-

23-

I t i s clear* from examination o f Table 7» that the
grad ations o f lig h t in te n sity * e sta b lish e d in the beginning
by means o f shading* m aintained w ith c lo s e approximation* as
the season advanced and ended*

While th e general in te n s ity

o f the su n lig h t in creased grad u ally fo r a ll* th e three
experim ental co n d ition s o f f u l l in ten sity * on e-h alf in te n sity *
and one-fourth in ten sity * continued to hold and to be
e ffe c tiv e *
General c o r r e la tio n o f th e s e c o n tr o lle d v a ria tio n s
in lig h t in t e n s it y - p rev io u sly shown w ith the d iffer en ce s
In growth responses of th e th ree lo t s o f p la n ts - i s ob vious.
The r e la tio n s h ip i s i t s e l f n egative in character fo r le a f area*
and stem elongation* and p o s it iv e fo r f r u it production* and
t o t a l p lant production*

In order to f a c i l i t a t e inspection*

c e r ta in fig u r e s from the preceding ta b le s are brought together
in Table 8*

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25-

However* th e a s s o c ia tio n which i s apparent in Table 8
can have v a lid it y only in case c e r ta in other fa c to r s which
are known to con d ition the p h otosyn th etic rate* and con­
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Table 9 g iv e s data resp ectin g th ree such

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con sid era tio n and attempted control*

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-2 7 These three fa c to r s are r e la t iv e ly uniform fo r each
b lock o f p la n ts*

The v a r ia tio n s o f th e se fa c to r s in the th ree

treatm ents i s probably regu lated somewhat by th e lig h t in te n sity *
This lig h t in te n s ity 'variation has e v id e n tly accounted fo r the
p la n t growth responses under each c o n d itio n , and the e f f e c t s
o f the oth er environmental co n d itio n s are regu lated by t h is
lig h t*
D a ily P eriods o f Measurement*
The data which have been presented show, beyond doubt,
a r e la tio n s h ip between the behavior o f the p la n ts and the
d iffe r e n t lig h t in t e n s it i e s under which they grew and matured*
The evid en ce, however, i s g en eral in nature and not such as to
be adequate fo r th ose mathematical p ro cesses which y ie ld
q u a n tita tiv e exp ression s o f co rrela tio n *
The p la n t m a teria ls manufactured by the p la n ts on 12
d a tes, spread over the p eriod o f growth, were determined on
seven p erio d s during each o f th e days*

This procedure fo r

estim a tin g the photo sy n th e tic a c t iv it y was that termed the
M odified SachS Method*

I t co n siste d of taking two square

centim eters o f f o lia g e from each p lant every two hours in
sample b o t t le s and w eighing in the fr e s h condition*

These

sm all d is c s o f the lea v es were then heated in the oven a t 70°C«
fo r 12 hours and then a t 95°C* fo r s ix hours, when they were
reweigjhed*

A fter drying they were put in c r u c ib le s and ashed,

-2 8and again reweighed.

The dry w eight minus the ash w eight was

th e amount o f phot osyn th esi zed product in each sample*
C orrection fo r r e s p ir a tio n and tra n slo ca tio n was determined by
adding th e average lo s s in w eight per two hours during th e night
to th e d iffe r e n c e in w eigh t o f the two hour samples during the
day*
l i g h t in each b lock o f p la n ts was measured at two hour
in te r v a ls w ith a Clement^ Photometer in which s o lio paper i s
used* and comparisons made w ith a standard*

On days when

p h otosy n th etic a c t iv it y was determined* the Macbeth illum inom eter
was a ls o used in order to g et readings in a ctu a l number of
fo o t-ca n d les*
12 *

She data are presented in Sables 10* 11* and

-29Table 1 0 .

S ate

Phot osynthate and Light I n t e n s it ie s fo r Unshaded P lan ts

V ariables

4-8

8-10

AM.

f
A p ril .7 Pho 10 synthat e
1 .6 0
Light In ten sity 200

P eriod o f Say
2 -4
10-12
12-2
P.M.

4 -6

6-8

1 .4 6
400

0 .1 6
444

2 .1 6
3S7

4 .2 0
256

1 .4 o
139

1 .6 4
87

A p ril 24

So
So

1 .1 2
606

1 .1 1
1200

1.60
l4 ll

2*96
2408

5 .3 4
2314

2 .4 4
1207

0 .3 6
117

A p ril 29

So
So

1 .6 4
732

3 .0 6
1464

3 .0 6
2553

4 .0 8
2553

1 .2 4
1445

2 .2 8
806

1.60
337

May 5

So
So

1 .4 8
212

1 .4 6
424

1 .4 4
393

1.60
394

1 .7 6
287

O.56
116

0 .2 7
87

May 13

So
So

1 .0 6
237

I .0 6
446

1 .4 4
286

2 .6 4
1285

0 .8 8
2408

0 .7 2
2408

0 .3 4
337

May l 4

So
So

1 .2 8
1164

1.4o
2078

2.00
2409

3*96
2503

1.68
1800

1 .8 0
687

l4o

May 23

So
So

2.'4S
1206

1.80
2172

0 .7 6
2588

1 .8 4
2937

7.34
2730

1 .2 4
1376

1 .68
736

May 24

So
So

1 .5 2
637

3.20
1230

4 .9 6
2499

1 .3 2
2435

2 .9 2
2100

1 .7 6
1350

1 .6 0
736

Jtme 9

So
So

3*76
739

3*32
1369

2 .7 6
2533

6 .o 4
2855

3 .0 0
2344

2.92
964

1.9 6
674

June 19

So
So

1 .8 4
100

l.4o 2 .7 6
186

277

4 .8 8
27s

2 .9 6
944

2*76
910

2 .1 6
121

So
So

1 .8 8
210

2 .7 6
330

5 .4 4
3480

7.20
3906

5 .1 2
3773

1 .6 0
3039

1674

June 17

So
So

4 .0 3
94o

2 .6 4
1734

I .52
1503

9 .2 0
3155

5 .1 2
4235

1.60
3115

1.08
1769

Average

Photosynthate

1 .8 9

2.05

2 .3 2

3 .9 9

3 .5 4

1 .7 5

1 .2 0

1086

1520

2091

2053

1426

578

June 16

Light In te n sity 582

0 .3 6

i.4o

-3 0 Table 11*

Photosynthate and Light I n t e n s it ie s for P la n ts Shaded
w ith 1 Layer o f C heesecloth

Date

V ariables

b—s
A«M*

8—10

Period o f Day
2-4
10-12 12-2

4 -6

6-8

PM.

A p ril 7

Phot 0 synthat e
Light In te n s ity

1#00
100

0 .7 6
202

0 .1 6
246

0 .2 8
236

3 .4 4
128

1 .4 4
49

0 .3 6
24

A p ril 2^

Do
Do

1.00
237

1 .0 6
61*8

1 .3 6
768

1.28
1245

2 .1 6
1135

2 .8 0
544

0 .8 0
49

A pril 29

Do
Do

0 .9 2
462

1 .6 0
739

1*72
1182

1 .8 4
1281

1 .8 4
813

2.00
739

0 .6 0
247

May 5

Do
Do

1 .4 2
104

0 .8 8
2l*l

1 .6 0
187

1 .0 4
187

1 .6 0
136

0 .1 2
S3

0 .1 6
64

May 13

Do
Do

0 .8 0
106

0 .6 0
207

1.80
182

1*32
677

O.5 2
U 63

0 .2 4
1231

0 .2 8
187

May l4

Do
Do

X.00
737

1 .6 4
1207

1 .6 0
1251

1 .1 6
1409

1 .7 2
914

0.88
352

0 .1 6
47

May 23

Do
Do

1 .6 8
737

1 .4 8
894

0 .1 6
1191

1 .2 8
1113

1 .32
1113

1 .1 2
689

o .o 4
306

May 2k

Do
Do

1.68
331

2 .2 4
532

1 .4 8
1170

1 .0 4
1180

1 .5 2
1259

O.96
532

0 .4 0
306

June 9

Do
Do

O.7 6
312

0 .1 2
692

1 .1 6
IO69

O.3 2
1424

1 .2 0
1177

2 .4 8
409

0 .1 6
394

June 15

Do
Do

0.1*0
100

1 .0 6
153

1.4o
128

2 .3 6
157

1 .2 8
476

2 .2 4
451

o .o 4
86

Do
Do

1 .4 6
96

2 .7 4
823

0 .1 2
1793

2 .6 4
1946

2 .4 0
1891

1 .2 8
1562

o.4o

Do
Do

4 .1 2
1*31

2.28
586

2 .8 0
796

4 .2 0
1025

4 .9 6
2124

2.08
1515

0 .1 8
815

Pho tosyn th ate

1*35

1*37

1 .2 8

I .5 6

1*91

1 .4 7

0 .2 9

Light I n te n s ity

321

577

830

990

1024

679

279

June 16
June 17

Average

815

T&ble 12*

Late
A p ril 7

-3 1-

Photosynthate and Light I n t e n s it ie s fo r P la n ts Shaded
w ith 2 Layers o f C heesecloth

V ariables
Phot osynthat e
Light In te n s ity

5-8
8-10
A*M*

Period of Bay
10-12 12-2
2-1*
P.M.

1+-6

6-8

0 .5 2
*7

0 .4 4
92

1*12
104

.00
l l *8

2 .0 8
96

1 . 2i*
**5

0 .3 2
16

A pril 2*1

Lo
Do

0*20
92

0 .4 4
341

0 .7 2
354

1 .2 4
68U

2 .6 0
578

2.68
257

0.1*0
26

A p ril 29

Lo
Lo

0*20
i4 i

1*32
371

1.68
600

1 .3 2
661

1 .2 0
1*07

1 .6 8
303

0 . 1*0
12l*

May 5

Lo
Lo

0*16
109

0*12
116

0 .6 0
101

1 .1 6
75

1 .2 4
72

1.00
37

0 .3 2
2l+

May 13

Lo
Lo

0*46

0*32
123

O.3 6
99

O.7 6
384

0.21*
561

0 . 21*
61*7

0 .2 8
90

May l 4

Lo
Lo

0 .3 6
361

0*20
683

0 .6 4
630

O.72
754

0.1*1+
1*56

0 .5 6
106

0 .2 6
2l*

May 23

Lo
Lo

0*68
337

1*56
491

0 .2 8
573

1 .0 8
602

2.80
520

1.1*1*
297

0 . 1*0
152

May 2*1*

Lo
Lo

1*12
113

0*96
225

1 .28
552

0 .7 2
519

2 .7 6
635

1 . 0U
225

0 .3 2
ll*7

June 9

Lo
Lo

0*60
185

0*06
386

0 .1 6
548

0.81*
760

0 .8 8
680

1.1*8
230

O.5 6
155

June 13

Lo
Lo

0*20

0*86
90

o.4o 3 .20
91

85

1 . 1*0
238

0 .7 2
213

0 .2 8
107

June 16

Lo
Lo

0*60

4o

0*37
490

0 .2 0
937

3«2l*
951

1 .3 6
91*6

I .3 6
744

0 .1 6
521

June 17

Lo
Lo

4*20
261

2.52
276

1 .0 4
1*67

I .S 3
52*4-

i+.oi*
1080

1 .3 2
7Si*

0 .1 0
521

Phot 0 synthat e

0*77

0*76

0 .7 0

I . 3U

1 .7 5

1.23

0 .32

Light In te n s ity

154

307

1*21

512

522

321*

158

Average

4o

-3 2 The source o f energy fo r th e p la n t world i s su n lig h t
and i t e v id e n tly r e g u la te s th e amount o f p lant food manu­
factu red according to i t s in te n s ity a s i s demonstrated in
Tables 10, 11, and 12*

I t appears th at t h is carbon a s sim ila tio n

changes grad u ally or v io le n t ly in r e la tio n to th e v a r ia tio n
in lig h t in te n s ity receiv ed by th e plants*
The unshaded p la n ts show that a g rea ter amount o f
lig h t i s n ecessary fo r each gram o f photosynthate manufactured*
Furthermore, the smounts o f p la n t food appear to in crease u n t il
12-2 P*H* when th e lig h t in te n s ity reaches i t maximum and then
decreases a t a r e la t iv e ly sim ila r r a te w ith the lig h t*

D iffe r ­

in g from t h is , th e p la n ts shaded show a slower in crease in
food manufacture r e la t iv e to the l i ^ i t in crease u n t il th e 2-4 P*M*
p eriod , when they reach the maximum, and then they decrease more
rap id ly in r a t io w ith the lig h t in te n sity *

Greater reduction in

lig h t shows a more gradual in crease in p hotosyn th esis and th ere
appears to be an accumulation o f p lant food over a longer p eriod ,
or a la g g in g in photosynthate manufacture, when the lig h t i s
decreased due to heavy shading*

This appears to r e s u lt in th e

p la n ts exposed only to lig h t o f low in t e n s it i e s having a much
lower b a sa l metabolism than no shade p lan ts*

The simple

c o e f f ic ie n t s o f c o r r e la tio n fo r photosynthate and lig h t s
r = .5454 i .05275 r « .3012 ± .0681 and r ■ .3034 i. .0679
demonstrate the importance o f the lig h t to th e plant food

-3 3 manufacture under the no shade con d itio n as compared w ith
shaded p lan ts*
While temperature and hum idity a re sim ila r or r e la t iv e ly
uniform fo r th e three l o t s of p la n ts, i t v a ried the same fo r
each, a s the day advanced*

N atu rally, t h is would he expected

to he tru e, due to th e ir r e la tio n s h ip to lig h t in te n s ity and
i t s v a ria tio n *

Consequently, data on temperature and humidity

were taken fo r each of the two hour periods*

The sim ple

c o e f f ic ie n t s of c o r r e la tio n fo r photosynthate and temperature:
r 3 .2968 ± .0 6 81; r . .1924 * .0725; and r * .1704 * .0727
s ig n if y th at th e temperature i s in c lo s e r e la tio n w ith the
l i g h t in te n s ity and probably i s in t ere or r e la te d w ith it *

Their

b ein g lower than those fo r lig h t demonstrates th e ir s lig h t e r
importance in photo sy n th etic a c tiv ity *
The sim ple c o r r e la tio n c o e f f ic ie n t s fo r photosynthate
and hum idity: r - -*2099 * *0714; r = -*4955 - *0565• an?i
r s -*3377 * *0663 in d ic a te s that humidity i s p o ssib ly too high
fo r proper p lant fo o d manufacture*

Their n egative character

s i g n i f i e s th a t th e high hum idity might have a tendency to hinder
p h otosy n th esis and the higher the n egative c o r re la tio n th e
greater i t i s reduced*

This appears to be one of the con trib u tin g

causes o f low er p lan t food manufacture when the p la n ts are shaded*
C orrelation C o efficie n ts*
In order to measure th e d irect e f f e c t o f lig h t in te n s ity

-3 4 on p h o to sy n th esis, i t i s n ecessary to know how much the other
environmental fa c to r s a f f e c t p h otosyn th esis and the r e la t io n
between a l l th e se fa cto rs*

I t appears that the true valu e o f

t h is r e la tio n s h ip cannot be obtained d ir e c t ly from the raw
fig u r e s , but an a n a ly s is o f the data must be completed in
order to determine th e numerical measurements*

This a n a ly sis

w i l l show the r e la t iv e importance o f th e v a r ia tio n in each
of th e se independent v a r ia b le s on the v a r ia tio n in the
dependent v a r ia b le and can b est be demonstrated by the
c o r r e la tio n c o e f f ic ie n t s given in Table 13*

=55-

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-3 6 A*

No Shade P lants*
The zero order c o e f f ic ie n t s seem to show a much

greater r e la tio n s h ip between Xg and X^ than between X^
and X^or Xjj, and X^»

The r e la tio n s h ip between X-j and X^

and X^ and X^ are q u estion ab le because o f th e p o s s ib ili t y
o f th e ir b ein g obscured by th e r e la tio n s h ip s between the
independent variab les*

Because o f t h is we have separated

the e f f e c t s o f the independent v a ria b le s in order to g et the
f ilt s t order c o e ffic ie n ts *

This separation tends to confirm the

te n ta tiv e con clu sion s reached w ith the zero order c o e ffic ie n ts *
(That th e major r e la tio n s h ip i s that between X^ and X^)*

The

con clu sion s appear to be s t i l l s lig h t ly questionable because
only two o f th e independent v a ria b le s have been considered at
a time*

Because o f t h is we s h if t to the second order

c o e ffic ie n ts *

This demonstrates th at when we consider the

e f f e c t o f v a r ia tio n in lig h t in te n s ity alone (both temperature
and hum idity being constant)* we can ex p la in 2

7

of th e

v a r ia tio n in photosynthate* w h ile v a r ia tio n in temperature exp lain s
but 1$ and v a r ia tio n in hum idity ex p la in s but
synthate v a ria tio n *

of the photo—

The c o e f f ic ie n t o f m u ltip le c o r r e la tio n

shows th a t th e th ree fa c to r s taken togeth er ex p la in 32**$ o f
th e photosynthate v a ria tio n *

L i^ it in te n s ity alone* as we have

Been* accounts fo r 27*7$► showing that temperature and humidity
are n e g lib le fa c to r s in p hotosyn th esis except where they are
co rrela ted w ith lig h t in te n sity *

B*

-3 7 One Layer o f C heesecloth Plants*
I t appears that th e humidity i s to o h igh fo r proper

u se o f lig h t by th e p lan t in p hotosyn th esis*

The humidity

i s c o n s is te n tly higher in t h is b lock o f p la n ts than in th e
other blocks*

(Table 9)*

This may account fo r th e red u ction

in photosynthate as compared w ith th e no shade block o f
p lan ts*

The c o r r e la tio n c o e f f ic ie n t s demonstrate t h is f a c t

in every case*
C*

Two Layers o f C heesecloth P lants*
The hum idity i s e v id e n tly too h ig h fo r the p la n ts

to u t i l i z e lig h t a t the b e st advantage*

I t appears th a t lig h t

in te n s ity i s p o ssib ly too low even a t th e b e st fo r proper
food manufacture*

The t o t a l e f f e c t o f lig h t* temperature and

humidity in t h is b lock o f p la n ts explained only 13*9

o f the

photosynthate v a r ia tio n which i s about o n e-h a lf that of the
no shade p lan ts*

This demonstrates th a t some other factors*

th at were not taken in to consideration* probably have a
d e f in it e e f f e c t on the photo sy n th etic a c tiv ity *

Supplementary C onsideration:

-3S-

Chlorophyll Content*

The r e la t io n of ra d ia tio n to pigm entation i s of very
great importance through the n e c e s s ity of lig h t fo r the
form ation o f pigments* and due to the f a c t th a t the pigments
absorb rad ian t energy which i s e s s e n tia l fo r the photo sy n th e tic
a c t iv it y o f th e p la n ts*

The n a tu ra lly occuring plant pigments*

which a re found in th e c e l l stru ctu re of th e p lan t fo lia g e are
chlorophyll* c a r o tin and xanthophyll*

These pigments per u n it

le a f area were determined fo r each group of plants* a t sev era l
p eriod s during th e experiment*

The m odified W illst& tter and

S t o ll method o f e x tr a c tio n (22) was used f o r th ese determ inations
and th e comparison w ith a standard was made w ith the DuBosc
Colorimeter*
The data are given in Table l4*

-39"
Table l 4 .

F o lia g e Pigments
No Shade

A p ril 15
A pril 15
May 10
May 10
June 17
June 17
June 21
June 21

.O36S
•0179

1 1 Laver C heesecloth
!2 Lavers Cheesecloth
Chlorophyll - Men. per s o • cm*

•0391

.0255
.0257
.0353
.0377
.0309
.0392
.0302
.0300

.0189
.0189
. 0111+
.0125
.0110
.0177
.0187
.0186

T otal
Average

. 31*55
. 01+32

. 251+5
.0318

.1277
.0159

A pril 15
A pril 15
May 10
May 10
June 17
June 17
June 21
June 21

.0024
.0028
.0020
. 0021+

.0015
. 0011+

.0008
.0008

.0028
.0028

.0017
. 0011+

.0010
.0012

Total
Average

.0132
.0023

.0060
.0015

.0038
.0009

•0477

.0463
.0474

.0437
.o4oi

Xanthochyll - lion, per s a . cm.

Carotin - Urns, per so • cm.
.OOlU
.0012
.0011
.0011

A pril 15
A pril 15
May 10
May 10
June 17
June 17
June 21
June 21

. 001U
.0017
. 0011+
.0011
.0013
.0012

.0011
.0010

.0006
.0006

Total
Average

.0081
.0013

.0069
.0011

.ooi+i+

.0010
.0010
.0006
.0006

.0007

bo­
l t appears that lig h t in te n s ity i s e s s e n tia l fo r
—

proper chromogenesis in th e tomato fo lia g e *

According to

ex p ecta tio n s, the no shade p la n ts con tain more ch lorop h yll
per square centim eter in le a f area, and th e shaded p la n ts
showed a v a r ia tio n according to the amount of lig h t the
p la n ts received *

She red u ction in photosynthate manufacture

due to shading i s r e la t iv e ly p rop ortional to the amount of
ch lorop h yll per square centim eter o f le a f area, although
t h is red u ction does not a f f e c t the ch lorop h yll e f f ic ie n c y ,
hut i t does appear to a f f e c t the t o t a l p la n t food manufacture*
Leaf Anatomy, - She in tern a l l e a f stru ctu re i s , in
g en era l, an adaptation to the co n d itio n s necessary fo r
photosynthesis*

Consequently, the s i x p la te s demonstrate

the m o d ifica tio n s o f p a lisa d e and spongy parenchyma that
a s s i s t in th e p lant food manufacture under the e x is tin g
shaded con d itions*

Explanation of Plates

P la te I*

A cro ss s e c tio n o f one o f th e lea v es growing w ith
no shade* This se c tio n was made A pril 30* The
c e l l development under t h is co n d itio n o f lig h t has
elongated p a lisa d e c e l l s , heavy epidermis and c u t ic le
and a th ic k mass o f spongy parenchyma c e l l s w ith a
rath er sm all amount o f a ir space*

P la te II* A cr o ss s e c tio n o f one o f the le a v es growing w ith
no shade* This s e c tio n was made June 17*. The
p a lisa d e c e l l s have become much more elongated ,
ch lo r o p la sts appear to have changed, the spongy
parenchyma c e l l s are decreased and the a ir spaces
have become more p le n tifu l* This le a f apparently
shows the e f f e c t s of age*
P la te III* A cro ss se c tio n o f one of the lea v es growing under
one la y er o f cheesecloth* This se c tio n was made
A p ril 30* The p a lisa d e c e l l s are somewhat elongated,
ch lo ro p la st are arranged f u l l len gth o f them, spongy
parenchyma c e l l s are rath er sc a tte r e d , w ith a la r g e
amount o f a ir space throughout the le a f*
P la te IT* A cr o ss s e c tio n o f one o f the lea v es growing under
one la y er o f ch eesecloth* This se c tio n was made
June 17* The epidermis and c u t ic le has in creased
as th e le a v e s become older* The p a lisa d e la y er re­
mains about th e same, but the spongy parenchyma c e l l s
have Increased in number as the p lant g e ts older*
P la te V*

A cro ss s e c tio n o f one o f th e le a v es growing under
two la y e r s o f ch eesecloth* This se c tio n was made
A pril 30* The epiderm is, c u t ic le , and in f a c t ,
th e e n tir e le a f appears rath er thin* The p a lisa d e
la y er o f c e l l s i s somewhat p oorly organized and not
elongated as when the le a v e s r e c e iv e a higher degree
o f lig h t in te n s ity * The spongy parenchyma c e l l s are
f a i r l y w e ll developed w ith a la r g e amount o f a ir
space between them* The ch lo ro p la st are in much
sm aller number than as shown in the previous p la te s*

P la te YI* A cro ss s e c tio n o f one o f the le a v es growing under
two la y e r s o f ch eesecloth* This se c tio n waB made
June 17* The c u t ic le , and epiderm is have in creased
somewhat in th ic k n ess, but th e gen eral th ick n ess o f
the l e a f remains about th e same as the p reviou s p la te*
The p a lisa d e c e i l s have became somewhat more elongated
a s th e le a f g e ts old er and the a ir space seems to
have in creased in quantity*

■>

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Plate V.

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Plate VI* 250x

In general* the c e l l development o f the le a v es under
the no shade con d ition i s normal, hut when shaded the c e l l s
in th e spongy parenchyma la c k r e g u la r ity in shape and are
arranged lo o sely * so th a t a la rg e part o f t h e ir surface i s
exposed to th e in te r c e llu la r spaces*

The g rea ter the shade*

the l e s s the p a lisa d e parenchyma c e l l s are developed*

This

demonstrates how the number of p a lisa d e la y e r s and th e d en sity
o f the c e l l stru ctu re depends largely* e ith e r d ir e c tly or
in d irectly * upon lig h t in te n sity *
These supplementary fa c to r s are rather d e f in it e ly
regu lated by th e amount o f lig h t receiv ed by th e plants* and
t h is appears to be in order w ith the v a r ia tio n o f photo sy n th etic
a c tiv ity *

V hen the p la n ts are exposed to the no shade con d ition

the p a lisa d e c e l l s are w e ll developed and t h e ir ch lo ro p la sts
seem to arrange them selves so as to decrease the su rface and
tra n sp ira tio n due to t he lig h t* but when shaded they are
d iff e r e n tly arranged so as to in crease the surface fo r r e ce iv in g
lig h t*

This la t t e r arrangement appears to in crease th e ch lo ro-

p la sts* e ffic ie n c y * and the greater the lig h t reduction the
more i t i s increased*

I t appears th at the reduction in photo**

sy n th etic r a te did not have an e f f e c t on the ch lo ro p la st
e ffic ie n c y *

As p rev io u sly stated* th e lig h t seems to have a

regu latory e f f e c t on the ch lorop last content and c e l l stru ctu re
of the leaves* and t h is i s one o f the con trib u tin g causes fo r a
decreased photo sy n th e tic a c t iv it y by the p la n ts when shaded*
* The c e l l s o f the upper part o f t h e th ic k le a f in the
no shade group removes enough red and v io le t lig h t
rays to reduce the e ffe c tiv e n e s s on th e lower le a f c e lls *
but t h is blocking e f f e c t i s not as apparent in th e th in
shaded leaves*

-U6DISCUSSION
This study has d e a lt p rim arily w ith the in flu en ce o f
lig h t in t e n s it y on photo sy n th etic a c t iv it y o f tomato p lan t
le a v e s, as measured by amount of growth, f r u i t production, and
in crea ses in fr e sh and dry weights*

The r e s u lt s show, a s would

be expected, th a t on th e whole there i s a c lo s e r e la tio n s h ip
between th ese sev era l fa c to r s , v iz i w ith decreased lig h t
in te n s ity there i s i (a ) greater v e g e ta tiv e growth, as measured
by le a f area, and both fr e s h and dry w eig h t o f tops and ro o ts,
(b) decreased f r u it production, and (c ) a decrease in th e toted
amount o f photosynthate produced by the plants*

However, the

in crea se in v e g e ta tiv e growth and the d ecreases in f r u it pro­
duction and t o t a l photosynthate produced are not d ir e c t ly pro­
p o rtio n a l to the decreases in lig h t in te n sity *

Thus, reducing

lig h t in te n s ity by a h a lf r e su lte d in only approximately a onefou rth in crea se in emount o f v e g e ta tiv e growth, a o n e-th ird
decrease in f r u it production, and a o n e-six th decrease in t o ta l
photosynthate production*

Beducing lig h t in te n s ity to

approximately one-fourth normal r e su lte d in only a Uo percent
in crea se in v e g e ta tiv e growth, a one-half decrease in f r u it
production and a on e-th ird decrease in t o t a l photosynthate pro­
duction*

(Table 15)*

This i s but another way o f saying that

the p art le d ly shaded le a v es used th e ir lim ite d supply o f lig h t
more e f f i c i e n t l y than the unshaded le a v e s used th e ir noxmal
supply*

That i s , a given quantity o f lig h t e ffe c te d a greater

-47-

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-Hgphotosynthate production in th e case o f th e shaded p la n ts
than was tru e in the ca se o f th o se unshaded*
Great, however, a s were th e d iffe r e n c e s between th e
growth r a t e s , le a f a re a s, and f r u it and photosynthate pro*
duct ion o f the sev era l groups of p la n ts exposed to the d iffe r e n t
lig h t in t e n s i t i e s , th ere were eq u ally great d iffe r e n c e s , between
d iffe r e n t p la n ts w ith in t h e same group, in th e ir apparent a b ilit y
to u t i l i z e th e ir lig h t supply fo r f r u it and photosynthate pro­
duction*

Indeed, some o f the in d iv id u a ls &g* No* 11) in the

m oderately shaded group produced n early as iroch photosynthate
per u n it o f le a f area as some o f those in th e unshaded group
and one o f th ose in the h e a v ily shaded group (No* l l ) produced
nearly as much photosynthate per u n it o f le a f area as the
average o f th ose in the m oderately shaded group and w ith in 30
percent as much as some of the le a s t e f f ic ie n t in the unshaded
group (Table 15)*
This la t t e r fa c t i s of e sp e c ia l s ig n ific a n c e fo r i t
su ggests the p o s s ib ili t y of developing a s tr a in o f p la n ts that
has a high degree o f p h otosyn th etic e f f ic ie n c y under co n d ition s
o f low lig h t in te n sity *

Obviously, the producer o f indoor-grown

tomatoes has no con trol over lig h t in te n s ity * a t le a s t he has
no p r a c tic a b le means o f in creasin g it *

However, i f he can obtain

which

th a t^ is e s p e c ia lly adapted to the low lig h t in t e n s it ie s and short
days o f the northern w inter season, a su b sta n tia l con trib u tion
w i l l have been made to thse so lu tio n o f the problem of p r o fita b ly

-^9growing tomatoes in th e greenhouse during the winter*
L i t t l e or no e f f o r t has thus fa r been d ire cted toward de­
v elo p in g such a p h y sio lo g ic a l s tr a in o f tomatoes, p resent
stocks apparently being heterozygous in t h is respect*
The stu d ie s here reported p oin t c le a r ly to some o f the
p o s s i b i l i t i e s that l i e in th is d irectio n *
SGMMABY
The e f f e c t o f lig h t in te n s ity on the p hotosyn th etic
e f f ic ie n c y o f tomato p la n ts was stu d ied by growing Grand
Bapids Forcing tomato p la n ts under three d iffe r e n t d a ily
average lig h t in t e n s it ie s of 1139*3 * 5S3*1* and 261*0 fo o t
candles*
1*

The r e s u lts were as fo llo w s:
The responses in stem elo n g a tio n and le a f area

expansion were both continuously and f in a l ly the g rea ter when
the lig h t in te n s ity was reduced, showing a n egative rela tio n sh ip *
2#

I t was in d ica ted that when the lig h t in te n s ity reached

a d e f in it e average the f r u it would s e t rather fr e e ly and develop*
of

3*

The per cent agesAdry m atter, ash m a teria l, w ater, fresh

weight and elaborated food m a teria ls c o r r e la te rather c lo s e ly w ith
the lig h t in te n s ity r e ce iv ed by the plants*

Light in te n s ity

v a r ia tio n i s the c h ie f cause o f d iffe r e n c e s in plant e ffic ie n c y *
H# B asal p lant metabolism and i t s con trib u tin g fa c to r s
are reg u la ted by the amount of lig h t re ce iv ed by the p lants*
3*

The increase in th e m u ltip le co r r e la tio n s (when the

elaborated food m a teria ls are the dependent v a ria b le and lig h t

-5 0 in te n s ity , h u m ility , and temperature are the independent
v a r ia b le s) over the sim ple c o r r e la tio n s under each degree of
lig h t in te n s ity i s evidence that there i s in te r r e la tio n between
fa c to r s r e g u la tin g the p lant food manufacture*

The c o e f f ic ie n t s

o f determ ination demonstrate that lig h t in te n s ity alone accounts
fo r 32**$ of the photosynthate v a r ia tio n and that temperature
and humidity are n e g lib le fa c to r s only when co rrela ted w ith
lig h t in te n s ity , - humidity becoming a c r i t i c a l fa c to r in
p h otosyn th esis when the lig h t in te n s ity i s reduced*
6*

The lig h t In te n s ity appeared to have a regu latory

e f f e c t on th e average amounts o f ch lorop h yll per square meter
o f l e a f area*

The ch lo r o p la sts in th e le a v e s arranged them­

s e lv e s so a s to g et the g r e a te st amount o f lig h t when i t was
reduced*
7*

The le a f anatomy shows abnormal c e l l development

when th e p la n ts are shaded*

This abnormality c o n s is ts of

lo o s e ly arranged, irreg u la r spongy parenchyma c e l l s and a
reduction in s iz e , d en sity and number o f p a lisa d e c e lls *
S*

I t i s evident th a t lig h t in te n s ity averaging

1139*9 fo o t-c a n d le s d a ily during the growth o f the tomato
p la n ts had a grea ter e f f e c t in promoting ch lorop h yll form ation,
f r u it production and photo sy n th etic e f f ic ie n c y than lig h t o f
a d a ily average o f 933*1 fo o t-ca n d les and t h is in turn had a
sim ila r g rea ter e f f e c t than that on the p la n ts re c e iv in g a
d a ily average lig h t in te n s ity o f 261*0 fo o t-ca n d les*

-5 1 LITERATURE CITED

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Amer* Jour* Bot* 17*^16-^82*

1930*
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Determ ination des in te n s ite s lumineuses

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Ann* Sc I* Nat* 3ot»* 9 :ser* ; 11*

1910.

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Influence de la

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P a ris 19S* U67-70*
Emerson, Robert*
syn th esis*
Folmer, Swith*

Compt* rend* acad* sc i*

1919*

The ch lo ro p h y ll fa c to r in photo­
Amer* Nat* 6*4-5252-260*

Researches on the in flu en ce o f natural

and a r t i f i c i a l lig h t on p lants*
Landbruckshoi sk o le * 13 s1 -5 •
Hayden, A*

193^*

Meldinger Fra Norges

1933•

The e c o lo g ic f o lia g e anatomy o f some p la n ts

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1919*

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Amer* Jour*

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Smithsonian Misc* Coll*

8*

Kostytschew, S . und H*

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9*

Lubimenko, V*N*

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In flu en ce o f lig h t upon growth and

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P lant Physiology*

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Comparative anatomy of leaf of cycads

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