Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 117
I'HOTOSYNTHESIS1
WINSTON M. MANNING
Wisconsin Geological and Natural History Survey, Madison, Wisconsin
Received May 8§, 1988
I. INTRODUCTION
In this review major emphasis will be placed on the photochemical
and kinetic aspects of photosynthesis. The influence of the various
physiological factors is more fully discussed in other recent reviews of
photosynthesis (23, 24, 58, 721.
Except for studies of the physical and chemical properties of extracted
plant pigments, investigations of photosynthesis are practically limited
to a study of living plants. This limitation introduces complications not
found in non-biological photochemical studies. In the first place, other
chemical reactions, thermal as well as photochemical, undoubtedly occur
during a period of photosynthesis. Unless proper allowance is made,
these reactions may be of sufficient magnitude to obscure the significance
of the photosynthesis measurements. In the second place, variations in
factors influencing photosynthesis must be limited to those which will
not kill or seriously injure the plant during the time when measurements
are being made. In many cases, moreover, a variation in a particular
environmental (external) factor may influence photosynthesis indirectly
through its effect in the plant on internal factors other than those directly
connected with the photosynthetic mechanism. This is particularly true
for experiments with the higher plants.
Emerson (23) has discussed more fully some of the limitations involved
in photosynthesis research.
II. THE MEASUREMENT OF PHOTOSYNTHESIS '
Only a brief outline of experimental procedure will be given here.
More extensive descriptions of the earlier methods which have been em-
ployed may be found in the monographs of Spoehr (70) and Stiles (741.
Details of later procedures may be obtained from the original papers.
Photosynthesis is usually measured by determining either the amount
of carbon dioxide consumed or the amount of oxygen liberated, or both.
~ Contribution No. 11 to the Third Report of the Committee on Photochemistry,
National Research Council.
117
OCR for page 118
118
WINSTON M. MANNING
Less direct methods may also be used, such as the measurement of the
change in dry weight or sugar content of the plant material under investi-
gation. However, results so obtained may involve factors other than the
photosynthetic process.
The measurement of photosynthesis in land plants
With land plants photosynthesis may be measured by using either an
entire plant or only part of the plant (often a single leaf). When a single
leaf is studied, it is often detached from the rest of the plant, although
many recent investigators have worked with attached leaves (41, 59, 60~.
In either case the material to be investigated is usually enclosed in a light-
transm~tting chamber.2 The change in carbon dioxide or oxygen com-
position during a period of illumination or darkness may then be deter-
mined. In practice a flow method is often employed in order to maintain
a nearly constant carbon dioxide concentration in the reaction chamber.
The change in gas composition during a period of darkness gives a
measure of respiration. It is usually assumed that, for a given tempera-
ture, this process proceeds at the same rate in light as in darkness. A
correction for respiration is therefore made in calculating absolute photo-
synthetic rates.
The change in oxygen and carbon dioxide may be determined by gas
analysis (17), but other methods, in which only carbon dioxide is deter-
mined, are more frequently used.
In the conductivity method (43, 59, 60, 71, 76), the carbon dioxide is
absorbed by a solution of alkali at constant temperature (~ 0.01°C.~.
The conductivity of the hydroxide-carbonate mixture gives an accurate
measure of the amount of carbon dioxide absorbed. With this method a
continuous record of photosynthetic (or respiratory) rate may be obtained.
Thomas and Hill (76) constructed a chamber large enough to enclose a
field plot of wheat or alfalfa plants 6 feet square, and used the conducting ity
method to obtain a continuous record of photosynthesis and respiration,
the measurements sometimes extending over a period of several weeks.
Instead of measuring the conductivity of the hydroxide-carbonate
mixture, some investigators have titrated the mixture with standard acid
(41~. This method requires less apparatus than the conductivity method,
but it is not as suitable for continuous measurements.
McAlister (53) has recently developed a very sensitive spectrometric
method for determining carbon dioxide. This method is well adapted for
continuous measurements of photosynthesis in land plants. A closed
2 In single leaf studies the chamber may be attached to the under side of the leaf,
with the leaf forming part of the chamber wall (41, 60~. This permits nearly normal
air circulation and transpiration.
OCR for page 119
PHOTOSYNTHESIS
119
system is used, with the gas rapidly circulating through the plant chamber
and through an optical absorption tube. The absorption by the 4.2~.3,u
band of carbon dioxide is determined with a rock salt spectrograph and a
vacuum thermocouple. A notable characteristic of this method is its
rapid response to changes in photosynthetic rate.
Photosynthesis measurements in algae and other aquatic plants
Photosynthesis measurements in aquatic plants are usually carried out
with the plant material suspended in water. However, both van den
Honert (77) and van der Pasuw (78) have worked with moist films of
Hormidium (a very small filamentous blue-green alga) attached to glass
plates. The rate of oxygen evolution by the suspended plant material is
usually used as the criterion of photosynthetic rate.
The manometric method of Warburg (84) has been widely used for
measuring changes in oxygen concentration in experiments with aquatic
plants. In this method the plant material is suspended in a carbonate-
bicarbonate buffer mixture (usually about 0.1 molar), and placed in a
closed glass vessel connected to a manometer. The buffer concentration
is high enough to maintain a practically constant carbon dioxide partial
pressure in the gas phase, so that the increase in gas volume due to oxygen
evolution can be read directly on the manometer. Vigorous shaking is
necessary to maintain equilibrium between gas and liquid phases. With
some modifications the manometric method can be applied to measure-
ments carried out with an unbuffered nutrient solution (9, 85~. This
modification requires the use of two vessels and depends upon the difler-
ence in solubility of carbon dioxide and oxygen in the nutrient solution.
With this differential type of manometer both oxygen and carbon dioxide
changes can be measured.
Another method frequently employed, especially in ecological studies,
is the determination of dissolved oxygen in closed vessels by titration
(Winkler method). This method avoids the use of the somewhat non-
physiological buffer mixture and is much simpler than the differential
manometric method. However, it does not permit a series of measure-
ments on a single sample.
Petering and Daniels (62) have recently applied the dropping-mercury
electrode to a determination of dissolved oxygen changes in photosynthesis.
As in the manometric method, repeated measurements may be made on a
single sample of plant suspension. In at least two respects, this method
is superior to the usual manometric method. It does not necessitate the
use of buffer mixtures and it does not involve an equilibrium between gas
and liquid phases. In speed of response it compares favorably with the
spectrometric method of McAlister (53~.
OCR for page 120
120
WINSTON M. MANNING
III. GENERAL DESCRIPTION OF THE PlIOTOSYNT1 IETIC PROCESS
The equation for the reaction involved in photosynthesis in green plants
is often written as follows:
CO2 + H20 ~1/6 (C6H1206) + 02
i\H = 112,000 cal. (I)
This equation, when reversed, becomes the one usually used to represent
the normal respiratory process.
It is usually assumed that formaldehyde is the first product of photo-
synthesis, and that subsequent polymerization is responsible for the forma-
tion of glucose or other carbohydrates. If formaldehyde is the first
product, then HI for equation I becomes 134,000 cal. The subsequent
polymerization reaction will then be exothermic. The necessary energy
for photosynthesis is assumed to come from light absorbed by the two
chlorophyll pigments in the plant, although it is possible that energy
absorbed by other pigments, such as carotene and xanthophyll, may
sometimes be utilized.3 It is rather remarkable that light of wave length
as long as 7000 A. (see figure 2), for which Nhv is equal to 40,500 car., will
bring about a reaction for which HI is 112,000 cal. Evidently photo-
synthesis involves a more complex series of endothermic reactions than
has been observed for any non-biological photoreaction.
Photosynthesis at different wave lengths
Figures 1 and 2 show a rather close correlation between relative rates
of photosynthesis atdiBerent wave lengths (Hoover (42~) and the absorption
spectra (in ether solution) of chlorophylls a and b (93~. Hoover's results
were obtained with young wheat plants, using equal incident light intensi-
ties (less than 300 foot-candles) at the various wave lengths. A wheat
leaf contains enough chlorophyll to absorb a considerable fraction of the
incident radiation, even in the region between 5000 and 6000 A.; this
accounts for the relatively high minimum in the photosynthesis curve
(figure 2~. Chlorophyll absorbs more strongly at 3660 A. than in the
region between 5000 and 6000 A. (90~. Consequently the low rate of
3 Except in the Myxophyceae (blue-green algae) the chlorophyll pigments are found
only in restricted regions of the plant cell, known as chloroplasts. These regions
also contain the yellow pigments carotene and xanthophyll. Frequently a large
number of the cells in a green leaf contain no chloroplasts, while in the remaining
cells the number may vary from one to many. Consequently only a small fraction of
the total leaf volume consists of chloroplasts and hence is able to carry on photosyn-
thesis. In many of the algae, however, the chloroplasts constitute a much larger
fraction of the total plant material. For example, the single chloroplast in the uni-
cellular green alga Chlorella probably occupies half of the cell volume.
This large proportion of photosynthetically active plant material constitutes one
reason for the widespread use of Chlorella in photosynthesis investigations.
OCR for page 121
PHOTOSYNTHESIS
121
photosynthesis at 3660 A. (figure 2) indicates a much lower quantum
efficiency for the process than in the region frown 4000 to 7000 A. Below
3000 A. ultraviolet radiation is distinctly injurious (57, 4~.
of _
90
so
70 _
60
a
v
~ 50
A
40
3
2
IC
I1 1°
l
, .:
? 1
l
i
-I!
-' 1
~ l '
o ~
I ~ ~ ~1 1 1 _
o 0 o ~
° ~ ~ ~ ~ 8 ~ ~ ~ a ~ ~ 8 ~ ~ ~
~ ~ ·n X ~ ~ ~ O
WAVE LENGTH IN A.
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
o 0 = COuPONENT A
——~——~ = COUPONE NT B
11
l
,tl
ill
1
i
1
1
!
_, ~ ~
~ ~ I 1_
1 1 1 1 1
100
90
So
To
60
so
30
20
FIG. 1. The absorption spectra of chlorophylls a and b in ether (Zscheile (93~)
The formaldehyde hypothesis
The occurrence of formaldehyde as an intermediate product of photo-
synthesis has been neither proved nor disproved (589. In any event its
concentration must be very low, even during rapid photosynthesis, since
OCR for page 122
122
WINSTON M. MANNING
concentrations higher than a few hundredths of a per cent are distinctly
toxic. The presence of small amounts of formaldehyde in green leaves
has been reported by a number of investigators. Others have found that
plants in the dark can utilize low concentrations of formaldehyde to form
carbohydrates. These observations add plausibility to the hypothesis
that formaldehyde is an intermediate product in the photosynthetic
process, but do not constitute proof of the hypothesis, since formaldehyde
350
300
o
a
° 150
ID
if:
o
--1 ~ 1 ' 1 --- 1 '-'-
1 :~1
250 _
200 _
100 _
I' ~
r' 4\
x
1 1 1
3500 4500 5~0
WAVE LENGTH
6500 7500
FIG. 2. Rate of photosynthesis as a function of wave length (low light intensity).
As, the corrected form of the curve obtained with large Christianson filters; B4, the
corrected form of the curve obtained with small Christianson filters. Points marked
X are the results obtained with line filters and quartz mercury arc. (Hoover (42~.
may be produced by other metabolic processes. Also, formaldehyde is
only one of several organic compounds which may be utilized by plants in
the formation of carbohydrates (58~.
The photosynthetic quotient
The ratio of the number of moles of carbon dioxide absorbed to the
number of moles of oxygen produced, /`CO2/AO~, iS called the photosyn-
OCR for page 123
PHOTOSYNTHESIS
123
thetic quotient.4 If glucose or some other carbohydrate is the final prod-
uct of photosynthesis, the photosynthetic quotient should have a value
of 1, as in equation I. Maquenne and Demoussy (56) and Willstatter
and Stoll (89) investigated a large number of different plants and found a
quotient very close to unity. Apparent values markedly different from
unity are sometimes obtained, but these may often be attributed to the
effects of abnormal respiration or other processes.
The value of the photosynthetic quotient constitutes the principal proof
that a carbohydrate is the first product of photosynthesis. Many investi-
gators have attempted to identify this carbohydrate product. From
present evidence it may be considered probable that a hexose is the first
product formed, though the prior formation of sucrose remains as a
possibility.5
In diatoms (and also in some other plants) the products of photosyn-
thesis are stored principally in the form of oils rather than as carbohydrates.
Barker (10) has measured the photosynthetic quotient in two species of
diatoms in an effort to determine whether the oils are formed as primary
products of photosynthesis or whether they are secondary metabolic
products. For complete conversion of the carbon dioxide to oils, the
quotient should be approximately 0.70 instead of 1.00. For both species
of diatoms Barker found a quotient close to 0.95, with a tendency for the
value to increase slightly with increasing light intensity. He concluded
that 10 per cent or less of the photosynthetic products appeared as stored
fat and that this fat production was probably the result of a secondary
reaction.6
The eject of light intensity and temperature on the rate of photosynthesis
Figure 3 shows diagrammatically the relation between the rate of
photosynthesis and light intensity at high carbon dioxide concentration.
Curve A represents the behavior at a relatively low temperature, perhaps
10°C.; curve B is for a higher temperature, such as 20°C.
While the type of behavior shown in figure 3 is characteristic of most
4 Some writers apply the term "photosynthetic quotient" to the reciprocal of this
ratio, i.e., /~02/^CO2.
5 More complete discussions of this problem are given by Spoehr (reference 70,
page 215) and Stiles (reference 74, page 151~.
6 When a simple alga, such as Chlorella, is grown under long-continued constant
conditions, including constant illumination, it is probable that an equilibrium is
reached between the rate (per unit of plant material) at which food is manufactured
by photosynthesis and the rate at which it is used in growth. Under these conditions
the apparent photosynthetic quotient (i.e., uncorrected for respiration) should give
a measure of the average state of oxidation of the entire cell material. The oxygen
eliminated during the reduction of nitrates to protein material should contribute
noticeably to a lowering of the apparent quotient.
OCR for page 124
124
WINSTON M. MANNING
plants which have been studied, the exact numerical behavior may vary
widely. Thus, in many land plants the maximum photosynthetic rate
may not be reached until the light intensity approaches that of sunlight,
while in other plants, particularly some of the algae, one-tenth of this
intensity may produce the maximum rate.7
ma. . .
fine snape of a rate-nght Intensity curve will depend on the fraction of
light absorbed by the plant material under investigation. With a thick
leaf containing an abundance of chlorophyll, or with a dense suspension
of algae, different portions of the plant material will be exposed to widely
~0
-
s
I
o
IL
o
350
/
1/
I
LIGHT INT ENSITY _
FIG. 3
300
.5
~ USA
200
t56
A'
IN
5G
906 _ ~
' ~ \
, _ ~
- / ~
6 ,7 1 1
~7
em,oerolure, °C
FIG. 4
FIG. 3. Rate of photosynthesis as a function of light intensity. A, low tempera-
ture curve; B. high temperature curve.
FIG. 4. Variation in rate of photosynthesis with temperature at high light inten-
sities for N~tzschia closterium (O) and Niteschia palea (my. Photosynthesis is ex-
pressed in cubic millimeters of evolved oxygen per hour per 10 cmm. of cells. (Bar-
ker (10~.
different light intensities. A very high incident intensity will then be
necessary to produce a maximum photosynthetic rate in the chloroplasts
farthest removed from the incident surface.
As shown in figure 3, the rate of photosynthesis is nearly independent
of temperature at low light intensities, but becomes temperature-depend-
ent at higher intensities. Thus, for Chlorella Warburg (82) found a
temperature coefficient (Q1O) of unity at approximately 6 X 103 ergs per
7 In some plants the rate of photosynthesis may increase to a maximum and then
decrease again as the light intensity is increased (48, 54, 72). The reason for a de-
crease in rate at high intensities is not definitely known. It may be caused by some
form of injury, or it may be caused by the simultaneous occurrence of photooxidation
processes.
OCR for page 125
PHOTOSYNTHESIS
125
cm.2 per second,8 but a coefficient of from 2 to 5 (depending upon the
temperature) at an intensity of approximately 100 X 103 ergs per cm.2
per second.
From the type of behavior shown in figure 3, as well as from other evi-
dence to be discussed below, it must be concluded that photosynthesis is
a complex cyclic reaction, involving at least one thermal reaction, usually
known as the Blackman reaction,9 in addition to whatever photoreactions
are necessary. According to this view, at low light intensities, where the
rate of photosynthesis is approximately proportional to the intensity, the
time between the absorption of successive photons, by that portion of the
mechanism involved in the reduction of a single carbon dioxide molecule,
will be sufficient to permit the Blackman reaction to be completed. Light
is then said to be the limiting factor. But at high intensities, where the
rate has practically reached its maximum value (so-called light satura-
tion), the rate will presumably be limited only by the rate of the Black-
man reaction.~° This implies that photons are absorbed so rapidly by the
mechanism that the average time between the completion of the Black-
man reaction and the next light absorption will be very short compared
with the average time required for the Blackman reaction. An increase
in temperature will increase the rate of the Blackman reaction without
appreciably altering the rate of the light reaction.
s In this research Warburg made only approximate estimates of absolute inten-
. .
sties.
9 The term "Blackman reaction" as used in this paper refers to any reaction or
reactions which contribute to the temperature dependence of the photosynthetic
process. It is named for F. F. Blaekman, who, in 1905 (12), formulated his principle
of limiting factors in the following words: "When a process is conditioned as to its
rapidity by a number of separate factors, the rate of the process is limited by the pace
of the 'slowest' factor." Application of this principle did much to clarify the eon-
tradietory results of earlier research on photosynthesis.
Many subsequent investigators appear to have interpreted this principle too
literally, i.e., they have assumed that for measurements under ideal conditions only a
single factor could influence the rate of photosynthesis at any one time. According
to this view, an increase in the intensity of a particular variable should result in an
abrupt transition from direct dependence of the rate on this variable to complete
independence. Such a transition is contrary to any reasonable kinetic formulation
of the process.
t°Aeeording to a recent mechanism of Franck and Herzfeld (34), the rate of the
Blaekman reaction is not limiting at high light intensity (see page 850~.
~i Warburg and Uyesugi (87) and Yabusoe (92) found some similarities between the
Blackman reaction and the rate of decomposition of hydrogen peroxide by the en-
zyme eatalase. There has therefore been a widespread belief that the Blaekman
reaction consists of a reaction between eatalase and a peroxide. Hence many pos-
tulated mechanisms have included hydrogen peroxide or organic peroxides as inter-
mediate products in photosynthesis. However, Emerson and Green (28) have re-
eently made further comparisons of the two reactions without finding any significant
similarity.
.,
OCR for page 126
126
WINSTON M. MANNING
\
Figures 4 and 5 illustrate the effect of temperature on the rate of photo-
synthesis at high light intensity and high carbon dioxide concentration.
Figure 4 gives the results obtained by Barker (10) with two species of
diatoms; figure 5 gives the results obtained by Craig and Trelease (21)
with Chlorella vulgaris. The lower curves in figure 5 are for rate measure-
ments in 99.9 per cent heavy water. (The heavy water experiments will
be discussed below, page 828.) The decrease in photosynthetic rate at
high temperatures is probably due to injury to the plant material. Fur-
ther evidence of injury is found in the fact that the photosynthetic rate
at high temperatures is not constant, but decreases with time. Injury
may be due to deactivation of enzymes (or of an enzyme-producing
mechanism) connected with the Blackman reaction, or it may be due to a
change in the physical condition of the chloroplasts.
The Arrhenius equation may be used to calculate apparent activation
energies for the temperature-sensitive reaction in photosynthesis. How-
G O
O4-~;r T~
o ~ ! 1 1 1 1 1
26 34 42 30
38 46 30 38 'C
FIG. 5. Rate of photosynthesis as a function of temperature. For H20 shown by
circles with vertical bars, and for D20 shown by circles with horizontal bars. (Craig
and Trelease (21~.
ever, in view of the complex system involved, the significance of the figures
so obtained is doubtful. The apparent activation energy varies for difler-
ent species and decreases with increasing temperature. The low tem-
perature value for Chlorella pyrenoidosa and Hormidium flaccidum has
been reported as being approximately 20,000 cal. (27, 78~. Barker (10)
found a value of about 30,000 cal. for the diatoms Nitzschia closterium and
Nitzschia palea (see figure 4~. Emerson and Green (27) found a value of
approximately 50,000 cal. for Chlorella vulgaris and for the marine alga
Gigartina harveyana.
The effect of carbon dioxide concentration on the rate of
photosynthesis
Variation in carbon dioxide concentration apparently affects the rate
of photosynthesis in a manner similar to variation in light intensity. Van
den Honert (reference 77, page 225), working with moist films of Hormid-
OCR for page 127
PHOTOSYNTHESIS
127
tom at 12° and 20°C., and using high light intensity, obtained curves
essentially like those of figure 3 (with light intensity replaced by carbon
dioxide concentration). In both curves the rate of photosynthesis reached
its maximum value at a carbon dioxide concentration of about 0.04 per
cent by volume. (Normal carbon dioxide concentration in air is approxi-
mately 0.03 per cent.) For young wheat plants Hoover, Johnston, and
Brackett (43) found that a higher carbon dioxide concentration, approxi-
mately 0.15 per cent, was necessary for a maximum rate of photosynthesis
(light intensity one-fourth of that of sunlight). Light saturation was
not reached in this work; at higher light intensities still higher carbon
dioxide concentrations would have been required. In general, the dif-
fusion of carbon dioxide into a chloroplast will encounter more resistance
in a leaf, where most of the chloroplast-containing cells are usually some
distance from the leaf surface, than in a small unicellular or filamentous
alga, where every vegetative cell is photosynthetically active and not
surrounded by other cells. A higher carbon dioxide concentration will
then be necessary for leaves in order to overcome this diffusion resist-
ance.~2
For Chl~rella in liquid suspension Emerson and Green (29) found that
a carbon dioxide concentration of 10 or 15 X 10-6 moles per liter was
sufficient for maximum photosynthesis.
Smith (68), using the water plant Cabomba, has studied the effect of
light intensity and carbon dioxide concentration on the photosynthetic
rate. He found that both variables affected the rate in the same way.
However, there is disagreement concerning the effect of temperature at
low carbon dioxide concentrations. According to van den Honert's
curves (reference 77, page 225), temperature is without effect on the rate
at low concentrations, but Warburg (82) and Emerson (23) found a high
temperature coefficient for Chlorella at low carbon dioxide concentrations.
Both Warburg and Emerson used the manometric method in their measure-
ments; the Chlorella was suspended in carbonate-bicarbonate buffer
mixtures and the concentration of free carbon dioxide was varied by vary-
ing the ratio of carbonate to bicarbonate. At least three possible objec-
tions may be raised to the use of these buff ers for regulating carbon dioxide
concentration: (~) In varying the carbon dioxide concentration, the pH
is also varied. The high pH necessary to obtain limiting carbon dioxide
concentrations may influence the photosynthetic rate. (2) A high con-
centration of carbonate or bicarbonate ions may inhibit photosynthesis.
(~) It is possible that water plants may be able to use carbonate or bicar-
bonate in the photosynthetic process (3~. Emerson and Green (29), work-
ing with phosphate buffers, found that within the range from pH 4.6 to
8.9, neither bicarbonate nor hydrogen ion influenced the rate of photo-
72 The problem of diffusion resistance has been discussed by James (47~.
OCR for page 146
146
WINSTON M. MANNING
ing of fluorescence by oxygen to the utilization of chlorophyll excitation
energy in the process of photosynthesis. Kautsky and Hormuth (49)
suggested that oxygen forms an addition compound with some substance
in the chloroplast, and that this compound serves to transfer the chloro-
phyll excitation energy to carbonic acid, which is thereupon reduced to
formaldehyde or carbohydrate.
Kautsky's interpretation is open to question from several points of
view. In the first place, recent experiments by Gaffron (39), with
Chlorella, indicate that oxygen is not essential for photosynthesis. Gaf-
fron observed that when Chlorella was illuminated following 15 hr. in
darkness in a nitrogen atmosphere, carbon dioxide consumption proceeded
normally. However, oxygen production did not occur for some time,
presumably because of oxidation of excess intracellular fermentation
products.
Secondly, Kautsky used the 3660 A. line of mercury to excite fluores-
cence. This wave length is relatively inefficient for photosynthesis (see
page 818~..
Thirdly, as Emerson (23) has pointed out, a plant exposed to high light
intensity is using only a small fraction of the absorbed energy in the
photosynthetic process. Consequently, changes in fluorescent intensity
might occur quite independently of the photosynthetic process.
Franck and Wood (36), while not considering oxygen as necessary for
photosynthesis, concluded that there is a close connection between the
processes of photosynthesis and fluorescence. They interpreted the vari-
ation in fluorescent intensity as being due to the formation and disap-
pearance of a strongly fluorescent radical.
According to the mechanism of Kautsky and Hormuth (49), it would
appear that, at low light intensities, the percentage yield of fluorescence
should increase with increasing intensity of the incident light, approach-
ing a maximum value after the attainment of a maximum photosynthetic
rate. If this type of relation could actually be established, it would
constitute strong evidence for a connection between photosynthesis and
fluorescence.
The relation between chlorophyll concentration and the maximum rate of
photosynthesis
Emerson (22) and Fleischer (33) have studied the effect of varying
chlorophyll concentration on the rate of photosynthesis in Chlorella.
Emerson grew Chlorella in a nutrient solution containing glucose, and
varied the chlorophyll concentration by varying the concentration of
iron in the nutrient solution. Fleischer varied the chlorophyll concentra-
tion in three different ways. In some of his series the iron concentration
was varied, while in other series the magnesium or nitrogen concen-
OCR for page 147
PHOTOSYNTHESIS
147
"rations were varied. Except for samples deficient in magnesium, Emer-
son and Fleischer both found that, at high light intensity and abundant
carbon dioxide concentration, the rate of photosynthesis per unit volume
of cells was approximately proportional to the amount of chlorophyll.
When chlorophyll deficiency was caused by insufficient magnesium,
Fleischer found an abnormally low rate of photosynthesis, and conse-
quently concluded that magnesium influences the process of photosyn-
thesis in some way other than by its influence on chlorophyll production.
Emerson and Arnold (26) have studied the relation in Chlorella between
chlorophyll content and the rate of photosynthesis in intermittent light
of high intensity. In these experiments the chlorophyll content was
varied by varying the light conditions under which the Chlorella was cul-
tured. As in previous work (25), they used light flashes which lasted only
about 10-5 sec., a period which is short compared with the duration of the
dark or Blackman reaction (see page 827~. Sufficient time was allowed
between flashes for completion of the dark reaction, so that light saturation
in their intermittent light experiments implied, not that the rate of the
dark reaction was limiting, as it is in the case of saturation with continuous
light, but that the photochemical mechanism was saturated. Under
these conditions the maximum rate was again found to be approximately
proportional to the chlorophyll concentration. Emerson and Arnold
calculated the ratio between the total number of chlorophyll molecules
present in the reaction vessel and the number of carbon dioxide molecules
reduced per single light flash. Instead of finding a value of 1 (or of 3
or 4 or 5, as would be expected in case a series of three or four or five al-
ternate photochemical and thermal reactions were required for the photo-
synthetic cycle), they found a value of approximately 2500. This figure
represented the average for all chlorophyll concentrations; its value was
apparently independent of chlorophyll concentration.
In similar ex-
,
periments Kohn (52) found that light saturation was reached when only
about one chlorophyll molecule in a hundred absorbed one or more quanta
in a single flash. Arnold and Kohn (6) have determined the minimum
value of the ratio of chlorophyll molecules to carbon dioxide molecules
reduced per flash for several different species in different divisions of the
plant kingdom. They found the value to lie between 2000 and 4000 in
all cases.
Emerson (23, 24) has suggested that carbon dioxide probably does not
combine with chlorophyll prior to the photoreaction, and that the ratio
of about 2500 probably represents the ratio between chlorophyll and some
other internal factor, perhaps the substance which combines with carbon
dioxide. However, according to this mechanism, one would expect a
very low quantum efficiency, even with low intensity continuous light.
The high efficiency values reported by Warburg and Negelein (86), or
OCR for page 148
148 WINSTON M. MANNING
even those found by Manning, Staufler, Duggar, and Daniels (55), would
thus be very difficult to explain.
The photosynthetic Unit
One way of reconciling the small yield of carbon dioxide reduced per
flash with the relatively high quantum efficiencies observed at low light
intensities is to assume that light absorbed by a large number of chloro-
phyll molecules can be made available to a single carbon dioxide molecule.
This assumption implies (unless one postulates an extremely long life for
excited chlorophyll) that, in the chloroplast, the chlorophyll molecules
are distributed in the form of groups or units, each containing a large num-
ber of individual molecules. Arnold and Kohn (6), Gaffron and Wohl
(40), and Weiss (88) have favored this interpretation. The postulated
size of such a unit would depend upon the number of successive photo-
chemical reactions which are required to reduce one carbon dioxide
molecule, and also on the number of carbon dioxide molecules associated
with each group. Weiss (88) has suggested that a single unit may have
carbon dioxide molecules adsorbed on the surface, with one carbon dioxide
molecule to each surface molecule of chlorophyll. Assuming four succes-
sive photoreactions, it would thus be necessary to have 500 or 600 mole-
cules in the interior for each surface molecule of chlorophyll. Weiss
calculated that, if such a unit were spherical, its radius should be of the
order of 0.4~. Globules of this general size have been observed in photo-
graphs of chloroplasts (44~. However, it is difficult to conceive of energy
transmission through a unit of this magnitude without enormous losses
in the form of heat.
Gaflron and Wohl (40) obtained additional support for the existence of
of a photosynthetic unit from approximate calculations in which the data
of numerous investigators were used. They found that the maximum
photosynthetic rate (light saturation) corresponded to the reduction of
one carbon dioxide molecule by each chlorophyll molecule every 10 or
20 sec. This period is approximately 1000 times as long as the average
time for the dark or Blackman reaction (25, 26~. Assuming a single
dark reaction in each cycle, this would indicate a unit of about 1000
chlorophyll molecules for each carbon dioxide molecule.
It has also been suggested (34, 91) that the existence of a photosyn-
thetic unit would account for the absence of a long induction period at
low light intensities. If a single chlorophyll molecule were obliged to
absorb four or more quanta before oxygen could be liberated, a long
induction period would be required at low light intensities. But, if a
unit of 500 or 1000 chlorophyll molecules were available for the reduction
of each carbon dioxide molecule, the necessary quanta would be absorbed
OCR for page 149
PHOTOSYNTHESIS
149
within a second or two, even at the lowest intensities that have been
used in studies of photosynthesis.
If the process of photosynthesis involves a series of photochemical
reactions, any thermal decomposition of intermediate products would
result in a lowering of the overall efficiency of the process. If one or more
of t.~.~n intermediate products were sufficiently unstable to decompose
· 1 ~ ·1 1 · _ ~ _1 _~ ~ ~ AL ~~ 1~~ d CAN ~ ~1~ ~ Am ~~
appreciably wlt~m a period of a minute or less, men a plant SIlOUl~ O~
greatly benefited at low light intensities by the cooperation of more than
one chlorophyll molecule in the reduction of a carbon dioxide molecule,
-
since the time between successive photoreactions would be shortened in
proportion to the number of chlorophyll molecules per unit.
As Gaflron and Wohl (40) have pointed out, a variation in the size of
the photosynthetic unit might account for the phenomenon of light
adaptation, by whICh plants of the same species may be conditioned to
efficient use of either high or low light intensities. A plant accustomed to
high intensities will often photosynthesize more rapidly in bright light
than will one accustomed to low intensities, and vice versa.
=7 ~
A large num-
ber of small units would permit more rapid photosynthesis in bright light,
while, with weak light, large units would serve to minimize the possible
losses due to decomposition of intermediates.
Thermal decomposition of intermediate products might account for
the type of behavior shown at low light intensities by the species of purple
bacteria which French (37) used in his measurements of quantum e~-
ciency (see page 838~. At low light intensities French's measurements
indicated a decreasing efficiency as intensity decreased. If thermal de-
composition of intermediates was responsible for this behavior, it would
be expected that an increase in temperature should increase the rate of
decomposition, and thus cause the diminished efficiency to occur at higher
light intensities. This effect was actually observed by French (reference
37, figure 5~. However, it should be remembered that the change in AH
is small for the reaction studied by French. The only reason for assum-
ing a series of photoreactions for the process is its apparent similarity to
green plant photosynthesis.
Perhaps the strongest argument against the existence of a photosyn-
thetic unit is the difficulty involved in picturing a model which would
permit an efficient transfer of energy from every region where visible
light could be absorbed to the point where it could be utilized for carbon
dioxide reduction.
Franck and Herzfeld (34) considered that the fluorescence of chloro-
phyll in living plants is an indication of unimolecular dispersion. Since
colloidal chlorophyll does not fluoresce in vitro, they argue that chloro-
phyll in units of 500 or more should not fluoresce in the chloroplast.
OCR for page 150
150
WINSTON M. MANNING
However, the very low intensity of fluorescence in vivo weakens the
argument. Rabinowitchi~ has suggested that the weak chlorophyll
fluorescence observed in living material may be emitted by a few "mis-
placed" chlorophyll molecules which may play no part in photosynthesis.
Wohl (91) has discussed in detail various arguments favoring the
existence of a photosynthetic unit. He also has suggested several ex-
periments with intermittent light which might add to our knowledge of the
photoprocess involved in photosynthesis.
The existence of a photosynthetic unit has thus far been neither proved
nor disproved. Its existence would offer an explanation for several difler-
ent groups of experiments, but on the other hand, various arguments,
largely based on physical grounds, can be offered against it.
The mechanism of photosynthesis
Several recent investigators have proposed kinetic mechanisms for the
process of photosynthesis, e.g., Arnold (5), Baly (7), Briggs (15), Burk
and Lineweaver (16), Franck and Herzfeld (34), and Smith (68~. Of
these, the mechanism of Franck and Herzfeld is the only one which in-
cludes a detailed consideration of the photochemical reactions which may
be involved. Other investigators have suggested a variety of chemical
mechanisms without going into detail regarding the kinetics of the proc-
ess ((:onant, Dietz, and Kamerling (20), Gaflron and Wohl (40), James
(47), Kautsky and Hormuth (49), Stoll (75), van Niel (79~.
In most of these mechanisms it is assumed that the quantum efficiency
for the photochem~cal reaction is 0.25, in accord with the observations of
Warburg and Negelein (86~. However, the low efficiency values found
by recent investigators (see page 835) suggest that the value 0.25 may be
too high.
It has also been generally assumed that the photosynthetic process
includes ~ series of four successive photochemical steps, AH for a single
step being limited to the energy supplied by a single quantum. In van
Niel's mechanism, however, the four photochemical steps are assumed to
be identical, resulting in each case in the formation of a hydrogen atom;
the four hydrogen atoms then presumably bring about the reduction of
carbon dioxide to formaldehyde. Van Niel's mechanism is represented by
the following equations:
4EChlorophyll.H20 + he > Chlorophyll.OH + Hi
(1)
CO2 + 4H ~ (CH20) + H20 (2)
2~2Chlorophyll.OH + H2A > 2Chlorophyll.H20 + A] (3)
. . _
(V)
CO2 + 2H2A + 4hY > (CH20) + H20 + 2A
18 Private communication.
OCR for page 151
PHOTOSYNTHESIS
151
For reaction 1 to be energetically possible, the chlorophyll H2O com-
pound would have to be a more intimate compound of chlorophyll and
water than in the case of an ordinary hydrate. An alternative way of
writing reaction 1 would be
4[Chlorophyll.H20 + he ~ Dehydrochlorophyll H2O + Hi
the hydrogen atom thus being split from the chlorophyll portion of the
hydrated molecule.
In reaction 3, H2A would be water in the case of algae and higher
plants, and hydrogen sulfide in the case of the green sulfur bacteria.
Without modification this series of reactions could not take place (ex-
cept perhaps in bacteria) with a quantum efficiency as high as 0.25, since
reaction 2 is exothermic to the extent of about 150,000 cal. The energy
thus wasted would be nearly equivalent to the total energy supplied by
four photons at 7000 A. For HERA = H20, reaction 3 would be endo-
thermic. The necessary energy would presumably have to be supplied
by additional photons, unless reactions 2 and 3 were considered to be
coupled in some manner so that the excess energy from reaction 2 could
be utilized for reaction 3.
The mechanism of Franck and Herzfeld (34) was designed to avoid the
necessity of assuming a photosynthetic unit. They assumed, as did Stoll
(75), that carbonic acid forms a complex with chlorophyll. This complex,
in turn, was assumed to be combined with an organic molecule ROH,
present in abundance. Franck and Herzfeld suggested that ROH may
be a protein which forms the main body of the chloroplasts. The fol-
lowing equations represent the forward reactions postulated by Franck
and Herzfeld. They state that the equations are suggested as a working
hypothesis, not as a final solution.
HOE
HO R
Chl >C=O,ROH + he - ~ Chl >C/O OH (1)
HO HO
Carbonic acid
HO\ /R
Peracid
HO
~ \
Chl >COO OH + enzyme ~ Chl + >C O + 202 + H20 (2)
HO R
Peracid
H/
Formic acid
HOE
R acid
HOE
Chl >C=0 + H20 + he ~ Chl >C=O,ROH (3)
R
R acid
HO\
H'
Formic acid
HO\ OR
Chl ~C=O,ROH + he ~ Chl
H
Peraldehyde
NC' O OH (4)
OCR for page 152
152
WINSTON M. MANNING
HO R
Chl ECHO OH + enzyme > Chl +
H'
Peraldehyde
HO R
Chl COHN + he ~ Chl +
H/
R aldehyde
HO R
\C~OH + 2O2 (5)
H/
R aldehyde
H\
TIC O + ROH (6)
H'
Formaldehyde
The two enzyme reactions, in which oxygen is evolved, constitute the
temperature-sensitive or Blackman reaction.
According to Franck and Herzfeld, the calculated binding energy of
OH to O in the two peroxides is so weak that the OH radical
will be split off whenever the peracid or peraldehyde complexes absorb a
photon. At low light intensities the probability of such an absorption
would be negligible, because of the speed of the enzyme reactions. But,
at intensities corresponding to light saturation in most leaves, they
estimated this probability to be about TOO- According to their postulation,
the photodecomposition of the permolecules into two radicals initiates
chain reactions (back reactions in this case), each chain resulting in the
decomposition of a large number of permolecules.
With these chains light saturation is thus reached at a much lower
intensity than would be required if the rate were limited by the Black-
man reactions. By assuming a sufficient chain length (~103 at light satu-
ration), saturation, both in flashing and in continuous light, can be ex-
plained without the assumption of a photosynthetic unit.
Franck and Herzfeld suggested that, after a long dark period, much
of the chlorophyll may be attached to intermediate respiratory products,
probably plant acids. Illumination could then cause photosynthesis,
as in the case of chlorophyll-carbonic acid or chlorophyll-formic acid
complexes. However, according to Franck and Herzfeld, photooxida-
tion of these plant acids, probably by a chain mechanism, should also
take place to a large extent. Until the accumulated respiratory products
were exhausted, this oxygen consumption would largely counterbalance
the photosynthetic oxygen production, thus producing an induction
period. Increased probability of oxidation at high light intensities would
cause a more noticeable underproduction of oxygen at high intensities
than at low intensities, in agreement with experimental results (53~.
The kinetic equations derived by Franck and Herzfeld are in reason-
able agreement with experimental facts. However, this cannot be con-
sidered as a criterion for the correctness of the mechanism, since equa-
tions derived by other investigators on the basis of various mechanisms
are also in fairly good agreement with experimental results.
The reverse chain reactions of Franck and Herzfeld have been subjected
OCR for page 153
PHOTOSYNTHESIS
153
to some criticism (91~. Aside from doubts concerning this part of their
mechanism, it is evident that to account for the high efficiencies found by
Warburg and Negelein, the various forward reactions in the mechanism
of Franck and Herzfeld would have to proceed with remarkable efficiency.
Not only would the quantum efficiencies for each photoreaction have to
be nearly unity, but the intermediate products would have to remain
practically unchanged during the periods between the absorption of
successive quanta by a single chlorophyll molecule. This period, accord-
ing to calculations of Gafiron and Wohl (40), was about 10 min. in many
of the experiments of Warburg and Negelein. But Franck and Herzfeld
explained the induction period by assuming that, in the dark, a consider-
able fraction of the chlorophyll molecules become attached to interme-
diate respiratory products. The experiments of McAlister with wheat
(53) indicated that 10 min. in the dark is sufficient to produce a notice-
able induction period. Thus, in its present form, the mechanism of Franck
and Herzfeld can hardly be reconciled with the results of Warburg and
Negelein. It could much more easily account for the efficiencies found
by Manning, Staufler, Duggar, and Daniels (55~. But, if the maximum
quantum efficiency for photosynthesis is actually only 0.06 instead of
0.25, then there is no very good reason for choosing a mechanism in-
volving four photoreactions, rather than one involving five or possibly
more reactions.
In some respects it may be more desirable to postulate a mechanism
involving only a single photoreaction, as in van Niel's mechanism (79~.
This result could also be obtained in a manner quite different from that
suggested by van Niel. The following scheme indicates a possible mecha-
nism of this type.
In the series of substances A:, As, An_l, A=, carbon dioxide or car-
bonic acid is represented by A: and formaldehyde or other product is
represented by An, with the other symbols representing intermediate
substances. Assuming for simplicity that there are three intermediates,
the following series of changes might conceivably take place:
Al + As ~ 2A3
2tA3 + As > 2A4]
4tA4+hv~A5]
(1)
(2)
(3)
The overall equation would then be Al + 4hv > As. Water, oxygen, and
other possible reactants are ignored in the above equations. Reactions
1 and 2 would presumably be enzyme reactions. The Blackman reaction
might consist of one or more reactions like reaction 1 or 2, or it might be
due to intermediate reactions not included in this or the following scheme.
OCR for page 154
154
WINSTON M. MANNING
To avoid postulating an enzyme reaction involving carbonic acid, one
might use the following type of scheme:
4[A1 + he ~ As]
2[2A2 ~ A1 + As]
2A3~A1 + A5
(1')
(2')
(3')
The sum of these reactions again gives the equation Al + 4hv ~ As.
Disproportionation reactions similar in principle to 2' and 3' are probably
necessary in the production, by a plant, of such compounds as fats and
oils, where presumably much of the energy comes from the oxidation
of glucose.
If the maximum quantum efficiency is much lower than 0.25, many
mechanisms of this type are possible, particularly ones involving more
than three intermediate products.
With a mechanism of the type just proposed, a relatively high quantum
efficiency could be maintained at very low light intensities without neces-
sitating stable intermediate substances, such as would be necessary with
a mechanism like that of Franck and Herzfeld.
Perhaps the principal value of this discussion has been to give an
indication of how little actually is known concerning the chemistry of the
photosynthetic process. When such entirely different mechanisms can
possess even a small degree of plausibility, the need for further definite
information becomes evident.
REFERENCES
(1) ALBERS, v. M., AND KNORR, H. v.: Cold Spring Harbor Symposia Quant. Biol.
3, 87 (1935~.
(2)
(3) ARENS, K. : Jahrb. wiss. Botan. 83,561 (1936~.
(4) ARNOLD, w. : J. Gen. Physiol. 17, 135 (1933~.
(5) ARNOLD, w.: Cold Spring Harbor Symposia Quant. Biol. 3, 124 (1935~.
`6> . --~
(7)
(8)
ALBERS, v. M., AND KNORR, H. v.: Plant Physiol. 12, 833 (1937~.
ARNOLD, w.' AND KOHN, H. I. : J. Gen. Physiol. 18, 109 (1934~.
BALY, E. C. C.: Proc. Roy. Soc. (London) B117, 218 (1935~.
BALY, E. C. C., STEPHEN, w. E., AND HOOD, N. R. : Proc. Roy. Soc. (London)
A116, 212 (1927~.
(9) BARCROFT, J. : J. Physiol.37,12 (1908~.
(1O) BARKER, H. A. : Archiv Mikrobiol. 6, 141 (1935~.
(1l) BEBER, A. J., AND BITER' G. O.: Paper read at Indianapolis meeting of Ameri-
can Society of Plant Physiologists (1937).
(12) BLACKMAN, F. F.: Ann. Botany 19, 281 (1905).
(13) BRIGGS, G. E.: Proc. Roy. Soc. (London) B106, 1 (1929~.
(14) BRIGGS, G. E.: Proc. Roy. Soc. (London) B113,1 (1933~.
(15) BRIGGS, G. E. : Biol. Rev. Cambridge Phil. Soc.10,460 (1935~.
(16) BURE, D., AND LINEWEAVER, H.: Cold Spring Harbor Symposia Quant. Biol.
3, 165 (1935~.
OCR for page 155
PHOTOSYNTHESIS
155
(17) BURNS, G. R.: Am. J. Botany 24,257 (1937).
(18) BURNS, G. R.: Am. J. Botany 25, 166 (1938).
(19) BURR, G. O., AND MILLER, E. S..: Paper read at IndianaPO1iS meeting Of Ameri-
Can Society Of Plant Physiologists (1937).
(20) CONANT, J. B., DIETZ, E. M., AND KAMERLING, s. E.: Science 73, 268 (1931).
(21) CRAIG, F. N., AND TRELEASE, s. F. : Am. J. Botany 24, 232 (1937~.
(22) EMERSON, R. : J. Gen. Physiol. 12, 609 (1929).
(23) EMERSON, R.: Ergeb. Enzymforsch. 5, 305 (1936).
(24) EMERSON, R.: Ann. Rev. Biochem. 6, 535 (1937).
(25) EMERSON, R., AND ARNOLD, w. : J. Gen. Physiol. 16, 391 (1932).
(26) EMERSON, R., AND ARNOLD, w. : J. Gen. PhYSiO1.16, 191 (1932).
(27) EMERSON, R., AND GREEN, L.: J. Gen. PhYSiO1.17, 817 (1934~.
(28) EMERSON, R., AND GREEN, L.: Plant PhYSiO1. 12, 537 (1937).
(29) EMERSON, R., AND GREEN, L.: Plant PhYSiO1. 13, 157 (1938).
(30) FISCHER, H. : Chem. Rev. 20, 41 (1937).
(31) FISCHER, H. , AND BREITNER, s. : Ann. 522,151 (1936) .
(32) FISCHER, H., AND HASENKAMP, J. : Ann. 515, 148 (1935).
(33) FLEISCEER, w. : J. Gen. Physiol. 18,573 (1935).
(34) FRANCK, J., AND HERZFELD, K. F. : J. Chem. Phys. 5, 237 (1937).
(35) FRANCE, J., AND LEVI, H.: z. physik. Chem. Bay, 409 (1935).
(36) FRANCE, J., AND WOOD, R. W.: J. Chem. Phys. 4, 551 (1936).
(37) FRENCH, c. s.: J. Gen. PhYSiO1.20, 711 (1937).
(38) FRENCH, c. s. : J. Gen. PhYSiO1.21,71 (1937).
(39) GAFFRON, H.: Biochem. z. 280, 337 (1935).
(40) GAFFRON, H., AND WOEL, K.: Naturwissenschaften 24, 81, 103 (1936~.
(41) HEINICKE, A. J., AND Hoffman' M. B.: Cornell unit. Agr. Expt. stat BU11. 577
(1933).
(42) HOOVER. w. H.: Smithsonian Misc. Collections 95, NO. 21 (1937~.
(43)
HOOVER, w. H., JOHNSTON, E. S., AND BRACEETT, F. S.: Smithsonian Misc.
Collections 87, NO. 16 (1933).
(44) HUBERT, B.: Rec. tray. botan. neerland. 32, 364 (1935).
(45) INMAN, o. L. : Science 86, 52 (1937).
(46) INMAN, o. L., ROTHEMIJND, p., AND KETTERING, c. F.: in Biological Effects of
Radiation, p. 1093. McGraw-Hill Book co., Inc., New York (19361.
(47) JAMES, w. o.: New PhYtoloRist 33, 8 (1934).
(48)
(49)
(50)
(51)
JENEIN, p. M. : J. Marine Biol. ASSOC. United Kingdom 2a, 301 (1937~.
KAUTSEY, H., AND HORMUTE, R. : Biochem. z. 291,285 (1937~.
KAUTSEY, H., AND MARX, A. : Biochem. z. 290,248 (1937~.
KNOW, H. V., AND ALBERS, v. M.: Cold Spring Harbor Symposia Quant. Biol.
3, 98 (1935~.
(52) KOHN, H. I. : Nature 137,706 (1936~.
(53)
(54)
(55)
MCALISTER, E. D. : Smithsonian Misc. Collections 96, NO. 24 (1937~.
MANNING, w. M., JUDAY, c.' AND WOLF, M. : J. Am. Chem. soc. 60, 274 (1938~.
MANNING, w. M., STAUFFER, J. F., DUGGAR, B. M., AND DANTELS, F.: J. Am.
Chem. soc. 60, 266 (1938~.
(56) MAQIJENNE, L., AND DEMoirssy, E.: Compt. rend. 156, 506 (1913~.
(57) MEIER, F. E.: Smithsonian Misc. Collections 95, No. 2 (1936~.
(58) MILLER, E. C.: Plant Physiology. McGraw-Hill Book co.' Inc., New York
(1931~.
(59) MITCHELL, J. W.: Botan. Gaz. 98, 87 (1936~.
(60) NUTMAN, F. J.: Ann. Botany [N.S.] 1, 353 (1937~.
OCR for page 156
156
WINSTON M. MANNING
OSTERHOUT, w. J. V., AND HAAS, A. R. c. : J. Gen. PhYSiO1.1, 1 (1918~.
PETERING, H. G., AND DANIELS, F.: In press.
PORRET, D., AND RABINOWITCH, E. : Nature 140, 321 (1937~.
PRATT, R., AND TRELEASE, s. F.: Am. J. Botany 26, 133 (1938~.
RABINOWITCH, E., AND WEISS, J. : Proc. ROY. SOC. (London) A162, 251 (1937~.
ROELEFSON, p. A.: Thesis, Utrecht, 1935.
ROTEEMUND, p. : CO1d Spring Harbor Symposia puant. BiO1. 3, 71 (1935~.
(61)
(62)
(63)
(64)
(65)
(66)
(67)
(68) SMITH, E. L.: J. Gen. Physiol. 20, 807 (1937~.
(69) SMITH, E. L.: J. Gen. PhYSiO1. 21, 151 (1937).
(70) SPOEHR, H. A. : Photosynthesis. Chemical Catalog co. ; InC., New York (1926~.
(71) SPOEHR, H. A., AND MCGEE, J. M. : Ind. Eng. Chem. 16, 128 (1924).
(72) SPOEHR, H. A., AND SMITH, J. H. C. : in Biological Effects Of Radiation, p.
1015. McGraw-Hill BOOk CO. InC., New York (1936~.
(73) STEELE, c. c. : Chem. Rev. 20,1 (1937~.
(74) STrLES, W.: Photosynthesis. London (1925~.
(75) STOLL, A. : Naturwissenschaften 24,53 (1936~.
(76) THOMAS, M. D., AND HILL, G. R. : Plant Physiol. 12,285,309 (19371.
(77) VAN DEN HONERT, T. H. : Rec. tray. botan. neerland. 27,149 (1930~.
(78) VAN DER PAACW, F. : Rec. tray. botan. neerland. 29,497 (1932~.
(79) VAN NIEL, c. B.: Gold Spring Harbor Symposia Quant. Biol. 3, 138 (1935~.
(80) VAN NIEL, c. B. : Archiv Mikrobiol. 7, 323 (1936~.
(81) VERMECLEN, D., WASSINK, E. c., AND REMAN, G. H.: Enzymologia 4, 254
(1937~.
(82) WARBURG, o. : Biochem. z. 100, 230 (1919~.
(83) WARBURG, o. : Biochem. z. 103, 188 (1920~.
(84) WARBURG, o.: tber die Katalyschen Wirkungen der lebendigen Substanz.
Berlin (1928~.
(85) WARBURG, o.' AND NEGELEIN, E. : Z. physik. Chem. 102, 236 (1922~.
(86) WARBIJRG, o., AND NEGELEIN, E.: Z. physik. Chem. 106, 191 (1923~.
(87) WARBURG, o., AND UYESUG1, T. : Biochem. z. 146,486 (1924~.
(88) WEISS, J.: J. Gen. Physiol. 20, 501 (1937~.
(89) WILLSTATTER, R., AND STOLL, A.: Untersuchungen uber die Assimilation der
Kohlensaure. Berlin (1918~.
(90) WINTERSTEIN, A., AND STEIN, G. : Z. physiol. Chem. 220, 263 (1933?.
(9l) WOHL, K.: Z. physik. Chem. B37, 209 (1937~.
(92) YABUSOE, M. : Biochem. z. 162, 498 (1924~.
(93) ZSCHEILE, F. P., JR. : Botan. Gaz. 96, 529 (1934~.
(94) ZSCHEILE, ~. p., JR. : Protoplasma 22, 513 (1935~.
Representative terms from entire chapter:
light intensities
