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Limitations to Photosynthesis Responsible for Differences Among Speciesl
J.
D. Hesketh 2
AMONG species, the CO 2 assimilation varies many-fold £l. in bright sunlight. In a previous paper, photosynthesis at 1.0 Iy min. -1 (langley or cal.cm. - 2 per minute) and 300 ppm CO 2 was shown to vary from 7 to 49 mg. CO 2 dm. -2hr. --1. Further, the photosynthesis of the species with the more rapid rates continued to rise as radiation density increased above 1.0 Iy min. - 1 (3). In this report, several possible causes for these important differences among spe­
cies are investigated. Theoretical causes are analyzed by Rabinowitch (7). His
equations predict that the maximum photosynthesis
.may be determined by chlorophyll content or the propor­
tionality between absorbed t}uanta and photosynthesis. This
. theory showed that any differences in these would be re­
flected in the response to dim light. Therefore, the response
to low light intensities by species that vary widely in Pmax
was examined. Chlorophyll concentrations were also deter­
mined.
Alternatively, the equations predict that Pl l" " may be
determined by factors in the "dark reactions," such as the
quantity or renewal of the acceptor for CO 2 or by the dif­
. fusion of CO 2 to the site of synthesis. The theory also
....
that any differences in these would be reflected in
e to CO, when the CO" concentration is low. There­
this response as well as some of the factors affecting
absorption were examined.
MATERIALS AND METHODS
photosynthesis of plants was measured in transparent, water·,:ooled chambers under artificial lights. The concentration of air entering and leaving the chambers was measured of an infrared gas analyzer. Air flow was measured with A fan was placed in the chamber to insure adequate un:ma(!(Jn. Details of this system for measuring photosynthesis already been presented (3). Thermocouples were fastened to bottom of the leaves by means of transparent tape. Castor bean, RichlUS commlmis L.; '.Mammoth Russian' sun­
Heliamhus allJIUUS L.; Conn. 870 maize, Zea mays L.; nU',,-lll!;"<"", Dt/elylis glomera/a L.; red dover, TI'ifolium pl'alellSe 'Havana Seed' tobacco, Nicotialla tabawtn L. were grown
pots outdoors in May-June. In addition, some maize was grown
the greenhouse. The sun-grown leaves of maple, AceI' sacchamtn
and red oak, Quercus mbra L., were excised under water
large trees in early June. The plants were in the vegetative
of growth. The maple, maize, oak, orchardgrass, and red
were growing vigorously; the castor bean, Russian sun­
flower, and tobacco were less vigorous than field grown plants.
An attempt was made to obtain leaves capable of the maximum
rate possible for the species. Leaves
excised for the tree speCies as leaves for potted seedlings
low photosynthetic
air-free water in
rates. T eave' of maize have been
the field without any change in photosynthetic rates of 50 to 60
1
1'1'(. CO" d'11.""'hr.- before and after excision (unpublished data).
For the photosynthetic studies between 0 and 1000 ppm CO"
plants were grown in the greenhouse in the fall.
To study response to light the leaves were exposed to 300 ppm
CO, and temperature was maintained within a range of 2° C.
while radiation was changed from 0 to 1.2 Iy min.-I. When radia­
tion was increased to 2.0 ly min.-i. the temperature rose 4° C.
To study response to
the leaves were exposed to 2.0 and 2..1
Iv min.-" except 1.2 Iy
-1 was used on maple anel oak because
. photosynthesis did not increase in brighter light.
The great differences among these species in photosynthesis in
bright light are illustrated in Figure 1. This figure exemplifies
34 curves. At 1 Iy min.-' the photosynthesis of maize was eight­
fold that of maple. Further, the rate of maize was still rising with
increasing I1ght while that of maple was constant at all light inten­
sities above 0.25 ly min.->' The response of sunflower resembles
that of maize; tobacco, red dover, and castor bean resemble that
of orchard grass. These can all be seen in the first 2 columns of
data in Table 1 where 34 observations are summarized by ranges
of
Thus these species provide a wide range in P a,.,: in which
causes
variation might be discovered.
RESULTS
Response to Light
As stated earlier, Pm.." may be determined by chlorophyll
content or the proportionality between photosynthesis and
absorbed quanta. These factors also determine the slope of
the light response curve at low radiation density I.
The rectangular hyperbola
Q _ Qm.."KI (1 + KI)-l
[1]
has satisfactorily fit Q, (net photosynthesis plus respiration
in the dark), when related to a wide range of 1. Q"H'X is
Q as I -;... 00. QmaxK is the slope of the response curve
as 1-+0.
Qm..x K is, therefore, the critical characteristic. Unfortu­
nately its variation is poorly revealed near the crowded
origin of graphs such as Figure 1. If, however, we trans­
form equation [1] into
IJQ - IJQm.." + I/Qm..J< K
[2]
Qm." K becomes the intercept, Figure 2.
Where I > 0.25, equation [2] fits the data well. Where
I < 0.25, the four sets of data in Figure 2 (as well as the
30 additional ones not shown in the figure) all require a
curvilinear relation rather than equation [2]. As light de­
creases to 0.25 ly min.
respiration becomes at least one­
fifth of Q. The departure from equation [2] where I <
0.25 may, therefore, be caused by the questionable estima­
tion of photosynthesis as net photosynthesis plus respira­
tion in the dark where the latter is relatively large.
Where I > 0.25, equation [2] does, nevertheless, fit
the data well. Extrapolation to I - 0 of the lines through
these data indicates that the lines all intersect the ordinate
60
'­ 50
Maize
..c
't
E
40
'0
0'30
o
Ol
-
E
20
I
1 Investigations supported by Natioml Science Foundation and
C0 .flflecticllt funds. Received Jan. 31, 1963.
- Formerly Assistant Crop Physiologist, The Connecticut
tuml Experiment Station. New Haven. now Assistant Plant
University of Arizona, Tucson.
Uy
min-I)
Figure I-Relation to light intensity I of the photosynthesis Q
of 4 species. The 2.0 Iy min.-1 is equivalent in quanta to
10,000 ft.-c. of sunlight.
493