Solar energy in photosynthesis

Solar Energy in Photosynthesis A 1966 Conference Paper

Dale N. Moss Crop Physiologist, Connecticut Agricultural Experiment. Station, New Haven., Connecticut

Solar energy provides the reducing power w i th in green leaves to convert CO~ and H20 in to sugars. The CO2 is suppl ied by the a tmosphere and enters the leaf by diffusion. Factors affecting the rate of pho tosyn th e s i s m u s t e i ther change the CO~ diffusive resistances or the CO2 concentra t ion gradi- en t a long the diffusion pathways . Therefore, these effects can be described in terms of diffusive control m e c h a n i s m s .

Light affects CO2 diffusion by in i t ia t ing ph o to - synthes i s , which removes CO2 at the chloroplast and establ ishes a diffusion gradient . Light also triggers s tomata l opening , thereby sharply decreasing the diffusive resistance. However, in tense radiat ion can cause des iccat ion of s tomata l guard cells, a m e c h a n i s m whereby the diffusive resistance increases.

During i l l u m i n a t i o n , leaf cells have bo th a source (respiration) and sink (photosynthes i s ) for CO2. Respirat ion in some species appears to be greatly s t imula ted by l ight . This addit ional internal CO2 flux is a possible reason for a lower efficiency of energy ut i l i za t ion t h a n in species whose respiration is n o t enhanced by l ight .

Physio logical growth responses or m o v e m e n t s o f ten occur t h a t pos i t ion leaves in the l ight . Plants lacking this capabil i ty are of ten excluded in ecological success ion in nature .

A GRICULTURE as a science is concerned with methods to achieve greater yields of specific products- -

red apples, high protein corn, or prime beef for examples. The economic value of these products can, in large measure, ultimately be traced back to the input of solar energy. Indeed, success is measured in tons of hay or bales of cotton per acre but in many cases could just as well be stated as calories stored in food or fiber per unit area per unit time. Although they may have originated through trial and error, many of the crop- ping practices in use today are successful because their design permits leaves to intercept a maximum of solar energy.

This paper will discuss some of the factors control-

Presented at the Solar Energy Conference, Boston, Massachu- setts, March 21-23, 1966.

ling the capture of solar energy by photosynthesis in plants, will present examples of how these principles are applied in agriculture, and will conclude with a brief look at some areas where knowledge is sparse and the efforts of current research are directed. This paper cannot extensively review the literature on photosynthesis; the literature is too vast for a comprehensive review in a short paper. For an extensive review, the reader is referred to Rabinowich who stated '° the problem of condensing the literature rather well when he writes, "Seventeen years and 2000 printed pages later, it is time to s top--even if this closure has to come in the midst of rapid and promising develop- ments . . . . " Rather, this discussion will be centered on some of the physical principles involved in the utilization of solar energy to reduce CO2 to carbohydrate by plants growing under natural conditions or in managed fields. Examples will be drawn from recent literature and, to some extent, from work done at the author's laboratory. This does not imply that these are the only examples which could have been used.

Photosynthesis, of course, is the combining of CO2 and H20 to yield carbohydrates. This reaction occurs in chlorophyll-containing tissue in light and results in a large increase in the chemical potential energy of the products, the energy being derived from radiation of the visible wavelengths. We will not discuss the pathway of carbon in photosynthesis or dwell on the biochemical reactions, important as they are, nor will we concern ourselves with the spectral response of photosynthesis. The fact that the action spectrum of photosynthesis has peaks in red or blue light is largely due to the fact that absorption by chlorophyll is greater at these wavelengths. In most agricultural situations, the concern is more with intensity than composition of light.

Since photosynthesis utilizes CO~ as a reactant, a simple measure of the net storage of energy by leaves is the amount of CO2 taken up by the leaf multiplied by the proper energy-conversion factor for CO2 to carbohydrate. CO~ enters the leaf as a gas. Therefore, any factor influencing the rate of photosynthesis must affect the diffusion of CO2 from the atmosphere into the leaf, and its effect can be described by Fick's law of diffusion. After Gaastra6:

Vol. 11, Nos. 3 and 4 1967 173

4 o

o~

d

,~2o I a .

60 HESKETH ~ " ~ Cot.

Sunflower

Tobocco

Dogwood

04 0.8 r2 INSOLATION, Ly/min

FIG. 1 Photosynthesis as a fimetion of sunlight intensity for four species of plants. (From Waggoner, el al. 14.)

p = [CO2]a - - [CO2]chl ( 1 )

(r,r + r~ + r,,~)¢o,

where P (cm a CO2 em -2 leaf sec 1) = photosynthesis or net

absorption of C02 from the air [CO~] (Cmacm -'~) = the concentration of carbon

dioxide in the air (a) or at the reactive site, the chh)rop]ast (chl)

(~'. "-[- rs -~- tree)CO 2 ( s e c c n l 1) = the diffusive resist- antes for CO2 successively encountered in the external air (to,), in the pores through the leaf surface, the stomata (rs) and, in the dissolved state, in the green mesophyl] (tells inside the leaf (r,,~).

I t is helpful to consider factors that influence the rate of photosynthesis in terms of how they affect diffusion of C02 into a leaf. They may alter either the diffusive resistance or the concentration gradient along the diffusion path or, in most cases, both resistance and ('on- centration.

l ']ant species differ markedly in their capacity to utilize sunlight to reduce CO2. The response of photosynthesis to increasing light fi)r four species is given in Fig. 1. These measurements of carbon dioxide absorption by leaves were nmde on attached leaves of plants growing in the field by enclosing the leaves in transparent envelopes and determining the difference in carbon dioxide concentration of an air stream before and after it was passed over the leaves. The higher light intensities are full sunlight as measured with "m Eppley pyrheliometer. The lower intensities were achieved by shading with neutral screens.

The top curve of Fig. 1 is for corn (Zea mays L.) and is typical of the light response of sugarcane (Sac- charum officinaru.m L.) and sorghum (Sorghum vulgate

Pers.). The maximum rate of photosynthesis per unit leaf area (50-60 mg CO2 dm -2 leaf hr -~ in normal air and full sunlight) is greater than for most plants. Also, photosynthesis increases as light intensity increases up to full sunlight. This high capacity for photosynthesis results in high yields of dry matter, and these species are some of the most productive plants on earth.

Sunflower (Helianthus annuus L.), the second curve of Fig. 1, has a somewhat lower rate of photosynthesis in bright light than corn. It, too, is a productive plant Its natural habitat is unshaded areas such as open fields. The physiological adaptations of corn and sunflower permit efficient utilization of intense light. These plants cannot exist in shaded environments.

The curve for tobacco (Nicotiana tabacum L.) is representative of the great majori ty of crop plants. These species have maximum rates of photosynthesis about 1 to ½ that of corn and their photosynthesis does not become faster as light-intensity increases above about -~ full sunlight. Although their capacity to utilize light is less than the maize group, they are "sun-loving" p~ants and normally exist only in unshaded habitats.

The final group, represented in Fig. 1 by dogwood (Cornusflorida L.), has a still h)wer maximum photosynthes!s that is achieved at 1 or less full sunlight. Although these plants can do well in open habitats, many are also capable of surviving in shade. Plants of this group have slow growth rates.

The reasons for these differences among species has not been elucidated clearly to now. Hesketh s found no difference in light intercepting ability (i.e. the concen- tr.~tion of chlorophyll in the leaves or their thickness) for species differing widely in (heir rate of photosynthesis in intense light. In some but not all cases, the differences appeared to be at least partially due to differences in diffusive resistances r.~ and r,,~. Thus, the photosynthetic CO2 response curves for corn and sunflower came together at 1000 ppm CO2 in the atmosphere. We will return to this problem.

To have a capacity to utilize sunlight to reduce CO2 is only one aspect of the process of production. The leaves must be able to intercept sunlight and a supply of CO2 must be available if photosynthesis is to occur, the light to supply energy for C02 reduction at the ehhwoplasts so that a diffusion gradient is established • rod the CO2, of course, as the raw material for the synthetic process. Many of the present practices-- planting density, species combinations in forages, planting dates--are successful because a maximum area of leaves within a plant commuifity are exposed to light. Quite apart from these manipulations of leaf exposure, plants themselves compete actively for sunlight with other plants or objects in their environment although they are anchored to one spot by roots. The leaning of a house plant toward a window is an example of a biological movement that has important

174 Solar Energy

survival value resulting in a greater leaf area being exposed to light.

In the field, there are numerous examples of plant movements to maximize interception of light. A famil- iar example is the changing orientation of sunflower leaves as the plant bends toward the sun during the course of the day. Thus, the upper leaves, at least, are oriented to normal to the sunlight for most of the day. Photosynthesis of sunflower as a function of sunlight intensity is shown in Fig. 1. Although the curve is be- low that of maize, it is similar in that photosynthesis continued to rise with increasing light.

The difference in incident solar energy for a horizontal compared to a normal surface can be seen in Fig. 2. z (In Fig. 2, the "normal" does not include diffuse radiation. A reasonably accurate estimate of the total energy incident on a normal surface would be the sum of the "normal" and "diffuse" curves of Fig. 2.) The incident solar energy of Fig. 2 and the light response of sunflower from Fig. 1 permit an estimate of the total daily photosynthesis for normal and horizontal leaves on (.lear days. This calculation predicts 26 percent more photosynthesis in June and 34 percent more in Sep- tember in the normally exposed than in the hori- zontally exposed suzdiower leaf. Thus, orientation toward the sun is important for individual plants whose leaves are capable of utilizing intense insolation. On the other hand, orientation of leaves normal to sunlight would be a disadvantage to many species whose leaves do not respond to intense light or for a dense covering of plants because fewer leaves would then receive a significant intensity.

In crowded plant communities, the most common plant movements that position leaves in light are growth responses. Some species in mixed communities survive because they are able to avoid shading by neighboring plants by rapid elongation of their stems. ~ This elongation is not the result of the plant seeking light, of course, and has a simple explanation in physiology. Nevertheless, the ecological succession of plants in unmanaged stands can be largely explained by whether or not the growth responses of the plant to a given light environment result in its leaves being exposed to light more favorable than those of its neighbors.

Elongation in response to shading also occurs in many crop plants, of course. An at tempt is often made to increase yields of crops by increasing the number of plants per unit soil area which increases the area of leaves to intercept light. If the crop canopy is not closed (i.e. the entire soil area covered with leaves), then striking increases in yield can be the result. Often these increases are proportional to the percentage increase in plant density. However, there are some prac- tical and physiological limits to increasing yields in this manner. In many crops, the need to get machinery through the stands requires that plants be in rows and

300l

~ 1 0 0

~0

_ . [ - ~ - _ ~ }

, ~ l ~_~Ver~ ] ~ I~

/1 \ I Z". A4arch a n d SepZember, 21

I 1 ] I I T I I I ~ . - { - ~ r ~ L I I I I I I I I I I I ] I ~ I ] l .J ) _ L , I I

I I L-.P'R,~I I I I I I I I i%..q"bd,"Y, fV 'Y ' l [ [ i -J:: ] t . , , r l . l I I I I I I i l l "N. I F'Ig.I I I I 1.-".~ I /.~.,,d I.-b..L.~ I I I I I I I I "N.i I IN,,. I I r I 1 / ] N " : , r ' l f l I I I I I i I I I "N,I i I " I , 1

v , f , ! ~ 4 ! ! ! ,~ , , ! ! ! . - l - ~ , , ~ ! ! ! ! ! , ~ ! ! X " l , , L , l , , r,I t I

I~ Lv l l I I ~ z l x J l_, L , ~ J l , , , InJ_. . l~ 1 5 I I / I / ] " b d .J" l £-t--j."~...J~"-~-'u~f.~l_l"kl I .L ,d~N. I "~ j I v b 4 ~ ] I [ I I " l~j '~ j / . .~." I '~ I I [ I 17 I"l,~[VCr,t I [ 1 1 I ] I q I I I T ' ? " ' t ~ 1

...7 5 6 7 8 9 lO 11 12 / 2 ,.7 4 5 • 7 8 orn Solar" 7?me June 21 prn

FIG. 2--Incident solar energy on clear days, Lat. 42°N. (Re- ported by I. F. Hand, U. S. Weather Bureau, Milton, Mass., Jan. 1950. Cited by Brooks (1), Fig. 2-13.)

limits the spatial distribution of plants. Whether in rows or distributed uniformly over a field, however, as plant numbers increase, leaves are shaded more and more by the surrounding plants. Elongation of stems is often the result but it has little or no adwmtage for light interception in a homogenous community of plants because the surrounding plants also elongate. Adding still more leaves per soil area simply shades other leaves. In the extreme case, too few leaves on each plant are exposed to sufficient light to supply the carbohydrate requirements for sustaining life of that plant and yields actually decrease as the plant popula- tion is increased further. Because of mortality of plants, yield decreases at high density are particularly noted when the yield measured is some storage product of the plant such as grain or fruit that accumulates only after the basic growth requirements of the plant for carbohydrate are met.

Another plant movement that affects solar-energy interception is the response to drought or water stress. These take many forms but the most common are curl- ing of leaves or drooping of leaves and petioles. The result is a drastically decreased surface exposed to radiation with the average orientation tending to be parallel rather than perpendicular to radiation. This orientation is probably best recognized for its survival value in that less heating of the leaves occurs that would increase the vapor pressure of water within the leaves and further increases water loss when the supply was already critically short. However, this orientation further decreases any remaining photosynthesis due to poor light interception.

Many reactions in plant communities, while the cause is attributed to some other factor, are really reactions to the light environment. An example is found in

V o l . 11, N o s . 8 a n d .~ 1967 175

,oo~ . ,.oo o ,.oo

"~ $ 0 [ " N , O

; I 0 , . . . . G . . . .

YIELD OF DRY MATTER (9 re'z) ,90 9 ,90 __l-tg°. ,.oo o ,oo

i N.: 20 ] N = 70

/ I ! Clover Gross

' ,6 ' 5'o ~ ,6o LIGHT DENSITY (% DAYLIGHT)

,oo ,o ,9o N : 200

Clover Gross

. . / , , I

50 ~bo

FIG. 3--Vertical distribution of the yield of grass and clover under four nitrogen treatments, together with the profile of light density relative to daylight (adapted from Stern and Donald, 11).

the work of Stern and Donald n with forage crops. Numerous popular forage mixtures contain both grass and clover species. I t is common knowledge among agronomists that the relative amounts of clover and grass in the mixture can be controlled by the intensity of nitrogen fertilization. With no added N, the clover predominates, probably because it has symbiotic nitrogen-fixing bacteria living in nodules on its root system, The grass, not having this nitrogen source, grows slowly, and the clover canopy closes over the grass. In Fig. 3, the accumulation of dry mat ter in each stratum of a mixed stand of clover and grass is shown together with the percentage of midday sunshine reaching that stratum. Lacking any added N, the grass yield (shown to the right of the yield base line) is low and the grass height is short. The reason practically no grass is found

7 25 ,.c:

. lO

~. 5

a. 0

Top :rgh? on 5.500 ft-c

= = ;' " I \ L CSeltom light off ~

Bottom light on ],000 f t -c

i o J 5 2'0 2'5 M i n u t e s

FIG. 4--The effect on photosynthesis of illuminating the lower surface of a tobacco leaf also illuminated from above. (From Moss, 1964, Crop Sci. ~,: 133.)

30

20 OD :z

X

Z >-

~ 1 0 0

FI6. 5---Response of the photos.yn- thesis of sugar cane to increasing light in calm (©) or turbulent (o) air containing 200 ppm CO~ and in calm air containing 300 ppm CO2 (X). (From Waggoner, et al., 14.)

on these infertile mixed plots is that the clover, which grows normally with its symbiotic source of nitrogen, shades the grass so severely (note the rapid decrease in light at the top of the clover canopy) that the grass is practically eliminated.

When abundant N is added, the grass and clover grow well together. However, at 200 lb of N per acre, the grass grows tall and dense. Here the clover is shaded and eliminated because it cannot avoid shade by elongation. Thus, in the presence of sufficient N, the turf will become all grass and is sometimes referred to as sod- bound. The clover is eliminated, not because it is harmed by N fertilization, but because it cannot suc- cessfully compete for light with the taller well-fer- tilized grass.

L e a f A c t i o n i n L i g h t

Thus, interception of solar radiation by the plant involves more than the plant just "being there". A plant adapts to its light environment and, in numerous cases, must actually compete with other plants for sunlight. Let's turn our at tention now to what happenc to a leaf that is in position to intercept light.

We have already seen that light affects diffusion of C02 into leaves by providing energy for the reduction of CO2 in the chloroplasts of green cells. Thus, C02 is removed from the system and a diffusion gradient is established from the external air toward the chloroplasts. However, light has another dramatic effect on diffusion in that it triggers (or supplies energy for)

the opening of the stomata or leaf pores. This markedly decreases the diffusive resistance re and permits greater flow of gas into the leaf.

ppm

0 J

J J

0.4 0.8 1.2 INCANDESCENT LIGHT, Ly/min

L 1.6

176 Solar Energy

A otm , ~

>

e c c o , sink

" l:;z ¢oa

source

FIG. 6--A diagram of the diffusion path for CO2 in a leaf cell. (From Moss, 9.)

Most obviously, sunlight ~peeds evaporation from leaves and, if the roots are not amply moistened, leaves may wilt and stomata close. However, illumination may also close stomata without any visible wilting of the leaf. An example is shown in Fig. 4. A tobacco leaf that received 5500 ft-c illumination on its top assimilated 25 mg CO2 dm -2 hr -~. Then only 1000 ft-c was directed on the bottom of the leaf and photosynthesis decreased to only 14 mg CO~ dm -2 hr -~. This decrease was due to stomatal closure and could be prevented by moist air. Evidently the stomatal guard cells are particularly sensitive to desiccation and, in this case, became flaccid even though the leaf was amply supplied with water and seemed fully turgid.

Air turbulence around a leaf also affects the rate at which CO2 diffuses into a leaf by changing the diffusive resistance of the air, r, in Eq. (1). A sugarcane leaf was placed in very still air with either 200 or 300 ppm CO2 in the atmosphere giving the two curves ~4 shown in Fig. 5. Stirring the air containing 200 ppm and thereby decreasing r~ made photosynthesis, as shown by the solid circles near the upper curve, as fast as increasing CO2 to 300 ppm in the quiet air. Now, we do not generally think of turbulence as being caused by solar radiation, but turbulence around a leaf follows when radiation is absorbed and heats the leaf. The warm leaf, in turn, heats the air layer nearest it, causing it to rise and be displaced by heavier and cooler air. The wind speed also generally increases as the sun warms the earth. The net result is replenishing the supply of CO2 at the leaf surface. Thus, radiation not only provides the energy for chemical fixation of CO2 inside the leaf and for opening of the stomata but also helps bring CO~ to the leaf by turbulent transport.

The efficiency of the photosynthetic mechanism in leaves is a source of controversy. Needless to say, some solar energy is employed in warming the system and per- mitting it to function. The net amount of energy stored as food or fiber is usually less than 2 percent of that incident on the leaves although this may rise above 10 percent in dim light.

However, since C02 is produced by respiration and recycled during photosynthesis, estimates of the actual efficiency of the photosynthetic system itself in utilizing radiant energy are made by adding the COs produced by dark respiration to the net uptake in the light to give " t r u e " photosynthesis. When a mole of sucrose is burned to yield CO2 and H20, 112,000 calories are released per carbon atom. Thus, the net energy stored in the reverse reaction of photosynthesis is easily determined. However, recent experiments suggest that some leaves respire at a greater rate in light than in darkness.

If more internal recycling of C02 occurs than origi- nally supposed, then previous estimates of the efficiency of energy utilization in photosynthetic CO2 reduction are inaccurate. 5{ore directly to the subject of this discussion, however, is the fact that to accurately estimate the magnitude of the diffusive resistance within leaves one must know the magnitude of the internal recycling CO2 flux. Furthermore, recent studies of respiration in illuminated leaves (respiration of leaves in the light is often referred to as 'photorespiration') show an interesting relationship between photorespiration and an aspect of CO2 diffusion into leaves which has been discussed earlier in this paper, namely the difference in C02 absorption among species when the leaves are in identical environments. I t seems that species which are poor absorbers of CO2 (in Fig. 1, tobacco compared to corn, for example) are species that have a large photorespiration. Ill contrast, the respiration of corn does not appear to be stimulated by light 4' 5, 9, 12, 13.15, 16 Thus, it would be reasoned, in the case of the above example, that the efficiency of the photosynthetic reactions in corn and tobacco may be identical. Since tobacco has a large photorespiration resulting in significant amounts of COs being released within the leaf ceils, however, the concentration gradient between the atmosphere and the internal cell solution is less than in corn. Therefore, less CO2 diffuses into the tobacco leaf than into a corn leaf when they are in identical environments. 9' 16

The evidence for increased respiration in the light comes from several lines of research. Decker 2' 3 reported a much higher rate of C02 evolution from leaves of several species for several minutes immediately after a period of light than the steady-state dark respiration. He interpreted this gush to be the overshoot or re- mainder of a much greater respiration occurring in light than in darkness. Again, in the case of the above example, this gush is found in tobacco but not in corn. 6' 9 Decker's observations have been confirmed by many including Tregunna, et a112' 13 who found that the magnitude of the dark surge was related to the preceding light intensity and was small from chlorophyll defi- cient tissue. Forrester, et al 4' 5 report that this dark

Vol. 11, Nos. 8 and ~ 1967 177

i

~- ioo

i

.......... f V

• ~/ /( ( ~

/ • d,¢ /

!J//

~e 4o 6o ao Minut l l

FIG. 7 - -The rate of CO2 evolut ion into CO2-free air from Pelargonium. Curve A was for the leaf in darkness preceding illu- mina t ion ; Curve B is the t ime course of evolut ion in 2500 ft-c i l luminat ion wi th the l ight on a t 0; Curve C is for darkness af ter 50 minutes of l ight ; Curve D is for darkness af ter 25 minutes of l ight ; Curve E is drawn th rough the maxima of a series of curves such as C and D. (From Moss, 9.)

Fro. 8 - -The effect of (~-OH-2-pyri- d inemethanesulfonic acid on the ra te of CO2 evolut ion into CO2-free air from a tobacco leaf wi th i l luminat ion beginning a t 0 time. (Unpublished da ta by Moss.)

14

0 2 4 6 8 10 MINUTES

surge, presumed to be photorespiration, was strongly suppressed by a low oxygen concentration (2 percent) in the atmosphere while ordinary dark respiration was not affected. They interpret this to mean that photorespiration and dark respiration are separate reactions with much different oxygen requirements.

To describe the effect of respiratory COs on diffusion of CO2 from the external air, Eq. (1) must be rewritten. In use of Eq. (1), the assumption is made that during photosynthesis in bright light [CO2]¢~, is zero, and r~ is estimated from evaporation of water from a piece of wet filter paper cut to the shape of the leaf and placed in the position and environment in which the leaf is placed during measurements of photosynthesis. The combined r, + r~ are estimated from the rate of evaporation from the leaf (transpiration). P and [C02]a are measured. Then r,ne, the only remaining quantity of Eq. (1), is calculated. Calculated in this way, rm~ is a residual quantity that includes any errors of either assumption or of the measurements.

For the present purpose, rm~ must be subdivided and it is convenient to visualize the average green cell of a leaf as a diffusion system such as depicted diagram- matically in Fig. 6. Here, A represents the atmosphere, C the chloroplast, and D the site of respiration. Within an illuminated cell containing chloroplasts, there are both a source (respiration) and a sink (photosynthesis) for CO2. During photosynthesis, C02 from respiration is being produced at D and diffuses to B as does C02 from the atmosphere A. This combined stream then flows from B to C. The COs concentration at B and therefore, the COs gradient from A to B (since A is a fixed atmospheric concentration) depends not only on the flow away from B to C over resistance R~ but also on the respiratory flux from D to B. Thus, if respiration increases, the concentration at B increases and the gradient, and therefore the flux, from A to B decreases. In this way, respiration can affect the net absorption of CO2.

To determine if respiration was greater in the light than in darkness, a leaf was placed in a transparent chamber and COs-free air was passed over the leaf in both light and darkness. 9 In the dark, there was no internal sink and all the COs produced by dark respira-

tion escaped into the atmosphere following path DBA and was measured. In the light, COs at C was utilized in photosynthesis and, if the concentration at C really approached 0, the gradient from B to C equaled the gradient from B to A. Then, if the amount of CO2 escaping to A should be greater than when the leaf was in darkness despite the fixation of part of the respiratory C02 in photosynthesis, this would be posi- tive proof of light enhancement of respiration.

The results of placing a pelargonium leaf in a C02- free atmosphere, as proposed above, are shown in Fig. 7. Curve A represents the COs evolution in darkness (3.2 mg CO2 dm -2 hr -1 plotted here at 100 and all other rates plotted relative to it). Curve B shows the time course of the rate of COs evolved with the lights on at time 0. The rate of C02 escaping at first decreased to one-third but then actually increased to 1.8 times the dark respiration. Thus, these leaves were losing almost twice as much CO2 in the light despite the fact that part of the CO2 was being fixed in photosynthesis.

Curve C shows the effect of turning off the lights after 50 minutes of illumination. COs "surged" from the leaf, and the rate of escape then gradually decayed to the original "steady-state dark rate", curve A. After a period of darkness, illumination caused the evolution of COs again to follow curve B with 0 time beginning at the new reference. This process was re- peated several times, for example, curve D at 25 minutes. Curve E was drawn through the maxima of the surges. Curve E is a minimal estimate of the respiration occurring in the light; the actual rate may have been much higher.

The data of Fig. 7 show positively that more C02 is evolved in the light than in darkness and suggests that light enhanced respiration is due to the formation in light of some substrate which is readily oxidized in light or darkness to yield C02. One known compound universally found in leaves that fills these requirements is glycolic acid. The pathway for synthesis of glyco]ie acid is unknown. It is synthesized only in the light and, therefore, is thought to be an early product of photosynthesis. Since it is formed in the absence of CO2, however, there must be other pathways for its synthesis. When its oxidation is Mocked with a-OH-2-pyri-

178 Solar Energy

dinemethanesulfonic acid which inhibits the enzyme, glycolic acid oxidase, considerable quantit ies of glycolie acid accumulate in leaves in l igh t - -none in darkness. 15 Therefore, I have recently tested the effect of glycolic acid oxidase inhibitor on COs evolution into COs- free air. The results of one experiment with tobacco are shown in Fig. 8. The top curve is the t ime course of CO2 evolution for a leaf with the petiole in water with the light beginning at t ime 0. The light period was 10 minutes after which t ime the leaf was darkened and the water was replaced with 0.01 M a-OH-2-pyridine- methanesulfonic acid. The leaf was then returned to C02-free air for 1 hour to allow the inhibitor to be dis- t r ibuted throughout the leaf. The lights were then turned on for 'mother 10 minutes with the results shown by the bo t tom curve of Fig. 8. The difference between the curves for leaves with and without inhibitor permits calculation of how much more CO2 was given off by the untreated than the t reated leaf. For Fig. 8, this difference was 5.5 gmoles of CO~.

Samples of the leaf were taken before and at the end of each period of light, and the concentration of glycolic acid in the tissue was determined. Practically no glycolic acid was found in either sample in the dark. After 10 minutes of light, the leaf, after t rea tment with inhibitor, contained 9.6 gmoles of glycolic acid. I t had only 1.5 gmoles after 10 minutes of light before t rea tment . Thus, the accumulation due to the inhibitor was 8.1 gmoles of glycolate whereas the difference in CO~ evolution was 5.5 gmoles. Since 1.5 #moles of glycolate accumulated for each mole of CO~ loss prevented by the inhibitor, not all glycolate was transferred into CO2.

In further tests, glycolate given to tobacco leaves through the petiole has increased dark respiration 5- fold while only slightly increasing the C02 loss from illuminated leaves. All these experiments indicate tha t glycolic acid may be the substrate tha t is formed in light and permits more rapid respiration than occurred in the dark.

Despite the efforts of many investigators, some of whose work we have discussed briefly, no one has suc- ceeded to now in designing an experiment tha t will show the magni tude of C02 recycling within illuminated leaves. Thus, this discussion ends without a complete picture of how the capture of solar energy by plants can be defined in terms of diffusion of CO2 from the atmosphere into leaves. However, these uncertainties

are in the hidden areas within the leaf cells. We have seen tha t mechanisms and factors tha t affect photosynthesis must be translated into physical phenomena controlling the diffusion of CO2 in the plant-a tmosphere system. T h a t being true, the effect of these phenomena can be adequately described by the simple mathemat ics of Fick 's law of diffusion.

REFERENCES

1. Brooks, F. A. 1959. "Introduction to Physical Microclima- tology." Univ. of Calif. Press, Davis.

2. Decker, J. P. 1955. "A Rapid Post-Illumination Decelera- tion of Respiration in Green Leaves." Plant Physiol. 80: 82-84.

3. Decker, J. P. and J. D. Wien. 1958. "Carbon Dioxide Surges in Green Leaves." Solar Energy 2: 39-41.

4. Forrester, Marlene L., G. Krotkov, and C. D. Nelson. 1966. "Effect of Oxygen on Photosynthesis, Photorespira- tion, and Respiration in Detached Leaves." I. Soybean. Plant Physiol. 41: 422-427.

5. Forrester, Marlene L., G. Krotkov, a/ld C. D. Nelson. 1966. "Effect of Oxygen on Photosynthesis, Photorespiration, and Respiration in Detached Leaves." II. Corn and Other Monocotyledons. Plant Physiol. 41: 428-431.

6. Gaastra, P. 1959. "Photosynthesis of Crop Plants as In- fluenced by Light, Carbon Dioxide, Temperature, and Stomatal Diffusion Resistance." Med. Landbouhogesch, Wageningen 59: 1-68.

7. Grime, J. P. and D. W. Jeffrey. 1965. "Seedling Establish- ment in Vertical Gradients of SuIflight." J. Ecol. 53: 621- 642.

8. Hesketh, J. D. 1963. "Limitations to Photosynthesis Responsible for Differences among Species." Crop Sci. 3: 493-496. Moss, D. N. 1966. "Respiration of Leaves in Light and Darkness." Crop ,~ci. 6: 351-354. Rabinowitch, Eugene I. "Photosynthesis and Related Processes." Vol. 1--1945; Vol. 2, part 1--1951; Vol. 2, part 2--1956. Interscience Publishers, New York. Stern, W. R. and C. M. Donald. 1962. "Light Relationships in Grass-Clover Swards." Aust. J. Agr. Res. 15: 599-614. Tregunna, E. B., G. Krotkov and C. D. Nelson. 1961. "Evolution of C02 by Tobacco Leaves during the Dark Period Following Illumination with Light of Different Intensities." Can. J. Bot. $9: 1045-1056. Tregunna, E. B., G. Krotkov and C. D. Nelson. 1964. "Further Evidence on the Effects of Light on Respiration during Photosynthesis." Can. J. Bot. 42: 989-997. Waggoner, P. E., D. N. Moss and J. D. Hesketh. 1963. "Radiation in the Plant Environment and Photosynthe- sis." Agron. J. 55: 36-39. Zelitch, I. 1964. "Organic Acids and Respiration in Photo- synthetic Tissues." Ann. Rev. Plant Physiol. 15: 121-142. ZELITCH, I. 1966. "Increased Rate of Net Photosynthetic Carbon Dioxide uptake caused by the Inhibition of Glyco- late Oxidase." Plant Physiol. 41: In press.

9.

10.

11.

12.

13.

14.

15.

16.

Vol. 11, Nos. 3 and 4 1967 179

Documents

Solar energy in photosynthesis