انجمن  زیست شناسی دانشگاه یاسوج

PHOTOSYNTHESIS IN RELATION TO STOMATAL FREQUENCY

R. 0 . FREELAND

(WITH ONE FIGURE)

Received May 4, 1948

The concept that stomates play an all-important role in controlling the

diffusion of carbon dioxide into leaves during photosynthesis seems to be

quite generally accepted by teachers and authors of botany texts. In a

plant physiology text by MAXIMOV (6) may be found the following, "That

the carbon dioxide enters the leaf mainly through the stomates may be

shown by a simple experiment. If on a certain portion of the leaf the

stomata are coated with vaseline, and the leaf is then exposed to light and

afterward treated with iodine, the blue color reaction will be observed only

in those portions where the stomata remained open." Also in the text by

MEYER and ANDERSON (7) one finds "Critical experiments have shown,

however, that the proportion of this gas (CO2) entering the leaves by this

route (directly through epidermis) is relatively small, and that practically

all of the carbon dioxide entering leaves diffuses in through the stomates."

Similar statements may be found in many other botanical books.

It seems that this concept stems mainly from the research of Blackman

and his contemporaries. BLACKMAN (1) in a review of the research regarding

this problem concludes that "Under normal conditions, practically

the sole pathway for carbon dioxide into or out of the leaf is by the stomata."

Further support and impetus were given to this idea by the classical

experiments of BROWN and ESCOMBE (2) which demonstrated and

partially explained the enormous diffusive capacity of small pores and

stomates. More recently MASKELL (5) working with Prunus lauro-cerasus

and NUTMAN (9) studying Coffea arabica have reported that photosynthesis

in these plants is directly related to stomatal movement.

On the other hand considerable evidence has appeared in the literature

which indicates that the rate of diffusion of carbon dioxide through epidermal

cells of leaves may be quite appreciable and should not be minimized

or ignored. Blackman mentions that Boussigault and Barthelmv

reported data to the effect that the carbon dioxide exchange during "assimilation"

was independent of stomatal distribution. MITCHELL (8)

found that the leaves of tomato and Pelargonium absorbed carbon dioxide

and accumulated carbohydrates in appreciable quantities although the

stomates appeared to be closed. Furthermore, the amount of carbon dioxide

absorbed by the leaves in which the stomates appeared to be closed was

approximately equal to the amount absorbed by the same leaves when

occasions when the rate of photosynthesis was quite high even though the

stomates were closed. In this laboratory, many measurements of the apparent

or no direct correlation between photosynthesis and stomatal distribution.

Some of these data will be presented in this paper.

The plants to be included in this report are yellow-green coleus, Coleus

blumei; avocado, Persea americana; poinsettia, Euphorbia pulcherrima;

4---<

rubber plant, Ficus elastica; begonia, Begonia sp.?; Rhoeo discolor; bean,

Phaseolus vulgaris (var. stringless green pod) ; tobacco, Nicotiana tabacum,

and geranium, Pelacrgonium zonale. These plants were selected because of

variation in stomatal distribution and thickness of cuticle. Six of the

species have stomates in the lower epidermis while the other three have

stomates in both the upper and lower surfaces of their leaves as shown in

table I. The amount or thickness of cuticle progresses from very little in

such plants as coleus and tobacco to much in poinsettia and rubber plant.

Apparent photosynthesis, in terms of milligrams of carbon dioxide used,

was measured from the upper and lower surfaces of the leaves. Glass cups

of the design shown in figure 1 were clamped opposite each other on several

leaves of the experimental plant. These cups each had a diameter of 4 cm.

and a depth of 8 cm. A little grafting wax applied around the rim of each

cup and warmed slightly at the time the cup was applied to the leaf provided

an air-tight seal. Natural air passed through these cups to absorption

towers containing 0.1N KOH for carbon dioxide determination after

the method of HEINICKE and CHILDERS (4). Using flowmeters and constant

suction, FREELAND (3), the rates of air flow were so regulated as to

temperature rise in the cups. A rate of air flow of about 0.5 cu. ft. per

window in direct sunlight where on hot days they were shaded with one

or two layers of cheesecloth. No other attempt was made to control the

and a Leeds and Northrup thermopotentiometer. Little or no difference

in temperature was ever found in the cups attached to the opposite

sides of a leaf. The duration of each experiment was four hours.

To test the accuracy of the apparatus and methods, a large number of

determinations of the carbon dioxide content of the air in the laboratory

Results and discussion

The experimental results obtained for the various plants are summarized

averaging the results from three or more determinations made upon different

in the lower epidermis provides a basis for evaluating the importance of

stomates in the absorption of carbon dioxide during photosynthesis. For

example, in coleus, avocado, and begonia, carbon dioxide diffused through

the upper epidermis in very significant amounts. In no case was the rate

of diffusion of carbon dioxide through the upper epidermis alone as great

as it was through the lower epidermis plus stomates. However, from the

known structure of leaves, it would appear to be a reasonable assumption

that carbon dioxide could diffuse through the lower epidermal cells, excluding

stomates, at a rate approximately equal to that through the upper

epidermis. On this basis the apparent photosynthesis calculated for the

PLANT PHYSIOLOGY

total leaf epidermis, excluding stomates, would really be twice the amounts

presented for the upper epidermis in table I. Applying this principle to

coleus, as an example, the apparent photosynthesis in mg. of CO2 used per

hour per square decimeter would be 6.2 through epidermal cells alone and

7.0 through stomates alone. The data for the other plants in this group,

poinsettia, rubber plant, and Rhoeo discolor, indicate that they belong in a

separate class. In these plants there is little or no diffusion of carbon

dioxide through the epidermal cells and the stomates are the primary pathway

through which carbon dioxide diffuses during photosynthesis. A partial

explanation for the difference between these two groups of plants with

respect to carbon dioxide exchange through the astomatous epidermis may

be found in the thickness of the epidermis or cuticle. There probably are

other factors as indicated by the data for begonia, which has a rather thick

cuticle. At any rate the data for these plants, having stomates in the

SUMMARY OF THE DETERMINATIONS OF APPARENT PHOTOSYNTHESIS IN TERMS OF THE

MG. OF CO2/DM.2/HR. STOMATES/CM.2

COLEUS ... .. 3.1 10.1 0 11,000

AVOCADO .... 4.1 5.9 0 15,200

POINSETTIA 0.6 4.1 0 13,000

RUBBER PLANT ......* 2.3 0 17,000

BEGONIA 2... 2.2 3.3 0 3,400

RHOEO DISCOLOR * 3.5 0 1,700

BEAN 3.8 1,000 7,000

TOBACCO 3.1 3.6 3,000 3,000

GERANIUM .... 1.3 3.7 2,300 15,000

* Not significant.

lower surface of the leaves, indicate that the statement of Blackman and

others that the stomates are practically the sole pathway for carbon dioxide

into and out of leaves under normal conditions is not true in many cases.

The data confirm the conclusions of Mitchell, Heinicke, and others to the

effect that some plants may carry on a rather high rate of photosynthesis

with the stomates closed.

The results for bean, toba2co, and geranium which have stomates in both

the upper and lower epidermis of the leaves must be examined from a different

point of view. For these plants the amounts of apparent photosynthesis

related to stomatal diffusion and epidermal diffusion respectively

cannot be separated. In tobacco there is a direct correlation between

stomatal frequency and apparent photosynthesis for the upper and lower

side of the leaves. As indicated above there is no way of determining how

much, if any, of the carbon dioxide exchange was due to epidermal diffusion

exclusive of stomates. For geranium the degree of correlation is

much less and for bean it is probably nonexistent. Therefore, one must

conclude from these data that for those plants having stomates in both the

upper and lower epidermis of their leaves the diffusion of carbon dioxide

into the leaves during photosynthesis may or may not show a direct correlation

with stomatal frequency and distribution. It seems probable that

in some plants, at least, some other factor (or factors) other than stomatal

frequency and distribution plays a considerable role in determining the

pathway of carbon dioxide exchange during photosynthesis.

1. This is a report of measurements made to determine the relative significance

of stomates versus the epidermis, exclusive of stomates, as routes

followed by carbon dioxide which diffuses into leaves during photosynthesis.

2. Plants were selected for the experiments to obtain wide variations in

stomatal frequency and distribution, and thickness of cuticle.

3. For those plants having stomates only on one side of their leaves,

considerable variation occurred with respect to the absorption of carbon

dioxide through the epidermal cells during photosynthesis. In all cases

the rate of apparent photosynthesis through the stomatal bearing epidermis

was greater than through the astomatous side. In some plants the

amount of carbon dioxide exchange through epidermal cells alone was very

significant sometimes being approximately equal to the amount which diffused

through the stomates. In other plants of this group, particularly

those with a thick cuticle, little or no apparent photosynthesis could be

detected through the epidermis without stomates.

4. The data for those plants having stomates in both the upper and

lower epidermis indicate that apparent photosynthesis, in terms of CO2

absorption, may or may not show a direct correlation with stomatal frequency

and distribution.

DEPARTMENT OF BOTANY

NORTHWESTERN UNIVERSITY

LITERATURE CITED

1. BLACKMAN, F. F. Experimental researches on vegetable assimilation

and respiration. II. On the paths of gaseous exchange between

aerial leaves and the atmosphere. Phil. Trans. Roy. Soc. B. 186:

2. BROWN, HORACE T., and ESCOMBE, F. Static diffusion of gases and

liquids in relation to the assimilation of carbon and translocation

in plants. Phil. Trans. Roy. Soc. B. 193: 223-294. 1900.

3. FREELAND, R. 0. Automatic electric switch for constant air pressure.

Sci. 102: 231-232. 1945.

4. HEINICKE, A. J., and CHILDERS, N. F. The daily rate of photosynthesis,

during the growing season of 1935, of a young apple tree

of bearing age. Cornell Univ. Agr. Exp. Sta. Bull. 201. 1937.

600 PLANT PHYSIOLOGY

5. MASKELL, E. J. Experimental researches on vegetable assimilation and

rates in cherry laurel. Proc. Roy. Soc. B. 102: 488-533. 1928.

6. MAXIMOV, N. A. Plant Physiology. McGraw-Hill Book Co. 1938.

7. MEYER, B. S., and ANDERSON, D. B. Plant Physiology. D. Van Nostrand

fixation by plants. Bot. Gaz. 98: 87-104. 1936.

9. NUTMAN, F. G. Studies of the physiology of Coffea arabica. II.

Stomatal movements in relation to photosynthesis under natural

conditions. Ann. Bot. N. S. 1: 681-694. 1937.

10. SCHNEIDER, G. W., and CHILDERS, N. F. The influence of soil moisture

on photosynthesis, respiration, and transpiration of apple leaves.

Plant Physiol. 16: 565-583. 1941.