The physiological function of the cereal awn

T H E P H Y S I O L O G I C A L F U N C T I O N O F T H E C E R E A L A W N

F. J. GRUNDBACHER* D e p a r t m e n t o[ A g r o n o m y

Un ive r s i t y o] Cal i]ornia

Davis, Cali]ornia

Introduction ................................................................................................................................................................. 366

Anatomy of the Cereal Awn .................................................................................................................. 366

Origin of the Carbohydrates Accumulated in Cereal Grain ................................. 368

Physiological Function of the Awn ................................................................................................... 370

Discussion ...................................................................................................................................................................... 375

Summary and Conclusions .......................................................................................................................... 378

Acknowledgments ................................................................................................................................................... 379

Literature Cited ................................................................................................................................................... 379

INTRODUCTION The physiological function of awns has long been of interest because

under certain climatic conditions awned varieties outyield awnless ones. Transpiration and photosynthesis have been considered to be possible functions of the awn in contributing to grain yield. As early as the previous century it was found that awns are sites of intensive water transpiration, and numerous later experiments have confirmed these findings. Only a few experiments have been carried out in regard to the photosynthetic activity of the awn. In the following discussion a brief description of the anatomical features of the awn will be included in order to obtain a broader outlook as to the potentialities of this organ. The present concepts concerning the sites and time of photosynthesis producing the organic matter for cereal grain also will be mentioned.

ANATOMY OF THE CEREAL AWN The anatomical attributes of a plant organ may provide some indi-

cation of its physiological functions. The awns of barley, wheat, oats, and rye are quite similar in structure, and the following anatomical description of the barley awn, based mainly on the works of Schmid (36) and Miller et al. (27), is given as an example.

*Presently USPHS Postdoctoral Fellow, Dept. of Human Genetics, University of Michigan, Ann Arbor, Michigan.

366

THE CEREAL AWN 367

In cross-section the barley awn has triangular form, becoming smaller towards the apex. The base'of the triangle is directed towards the rachis. Four types of tissue are always present: epidermal, sclerenchyma, con- ducting, chlorenchyma. The epidermal cells are short and thick-walled, and have a high silicon content, especially as the plant approaches ma- turity. Sharp, hooked epidermal hairs are a characteristic of rough- awned barley. The exterior walls of the epidermal cells are very thick and frequently perforated with pits or canals which extend to the cu- ticle. Each of the two outer faces of the awn possesses two to four rows of stomata, while stomata are entirely lacking on the inner face of the awn. The stomatal pores have their long axis parallel to that of the awn.

The chlorenchyma is located underneath the epidermis on the outer faces of the awn and constitutes approximately one-third of the total awn. It consists of two strands which remain separate along the length of the awn and become fused only near the apex. The cells of the chlorenchyma contain great numbers of chloroplasts.

The mechanical tissue lies below the epidermis, with the exception of the two places where the chlorenchyma strands are located. The mechanical tissue has two main functions (34): the cells nearest the epidermis give support, and those with numerous pits lying near the center function in conduction, especially of water.

One large and two small vascular bundles are present in each awn. The large bundle is actually the continuation of the vascular bundle of the lemma (26). It is a typical monocotyledonous type, while the smaller ones are often much reduced, especially in the distal part of the awn (34).

Comparing the awn surface to the total exposed surface, Schmid (36) found that this ratio varies widely, depending on the size of the awns. For example, in a four-rowed barley with long awns, the total exposed surface of the spike was about 125 cm 2, of which the awns contributed 75 cm 2 or three-fifths of the total spike. The stomata-free inner surfaces of the awns contributed 33 cm 2, and the two outer sides 42 cm 2. For the same plant the leaf blades amounted to 173 cm 2 per plant, but Schmid noted that some leaves appeared yellowish, and their functional ability was probably not at full potential any more. Accord- ing to Schmid, the glumes and the lemma of the spikelets have relatively few stomata; on the other hand, stomata are comparatively numerous on the awns, suggesting possible differences in their potential

368 THE BOTANICAL REVIEW

for gas exchange. Thus, calculations based on area alone may not be suitable for estimating photosynthetic or transpirational potential, especially when one notes position in canopy and relative illumination.

Perlitius (29) investigated the distribution of stomata on various parts of the cereal floret. On the average, wheat and barley awns possess 55-60 per cent and 72-75 per cent, respectively, of the total number of stomata of the spikelet. He calculated that the awns of Squarehead wheat increased the plant surface adjusted to equal stomata densities by one m 2 per m 2 of soil surface. For barley the increase of active plant surface by the awns was even greater, and in a large-awned variety it may amount to three m 2 per m 2 of soil surface. This increase was considered to amount to about ten per cent of the total active plant surface. This is a remarkable increase, especially when it is considered that at the time of heading, some of the leaves are already senescent or heavily shaded.

Cereal awns have been regarded as rudimentary leaves ( 1, 29, 30, 35, 40) because of the presence of stomata on them. However, there exists some disagreement to this view, since stomata are not limited to leaves (9). The presence of chlorenchyma tissue, stomata, and vascular bundles in the awns is an indication of the photosynthetic capacity of these organs (26, 27). Favorable conditions exist for photosynthate translocation, inasmuch as the vascular route from an awn to the developing kernel is short and direct (26).

ORIGIN OF THE CARBOHYDRATES ACCUMULATED IN CEREAL GRAIN

According to Porter et al. (32), the dry weight of the mature barley spike constitutes about 50 per cent of the total dry weight of the aeriat parts, and this large fraction of the dry weight is accumulated in the main over a period of four to five weeks, as compared to about ten weeks required for growth of the vegetative organs. They pointed out that there must be a considerable transfer to the spike of carbohydrates synthesized during the vegetative stage of growth, or that very active assimilation must accompany growth of the spike. According to Arch- bold (3) , an early view that leaves supply this carbohydrate from current photosynthesis was superceded early in the present century by the concept that reserve materials are stored mainly in the stem during the vegetative stage of growth and form the principal source of carbon for

THE CEREAL •WN 369

the spike after heading. This latter view was based on the observation by Deherain and Dupont (15) in 1901 that during active starch synthesis in the grain, the leaves of cereals are senescent and thus unlikely to be capable of producing the necessary sugar by photosynthesis.

Both views overlooked the possibility of new sites of photosynthesis. It thus was necessary to consider the functional importance of other green surfaces as photosynthesizing organs, in particular the spike itself. Boonstra (12), by shading the spike and other parts of his plants, and by using various defoliation methods, found that the spike is important in the synthesis of carbohydrates. He concluded from his studies that, on the average, the spike of wheat contributes 30 per cent of the dry matter required for filling of the grain, the peduncle 10 per cent, the stem and leaf sheathes 35 per cent, and the leaf blades 25 per cent. He concluded further that the spike has a much greater effect on yield than was usually assumed and that the carbon needed for starch synthesis in the grain is supplied by current photosynthesis.

Archbold and his associates (2, 4) came to similar conclusions which were based on a long series of experiments dealing with sugar metabolism in barley. They did not find any evidence for migration of sugar from the stem to the spike of a magnitude which would be effective. The free sugars in the barley stem therefore are not considered to be precursors of starch in the spike.

Asana and Mani (5, 6) estimated that in wheat the contribution of the spike to the carbohydrate supply of the grain falls within the range of 11-46 per cent depending on season and variety.

The first direct measurements of CO2 assimilation by barley spikes under natural conditions were made by Porter et al. (32). Their results, that about 30 per cent of the dry weight of the spike arose from auton- omous assimilation, confirmed previous estimates from shading experiments (2, 6, 12, 13, 32, 41, 42).

Watson et al. (41) studied the causes of differences in grain yield among three barley varieties. The varieties did not differ in leaf area index nor in net assimilation rate before spike emergence, so that all had the same total dry weight at heading. This led them to conclude that the difference in yielding ability was conditioned by additional photosynthesis in parts of the plants other than the leaves, i.e., in the spikes themselves. Averages of the three varieties showed that 26 per


cent of the dry matter in the grain at harvest originated from photosynthesis in the spikes, including ten per cent from the awns.

Translocation studies with 14CO2 have shown that only the upper- most leaves of barley contribute to any remarkable extent to the accumulation of carbohydrate in the grain (13, 17, 23). In addition to con- ducting studies with C t~, Buttrose and May (13) applied the shading technique to barley. Shading of the spike of awnless, single-awned, and triple-awned varieties of barley led to significant reductions in yield of 9, 23, and I8 per cent, respectively. The authors state that the relatively low value obtained for the photosynthesis of the spike might have been due to increase in translocation from other parts of the plant.

The results of the experiments mentioned above indicate that the cereal spike is an important site of photosynthesis. It was estimated that on a broad average the spikes of wheat and barley contribute about 30 per cent to the accumulation of organic matter in the grain; however, for barley maximum values of 50 per cent (6) and 76 per cent (16) have been mentioned. The contribution of the spike varies, depending on the peculiarities of a variety (42).

PHYSIOLOGICAL FUNCTION OF THE AWN

The physiological importance of the cereal awn was studied in both laboratory and field experiments. Investigations on awn activities, such as transpiration and photosynthesis, were mainly confined to laboratory experimentation, while in field trials the relative contribution of awns to grain yield was measured. Field experiments consisted of three types: comparison of a large number of awned versus awnless varieties, the effect of awn removal, and genetic studies.

Numerous field trials have shown that awned cereal varieties outyield awnless ones, especially under warm and dry climatic conditions. In certain cooler and more humid regions, however, awns had apparently no or little effect on grain yield. The literature on this subject has been reviewed by Miller et al. (27) and Vervelde (40). There- fore, in the present discussion only one experiment is mentioned as an example of a comparison between awned and awnless varieties. Gran- tham (18) in Delaware reported the results of 26 tests covering ten years and including 1986 varieties and strains. The awned varieties out- yielded the awnless ones by 3.3 bushels per acre. The difference in yield was caused largely by the poorer quality of the grain from the awnless

THE CEREAL AWN 371

varieties. The smooth wheats had a much higher percentage of shrunken kernels than the awned ones, especially under unfavorable conditions.

As early as 1882 Zoebl and Mikosch (43) published the results of studies in which they found that the presence of awns had a favorable influence on the kernel size. This finding stimulated investigations concerning the physiological activity of awns. They found that awns of barley are structures carrying out intensive transpiration, since spikes whose awns had been removed transpired only one-fifth to one-fourth as much as unclipped heads under the same conditions. The studies further revealed that transpiration from the spikes was most intense when the accumulation of materials into the spike was at its maximum. Hence, it was concluded that the rate of transpiration was in direct relation- ship to the intensity of metabolism in the spike.

Schmid (36), in his transpiration studies, confirmed the results of Zoebl and Mikosch. However, he pointed out that the amount of water transpired by the awns is variable, depending on external factors. Also dead awns were able to transpire at one-half the rate of living awns. The stomata of the dead awns were closed; thus water loss was attri- buted to cuticular transpiration.

Awnless barley and de-awned spikes behaved similarly as far as the amount of water transpired was concerned. Schmid concluded that the awnless barley had not developed a replenishment within the spike, and he expressed the opinion that a compensation effect from the other parts of the plant may account for the reduced transpiration due to the absence of awns.

Despite a reduction of transpiration of the spike by about 70 per cent after de-awning, the mineral content of grains from awned and de-awned spikes was practically the same. In this connection Schmid pointed out that at the stage of milk ripeness, 95 per cent of the min- erals have been taken up by the plant; and he concluded that the high transpiration activity of the awns could hardly indicate a high rate of mineral fixation in the spike. Likewise, Miller et al. (27) did not find any alteration in the mineral content of grains of spikes whose awns had been removed. Hence, these experiments do not indicate that transpiration has an effect on mineral transport; however, even today this problem remains unsolved and conflicting evidence has been presented ( 10, 24).

Photosynthetic activity of the awns was first measured by Schmid


(36) who determined the C Q assimilation in a closed system. From the difference of the amount of CO., introduced into the system and the amount of CO,, recovered after one-and-a-half to three hours, he calculated the assimilatory activity. His results clearly show that the awns carry out photosynthesis. For example, in the four-rowed barley Mam- muth the assimilation rate of the awned spike was four times that of a de-awned spike, intact spikes of the two-rowed variety Chevalier assimi- lated twice as much as their de-awned comparison, and an awnless barley behaved similar to that of de-awned spikes. He estimated that photosynthesis of the awns during formation of the grain contributed up to one-sixth of the total photosynthetic activity of a barley plant. In all cases CO,, assimilation was higher in awned spikes than in de- awned or awnless spikes. The chlorenchyma of the spikes contained small amounts of starch, even after warm and sunny days. Also, the awns showed high respiration rates.

According to Schmid's conclusions, awns have in general a dual role: one is manifested in protection against animals and as a mechanism of seed dispersal; another, physiological activity, makes a considerable contribution to the formation of the fruit. The contribution of awns in metabolism is positively correlated to their size (27, 29, 36, 40).

Radioactive carbon as a tool of investigation was used by McDonough and Gauch (26) on awned Durum wheat. They studied photosynthesis and translocation by exposing parts of the plants to 14CO2. These studies showed that appreciable amounts of photosynthate were translocated from the awns to the kernels under conditions of intermediate soil moisture. The contribution by the awns to total kernel weight was 12 per cent of that by the entire plant. At soil moisture levels close to the wilting point the percentage contribution of the awns to kernel dry weight increased, while the contribution of the flag-leaf blade decreased. They concluded that this finding explains how and why awned varieties outyield awnless ones in semi-arid regions and why there is little or no advantage in terms of yield of awned over awnless varieties in humid regions.

Removal of awns was used by several investigators in order to de- termine their contribution on grain yield. Harlan and Anthony (19) found that plants from which the awns were removed produced only 75 per cent of the yield of normal plants. The grain from de-awned spikes had a smaller volume and a lower test weight than did those

THE CEREAL AWN 373

from awned spikes. This difference in yield was not considered to be due to injury or shock of removing the awns. About one week after flowering, which is near the period of rapid starch formation, the accumulation of dry matter in the grain of the awned spikes began to exceed that in the grain of de-awned spikes. The daily deposition of nitrogen and ash was more nearly equal in the two types of spikes than was the increase in starch. Harlan and Anthony further observed that the rachises of clipped spikes contained about 25 per cent more ash than the rachises of normal spikes. They suggested that the additional ash in the rachises of the clipped spikes might have been responsible for the tendency of these spikes to break and, hence, elimination of the awns could result not only in lower kernel weight but in increased shat- tering as well. Rosenquist (33) observed that the grain of clipped heads of wheat weighed only 82.7 per cent as much as those from the awned ones.

Miller, Gauch, and Gries (27) included in their de-awning experiments four stages for removal of the awns: 7 to 10 days prior to anthesis, at anthesis, one week after anthesis, and two weeks after anthesis. The weighted average of the percentage decreases in grain weight for three years was 11.15, 9.37, 4.37, and 2.33 respectively, for the four stages of de-awning. The decrease in grain weight represented 50 to 80 percent of the decrease in grain yield due to de-awning. The number of grains formed per head was reduced slightly by de-awning. The earlier the de-awning was performed, the greater was the reduction in the number of kernels.

Bonnet (11) showed in his emasculation studies that injury of a part of the floret may result in reduction of the kernel weight. For example, normal florets produced kernels of an average weight of 49.1 mg. Florets whose flowering glume had been split formed kernels of an average weight of 41.6 mg., indicating a remarkable decrease in kernel weight due to injury.

Some genetic studies also revealed that the awns may have a favorable effect on grain yield, while in others no difference could be found between awned and awnless cereal strains. Moskalenko (28) investigated the hybrids of winter wheats with regard to awnedness as a factor of productivity. From a study extended over six years he found no re- lationship between the presence of awns and productivity under the climatic conditions of the Ukraine.


Suneson et al. (38) compared awnless Baart with awned Baart and awned Onas with awnless Onas wheat derived by reciprocal backcross- ing from a cross of Baart x Onas. The field trials were carried out at 16 locations in North America. In general, the awned forms were superior in yield, test weight, and kernel weight under both irrigated and semi-arid conditions. Exceptions to the general responses occurred at a few stations. But only at three stations was the mean yield of an awnless strain (tested during one, two, or three years) higher than that of its awnless counterpart. This occurred at two irrigated stations and in Ottawa, Ontario. The latter station was located the farthest east of all the locations at which the tests were carried out. The exceptions were considered by Suneson to have occurred by chance alone. However, the difference, e.g., of the climate in Ottawa, Ontario, from that of the West might account for the different effect of awnedness in comparison to the other stations.

Vervelde (40) published the results obtained from F2 segregates resulting from numerous crosses between awned and awnless varieties of various cereals. Under the prevailing experimental conditions the contribution of the awns to the grain yield was about ten per cent for spring barley, three to five per cent for winter wheat, and less than two per cent for winter rye.

Atkins and Norris (7) determined the effect of wheat awns on yield by means of ten pairs of specially developed isogenic awned and awnless lines obtained by ten generations of selfing. The authors assumed that each line within a pair was identical phenotypically and genetically, except that one line of the pair was awned and the other awnless. The awned lines produced significantly higher yields, heavier kernels, and higher test weights. The favorable influence of the awns on yield and weight of kernels was expressed more distinctly in drought years when the crop was under stress. Atkins and Finney (8) extended the studies to the influence of awns on several quality characteristics of wheat. The properties of the kernels of awned and awnless lines appeared identical with two exceptions: the test weight of the awned segregate was usually higher, whereas the bread loaf volume (a quality factor related to protein) of the awnless segregate was generally slightly higher.

A particular barley mutant, elburata (with green awns and white glumes), gave further indications on the importance of the contribution of the awn on grain yield. Sagromsky (34) observed that the mu-

THE CEREAL AWN 375

tant elburata had a lower kernel weight than the original form with green awns and green glumes. Removal of the awns resulted in a much greater reduction of the kernel weight in the elburata mutant than in normal barley. It was concluded that photosynthesis of the awns and glumes itself or related processes played a major factor in the formation of the cereal fruit. Sagromsky further noted that removal of the awns at bloom or shortly thereafter causes omission of nutation.

DISCUSSION

The anatomical properties of the awn indicate great capacities for transpiration as well as for photosynthesis (26, 27, 36), and experimental results have revealed that cereal awns are important sites of water transpiration and photosynthesis (13, 26, 29, 36, 43). Reports in the literature indicate that, especially under semi-arid conditions, awns have a favorable effect on grain yield. The question arises as to why the awned are superior to the awnless cereal varieties under warmer and dryer conditions, while in a cool and humid climate there appears to be no difference or occasionally yield-reducing effects of awns have been observed. Increase in productivity in dry conditions indicates that the awns are better adapted to such conditions than other organs of the plant which contribute to the accumulation of carbohydrates in the grain.

As a physiological basis for this advantage, Sande-Bakhuyzen (35) mentioned that the awn is a young organ which is in its most physio- logically active stage during seed development. Furthermore, he stated that awns have xeromorphic characters. Like others ( 1, 29, 30, 36, 40) he considered the cereal awn to be a reduced leaf where slightly more than the midvein has remained. As Vervelde (40) pointed out, by such reduction numerous xerophytes have attained a favorable water economy in photosynthetic tissues. Xerophytes are able to produce more dry weight per unit of water used (39). Furthermore, organs with xerophytic properties have a greater ability to withstand desiccation without permanent injury (25), a factor which may have considerable importance under semi-arid conditions.

Another reason why awns may have a favorable influence on yield under high temperature and low rainfall is the increase of the assimilatory tissue of the plant by the awns. The life of a cereal plant in a dry and hot climate usually is shorter than the life of a plant in a cooler and

376 T H E BOTANICAL REVIEW

more humid climate. Under semi-arid condtions, in which the plant has a shorter time available for production of carbohydrates for fruit formation, additional assimilatory capacities of the awns may become of greater importance. Since awns are located in close proximity to the developing caryopsis, no difficulties should arise in translocation of photosynthates from the assimilatory tissue of the awn to the fruit (26). The awn may still be capable of photosynthesis and translocation of the assimilates to the cereal fruit for several days after the foliage leaves have desiccated. Since the stem yellows later than the leaves, the essen- tial water supply for photosynthesis may still be translocated to the awns after the leaves have ceased their metabolic activities.

Some experiments have indicated that awns had a negative effect on grain yield. Vervelde (40) considered the apparently negative effects of awns on yield as being due to sampling error. However, the possibility should not be excluded that awns may have an unfavorable indi- rect influence on commercial yield, especially under humid conditions. For example, in more humid areas, such as northwestern Europe, lodging is one of the main limiting factors for high yield. It is likely that awned varieties have a greater tendency for lodging than awnless ones due to excessive accumulation of moisture by the awned spikes during rain. Furthermore, an awned spike offers a greater surface of attack to the pushing wind which usually accompanies rain. Schmid (36) observed quite early that awns may be heavily attacked by fungi, such as rust and mildew. Since fungi thrive better under humid conditions, it may be expected that awns become more damaged by disease in a humid climate than under semi-arid conditions.

Accurate estimates of the photosynthetic contribution of awns to the accumulation of carbohydrates in cereal fruits are difficult to obtain, and the results may vary, depending on the method of investigation. Some methods of study have secondary effects which may bias the results. For example, Watson and Norman (41) observed in their experiments that shading of the spikes decreased the water content of the spike and possibly upset the metabolism of this organ. Besides a decrease in photosynthetic activity, a disturbed metabolism in the spike may be responsible for a decrease in grain yield by shading. Also, secondary effects from injury by removal of the awns have been mentioned (11). Quantitative estimations of the contribution of awns on yield by use of isogenic lines, as was done by Atkins and his co-workers (7, 8), may

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give more reliable results than shading or clipping of awns because of the absence of secondary effects. Likewise, the technique used by Mc- Donough and Gauch (26), measuring the photosynthetic activity of awns by means of 14CO.,, may give unbiased results.

Cereal awns are in a very favorable photosynthetic position in so far as they are located far above the foliage leaves. In this position the awns are relatively free from mutual shading, as may be the case with the foliage leaves.

Increased weight of the grain of awned varieties in comparison to awnless ones is mainly due to an increase of starch content (8, 19, 22, 29, 36), i.e., an increase in photosynthates. Therefore, the remarkably higher yield of awned cereals under semi-arid conditions appears to be the result of the assimilatory contribution of the awns. The presence of relatively large proportions of chlorenchyma in the awns, and the fact that the awns are relatively young organs and their most produc- tive stage coincides with the time of most intense carbohydrate accumulation in the fruit, would support such a conclusion.

The effects of water transpiration from awns are unclear, and con- tradicting views have been expressed. Vervelde (40) stated that awns can effect the transport of assimilates towards the developing seed only under conditions of extremely rapid evaporation. He also mentioned that there is little reason to expect that awns will increase water consumption of a cereal crop under field conditions, as one might be in- clined to conclude from laboratory tests with single plants or spikes, since the water consumption of a closed vegetation is predominantly controlled by atmospheric conditions. Furthermore, he concluded that a greater part of the evaporated water of awned plants led through the spike will result in less transpiration by the other parts of the plant. According to Vervelde (40), the transpiration of the awns will, therefore, not have such vital consequences for the plants, i.e. to explain the generally observed influence on yield.

An opposite view was expressed by Schulte (37) who considered the regulation of the water economy of the flower, not the photosynthetic activity, to be the main function of the awns. However, awnless cereals are also able to regulate their water economy. Furthermore, the fact that grains of awned cereals merely have a higher starch content while the other compounds of the grain are practically the same (as


in the grain of awnless cereals) is above all an indication of differences in photosynthetic rate between awned and awnless plants.

Huber (20), reviewing the evidence for the effect of the transpiration stream, concluded that ions can be moved by mass flow. However, this question is still under discussion and opposing opinions exist ( i0, 24). In this connection, the fact that practically no difference was found in the absolute mineral content of awned and de-awned spikes, in spite of great differences in transpiration rates (19, 36), does not support Huber's view.

Little is known about the importance of transpiration. Kramer (24) expressed the opinion that transpiration per se contributes very little directly to plants and can, therefore, best be described as an "unavoidable evil". It is unavoidable because of the particular structure of plants. However, he remarked that plants which have very low rates of transpiration usually also have low rates of photosynthesis and grow slowly. For example, Polster (31) measured the rates of transpiration and photosynthesis of seven tree species, and found that those with high rates of transpiration also showed high rates of photosynthesis and those with low rates of transpiration also had low rates of photosynthesis. These findings are in agreement with the suggestion by Hygen (21) that, for a given set of conditions, the rates of transpiration are an indication of gas exchange and rates of photosynthesis.

Clum (14), investigating a possible cooling effect resulting from transpiration of foliage leaves, found that its magnitude is very small compared with effects of light intensity, angle of incidence, radiation, and air currents. Since cereal awns are so extremely exposed to radiation and air currents, transpiration would have very limited cooling effect.

As far as transpiration by awns is concerned, no concrete conclusions are possible. More basic studies on the effects of transpiration in general are desirable. As soon as more knowledge has been obtained on this matter, it may be possible to conclude whether or not the relatively high transpiration rate of the cereal awn has a favorable influence on grain yield.

SUMMARY AND CONCLUSIONS The cereal spike is a site of photosynthesis which contributes, on

the average, about 30 per cent of the carbohydrates accumulated in the mature cereal grain.

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Cereal awns consist of about one-third chlorenchyma tissue which is

overlain by stomata. The main physiological functions of the awns are

photosynthesis and transpiration. The significance of transpiration of

awns is still unclear. Awns as assimilatory organs may contr ibute more

than ten per cent of the total kernel dry weight. The contr ibut ion of

awns depends mainly on their size and environmenta l conditions. Es-

pecially in a warm and semi-arid climate, awns have a favorable in-

fluence on grain yield, possibly because awns, due to their xeromorphic

structure, are better adapted than leaves to such conditions.

A C K N O W L E D G M E N T S

The author wishes to express his thanks to Dr. R.S. Loomis and

Mr. C.O. Qualset for critical review of this article in manuscript .

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THE CEREAL AWN 381

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The physiological function of the cereal awn