UUL 0:319”! . d 4" ‘ \fWA A fr")7|I ‘w‘. .."Illvvl-‘ ENVIRONMENTAL FACTORS AFFECTING THE GROWTH OF CUCUMIS SATIVUS L., WITH SPECIAL REFERENCE TO CARBON DIOXIDE BY Herbert J;\Hopen AN ABSTRACT OF A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1962 / " / ’ Approved ‘2 K/lZJ/p ABSTRACT ENVIRONMENTAL FACTORS AFFECTING THE GROWTH OF CUCUMIS SATIVUS L., WITH SPECIAL REFERENCE TO CARBON DIOXIDE by Herbert J. Hopen Recent advances in improved varieties and plant growing technology have resulted in larger and higher yielding economic crop plants. To obtain maximum growth and yields from economic crop plants, factors affecting field growth have been intensified to a high degree. A study of the field growth of pickling type cucumbers showed that yields and plant growth were not greatly in- creased by intensification of common cultural practices over yields obtained when current recommended cultural practices are followed. Thus, growth factors other than the common cultural practices must have a large effect on yield. Carbon dioxide and light functioned as compensative factors in the growth of cucumber seedlings. In growth chamber studies added carbon dioxide at a level slightly above the normal atmospheric carbon dioxide concentration was shown to increase the plant height, fresh and dry weights and number of internodes developed of pickling cucumber seedlings in direct proportion to the amount of Barbert J. Hopen - 2 carbon dioxide added. Carbon dioxide concentrations up to seven-fold normal increased seedling growth, but at a lower rate per unit of carbon dioxide supplementation. The largest benefits from carbon dioxide enrichment were reached at 1400 and 1000 foot candles compared to smaller benefits at 700 and 300 foot candles. As light levels were intensified (up to 1400 foot candles) the growth of cucum- ber plants increased at a more rapid rate with high levels of carbon dioxide than under low levels. A higher starch content was present in leaves grown in enriched carbon dioxide atmospheres. The atmosphere at sunrise over muck soil was shown to contain more carbon dioxide than over mineral soil. A fruit yield increase could not be claimed from the higher carbon dioxide concentration due to lack of control of the soil and temperature effects on the two soil types. In— creased seedling growth and fruit yield were obtained during the early growing season, probably from carbon dioxide evolved from decomposing manure in a field enclosure. ENVIRONMENTAL FACTORS AFFECTING THE GROWTH OF CUCUMIS SATIVUS L., WITH SPECIAL REFERENCE TO CARBON DIOXIDE BY {x 3 Herbert J5 Hopen A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1962 é: 51f.“ -”3 / 15/24/52. ACKNOWLEDGMENTS The writer wishes to express his appreciation to Dr. S. K. Ries for assistance and guidance in the planning and analysis of this study and to the other members of the guidance committee; Dr. R. L. Carolus, Dr. S. H. Wittwer, Dr. R. E. Lucas, Dr. D. R. Isleib and Dr. C. M. Harrison. Appreciation is also expressed to other members of the Horticulture Department for advice and assistance during the course of this study and the National Pickle Packers Association for partial financial support. The author wishes to acknowledge the help of Mr. Max E. Austin for photographic assistance. ii H53, DIS: SUM} LIT} TABLE OF CONTENTS ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . LIST OF TABLES . . . . . . . . . . . . . . . . . . LIST OF FIGURES . . . . . . . . . . . . . . . . . . INTRODUCTION . . . . . . . . . . . . . . . . . . . REVIEW OF LITERATURE . . . . . . . . . . . . . . . Introduction Growth Factors Other Than Carbon Dioxide Carbon Dioxide Summary MATERIALS AND METHODS . . . . . . . . . . . . . . . Exploratory Survey The Effect of Carbon Dioxide and Light and Their Interaction Comparison of the Carbon Dioxide Concen- tration Over Muck and Mineral Soils with the Growth of Pickling Cucumbers in the Field RESULTS . . . . . . . . . . . . . . . . . . . . . . Exploratory Survey The Effect of Carbon Dioxide and Light and Their Interaction Comparison of the Carbon Dioxide Concen- tration Over Muck and Mineral Soils with the Growth of Pickling Cucumbers in the Field DISCUSS I ON n o o o o o c o o 0 a I o o o o o a I O SIJMMARY o o o o o 0 o I o a o I 0 0 I o I I o o n I LITERATURE CITED 0 O o o o o o o o o O O o o o I o Page ii vi \OO‘WLAJLAJH p.- 21 25 34 39 39 45 65 73 82 84 10. 11. 12. 13. LIST OF TABLES Pounds of elemental nutrients applied per acre in nutrient source experiment . . . . . Yield of cucumbers with two levels each of fertility and spacing . . . . . . . . . . Fruit yield in the manure and commercial fertilizer experiment . . . . . . . . . . . "Possible sunshine" during the 1959 growing season at East Lansing, Michigan . . . . . . An example of light intensities on July 7 ’ 1959 O O I I O O O O O O O I O O O 0 Pounds of fruit harvested per plot following a low light intensity period and total yield for seas0n from differential light levels . Dry weight, height and internodes developed at 350 ppm and 450 ppm of carbon dioxide . . Dry weight, height and internodes developed at the four radiant energy levels with 350 ppm and 450 ppm carbon dioxide levels combined . . . . . . . . . . . . . . . . . . Dry weight of seedlings grown for 21 days in 450 ppm and 1250 ppm carbon dioxide . . . . Dry weight, height and internodes developed at 500 ppm and 2150 ppm of carbon dioxide . Dry weight, height and internodes developed at the four radiant energy levels with 500 ppm and 2150 ppm carbon dioxide levels combined . . . . . . . . . . . . . . . . . . Average dry weight and number of fruit developed at four light intensities for 21 and 35-day old plants at 450 ppm and 1350 ppm carbon dioxide . . . . . . . . . . Dry weight and number of fruit developed by 42—day old cucumber plants grown in 450 ppm and 1350 ppm carbon dioxide for one week . . iv Page 24 4o 41 42 43 44 46 47 51 54 55 63 64 LIST OF TABLES -- continued Table 14. 15. l6. 17. Page Daily maximum temperatures and precipitation for June, July, August and September, 1961, at the Michigan State Horticulture and Muck Experimental Farms . . . . . . . . . . . . . 67-68 Average bushels of fruit harvested per acre at two locations from 2 and 4 foot between- row spacings . . . . . . . . . . . . . . . . 69 Plant measurement and yield from two chambers in the field subjected to different carbon dioxide levels . . . . . . . . . . . . . . . 71 Average total number and pounds of fruit harvested per plot in the replicated enclosure study . . . . . . . . . . . . . . 72 4.; ll. LIST OF FIGURES Figure Page 1. Field light manipulation experiment show- ing shaded, supplemental light and normal light plots . . . . . . . . . . . . . . . . . 23 2. Mylar growth chambers covered with black polyethylene . . . . . . . . . . . . . . . . 26 3. Average dry weight and internodes developed for seedlings grown at 350 ppm and 450 ppm of carbon dioxide . . . . . . . . . . . . . . 48 4. Cucumber seedlings after 21 days at 1400 f.c., and 350 ppm and 450 ppm of carbon dioxide . . 49 5. Cucumber seedlings after 21 days at 1000 f.c., and 1250 ppm and 450 ppm carbon dioxide . . . 52 6. Cucumber seedlings after 21 days at 700 f.c., and 1250 ppm and 450 ppm of carbon dioxide . 52 7. Average dry weight of seedlings grown at 500 ppm and 2150 ppm of carbon dioxide and the interaction of carbon dioxide with light and time . . . . . . . . . . . . . . . 57 8. Dry weight at four radiant energy levels with 500 ppm and 2150 ppm of carbon dioxide 58 9. Cucumber seedlings after 21 days at 700 f.c., and 500 ppm and 2150 ppm of carbon dioxide 59 10. Cucumber seedlings after 21 days at 300 f.c., and 500 ppm and 2150 ppm of carbon dioxide 59 11. Dry weight of cucumber seedlings grown at 450 ppm and 1350 ppm of carbon dioxide in "age of plant" study . . . . . . . . . . . . 62 12.. Average carbon dioxide levels at sunrise, 2 inches above soil surface, at a mineral and muck soil area(1lndles apart) during the summer of 1961 . . . . . . . . . . . . . 66 vi .qu INTRODUCTION Michigan is the leading state in the production of pickling cucumbers (60). Pickling cucumbers also account for a significant part of the vegetable processing industry in Wisconsin, California and North Carolina. Numerous factors may limit the growth of plants, speCi— fically cucumbers. Growth factors which lend themselves to field experimentation and ultimate control under field conditions have been studied extensively, but there are still ecological factors in the growth of plants which cannot be regulated with ease in natural plant habitats. As more factors become regulated, more research should be oriented towards the unregulated factors. What are the factors limiting further growth increases if elevated levels of the controllable field growth factors do not provide a significant increase? The objective of this research began as a study of the growth of cucumbers under field conditions as "near optimum" as was attainable. When intensification of field growth factors did not pro- vide increases in plant growth of any significant magnitude, investigation of a largely uncontrolled factor in field growth——carbon dioxide concentration--was undertaken. Carbon dioxide and radiant energy interactions in cucumber 1 growth constituted a large share of the experimentation because the utilization of carbon dioxide varied with the incident radiant energy level on a leaf. REVIEW OF LITERATURE Introduction In the early twentieth century Blackman published a theory called "Optima and Limiting Factors" (6). The five controlling factors of a chloroplast engaged in photosyn- thesis are; the amount of carbon dioxide available, the amount of water available, intensity of available radiant energy, amount of chlorophyll present in the leaf and the temperature of the chloroplast. He further stated that "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." Growth Factors Other Than Carbon Dioxide Factors other than carbon dioxide may limit the growth of plants. Among these factors are available radiant energy, available moisture and nutrients, the intensity of temperature, wind velocity, plant population and photoperiod. Radiant energy has been described as the factor most frequently limiting photosynthesis of the full leaf surface of an apple tree (41). The maximum productivity of forest stands is set by the efficiency with which light is used in photosynthesis (43). The light intensity available to many cultivated crops varies from 2000 to 4000 foot candles (20). 3 8262 “'6' by. g is ”8”“ , 5 .3“ tior . ‘ . 3.5‘1: fl 1 .e, ““u I ‘ r .C e “E - "1 n. 01-. ‘l‘: Research workers do not agree on the amount of radiant energy required by a leaf for maximum assimilation. The optimum light intensity for maximum assimilation varies with species. This may fluctuate from three-fourths to full sunlight (7). Many investigators feel that 25 percent of full sunlight or about 2500 foot candles is adequate (1, 87, 91). Other scientists believe that 1000 to 1200 foot candles may be sufficient (42, 95). Shirley (83) states that "throughout the range of 1 to 15 percent of full sunlight (100 to 1500 foot candles) photosynthesis is almost directly proportional to light intensity, pro— vided other factors essential for rapid photosynthesis are available." Leaves require from 300 to 500 foot candles in order to maintain a balance between respira— tion and photosynthesis (l, 46, 68). Spectral energy distribution, as well as intensity, constitutes an impor— tant role in plant growth (46, 92, 94). The soil functions as a moisture reservoir for plants. Moisture stress may at times cause cessation of plant growth (93). Certain minerals are essential for plant growth and have been extensively discussed by Scarseth and Volk (80). The mineral nutrient levels for the growth of pickling cucumbers have been studied by several investigators (60, 61, 62, 74, 76, 77, 82, 99). There is no general agreement as to the nutritional conditions necessary for satisfactory yields of pickling cucumbers. This can be accounted for by the environmental conditions and cul- tural practices associated with the nutritional experiments on pickling cucumbers (60). Photosynthesis has a temperature coefficient of 2.0 or more under laboratory conditions. However, the tem- perature effects are obscured and complicated by other factors under field conditions (20). Temperature may interact with photoperiod to significantly affect yield and quality determination in pickling cucumbers (60). Excessive air movement may cause mechanical injury and desiccation (97). Windbreaks are often used under field conditions in an attempt to control air movement (25). Much research on cultural practices has been carried out with the intensity of plant population. Ries (76) obtained the highest early-season yields with pickling cucumbers at close spacings. The maximum population has been predicted in certain plant species as measured by carbon dioxide uptake and Leaf Area Index (LAI) (1, 13, 14). Brougham (14) has shown that maximum growth rates of pure stands of several speciesame closely related to the content of leaf and stalk chlorophyll above 95 percent light inter- ception at local noon. Therefore, the limiting factors for plant growth are the amount of chlorophyll exposed per unit area of land and the carbon dioxide content of the air surrounding the leaf. The total plant population, thus, L. must be distributed in such a manner as to provide the maximum area per leaf and plant surface. Nutrient, light, root and moisture competition are all factors in determin- ing competition in forest succession (83). All of these factors must be considered in plant population evaluation, whether it be for field crops or forest canopy. Carbon Dioxide The numerous factors previously discussed, which may limit growth, can be controlled or regulated under natural conditions by either geographic location or practical cul— tural procedures, but carbon dioxide is a factor which is very difficult to regulate and control under field condi- tions. It is very important to recognize that the carbon dioxide level is at times the minimum factor and thereby limits the amount of plant productivity (51). The normal atmospheric level of carbon dioxide is generally stated as 0.03 percent on a volume basis (20). However, this 0.03 percent volume basis is seldom main— tained in areas of vegetation during daylight hours (18) as the content varies from one-half to several times the normal value (18, 29, 57, 59, 67). After sundown, the level often rises slightly above 0.03 percent (18). Carbon dioxide and moisture are the only aerial compo- nents which vary appreciably in concentration throughout the vertical layers of the atmosphere. Oxygen remains proportionally constant to the drop in density at higher altitudes (27). Carbon dioxide has been recognized as important in plant growth since Priestly's discovery in 1772 that green plants "revitalize air." Subsequently, there have been papers published on the carbon dioxide content of the atmosphere for 200 years (19). Recently several studies have shown that the carbon dioxide level in the atmosphere is increasing (9, 12, 73). Fossil fuel combustion has added large amounts of carbon dioxide to the atmosphere in the last century (9). Some areas presently report an atmospheric content considerably above the normal level of 0.03 percent (9,65). The present increase in the atmosphere is probably on the order of 0.1 to 0.4 percent of the total atmospheric carbon level (9, 73). Revelle (73) summed up the situation as "within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedi— mentary rocks over hundreds of millions of years." In early geological periods much of the carbon dioxide was used for the deposition of carbonaceous materials (52). If the build-up of carbon dioxide continues at its present rate, it will become an increasingly important factor to be accounted for in the future analysis of plant growth. Several investigators have agreed that the present normal atmospheric concentration of carbon dioxide limits photosynthesis (ll, 51, 57). As the light level increases to 1000 foot candles, carbon dioxide often may become a limiting factor because increasing amounts can be uti- lized (47). The available light energy fixes an upper limit of carbon dioxide that can be utilized (6). Under normal light intensities a plant is more sensitive to variations in the carbon dioxide content of the air than variations in light intensity (57). It is possible that carbon dioxide may be more of a limiting factor to plant growth than low light levels during winter months under glasshouse conditions (34). The exact amount of carbon dioxide needed to reach leaf saturation has not been agreed SCI: 0.15 Blac stat such upon. Blackman (6) states that 1 percent is needed to reach an upper limit in direct sunlight while Nielsen (72) sub— scribes to the idea that land plants are saturated at 0.15 percent, unicellular algae at 0.05 percent and aquatic plants at a carbon dioxide concentration of 1.1 percent. Various modifications and concepts have emerged from Blackman's original theory. One of these modifications states that "It is possible for several factors in a process such as photosynthesis to lie in an intermediate range of supply simultaneously. An increase in any one of these factors will result in detectable yield increases. In an 'intermediate concentration' of carbon dioxide, carbon dioxide utilization is detectably less than 100 percent and an increase of other factors may enable greater efficiency of carbon dioxide utilization and higher yields“ (89). The original axiom may have been oversimplified. With a low concentration of carbon dioxide, the yield is linearly proportional to supply. In a higher concentration of carbon dioxide the utilization is such a small part of the total supply that carbon dioxide concentration has no detectable influence on yield. Thus, when one factor such as light is varied, another factor carbon dioxide, may be in abundance or scarce according to absolute values (89). 10 Even with the relatively low level of carbon dioxide in the atmosphere, an apple tree uses only about 12 percent of the carbon dioxide in normal air (41). By the develop- ment of large foliar leaf surfaces, the plant is able to assimilate the mass of carbon dioxide necessary for devel- opment (4). The assimilation rate of apple leaves has been shown to decrease if less than 2 liters of air per hour were supplied per square centimeter of leaf (42). It is difficult to reach an optimum utilization of carbon dioxide at any energy level (6) due to the fact that carbon dioxide utilization is never total (88). Two leaves, side by side, may not show the effect of a treatment because both may be limited by the carbon dioxide supply in the ambient atmosphere (6). With Sambucus pigga L. the thres— hold value or lowest atmospheric level of carbon dioxide available to the leaf was approximately 0.01 percent (30). Growth is materially decreased by levels as low as 0.02 percent (34). Temperature, stomatal movements and the uptake of mineral salts must be taken into account in assessing the action of carbon dioxide in assimilation (57). Carbon dioxide free air causes stomatal opening and pure carbon dioxide slowly closes the leaf stomata (39, 40, 81). A ll deficiency of moisture has been shown to decrease the assimilation of carbon dioxide. The assimilation decreases before wilting is noticeable (2). Species variation should not be overlooked when photo— synthetic rates under comparable conditions are being considered (90). During the past several decades, plant growth has been shown to benefit from carbon dioxide added to the atmosphere. Bolas (8) has shown increases in the growth of cucum- bers in small laboratory enclosures. No mathematical evaluation of these preliminary experiments was attempted. Short—time field experiments have shown increased pro- duction due to added atmospheric carbon dioxide. With a carbon dioxide enrichment of the air of up to 12 times the normal concentration, there was nearly a linear increase in apparent assimilation. A lO-fold enrichment (6 hours a day for 2 weeks) caused leaf chlorosis in tomatoes. The chlorosis was intensified by a general low level of mineral nutrients and decreased in normal carbon dioxide levels (87). With the aid of individual "leaf envelopes," over—all average rates of carbon dioxide absorption by potato leaves increased directly with elevated levels under field condi- tions. It was shown that with potato leaves at low light 12 intensities carbon dioxide absorption is retarded by insufficient light and low carbon dioxide levels. However, at light intensities above saturation, photosynthesis was limited mainly by the carbon dioxide level (20). Two times the normal content of carbon dioxide pro- duced twice as much growth as a normal atmosphere with carnations under glasshouse conditions (44). The amount of carbon dioxide inside glasshouses is much more variable in a diurnal analysis than outside the glasshouse. A linear increase in growth with increased carbon dioxide levels has been observed with carnations (34). Larger yields were obtained per unit of supplemental carbon dioxide at 350 ppm than at 550 ppm and showed a yield increase of 38 and 30 percent respectively when compared to 200 ppm. An increase in carbon dioxide concentration had an inverse relationship to stem length of carnations (33), and an increase in the longer grades and a decrease in shorter grades of roses was obtained in a glasshouse rose study (45). Frequently winds are not adequate to cause sufficient turbulence for mixing the carbon dioxide with the air sur- rounding crops. Wind speed and the carbon dioxide content, of the air surrounding corn leaves, have been shown to vary at the same rate (55). If the flow of air is horizontal 13 over wide areas covered by vegetation, there may be little replacement of carbon dioxide (57). The question of how critical turbulence is to crop production is probably still unanswered (55). The atmosphere is a reservoir of carbon dioxide which is only directly available to a limited extent for vege- tation (57). There are 18 to 20 tons of carbon dioxide over each land acre. The oceans contain 100 times as much carbon dioxide as the troposphere (28). Depending on the turbulence, the atmosphere at the disposal of the plant extends upward from 100 to 150 meters (19, 57). A deficit of carbon dioxide assimilation over soil respiration creates a vertical flow of carbon dioxide from upper air layers. There is always a diffusion gradient between the upper and lower leaves of a plant so that the lower leaves are usually in a higher carbon dioxide concentration than the upper leaves (57), with a steep gradient of carbon dioxide concen— tration from the soil surface to nearly 1 meter above the soil surface (29). However, above 8 centimeters from the soil surface, the level seldom rises above the normal 0.03 to 0.04 percent (29). Except for prostrate species, the strata of higher carbon dioxide air immediately above the soil would seem to have no appreciable effect on plant 14 growth. Below the 8 centimeter level above the soil surface, carbon dioxide may not be as serious in limiting plant growth as above this level (29). Lundeggrdh believes that when carbon dioxide is dif- fusing from the soil to the atmosphere, the practical maximum level of carbon dioxide, under field conditions, has already been reached (57). The carbon dioxide con— tent of soil air and its implications in plant growth has largely been based on inference rather than quantitative data in the majority of cases. This is due to the lack of satisfactory methods of characterizing soil aeration in terms of measurements that are significant in plant growth. However, soil aeration is still a major factor in plant growth. The quantity of carbon dioxide and water vapor are normally higher in the soil air than in the atmosphere (79). Carbon dioxide of the soil air increases with an increase in soil depth, density, moisture content and cropping. The carbon dioxide of soil air decreases with an increase in the number of large pores. The air of peat soils contains more carbon dioxide than mineral soils (3). An elevated level of carbon dioxide and a correspond- ingly low level of oxygen may be a factor in root 15 competition in poorly aerated or heavy soils (24, 71) as evidenced by its effect on the uptake of mineral nutrients and water (17, 36, 37). In general, plant processes were not appreciably reduced until the carbon dioxide concen- tration exceeded the oxygen concentration, but were sharply reduced after the carbon dioxide to oxygen ratio became larger than one (37). Specific species have shown toler- ance at a larger ratio (16). Kursanov (54), feels that carbon dioxide absorbed by roots and translocated to the leaves makes up an appreciable amount of the carbon dioxide assimilated. Stalwijk (84), estimates the amount to be quite low. The annual production of carbon dioxide by the soil microflora is approximately the same as the amount used in photosynthesis (l9). Plots manured over periods of 10 to 12 years had a considerably higher carbon dioxide evolution than unmanured soil (53). Cultivation is beneficial to the release of carbon dioxide as it maintains a looseness of structure (57). Chapman (18), has assumed that soil organic matter, moisture and temperature markedly affect the build—up of carbon dioxide during the night. His assumptions were based on measurements taken under western Nebraska and Iowa climatic conditions. 16 Actual fluctuations of the carbon dioxide content of the air may exceed 10 percent over short time periods (23). The content decreases during the hours after sunrise, then there is a slight increase in the afternoon which continues into the night (32, 88). The maximum atmospheric concen- tration occurs sometime after midnight and before sunrise, while the minimum concentration occurs at about mid—day (65). The carbon dioxide produced by and liberated from the soil during calm nights stratifies in the lower air layers (19, 65). The concentration is measurably higher on foggy than clear mornings, with a correspondingly higher photo— synthetic rate on foggy as contrasted to clear mornings, if no other factors are limiting (98). The carbon dioxide content is higher in certain micro- climates as was shown in a heavy fog in London, England, where the concentration was twice the normal level (58). Collendar (23) states that "air samples in towns contain 5 to 20 percent more carbon dioxide than uncontaminated air," and in Los Angeles, California, the atmospheric carbon dioxide content has been measured as high as 0.054 percent during the evening hours (35). Higher carbon dioxide concentrations have been reported in a cropped field than over bare soil (due to entrapment of 17 evolved carbon dioxide by foliage), during the night compared to daytime levels (65), at the soil surface of forest and forested river bottom than at the soil surface of grassland (29). "Swamp gas" or air in low land ele- vations possibly has a higher than normal carbon dioxide content (50). Annual fluctuations of carbon dioxide may be corre- lated with climate. After rainy summers a high value is to be expected since humidity favors soil respiration and cloudiness decreases assimilation. In autumn the concen- tration increases with a decrease in carbon dioxide assimilation. In the spring the concentration drops due to vigorous assimilation by vegetation (57). Misra (65) specifies early spring and Lundeggrdh (57), believes July to be the time when the minimum annual carbon dioxide level occurs. This variation was possibly due to the climatic area sampled. In contrast to current theories, a portion of the lit- erature in the early twentieth century dealt with the detrimental effects on plants by small increases in the carbon dioxide content of the atmosphere. Two to four times the ordinary carbon dioxide level increased photo— synthesis, but the gain in dry weight was less than in 18 ordinary air (15). It was also thought that nearly all flowering plants would be destroyed by an increase in carbon dioxide not much richer than the existing natural level (22). The results, which led to these conclusions, may have been due to the impurities in the carbon dioxide used. Theories were produced later which stated that below 1 percent carbon dioxide has no harmful effect on terrestrial plants, but at higher concentrations the gas becomes toxic (57). Miller has extensively reviewed the application of "carbon dioxide fertilization" in agricultural practice (63). In 1926 Rippel (78), stated "While the application of carbon dioxide apparently has possibilities for in— creasing crop production, the experimental evidence can- not yet be welded into a scientific system or into a working basis.” This statement may be less applicable at present because of the engineering advancements with regard to carbon dioxide production and handling. Several investigators have shown a benefit in plant growth from burning fuel or adding gaseous carbon dioxide to plants in glasshouses or open top chambers (10, 45, 96). The use of "gas fertilization" may be more important in the future, if the population continues to increase (51). 19 Kilbinger has speculated on the possible use of "swamp gas" or "scrubbed" industrial gas for the "carbon dioxide fertilization" of glasshouses (50). It has been stated recently that it is possible for a carbon-dioxide—introduced, growth-promoting action to be attained even with cucumbers grown in organic material (85). It should be realized that only by increasing both carbon dioxide and mineral fertilization can growth be increased to an optimum (51). Mineral nutrient requirements increase as the atmospheric carbon dioxide level is increased, and therefore optimum growing conditions may have to be re—evaluated (34). Greater response to "carbon dioxide fertilization" occurred at higher light intensities, therefore Moss, g; 21. believe that "any economical fertilization with carbon dioxide would have to be applied during mid-day hours on clear days" (68). Summary It appears on the basis of present knowledge that plants are more sensitive to variations of the carbon dioxide content of the air than variations in light intensity, if leaves are saturated with light at approxi- mately one-four of full sunlight. Carbon dioxide is 20 frequently the factor limiting plant growth, but its relative deficit is dependent on other climatic factors. Supplemental carbon dioxide has been shown to in- crease the growth of plants under controlled conditions. The degree of benefit varies with the species and amount of "carbon dioxide fertilization" practiced. The carbon dioxide content of the atmosphere in nature is currently increasing at the rate of 0.1 to 0.4 percent annually. If this trend towards higher atmospheric levels in specific microclimates continues, it will constitute an increasingly important factor in plant growth. MATERIALS AND METHODS Exploratory Survey The cultural practices in these exploratory experiments included some of the common factors which have been shown to limit plant growth. Four separate tests in 1959 using the pickling cucumber variety Wisconsin SMR-18 included the following factors: soil moisture level, soil fertility, plant population, wind protection, light intensity and manure versus com- mercial fertilizer. The irrigation experiment was conducted on a Wauseon fine sandy loam soil, the tests comparing manure and commercial fertilizer and use of windbreak as protection on a Spinks sandy loam and the light study on a Hillsdale sandy loam. The first experiment included irrigation, fertility levels and two plant populations. The design of this test was a split plot with the order of randomization being irrigation, fertility levels and spacing. The plots were irrigated with one-fourth acre-inch of water twice and one—half acre—inch twice during the early growing season. No irrigation was applied in late July and August due to the heavy rainfall during this period. 21 22 The original soil fertility equaled 170 pounds phos— phorus and 70 pounds potassium per acre by the reserve test (Spurway). Two levels of fertility were evaluated on this soil; low, with no fertilizer applied, and high, with 500 pounds per acre of 5—20—20. Plant spacings studied included rows 2-1/2 or 5 feet wide with plants spaced either 6 or 12 inches apart in the row. The effect of a windbreak on the growth of cucumbers was evaluated in the second experiment. A 4—foot high, double layer, lath snowfence was erected around the 25 by 55 foot area in each of the two replicates. A similar area was left unenclosed. Experiment number three included supplemental field lighting and shading with three treatments as follows: 1) three 400-watt mercury vapor lamps per 20 feet of cucumber row with the lamps on from 0430 to 2000 hours; 2) shaded with cheesecloth; 3) control plot exposed to normal sunlight (Figure l). The treatments in the final experiment consisted of the application of plant nutrients from cattle manure and inorganic nutrient sources in a randomized block design with three replicates, as shown in Table l. Figure l: 23 Field light manipulation experiment showing shaded, supplemental light and normal light plots; ballast and time control shelter is at the right 24 ‘Jisual ratings of plant vigor were made on the treat- ments four times from mid—season until senescence. Fruit harvest records were taken in all experiments in accordance with the commercially established, hand picking procedure of complete removal of all marketable fruit at least two times a week depending on climatic conditions. Data were statistically evaluated by the Analysis of Variance. Linear and quadratic effects were partitioned out where specific interactions or main effects required it. TABLE 1. Pounds of elemental nutrients applied per acre in nutrient source experiment Treatment N P K No fertilizerl/ —— __ __ 500 lbs. per acre 5-20-20 25 44 82 1000 lbs. per acre 5-20-20 50 88 164 10 ton manure per acre 109 20 96 20 ton manure per acre 218 40 192 40 ton manure per acre 436 80 384 1/ Original soil fertility tested 80 pounds nitrate, 97 pounds phosphorus and 135 pounds of potassium per acre by the reserve test I [if |F| I r '11. 25 The Effect of Carbon Dioxide and Light and Their Interaction Two growth chambers located in a laboratory were used in this series of experiments. Each was 8 feet long, 6 feet wide and 7 feet high (Figure 2). The size was designed so one person could work within the chambers with relative ease. The framework was 1 by 2 inch wood covered with 10 mil. weatherable Mylar* (polyester film). Mylar was chosen because of its low carbon dioxide diffusion pro- perty of about 240 cubic centimeters per square meter per 24 hours per atmosphere. -Black plastic was placed over the Mylar to maintain a constant photoperiod. The photoperiod was maintained at 15-1/2 hours through— out all experiments (time clock control from 0500 to 2030 hours) with high-pressure mercury vapor lamps used as a source of radiant energy. This type of lamp, as shown by Lindstrom, EEIQL. (56), is rich in the yellow and green portion.of the visible spectrum. Two 400—watt westinghouse HL—LB/G bulbs were employed in each chamber. Radiant spectral energy measurements of the visible spectrum were recorded in foot candles with a weston light meter and in microwatts per square centimeter (56). Three or four radiant energy levels were used in the experiments. A * Trademark, E. I. DuPont Co. Figure 2: Mylar growth chambers covered with black polyethylene; air sample evacuation pump shown in foreground; "mixing box" and air-inlet fan at top of chamber; second chamber in the background 27 “step method" of varying the distance from the light source to the plant was employed to obtain the various energy levels. The four approximate energy levels used (at top of clay pot) were 1400, 1000, 700 and 300 foot candles or about 6000, 4000, 2500 and 1250 microwatts per square centimeter, respectively. Four pots containing three plants each for the times of harvest were evaluated at each of the radiant energy levels in all tests. For statistical purposes, the mean of the four plants was used. A positive pressure of air moving into the growth chambers was maintained at all times. One chamber con- tained a normal atmosphere with the second chamber con— taining a plus or minus carbon dioxide level in relation to the normal atmosphere. The chambers were alternated for carbon dioxide treatments between experiments. "Normal atmosphere" refers to air which had neither an addition nor removal of carbon dioxide. The data were statistically evaluated by using time as replication so that the normal atmosphere and plus or minus carbon dioxide were evaluated in both chambers in each experi- ment. Time was used as the replication for all of the carbon dioxide concentrations studied. 28 A measured amount of air (adjusted with a velometer) together with the carbon dioxide was constantly forced into the plus carbon dioxide chamber. The same rate of air was forced into the normal atmosphere chamber. Carbon dioxide and air were mixed together in a "mixing box" before forcing into the chamber. Air flow was regulated by the size of the orifice in the mixing box and the carbon dioxide was metered from a cylinder source through two valves (in series) and a flowmeter. The carbon dioxide flow was adjusted frequently with the aid of the flowmeter to compensate for loss of pressure as the cylinder became empty. In the experiments where carbon dioxide was removed from the air, a portion of the air was drawn through sodium hydroxide to partially remove the carbon dioxide from the air inside one of the chambers. The same percent— age carbon dioxide was maintained in each chamber through— out light and dark periods. The carbon dioxide source used consisted of carbon dioxide 99.79 percent, oxygen .10 percent, nitrogen .10 percent, oil 25 ppm, dew point -75° F. (70). Concentrations of carbon dioxide maintained for the growing atmosphere in the 21—day tests were as follows: 350 ppm and 450 ppm of carbon dioxide (runs eight and 29 nine); 450 ppm and 1250 ppm of carbon dioxide (runs four and five); and 500 ppm and 2150 ppm of carbon dioxide (runs six and seven). The "age of plant" studies (runs 10 and 11) were maintained at 450 ppm and 1350 ppm of carbon dioxide. Three fans were utilized in each of the chambers. One fan was for the air intake. A 12-inch floor fan was employed to maintain turbulence within each chamber. The third fan cooled the area immediately below the two lights. Measurements of the carbon dioxide content of the chamber atmospheres were made colorimetrically by using a brom—thymol blue, sodium bicarbonate method of carbon dioxide determination as described by Claypool and Keefer (21). To collect air samples drawn from the chambers by a vacuum pump, the air collection tubes were kept in opaque waterbaths at a constant temperature. The air evacuation pattern was set up in such a way that the air sample passed through the determination solution before passing through the pump. Air samples were determined immediately before the soil moisture was adjusted each morning or at least 1 hour after "opening” the chamber for observations or measurements. Entry into the chambers was kept to a minimum for maintenance of the partial 30 pressure of carbon dioxide. The air from both chambers was sampled simultaneously. Optical density readings were taken with an Evelyn colorimeter at 620 millimicrons. The carbon dioxide level in the normal atmosphere chamber was usually slightly above 0.03 percent because the chambers were located inside an occupied building in an urban area. Carbon dioxide was also released into the adjacent chamber in the majority of experiments. Carbon dioxide measurement was sensitive to internal chamber conditions. One soil mixture used in February, 1961, was very high in organic matter and the atmospheric carbon dioxide measurements for the first day were very high due to initial decomposition. This level of carbon dioxide was never reached again in the normal atmosphere chamber. A single seed lot of Wisconsin SMR-18 pickling cucumber was used for all experiments except one, where the variety Ohio MR—200 slicing cucumber was employed. Cucumbers in all but the "age of plant" tests were seeded and grown within the growth chambers in a sand, peat and soil potting mixture with a ratio of 1-1—2. Five or six seeds were planted in each 4-inch clay pot so that the seedlings could be thinned to three for uniformity. Watering and 31 nutrient application was carried out by bringing the soil to a constant weight early each morning utilizing a small platform scale. Seedlings were fertilized with a 0.5 percent solution of 10-52-17 water soluble fertilizer every second day. Plants in the age of plant studies (runs 10 and 11) received a 0.5 percent 10-52-17 water soluble fertilizer plus 0.25 percent potassium nitrate solution every second day because they were grown until they produced fruit. Wisconsin SMR—18 cucumbers were grown outside the replicated, within—chamber design to test for the starch content of leaves grown in the various carbon dioxide levels. The starch content of cucumber leaves was deter- mined by boiling the detached leaves in ethanol to extract the plant pigments. The leaves were then covered with a potassium iodine solution containing free iodine to develop the characteristic purple color which indicated, by the color intensity, the amount of starch present in the leaves. Constant hydro-thermograph records were kept in both chambers. The usual diurnal temperature pattern was a high of approximately 95° F. at mid—day and a low of 750 F. throughout the dark hours. Very little temperature varia— tion was recorded between chambers. A usual daily average 32 Of 40 percent relative humidity was recorded when the air inside the chambers was renewed in approximately 1 hour. In the minus carbon dioxide tests (runs eight and nine), where the air was not renewed as frequently, the usual daily average relative humidity was 60 percent. Chamber number two had a constantly higher relative humidity (3 to 4 percent) throughout the majority of the experi- ments which was an important reason for alternating carbon dioxide treatments between chambers. An attempt was made to control all factors except the intended variation of carbon dioxide and light levels. The hazards of not controlling all the factors in a study such as this are discussed by Bolas (8). The data from the three earlier runs were discarded due to the high and inconsistent light intensities on the high light level plants. The close proximity of these plants to the light source caused a differential heat distribution between the light levels in these earlier experiments. In subsequent tests there was a 1.50 F. maximum temperature differential between the two extreme light levels as measured with a thermocouple-potentiometer system. The growth measurements determined on each plant at harvest were as follows: 1) height of plant (soil surface 33 to terminal bud), 2) number of internodes developed, 3) fresh weight of top growth, 4) dry weight of top growth (dried at 1500 F.), 5) number of fruit developed (with those plants grown to an advanced stage of growth). Plants were harvested by severing them at the soil surface with a knife. The fresh to dry weight ratio did not vary greatly throughout the carbon dioxide levels evaluated. Therefore, for simplicity of presentation, only dry weights are pre- sented graphically. Dry weights presented were determined on a Mettler-HS milligram balance. Growth measurements were recorded in the 21-day studies as follows: 9 days after seeding (first true leaf expand- ing), 15 days after seeding (two internodes usually devel— oped), 21 days after seeding (four internodes usually developed). One experiment was designed to grow plants to three physiological ages before subjecting them to differential carbon dioxide levels (runs 10 and 11). Cucumber seedlings were grown and maintained in a glasshouse until the appro- priate treatment date. The stages of plant development were as follows: first true leaf starting to expand, sub- jected to the carbon dioxide treatments for 2 weeks (21 days old); third true leaf starting to expand, subjected 34 t0 the carbon dioxide treatments for 2 weeks (35 days old); 14 inches tall with an average of six and a half internodes developed, subjected to the carbon dioxide treatments for 1 week (42 days old). Only one of the age of plant studies was carried through the third time of treatment (42 days) due to differential growth of the plants from a seasonal glasshouse temperature variation. Chamber experimentation occurred during the time period of July, 1960, to May, 1961. Data were statistically evaluated by the Analysis of Variance. Where further evaluation of main effects or interactions seemed desirable Duncan's New Multiple Range Test was used or linear and quadratic effects were partitioned. Comparison of the Carbon Dioxide Concentration Over Muck and Mineral Soils with the Growth of Pickling Cucumbers in the Field Field research during the 1961 growing season was under— taken to correlate the growth pattern of pickling cucumbers with field carbon dioxide levels when the cucumbers were grown under as “near optimum" conditions as possible. These tests were used as a followup of the indoor growth chamber Studies to determine carbon dioxide differences in different microclimates. 35 VVisconsin SMR—18 or SMR—lS pickling cucumber varieties were used in all tests at the Michigan State Horticulture Farm, while Spartan Dawn pickling cucumber was grown at the Michigan State Muck Experimental Farm. Atmospheric carbon dioxide samples were collected with a portable field measurement apparatus consisting of two Evans fuel oil, eccentric-type motors powered by a 6-volt storage battery. Two separate installations were used to take air samples at the same time. Air samples were collected at five points within a plot, 2 inches.above-the soil surface. Measurement of the carbon dioxide was always at sunrise to have a constant time for the measure- ments and less turbulence on fair days (5). The appro- priate time for the measurements varied from 0500 hours in June to 0600 hours in September. Absorption of the carbon dioxide from the air sample was in sodium hydroxide through fritted glass aerators contained in l6-inch test tubes. The sodium hydroxide was then titrated to neutrality with standardized hydro— chloric acid by an electrometric method. Two blanks, or non—aerated sodium hydroxide samples, were titrated in each set of determinations. Determination of the carbon dioxide in the air sample was then calculated by the 36 milliequivalents of carbon dioxide and hydrochloric acid method and the percentage carbon dioxide calculated on a milligram weight basis. One experiment was carried out for the purpose of measuring the carbon dioxide over muck and mineral soils. The Horticulture Farm was about 50 feet higher above sea level than the Muck Farm. The mineral soil at the Muck Experimental Farm was 4 feet higher than the muck soil (49). Another objective of this experiment was an attempt to increase the carbon dioxide concentration of the air within a small snowfence enclosed area. Decomposing manure was used as a source of supplemental carbon dioxide. Between- row spacings of 2 and 4 feet with 8 to 12 inches within rows were employed as a split plot within the main treat- ments. Plots on both mineral and muck soils at the Muck Farm and mineral soil at the Horticulture Farm were enclosed by a double layer of snowfence in an attempt to reduce the turbulence within the area during the sample collection. Muck and mineral soil areas were separated by 11 miles. The treatments on muck and mineral soils were: 1) 20 tons of manure per acre disked into the soil, 2) equivalent amounts of commercial nitrogen, phosphorus and potassium disked into the soil as in treatment number 1, and 37 3) commercial fertilizer applied as in treatment number two plus 20 tons per acre of manure which was kept moist in steel pans placed inside the enclosed areas. The soil type at the Horticulture Farm was a Wauseon fine sandy loam with 3.7 percent organic matter. The mineral soil at the Muck Farm was a loamy sand with 6.0 percent organic matter. The muck soil was a Houghton Muck containing 80.5 percent organic matter. The Mylar growth chambers used in the indoor studies were erected in the field on an Allendale sandy loam soil. Eighty pounds of cattle manure in four steel pans were placed inside one of the 8 by 6 foot chambers. The chambers had four 7-foot high sides with the top left open. Air samples were drawn from the bottom of the chambers. The two outer guard rows were eliminated at the time of the first fruit harvest and data compiled on one 8-foot row per chamber. Mylar enclosures 8 feet long, 6 feet wide and 3 feet 4 inches high were arranged in an open field and inside a large Mylar greenhouse, each part having two replicates. The greenhouse was used to protect against loss of carbon dioxide evolved from the manure placed in the enclosures. 38 Treatments in this experiment were no enclosure, enclosure with 80 pounds of manure in four steel pans and enclosure with no manure. Air samples were drawn out the bottom of the enclosures. The two outer guard rows were eliminated at the time of first fruit harvest and data compiled on one 8—foot row per plot. Field chambers refer to structures with 7-foot high walls and enclosures refer to structures with 3 foot 4 inch high walls. As in the exploratory survey, fruit harvest records were taken in all experiments in accordance with the commercially established, hand picking procedure. Data were statistically evaluated by the Analysis of Variance. No treatments will be described in the three sections as having an effect in replicated tests unless they were proven to be statistically significant. RESULTS Exploratory Survey In the test incorporating two moisture levels, pickling cucumber yields were not increased by the use of supple— mental irrigation. In the same experiment fruit yields were increased by the addition of 500 pounds of 5-20—20 per acre compared to treatments receiving no fertilizer. The yields for both treatments in these plots were very high when compared to the average yield of 192 bushels per acre in 1959 for the State of Michigan.* The narrower row spacing (2—1/2 feet between rows) gave yields which were greater than the wider row spacing as seen in Table 2. The 2-1/2 foot spacing became a dense mat of vegetation; and although this area had a greater yield than the 5-foot spacing, a high percentage of fruit deterioration prevailed due to the constant high moisture level maintained by the vegetation. There was no increase in yield of fruit with plants protected from the wind compared to unprotected plants. *Agricultural Statistics, U. S. Department of Agriculture 39 40 TABLE 2. Yield of cucumbers (bushels per acre) with two levels each of fertility and spacing (includes average of two irrigation levels and 12 harvests) 1/ Treatment 2.5' row 5' row Average 500#/acre 5-20—20 1002 728 865 No fertilizer 853 483 668 Averageg/ 928 606 1/ F value for difference between fertility levels significant at 5% level g/ F value for difference between spacings significant at 1% level 41 The manure and commercial fertilizer test showed an increase in yield with large nutrient additions from manure as seen in Table 3. There was an increase in yield from the closer row spacings. TABLE 3. Fruit yield in the manure and commercial fertilizer experiment (six harvests) Yieldl/ Treatment (bu./acre) No fertilizer 264 b 500#/acre 5—20-20 268 b 1000#/acre 5—20-20 243 a 10 ton manure/acre 345 c 20 ton manure/acre 271 b 40 ton manure/acre 365 d 2—1/2' between—row spacing 366 2/ 5' between-row spacing 218 E Yields not containing a common letter are significantly different at the 1% level 2/ F value for yield differences between spacings was significant at the 1% level No difference could be observed in vigor between the Check, inorganic fertilizer and manurial treatments from 42 the \nisual ratings made. The plants within the plots were infected heavily with powdery mildew late in the growing season and senescence was rapid. As shown in Table 4 the "possible sunshine" was low for July and August, 1959. The effect of light manipula- tion on the light intensity for the various plots is shown in Table 5. It can be seen that supplemental light levels were slightly greater at 0915 in the example cited. TABLE 4. "Possible sunshine"l/ during the 1959 growing season at East Lansing, Michigan Normal 1959 for the month May 66% 67% June 69% 71% July 65% 74% August 62% 69% l/ U. S. weather Bureau measurement of "possible sunshine" as measured by the amount of time the light was of sufficient intensity to cast a shadow 43 TABLE: 5. An example of light intensities in field light experiment plots on July 7, 1959 Foot candlesl/ Time Shaded Normal Supplemental 0915 3,700 5,700 6,000 1430 6,200 10,000 + 10,000 + 1/ Foot candles recorded by a weston light meter In the erection of the mercury vapor lamps, the plant population in the supplemental light plots was damaged to the extent that some of the area had to be transplanted with cucumber seedlings, however, in the final analysis yields were still higher in these plots as shown in Table 6. The total yield increased in a linear fashion from shaded to normal and supplemental light. On August 13 all plots were harvested and the four days which followed were cloudy and rainy. All plots were harvested again on August 17 and the yield on this date was higher from the supplemental light plots compared to the normal plots as seen in Table 6. 44 TABLE 6. Pounds of fruit harvested per plot following a low-light intensity period and total yield for season from differential light levels (eight harvests) August August Total l3 17 yield;/ Shaded 4.85 4.75 20.25 Normal 6.05 8.75 30.20 Supplemental 5.50 9.15 32.15 1/ F value for linear response of total yield with light level significant at 5% level 45 The Effect of Carbon Dioxide and Light and Their Interaction Emergence of cucumber seedlings was complete and rela- tively uniform in all chamber tests after three days. The fresh and dry weights of cucumber seedlings were increased in all tests where carbon dioxide was added to the ambient atmosphere. The addition of approximately one-fourth more atmos— pheric carbon dioxide at realistic natural levels (runs eight and nine) increased the dry weight, number of inter— nodes developed and plant height (Table 7 and Figure 3). Dry weight increase was linear with time. This increase was greater at 450 ppm than at 350 ppm (Figure 3 and Table 7). More internodes developed at the two later harvests with 450 ppm than at 350 ppm carbon dioxide (Table 7). Internodes developed and dry weight showed a positive response with an increase in light intensity, as shown in Table 8. All growth measurements of the plants increased with time at 700, 1000 and 1400 foot candles. At all light intensities, except 300 foot candles, the dry weight was greatly increased by adding carbon dioxide. Plants at 300 foot candles and 450 ppm survived longer than with the corresponding light intensity at 350 ppm. The extreme example occurred in one replicate where all 46 TABLE 7. Dry weight, height and internodes developed at 350 ppm and 450 ppm of carbon dioxide (average of eight plants at four radiant energy levels each in two experimental runs) Concentration of Days from seeding carbon dioxide 7 15 21 Average Dry weight (grams) 350 0.04 0-13 0.19 0.12 2/ 450 0.05 0.18 0.34 0,19 Average l/ 0.05 0.16 0.27 Height (inches) 350 2.38 2.95 2.96 2.76 1/ 450 2.52 3.20 4.30 3.34 Average 2/ 2.45 3.08 3.63 Internodes developed 350 0.00 1.48 2.60 2.04 1/ 450 0.00 1.95 3.41 2.68 Average 2/ 0.00 1.72 3.01 1/ F value for linear response as time increased significant at 5% level 2/ F value for linear response as time increased significant at 1% level E/ P value for difference between means significant at 5% level 47 TABIJB 8. Dry weight, height and internodes developed at the four radiant energy levels with 350 ppm and 450 ppm carbon dioxide levels combined (average of 48 plants in two experimental runs) 2 Dry weightl/ Heightl/ Internodes—/ Foot candles (grams) (inches) developed 1400 0.36 a 2.94 a 4.32 a 1000 0.18 ab 3.03 a 3.32 b 700 0.05 b 3.10 a 1.44 c 300 0.03 b 3.14 a 0.37 d 1/ Average for 7, 15 and 21 days from seeding 2/ Average for 15 and 21 days from seeding 3/ Measurements not containing a common letter are significantly different at the 1% level lfour plants died at a concentration of 350 ppm, but there was only one casuality at 450 ppm. Etiolation of the plants caused an increase in plant height at the 300 foot candle light level at 7 and 15 days, but at 21 days from seeding they were highest at 1400 foot candles closely followed by 1000 foot candles. The number of internodes increased with the highest levels Of radiant energy and the higher carbon dioxide level (Tables 7 and 8 and Figure 4). More internodes were 48 4 Internodes developedl/ .—. 350 ppm /. 2 ~ / I — l/ F value for linear effect of carbon dioxide level with time significant 0 v 11] at 1% level 7 I5 2| Days from seeding 0.4- , Dry weighty E b ()2'- o G / OJ — 2/ F value for linear effect of carbon dioxide level with time significant 0 l l ##J at 5% level 7 I5 2| Days from seeding Figure 3. Average dry weight and internodes developed for seedlings grown at 350 ppm and 450 ppm of carbon dioxide 48 Internodes developedy .—. 350 ppm /. 2 — / I — i/ F value for linear effect of carbon dioxide level with time significant 0 v lllJ at 1% level 7 l5 2| Days from seeding 0.4- Dry weighty E O ()2'- o 5 / OJ — g/ F value for linear effect of carbon dioxide level with time significant 0 l l 44‘ at 5% level 7 l5 2| Days from seeding Figure 3. Average dry weight and internodes developed for seedlings grown at 350 ppm and 450 ppm of carbon dioxide Figure 4. Cucumber seedlings after 21 days at 1400 f.c., (left) 350 ppm, (right) 450 ppm of carbon dioxide; seedling grown at 350 ppm is chlorotic and less vigorous than at 450 ppm 50 develxxped at the 350 ppm treatment of a light level than at the next lower light level of a 450 ppm treatment. A second experiment (runs four and five) was designed to maintain atmospheres containing 450 ppm and 1250 ppm of carbon dioxide. Three light intensities were included in this test, the first in which temperature was not con- founded with light intensities. The 300 foot candle light intensity was not evaluated until experiment six. Dry weight at the 1250 ppm treatment exceeded the dry weight at the corresponding light levels in the 450 ppm carbon dioxide atmosphere (Table 9). Height of seedlings did not differ at the light and carbon dioxide levels evaluated. Carbon dioxide utilization was greater as the radiant energy increased at 1250 ppm than at 450 ppm of carbon dioxide as shown by the dry weight in Table 9 and Figures 5 and 6. 51 TABLE 9. Dry weight of seedlings grown for 21 days in 450 ppm and 1250 ppm carbon dioxide (average of eight plants in two experimental runs) Foot candles 1250 ppm 450 ppm Averagel/ Dry weight (grams) 1400 1.51 0.68 1.10 1000 1.32 0.42 0.87 700 ngl Qplg 0.35 Averageg/ 1.11 0.43 1/ F value for difference between means of light intensities significant at 1% level g/ F value for difference between means of carbon dioxide concentrations significant at 5% level 52 Figure 5. Cucumber seedlings after 21 days at 1000 f.c., (left) 1250 ppm, (right) 450 ppm carbon dioxide Figure 6. Cucumber seedlings after 21 days at 700 f.c., (left) 1250 ppm, (right) 450 ppm of carbon dioxide 53 In another experiment (runs six and seven) where the carbon dioxide content was increased to 2150 ppm, the same amount of increase per added unit of carbon dioxide did not occur as described in the previous experiments. This can be seen by comparing weights in Tables 7 and 10, particu- larly at 21 days from seeding at the two highest light intensities (Tables 8 and 11). The levels in this test were 500 ppm and 2150 ppm of carbon dioxide. Four light levels were evaluated in each of these atmospheres. The largest fresh and dry weights obtained in all 21-day experiments were with 2150 ppm and 1400 foot candles. The average dry weight was approximately twice the weight at 500 ppm; which was a two—fold increase in weight with a four-fold increase in atmospheric carbon dioxide. Figures 9 and 10 show the large growth benefit at 2150 ppm. The dry weights indicate that carbon dioxide utilization increased at a more rapid rate with time at 2150 ppm than at 500 ppm carbon dioxide (Figure 7). Height did not vary between 500 ppm and 2150 ppm. The 2150 ppm level compensated for the low light intensity and a greater height was obtained at 300 foot candles. More internodes developed at 2150 ppm than 500 ppm carbon ‘dioxide as seen in Table 10. 54 TABLE: 10. Dry weight, height and internodes developed at 500 ppm and 2150 ppm of carbon dioxide (average of eight plants at four radiant energy levels in two experimental runs) Concentration of Days from seeding carbon dioxide 7 15 21 Average Dry weight (grams) 500 0.03 0.10 0.31 0.15 2/ 2150 0.04 0.17 0.71 0.31 Average;/ 0.04 0.14 0.51 Height (inches) 500 2.43 2.17 2.40 2.33 2150 2.36 2.84 3.67 2.96 Average 2.40 2.51 3.04 Internodes developed 500 0.00 0.88 2.60 1.74 _2_/ 2150 0.00 1.22 3.75 2.49 Averagel/ 0.00 1.05 3.18 1/ F value for linear response as time increased significant at 5% level 2/ F value for difference between means significant at 5% level 55 Table 11" Dry weight, height and internodes developed at the four radiant energy levels with 500 ppm and 2150 ppm carbon dioxide levels combined (average of 48 plants in two experimental runs) Dry weightl/ Heightl/ Internodesg/ Foot candles (grams) (inches) developed 1400 0.54 a 2.41 a 3.63 a 1000 0.29 b 2.60 a 2.91 b 700 0.06 C 2.94 a 1.41 c 300 0.02 c 2.63 a 0.51 d Average for 7, 15 and 21 days from seeding Average for 15 and 21 days from seeding Measurements not containing a common letter are significantly different at the 1% level RTE Light levels resulted in no difference in plant height as shown in Table 11. Internodes increased as light inten- sity became greater (Table 10). Table 11 presents the in- crease in dry weight with elevated light intensities. The utilization of carbon dioxide was greater at 2150 ppm than 500 ppm carbon dioxide as the radiant energy increased as shown in Figure 8. All plant measurements, except height, increased with days from seeding. 56 The differential utilization of carbon dioxide with an increase in radiant energy and time is shown graphically in Figure 7. Carbon dioxide at 2150 ppm was at a suffi— ciently high concentration to produce an equal weight with a lower radiant energy level than was produced with a higher radiant energy level with a carbon dioxide concen— tration of 500 ppm as the plants became larger. As time progressed high levels of carbon dioxide and light gave increasingly greater dry weights, while low levels of these two factors produced the lowest weights. One low and one high level of either factor produced comparable increases in weight as time progressed (Figure 7). 57 Time, light and |.6— carbon dioxidel/ '— 2|50 ppm m — ° ' SOOPDm E E o o 00 I400 f.c. (a 013— . o :1 Do I000f.c. 04- o y F value for inter— . action of time, light _ and carbon dioxide levels significant 0 at 1% level 7 I5 2| Days from seeding 0'8- Dry weight 2/ 0-6‘ ——0 2|50 ppm - 0—0 500 ppm E E 0!}- ‘3 .— /O 02__ y F value for linear ' effect of carbon _ dioxide level with time significant at O I 5% level 7 I5 2| Days from seeding Figure 7. Average dry weight of seedlings grown at 500 ppm and 2150 ppm of carbon dioxide and the interaction of carbon dioxide with light and time 0.8 0.7 0.6 0.2 58 F value for interaction of light and carbon dioxide levels significant at the 1% level *—0 2|5O ppm Figure 8. 0 500 ppm 1 l l 700 I000 I400 Foot candles Dry weight at four radiant energy levels with 500 ppm and 2150 ppm of carbon dioxide Figure 9. Cucumber seedlings after 21 days at 700 f.c. (left) 500 ppm, (right) 2150 ppm of carbon dioxide Figure 10. Cucumber seedlings after 21 days at 300 f.c. (left) 500 ppm, (right) 2150 ppm of carbon dioxide 60 The age of plant studies (runs 10 and 11) were carried out in atmospheres of 450 ppm and 1350 ppm carbon dioxide. Experimentation was limited in this study by the available chamber space because of the climbing habit of the cucumber plant. Dry weight was greater at both 21 and 35 days from seeding at the higher carbon dioxide level (Figure 11). The average pooled dry weight value for the two dates is shown in Table 12. A larger number of fruit was developed on plants at 1400 foot candles subjected to 1350 ppm carbon dioxide for two weeks than on plants subjected to 450 ppm (Table 12). The number of fruit developed was not significant between carbon dioxide concentrations because of the lack of differences at foot candles below 1400. The plants at 1350 ppm were at a more advanced physiological age than those at 450 ppm of carbon dioxide. Dry weight increased at a greater rate with 1350 ppm than at corresponding light levels at 450 ppm carbon dioxide. The plants in the third segment of this test showed a growth increase with added carbon dioxide in the measure— ments of plant height, dry weight and number of fruit developed. This experiment was not replicated in time because of seasonal growth variation, thus, the data in Table 13 were not statistically evaluated. 61 The 42—day old plants were larger after being subjected to 1350 ppm carbon dioxide than those grown at 450 ppm. Because they were larger plants, more fruit was developed to the minimum acceptable picking size. As shown in Table 13, the number of fruit was greater at the highest light intensities. There was no difference in weight between plants grown at 300 foot candles at 1350 ppm and 450 ppm carbon dioxide. The starch content of cucumber leaves was shown to be greater at the end of the light period in 1350 ppm and 2150 ppm than 450 ppm and 500 ppm atmospheres respectively. Leaves grown in the carbon dioxide supplemented atmospheres had a distinctly darker purple color when developed with the potassium iodine than did the leaves from a normal atmosphere. An attempt was made to adapt a quantitative perchloric acid, colorometric starch test, used for apple fruit, to the cucumber leaves. The test proved to be inconsistent probably because of the difference between the two types of tissue. Leaf size of the cucumber plants was larger when carbon dioxide was added to the atmosphere. However, quantitative measurements of leaf area were not recorded. Grams 2.4 2.0 0.8 0.4 62 F value for interaction of time and carbon dioxide levels significant at 1% level - 0—0 l350 ppm '— .—. 450 ppm . h. C I l 2| 35 Plant age(days) Figure 11. Dry weight of cucumber seedlings grown at 450 ppm and 1350 ppm of carbon dioxide in "age of plant" study Grams 2.4 2.0 0.8 0.4 62 F value for interaction of time and carbon dioxide levels significant at 1% level 2| 35 Plant age(days) Figure 11. Dry weight of cucumber seedlings grown at 450 ppm and 1350 ppm of carbon dioxide in "age of plant" study 63 TABLE 12. Average dry weight and number of fruit developed at four light intensities for 21 and 35 day old plants at 450 ppm and 1350 ppm carbon dioxide (average of 16 plants each in two experimental runs) Foot candles 1400 1000 700 300 Plant age (days) 21 35 21 35 21 35 21 35 450 ppm Fruit developed 0.0 0.5 0.0 0.1 0.0 0.0 0.0 0.0 1 Dry weight—/ (grams) 1.4 1.1 0.5 0.3 1350 ppm Fruit developed 0.0 1.3 0.0 0.5 0.0 0.1 0.0 0.0 1 Dry weight—/ (grams) 3.1 2.1 0.8 0.3 1/ F value for interaction of light and carbon dioxide levels significant at the 1% level TABLE 13. Dry weight and number of fruit developed by 42—day old cucumber plants grown in 450 ppm and 1350 ppm carbon dioxide for one week (average of four plants in one experimental run) Foot candles 1400 1000 700 300 450 ppm Dry weight (grams) 3.2 2 3 2.4 2.0 Fruit developed 1.8 0.8 0.8 0.5 1350 ppm Dry weight (grams) 4.8 3 8 2.8 1.8 Fruit developed 4.0 3.8 1.0 0.5 64 TABLE 13. Dry weight and number of fruit developed by 42—day old cucumber plants grown in 450 ppm and 1350 ppm carbon dioxide for one week (average of four plants in one experimental run) Foot candles 1400 1000 700 300 450 ppm Dry weight (grams) 3.2 2.3 2.4 2.0 Fruit developed 1.8 0.8 0.8 0.5 1350 ppm Dry weight (grams) 4.8 3.8 2.8 1.8 Fruit developed 4.0 3.8 1.0 0.5 65 Mison of the Carbon Dioxide Concentration Over Muck and btlneral Soils with the Growth of Pickling Cucumbers in the Field Measurement of the atmosphere of the fertilizer and decomposing manure treatments within the areas enclosed by snowfence did not indicate a difference for carbon dioxide levels between treatments. The soil treatments did not show a differential response between muck or mineral soil. Muck and mineral soil areas showed an appreciable natural difference in the atmospheric con- centration of carbon dioxide. As shown in Figure 12 the air over muck soil and the adjacent mineral soil area contained more carbon dioxide than over the mineral soil at the second location. The atmosphere of two adjacent areas at the mineral location and the mineral and muck areas at the muck location did not differ in carbon dioxide content, but differed from each other. Figure 12 presents the average values obtained for each of the locations. The carbon dioxide levels determined over a bare soil area adjacent to the cucumber plots on muck and mineral soils were usually lower than within the vegetated area. However, the differences were not large enough to claim a valid or consistent difference. No consistent correlation between average daily temperature, rainfall (Table 14) and carbon 66 0. Hood mo HwEESm map msHMSU Auuwmm meHE Haw mane HHOm £058 pcm appease m um w>onw masosfl N .wmaussm pm mHm>mH woaxoap conumo mmmnw>< .onEmEom 53944 n .5 mm ON 9 o. m. ___n _ mm _ .wommusm HHOM ON 0 o. .NH magmas :2. of... 0 _On nm _N __ I _ __ _ a l .- 88 3.6 F :8 .6... .0552 x032 Ho>ws as ms» um Hflom HmumsHE ou Umnmmeoo HHOm £058 H0>o pummoum on Op opflxoflp connmu mHoE pozosw pmuuoam moEHu ebb um mam>wa Mom 03Hm> m 00. On. OON omN Noo 00m Eaa 0mm 00¢ 00¢ 00m 67 TABLE} 14. Daily maximum temperatures and precipitation for June, July, August and September, 1961, at the Michigan State Horticulture and Muck Experimental Farms June July Temp.l/ Prec.g/ Temp. Prec. Date H M H M H M H M 1 74 71 .20 .27 90 87 —— —— 2 78 74 —— —- 90 78 52 .30 3 77 68 T;/ —- 80 72 -— —- 4 72 76 —— -— 72 72 -— —- 5 84 83 —— —— 79 78 —— —— 6 83 74 —— -- 83 81 -- —— 7 77 74 .45 —— 81 76 —— -— 8 77 72 .08 .30 76 76 —— —- 9 77 72 .03 .10 78 79 —— —- 10 84 72 .02 —- 78 No T —- 11 87 —— .03 02 85 R —— —— 12 90 88 —— -— 87 E —— —- 13 90 84 .24 .56 84 C .07 —— 14 71 —- .27 .23 79 O T -— 15 64 -— -— —— 83 R —— —— 16 72 —- —— —— 82 D .56 .65 17 77 —- —— —— 82 78 -— —— 18 79 —— —— —— 86 85 -— —— 19 79 76 T .23 85 80 -— -- 20 66 63 .41 .61 86 83 51 —- 21 72 71 —— —— 85 84 —- —— 22 73 69 T —— 85 83 T —— 23 70 60 .28 .40 85 84 18 —- 24 69 66 .14 —— 82 78 .01 30 25 69 66 T .08 81 73 —— —— 26 79 77 -— —— 85 84 —— —— 27 81 79 —— —— 85 83 —— —— 28 87 85 —— —— 85 79 .02 1.25 29 9O 88 T —— 85 82 67 —— 30 92 91 .02 —— 87 83 —— .09 68 TABLE 14.--Continued August September Temp.l/ Prec.2/ Temp. Prec. Date H M H M H M H M 1 74 66 12 1.42 87 74 .06 -— 2 78 75 .04 —- 88 81 .23 .27 3 83 78 —— .60 86 83 —- —- 4 82 79 49 .12 85 80 .06 13 5 79 70 08 —- 82 78 —- -- 6 77 74 T;/ .07 82 79 .37 .02 7 78 77 T —— 83 81 —— —- 8 84 80 -— —— 83 82 —— —— 9 86 83 —— —- 82 84 —— —— 10 84 81 -— 29 88 84 .05 -— 11 83 79 21 47 12 79 74 -- —— 13 72 69 —— -- 21 71 72 -- -— 22 74 73 —— -— 23 73 64 64 .50 24 75 72 49 80 25 78 73 .07 —— 26 78 74 27 45 27 81 77 08 —- 1/ Degrees Fahrenheit 2/ Inches of precipitation 3/ Trace of precipitation 69 dioxide level at the locations could be established. However, the high levels over muck soil in July may have been due to the preceding warm period and relatively less plant growth than later in the season. No fruit yield differences were found within locations as a result of the treatments. Yields from this experi— ment are presented in Table 15. the 2—foot between row spacing produced a larger yield than the 4—foot spacing on mineral soil (as in exploratory survey). Deterioration of the fruit was a major problem on the muck area where the vegetation completely covered the soil area, particularly TABLE 15. Average bushels of fruit harvested per acre at two locations from 2 and 4 foot between-row spacings (10 harvests) 2 feet 4 feet Muck farml/ 265 g/ 328 Horticulture farmz/ 347 g/ 271 Spartan Dawn pickling cucumber Wisconsin SMR-18 pickling cucumber F value for difference between spacing significant at the 1% level BEE 70 at the 2-foot between row spacing. The vines were con- stantly moist and were separated with difficulty at picking time and often with much harm to the foliage and consequently to yield as seen in Table 15. Cucumbers were seeded in Mylar chambers located in the field on June 8, 1961. Fruit yield and plant measurements are shown in Table 16 where it can be seen that the chamber with manure pans had the greatest seedling height, fresh and dry weights. This chamber also produced the larger number and the greater weight of fruit. Although in the majority of measurements of the chamber air, more carbon dioxide was present in the air from the chamber with the manure included, about one—third of the measurements showed that the no—manure chamber had a slightly greater carbon dioxide concentration. The treatments in this chamber study were not repli— cated, and a low—walled enclosure study was started June 30, 1961, when the initial results of the chamber study appeared promising. 71 TABLE 16. Plant measurement and yield from two chambers in the field subjected to different carbon dioxide levels (7 harvests) Plant measurements Height Fresh wt. Dry wt. (in.) (gms.) (gps.) Nit/1(a) M03) NM M NM M 2—week old seedlings 1.1 1.2 1.1 1.9 0.13 0.22 3-week old seedlings 2.4 3.8 4.4 10.7 0.63 1.33 Fruit yield Pounds of fruit Number of fruit 6.2 9.6 88 102 (a) NM — chamber without manure (b) M - chamber with manure 72 No consistent atmospheric carbon dioxide differences between the treatments in the enclosure study were found. Fruit number and total yield are shown in Table 17. These measurements did not differ between the treatments within the field enclosures and the enclosures in the greenhouse part of the experiment. Seedling growth showed no benefit from decomposing manure treatments. The total yield and number of fruit were greater in the open field because of the higher light intensities. TABLE 17. Average total number and pounds of fruit harvested per plot in the replicated enclosure study Numberl/ Poundsl/ Greenhouse 14 0.93 Field 47 4.87 1/ F value for difference between number and pounds of fruit between the two sections was significant at the 5% level DISCUSSION The exploratory survey indicated that cucumber produc— tion could not be greatly increased by intensification of selected cultural factors. Although these factors are fre— quently limiting for plant growth, it is possible to reach optimum economic levels with high fertility levels and a large plant population. The question of what limits the growth of pickling cucumbers under these intensified con— ditions essentially remained unanswered in the exploratory survey. Plants protected with a windbreak did not show in— creased yields. Possibly this was because of the small wind—exposed plant surface due to the decumbent growth habit of the cucumber plant or the low wind velocities during the growth period. An elevated moisture level did not increase yields possibly because of the relatively high rainfall during the latter part of the 1959 growing season. One exploratory field test did show that with supplemental light in the field there was an increase in fruit yield. The additional light supplied must have had an effect over the periods of the growing season when the 73 74 light intensity was below full sunlight for a period of time, as demonstrated by yield records during a period of several days in August, 1959. In two other experi— ments yields were greater with a large addition of plant nutrients. Plants grown at row spacings closer than the com— mercially used spacing produced larger total yields, but were objectionable for a season characterized by luxuriant growth on the basis of a high percentage of fruit deterioration. In the laboratory large growth increases were observed with the presence of a small amount of additional carbon dioxide (450 ppm). There was a decrease of the benefit per unit of carbon dioxide with increased levels up to seven times normal. Elevated levels of atmospheric carbon dioxide increased plant height, the number of internodes developed, fresh weight, dry weight and the number of fruit developed in a given period of time. Very little chlorosis, as reported in the literature with tomatoes, was observed with all elevated carbon dioxide levels. This was possibly due to the frequent application of nutrients to the soil. Plants grown in a higher carbon dioxide atmosphere were observed to develop more leaves and a larger foliar surface 75 which facilitated the utilization of the additional ambient carbon dioxide as indicated by the increased values of the plant measurements reported. The results from the exploratory survey posed the question; Is the carbon dioxide content of the atmosphere the limiting factor when the readily controllable field— growth factors are at an optimum? The laboratory data which were reported indicated that it could be one of the important factors. Under shaded conditions in the field, or large plant populations, the trends at the 300 and 700 foot candle light levels may apply. The 300 foot candle level is thought by many scientists to be near the compensation point of respiration for a wide range of plant species. This is borne out from the data when at 350 ppm the seed— lings lost weight with time, while at 450 ppm a net gain in weight occurred. Verduin's theory(89) that the photo- synthetic factors vary independently seems the logical explanation for supplemental carbon dioxide benefit extend— ing to such a comparatively low light level. The work reported here showed that carbon dioxide and light functioned as compensative factors in plant growth. Blackman (6), however, implied that one factor at a given time is the 76 absolute limiting factor. Therefore, "carbon dioxide fertilization" under low light intensities may be feasible for increased production of intensively cultivated crops. Under intensified or glasshouse growing conditions, more studies are needed on the specific effect of supplemental carbon dioxide on the vegetative and fruiting development of each species. Additional studies are needed to corre— late carbon dioxide concentrations with light intensity in relation to economic returns. Light was shown to be the limiting factor at 1400 foot candles and 2150 ppm carbon dioxide when related to growth measurements at 1400 foot candles and 500 ppm carbon dioxide. The nature of the experimental apparatus prevented exposure to light intensities which exceeded the "borderline” zone commonly thought to be at light saturation. Thus, at full light saturation comparable levels of supplemental carbon dioxide would undoubtedly have a greater growth—promoting action. Under conditions of full sunlight the upper limit of carbon dioxide that can be utilized is not reached under field conditions (6), but as the radiant energy is re- duced, it becomes a limiting factor. Depending on the species evaluated, as light is reduced it becomes more and carbon dioxide less of a limiting factor. Lundeggrdh (57) states that the lower leaves are subjected to a 77 higher carbon dioxide concentration. With mutual shading of plants, the lower leaves are subjected to a lower radiant energy level and a higher carbon dioxide concen- tration under ideal conditions. The relationship would be for carbon dioxide to compensate for reduced radiant energy. Plant growing technology and the understanding of plant growth has increased greatly during the past decades with the result that our cultivated vegetative and fruit yields have been increased. With these increased potentials, the atmospheric carbon dioxide may be a more serious limiting factor than previously, at least during vigorous growth periods and in specific microclimates. Conversely, although carbon dioxide may be more limiting to vigor— ously growing vegetation than slow growing and lower yielding plants, how much of our recent increased pro- duction and plant growth has been brought about by ele- vated carbon dioxide levels from fossil fuel combustion in restricted microclimates? What will the effect of further elevated carbon dioxide levels be on future plant growth? Will these increased levels be limited to restricted microclimates? To answer these questions, we should know the response of commercially important 78 varieties of past decades, present and future varieties to standardized levels of atmospheric carbon dioxide. The geologically recent atmospheric carbon dioxide increase is limited by the gas exchange rate with the oceans (9). The microclimatic build—up is further limited by rapid and large scale air movement. In high value or intensively cultivated crops, we should determine the levels of natural carbon dioxide, in various plant environments, to understand its influ— ence on plant population, photosynthesis and the resultant storage compounds. This may be a factor in yield and quality of vegetation that has not been considered. As an example, certain root crops are thought to contain more starch when grown under relatively high plant popu- lation conditions. Entrapment of evolved carbon dioxide by a greater foliage cover may be an important factor in increasing the build—up of starch, as was true in the higher starch composition of leaves grown in enriched carbon dioxide atmospheres. The difference between vari- etal response for growth and yielding capacity may be due to their greater capacity for carbon dioxide utilization at low carbon dioxide levels. Within a "safe atmospheric range" of carbon dioxide for a species, the lower the carbon dioxide content of the air 77 higher carbon dioxide concentration. With mutual shading of plants, the lower leaves are subjected to a lower radiant energy level and a higher carbon dioxide concen— tration under ideal conditions. The relationship would be for carbon dioxide to compensate for reduced radiant energy. Plant growing technology and the understanding of plant growth has increased greatly during the past decades with the result that our cultivated vegetative and fruit yields have been increased. With these increased potentials, the atmospheric carbon dioxide may be a more serious limiting factor than previously, at least during vigorous growth periods and in specific microclimates. Conversely, although carbon dioxide may be more limiting to vigor— ously growing vegetation than slow growing and lower yielding plants, how much of our recent increased pro— duction and plant growth has been brought about by ele— vated carbon dioxide levels from fossil fuel combustion in restricted microclimates? What will the effect of further elevated carbon dioxide levels be on future plant growth? Will these increased levels be limited to restricted microclimates? To answer these questions, we should know the response of commercially important 78 varieties of past decades, present and future varieties to standardized levels of atmospheric carbon dioxide. The geologically recent atmospheric carbon dioxide increase is limited by the gas exchange rate with the oceans (9). The microclimatic build—up is further limited by rapid and large scale air movement. In high value or intensively cultivated crops, we should determine the levels of natural carbon dioxide, in various plant environments, to understand its influ— ence on plant population, photosynthesis and the resultant storage compounds. This may be a factor in yield and quality of vegetation that has not been considered. As an example, certain root crops are thought to contain more starch when grown under relatively high plant popu— lation conditions. Entrapment of evolved carbon dioxide by a greater foliage cover may be an important factor in increasing the build—up of starch, as was true in the higher starch composition of leaves grown in enriched carbon dioxide atmospheres. The difference between vari- etal response for growth and yielding capacity may be due to their greater capacity for carbon dioxide utilization at low carbon dioxide levels. Within a "safe atmospheric range” of carbon dioxide for a species, the lower the carbon dioxide content of the air 79 the less carbon dioxide is available for a plant to utilize in a given time with the same turbulence. Because plants can utilize only a portion of the carbon dioxide contained in a given air volume (30), the greater the carbon dioxide content above the threshold level, the more there would be available for utilization by the vegetation. Carbon dioxide is hard to control or regulate under natural conditions. However, in this experiment a natural differential concentration was determined over muck and mineral soils. The atmospheric carbon dioxide content was measured at sunrise, because the equipment available did not lend itself to "continuous measurement" of the atmosphere. The levels at sunrise can be taken, however, as a measure of the relative carbon dioxide producing potential of a microclimate. The potential advantage would benefit photosynthesis when the turbulence was not great enough for total dissipation of the positive carbon dioxide increase. Measurements taken at times other than sunrise agreed with the diurnal variation as reviewed in the literature (65, 88). Higher atmospheric carbon dioxide levels have been reported over vegetated than over 79 the less carbon dioxide is available for a plant to utilize in a given time with the same turbulence. Because plants can utilize only a portion of the carbon dioxide contained in a given air volume (30), the greater the carbon dioxide content above the threshold level, the more there would be available for utilization by the vegetation. Carbon dioxide is hard to control or regulate under natural conditions. However, in this experiment a natural differential concentration was determined over muck and mineral soils. The atmospheric carbon dioxide content was measured at sunrise, because the equipment available did not lend itself to "continuous measurement" of the atmosphere. The levels at sunrise can be taken, however, as a measure of the relative carbon dioxide producing potential of a microclimate. The potential advantage would benefit photosynthesis when the turbulence was not great enough for total dissipation of the positive carbon dioxide increase. Measurements taken at times other than sunrise agreed with the diurnal variation as reviewed in the literature (65, 88). Higher atmospheric carbon dioxide levels have been reported over vegetated than over 80 bare soil areas in the past (65). Similar tests in these experiments were inconclusive in confirming these reported results. It seems possible that with further experimentation, all other factors being equal, specific natural micro— climates may prove useful for more beneficial carbon dioxide utilization by specific plant species. At least a portion of plant growth increases in recent times, in specific microclimates, may be due to an increased natural carbon dioxide concentration. Through a better understanding of the variation of the natural atmospheric content of carbon dioxide, we might be able to manipulate it by feasible practices so that plant growth can be increased. As an example of this, the difference in plant growth and yield obtained in the field chambers may be due to the higher walls (7 feet) and growth during the early part of the growing season (June 1) when carbon dioxide levels are thought to be lower. The 3—1/2 foot high wall enclosure experiment initiated on June 30 showed no such benefit. Cucumber seedlings have shOWn growth benefits at atmospheric carbon dioxide levels up to seven times the normal atmospheric content. 81 Large volume additions of carbon dioxide to a restricted plant growing atmosphere would certainly be uneconomical. However, small volume additions were shown to result in proportionally large growth increases. Therefore, with cucumbers the most practical level of carbon dioxide fertilization would seem to lie in the range of 400 ppm to 600 ppm. SUMMARY Environmental factors for the growth of cucumbers studied in the exploratory survey were irrigation, fertil— ity level, plant population, nutrient source, wind pro- tection and field light intensity. The survey indicated that economically feasible intensifications of the factors did not result in growth or yield increases of a large magnitude over that obtained with current recommended cultural practices. Supplemental field lighting resulted in slightly larger fruit yields compared to shaded and natural sunlight areas, while very large additions of plant nutrients increased yield appreciably. Cucumber seedlings grown with a supplemented atmos— pheric carbon dioxide level showed an increase in fresh and dry weight and the number of fruit developed. The number of internodes developed and plant height were also increased in some tests. The benefit per unit of supple— mental carbon dioxide ranged from a linear increase per 100 ppm added at normal air concentrations to appreciably less than a linear benefit at four and seven times the normal air content. Carbon dioxide and light functioned as compensative growth factors. 82 83 As light levels were intensified (up to 1400 foot candles) the growth of cucumber plants increased at a more rapid rate with higher levels of carbon dioxide than under low levels. At the highest carbon dioxide level studied (2150 ppm), light was shown to be a limiting factor at a light intensity range from 1000 to 1400 foot candles. Cucumber leaves from supplemented carbon dioxide atmos— pheres were shown to contain a greater amount of starch than leaves grown in normal atmospheres. Plants grown in carbon dioxide enriched atmospheres developed larger foliar surfaces than in normal atmospheres. 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