Light In Its Relation to Greenhouse Culture
( Originally Published 1920 )
OF the many factors which are intimately inter-woven with the growing of greenhouse crops, light is a very important one. Unfortunately, this subject has received scant attention. However, Dr. Stone of the Massachusetts Agricultural Experiment Station has contributed greatly to our knowledge on this subject. It is apparent from his work that success with greenhouse crops goes hand in hand with a thorough understanding of the light requirements of plants. The problem of light has. a direct bearing on the physiology and pathology of hot-house crops.
PHYSIOLOGICAL RELATIONSHIP OF LIGHT
To realize the importance of this subject we must be aware that nearly ninety-five per cent of the substances contained in the plant is derived from the atmosphere. These substances are manufactured through the action of light on the green matter (chlorophyll) located primarily in the leaves. This process is known as photosynthesis. It consists of an intake and assimilation of carbonic acid by the plant, and a simultaneous liberation of oxygen. The carbonic acid breaks down and combines with the water to form the sugars and starch. The spectrum rays which are most concerned with the manufacture of starch are the orange and the red. The blue rays affect growth. Success with greenhouse plants depends largely on the intensity and the nature of the light rays which are permitted to penetrate through the glass. At best, these rays differ materially from the normal sunlight.
Contrary to general belief, plants make most growth at night or in the dark. On the other hand, photosynthesis takes place during the daytime and under the direct influence of light. While light does not favor growth, it assists in the development of supportive tissue which enables the plant to resist attacks of various diseases. The lack of a proper amount of light in the greenhouse causes the plants to possess little or no resistance to disease. This is especially true in the winter months. However, while insufficient light is conducive to disease, an excess of it, such as occurs in the summer months, is also detrimental to plant health in the hothouse. In that case, shading the glass becomes necessary. Moreover, there are numerous hothouse plants, such as palms for instance, which naturally require less light. On the other hand, lettuce, tomatoes, cucumbers, roses or carnations require more light in the winter months than the ordinary hothouse is able to furnish.
PATHOLOGICAL RELATIONSHIP OF LIGHT
While it is true that plants grow in the dark, they must have light to thrive. The growth made in darkness exclusively is of a soft nature. The long, whitish, slender sprouts of potato kept in the dark are a good illustration. Numerous diseases of plants grown under glass may undoubtedly be traced to improper light conditions. Cucumbers, for instance, when grown in poorly lighted houses, become slender, producing elongated petioles and stunted leaves with little green color in them. Such plants, too, are soft, and possess little of the solid or resistant tissue. Poor light also makes cucumbers, as well as most other hothouse plants, susceptible to mildew, blight and leaf spots. Poor light and wet soils are responsible for the burning of the foliage of hothouse plants under fumigation. Too much light often affects the transpiration of plants and causes them to wilt unduly. Blossom end rot of tomatoes under glass is more severe under bright light than under partial shading. No fixed rules can be given as to the light requirements of greenhouse crops. Until more definite knowledge is obtained on this important subject, the greenhouse manager will of course depend on his common sense, observations and experience to guide him.
CONSTRUCTION AND MANAGEMENT OF HOTHOUSES AS AFFECTED BY LIGHT CONDITIONS
From the previous discussion, it is now evident that to improve light conditions indoors will tend to produce normal growth and to hasten maturity. The greater the photosynthesis, the more rapid the assimilation of plant food, hence the quicker the growth. An increased amount of heat can never replace the normal effect of an increase of light for those crops which most require it. Many growers, especially those who possess poorly constructed houses, often attempt to substitute heat for light in forcing. The result is generally a failure, because diseases of all sorts find the tender weak plants an easy prey to their attacks. The modern greenhouse man is partly solving the light problem by constructing larger houses and using larger glass. As a result of this, more air space and more uniform moisture distribution is assured. The double-thick, third quality glass, used in previous years, is now being replaced by a good grade of double-thick, second quality glass. Improvements are also being introduced in the roof angles, for these, too, mean added and better light. The more closely the angle of the roof coincides with the right angle cast by the sun's rays, the greater the amount of light that may reach the indoor plants. In the old form of houses, many of which are still in existence, the glass used was from two to seven inches long and two to five inches wide, and was often lapped more than an inch. This system practically excluded fifty per cent. of the light. The modern house uses glass varying in dimensions from 16 inches by 24 inches, 20 inches by 30 inches to 24 inches by 24 inches. With the use of the larger glass, the diminished lapping results in a considerable saving of light. To prevent breakage of the larger glass a house must, of course, be solidly built and well purlined.
The location of the house, too, influences the amount of light taken in. Houses located north and south are benefited by the morning light only, whereas those running west receive only the after-noon light. The ideal location from the light view-point would be to set the house on a line running 20 to 25 degrees north of east.
Houses with greater roof angles naturally receive more light. It is also a well conceded fact that light will pass through a transparent object more easily if it is placed at right angles to the light rays. This fact is not often taken advantage of by greenhouse builders. However, it cannot be denied that the sunlight strikes the house at different angles during the day and likewise at different seasons of the year, thus producing considerable variation in the amount of light reflected. To obviate this, houses should be built with greater roof angles, a plan which will insure less reflection and thus allow a greater amount of light to penetrate. During January, for instance, when the normal sun-light is naturally less, glass placed at an angle of 60 degrees will absorb far more light than if placed at angles of 10 or 30 degrees.
It has been the general belief that the light in the hothouse was greatest nearest to the glass. Experience has disproved this. Modem houses are built larger, which means a greater distance between glass and plants.
EFFECTS OF VARIOUS COLORED LIGHTS
That plants require light for their normal development and for the proper performance of their function no one doubts. Crops grown outdoors naturally receive their light from the sun's rays. Plants within the greenhouse do not always receive the rays of the sun in a normal way. As a result, the health of these plants may often be impaired, or the quality of the product greatly affected. The researches of Flammarion* on this subject are of particular interest to the greenhouse man.
TEMPERATURE INFLUENCED BY THE COLOR OF THE GLASS
Flammarion has shown that temperature is affected by the color of the glass. Houses differentiated by the following sorts of colored glass were tried: blue, approaching closely to violet; red traversed by a little orange; green and ordinary white glass. These four houses were placed side by side and equal conditions of care and culture were observed in all of them, approaching natural conditions as nearly as possible.
It is evident that the ability of the glass to absorb the sun's rays determines the heat in the hothouses. All rays are able to pass through white glass, which explains why the highest temperatures were found in this house. The lowest temperatures were found in the blue house, blue having the greatest absorbing power. It is striking that the temperature was apparently the same in all the hothouses during, the cloudy weather and when the sun's rays did not penetrate directly.
PLANT GROWTH INFLUENCED BY THE COLOR OF THE GLASS
Experiments on sensitive plants showed the following results: Plants placed in a red house developed a height fifteen times as great as that in the blue house, where practically no growth was made. The red light in this case acted as a fertilizer. Moreover, the sensitiveness of the plants grown in the red house had increased considerably. The slightest movement or breath was sufficient to cause the leaflets to close, or the pedicels to droop. The sensitiveness diminished under the white or green color, while under the blue glass the sensitiveness was almost lost. The plants in the red house were first to bloom. In the white house they increased in stockiness and in vigor, but did not seem to in-crease in height. The plants in the house with the red glass possessed foliage which was lighter than those grown in the white house, while under the blue glass the foliage was much darker. After three months the height of the plants in the different houses.
From the preceding table it is seen that the plants in the hothouse with the red glass attained greater height and exhibited more sensitiveness than those in the white house. The sensitive plants in the green hothouse made a little headway at first and then came to a standstill. In the hothouse with the blue glass practically no headway was made. In comparing the weight of the plants in the various hothouses with that of the height as previously mentioned, the results will be found to be different.
It is very curious to find that the plants in the red hothouse, although the highest, were not the heaviest. The weight was almost double in the white hothouse, although in height the plants did not compare to those in the red hothouse.
Experiments on lettuce, similar to those on the sensitive plants, yielded like results. Lettuce grown in the white hothouse produced large thick leaves with well rounded heads, in fact the plants here did not differ from those grown in the open. Lettuce grown in the red house was drawn, its leaves long, straight, blanched and drooping. Those grown in the hothouse with green glass made a slight growth, but the leaves were more curled than those in the red house. In the blue hothouse, the lettuce plants added only a few leaves, without increasing the height attained in the first two weeks (fig. 9, i-j.).
Experiments on peas and beans yielded similar results. In both plants the normal and most vigorous growth was found to occur in the white hot-house. The plants in the red hothouse were taller but thinner, while in the blue hothouse the minimum of growth occurred. The beans bloomed and fruited equally well in the white and in the red hothouses. In the green and in the blue houses the plants soon died. With the peas, blooming and fruiting seemed to be normal both in the white and in the red hot-houses. In the house with the green glass, the peas remained in bloom for three weeks, but did not fruit. In the blue house, the peas failed to bloom altogether.
Experiments with ornamental plants, such as Coleus, yielded similar results. The Coleus in the white hothouse produced a normal well developed plant. In the red house there was an increase in height with a decrease in foliage. In the green and blue hothouses there was very little development (fig. 9, a-h.).
Root Development Influenced by the Color of the Glass
It has already been seen that different rays of the solar spectrum may modify the parts of the plant above ground. The same effects may also appear in the root system of such plants. The root system is considerably smaller in the red hothouse than it is in the white hothouse. In the hothouse with green glass, the root system is very poorly developed, while in the blue there is almost no root system.
ANATOMY OF THE PLANT AS INFLUENCED BY DIFFERENT RAYS OF LIGHT
As we have seen, different rays of light are capable of influencing the growth of plants. The same is also true of the structure of plants. Flammarion has found that sensitive plants, for instance, when grown in a white hothouse, possess a thicker epidermis, more numerous wood fibers in the stem, and the pith was much less developed than was the case with similar plants grown in red, green or blue hot-houses.
EFFECT OF VARIOUS LIGHT RAYS ON THE COLOR OF PLANTS
It is well known that the green color of leaves which is due to chlorophyll can only be produced in the light. Other plant colors such as red, yellow, blue, may be due to the presence of colored pigments, or to color in the cell sap itself.
That light and not temperature is capable of changing the colors in plants has been proven by the investigations of Flammarion. He found that lilac blossoms in a white hothouse became pink, and in red, green and blue hothouses the blossoms became white. If the lilac blossoms already colored are placed under a dark bell jar, they will turn from pale blue to clear red violet. This change of color is not due to temperature. It is due solely to the effect of various light rays. Lilac blossoms, for in-stance, if enclosed in a dark chamber will become discolored, irrespective of surrounding temperatures.
Coleus plants grown in a white hothouse will produce leaves with the normal amount of red color and pigmentation. In a red hothouse, the red pigment of the Coleus decreases, the leaves are more spread, and their form is changed. Coleus grown under green-colored glass produces leaves of small sizes, the pigments almost disappear, and give place to a yellow coloration. The same is true also when Coleus is grown under blue glass. In this case, however, the red pigments disappear almost completely (fig. 9, a-d.). In substantiation of the fact that light is capable of transforming plants, Flammarion refers us to the following experiment: Coleus plants may gradually be transformed when grown under a slightly diffused light through a garden frame, in diffused light, and in still weaker light. The plants grown in the open are of course normal The most curious transformation occurs in the diffused light. Here the leaves enlarge considerably and the red pigments diminish in the ten-ter. Under a weak light the Coleus leaves become stunted, and the color changes from poppy red with a dark edge to yellow with a light green edge (fig. 9, e-h.). Purple leaves of Alternanthera amena will become green under red glass. In the open, geranium leaves possess a reddish brown tone. This color changes under red, green or blue ray.
These experiments seem to indicate that light is of itself able to modify plants.
EFFECT OF LIGHT ON DISEASE RESISTANCE
It has already been seen that light is an important factor in plant culture. It alone seems able to change the form of a plant as well as its color. Moreover, the resistance of a plant to a disease may be modified by light. Damping off, for instance, a prevalent disease in greenhouses, is more virulent on dark cloudy days, when the light in the hothouse is abnormal and weak. A temporary change in the normal functions of the metabolism of the plant occurs and is followed by a sudden lowering of vitality. The greenhouse man cannot overlook the importance of the light requirement of plants under glass. Experience has shown that white glass is the only one capable of furnishing the necessary light rays to the plant. Needless to say, that only the best quality should be secured, for economies in the quality of glass may not always be the wisest nor the cheapest in the end.
The field of greenhouse culture is a plastic one. Electricity is undoubtedly capable of influencing plant growth. The greenhouse man who is conscious of the possibilities of control in his plastic domain will not neglect this phase of plant culture.
EFFECT OF ELECTRIC LIGHT
It has always been a question whether growth under hothouse conditions could not be hastened by using artificial light at night. The work of Rane* has shown that such is the case.
The beneficial effect on lettuce seems to be especially marked for the Grand Rapids variety first, next for the Hanson and thirdly for the Tennis Ball, the only three varieties experimented on by Rane. The lettuce in the house lighted with electricity seemed more erect, vigorous, and the soil freer from damping off and rot-producing organ-isms. This is indeed an important consideration. Moreover, the lettuce in the electrically lighted house matured about twelve days earlier than that grown otherwise. Greenhouse spinach, like lettuce, seems also to be benefited by electric light at night. On the other hand, cauliflower reacts poorly to this treatment. Although the plants are taller, the quality of the head is of an inferior grade. Radishes develop more tops than roots. The practical-minded greenhouse man will use electric light at night to induce extra stimulation for those green-house crops that respond favorably to it. The cost of installing the system certainly cannot be considered as a real drawback. Electricity in these days may be obtained at a reasonable price. This is especially true of greenhouses situated near large cities. But more extensive investigations are needed to convince the grower of the practicability of the attempt.
EFFECT OF SHADING
It has been shown in the previous chapter that certain light rays, such as green or blue, are detrimental to plant growth. On the other hand, the normal sun rays from outdoors or as they come through white glass are most conducive to normal plant culture. The practical man, however, realizes that at certain times of the year, especially during the summer months, the white glass must be shaded to prevent an excess of sunlight. This is accomplished by whitewashing the glass. That this procedure is necessary no one can question. However, it must be admitted that the method itself is still a crude one, inasmuch as the various plants in the hothouse are subjected alike to the same amount of shading. Shantz* has shown that while a certain amount of shading is beneficial to plant growth, yet not all plants are benefited alike by this treatment.
EFFECT OF DIFFERENT LIGHT INTENSITIES ON PLANTS
The work of Shantz distinctly shows that all plants do not tolerate the same amount of shading. To prove this he grew various crops in a bed covered with cloths of different textures. The arrange-ment of the cloth and the amount of light penetrating it are shown.
As an explanation to Table 11, in referring for instance to column marked 2, n/15, we mean the second section of the bed where light canvas cloth (black) was used for shading, and where the light capable of passing through was equivalent to 1/15 or n/15, z or n represents the normal. The same interpretation is given to the other sections of Table 11, all of which really correspond to the sections of the bed experimented with, it being remembered that z or n represents normal light.
EFFECT OF DIFFERENT LIGHT INTENSITIES ON LETTUCE
According to Shantz, lettuce could not grow in section 1 under n/93 illumination, as the seedlings died as soon as the reserve food material in the cotyledons was consumed. In section 2, n/15 illumination, growth was barely possible. The plants in this case were emaciated and worthless. It seems, therefore, evident that lettuce cannot stand shading where the light is reduced to n/15. The greatest amount of gain is made by lettuce when the intensity of the light ranges between n/7 and n/5. At this point the stimulation is greater than in those grown under normal light or even at n/2. In full light lettuce plants are smaller than in n/7 or n/5 light. In flavor, a very slight change only may be noticed between plants grown under full light and those receiving n/2 light. However, under n/5 illumination the strong taste seems to disappear entirely. When the light is reduced to n/7 the flavor seems to improve even more. Moreover, in this case the plants acquire the particular form of growth required by the market more consistently than do those that are produced in brighter light.
EFFECT OF DIFFERENT LIGHT INTENSITIES ON RADISH
Young radish seedlings seem capable of standing about 30 days under n/93 illumination. At the end of that time, however, they die. In n/15 light there is almost no growth. The best gains seem to be made under a light of n/2 or n/5. In this respect, the shade tolerance of the radish is somewhat similar to lettuce. However, the effect of shade is not noticeable in the flavor of the radish.
From the above evidence it is apparent that plant growth is assisted by shading, the degree of which must be worked out for each specific crop. Ordinarily, under greenhouse conditions, shading is accomplished by means of whitewashing the glass of the hothouse. It would seem more desirable to use chambray, light chambray or voile cloth instead of whitewashing. . The cloth could be installed on a system of rollers, so that when shading is necessary it could be spread out on the glass and when not needed it could be rolled up again. This would enable the greenhouse man to retain the full amount of normal light on cloudy days, an advantage which cannot be obtained when the glass is whitewashed. The substitution of cloth for whitewashing offers a good field of experimentation, both for the laboratory and for the practical man.
The heat requirement of indoor crops demands the closest attention and study. As is well known, no two crops require the same temperature for their maximum development and production. Moreover, the same plant requires different temperatures in its various stages of growth. To appreciate thoroughly the relationship of heat to plant life, we may liken the plant to a steam engine. With very slight steam pressure, the engine remains "dead," because it is not able to overcome the friction of its own parts and hence is capable of no work. With the proper amount of steam pressure, the engine is capable of a maximum amount of work.
However, if an excess of steam pressure is used, the engine under an excessive strain will break or explode. The same is true with plants. A low temperature may not suffice to awaken the active life processes. With increased temperature, the plant becomes capable of maximum activity. Temperature beyond the normal requirements causes it to suffer or even to die from weakness and disease.
The ideal development with forced plants is possible only if we consider the relationship of the soil temperature to that of the air. For instance, the bum of lettuce is brought about by rapid evaporation of moisture from the leaves at a time when , the roots are unable to supply this excessive demand of water. If the soil is cold, or its temperature different from that of the house air, the roots will be unable to supply fast enough the water needed by the foliage. This will result in their collapse or burning, and the ruin of the lettuce crop. The proper temperature requirement for each crop will be taken up when we consider further the cultural requirements of each.