The Effects of Light on Plant Growth
Most gardeners are familiar with seed potatoes that have been chitted up in the light — the sprouts are sturdy and blue-green in colour. Now compare these with potato sprouts that have been stored for several months in a sack in a dark corner of a shed. They are etiolated — pale and sometimes up to 15cm (6 in) in length. This is a very clear example of what happens to plants when they are deprived of natural light.
Unfortunately the problem of making up a plant’s light deficiencies cannot necessarily be solved by simply suspending an ordinary light bulb over it. The reaction of different species and varieties of plants to light is very complex — no two species react in the same way; some plants prefer full sun while others like shade (sometimes even heavy shade) and some flower in summer while others flower in spring, autumn or even winter, each one adapting itself to particular environmental needs and conditions.
Before you can use artificial light intelligently with the object of supplementing daylight and assisting plant growth, you will need more precise knowledge of the principles of light and the effects of light on plant growth. You will then also be able to appreciate why some sources of artificial light are better than others.
The effect of latitude
The difference between the amount of solar light radiation (sun’s energy reaching the Earth) that is received in summer and winter is due largely to latitude. The farther from the equator your garden lies the lower the amount of radiation it receives. In Britain, even in the extreme south of England, the sun never rises more than 15 degrees above the horizon in mid winter (December); as it then has to penetrate a much thicker layer of the Earth’s atmosphere, its power is reduced. In fact the total solar energy supplied in a winter month may be only 10 per cent of that experienced in a summer month.
In addition to the effect produced by latitude there are also local conditions to take into account. Cloud cover, for instance, which can vary over quite a short distance, and industrial pollution, can be important factors in reducing solar radiation. Taking the average figures for the British Isles as a whole, there is not sufficient natural light (not enough daylight hours) to grow many plants effectively between late autumn and mid spring (October and March).
In the late 1940s W. J. C. Lawrence of the John Innes Institute noted that serious light deficiencies existed during the winter months and he started using artificial light on tomato seedlings, not only to produce better plants but to shorten the time between pricking out and planting. Although the results were rewarding, the cost of the lamps used at the time (high pressure mercury) did not convince growers that the expense was worthwhile.
Since that time more efficient lamps have been produced, commercial growing rooms introduced and the technique of inducing plants (chrysanthemums in particular) to flower throughout the year has become established practice.
Principle of light
Any source of radiation — whether the sun or an artificial radiator (lamp or heater, for example) — emits a certain amount of energy consisting of electromagnetic vibrations (such as visible light, radio waves or gamma rays). The wave motion of these vibrations has a constant speed but the length of the waves may vary, and it is the length of the waves that determines what kind of energy is produced. Some wave-lengths — the relatively narrow waveband between 380nm and 780nm (one nm, or nanometer, is one millionth of a millimeter) — are visible to the human eye. It is this visible part of the spectrum that is called light.
Below 380nm comes ultra-violet radiation; this causes the skin to tan and can be dangerous to plants. Above 780nm is infra-red radiation (heat).
Within the visible spectrum the wave length of the radiation determines the colour of light. As the wave-length increases, you see the following sequence of colours — violet, indigo, blue, green, yellow, orange and red. ‘Beyond’ red is ‘far-red’ — partly outside the visible spectrum but important to plants. For convenience the spectrum can be simplified into four spectral divisions: blue 400 — 500nm, green 500 — 600nm, red 600 — 700nm and far-red 700 — 750nm.
Light can be considered not only as a wave motion but also as a flow of light particles. Each of these light particles represents a certain amount of energy that depends upon the wavelength (shown by colour) of the type of light; the shorter the wavelength the higher the amount of energy per light particle. Thus a blue light particle contains a greater amount of energy than a red one.
Plants react to light in three ways: to the intensity of light, to the duration of light and to the colour of light. Each of these reactions will now be considered in greater detail.
Light and photosynthesis
The energy necessary for plant growth is almost all derived from the radiant energy that is absorbed by the parts of the plant growing above ground. The carbon required by the plant for food and for cell structure is derived from the carbon dioxide in the air. Photosynthesis is the process by which light energy is used to reduce this carbon dioxide to sugars that can later be converted to a variety of different structural and food materials. Photosynthesis is dependent on the particular pigments (the chlorophylls) that produce the basic colour of the green leaves. Chlorophylls absorb mainly blue and red light; green is largely reflected.
Not all the light available to a plant is absorbed by the leaf; a proportion of it is reflected at the leaf surface and some light is transmitted through the leaf without being absorbed.
Allowing for this wastage, for photosynthesis to take place a great deal of light must fall on the leaf surface. Although this is likely to happen in summer, in winter the amount of sunlight available to greenhouse plants is greatly reduced.
In commercial practice the aim is to maintain between 5,000 and 10,000 lx (the amount of light falling on an area one metre square is measured in lx or lux). This will be dealt with in more detail in the following sections when we look at different kinds of lamps and their uses.
Light and photoperiodism
Photoperiodism is the effect of day-length on plant development. It has been known for more than 50 years that the flowering habits of some plants depend on the relative length of day and night and that these habits vary widely from one kind of plant to another.
This knowledge has led to a broad classification of plants into three groups. First there are those that will only flower (or will flower more readily) when the daily period of light exceeds a certain critical minimum; these are known as ‘long-day’ plants. In the second group are those that will flower only when the day-length is less than that critical minimum —’short-day’ plants. The third group contains plants that flower equally readily in any day-length — `day-neutral’ plants.
The position for individual plant species, however, cannot be stated quite so simply. Some plants require a certain day-length for bud initiation and a different period for flower development (short-day/long-day plants or long-day/short-day plants). In others, sensitivity to day-length varies with temperature. The effects of relative day-length on flowering behaviour first drew attention to the phenomenon of photoperiodism and it is in the control of flowering that artificial lighting techniques have so far been used most successfully.
Sufficiently long photoperiods can be provided during naturally short days by using artificial light to shorten the night. This can be done by switching lights on at dusk to lengthen the day or turning them on for a few hours before dawn to produce the same effect at the opposite end of the day, or using them for a short period during the middle of the night — known commonly as night-break.
Only relatively low illuminance levels are needed to achieve the desired effect, the actual level depending on the type of plant concerned. Simple tungsten-filament lamps (domestic light bulbs) are generally used. You need not be too fussy about the arrangement of the lamps over the growing area so long as it provides the required illuminance as evenly and cheaply as possible. Generally a minimum of 50-100 lx is considered adequate.
Light and photomorphogenesis
The rate at which photosynthesis can take place depends on the number of suitable light energy particles received. Red light contains more light energy particles per unit of energy than any other suitable wavelength. From this it could be assumed that a pure-red light source would be the most beneficial for good plant growth. But there are other light requirements (in addition to photosynthesis) that must be taken into account if the plant is to develop satisfactorily. As well as red light (about 660nm), blue light (about 450nm) and far-red light (about 735nm) also exert controlling influences on plant development; this is known as photomorphogenesis — the effect of different light wavelengths on plant growth. Plants grown entirely in blue light have a suppressed, hard, dark appearance and are inclined to ‘rosette’, while those grown in red light tend to be softer, and suffer some degree of stem elongation. Red light can also suppress the elongation or etiolation that occurs in darkness, but far-red light cannot. Thus the effect of light on vegetative growth (stems and leaves) is complex. This, then, is the reason why the careful choice of a light source for growing plants in the greenhouse — or the house — is so important.
Right lamps for the light
Suitable lamps for artificial illumination are expensive and it would be unfortunate if you chose the wrong lamp for the lighting technique you had in mind. Photosynthesis, photoperiodism and photomorphogenesis are only three, albeit extremely important, factors to be taken into consideration. There are others, such as temperature, size of greenhouse, bench or bed widths, height of roof above the growing area, and number, type and stage of development (seedling or mature stage) of the plants and the proximity of other plants in the growing area. These aspects will now be covered in more detail.