Author: Prof. Dénis van Rensburg.
The role of light (or radiation) on the growing of orchids
Green plants possess the amazing ability to manufacture their own food through the process of photosynthesis. It is this basic process upon which the entire world of living creatures depends for its own food, either directly by eating plants, or indirectly, by feeding on other organisms that have fed on green plants. The process of photosynthesis, or the manufacture of food using the energy of sunlight, is often expressed by a simple chemical equation (see above).
Carbon dioxide and water are the two main nutrients of plants and for plants to "eat" or absorb these nutrients, sunlight (or energy that is contained in sunlight) is absolutely necessary. Sunlight contains many different wavelengths or packets of energy. Just like a radio or TV set that only "works" when it is tuned to a certain wavelength (or energy packet), so do plants only react to certain wavelengths and use that specific energy to manufacture food through photosynthesis. All the other light (wavelengths) are totally useless to the plant and may even be harmful. In Figure 1 all the wavelengths to be found in sunlight are shown. This wavelength range is commonly referred to as the "electromagnetic spectrum". From this figure it can be seen that the sunlight that reaches our shade houses, apart from ordinary "visible" light, also contains X-rays, microwaves, radio waves etc.
Because oxygen is produced when plants photosynthesize, it is fairly easy to determine which wavelengths present in light are indeed preferred by the plant. All you need to do is expose a plant to each different wavelength and measure the oxygen produced. If no oxygen is produced, that specific wavelength has no effect on the chemistry of the plant. Figure 2 shows such an oxygen versus wavelength graph for the "visible" light region. Photosynthesis seems to be very high in the "blue" and "red" wavelength ranges and much less effective in the "green" region, and absent in all the other wavelength regions.
This effect of light on photosynthesis or plant growth could have been deduced from the green colour of the leaves. The colour of an object arises when the object reflects those wavelengths present in visible light that it does not want or cannot use. If we pass light from the visible part of the electromagnetic spectrum through a prism we can "see" all the coloured components of so called "white light". The red component has slightly less energy than the green, while the blue and violet components have the hightest energy (lowest wavelengths) in "white light" (see Figure 3).
Hence an object such as an apple will appear to our eyes as yellow because it absorbs all the other colours and only reflects the light it does not want - the yellow (see Figure 4). An object appears red to our eyes because it absorbs or "likes" all the other colours of the spectrum but "hates" red and hence reflects it. A white object reflects all the colours and hence appears white, while black objects appears black because it absorbs all the colours. Hence, strictly speaking, black is not a colour! (see Figure 5). By the same argument green plants hate green light, therefore they reflect the green light and appear green to the human eye.
This brings an interesting aspect to the fore when it comes to the choice of shadecloth for your orchids - green would not be the ideal choice but rather blue or red, when we look at the production of oxygen curve.
For reasons beyond the scope of this article, the energy of radiation is inversely proportional to the wavelength so that UV-light has more energy that white or visible light and causes much more damage to plastics, human skin and other substances. There is also more energy in "blue" than "red" light and blue light or shadecloth should, in theory, produce little more heating in plants and the shade house than similar red shadecloth. Infrared light has low energy but the energy is sufficient to cause heating if absorbed. Due to their water content, most plants are effective reflectors of infrared and are not greatly heated by it. Where shadecloth with a low percentage shade is used, the colour of the cloth is not very important because more than adequate white light will reach the plants for photosynthesis - with high percentage shade shadecloth the colour of the cloth does come into play.
Klein, Edsall and Gentile (1965) published an interesting paper in the effects of near UV and green light on plant growth, not specifically on orchids, but there is no reason why the results should not be applicable. The conclusions reached are that the near UV and green wavelengths are capable of suppressing the growth of plants which otherwise receive adequate levels of those wavelengths necessary for photosynthesis and normal development. Conversely, the selective removal of near UV and green wavelengths from white light gave enhanced growth. From these scientific experiments it is evident that green shadecloth should, as a matter of routine, not be used as a covering in shade houses. Due to the total reflectance of white shadecloth varieties, a higher than normal shade percentage should be used when the shadecloth is white.
The amount of pure sunlight (in the visible region) needed by orchids to manufacture their food has always been an unknown factor usually obtained by trial and error. In experiments conducted by Williams et. al. in 1983 using Paphiopedilum insigne at leaf temperature of 20ºC, it was found that less than 10% of full sunlight gave maximum photosynthesis.
The effect of temperature on photosynthesis
The effect of temperature on photosynthesis is a very important consideration. Though the photochemical reaction is not dependent on temperature, the rate of photosynthesis does increase with increasing temperature. Figure 6 shows this effect.
While gross photosynthesis rises with temperature, so does respiration and whereas the photosynthesis rate tends to flatten at about 25ºC, respiration continues to rise rapidly above this temperature. Consequently the nett photosynthesis (the production of energy compounds minus their use by respiration including photorespiration) must continuously be considered. It can even be better understood if we add a curve depicting respiration rate against temperature to the one shown in Figure 6 (see Figure 7). Respiration increases strongly with temperature (same as with us humans) and at temperatures above about 35°C all the food manufactured is used to support respiration. At temperatures higher than 35ºC, plants hence use more food than they produce to respirate which leads to deterioration and ultimate death (if the condition is allowed to continue for too long, of course).
In their natural habitats, plants are subjected to a day and night temperature differential and it is important to imitate this as far as possible. Photosynthesis is effected by temperature as well as light intensity. As long as it doesn't become too hot, the rate of photosynthesis, and hence growth, increases with rising temperature, but only as long as there is sufficient light (and carbon dioxide). Not only is there no point in increasing the temperature of the greenhouse beyond 25°C if there isn't sufficient light, it is positively harmful as the rate of photosynthesis will, even at 25°C, fall below the rate of transpiration.
Technically speaking, when gains due to photosynthesis match losses due to respiration, this is termed the "compensation point". The practical value of appreciating this is that if nighttime temperatures are too high, plants will gradually become depleted of their glucose and starch reserves as losses due to respiration regularly exceed gains due to photosynthesis during the day.
Diffuse radiation and albedo values
Diffuse or sky radiation is that radiation which reaches the earth's surface after being scattered from the direct beam by molecules in the atmosphere or reflected from clouds or other objects. On clear days it increases with solar elevation up to 30° but after this it remains constant. It is diffuse radiation which gives the light in shaded areas.
Diffuse radiation is very important for places of high latitude where low solar elevations reduce the direct solar energy due to thicker atmosphere passage, hence more absorption. For example, in England diffuse radiation may contribute from 50 to 100% of the total radiation used by plants. This has produced the typical English glasshouse, which has the maximum amount of glass in walls and roof to catch this omni-directional radiation.
In South Africa our proportion of diffuse radiation is much lower but we still build English-European style glasshouses, then promptly cover them with shadecloth to reduce the intense direct radiation on to the glass. What makes it even worse is that we use green shadecloth.
Figure 8 shows the various light inputs to a typical horizontal leaf. These are:
- direct radiation from the sun (SA)
- diffuse radiation from the sky (SD) - on overcast days this may be the main source of light
- Both direct and diffuse light reflected from the ground: R (SA + SD) where R is the reflectance from the ground or albedo. These albedo values can be of great help when we have a problem with too much or too little light.
The correct unit to measure radiation energy is in watts per square metre. Because foot candles makes more sense to me and because my light metre is calibrated in foot-candles, I will use this non-SI unit. Just to give some idea of the values involved on a clear day without much smog, with a high sun angle (warm temperate or tropical) and little cloud we have SA=10 000 foot candles; SD=1 370 foot candles and R (SA + SD) = 1 580 foot candles.
The rate of photosynthesis is proportional to the light intensity received by a plant up to a maximum of about 5 000 foot candles. At 5 000 foot candles most plants are at 100% photosynthesis efficiency and light intensity levels above this value are of little or not benefit and can only cause heat increase, plant exhaustion and undue drying of the plant. The light intensity of a full sun on a clear day is approximately 10 000 foot candles - where a foot-candle is the amount of light cast by one candle at a distance of one foot. At 5 000 foot candles most common outdoor plants are at 100% efficiency and light levels above can only lead to heat increase, plant exhaustion and undue drying of the plant.
Plants of our temperate zone can be categorized into day, neutral and long-day plants. The dividing line between day lengths favourable to vegetative growth and those to cause seed and flower formation is called the critical light period. For most species the critical light period is between 11 and 16 hours.
The intensity of the light and the duration of exposure combine to let us know the quantity of light received by the plant. The intensity of 1 000 foot candles usually is the minimum light intensity for ordinary plants and the minimum quantity of light is 15 000 foot candle hours (light intensity multiplied by duration of exposure in hours).
The relative length of the daily light and dark periods controls flowering of many kinds of plants. This phenomenon is called photoperiodism. Hence photoperiodism is the length of time a plant is exposed to light. Some plants, such as certain varieties of chrysanthemum, poinsettia and morning glory are short-day plants and flower in nature only when the days are short and the nights long. Certain varieties of spinach, beet, barley and tuberous-rooted begonia are examples of long-day plants that flower in nature only when the days are long and the nights short. Flowering of many other kinds of plants is hastened but not absolutely controlled by the appropriate day length.
Bulbing and tuber formation is also controlled by day length. Tuberous-rooted begonia, which is a long-day plant for flowering, produces tubers on short days but not on long days. Onions, on the other hand, produce bulbs on long days but not when the days are short.
As a general rule, orchids require between 38 000 and 15 000 foot candle hours to grow effectively. Provided the light is not unduly "green" and contains enough of the main blue and red wavelengths needed for photosynthesis, orchids such as Vandas and Cattleyas will excel in the higher foot-candle hour regions and Paphiopedilums in the lower regions.
Recent research done in America found that the optimal light requirements of Phalaenopsis was between 1 000 and 1 500 foot candles with noticeable growth and flowering inhibitions commencing at light intensities of 2 000 foot-candles and higher. Growing Phalaenopsis under the "ideal" light conditions mentioned above (while maintaining temperatures between 20 and 30°C), reduced the duration of the juvenile period to between 1 and 1,5 years following removal of plants from in vitro culture. (Normal juvenile period is 3 to 4 years).
The effect of poor or low light on the growth of Phalaenopsis (Light intensities of 400 to 800 foot-candles at noon on a sunny day) was also researched. Plants grown under these conditions showed delayed spiking and flowering as well as fewer flower counts than those grown under 1 300 to 1 600 foot candles. In addition, the low light plants had significantly less weight in their leaves and roots than the control group.
Making more use of diffuse and reflected radiation in your shade house
On a sunny day the relative contribution of diffuse light (SD) is 13,7% of that of direct light (SA) and the reflectance (albedo) contribution is 15,8% of SA. On an overcast day the albedo value can be as high as 40% of the 'direct' radiation. I have found that on cloudy days and early morning or late afternoon, the contribution of reflected light to my orchids (relative to the incident radiation) more than trebles for reflected light and nearly doubles for diffused light.
Many leaves are not horizontal hence they present a much smaller effective area to direct radiation. This means that they absorb much less radiation than a horizontal leaf. The absorption of diffuse and reflected radiation remains approximately the same whether the leaf is horizontal or not. we should try and use these two sources of radiation more effectively in our hothouses because they reach the total available area of the leaves. We should consider this when buying plastic pots for our plants and considering paint on our walls and materials for the floors. For example, white pots will reflect all light and will meaningfully contribute to diffuse values where-as black pots will significantly reduce diffuse radiation, because they absorb all the light. Green ports would increase the unwanted and unhealthy wavelengths in the reflected radiation.
These facts present some interesting possibilities. If you have a light problem in your hothouse (too little light), the use of a floor covering with a higher reflectance value (albedo value), white pots and white paint would ensure that the light missing the orchids and striking the floor, pots and other surfaces could be reflected and then be utilized as reflected radiation by the plants. Bearing in mind the two colours that orchids love are blue and red and that red causes less heating than blue, then a red brick floor would contribute to much better photosynthesis and plant health. Red light is the most efficient type of light for inhibiting stem elongation and promoting leaf expansion. The far-red (infrared) light immediately following the red reserves the potential effect of red irradiation and stems become long again. Use of white paint on walls and benches will increase the amount of reflected radiation to plants, while the use of red paint would have the same effect but would result in less heating of the shade house.
In most cases in South Africa, it is not a lack of light that is the problem, but the heating caused by too much light entering the hothouse. In such a case the reduction of the direct solar radiation SA by heavy shading will have a marked effect. Decreasing the albedo will also have an effect and in this case an asphalt floor or dark coloured soil floor would be advantageous. I personally feel that multi-directional reflected radiation is a great asset in a hothouse and if we can utilise this light source through use of the right materials and the right colour of these materials, it will have a positive effect on plant growth, flowering and general health.