Photosynthesis - How many hours of sunlight does your rose need?

by Richard Chew


How many hours of direct sunlight does the roses require? This is the most common question. I asked the same question when I started growing roses.


After doing some research on plant psychology, I learned that ensuring the minimum hours of indirect sunlight is as important as determining the minimum direct sunlight. The reason is in Malaysia we are blessed with long hours of sunlight, having direct sunlight is not so crucial.


Before we determine the number of hours of sunlight for our roses, we need to understand how and what is the optimum rate of photosynthesis.

A commonly used slightly simplified equation for photosynthesis is:

6 CO2(g) + 12 H2O(l) + photons → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)

carbon dioxide + water + light energy → glucose + oxygen + water

The equation is often presented in introductory chemistry texts in an even more simplified form as:

6 CO2(g) + 6 H2O(l) + photons → C6H12O6(aq) + 6 O2(g)

What this means is that during photosynthesis, the plant will use light energy to manufacture glucose. At the end of this process, glucose is stored for the purpose of plant growth and production of carbohydrate.

2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2

While knowing photosynthesis is at work during sunlight, there is another process that is at work all the time. This process is known to be light-independent process. That means during night or darkness, the plant will draw energy stored to use CO2 for the production of carbohydrate.

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

This process of respiration happens all day and night.

Another critical factor to take note is that when sun light is very intensed and temperatures rises, photosynthesis rate shall decline. However respiration rate will continue to rise, therefore reduces the net photosynthesis (lower photorespiration - in shaded red below).

Therefore understanding this effect is important to determine the right amount of direct sunlight and at which of part of the day is crucial, especially trying to grow roses in tropical country like Malaysia. As Malaysia weather is known to have average of 32 to 33 degress C outdoors, it is crucial not to over exposed the roses with too much heat. It doesn't harm that much if its for short period, as roses are known to be quite tolerant of direct sun.

I would prefer to place roses at a location where it can draw direct morning sun for about 2 to 4 hours as temperature is at lowest of the day, and indirect sunlight for 5 to 7 hours. The rose plant must get indirect sunlight from opened sky and not from opened window (or any location that is shielded, it can never grow indoor). In such condition, you provide a controlled temperature environment within 28 to 32 degress C.

A practical way to artificially create this effect is by growing roses next to a wall or a taller tree/plant that will shield your roses from the intensed afternoon sun.

How much light does my roses get?

My garden usually gets direct sun between 10am to 12pm, my estimation is about 100,000 lux (light intensity) at clear sky. It is believed that at this light intensity, photosynthesis will reach light saturation (photosynthesis will cease if reach light saturation of 32,000 lux for upper leave and 100,000 lux for whole plant)

Thankfully Malaysia's weather is usually cloudy, therefore will diffuse the direct sunlight. However it is necessary to take pre-caution if weather is dry and hot for long periods as the plant will be exposed to high temperature thus raise the rate of respiration. That is why, I feel it is necessary to place roses at location that is shielded from hot afternoon sun, but not totally shielded from indirect sun.

Just to ensure that your roses receives sufficient sunlight, especially if your garden has limited space and suspicion of insufficient light, just do a simple calculation.

8am to 5pm - 9 hours x 30,000 lux (indirect sunlight) = 270,000 lux hours

10am to 12pm - 2 hours x 100,000 lux (direct sunlight) x 30% (below upper leave) = 60,000 lux hours

Total is 330,000 lux hours.

In case you are unsure how many lux (light intensity) in your garden, in general you can use 100,000 lux for direct sunlight, 30,000 for indirect sunlight and 5,000 for low light (indoor light. You will need to multiply the ratio of 0.3 (30%) for direct sunlight, as only the leaves below the upper leave will benefit from the sunlight. It is believed that at high intensity the upper leaves will cease photosynthesis due to higher light saturation.

For the plant to survive it requires a minimum of 160,000 lux hours of sun light.

If your light calculation exceeds the minimum of 160,000, your rose should thrive. If you have read some books on rose growing, you may notice that most say that roses require direct sunlight, that is because the weather is different from our tropical weather in Malaysia. Averagely Malaysia outdoor weather is at 32 degrees C, if at direct sunlight, the temperature may increase for another 2 to 3 degrees C, which lower the net photorespiration rate.

Also if the heat reaches the soil, the good bacterias in the soil will also cease their activities thus will affect the plant in the long run. It won't do much harm if exposed for short periods, as it reduces the risk of fungal infections. But if for long periods, the plant roots will deplete in its nutrition as the bacterias stop breaking down the nitrogen components.

You may want to check the earlier posting that is extracted from external site. I posted in this blog, in case you wish to know about plant leaves anatomy, photosynthesis and light.

http://rosegrowing.blogspot.com/2008/10/chemistry-in-shadehouses-ext.html

http://rosegrowing.blogspot.com/2008/10/photosynthesis-leaves-light-ext.html

This posting explains the impact of diffuse light on photosynthesis rate. It dispel the myths of having direct sunlight leads to increased in photosynthesis rate.

http://rosegrowing.blogspot.com/2008/10/large-volcanic-eruptions-help-plants.html

Photosynthesis, leaves & light - (EXT)

Extracted from http://www.helpsavetheclimate.com/photosynthesis.html#a5


This source is good information on the plant leave anatomy and photosynthesis.


Help Save the Climate


This page introduces the process of photosynthesis, one of the most important biological processes on Earth. The plant structures involved are described and also some of the molecules of particular relevance to our project. We also briefly describe some features of light.
Photosynthesis is the process by which plants, algae (and certain types of bacteria) use sunlight to convert inorganic components (mostly carbon dioxide and water) into sugar molecules. They then use these molecules to build further organic molecules that make the plants' structures, as well as providing chemical energy.

Photosynthesis is at the start of 99% of the food chains on Earth and plant materials are used in many other ways, such as fuel and as building materials. Many animals eat plants to obtain energy and matter, and in turn these animals are often eaten by others. Ultimately the Sun provides all of the metabolic energy on planet Earth (except for some very small, specialised food chains in the deep oceans around hydrothermal vents).

The basic photosynthesis equation


The key chemical pathway in photosynthesis is the conversion of carbon dioxide (CO2 from the air) and water (H2O) into carbohydrate molecules ([CH2O]n such as sugar) and free oxygen (O2), using light as an energy source for the reactions, as shown here:
the basic form:

CO2 + H2O + light => sugars + O2

the more chemically accurate form:

6CO2 + 6H2O + light => C6H12O6 + 6O2

These carbohydrate molecules, such as sugars, contain more energy than the starting molecules, in other words they act as chemical batteries for solar energy. The carbohydrate molecules are often then used to construct more complex molecules, such as cellulose and lignin, that make the plant structure.

To release stored energy, both plants and animals use oxygen from the atmosphere to convert carbohydrates back into CO2 and H2O. This process is called respiration. However the rate of photosynthesis is usually 30 times the rate of respiration, so plants produce large net quantities of oxygen that is then used by animals that in turn produce CO2 which feeds the plants, in a cycle.


Leaves, cells and chloroplasts

Leaves are the structures in plants that have evolved primarily to absorb sunlight and to convert this energy into carbohydrates. Figure 1 shows a cross-section through a typical leaf.







Figure 1. Cross-section through a typical leaf illustrating types of cells and also the intracellular organelles called chloroplasts, where photosynthesis takes place. The cuticle is a waxy coating, layers of cells (epidermis, palisade and spongy mesophyll) and vascular bundles (xylem and phloem, which are specialised cells) that transport water and nutrient solution. The stoma (plural: stomata) is a pore that allows the entry of air and therefore CO2 into the leaf (image source: Oregon State, botany)



Figure 2 shows a typical plant cell. The nucleus contains the DNA of the cell that encodes the information that the cell needs to function. Respiration takes place within mitochondria (singular: mitochondrion). The vacuole is a fluid-filled bag that often takes up more than 80% of plant cell volume. The major role of vacuoles in most plants is to maintain turgor pressure i.e. to keep the cell rigid, thus helping to support the plant, along with the cell wall (notice how many plants wilt when they lack water).




Figure 2. A typical plant cell, illustrating some key organelles (image source: here)

Photosynthesis takes place within chloroplasts (figures 2 and 3) contained within the green cells of plants and algae (photosynthesizing bacteria have other, simpler structures). Chloroplasts are flattened disc shaped organelles usually 2-10 μm in diameter and 1 μm thick (click here to open a note on units in a new window).

The chloroplast has a two membrane envelope termed the inner and outer membranes, with a space inbetween. The fluid within the chloroplast is called the stroma. Within the stroma are stacks of thylakoids, the sub-organelles where photosynthesis actually takes place. A stack of thylakoids is called a granum (plural: grana). A thylakoid looks like a flattened disk, and inside is the thylakoid space or lumen. The photosynthetic reaction takes place on the thylakoid membrane.


Figure 3. A chloroplast. Thylakoids are a phospholipid bilayer membrane-bound compartment. A granum is a stack of thylakoids folded on top of one another. The stroma is the fluid space within the chloroplast. The lumen is the fluid filled space within a thylakoid.




Light

Before introducing the photosynthetic molecules, we will briefly discuss light. What we call "light" is actually part of the spectrum of electromagnetic radiation. This covers a range of rays or waves from cosmic rays, with very short wavelengths all the way to radio waves (figure 4).



Figure 4. The electromagnetic spectrum, with an expanded visible light region, to show more detail. Key: IR, infrared; UV, ultraviolet. (image source: here)

The wavelength is the distance between the peaks of 2 adjacent waves (figure 5). Rather confusingly, light has some properties of waves and some of particles. A light particle is called a photon. In some situations it is more convenient to think of light as photons, and in others to treat it like waves.






Figure 5. A representation of a light wave showing the electric and magnetic field components (don't worry, there won't be a test on this!)
There is a direct relationship between wavelength, frequency and the speed of light: v = f λ

v = the speed of light, 3 x 108 m/s (meters per second)
f = frequency
λ = wavelength (lambda, units of meters or nanometers)


The speed of light doesn't vary, so the longer the wavelength the smaller the frequency.


There is also a relationship between frequency and the energy of each wavelength of light: E = hf

E = energy of 1 photon or light particle
h = Planck's constant
f = the frequency of the light



The higher frequency (shorter wavelength) light has more energy. When calculated, the energy levels are as shown in table 1. You can see that red light has considerably less energy than blue light.



Table 1. Energy levels of visible light expressed as Joules per mole of photons (a mole is 6.023 x 1023 particles or in this case, photons). λ (lambda) is wavelength of light. Violet light has about twice the energy of red light (Adapted from Hall and Rao, 1999)

An example that shows the difference in energies of the colours of light is seen on older cars (automobiles). Red cars absorb the higher energy blue photons, but reflect the lower energy red photons, that's why the paint looks red. This absorption of more energetic photons leads to faster oxidation of the paintwork.

Photons with high enough energy can be absorbed by particular molecules in the plant, producing energised electrons. It is important to note that 1 photon can only supply its energy to 1 electron and also that photons with too low energy will not be used in photosynthesis. The red photons have less energy than the blue photons, this explains why infrared light is relatively poorly used in photosynthesis.



The absorption of light


The absorption of light in chloroplasts is by a range of pigments, mainly chlorophylls (which provide the green colour) and carotenoids. These different types of pigment absorb light of different wavelengths. A large number of these pigment molecules are arranged into an antenna to capture most of the photons of the correct energy. This energy is then passed through a chain of pigment molecules until it reaches a specialised molecular complex called the reaction centre. The pigment antenna complex/reaction centre together are called a photosystem (figure 6).








Figure 6. A photosystem, showing the absorption of a photon by a pigment molecule and the transfer of this energy to the reaction centre. The path taken (shown in yellow) is only an example. The reaction centre then ejects an electron, that is replaced by another from an electron donor molecule (not shown).

There are 2 types of photosystem in plants, linked together in a 2 stage process. Light is absorbed at each of the photosystems to provide enough energy to manufacture carbohydrates (figure 7). There are millions of these photosystems embedded in the thylakoid membranes within the chloroplasts.








Figure 7. A greatly simplified scheme showing the 2 photosystems and electron transport between them. Electrons flow from water, through the 2 photosystems and other molecular complexes and eventually enable carbohydrate formation, from CO2. The energy driving the process comes from the photons absorbed by the photosystems. Key: Chl, chlorophyll; Cyt b6f, cytochrome b6f electron transport complex; 4e-, 4 electrons; Pc, plastocyanin (e- transport molecule); Pq, plastoquinone (e- transport molecule); T.M, thylakoid membrane.


For more detail I recommend Photosynthesis (Hall and Rao, 1999). This is an excellent introduction to the subject at an undergraduate level. For a general grounding in cell biology I recommend the book Molecular Biology of the Cell, (Alberts et al. 2001) which also has an introduction to photosynthesis.



The scale of photosynthesis


The scale of the processes of photosynthesis and respiration is quite staggering. For instance around 10,000 tonnes of oxygen is consumed every second by respiration and the burning of fuels. If plants didn't balance this loss by their production, oxygen supplies in the atmosphere would only last about 3000 years. The amount of plant biomass produced every year has been estimated at 200 billion tonnes.


How much solar energy is absorbed by photosynthesis, or what percentage of sunlight falling on the Earth's surface is converted into plant matter?


The sunlight hitting the atmosphere every year is about 5.6 trillion trillion Joules, or 5.6 x 1024 J (a note on units and exponents is here, opens in a new window). This is equivalent to a continual flow of 1.8 x 1017 Watts. About half of this reaches the Earth's surface, the rest being reflected by the atmosphere and clouds. Of this 50%, some is reflected from the oceans and other surfaces leaving 30%. But only about half of this light is of the correct wavelength to enable photosynthesis. This leaves 15% of the original sunlight available for absorption by plants (8.4 x 1023 J).

Assuming the primary biomass production (plants) of Earth is 2 x 1011 tonnes per year, this equates to about 3 x 1021 J of energy stored as biomass.

Therefore the percentage of usable light stored by photosynthesis globally is: 0.4%


So why the difference in figures?


The measured efficiencies of energy conversion (i.e. sunlight into biomass) for various plants range from as low as 0.1%, to around 1.5% (most crop plants) to sugarcane at 8% (the exception). The reasons for this are complex, and range from the behaviour of light energy as it penetrates leaves, to limiting factors in the plant's environment e.g. temperature, micronutrients etc. There is also much room for error in calculating such enormous processes.


If plants could be made more efficient at converting light into plant matter than the productivity of the world's agriculture could be improved vastly. Numerous research projects are focusing on achieving this with a variety of plant species.

Chemistry in Shadehouses - (EXT)

Extracted from http://www.ont.co.za/new_page_1.htm

This is good material to get an understanding of light & photosynthesis.

Author: Prof. Dénis van Rensburg.

Introduction

It is the chemistry that happens in our shade houses that ultimately has a huge effect on the success of our orchid cultivation. Chemistry has to do with reactions and reagents (substances). But all chemical reactions are in some way or other affected by temperature and/or light. If these two variables can be controlled in our shade houses it can and will have a marked effect on substances and reactions that determine the quality of the plants.

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).

Green plants have the unique ability to utilize the gaseous carbon dioxide (CO2) of the atmosphere, plus water (H2O), in the presence of an energy source (sunlight) and chlorophyll (in the leaves of plants) to produce glucose (C6H12O6, a kind of sugar that is a basic food for the plant) and give of oxygen (O2) into the atmosphere as a byproduct. The process of photosynthesis is much more complex than the equation would suggest, and a textbook published in 1992, telling the whole story, required 21 pages to do so.

The important fact is that plants do not eat; we cannot "feed" them. They manufacture their own food, beginning with the glucose produced during photosynthesis, and use the mineral salts in fertilizers (or the naturally occurring mineral salts in the environment) to complete the manufacture of food, to fabricate plant parts and to perform metabolic processes. A critical distinction between food and mineral nutrients, required by both plants and animals, is that food provides organisms with energy, minerals do not.

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.

Photoperiodism

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.

LARGE VOLCANIC ERUPTIONS HELP PLANTS ABSORB MORE CARBON DIOXIDE FROM THE ATMOSPHERE [EXT]

I extracted this from NASA Goddard Space Flight Center site. I find this discovery interesting as it gives new insight about diffuse light on the efficiencies of photosynthesis rate.

http://www.gsfc.nasa.gov/gsfc/earth/pictures/co2/volcanom.jpg



LARGE VOLCANIC ERUPTIONS HELP PLANTS ABSORB MORE CARBON DIOXIDE FROM THE ATMOSPHERE

New NASA-funded research shows that when the atmosphere gets hazy, like it did after the eruption of Mt. Pinatubo in the Philippines in June 1991, plants photosynthesize more efficiently, thereby absorbing more carbon dioxide from the atmosphere.

When Mount Pinatubo erupted, scientists noticed the rate at which carbon dioxide (CO2) filled the atmosphere slowed down for the next two years. Also during 1992 and 1993, ash and other particles from the volcano created a haze around the planet and slightly reduced the sunlight reaching Earth's surface and made the sun's radiation less direct and more diffuse.

Many scientists previously thought the reduction in sunlight lowered the Earth's temperature and slowed plant and soil respiration, a process where plants and soil emit CO2. But this new research shows that when faced with diffuse sunlight, plants actually become more efficient, drawing more carbon dioxide out of the air.

"There is evidence indicating that the drop in the atmospheric CO2 growth rate was probably too big to be explained by a reduction in respiration alone," said the study's lead author, Lianhong Gu, a researcher at the University of California Berkeley's Department of Environmental Science, Policy and Management.

Gu added that the respiration rates of plants and soil are sensitive to temperature changes. But "in order to explain the drop in atmospheric growth rate of CO2, we would need an average drop in global temperatures of about 3.6 degrees Fahrenheit (2° C), but the temperatures only dropped by about one degree (0.9) Fahrenheit (0.5°C) globally."

Plants take in carbon dioxide during photosynthesis in the day, and release it during respiration at night. But they don't necessarily photosynthesize and respire at the same rates. Since decreased plant and soil respiration could not explain the drop in carbon dioxide entering the atmosphere in 1992 and 1993, Gu and his colleagues deduced that enhanced photosynthesis by plants must be involved.

After Mount Pinatubo erupted, while overall solar radiation was reduced by less than five percent, data showed a reduction of direct radiation by as much as 30 percent. So, instead of direct light, the sun's rays were reaching leaves after colliding with particles in the air.

"Diffuse radiation has advantages for plants," Gu said. That's because when plants receive too much direct light, they become saturated by radiation and their ability to photosynthesize levels off. In the layers of leaves from top to bottom, called the plant canopy, only a small percentage of the leaves at the top actually get hit by direct light. In the presence of diffuse light, plants photosynthesize more efficiently and can draw more than twice as much carbon from the air than when radiated by direct light.

Gu and his colleagues tested the CO2 uptake in various plant ecosystems around the world-including Aspen forests, mixed deciduous forests, Scots pine forests, tallgrass prairies, and a winter wheat field-based on the amount of solar radiation striking the leaves. From these analyses, they generated parameters necessary for evaluating impacts of the Pinatubo eruption. On clear days following the eruption, they found that in all of the ecosystems, photosynthesis increased under the diffuse light.

While large volcanic eruptions are rare, this research has big implications for more regular phenomena such as the effects of aerosols and clouds on an ecosystem's ability to pull carbon from the atmosphere. Aerosols, or microscopic particles like soot or black carbon in the air, occur naturally but have also been increasing due to human activities since the industrial revolution. Gu's research indicates that the maximum uptake of carbon dioxide by plant ecosystems occurs when cloud cover is about 50 percent.

The research will be presented at a poster session of the American Geophysical Union (AGU) Fall Meeting in San Francisco, Calif. on December 14, 2001. A paper will be published soon in the Journal of Geophysical Research.

Aside from NASA, the study was also funded by the National Oceanic and Atmospheric Administration (NOAA), the Department of Energy, and other organizations, through the FLUXNET program.

Editor's Note: AGU Title, Time and Location"Roles of Volcanic Eruptions, Aerosols and Clouds in Global Carbon Cycle"Friday, December 14, 2001, 8:30 AM, Moscone Center Hall