Light is essential to the growth of plants, both for the metabolic process - photosynthesis, and to interact with their surroundings - phototropism and photoperiodism. It was in the eighteenth century that it was concluded that plants used light.
Up to this point it could be dangerous to express such ideas as the all-powerful Church authorities thought it was at the will of God that life existed. The Flemish chemist and physician, Jan Van Helmont (1580-1644) was locked up and subjected to the Spanish Inquisition for experimenting on plants. He disproved the belief that plants 'ate' soil to grow by placing a small willow tree in a container of soil which he weighed over a five year period. The soil weighed the same at the end, but the tree had grown. He mistakenly concluded that it had gained the weight from water. His work was published after his death due to his earlier experiences with the Church.
Jan Ingenhousz (1730-1799), a Dutch-born physician who took a break from his work as a smallpox inoculator in 1779 to do some research on plants, found that they released a gas which he showed to be oxygen, only when in the presence of light. It was the German botanist and plant physiologist Julius von Sachs (1832-1897) who worked out that light, water and gasses were processed in the chloroplasts within plant cells to produce starches. The final link in the chain of discovery is the Calvin Cycle which describes the process in detail. This discovery is cloaked in controversy as although it bears the name of Melvin Calvin (1911-1997) who was awarded a Nobel prize for it, most of the work was done by his co-worker Andrew Benson using radioactively labelled carbon - he was sidelined and did not get a mention in Calvin's autobiography.
Photosynthesis
Within most active plant cells the organelles called chloroplasts which contain green chlorophyll molecules, are the 'engines' which capture light energy, a process called photosynthesis. In the early years of life on earth billions of years ago, these organelles were separate entities like Cyanobacteria which did not have a nucleus bound by a membrane known as prokaryotic cells (from the Greek meaning before nuclei). The chloroplasts still retain their own DNA so that they divide and increase their numbers independently of the cell in which they dwell. Another organelle in plant cells is the mitochondrion which releases energy to the cell so that it can carry out its functions (respiration). The Cyanobacteria are still around to-day and are similar to the early organisms that started life 3.5 billion years ago.
The incorporation of these individual entities to make up what we now call the eukaryotic cell, meant that each cell could function on its own. (Eukaryotic - having a nucleus). The mitochondria were probably incorporated in cells before the chloroplasts as they are found in animal cells as well, then the two Kingdoms went their separate ways.
When all cells carry out the processes of life they produce byproducts and in the case of chloroplasts the glucose produced could be used by others, so the cell which 'captured' them was at a great advantage and this was the beginning of plant life - some animals such as coral contain similar organelles which can utilise light energy.
The chloroplasts are found in greater concentrations in the cells on the upper surfaces of leaves. In plants with purple foliage they are still green but are masked by the pigment, if the leaves are in shade the other colour fades and the green chloroplasts become more visible. Some foliage starts off with purplish hues due to the presence of anthocyanins which act as a screen to prevent the chloroplasts from overheating and the leaves turn green as they mature and the sun is less intense - herbaceous Paeonies are an example. In total blackout the chloroplasts become yellow etioplasts due to the conversion of chlorophyll to protochlorophyll which is yellow, but revert to green chloroplasts if light returns - this process is mediated with plant hormones called cytokinins.
The green chlorophyll molecule has a Magnesium ion (Mg+) at its centre. Plants can become pale or chlorotic, and unable to thrive if there is a lack of these ions, a foliar feed of Magnesium Sulphate (Epsom Salts) should remedy this. Another cause of chlorosis can be alkaline soil conditions which prevent the uptake of Iron (Fe++) essential for making chlorophyll. Interestingly the Haemoglobin molecule which carries oxygen in our blood has a similar structure to chlorophyll with an Iron ion (Fe++) at its centre, but despite some health-food producer claims there is no way consuming juiced plant tissue such as the so-called superfood wheat grass, can increase the oxygen carrying capacity of the blood - and certainly not if given as an enema!.
The association of Iron with chlorophyll production has led scientists to suggest that 'seeding' the oceans with iron filings would cause enough of an increase in phytoplankton near the surface which would use up atmospheric carbon dioxide and help with climate change.
The energy captured by plants from the sun is used throughout the living world and is released as the carbon cycle proceeds. This solar energy is the primary source for all life except a few bacteria which live near the hydrothermal vents of underwater volcanoes and use chemical energy from minerals in a process called chemosynthesis. Other cave-dwelling bacteria use hydrogen sulphide molecules instead of oxygen to obtain their energy.
Herbiverous animals consume plants to extract that same stored energy and in turn carnivores at the top of the food chain are consuming solar energy indirectly when they eat their prey. Fungi, which do not photosynthesize, derive energy by breaking down material from other organisms, but ultimately this energy will have been solar in origin. The mycorrhizal fungi live symbiotically with plant roots taking carbohydrates, but improve the ability of the plant to take up water and nutrients. Other fungi live in a symbiotic relationship with algae in the form of a Lichen - the algae can photosynthesize to provide the energy for the fungi which provide a support structure.
Even after millions of years the same energy trapped in fossil fuels is still available to power our modern world - as the fuel burns the oxygen recombines with the carbon and hydrogen releasing the trapped energy, re-releasing the water and the carbon dioxide which is building up in the atmosphere to cause global warming. To appreciate the energy which has been captured from the sun you need only feel the heat of a fire and observe the hot gases in the flames.
The terms 'carbon neutral' and 'adding to the carbon load' are used to describe how our use of this energy is effecting our atmosphere by releasing carbon dioxide, which in turn influences the climate. The carbon taken up millions of years ago is locked away in the coal, oil and gas, but when they are used it is released. Carbon neutral fuels such as bioethanol and wood pellets are produced from plant material grown recently so they used up CO2 from the present atmosphere which is released again on burning, so the effect is neutral.
Scientists are currently working on a process to mimic photosynthesis in vitro which they hope will produce carbon neutral fuels. The trick is to carry out the process using less energy than can be obtained from the resulting compounds.
Photoperiodism
Light also determines when many plants start growing, flowering and producing seed. Special chemicals called phytochromes are produced by plants in response to the intensity or the absence of light and these are a controlling factor of their growth. It used to be thought that day length controlled the production of these phytochromes, but investigations have shown that it is periods of complete or broken darkness which are important.
Flowering is controlled by 'day length' in many plants and they have been categorised as Long-day, Short-day and Day-neutral depending on their response.
- Short-day (or more correctly long-night) plants flower in spring or autumn when the period of uninterrupted darkness is greater than 12 hours, eg. chrysanthemums.
- Long-day (short-night) plants flower when the period of uninterrupted darkness is less than 12 hours this category contains the summer flowering plants from temperate regions where the days vary greatly from season to season.
- Day-neutral plants flower independently of day length, eg. tomatoes.
This phenomenon is known as photoperiodism and some gardeners have had problems where the garden is adjacent to street lighting, as the plants become confused when light is around for 24 hours. Permanent weak light can reverse the phytochrome influence in Long- and Short-day plants.
It is possible to control the growth pattern by introducing artificial light or blocking out natural light. Seedlings grown indoors can become etiolated and flop over, using artificial light will prevent this happening. A light box with the correct intensity and wavelength of light will produce more robust seedlings and later, flowers and fruit at the required time. Many of the plants grown for garden shows are controlled in special light conditions so that they perform on the day and can flower completely out of season. The Poinsettia (Euphorbia pulcherrima) requires heat and about 14 hours of darkness to produce its bright red bracts so has to be grown in special light conditions to be ready for Christmas - the glasshouses where they are raised have black-out blinds which are drawn over to adjust the light.
There are special grow-light bulbs which give off the correct wavelength, but an equal number of cool white and warm white fluorescent tubes should give off a balanced spectrum sufficient for the job. They should be arranged 10 to 20 cm above the seedlings or cuttings in a box lined with reflective material. A larger scale arrangement is required for growing-on the plants. Most actively growing plants perform at their best with 18 hours of light per day, but those from tropical regions respond well to 12 hours as they would receive in the wild. It is possible to grow healthy plants without any natural light at all (as growers of illicit substances have discovered!).
Phototropism
Light also allows plants to interact with their surroundings. They are able to respond to shading by growing towards light sources. This is controlled by hormones produced at the shoot tip called auxins (indole-3-acetic acid or IAA) which influence cell division, so if the light is to one side the cells on the dark side receive more auxin and divide more rapidly forcing the plant to bend towards the light. This is phototropism. If the plants are grouped together and the light is from above they will all grow upwards at a faster rate, the winners will be those which are best adapted to local conditions. These are usually the weeds, but the phenomenon is used to advantage in forestry, producing straight-stemmed trees. The IAA also suppresses the growth of side buds so the effort is concentrated at the main tip of the plant - this is apical bud dominance a phenomenon we try to influence when pruning.
The parts of a plant which are above the ground are said to show a positive phototropic reaction and the roots show a negative phototropic reaction as they grow away from light. Small seed do not have the energy to support a long initial shoot or plumule, so need to be near or on the surface. They are responsive to light as well as temperature and moisture, so germination occurs only when all three are correct.
An additional effect of light which is a concern to gardeners is the ultra violet part of this radiation. It is not only damaging to our skin, but the many plastic tools and other equipment used in the garden can suffer as well. Some plastic watering cans can be so affected that one day the handle comes away leaving the rest of the can on the ground. Also the little O-ring washers which make the clip-on hose fittings watertight become brittle and crack if they are exposed to it for too long, therefore it is advisable to keep them indoors or if not possible then keep them attached to something even a blank fitting on the end of a tap when not in use - this is the usual cause of leaks from the joints.
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