Light is essential to the growth of plants, both for metabolic processes and to interact with their surroundings.
Photosynthesis
Within most active plant cells are organelles called chloroplasts which contain green chlorophyll molecules and these are the 'engines' which capture light energy, a process called photosynthesis. In the early years of life on earth these organelles were separate entities like bacteria and still retain their own DNA so that they divide and increase their numbers independently of the cell in which they dwell. Another organelle is the mitochondrion which releases energy.
During photosynthesis light is used to break water molecules apart to release more energy from the chemical bonds in the water. The hydrogen from the water combines with carbon from carbon dioxide (CO2) in a 'dark' reaction in the body of the chloroplast to produce sugars and these are used by the cell during respiration, stored for later use in the form of starch, or used to build the plant structure.
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 mitochondria were probably incorporated in cells before the chloroplasts as they are found in animal cells as well.
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 use chemical energy from minerals. Higher herbiverous animals consume plants to extract the stored energy and in turn carnivores at the top of the food chain are consuming solar energy indirectly.
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, reproducing the carbon dioxide and water. This is were the terms 'carbon neutral' and 'adding to the carbon load' come from. The carbon dioxide 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.
Fungi, which do not photosynthesise, derive energy by breaking down material from other organisms, but ultimately this energy will have been solar in origin. Some fungi live in a sybiotic relationship with algae in the form of a Lichen - the algae can photosynthesise to provide the energy for the fungi which provide a support structure.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. 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.
The green chlorophyll molecule has a Magnesium ion (Mg+) at its centre. Plants can become pale 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 a Fe++ ion at its centre.)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 and 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 independantly 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.
There are special grow-light bulbs, 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.
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