Role of Light in Photosynthesis

Light is a factor required in order that photosynthesis can occur. In any series of chemical reactions where one substance combines with another to form a larger compound, energy is needed to fuel the reactions. Energy for photosynthesis is provided by light from the sun or from artificial lamps. As with carbon dioxide, the amount of light energy present is important in determining the rate of photosynthesis - simply, the more light or greater illuminance (intensity) absorbed by the plant, the more photosynthesis can take place. Light energy is measured in joules per square metre, but for practical purposes the light for plant growth is measured according to the light falling on a given area, i.e. lumens per square metre (lux).

Radiant energy (irradiance) is a less useful method of measurement because it includes a significant quantity of energy from wavelengths that do not contribute to photosynthesis. However, photosynthetically active radiation (PAR) is the most useful method as it is the energy that can be used for photosynthesis (units _W/m2).Whilst the measurement of illuminance is a very useful tool for the grower, it is difficult to state the plant's precise requirements, as variation occurs with species, age, temperature, carbon dioxide levels, nutrient supply and health of the plant.

However, it is possible to suggest approximate limits within which photosynthesis will take place; a minimum intensity of about 500-1000 lux enables the plant's photosynthesis rate to keep pace with respiration, and thus maintain itself.The maximum amount of light many plants can usefully absorb is approximately 30 000 lux, while good growth in many plants will occur at 10 000-15 000 lux. Plant species adapted to shade conditions, however, e.g. Ficus benjamina, require only 1000 lux. Other shade-tolerant plants include Taxus spp., Mahonia and Hedera.

In summer, light intensity can reach 50 000-90 000 lux and is therefore not limiting, but in winter months, between November and February, the low natural light intensity of about 3000-8000 lux is the limiting factor for plants actively growing in a heated greenhouse or polythene tunnel. Care must be taken to maintain clean glass or polythene, and to avoid condensation that restricts light transmission. Intensity can be increased by using artificial lighting, which can also extend the length of day, which is short during the winter, by supplementary lighting.This method is used for plants such as lettuce, bedding plants and brassica seedlings. Total replacement lighting. Growing rooms which receive no natural sunlight at all use controlled temperatures, humidities and carbon dioxide levels, as well as light.Young plants which can be grown in a relatively small area, and which are capable of responding well to good growing conditions in terms of growth rate, are often raised in a growing room.

The type of lamp. Lamps are chosen for increasing intensity, and therefore more photosynthesis. All such lamps must have a relatively high efficiency of conversion of electricity to light, and only the gas discharge lamps are able to do this. Light is produced when an electric arc is formed across the gas filament enclosed under pressure inside an inner tube. Light, like other forms of energy, e.g. heat, X-rays and radio waves, travels in the form of waves, and the distance between one wave peak and the next is termed as the wavelength. Light wavelengths are measured in nanometres (nm); 1nm _ one thousandth of a micrometre. Visible light wavelengths vary from 800 nm (red light - in the long wavelength area) to 350 nm (blue light -in the short wavelength area), and a combination of different wavelengths (colours) appears as white light.

Each type of lamp produces a characteristic wavelength range and, just as different coloured substances absorb and reflect varying colours of light, so a plant absorbs and reflects specific wavelengths of light. Since the photosynthetic green pigment chlorophyll absorbs mainly red and blue light and reflects more of the yellow and green part of the spectrum, it is important that the lamps used produce a balanced wavelength spectrum to include as high a proportion of those colours as possible, in order that the plant makes most efficient use of the light provided. The gas included in a lamp determines its light characteristics. The two most commonly used gases for horticultural lighting are mercury vapour, producing a green blue light with no red, and sodium, producing yellow light.

This limited spectrum may be modified by the inclusion of fluorescent materials in the inner tube, which allow the tube to re-emit wavelengths more useful to the plant emitted by the gas and re-emit the energy as a shorter wavelength. Thus, modified mercury lamps produce the desirable red light missing from the basic emission. Low-pressure mercury-filled tubes produce diffuse light and, when suitably grouped in banks, provide uniform light close to plants. These are especially useful in a growing room, provided that they produce a broad spectrum of light as is seen in the ‘full spectrum fluorescent tubes'. Gas enclosed at high pressure in a second inner tube produces a small, high intensity source of light. These small lamps do not greatly obstruct natural light entering a greenhouse and, while producing valuable uniform supplementary illumination at a distance, cause no leaf scorch.

Probably the most useful lamp for supplementary lighting in a greenhouse is a high-pressure sodium lamp, which produces a high intensity of light, and is relatively efficient (27 per cent). Carbon dioxide enrichment should be matched to artificial lighting in order to produce the greatest growth rate and most efficient use of both factors.