Requirements for photosynthesis

Carbon dioxide

In order that a plant may build up organic compounds such as sugars, it must have a supply of carbon which is readily available. Carbon dioxide is present in the air in concentrations of 330 ppm (parts per million) or 0.03 per cent, and can diffuse into the leaf through the stomata. Carbon dioxide gas moves ten thousand times faster in air than it would in solution through the roots. The amount of carbon dioxide in the air immediately surrounding the plant can fall when planting is very dense, or when plants have been photosynthesizing rapidly, especially in an unventilated greenhouse.

This reduction will slow down the rate of photosynthesis, but a grower may supply additional carbon dioxide inside a greenhouse or polythene tunnel to enrich the atmosphere up to about three times the normal concentration, or an optimum of 1000 ppm (0.1 per cent) in lettuce. Such practices will produce a corresponding increase in growth, provided other factors are available to the plant. If any one of these is in short supply, then the process will be slowed down. This principle, called the law of limiting factors, states that the factor in least supply will limit the rate of the process, and applies to other non- photosynthetic processes in the plant. It would be wasteful, therefore, to increase the carbon dioxide concentration artificially, e.g. by burning propane gas, or releasing pure carbon dioxide gas, if other factors were not proportionally increased.


Light is a factor required for photosynthesis to 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/square metre, but for practical purposes the light for plant growth is measured according to the light falling on a given area, that is lumens per square metre
  • lux). More recently, the unit 'microwatts per sq. metre' has been introduced. One lux, in natural sunlight, is equal to microwatts per sq. metre. Whilst the measurement of illuminance is a very useful tool for the grower, it is difficult to state the plant's precise

Figure 8.5 CO2 burner enriches the requirements, as variation occurs with species, age, temperature, glasshouse ^ronr^nt s° Mpmg pi™de an carbon dioxide levels, nutrient supply and health of the plant. optimum growing environment

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

What Limits Plant Respirati0n

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.

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  • sasha
    Why clear polythene is required fir photosynthesis?
    7 years ago
  • alice
    What are the requirements of photosynthesis?
    6 years ago
  • haylom
    What gardening method gives optimum photosynthesis?
    6 years ago
  • niklas
    What is the lumens requirement for photosynthesis process?
    5 years ago
  • Bernardo
    What are the requirements for photosynthesis to occur?
    5 years ago
  • Stephanie
    What is the minimum amount of carbon dioxide needed for photosynthesis to take place?
    4 years ago
  • christopher
    How many parts per million of carbon dioxide are required for photosynthesis to occur?
    4 years ago
  • Becky Watson
    What level of co2 is required for photosynthisis?
    2 years ago
  • eloy
    What is the minimum amount of co2 needed in the soil for it to be sustainable?
    2 years ago

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