LAW OF LIMITING FACTORS

The Law of Limitation (Blackman 1905) – the rate of photosynthesis limited by the slowest factor.

armers can use their knowledge of these limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide and heat too.

If the temperature is at too low or too high a temperature – photosynthesis stops. All plants have an optimum temperature regime, within this regime the rate will increase with temperature until it reaches its upper limit. As the temperature rises the rate of molecular activity rises (so photosynthesis) until an optimum for the plant is reached. If the temperature continues to rise the enzymes controlling the process are denatured until it stops.

Light: Quality and duration of light affect the rate of photosynthesis as it is the energy from sunlight needed for the process. Normal summer bright sunlight is the best quality and longest duration = good photosynthesis; and the long duration of the production of sugar = good plant growth. Winter’s poor quality light for a shorter time = poor plant growth.

Carbon Dioxide: Carbon dioxide is a raw material for the process. If limited in concentration the rate of photosynthesis will drop.

Water: Water is a raw material for photosynthesis and needs to be in an optimum supply. Too little and the plant will not be getting a supply of raw material, too much/over watering and the roots can be damaged so decreasing the rate of water uptake. If the plant wilts the stomata close and this cuts off the entry of CO2.

Mineral Nutrients: As chlorophyll is made up of several minerals and elements, deficiency of any will cause a lack of good chlorophyll. Both iron and magnesium are needed in larger amounts than some of the others. If either of these is low then chlorosis develops (yellowing of the leaves): iron = yellow young leaves; magnesium = inter-veinal yellowing of old leaves.

How the grower can optimise the conditions for photosynthesis.

All need to be maintained in a general equilibrium to each other. CO2 enrichment only works in producing better/bigger plants if the light duration and quality is good, there is a good supply of water to the plant, a suitable (optimum) temperature for the specific plant is maintained and there is an adequate supply of minerals.

Temperature: Use of thermostatically controlled heating systems to provide the plant with its optimum growing temperature; Use of thermostatically controlled ventilation/shading (reduces light??) to stop the temperature going too high.

Light: In winter the quality of light can be improved by the use of artificial lighting over plants during the natural period of light. The duration of light hours is increased by having the lights on during the period of natural dark.

The types of bulbs used are sodium, mercury and fluorescent (Growlux tubes).

Water needs to be maintained at a suitable level for the plant. Some plants are suited to hydroponic systems where a constant water supply is maintained.

Others are suited to a wet/dry growing provided by an “ebb and flow” system.

And others normal manual watering methods.

NEVER LET PLANTS WILT!

Minerals: Keep plants fed with fertilisers as required.

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