13.10 Factors Affecting Photosynthesis

An understanding of the factors that affect photosynthesis is necessary. The rate of photosynthesis is very important in determining the yield of plants including crop plants.

Photosynthesis is under the influence of several factors, both internal (plant) and external.

The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll.

The plant or internal factors are dependent on the genetic predisposition and the growth of the plant. The external factors would include the availability of sunlight, temperature, CO2 concentration and water.

As a plant photosynthesises, all these factors will simultaneously affect its rate. Hence, though several factors interact and simultaneously affect photosynthesis or CO2 fixation, usually one factor is the major cause or is the one that limits the rate.

Hence, at any point the rate will be determined by the factor available at sub-optimal levels. When several factors affect any [bio] chemical process, Blackman’s (1905) Law of Limiting Factors comes into effect.

This states the following: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.

For example, despite the presence of a green leaf and optimal light and CO2 conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.

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