3.4 Factors affecting photosynthesis

Limiting Factors

The law of limiting factors:  
At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value. 

The rate of photosynthesis is usualy measured in one of two ways:

1. the volume of oxygen released by a plant
2. the volume of carbon dioxide taken up by a plant.

When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity.  

As light intensity increases, the volume of oxygen produced and carbon dioxide absorbed due to photosynthesis will increase to a point at which it is exactly balanced by the oxygen absorbed and the carbon dioxide produced by cellular respiration.  At this point, there will be no net exhange of gases into or out of the plant. This is known as the compensation point.  

Further increases in light intensity will causes a proportioinal increase in the rate of photosynthesis and increasing volumes of oxygen will be given off and carbon dioxide taken up. A point will be reached at which further increasses in light intensity will have no effect on photosynthesis. At this point some other factor, such as carbon dioxide concentration or temperature, is limiting the reaction.  

The optimum concentration of carbon dioxide for a consistency of hight rate of photosynthesis is 0.1% and growers of some greenhouse crops, such as tomatoes, enrich the air in the greenhouses with more carbon dioxide to provide higher yields. 

Carbon dioxide concentration affects enzymes activite, in particular enzymes that catalyse the combination of ribulose biphosphate with carbon dioxide in the light-dependent reaction. 

Provided that other factors are not limiting, the rate of photosynthesis increase in direct proportion to the temperature. Between the temperatures of 0oC and 25oC the rate of photosynthesis is approximately doubled for each 10oC rise in temperature. in many plants, the optimum temperature is 25oC above which the rate levels off and then declines - largely as a result of enxyme denaturation.