#105 Limiting factors in photosynthesis

A limiting factor is a factor that controls a process. Light intensity, temperature and, CO2 concentration and availability of H2O are all factors which can control the rate of photosynthesis.










Usually, only one of these factors will be the limiting factor in a plant at a certain time. This is the factor which is the furthest from its optimum level at a particular point in time. If we change the limiting factor the rate of photosynthesis will change but changes to the other factors will have no effect on the rate.

If the levels of the limiting factor increase so that this factor is no longer the furthest from its optimum level, the limiting factor will change to the factor which is at that point in time, the furthest from its optimum level. For example, at night the limiting factor is likely to be the light intensity as this will be the furthest from its optimum level. During the day, the limiting factor is likely to switch to the temperature or the carbon dioxide concentration as the light intensity increases.


Effects of changes in light intensity, CO2, H2O and temperature on the rate of photosynthesis

1. Light intensity

• This affects the rate of the light-dependent reaction. The energy that drives this process is light energy.
• When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. 

2. Temperature

• This affects the rate of the light-independent reaction. The energy that drives this process is heat energy. 
• At higher temperatures, molecules have more kinetic energy so collide more often and are more likely to react when they do collide. 
• Many enzymes are involved during the process of photosynthesis. At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected.

3. CO2 concentration 

• CO2 is a reactant in photosynthesis. Normal air contains only about 0.04% CO2.
• When the CO2 concentration is low, the amount of glycerate 3-phosphate produced is limited as CO2 is needed for its production and therefore the rate of photosynthesis is affected. 

4. Availability of H2O

H2O is a reactant in photosynthesis, but there is usually far more H2O available than CO2, so even if water supplies are low this is not usually a problem. However, water supply can affect the rate of photosynthesis indirectly, because a plant that is short of water will close its stomata, preventing CO2 from diffusing into the leaf.

lf the level of anyone of these factors is too low, then the rate of photosynthesis will be reduced. The factor that has the greatest effect in reducing the rate is said to be the limiting factor.

Economics of greenhouses

Farmers can use their knowledge of factors limiting the rate of photosynthesis to increase crop yields. This is particularly true in greenhouses, where the conditions are more easily controlled than in the open air outside:

• The use of artificial light allows photosynthesis to continue beyond daylight hours. Bright lights also provide a higher-than-normal light intensity.
• The use of artificial heating allows photosynthesis to continue at an increased rate.
• The use of additional CO2 released into the atmosphere inside the greenhouse also allows photosynthesis to continue at an increased rate.

Artificial light in the green house.

However, the additional cost of providing extra lighting, heat and CO2 has to be weighed against the increased crop yield and the extra income it will provide. The cost of should not exceed the additional income it generates for the farmer.

In practice, the farmer will need to find the optimum growing conditions for the crop, given the costs of providing extra lighting, heat and CO2. Paraffin lamps have traditionally been used in greenhouses. Their use increases the rate of photosynthesis because as well as the light generated from the lamps, the burning paraffin produces heat and CO2 too.

Investigating the effect of environmental factors on the rate of photosynthesis

One way to measure the rate of photosynthesis is to measure the rate at which oxygen is given off by an aquatic plant. There are various ways in which oxygen can be collected and measured. One method is shown in the diagram below.


Alternatively, you can make calcium alginate balls containing green algae and place them in hydrogencarbonate indicator solution. As the algae photosynthesise, they take in carbon dioxide which causes the pH around them to increase. The indicator changes from orange, through red to magenta.

Whichever technique is used, you should change one factor (your independent variable) while keeping all others constant (the control variables). The dependent variable will be the rate at which oxygen is given off (measured by the volume of oxygen collected per minute in the capillary tube) or
the rate at which carbon dioxide is used (measured by the rate of change of colour of the hydrogencarbonate indicator solution).

The independent variables you could investigate are:

• Light intensity. You can vary this by using a lamp to shine light onto the plant or algae. The closer the lamp. the higher the light intensity.
• Wavelength of light. You can vary this by placing coloured filters between the light source and the plant. Each filter will allow only light of certain wavelengths to pass through.
• CO2 concentration. You can vary this by adcting sodium hydrogencarbonate to the water around the aquatic plant. This contains hydrogencarbonate Ions, which are used as a source of carbon dioxide by aquatic plants.
• Temperature. The part of the apparatus containing the plant or algae can be placed in a water bath at a range of controlled temperatures.

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