[Explained] Factors Affecting Photosynthesis

There are many factors that affect the rate of photosynthesis, and it very important to understand how each factor affects the rate of photosynthesis. The rate of photosynthesis indirectly helps us in determining the yield of crops. 


There are internal factors like number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll which play a major role in regulating the rate of photosynthesis.


There are also some external factors that influence the rate of photosynthesis. These are namely the availability of sunlight, CO2, water and temperature. 


We can say that though many factors simultaneously affect CO2 fixation. There are usually one or two limiting factors which eventually govern the rate. Hence, at any point the rate will be determined by the factors available at sub-optimal levels.


When several factors affect any biochemical process, Law of Limiting Factors comes into effect.


This law was proposed by a British plant physiologist named Fredrick Blackman in the year 1905.

It states that: 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, if the temperature is not optimal for the plant to undergo photosynthesis, no matter how much light, water or CO2 is available, the plant will not be able to carry out the process of photosynthesis.


Now let us look at how the external factors affect the rate of photosynthesis.

Graph of how light affects the rate of photosynthesis
Graph Of How Light Affects The Rate Of Photosynthesis


Light

When we discuss light as a factor that affects photosynthesis we try to differentiate it into three parts namely light intensity, light quality and the duration of exposure to light. There is a linear relationship between incident light and CO2 fixation rates at low intensities. At higher intensities the rate does not keep increasing exponentially because the other factors such as water, CO2 concentration and temperature limit the rate gradually.



It is interesting to know that light saturation occurs at 10 per cent of the full sunlight. What this means as a factor for photosynthesis is that light is rarely a limiting factor only exceptions being the plants that stand in shade or plants in thick forests. Trees in those forests need to compete each other for available resources just like in a normal population of humans.


Increase in incident light beyond a point results in the decrease of photosynthesis due to the breakdown of chlorophyll.

Carbon Dioxide Concentration

The major limiting factor for photosynthesis is Carbon Dioxide (CO2). As the carbon dioxide concentration is very low in the atmosphere. In percentage terms it can be put somewhere between 0.03 to 0.04 per cent.


Just like any other factor, too much of carbon dioxide is also detrimental to the plant. If the concentration reaches 0.05 per cent it results in the increase of the rate of photosynthesis.


The C3 and C4 plants respond differently to CO2 concentrations. At  low light conditions neither group responds to high CO2 conditions. At high intensities, both C3 and C4 plants show increase in the rates of photosynthesis. However there are also few differences that these plants have. The C4 plants show saturation at about 360  , while the C3 plants responds to increased CO2 concentration and saturation is seen only beyond 450. Thus, it can be said that the availability of CO2 levels is a limiting factor for the C3 plants.


You might have heard of greenhouse crops. When greenhouse crops like tomatoes and bell pepper are grown in these, the availability of CO2 is very high which results in a higher yield.
And the fact that C3 plants responds to higher CO2 concentration helps in achieving higher productivity.


Temperature

The dark reactions being enzymatic are temperature controlled. Light reactions are also temperature sensitive but they are not much affected by it. The C3 plants have a lower temperature optimum while the C4 plants respond to higher temperatures and show higher rate of photosynthesis.


Habitat also plays a major role in deciding the temperature optimum of a plant. The temperature optimum for a plant is the temperature that they adapted to. Temperate plants have a lower temperature optimum than the plants that are adapted to tropical climates.

Water

Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. Water stress causes the stomata to close which in turn reduces the availability of CO2 to the plant. 


Water stress also makes leaves wilt, thus reducing the surface area of the leaves and their metabolic activities as well.

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