Factors affecting photosynthesis, a critical analysis

The process of photosynthesis is affected by different factors.
These factors are broadly classified into two types, that is:-
Internal factors or plant factors: size, number, and orientation of leaves, mesophyll cells and chloroplast, internal CO2 concentration, and amount of chlorophyll. Internal factor is genetically determined.
External Factors:- availability of sunlight, temperature, CO2 concentration, and water.
Law of limiting factors
When several factors affect any [bio] chemical process, Blackman’s (1905) Law of Limiting Factors comes into effect.
The law 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 that directly affects the process if its quantity is changed.
Light has three characters
Quality
Intensity and
Duration
There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
At higher light intensities, gradually the rate does not show further increase as other factors become limiting.
Light saturation occurs at 10 percent of the full sunlight.
Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
An increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.
CO2 concentration
Carbon dioxide is the major limiting factor for photosynthesis.
The concentration of CO2 in the atmosphere is between 0.03 and 0.04 percent.
An increase in concentration up to 0.05 percent can cause an increase in CO2 fixation rates; beyond which decrease the rate.
The C3 and C4 plants respond differently to CO2 concentrations.
At low light, neither group responds to high CO2 conditions.
At high light intensities, both C3 and C4 plants show an increase in the rates of photosynthesis.
C4 plants show saturation at about 360 micro lL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 micro lL-1.
Thus, the availability of CO2 levels is limiting to the C3 plants.
The fact that C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper.
They are allowed to grow in a carbon dioxide-enriched atmosphere that leads to higher yields.
Temperature
The dark reactions are enzymatic, hence temperature affects them more than light reactions.
The C4 plants respond to higher temperatures and show a higher rate of photosynthesis while C3 plants have a much lower temperature optimum.
The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to.
Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
Water
The effect of water as a factor for photosynthesis is more through its effect on the plant, rather than directly.
Water stress causes the stomata to close hence reducing the CO2 availability.
Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.