Factors affecting Photosynthesis: Light, Temperature & Water

Photosynthesis is the process by which plants prepare their food in the presence of water, sunlight, chlorophyll, and carbon dioxide. The photosynthesis process mainly occurs in the leaves of the plants while in some plants stems also perform photosynthesis. The rate and efficiency of photosynthesis is dependent on various factors such as light, the concentration of carbon dioxide, water, temperature, oxygen, etc. 

Key Terms: Photosynthesis, Carbon dioxide, chlorophyll, chloroplast, oxygen, light, leaves, plants

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Factors affecting photosynthesis

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The main factors that affect photosynthesis include - 

Light

Light is one of the main factors affecting photosynthesis. Photosynthesis can’t occur in the dark. Plants receive their light from sunlight. The three attributes of light that are important for the process of photosynthesis are -

Duration

The photosynthesis process continues as long as the plant is exposed to light. As long as the temperature of the plant is balanced, photosynthesis will keep on occurring.

Intensity

Initially, the process of photosynthesis starts at a low light intensity and then increases till it becomes maximum at the brightest time of the day.

• The amount of light that is required varies for different plants. 
• Photosynthesis uses a maximum of up to 1.5 % of light in the process and hence light is generally not a limiting factor at high intensity. 
• However, as light is a requirement hence it becomes a limiting factor in low intensity as without light photosynthesis cannot occur. 
• At high intensities of light, plant temperature increases leading to increased transpiration in the plant. 
• The stomata close due to which the intake of carbon dioxide is also reduced.
• As the CO₂ intake is reduced, it leads to reduction and finally to the stoppage of the photosynthesis process. Hence, excess light also stops photosynthesis.

Quality

Chlorophyll absorbs blue and red wavelengths most efficiently from the entire light spectrum as per the experiments conducted by Engelmann. Therefore, when plants are exposed to the light of blue and red wavelengths, maximum photosynthesis occurs at that point in time.

Factors affecting photosynthesis

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Temperature

All biological and biochemical processes occur best in a certain optimal temperature range. 

• When photosynthesis is not limited by CO₂ or light, the rate of photosynthesis increases with an increase in temperature to the optimum level.
• Beyond the optimum temperature levels on the positive and negative side of the normal range, enzymes are deactivated or destroyed and the process of photosynthesis stops.

Oxygen

Optimum levels of oxygen are needed for photorespiration in C3 plants. The by-product of photorespiration is CO₂ which is essential for photosynthesis. 

• The energy generated during oxygen respiration is needed for photosynthesis.
• However, an increase or decrease in the oxygen levels beyond the optimum level for plants leads to inhibition of the photosynthesis process.
• Oxygen breaks down the intermediaries formed in the process of photosynthesis.
• Increased levels of oxygen will mean that RUBISCO will combine with oxygen to start photorespiration and therefore, the process of photosynthesis will slow down.

Water

Water is considered one of the most important factors that indirectly affect photosynthesis. 

• In case of reduced water availability, stomata start to close in order to avoid the loss of water during transpiration.
• When the stomata close down, the intake of carbon dioxide will also stop, thereby affecting the process of photosynthesis.

Concentration of Carbon Dioxide 

The atmosphere is made up of 0.03% of carbon dioxide along with other gases. 

• Plants intake carbon dioxide from the atmosphere.
• However, since the amount of CO₂ in the air is less, it is a limiting factor for photosynthesis. 
• When light and temperature are not limiting factors, increasing the levels of CO₂ leads to an increase in the rate of photosynthesis. 
• Beyond a certain limit, carbon dioxide starts accumulating which slows down the process. So, excessive CO₂ stops photosynthesis especially when it accumulates.

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Blackman’s Principle of Limiting Factors

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Blackman’s Principle of Limiting Factors states that when a process is governed by more than one factor, the rate of the process is governed by the factor closest to its minimum value.

For example, if a leaf is exposed to a constant temperature, a certain amount of light intensity, however, less CO₂ available is less, the rate of photosynthesis will not increase with an increase in the light intensity. Hence, in this case, carbon dioxide is the limiting factor.

Blackman’s Principle of Limiting Factors

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Internal Factors Affecting Photosynthesis

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Some of the important internal factors affecting the rate of photosynthesis are as follows - 

Protoplasmic Factors

An unknown factor in protoplasm influences the photosynthesis rate. This factor also has an impact on dark reactions. The decrease in the rate of photosynthesis at above 30°C temperatures or at high intensities of light in many plants suggests that the unknown factor is an enzyme.

Chlorophyll Content

Chlorophyll is an important internal factor in photosynthesis. The amount of CO₂ fixed by one gram of chlorophyll in an hour is the assimilation or photosynthetic number. The number is usually constant for a species of plant, but it can vary on occasion. 

Accumulation of End Products

The accumulation of food or the end products in the chloroplasts slows down the photosynthesis process.

Structure of Leaves

The structure of leaves such as the size of stomata, behavior, position, and the number of intercellular spaces, determine the amount of CO₂ reaching the chloroplasts

The quality and intensity of light reaching the chloroplasts are also influenced by various factors or characteristics such as the thickness of the cuticle, presence of epidermal hair, epidermis, amount of mesophyll tissue, etc.

CO₂ Compensation Point

The compensation point of carbon dioxide is the value of light intensity and atmospheric CO₂ concentration at which the rate of photosynthesis is equal to the rate of respiration in photosynthetic organs, which results in no net gaseous exchange. 

A plant cannot survive for long at the compensation point as a net loss of organic matter occurs due to non-green organs and dark respiration.

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Things to Remember

• Plants prepare their food through photosynthesis while being exposed to water, sunlight, chlorophyll, and CO₂. 
• Photosynthesis starts at low light intensities and increases gradually until it reaches its peak at the brightest daytime.
• Water is an important factor influencing photosynthesis. On a decrease in water intake, the stomata close and prevent any water loss during transpiration.
• The photosynthesis process continues at optimal oxygen levels. 
• The amount of carbon dioxide reaching the chloroplasts is determined by the structural features of the leaves.

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Previous Year Questions

1. The Law of Limiting Factors was proposed by ______… [KCET 2004]
2. Emerson's enhancement effect and Red drop have been instrumental in the discovery of… [NEET 2016]
3. The first carbon dioxide acceptor in C4-plants is… [NEET 1990]
4. Anoxygenic photosynthesis is characteristic of… [NEET 2014]
5. Photosynthetic Active Radiation (PAR) has the following range of wavelengths… [NEET 2005]
6. Who provided the evidence that glucose is formed during photosynthesis and is then stored in the form of starch?
7. Moll's half-leaf experiment proves that_____ is essential for photosynthesis to take place
8. Ingenhousz in an experiment showed that in bright sunlight, small bubbles were formed around the green parts of the plant, while in the dark, they did not. He identified these bubbles to be of…
9. During non-cyclic photophosphorylation, electrons are continuously lost from the reaction centre of PSII. Which source is used to replace these electrons?
10. CO compensation point for C plants is…
11. Chlorophyll a appears_____ in colour and chlorophyll b appears_____ in colour in the chromatogram.
12. The external factors that affect photosynthesis are number, size, age and orientation of leaves, mesophyll cells and chloroplasts and the amount of chlorophyll. …
13. Photorespiration is a wasteful process. In photorespiratory pathway, there is no synthesis of sugars or ATP…
14. Dark reactions are called biosynthetic phase of photosynthesis. Dark reactions do not directly depend on the presence of light but are dependent on the products of the light reaction…
15. The movement of electrons from chlorophyll molecules to NADP occurs in…