Factors Affecting Photosynthesis: Photosynthesis is the process through which a plant converts light energy into chemical energy in order to create food. Plants create food or energy in the form of sugar using water, carbon dioxide, and sunshine in the presence of chlorophyll and emit oxygen as a byproduct. Understanding the photosynthesis process necessitates a thorough understanding of the many cellular and chemical activities within the plant body. 

Photosynthesis is derived from a Greek term that means both light and synthesis. This indicates a synthesis or a chemical reaction that uses light energy as a catalyst. Synthesis occurs in the chloroplast, a key organelle in green plants and algae containing the pigment chlorophyll. Their leaves, stems, flowers, sepals, and even plastids contain chlorophyll. In this article, we will explain the external factors affecting photosynthesis. Read this article to know more. 

What are The Factors Affecting Photosynthesis?

The rate of photosynthesis is defined as the rate of oxygen production per unit mass of green plant tissues or per unit weight of total chlorophyll. The factors affecting photosynthesis refer to those factors which either increase or decrease the rate of photosynthesis by increasing or decreasing their concentration level.
The factors affecting photosynthesis are of two kinds- Internal (or plant) and External (or environmental) Factors. An understanding of these factors affecting photosynthesis is essential in order to understand the rate of photosynthesis.

What are The Internal Factors Affecting Photosynthesis?

Let us look at some of the internal factors affecting photosynthesis:

1. The internal or plant factors greatly depend on the genetic predisposition and basically on the growth of the plant.
2. These factors include the number, size, age, and orientation of leaves. It also includes chlorophyll content, accumulation of byproducts, and hormones.
   (i) Chlorophyll Content of Leaves: The chlorophyll only acts as a biocatalyst. Hence, only a small quantity of chlorophyll is enough to carry out photosynthesis. So, the rate of photosynthesis is not proportional to the quantity of chlorophyll present in the leaves.
   (ii) Accumulation of Byproducts: Accumulation of sugars (final product of photosynthesis) in the cells of leaves slows down the process of photosynthesis. The process of photosynthesis takes place continuously as long as the concentration of the products formed (sugars) is maintained.
   (iii) The Internal Structure of Leaf: The thickness of the cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the stomata also affect the rate of photosynthesis.
   (iv) Protoplasmic Factor: Some internal factors present in the protoplasm prevent the plants from doing photosynthesis. It can be seen in the plants which are transferred from dark to light that do not start photosynthesis immediately.
   (v) Age of Leaf: The rate of photosynthesis increases with the age of the leaf till the leaf gets fully expanded. But after this, the rate of photosynthesis decreases because of ageing and senescence bringing about the deactivation of enzymes and degeneration of chlorophyll.
   (vi) Hormones: The hormones like auxins, cytokinins, and gibberellins stimulate photosynthesis. Abscisic acid reduces the rate of photosynthesis because of the closure of stomata.

Blackman’s Law of Limiting Factors 

1. For studying the external factors affecting the rate of photosynthesis, it is essential to have a knowledge of Blackman’s Law of Limiting Factors.
2. Before \(1905,\) early scientists attempted to determine the effect of individual factors on the photosynthetic rate in terms of minimum, optimum and maximum values which are known as cardinal points.
3. The simultaneous effects of other factors also have an importance in determining the rate of photosynthesis.                       

Fig: Concept of three cardinal points

4. But Blackman criticized the above-said concept and stated the Law of Limiting Factors which states that “when a process is controlled by more than one factor, then the rate of the process is limited by the factor which is nearest to its minimal value, i.e., limited by the pace of the slowest factor.”
5. The limiting factor is the factor that will affect the rate of photosynthesis if its quantity is changed.
6. For example, despite the presence of a green leaf and optimal light and carbon dioxide concentrations, the plant may not photosynthesize till the moment the temperature is very low. But if the optimal temperature is provided again to this condition, it will surely start photosynthesizing.

Fig: Blackman’s Law of Limiting Factors

What Are the External Factors Affecting Photosynthesis?

Let us explain the external factors affecting photosynthesis:

1. The external factors affecting photosynthesis include environmental factors like availability of sunlight, temperature, the concentration of carbon dioxide and water.
2. Factors available at suboptimal levels help in determining the rate of photosynthesis at any point.
3. Some of the factors which affect the photosynthetic rate are as follows:
   (i) Light
   a) Light is the main source of energy for photosynthesis that is mainly obtained from the sun.
   b) It is one of the major factors that affect the rate of photosynthesis.
   c) The three characteristics of light that influence the rate of photosynthesis are the intensity, quality, and duration of light.
   d) The light waves between the wavelength of \(400\,nm\) and \(700\,nm\) nm are the most effective for the process of photosynthesis, and this light is called photosynthetically active radiation (PAR).
   e) At low light intensities, the linear relationship between the incident light and the rate of carbon dioxide fixation occurs.
   f) At the higher intensity of light, the rate of photosynthesis fails to show further increase as other factors become limiting (saturation point).
   g) At \(10\% \) of full sunlight, the saturation of light occurs. Thus, excepting the shade and the dense forests, light is a limiting factor in very rare conditions in nature.
   h) If an increase of light beyond a certain limit or point occurs, it causes the breakdown of chlorophyll and decreases the rate of photosynthesis.
   i) If low light conditions are prevailing, both \({C_3}\) and \({C_4}\) plants fail to respond to high carbon dioxide conditions, while at high light intensities, both show an increase in the photosynthetic rates but up to a certain limit.

Fig: Effect of light intensity on the rate of photosynthesis

(ii) Carbon Dioxide Concentration
a) In \({C_4}\) plants, the photosynthetic rate increases as the carbon dioxide concentration increases, but at a much lower concentration of carbon dioxide, these plants tend to attain saturation, while the \({C_3}\) plants become saturated at much higher carbon dioxide levels.
b) Thus, the availability of carbon dioxide levels is a limiting factor for the \({C_3}\) plants. Scientists have demonstrated that the \({C_3}\) plants can grow much faster and lead to higher productivity due to a higher rate of photosynthesis.
c) Thus, in order to obtain a higher yield, some greenhouse crops like tomatoes, bell peppers, etc., are allowed to grow in an atmosphere enriched with carbon dioxide.

(iii) Temperature
a) Enzyme controlled dark reactions are affected by a change in temperature. Photosynthesis occurs in a very wide range of temperatures.
b) The \({C_4}\) plants respond to a higher temperature, showing a higher rate of photosynthesis, whereas the \({C_3}\) plants have a much lower optimum temperature range.
c) When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every \(10^\circ C\) rise in the temperature. 
d) But when the temperature reaches above the optimum range, the rate of photosynthesis shows an initial increase for a short period of time which later declines.
e) Different plants have different optimum temperature ranges for photosynthesis which depends on their habitat.
f) Tropical plants have a higher temperature range for photosynthesis than temperate plants.

Fig: Effect of temperature on the rate of photosynthesis

(iv) Water
a) Availability of water is not only an important factor for the process of photosynthesis but for the whole plant body as well.
b) In water stress conditions, the stomata become closed which reduces the availability of carbon dioxide to the plants, thereby causing reduction in the rate of photosynthesis.
c) Besides these, the decrease in water availability causes leaves to wilt, thus reducing the surface area of the leaves and ultimately reducing their metabolic activity.

Summary About Factors Affecting Photosynthesis

The process through which plants transform solar energy into chemical energy in the form of food is known as photosynthesis. Photosynthesis is the optimal mechanism for a living entity that lacks the capacity to actively move around in search of food. Photosynthesis is equally crucial for the food web’s consumers, and it serves as the beginning point for all ecological food chains. As a result, photosynthesis is an important biological mechanism for both plants and ecosystems. Photosynthesis is necessary for all life on Earth, not just for food, but also for the oxygen in the atmosphere, which they need to breathe. Photosynthesis is a process that cleans the atmosphere by removing carbon dioxide from the environment and converting it to oxygen.

Thus, the various internal and external factors affecting Photosynthesis determine the rate of photosynthesis which in turn determines the yield of plants. The factors affecting photosynthesis refer to those factors which either increase or decrease the rate of photosynthesis by increasing or decreasing their concentration level. The internal or plant factors greatly depend on the genetic predisposition and basically on the growth of the plant. For studying the external factors affecting the rate of photosynthesis, it is essential to have a knowledge of Blackman’s Law of Limiting Factors.

Frequently Asked Questions (FAQs) on Factors Affecting Photosynthesis

Let us look at some of the frequently asked questions on internal and external factors affecting photosynthesis:

Q.1. Name the factors affecting photosynthesis.
Ans: The top five factors affecting the rate of photosynthesis are chlorophyll content of leaves, the internal structure of leaves, light intensity, carbon dioxide concentration, and temperature.

Q.2. What do you mean by the law of limiting factors?
Ans: The law of limiting factors states that “when a process is controlled by more than one factor, then the rate of the process is limited by the factor which is nearest to its minimal value, i.e., limited by the pace of the slowest factor.”

Q.3. What is the best theory that explains the factors affecting photosynthesis?
Ans: Blackman’s Law of Limiting Factors is the best law that explains the factors of photosynthesis.

Q.4. What are the three main limiting factors affecting photosynthesis?
Ans: The three main limiting factors affecting photosynthesis are light intensity, carbon dioxide concentration, and temperature.

Q.5. What are the internal factors affecting photosynthesis?
Ans: The three internal factors affecting photosynthesis are chlorophyll content of leaves, accumulation of byproducts, and internal structure of leaves.

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