Factors Affecting Photosynthesis 

Photosynthesis is a complex process influenced by a variety of internal and external factors. The rate of photosynthesis is determined by the factor that is in the least favourable quantity, as described by Blackman's Law of Limiting Factors(1905). This Law states, "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.

1.0Definition of Photosynthesis

Photosynthesis is the process by which chlorophyll-containing organisms capture light energy and use it to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂).

2.0External Factors Affecting Photosynthesis

1. Light

Light is essential for the light-dependent reactions of photosynthesis. Its impact can be analysed based on three aspects:

• Light Intensity: At low light intensities, the rate of photosynthesis is directly proportional to the light intensity. As light intensity increases, the rate of photosynthesis also increases. However, a point is reached, known as the light saturation point, where the rate of photosynthesis plateaus because other factors (like CO₂ or temperature) become limiting. At very high light intensities, chlorophyll can be damaged, leading to a decrease in the photosynthetic rate.
• Light Quality (Wavelength): Plants absorb different wavelengths of light for photosynthesis. Chlorophyll molecules absorb light most efficiently in the blue-violet and red regions of the visible spectrum. The green light is reflected chiefly, which is why most plants appear green.
• Duration of Light: The duration of light exposure affects the total amount of photosynthesis that can occur, but not the rate at which it happens (unless it is so short that the plant doesn't have time to reach its maximum rate).

2. Carbon Dioxide (CO₂) Concentration

CO₂​ is a major reactant in the Calvin cycle (the dark reaction). Its concentration in the atmosphere is relatively low (0.03% to 0.04%).

• An increase in CO₂​ concentration leads to a higher rate of photosynthesis, provided that other factors like light and temperature are not limiting.
• However, after a certain point (typically around 0.05%), increasing CO₂​ concentration further can become damaging to the plant over long periods.
• C₃ plants are generally more responsive to increased CO₂​ concentration than C₄ plants because they lack an efficient carbon-concentrating mechanism and are more limited by the low atmospheric CO₂​ levels.

3. Temperature

Temperature affects the rate of photosynthesis primarily because the dark reactions are enzymatic and thus highly temperature-sensitive.

• The rate of photosynthesis increases with temperature up to an optimal level.
• Beyond the optimum temperature, the rate of photosynthesis decreases sharply because the enzymes involved in the process, especially RuBisCO, begin to denature.
• The optimum temperature for photosynthesis varies among plant species, depending on their native habitat. For example, tropical plants have a higher temperature optimum than plants from temperate regions.
• While the light reactions are also affected by temperature, they are much less sensitive compared to the dark reactions.

4. Water

Water is a reactant in the light-dependent reaction, but its effect on the rate of photosynthesis is more indirect.

• Water stress leads to the closing of stomata to conserve water. This reduces the availability of CO₂ for the Calvin cycle, thereby limiting the rate of photosynthesis.
• Severe water stress can also cause leaves to wilt, reducing their surface area and metabolic activity, further impacting photosynthesis.

3.0Internal Factors Affecting Photosynthesis

1. Chlorophyll Content

• Chlorophyll is the primary pigment responsible for absorbing light energy. 
• A higher amount of chlorophyll in the leaves allows for more light absorption, leading to a higher rate of photosynthesis. 
• Factors like mineral deficiency (e.g., magnesium and iron, which are crucial for chlorophyll synthesis), plant age, and diseases can reduce chlorophyll content and thus decrease photosynthetic efficiency.

2. Leaf Anatomy

• The structure of the leaf plays a crucial role in regulating photosynthesis. Factors like the size and position of stomata, as well as the number of intercellular spaces, influence the intake of CO_2. A thicker cuticle, for instance, may limit gas exchange.