50. Factors Affecting the Rate of Photosynthesis

The rate at which a plant photosynthesizes is determined by several environmental and biological factors. When one of these factors is in short supply, it "limits" the rate of the reaction and is known as a limiting factor.

1. Key Limiting Factors

Chlorophyll

• Role: The pigment that absorbs light energy.

• Variable: Levels can decrease due to disease (e.g., Tobacco Mosaic Virus), environmental stress, or lack of nutrients (e.g., magnesium).

• Impact: Less chlorophyll means less light is absorbed, lowering the rate of photosynthesis.

Light Intensity

• Effect: As light intensity increases, the rate of photosynthesis increases because there is more energy available for the reaction.

• The Plateau: Eventually, the rate levels off (plateaus). At this point, light is no longer the limiting factor; instead, temperature or CO₂ concentration is holding the rate back.

Carbon Dioxide (CO₂) Concentration

• Effect: As one of the reactants, increasing CO₂ increases the rate of reaction.

• The Plateau: Similar to light, the graph will eventually flatten out when CO₂ is plentiful and another factor becomes limiting.

Temperature

• Initial Effect: Increasing temperature increases the rate because enzymes work faster and molecules have more kinetic energy.

• The Peak and Drop: If it gets too hot (around 45°C), the enzymes involved in photosynthesis denature (lose their shape and function). The rate then drops rapidly to zero.

2. Interpreting Limiting Factor Graphs

Scientists often plot multiple curves on one graph to identify which factor is limiting:

• If two curves for light intensity show different maximum rates at different temperatures, temperature was the limiting factor for the lower curve.

• If two curves at the same temperature show different rates at different CO₂ levels, CO₂ concentration was the limiting factor.

3. Maximizing Yield in Farming

Farmers use knowledge of limiting factors to create optimum conditions for crop growth, often using greenhouses:

• Temperature: Greenhouses trap heat; paraffin heaters can be used to add warmth.

• Light: Artificial lighting allows photosynthesis to continue overnight or during winter.

• CO₂: Paraffin heaters or CO₂ pumps increase the concentration of this reactant.

• Protection: Enclosed structures keep out pests and pathogens.

• Nutrients: Fertilizers provide essential minerals to ensure maximum chlorophyll production.

Summary Table: Limiting Factors