Factors Affecting Photosynthesis
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Internal (Plant) factors: Number, size, age, and orientation of leaves, mesophyll cells, and chloroplasts, as well as internal CO₂ concentration and the amount of chlorophyll.
Plant factors depend on the genes and growth of plant.
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External factors: Sunlight, temperature, CO₂ concentration, and water.
Blackman’s Law of Limiting Factors (1905)
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“If a biochemical process is affected by more than one factor, its rate is determined by the factor nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.”
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For example, a plant with green leaves, optimal light, and CO₂ may not photosynthesize at very low temperatures. Providing optimal temperature enables photosynthesis.
Light
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Light quality, light intensity, and duration of exposure influence photosynthesis.
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At low light intensities, there is a linear relationship between incident light and CO₂ fixation rates. At higher intensities, the rate plateaus as other factors become limiting.
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Light saturation occurs at 10% of full sunlight, so light is rarely limiting in nature, except for shade plants or dense forests.
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Excessive light can break down chlorophyll, reducing photosynthesis.
Carbon Dioxide Concentration
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CO₂ is the major limiting factor for photosynthesis, with atmospheric levels at 0.03–0.04%.
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Increasing CO₂ up to 0.05% boosts fixation rates, but higher levels can be damaging over time.
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At low light, C3 and C4 plants show no response to high CO₂. At high light, both show increased photosynthesis rates.
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C4 plants reach saturation at about 360 μl L⁻¹, while C3 plants respond to higher CO₂, saturating beyond 450 μl L⁻¹, indicating current CO₂ levels limit C3 plants.
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C3 plants’ response to elevated CO₂ increases photosynthesis and productivity, utilized in CO₂-enriched greenhouses for crops like tomatoes and bell peppers.
Temperature
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Dark reactions, being enzymatic, are temperature-sensitive, while light reactions are less affected.
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C4 plants respond to higher temperatures with increased photosynthesis rates, while C3 plants have a lower temperature optimum.
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Temperature optimum of plants also depends on their habitat. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
Water
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Water stress closes stomata, reducing CO₂ availability.
Water stress also causes leaf wilting, decreasing surface area and metabolic activity.