Thursday, October 8, 2020

Photosynthesis - Notes | Class 11 | Part 5: Factors Affecting Photosynthesis

13. PHOTOSYNTHESIS

FACTORS AFFECTING PHOTOSYNTHESIS


Internal (plant) factors:
Number, size, age & orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and amount of chlorophyll.

Plant factors depend on the genes and growth of the plant.

External factors: 

Sunlight, temperature, CO2 concentration and water.

Blackman’s Law of Limiting Factors (1905): 

“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.”

E.g. a plant with green leaf, optimal light & CO2 may not photosynthesize in very low temperature. If optimal temperature is given, it will start photosynthesis.

Light 

Light quality, light intensity and duration of exposure to light influence photosynthesis.


There is a linear relationship between incident light and CO2 fixation rates at low light intensities.

At higher light intensities, the rate does not show further increase because other factors become limiting.

Light saturation occurs at 10% of the full sunlight. Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.

High increase in incident light breaks down chlorophyll. It decreases photosynthesis.

Carbon dioxide Concentration 

CO2 is the major limiting factor for photosynthesis.

CO2 concentration is very low in the atmosphere (0.03- 0.04%). Increase up to 0.05% cause increase in CO2 fixation rates. Beyond this level, it becomes damaging over longer periods.

At low light, C3 and C4 plants do not respond to high CO2. At high light, they show increased rate of photosynthesis.

C4 plants show saturation at about 360 mlL-1.

C3 plants respond to increased CO2 concentration and saturation is seen only beyond 450 mlL-1. Thus, current availability of CO2 levels is limiting to the C3 plants.

Due to response to higher CO2 concentration, C3 plants show increased photosynthesis and higher productivity. This fact is used for some greenhouse crops (tomatoes, bell pepper etc). They are grown in CO2 enriched atmosphere.

Temperature 

Dark reactions, being enzymatic, are temperature controlled. Influence of temperature on Light reactions is very less.

The C4 plants respond to higher temperatures and show higher rate of photosynthesis.

C3 plants have a much lower temperature optimum.

The temperature optimum of plants also depends on their habitat. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Water 

Water stress closes the stomata hence reduce the CO2 availability. Water stress also wilts leaves, thus reduce the surface area of the leaves and their metabolic activity.

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