Bachelor’s Degree Programme (BSCG) – Plant Physiology and Metabolism Assig. Answer | BBYCT–137(Q-4)

Bachelor’s Degree Programme (BSCG) – Plant Physiology and Metabolism Assig. Answer | BBYCT–137(Q-4)

4. a) Describe the phenomenon of Emerson’s enhancement effect and Red drop effect with the help of a labelled diagram.
b) Briefly discuss the ‘Law of limiting factors’.
c) Name two CAM plants.


a) Emerson’s Enhancement Effect: The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to the light of wavelength 680 nm (deep red spectrum) and more than 680 nm (far red spectrum). When simultaneously exposed to a light of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far-red light photosynthesis rates.
Red Drop Effect: Wavelengths beyond 700nm are apparently of insufficient energy to drive any part of photosynthesis. So a huge drop in efficiency has been noticed at 700nm. This phenomenon is called as RED DROP EFFECT. In other words, there is a sharp decrease in quantum yield at wavelengths greater than 680nm.

b)  Law of limiting factors

  • A plant physiologist Blackman studied on limiting factors on the photosynthesis system of plants. He stated that the biological factors are affected by a number of factors, but the amount in which they affect the whole process is different. Let us take the example of photosynthesis. Plants require adequate amounts of water, sunlight, chloroplast, temperature, carbon dioxide and chlorophyll to carry out photosynthesis. The scarcity of any of these components will affect the process of photosynthesis.
  • Any physiological process which is affected by more than one factor is governed by the law of limiting factor.
  • The relative magnitude of factors is more important than the absolute magnitude.
  • A factor which is present in the less quantity is the limiting factor.
  • When the rate of the process becomes constant due to a limiting factor, it can be regulated by regulating the amount of the factor only which is limiting. For eg., a leaf which can utilize 5mg of CO2 per hour in photosynthesis is exposed to certain light intensity. If only 1mg of CO2 enters the leaf in an hour, the rate of photosynthesis is limited due to the CO2 factor. As the concentration of CO2 increases, the rate of photosynthesis is also increased. Any further increase in the CO2 concentration will not affect the rate of photosynthesis. It will only increase if the intensity of light is increased.

c) CAM Plants – Orchids & Cactus.

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Ignou Assignment Solutions Asked question February 27, 2022