Discuss Red Drop and Emerson Enhancement Effect.

Emerson Red Drop and Enhancement Effect - The phenomenon of red drop shown by Emerson and Lewis was quite puzzling. This is not due to decrease in light absorption because the quantum yield measures only light that has actually S been absorbed.

This indicates that light of £ f. wavelengths greater than 680 nm is much less efficient than light of shorter wavelengths. In subsequent experiments, Emerson and his colleagues measured photosynthesis using red and far-red light after adjusting their fluence rates to give equal rates of photosynthesis

They observed that the quantum yield obtained using both red and far-red light simultaneously was much higher than the sum of the yields obtained with red and far-red light separately

This phenomenon is known as Emerson enhancement effect or often as Emerson effect. 

These puzzling phenomena of red drop and enhancement effect led to the conclusion that two different reaction centers or photochemical events are involved in photosynthesis. One event is driven by red light (= 680 nm) and the other driven by far-red light (>680 nm). Optimal photosynthesis occurs when both events are driven simultaneously or in rapid succession. These two photochemical events are now known as Photosystem II and Photosystem I and they operate in series to carry out photosynthesis optimally. Photosystem II absorbs red light of 680 nm well and is driven very poorly by far-red light. On the other hand Photosystem I absorb preferentially far-red light of wavelengths greater than 680 nm.

•The rate of photosynthesis is measured as the number of O2 molecules produced per quantum of light absorbed

• Robert Emerson et al (1958):Experimental material Chlorella Emerson Experimental material Chlorella

• Studying rate of photosynthesis at different wavelengths of light (390-760 nm)   

• Monochromatic light of different wavelength is used and rate of photosynthesis is measured

• The sudden fall in photosynthetic yield in the far red region (greater than 680 nm) compared to the red region of the electromagnetic spectrum is called “Emersons red drop”

• The increase in photosynthetic yield by the combined effect of red (680 nm) and far red light (700 nm) is called Emerson enhancement effect or Emerson effect.

• This work provide experimental evidence for the presence of two photo systems-PS I & PS II  

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