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 oMy 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