Plant Physiology

Effects of Red Drop and Enhancement
Emerson and Lewis’ demonstration of the red drop proved perplexing. Because the quantum yield only measures the light that has been absorbed, this is not attributable to a decrease in light absorption. This means that light with wavelengths longer than 680 nm is inefficient compared to light with shorter wavelengths. After modifying their fluorescence rates to give equivalent rates of photosynthesis, Emerson and his colleagues evaluated photosynthesis using red and far-red light in subsequent tests. The quantum yield obtained by combining red and far-red light was substantially higher than the sum of the yields obtained by combining red and far-red light separately. Emerson enhancement effect, or Emerson effect, is the name given to this occurrence.
These perplexing red drop and amplification effects led to the conclusion that photosynthesis involves two separate reaction centres or photochemical activities. Red light ( 680 nm) drives one event, while far-red light (> 680 nm) drives the other. When both activities are driven simultaneously or in rapid succession, optimal photosynthesis occurs. These two photochemical reactions are now referred to as Photosystem II and Photosystem I, and they work in tandem to optimise photosynthesis. Photosystem II absorbs red light with a wavelength of 680 nm well but is poorly driven by far-red light. Photosystem I, on the other hand, prefers to absorb far-red light with wavelengths larger than 680 nm.