Emerson's enhancement effect and Red drop have been instrumental in the discovery of

Emerson's enhancement effect and Red drop have been instrumental in the discovery of two photosystems operating simultaneously

R. Emerson attempted to determine the action spectra for photosynthesis in Chlorella. They observed that there was an unexpected drop in the quantum yield at wavelengths greater than 680 nm, even though chlorophyll still absorbed in that range. The drop in Quantum efficiency in the long red portion of the spectrum was called the red drop.

In another experiment, Emerson and his colleagues set up two beams of light—one in the region of 650 to 680nm and the other in the region of 700 to 720 nm. The fluence rates of both beams were adjusted to give equal rates of photosynthesis. Emerson discovered that when the two beams were applied simultaneously, the rate of photosynthesis was two to three times greater than the sum of the rates obtained with each beam separately. This phenomenon has become known as the Emerson enhancement effect.

The enhancement effect suggests that photosynthesis involves two photochemical events or systems, one driven by short-wavelength light (≤680 nm) and one driven by long-wavelength light (>680 nm).