The Emerson effect, described by Robert Emerson in the 1950s, has two parts. The "red drop" is the sharp fall in photosynthetic efficiency for light beyond about 680 nm on its own. The "enhancement effect" is the finding that adding shorter-wavelength light restores and boosts that efficiency, so the combined rate exceeds the sum of the two lights separately. It was key evidence that photosynthesis runs two photosystems in series: photosystem I absorbs far-red well, photosystem II does not.
In practice
For growers, the Emerson effect is the reason far-red photons are not wasted when a fixture also delivers plenty of 400–700 nm light. Zhen and Bugbee (2020) measured far-red photons up to about 750 nm contributing to canopy photosynthesis at near-equal efficiency when mixed with PAR. This supports counting ePAR (400–750 nm) for whole-canopy work. Established in model crops; cannabis-specific canopy measurements remain few.
It does not mean you can replace PAR with far-red. A far-red-only source gives very little photosynthesis and a lot of stretch. Typical fixtures with far-red keep it to a minority of total photons, often under 10–15%.
Don't confuse with
- Red:far-red ratio: a developmental signal via phytochrome, separate from the photosynthetic enhancement.
- Far-red: the waveband; the Emerson effect is how it interacts with other light.
