One major benefit of LED technology is the ability for manufacturers to custom tune the spectra of light to provide to your crop. The first versions of LED-based horticulture lighting systems mainly consisted of red (660 nm) and blue (450 nm) diodes to achieve the highest photon efficacy possible. While red LEDs have the highest photon efficacy,plants did not evolve in nature under narrow band wavelengths, so they are not always the most efficient spectra with regards to optimizing plant growth and development. This is especially true in situations where lighting systems are used for sole-source lighting, as compared to supplemental greenhouse lighting.

One problem that growers complained about was inconsistent plant morphology during different growing seasons due to varying percentages of total red/blue light provided when the amount of ambient sunlight changes. On cloudy days plants receive a much higher percentage of light from a supplemental light source, and narrow band wavelengths can cause plants to have varying morphological responses during production, as you are significantly changing the spectra of light from day to day. Read More

Initial studies by a Wisconsin group demonstrated the need to supplement high-output red LEDs with some blue light to get acceptable plant growth ( Hoenecke et al., 1992 ). Subsequent studies at the Kennedy Space Center showed wheat seedlings germinating under 500 μmol·m−2·s−1 of red LED light failed to develop chlorophyll but that supplementation with only 30 μmol·m−2·s−1 of blue light, or just reducing red PPF to 100 μmol·m−2·s−1, restored chlorophyll synthesis ( Tripathy and Brown, 1995 ). Further studies have confirmed that some blue light is typically necessary to improve growth and minimize shade avoidance responses, including excessively elongated stems (Snowden et al. 2016). Blue light activates the cryptochrome system and matches chlorophyll and carotenoid absorption spectra, thus having significant effects on green vegetable morphology, growth, photosynthesis, and antioxidant system response ( Olle and Viršilė 2013 ).

Green light also has valuable physiological effects ( Olle and Viršilė 2013 ). In several studies, 510–530 nm LED light (Johkan et al. 2012; Son and Oh 2015), comparably to green fluorescent lamps, supplemental for red and blue LEDs (Kim et al. 2004), promoted growth. Snowden et al. (2016) and Bugbee (2016) explained that green light penetrates deeper into leaves and canopies, thereby altering plant growth and development (128 A. Viršilė et al.). Green is characterized by better transmission through leaf tissue, as compared to red or blue light wavelengths (Massa et al. 2015).

In greenhouse environments, the impact of supplemental light quality seems to be diminishing, especially when background solar irradiance provides sufficient photosynthetically active photon flux (Mitchell et al. 2015). It is likely that there is a threshold background solar daily light integral (DLI) or a relative level of supplemental DLI that requires the additional blue photon flux through supplemental lighting (Hernández and Kubota 2012; 2014a, b; Mitchell et al. 2015).

New questions continue to arise when considering LEDs for horticultural lighting in view of recent studies, such as what levels/proportions of red, green, and blue light will be required for particular crops? This has prompted a few horticulture lighting companies to implement white LEDs (which are blue LEDs with a yellow phosphor coating). Better productivity generally is seen with the additional wavelengths and broadening of the spectrum achieved with white light. LEDs used as supplements to sunlight or other types of lighting in greenhouses or growth chambers could modify crop growth or development in a desired direction without depriving crops of necessary wavelengths (Massa, 2008).