Light Quality

The quality of light is its color, or spectrum. Light spectrum is useful for directing the plant’s growth habits, as well as contributing to healthy photosynthesis. Comparing this to a car, if light intensity is the gas pedal to increase speed of photosynthesis, light quality would be the steering wheel. Using different colors of light allows growers to affect the plant in terms of yield, flavor, color, growth, flowering, and even the severity of pests and diseases (Davis and Burns 2016).

Light color, as described in this text, is part of the electromagnetic spectrum that lies between wavelengths of 280 and 800 nanometers (nm) and is influential for plant growth and development. In horticulture, the rainbow of colors is abbreviated to only a few: blue (400–499 nm), green (500–599 nm), and red (600–699 nm). White light is created by combining all wavelengths between 400 nm and 700 nm. Ultraviolet light is between 200 nm and 399 nm, with UVC (200 to 279 nm), UVB (280 to 315 nm), and UVA (315 to 399 nm). In addition, far-red light lies from 700 to 750 nm, although most far-red light sources emit at 730 nm (Both et al. 2017). The reality of defining light by color is far more complex, as wavelengths within a color (425 vs. 450 nm) can affect growth differently, and different wavelengths are also known to have synergistic effects with one another.

Photosynthetically Active Radiation (PAR)

Several defined regions of the spectrum and their associated acronyms are used to describe the wavelengths of light for plant growth, specifically photosynthetically active radiation. PAR defines light that plants can use to gain energy for photosynthesis, from 400 nm (blue) to 700 nm (red), although wavelengths of light up to 752 nm can still positively affect photosynthesis (Cathey 1980; Sager 1984; Hogewoning et al. 2012; Zhen et al. 2019). Light from 280–399 nm and 700–750 nm can elicit responses in plants, such as the production of terpenes and cannabinoids, or affect flowering, while not necessarily contributing to photosynthesis. These wavelengths are sometimes included in a wider spectral range than PAR, which is referred to as photo-biologically active radiation (PBAR) because the wavelengths can affect more than just photosynthesis. Understanding the differences in light spectra and the effect they have on plants can help the careful cultivator ensure that the plants are getting everything they need to thrive.

This chart represents relative absorbance of various plant pigments across the spectrum of photosynthetically active radiation. These pigments primarily absorb light in the blue and red spectra, however there is still absorbance in the green/yellow/orange spectra that also drives photosynthesis.

The McCree Curve depicts the photosynthetic quantum efficiency across the spectrum of photosynthetically active radiation. It shows a plant’s real world photosynthetic efficiencies. This curve provides insight when designing spectral output from artificial lighting. SOURCE: Smart Grow Technologies

Light Spectrum Effects

White (400–700 nm)

White light is a combination of all wavelengths of visible light, sometimes referred to as broad spectrum. The sun emits white light, and plants have evolved to use this light effectively. Since plants have evolved to use a broad spectrum of light, they require some variation in light color for healthy, optimal growth.

Red (600–699 nm)

Red light is the most effective light for promoting photosynthesis (McCree 1971). Plant lighting favoring high ratios of red light grows plants more effectively than other spectra of light; however, it is not an effective light to use by itself (Massa et al. 2008; Hernandez et al. 2016).

Far-Red (700–750 nm)

Far-red light has important implications for photosynthesis, photomorphogenesis, and flower induction in plants. Far-red light increases the photosynthetic rate of other light sources in a synergistic way that is more than the addition of these light sources independently, despite being outside the PAR range (Zhen and van Iersel 2017; Hogewoning et al. 2012). In addition, far-red light has also been shown to be beneficial for inducing flowering responses in short-day plants (Cathey and Borthwick 1957).

Red to Far-Red Ratio

One of the best-known spectral effects comes from the ratio of red to far-red light. A low ratio (i.e., high amounts of far-red light) causes plants to stretch and leaves to expand. Using incandescent bulbs for night interruption will cause plants to stretch for this reason. Having a high ratio of red to far-red light will help keep plants stout (Franklin 2008). Using far-red at the start of the dark period in an attempt to promote flowering causes stretching and taller plants (Lund 2007).

Phytochrome

The main active photoreceptors for red light are phytochromes. Phytochromes come in two interchangeable forms: “red absorbing” Pr and “far-red absorbing” Pfr. If Pr absorbs light, it will shift to Pfr and vice versa. Although Pr absorbs most strongly at 660 nm and Pfr absorbs most strongly at 730 nm, both states absorb some light anywhere from 300 nm to at least 750 nm (Butler et al. 1964; Stutte 2009). Phytochromes can be induced to Pfr with green and yellow wavelengths (Shinomura et al.1996; Wang and Folta 2013). The balance of Pr to Pfr is known as the phytochrome photostationary state (PSS) and is altered anytime light is present. This is important for some of the differences in growth habits observed with different spectra. In addition to absorbing light, Pfr will naturally transition to Pr in darkness, and this forms the basis of dark periods being required for flowering (Borthwick and Cathey 1962).

Blue (400–499 nm)

Blue light has been shown to be critical for producing healthy plants, as chlorophyll development needs blue light and will not properly occur under only red light (Massa 2008). High ratios of blue light help keep plants stout (Hernandez 2016a, 2016b). In addition, blue light helps induce secondary metabolite production similar to having a higher light intensity (Hogewoning 2010). Although research has not been validated with cannabis, more blue light may increase THC and CBD production.

Cryptochromes, Phototropins & Light-Oxygen-Voltage Sensors

Blue light is effective for promoting photosynthesis while also activating a myriad of critical photoreceptors, including cryptochromes, phototropins, and light-oxygen-voltage sensors (LOV). These photoreceptors are important for chloroplast development, stomatal opening, circadian rhythm, and secondary metabolite production (Cashmore 1999; Ouzounis 2015; Pocock 2015; Kopsel et al. 2015).

Green (500–599 nm)

The perception and use of green light by plants is a commonly misunderstood topic. Photosynthesis driven by absorbing green light is almost as efficient as blue light and may in fact contribute more to full-plant photosynthesis, as it has greater leaf and canopy penetration than red and blue light (McCree 1972; Smith et al. 2017). There is even a green-light-specific photoreceptor theorized to exist (Pocock 2015; Mccoshum and Kiss 2011; Folta and Carvalho 2015; Zhang and Folta 2012). Green light is not wasted light, or invisible to plants, and plays an important role in both photosynthesis and signal transduction in plants.

Indoor lighting – it’s a whole new world by Ed Rosenthal from the Cannabis Grower's Handbook

Light Quality