Light Spectrum Effects
White light is a combination of all wavelengths of visible light, sometimes called broad spectrum. The sun emits white light and plants have evolved to use this light efficiently. Since plants have evolved to use a broad spectrum of light, they require some variation in light color for healthy and optimal growth.
Red light is the most effective light for promoting photosynthesis (McCree 1971). Plant lighting favoring high ratios of red light makes plants grow more efficiently than other light spectra; however, it is not an effective light to use alone (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 synergistically, which is more than adding these light sources independently, despite being outside the PAR range (Zhen and van Iersel 2017; Hogewoning et al. 2012). Additionally, far-red light has also been shown to be beneficial in 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 light to far-red light. Low ratio (i.e. high amounts of far-red light) causes plants to stretch and leaves to expand. Using incandescent bulbs for nighttime interruption will cause plants to stretch for this reason. Having a high ratio of red to far-red light will help keep plants strong (Franklin 2008). Using far red at the onset of the dark period in an attempt to promote flowering causes stretching and taller plants (Lund 2007).
The main active photoreceptors of red light are the phytochromes. Phytochromes come in two interchangeable forms: Pr “absorbing red” and Pfr “absorbing far-red”. If Pr absorbs light, it will pass to Pfr and vice versa. Although Pr absorbs most strongly at 660 nm and Pfr absorbs most strongly at 730 nm, both states absorb light between 300 nm and at least 750 nm (Butler et al. 1964; Stutte 2009). Phytochromes can be induced in Pfr with green and yellow wavelengths (Shinomura et al. 1996; Wang and Folta 2013). The balance of Pr to Pfr is known as the photostationary state of phytochrome (PSS) and is changed whenever light is present. This is important for some of the differences in growth habits seen with different spectra. In addition to absorbing light, Pfr will naturally change to Pr in the dark, which forms the basis of the dark periods necessary for flowering (Borthwick and Cathey 1962).
Blue light has been shown to be essential for producing healthy plants, as chlorophyll development needs blue light and will not occur properly under red light alone (Massa 2008). High ratios of blue light help keep plants strong (Hernandez 2016a, 2016b). Additionally, blue light helps induce the production of similar secondary metabolites at higher light intensity (Hogewoning 2010). Although research has not been validated with cannabis, more blue light can increase THC and CBD production.
Cryptochromes, phototropins and light-oxygen-voltage sensors
Blue light is effective in promoting photosynthesis while activating a myriad of critical photoreceptors including cryptochromes, phototropins, and light-oxygen-voltage (LOV) sensors. These photoreceptors are important for chloroplast development, stomatal opening, circadian rhythm and production of secondary metabolites (Cashmore 1999; Ouzounis 2015; Pocock 2015; Kopsel et al. 2015).
The perception and use of green light by plants is a subject that is often misunderstood. Photosynthesis driven by absorption of green light is almost as efficient as blue light and may actually contribute more to complete plant photosynthesis, as it has greater leaf and canopy penetration than red light and blue (McCree 1972; Smith et al. 2017). There is even a photoreceptor specific to green light (Pocock 2015; McCoshum and Kiss 2011; Folta and Carvalho 2015; Zhang and Folta 2012). Green light is not wasted, or invisible light to plants, and plays an important role in both photosynthesis and signal transduction in plants.
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