To receive light, plants have 4 main photoreceptor classes: phytochromes, which respond to red and far-red light, cryptochromes, and UV-A and UV-B receptors (Quail 1994; Quail et al. 1995; Spalding and Folta 2005). Phytochrome cycles between active and inactive state through the absorption of red and far-red light respectively. Therefore, the ratio of red-to-far-red determines the response. Sunlight has a red-to-far-red ratio of around 1.15 (Smith 1982). A shading leaf absorbs the red part of the spectrum, but not the far-red portion. The leaf below will receive a lower red-to-far-red ratio, resulting in induced growth through a phytochrome regulated signalling pathway that leads to shade avoidance response. The red-to-far-red ratio is determined as the ratio of photon irradiance from 655 to 665 nm and photon irradiance from 725 to 735 nm. Because other wavelengths can also deactivate phytochrome, it is preferred to use the phytochrome photostationry state (PSS) (Sager et al. 1988), instead of the red-to-far-red ratio. It is a parameter that represents the balance between active and inactive phytochrome considering the absorption and quantum yield spectra of both active and inactive phytochromes. Sunlight has a PSS value of around 0.72, which means that under sunlight 72% of phytochrome molecules will be in the activated state in vitro. Recently, a role for green light was described, which is contrary the belief that plants cannot ‘see’ green light (Sun et al. 2012). It is believed that different light spectra can initiate a specific biochemical response, similar as with high intensity light. Inducing effects of light are always fluence-rate dependent and could already be altered by small changes in spectrum. Changes in light spectrum such as addition of blue can also be interpreted as a reduction of red, or a change in ratio. Existing literature is not consistent on this; therefore, it is important to consider the exact conditions. Addition of a light colour can increase the intensity and change the ratio. However, it is also possible to add a light colour, and reduce another one, keeping the total light energy the same. Changes to the ratio of light spectra will be different in that case. Responses are also genotype dependent. There are many examples:
A short addition of blue light during the last stages of preharvest increased β-carotene accumulation, and red light led to an increase of lutein, in leafy greens and broccoli (Lefsrud et al. 2006, 2008; Li and Kubota 2009). Added far red light reduced the total carotenoids concentration in lettuce (Li and Kubota 2009). Flavonoids can also be manipulated by manipulating light spectra. Change to the ratio of red:blue spectra could be used to manipulate phenolic compounds and flavonoids content, antioxidant activity and nitrate content of basil leave(Choi et al. 2000; Lefsrud et al. 2006, 2008)s and strawberry fruit (Piovene et al. 2015), and red lettuce leaves (Johkan et al. 2010). Green light was able to reduce anthocyanin accumulation in Arabidopsis seedlings (Zhang and Folta 2012). In lettuce, blue light increases anthocyanin accumulation 3-4-fold compared with white light. However, that increase could be completely abolished by the addition of green light (Zhang and Folta 2012). Supplementary green and blue light can increase antioxidant components (vitamin C, phenolic compounds and tocopherols content) in red, green, light green and romaine baby leaf lettuce (Samuoliene et al. 2013). In other microgreens of kale, broccoli, and beet, anthocyanins were induced with far-red light. High-fluence green light could reduce this in some cases. A higher portion of blue-light also induced anthocyanin, same as lettuce, and accumulation was inhibited by the addition of green wavebands (Carvalho and Folta 2016). Enhanced blue light on green and red lettuce led to increased phenolic compounds (cichoric acid, quercetin and their derivatives) and pigments (Ouzounis et al. 2015; Taulavuori et al. 2016). Other phenolics, such as fragrance volatiles can be manipulated by the use of different spectra, although it is possible that these changes are also due to changes in intensity (Carvalho and Folta 2016).
There are two explanations for the change in metabolites by light spectra. First one is the energy of the light. Blue light is more energetic that red light, and this could initiate a defence response. The other possible explanation is the signalling function of light. During the day, and over the seasons, the light spectra has slight changes. In the morning and in spring, the light spectrum contains more blue, and far-red levels are higher at the beginning and the end of the day, and there is also a seasonal effect. Far-red light also informs the plant about their growth habit, and surroundings by other plants due to an adaptive response initiated by far-red light. Green and other spectra can also inform the plant about the time of day, as well as their situation in the field. Fluctuating light, due to clouds and can also lead to adaptive response in carotenoids composition and other compounds (Kromdijk et al. 2016).
Possible applications of light treatments to increase phytonutrients are currently in the experimental phase. Further investigation to determine the physiological, biochemical and molecular changes that can be induced by specific light recipes. Optimal values should be determined experimentally for each species and cultivar, it is not possible to provide a generalized recommendation. For more examples, it is recommended to read the excellent review by Poiroux-Gonord et al. (2010).