For discussion and reference purposes.
Blue (~450–520 nm) and UV (< 400 nm) Light
Blue and UV-A light triggers cryptochrome (320
–500 nm) and phototropin (phot1 and pho2; 320–500 nm) function (
Jones, 2018). These two photoreceptors regulate various physiological and developmental processes including chloroplast relocation, germination, elongation, and stomatal opening, which impacts water transpiration and CO2 exchange (
Cosgrove, 1981;
Schwartz and Zeiger, 1984). Blue light mediates chlorophyll and chloroplast development, enzyme synthesis, and plant density, and regulates responses to biotic environmental stresses (
Goins et al., 1997;
Schuerger et al., 1997).
Walters and Horton (1995) reported that blue light deficiency can impact the light saturation rate of photosynthesis and can change the Chl
a/b ratio in
Arabidopsis thaliana. Blue light causes thickness of the epidermis and palisade mesophyll cells in
Betula pendula (
Sæbø et al., 1995).
Lee et al. (2014) concluded that shorter blue wavelengths (<445 nm) promote stem growth, plant height, and anthocyanin synthesis in green perilla (
Perilla frutescens var.
japonica Hara cv.
Soim) plants. Cannabis plants grown under blue light with a short photoperiod (12 h light:12 h dark/flowering stage) improved cannabinoid content (
Magagnini et al., 2018). This same study suggested that there is a synergy between UV-A and blue wavelengths that induces cannabigerol accumulation in cannabis flowers.
Blue light activates Zeitlupe (ZTL) family function, a group of proteins that plays a role in circadian clock regulation, wherein their light-dependent function allows modulation of internal timing signals (
Kim et al., 2007). Accordingly, optimal lighting regimes for cannabis growth and production should take advantage of this temporal regulation initiated by the circadian clock and light-sensitive ZTL protein function.
Wavelengths of light that are shorter than the PAR spectrum [e.g., violet light and UV (<400 nm) radiation] have limited photosynthesis; however, discrete photomorphogenic effects are observed when UV-B (290
–320 nm) sensing systems are triggered (
Frohnmeyer and Staiger, 2003;
Folta and Carvalho, 2015). UV-B radiation is perceived
via the UV-B photoreceptor UV resistance locus 8 (UVR8). Although UV-B represents a threat to plant integrity in large quantities, smaller quantities of UV-B have important benefits such as promoting pest resistance, increasing flavonoid accumulation, improving photosynthetic efficiency, and serving as an indicator of direct sunlight and sunflecks (
Ballaré et al., 2012;
Wargent and Jordan, 2013;
Zoratti et al., 2014;
Moriconi et al., 2018). Further to this, some UV-B responses can also be modulated by a UVR8-independent signal and UV-A radiation, since plants’ responses to UV-B light are regulated by both UVR8-dependent and -independent pathways (
Morales et al., 2013;
Li et al., 2015;
Jenkins, 2017). UV-B light reportedly elicits THC accumulation in both leaves and buds (
Pate, 1983;
Lydon et al., 1987;
Potter and Duncombe, 2012).