Soon there will be new far red info to add.
And
@MIMedGrower said let there be red and far red info. An there was red and far red info
Red (~625–700 nm) and Far-Red (> 700 nm) Light
Red light impacts photomorphogenesis, leaf nutrient content, and stem growth. It is essential for chlorophyll synthesis and for straightening the epicotyl or hypocotyl hook of dicot seedlings (
McNellis and Deng, 1995;
Goins et al., 1997;
Poudel et al., 2008;
Johkan et al., 2012). These processes are under the influence of phytochrome control. Phytochrome is sensitive to red (~650–670 nm) light and far-red (FR) light (~705–740 nm), and to a lesser extent, blue light (~400–500 nm). For any one phytochrome, there exists a photoequilibrium of two interconvertible forms, red and FR absorbing forms (also known as Pr and Pfr, respectively). Pfr is the active form of phytochrome and it elicits physiological responses (
Shinomura et al., 2000). Pr, the other form of phytochrome, is the inactive form that switches to Pfr upon absorbing ~650–670 nm light (
Nagatani, 2010;
Folta and Carvalho, 2015). In long day plants, various experiments suggest that flowering is promoted mostly when red light (or light creating a high Pfr/Pr ratio) is delivered during the early part of the photoperiod and when FR light (or light creating a lower Pfr/Pr ratio) is delivered toward the end of the photoperiod (
Lane et al., 1965;
Evans, 1976;
Kadman-Zahavi and Ephrat, 1976;
Thomas and Vince-Prue, 1996). However, certain cannabis genotypes such as “G-170” are insensitive to changes in the R:FR ratio, and no effect on flowering has been observed (
Magagnini et al., 2018). The authors concluded that a low R:FR ratio during a long photoperiod (18 h light, 6 h dark/vegetative stage) is beneficial to the development of mature cuttings, contradicting popular belief in the cannabis industry.
The effect of red light on plant physiology has been investigated (
Poudel et al., 2008;
Vu et al., 2014).
Poudel et al. (2008) reported that red light induced an increase in rooting percentage and root numbers in grape (
Vitis vinifera) plants.
Wu and Lin (2012) showed that king protea (
Protea cynaroides L.) plantlets grown in red light produce a higher number of roots and new leaves.
Vu et al. (2014) reported that “
Lapito” tomato plants grown solely under red LED light produce a higher total root surface area, length, and number of root tips in comparison with other light treatments. Lower leaf nitrogen content was found in rice (
Oryza sativa L.) and spinach (
Spinacia oleracea L., cv. Megaton) grown under red light treatment (
Matsuda et al., 2004;
Ohashi et al., 2005;
Matsuda et al., 2007). In addition, photosynthetic rate reductions observed for plants grown under red light are reportedly due to stomata being controlled more by blue light than by red light (
Sharkey and Raschke, 1981;
Zeiger, 1984;
Bukhov et al., 1996).
Red light further regulates flowering quality, quantity, and flowering duration (
Bula et al., 1991;
Tennessen et al., 1994). According to
Guo et al. (1998) and
Thomas and Vince-Prue (1996), inhibition of flowering with red light is effected by red light receptors including phytochromes (
Kelly and Lagarias, 1985). The number of visible flower buds in marigold plants was approximately five times higher when grown with fluorescent light supplemented with red LEDs, as well as under fluorescent light, when compared to monochromatic blue or red light. No flower buds formed in salvia plants when grown under monochromic blue or red light or when fluorescent light supplemented with FR light was used for marigold (
Tagetes minuta) plants.
Plants grown under canopy shade conditions or in the proximity of other plants show a range of responses to changes in R:FR ratios of ambient light. This response, known as shade avoidance or the near neighbor detection response, is characterized by an acceleration of flowering time (i.e., becoming visible within the expanded floral bud) and rapid elongation of stems and leaves (
Halliday et al., 1994;
Smith, 1994).
Kasperbauer (1988) determined that FR light reflected from neighboring seedlings increased the R:FR ratio plants received, inducing a density-dependent increase in stem length, chloroplast content, chlorophyll
a/b ratio, and CO2 fixation rate, along with decreased leaf thickness. In recent years, the effect of FR light (or a low R:FR ratio) has been intensively investigated in different plant species and development stages (
Li and Kubota, 2009;
Finlayson et al., 2010;
Mickens et al., 2018;
Park and Runkle, 2018). Supplemental FR treatments increased dry mass for many greenhouse crops during vegetative development (
Hogewoning et al., 2012;
Lee et al., 2016;
Mickens et al., 2018;
Park and Runkle, 2018), but conflicting results on leaf area were reported.
Hogewoning et al. (2012) reported no significant difference in leaf area for tomato (
L. esculentum “Mecano”) and cucumber (
Cucumis sativus “Venice”), whereas an increase in leaf area was observed for lettuce, petunia (
Petunia × hybrida), geranium (
Pelargonium × hortorum), and coleus (
Solenostemon scutellariodes) (
Lee et al., 2016;
Mickens et al., 2018;
Park and Runkle, 2018). Such differences in leaf area responses among species are still unknown and need to be addressed. For an extensive examination of FR light, the reader is referred to a recent review (
Demotes-Mainard et al., 2016).
