General introduction 13 1 will be reflected whereas the red light will be absorbed, increasing the amount of FR light in the canopy relative to other wavelengths, especially red (R). This difference in the light spectrum is detected by the plant, enabling it to perceive their neighbours and (impending) shade (Beall et al., 1996). In short, the shade consists of light spectrum change and lower light level, and in our experiments, we particularly focus on the signals perceived by plants through low R:FR light ratios. In order to outcompete the neighbouring plants, shade-avoiding plants will adopt the SAS (Ballaré and Pierik, 2017), which involves a number of phenotypic changes that generally make the plant taller and more erect. One of the most significant phenotypic SAS changes observed is stem and petiole elongation. This phenomenon was originally identified to be associated with far-red light transmission in tobacco canopies in the 1970s (Kasperbauer, 1971): the stem elongated more with a lower R:FR light ratio in the canopy than in the daylight. This phenomenon has been observed in many species. For example, in Arabidopsis, both hypocotyl and petiole elongate (Reed et al., 1993; Devlin et al., 1996) and the elongation of the petiole can be induced especially by an end-of-day FR light treatment (Reed et al., 1993). In Arabidopsis, petiole elongation can also be induced by far-red treatment to the leaf tip only (Pantazopoulou et al., 2017). In Brassica rapa, FR light sensing in the cotyledon leads to the elongation of the hypocotyl (Procko et al., 2014). In tomato, significant stem elongation and leaf architecture differences have been observed (Chitwood et al., 2015; Maloof, 2015). Suppression of branching was also found in tomato (cv. Amberley Cross) by end-of-day FR-treatment (Tucker, 1975). In soybean (Glycine max), far-red light perceived on nodes can reduce the fruit production on the very same branch (Green-Tracewicz et al., 2011). Plants detect light with various molecules. Photoreceptors are specialized proteins that absorb specific wavelengths of light, enabling plants to sense changes in light quality and quantity. Cryptochromes respond to blue and ultraviolet-A light and are involved in regulating plant growth, circadian rhythms, and photoperiodic flowering (Lin, 2002). Phototropins perceive blue light and are responsible for phototropism (bending toward light) and stomatal opening (Briggs and Christie, 2002). UVR8 is a photoreceptor that detects ultraviolet-B (UV-B) light and mediates UV-B responses, such as stress protection and regulation of pigmentation (Jenkins, 2017). Phytochromes detect red and far-red light and are involved in various processes such as seed germination, shade avoidance, and flowering (Reed et al., 1994). Phytochromes are 120-kD soluble proteins that have a covalently linked linear tetrapyrrole chromophore. They exist in two photointerconvertible forms, Pr and Pfr. Pr is the red light-absorbing form, which can be
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