One of the environmental concerns in the production of potted plants is the use of plant growth retardants (PGRs) for the control of plant height. In the search for alternatives to PGRs, changes in light quality [e.g., increased red to far-red ratio (R:FR ratio)] have been shown to limit elongation growth (Khattak and Pearson, 2006; Mortensen and Strømme, 1987; Rajapakse and Kelly, 1992). However, the effect of changes in light quality is not always positive. A decrease in the R:FR ratio at the end of the day occurs naturally during twilight. This has been shown to increase stem elongation in different plant species (Blom et al., 1995; Blom and Kerec, 2003) comparable to end-of-day far-red (EOD-FR) treatments.
The photoreceptor, phytochrome, is responsible for the physiological responses incited by changes in red (600–700 nm) and far-red (700–800 nm) light. The phytochrome molecule exists in two photoreversible states, Pfr and Pr. Irradiation with high levels of far-red light increases the proportion of the molecule in the Pr state, whereas a high amount of red light increases the proportion of the Pfr form. Light quality determines the R:FR ratio and phytochrome photoequilibrium (ϕ), which in turn determines plant morphology (Holmes and Smith, 1977). In Arabidopsis, phytochrome is represented by five members: phytochrome A (phyA) to phytochrome E (phyE) (Quail, 2002; Smith, 2000). Phytochrome B is assumed to play a role at all stages of the life cycle (Smith, 2000), including the increase in stem elongation in response to EOD-FR treatments (Smith, 2000; Smith and Whitelam, 1990).
Under outside conditions, the R:FR ratio changes from ≈1.15 during daylight to 0.7 during twilight in the evening (Holmes and Smith, 1977). Outside measurements at lat. 42°N (Feb.1) showed that the R:FR ratio was 1.37 at official sunset with photosynthetic photon flux (PPF) of 3.8 μmol·m−2·s−1, whereas R:FR decreased to 0.69 with PPF 0.06 μmol·m−2·s−1 (near darkness) over a period of ≈30 min (Blom et al., 1995). The twilight period depends on the time of the year and latitude. Near summer and winter solstices at lat. 56°N, it lasts for up to 54 and 44 min, respectively, whereas year-round twilight lasts for ≈20 min near the equator (List, 1971). Eliminating twilight using black-out curtains in experiments during early spring in Canada (lat. 42°N) resulted in a decrease of 10% to 25% in height of easter lilies compared with ambient (Blom et al., 1995; Blom and Kerec, 2003). A similar experiment in Norway (lat. 60°N) with chrysanthemum and tomato had no or only a small effect on plant height (Mortensen and Moe, 1992). The latter experiment was done during summer with photoperiods varying between 12 and 18.5 h. Knowledge from experiments with EOD-FR is useful in understanding the effect of twilight, and EOD-FR response has been suggested to be the most effective in increasing the stem elongation rate during short photoperiods (Downs et al., 1957; Vince-Prue, 1977).
The effects of twilight on elongation are not clear and might depend on factors like the season and irradiance. Photoreceptors serve not only as quality detectors, but also as photon counters (Smith and Whitelam, 1990). Therefore, we wanted to test whether small differences in irradiation for the same ratios of R:FR would affect plant growth and/or development.
The objective of this study was to investigate whether EOD light quality as found during twilight affects growth and developmental characteristics of potted chrysanthemums and whether elongation depends on R:FR alone or on the level of irradiance of R and FR as well. For this purpose, it was decided to create artificial twilight using light-emitting diodes (LEDs) in growth chamber experiments. Light-emitting diodes give the possibilities of small and precise changes in intensity and spectral distribution of the EOD light treatment.
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