Radiation within the 400 to 700 nm waveband drives photosynthesis and is referred to as photosynthetically active radiation (PAR). By definition, all wavelengths within this range are considered to stimulate photosynthesis equally. However, McCree (1972) produced a relative quantum efficiency (QE) curve between 350 and 750 nm based on the photosynthetic activity of 22 crop species. The relative QE (RQE) curve has a primary peak at 620 nm and a secondary peak at 440 nm, which establishes that red (R; 600–700 nm) and blue (B; 400–500 nm) wavebands are more efficient in eliciting a photosynthetic response than wavelengths between 500 and 600 nm (green and yellow light). The peak RQE of R light is 30% higher than the B peak, and R light from 600 to 640 nm has the highest quantum yield (Evans, 1987; Inada, 1976; McCree, 1972).
Physiologically, the different QEs of PAR are because of the absorption spectra of plant pigments and the overexcitation of photosystem I (PSI), compared with photosystem II (PSII). Photosynthetic photons stimulate the excitation of PSI and PSII and the ratio of absorbed photons (≥580 nm) between the photosystems influences the RQE (Hogewoning et al., 2012). Hogewoning et al. (2012) grew cucumber (Cucumis sativus) plants under an artificial sunlight spectrum, an artificial shade spectrum [greater far-red (>680 nm) light], and under B LED (peak = 445 nm) light (PPF = 100 μmol·m−2·s–1; 16-h photoperiod). The highest QE recorded was R light between 620 and 640 nm. Similar to McCree (1972), the quantum yield was 70% of the maximum between 427 and 560 nm because of the lower absorbance of these wavelengths and the lower QEs. The overexcitation of PSI and greater QE occurred in cucumber under artificial shade, whereas PSII was overexcited with greater QE in plants grown under artificial sunlight and B light (Hogewoning et al., 2012). In contrast to Emerson et al. (1957), Hogewoning et al. (2012) also concluded that a combination of wavelengths within PAR could increase quantum yield and thus, plant growth.
Although R light can be the most effective in stimulating carbon fixation in photosynthesis, plants accumulate biomass faster and have a normal morphology with the addition of B light, G light, or both (Eskins, 1992; Kim et al., 2004). In some instances, the addition of green light can also increase plant biomass accumulation. Lettuce (Lactuca sativa) plants accumulated more biomass with the addition of up to 24% green light (510–610 nm) from green fluorescent bulbs or LEDs when the PPF was 150 μmol·m−2·s–1 (Kim et al., 2004). Plants grown under R light alone can develop abnormal morphological traits, such as in lettuce, where hypocotyls were elongated (Hoenecke et al., 1992). In addition, pepper plants (Capsicum annuum) grown under sole R light or less than 10% to 15% B light developed edema, a physiological disorder (Massa et al., 2008).
The emission of narrow-waveband light by LEDs provides the opportunity to test the effects of specific wavebands of light on plant growth and development. LEDs used for sole-source photosynthetic lighting can enable commercial growers to produce plants with desired characteristics and to optimize the spectra for each crop and stage of development (Folta and Childers, 2008; Stutte, 2009). LEDs are well-suited for commercial plant production due to their improving energy efficiency, spectral specificity, and longer lifetimes than the current industry standard lamps (e.g., fluorescent and high-pressure sodium) (Bourget, 2008; Morrow, 2008). LEDs emitting photons with greater RQEs could increase photosynthesis (Stutte, 2009) and potentially decrease commercial plant production time and costs compared with less efficient wavelengths of light.
To our knowledge, no studies have been published that compared the effect of different wavebands of R light on plant growth. Conclusions and interpretations from photosynthesis research using different R wavelengths may not necessarily apply to plant growth over time, because plants acclimate to their light environment. We grew seedlings of ornamental plants at two PPFs under different ratios of R (peak = 634 nm) and HR (peak = 664 nm) light, as well as under orange light (peak = 596 nm), to determine whether a particular wavelength or a combination of wavelengths of R light increased plant growth. We postulated that growth attributes of young plants would be similar under the same PPF as long as equal amounts of background B and G were provided in all treatments.
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