Abstract
Despite decades of research, the effects of spectral quality on plant growth, and development are not well understood. Much of our current understanding comes from studies with daily integrated light levels that are less than 10% of summer sunlight thus making it difficult to characterize interactions between light quality and quantity. Several studies have reported that growth is increased under fluorescent lamps compared to mixtures of wavelengths from LEDs. Conclusions regarding the effect of green light fraction range from detrimental to beneficial. Here we report the effects of eight blue and green light fractions at two photosynthetic photon fluxes (PPF; 200 and 500 μmol m-2 s-1; with a daily light integral of 11.5 and 29 mol m-2 d-1) on growth (dry mass), leaf expansion, stem and petiole elongation, and whole-plant net assimilation of seven diverse plant species. The treatments included cool, neutral, and warm white LEDs, and combinations of blue, green and/or red LEDs. At the higher PPF (500), increasing blue light in increments from 11 to 28% reduced growth in tomato, cucumber, and pepper by 22, 26, and 14% respectively, but there was no statistically significant effect on radish, soybean, lettuce or wheat. At the lower PPF (200), increasing blue light reduced growth only in tomato (41%). The effects of blue light on growth were mediated by changes in leaf area and radiation capture, with minimal effects on whole-plant net-assimilation. In contrast to the significant effects of blue light, increasing green light in increments from 0 to 30% had a relatively small effect on growth, leaf area and net assimilation at either low or high PPF. Surprisingly, growth of three of the seven species was not reduced by a treatment with 93% green light compared to the broad spectrum treatments. Collectively, these results are consistent with a shade avoidance response associated with either low blue or high green light fractions.
Highlights
Photobiology research began more than 200 years ago [1] with studies using primitive light sources and colored filters [2]
But not all, species was significantly better in the multiple wavelength treatments than in narrow band blue, green, or red light
At the lower photosynthetic photon flux (PPF), dry mass was reduced by 41% in tomato, but the effects of blue light (BL) on the other species were less than 6% and were not statistically significant
Summary
Photobiology research began more than 200 years ago [1] with studies using primitive light sources and colored filters [2]. Goins et al [9] used identical light treatments to Yorio et al [7] and found that dry mass of wheat with 10% BL was comparable to a white light control (33% BL) These results suggest that BL responses may be species dependent, but the effects of red/blue ratios and interactions with intensity remain unclear. With the exception of Son and Oh [16], these recent studies confirm early studies by Bula et al [17] and Hoenecke et al [18] who found that it was necessary to supplement red LEDs with blue fluorescent light to achieve highest dry mass in lettuce. Supplementing fluorescent lamps with either RL or BL generally increased lettuce dry mass at the same photosynthetic photon flux (PPF), but the studies were conducted at a PPF of only 135 μmol m-2 s-1, which resulted in unusually slow growth in all treatments
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