Sensitivity of plants to light is well-known. An important issue for LED light in horticulture concerns their economic viability. With advancing technology developments, LEDs are poised to become the light source with the highest electrical energy conversion ratio. LEDs might be used in greenhouses for lighting with selected wavelengths or for night breaks in off-season production of long-day crops [1]. The use of red LED light to power photosynthesis has been widely accepted for two primary reasons [2]. First, it is indicated that red wavelengths (600 to 700 nm) are efficiently absorbed by plant pigments; second, early LEDs were red with the most efficient emitting at 660 nm, close to an absorption peak of chlorophyll. They also saturated phytochrome, creating a high-Pfr photostationary state in the absence of FR or dark reversion. The other main wavelength included in early studies has been in the blue region (400 to 500 nm) of the visible spectrum. The source [1] describes on effects of infrared (IR) LEDs of 880 nm and 935 nm on etiolated oat seedlings. Spectroradiometric analysis of those long-wavelength sources showed that actual peak emission wavelengths averaged 916 nm and 958 nm, respectively. Compared with dark-grown controls, seedlings grown with 880 (916)-nm LEDs had shorter overall length but more advanced leaf emergence than either dark- or 935 (958)-nm-grown seedlings. Also, the proportion of mesocotyl tissue was significantly higher for seedlings grown with either IR source or dark grown, whereas the proportion of coleoptile tissue was significantly lower. An ancillary observation was that the IR LED radiation made seedlings significantly straighter and trained them to the gravity vector. The authors proposed the activation of a “gravitropism photon-sensing system” with potential involvement of phytochrome [3].

Red and blue light best drive photosynthetic metabolism, so it is no surprise that these light qualities are particularly efficient in advancing the developmental characteristics associated with autotrophic growth habits. Photosynthetically inefficient light qualities also impart important environmental information to a developing plant. For example, far-red light reverses the effect of phytochromes, leading to changes in gene expression, plant architecture, and reproductive responses. Recent evidence shows that green light also has discrete effects on plant biology, and the mechanisms that sense this light quality are now being elucidated. Green light has been shown to affect plant processes via cryptochrome-dependent and cryptochrome-independent means. Generally, the effects of green light oppose those directed by red and blue wavebands [4]. Similar effects investigated by NASA Biological Sciences research group at Kennedy Space Center, which performed several experiments with lettuce, one of the Advanced Life Support candidate crops, to evaluate the effects of green light in a controlled environment. Lettuce showed similar growth and photosynthetic rates with the addition of 5 % supplemental green light compared to the red and blue LEDs only grown plants. The addition of green light provided an aesthetic appeal of a green appearance. However, light sources with a higher fraction of green photons (> 50 % of total PPF) resulted in the reduced plant growth [5]. It is also demonstrated that under influence of dedicated combination of LED spectra, there is a development of a larger biomass of plans, e.g. [6]. The effect of LED light of different wavelengths was studied not only by using plants but also with mushroom. For instance [7] reports on the mycelial biomass production of medicinal Ling Zhi or Reishi mushroom Ganoderma lucidum. Optimum production was obtained at wavelengths between 425 and 475 nm, which correspond to the blue light, followed in order by white light, darkness, red light, and yellow light.

References

[1] Gioia D. Massa, Hyeon-Hye Kim, Raymond M. Wheeler, and Cary A. Mitchell, Plant Productivity in Response to LED Lighting, HortScience December, 43:1951-1956, 2008

[2] Mirela Maria Matioc-Precup, Dorina Cachiţă-Cosma, The Germination And Growth Of Brassica Oleracea L. Var. Capitata F. Rubra Plantlets Under The Influence Of Colored Light Of Different Provenance”, Studia Universitatis “Vasile Goldiş”, Seria Ştiinţele Vieţii Vol. 22, issue 2, 2012, pp. 193-202,  Vasile Goldis University Press, 2012

[3] Johnson, C.F., Brown, C.S.,Wheeler, R.M., Sager, J.C.,Chapman, D.K.,Deitzer, G.F. Infrared light-emitting diode radiation causes gravitropic and morphological effects in dark-grown oat seedlings. Photochem. Photobiol. 63:238–242, (1996)

[4] Kevin M. Folta1, Stefanie A. Maruhnich, Green light: a signal to slow down or stop, Journal of Experimental Botany, Vol. 58, No. 12, pp. 3099–3111, doi:10.1093/jxb/erm130, 2007

[5] Kim, H.H., Wheeler, R.M., Sager, J.C., Gains, G.D. and Naikane, J.H. 2006. Evaluation Of Lettuce Growth Using Supplemental Green Light With Red And Blue Light-Emitting Diodes In A Controlled Environment - A Review Of Research At Kennedy Space Center. Acta Hort. (ISHS) 711:111-120

[6] Duong Tan Nhut1, T. Takamura1, H. Watanabe2, K. Okamoto3 & M. Tanaka, Responses of strawberry plantlets cultured in vitro under superbright red and blue light-emitting diodes (LEDs), Plant Cell, Tissue and Organ Culture 73: 43–52, 2003

[7] Zapata, Paola A, Rojas, Diego F., Ramirez, David A., Fernandez, Carlos, Atehortua, Lucia, Effect of Different Light-Emitting Diodes on Mycelial Biomass Production of Ling Zhi or Reishi Medicinal Mushroom Ganoderma lucidum (W. Curt.: Fr.) P. Karst. (Aphyllophoromycetideae), Begell House, V.11, N 1, P 93-99, 10.1615/IntJMedMushr.v11.i1.110, 2009

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