Light-emitting Diode Light Transmission through Leaf Tissue of Seven Different Crops

Significant advances in controlled-environment (CE) plant production lighting have been made in recent years, driven by rapid improvements in light-emitting diode (LED) technologies. Aside from energy efficiency gains, LEDs offer the ability to customize the spectrum delivered to a crop, which may have untold benefits for growers and researchers alike. Understanding how these specific wavebands are attenuated by plant tissue is important if lighting engineers are to fully optimize systems for CE plant production. In this study, seven different greenhouse and field crops (radish, Raphanus sativus ‘Cherry Bomb II’; red romaine lettuce, Lactuca sativa ‘Outredgeous’, green leaf lettuce, Lactuca sativa ‘Waldmann’s Green’; pepper, Capsicum annuum ‘Fruit Basket’; soybean, Glycine max ’Hoyt’; cucumber, Cucumis sativus ‘Spacemaster’; canola, Brassica napus ‘Westar’) were grown in CE chambers under two different light intensities (225 and 420 mmol·m·s). Intact, fully expanded upper canopy leaves were used to determine the level of light transmission, at two to three different plant ages, across seven different wavebands with peaks at 400, 450, 530, 595, 630, 655, and 735 nm. The photosynthetic photon flux (PPF) environment that plants were grown in affected light transmission across the different LED wavelengths in a crop-dependent manner. Plant age had no effect on light transmission at the time intervals examined. Specific waveband transmission from the seven LED sources varied similarly across plant types with low transmission of blue and red wavelengths, intermediate transmission of green and amber wavelengths, and the highest transmission at the far-red wavelengths. Higher native PPF increased anthocyanin levels in red romaine lettuce compared with the lower native PPF treatment. Understanding the differences in light transmission will inform the development of novel, energy-saving lighting architectures for CE plant growth. Providing the optimal quality, quantity, and distribution of light in CE plant production, including bioregenerative life support systems and vertical farming applications, arguably remains the most significant hurdle in realizing the full economic and research potential of these systems. Electric lamps such as fluorescent, high-pressure sodium (HPS), and metal halide (MH) have been used for decades to grow plants in controlled environments (Sager and McFarlane, 1997; Wheeler, 2008). Typically, HPS and MH lamps are high-powered (e.g., 400 to 1000 W) and act as relative ‘‘point’’ sources that can create light distribution challenges (Cathey and Campbell, 1980). Depending on the lamp and ballast type, these arc-discharge lamps can have a relatively short operational life [e.g., less than 5000 h for very high output (VHO) fluorescent; up to 25,000 h for HPS], which translates to high operation and maintenance costs. Furthermore, these lamps have a fixed spectral output, not accounting for any degradation over time, and the spectral quality cannot, in any practical sense, be altered through the cropping cycle (Cathey and Campbell, 1980). LEDs are currently one of the most promising technologies for CE plant lighting systems (Massa et al., 2008) and especially for space crop growth applications (Barta et al., 1992; Massa et al., 2007; Mitchell, 2012; Olle and Viršile, 2013; Poulet et al., 2014). Compared with traditional arc-discharge lamps, LEDs have exceptionally long operational life expectancies (e.g., 50,000 to 100,000 h) (Yeh and Chung, 2009). Furthermore, LEDs operate at lower temperatures, emit comparably little long wave radiation, are intrinsically safer from a workplace safety perspective, and lend themselves to a wide range of system designs (Morrow, 2008; Olle and Viršile, 2013). Perhaps most important commercially, current state-of-the-art red and blue LEDs have efficiencies that approach or exceed 40% to 50%, which is a significant improvement from where they were just 10 years ago (Bourget, 2008; Haitz and Tsao, 2011). Because LEDs have a relatively narrow spectral output, use of different colored LEDs also offers the potential for creating or changing spectral combinations throughout the growth cycle (Morrow, 2008; Yeh and Chung, 2009). Light-emitting diodes are easily configured into different form factors and, for example, have been used to provide intracanopy lighting where whole plant canopies can be irradiated from both above and from within (Gómez et al., 2013; Lu et al., 2012; Massa et al., 2005; Olle and Viršile, 2013; Trouwborst et al., 2010). Such intracanopy approaches need further research to develop adaptable concepts for a variety of crops and production systems, and hence overhead lighting systems will likely remain the preferred approach for CE crop production for some time. Plants are capable of absorbing photons throughout the photosynthetically active radiation (PAR) waveband (McCree, 1971) but it is has long been understood that as a result of chlorophyll, absorption in the blue and red regions is particularly strong (McCree, 1971; Rabideau et al., 1946). Ancillary photosynthetic pigments (e.g., carotenoids) are capable of absorbing wavelengths between these red and blue bands; the energy captured by these ancillary pigments can be transferred to the photosynthetic reaction centers (Salisbury and Ross, 1992). Because other wavelengths are not as readily absorbed as red and blue, the potential exists for greater flux of these wavelengths through the upper canopy leaves (Kim et al., 2004; Moss and Loomis, 1952; Rabideau et al., 1946; Wang and Folta, 2013). Lower attenuation of these wavelengths would allow penetration deeper into the canopy where they would contribute to the overall photosynthetic capacity of the plant through their absorption by lower canopy leaves. The degree of light transmission into the canopy is determined by a range of factors including crop type, leaf age, leaf morphology, canopy architecture, and angle of incident light (Larcher, 1975; Monteith and Unsworth, 2007). Erectophile grasses, for example, have highly inclined leaves that allow light to penetrate directly, or by reflectance, deep into the canopy relative to species with more planophile leaf angle distributions (e.g., pepper) (Larcher, 1975). Although general leaf geometry (size, shape, and arrangement) is of primary importance in determining the light extinction coefficient for a plant canopy, spectral quality or composition can also have a significant influence (McCree, 1971; Monteith and Unsworth, 2007; Niinemets, 2010). This is generally not a concern in field crop production where plants are grown under broad-spectrum sunlight. When production is moved to a CE facility where light is delivered, in whole or part, by electric lighting systems, then the Received for publication 20 Oct. 2014. Accepted for publication 24 Nov. 2014. Support for this work was provided through the Florida Space Grant Consortium, Oak Ridge Associated Universities (ORAU) NASA Post-Doctoral Research Fellowship Program, Kennedy Space Center (Gioia Massa; Thomas Graham), and Orbital Technologies Corporation (ORBITEC) Madison, WI (LED supplier). Cedric Flemming was supported through the NASA CIPAIR program. We thank Larry Koss for help with plant chamber maintenance and light stand assembly. Co-first authors. To whom reprint requests should be addressed; e-mail gioia.massa@nasa.gov. HORTSCIENCE VOL. 50(3) MARCH 2015 501 MISCELLANEOUS