Spectral Effects of Artificial Light on Plant Physiology and Secondary Metabolism: A Review

By Theoharis Ouzounis, Eva Rosenqvist, C. Ottosen. 2015

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With the expeditious development of optoelectronics, the light-emitting diode (LED) technology as supplementary light has shown great advancement in protected cultivation. One of the greatest challenges for the LED as alternative light source for greenhouses and closed environments is the diversity of the way experiments are conducted that often makes results difficult to compare. In this review, we aim to give an overview of the impacts of light spectra on plant physiology and on secondary metabolism in relation to greenhouse production.We indicate the possibility of a targeted use of LEDs to shape plants morphologically, increase the amount of protective metabolites to enhance food quality and taste, and potentially trigger defense mechanisms of plants. The outcome shows a direct transfer of knowledge obtained in controlled environments to greenhouses to be difficult, as the natural light will reduce the effects of specific spectra with species or cultivar-specific differences. To use the existing highefficiency LED units in greenhouses might be both energy saving and beneficial to plants as they contain higher blue light portion than traditional high-pressure sodium (HPS) lamps, but the design of light modules for closed environment might need to be developed in terms of dynamic light level and spectral composition during the day to secure plants with desired quality with respect to growth, postharvest performance, and specific metabolites. Plants are capable of perceiving and processing information from their biotic and abiotic surroundings for optimal growth and development (Fankhauser and Chory, 1997). Light is one of the most important environmental cues that affect the developing plant and regulate its behavior (Whitelam and Halliday, 2007). Since plants are sessile, they need to be particularly plastic in response to their light environment. The diverse responses of plants to light entail sophisticated sensing of its quantity (fluence rate), quality (wavelength, i.e., color), direction, and duration (photoperiod) (Christie et al., 1999; Fankhauser and Chory, 1997). In many greenhouses in northern climates, supplemental lighting is needed from fall to spring to secure plant growth and development, as well as to obtain year-round high production and good quality plants. To date, the quality of most herbs and vegetable crops grown during the winter is far from the quality of summergrown crops. In commercial practice, greenhouse plants are supplied with supplementary light for up to 16–20 h per day and the light intensity ranges between 100 and 200 mmol·m·s (Paradiso et al., 2011), but lower levels are used for shade adapted species. Further north in Scandinavia, commercial installations of 300–500 mmol·m·s are used for tomatoes (Lycopersicum esculentum) (M. Verheul, personal communication). The predominant greenhouse lighting sources are the HPS lamps because of their high efficiency (1.9 mmol·m·W) in converting energy into photosynthetically active radiation (PAR) (van Ieperen and Trouwborst, 2008). However, they are neither spectrally nor energetically optimal. They emit most radiation in the yellow and orange region with some red between 550 and 650 nm, and only around 5% in the blue region between 400 and 500 nm (Sager and McFarlane, 1997). Because of the low levels of blue light fraction and other photosynthetic sensitive wavelengths, they are not the most efficient light sources in terms of light quality (Marcelis et al., 2006). A recent review by Nelson and Bugbee (2014) claims lack of economic benefits of LED units comparing mainly U.S. produced lamps, where all tested HPS and LED units had a photon efficiency #1.7 mmol·W. In northern Europe (UK and Denmark), independent measurements have shown efficiencies of commercially available Dutch and Danish LED fixtures of 2.2–2.4 mmol·W, whereas the newest HPS (1000 W) show up to 2.1 mmol·W (C. Dam-Hansen and S. Pearson, personal communication). Their light distribution is equal to or better than that of HPS lamps, so they are fully implementable in commercial scale. With the current energy prices, the payback time is now realistic— especially as some LED fixtures allow dynamic output with respect to intensity and spectra. LEDs and Use of Spectra Manipulation of the light spectrum of the lamps could trigger potential benefits by enhancing plant growth (Carvalho and Folta, 2014), but the HPS lamps do not provide the possibility for spectral manipulation or even dimming. As a consequence, the LED technology has emerged and developed rapidly in the past decades as alternative light sources (Massa et al., 2008). LEDs are solidstate and durable light sources providing a narrow spectrum of light (Stutte et al., 2009) in the range from ultraviolet to infrared. Their lifetime could reach up to 100,000 h, in comparison with the HPS lamps with a lifetime ranging from 10,000 to 20,000 h (Bourget, 2008; Morrow, 2008; Sager and McFarlane, 1997). As LED use in greenhouses is developing, the prices are expected to gradually decrease and there has been a renewed interest in the use of LEDs as a tool in greenhouse research (Folta and Childers, 2008). Because of its higher frequency and hence shorter wavelength, blue light contains more energy than red according to E = hn = hc/l, where E is the energy content of the photon (J), h is Planck’s constant (6.626 · 10 Js), n is the frequency (s) of the light wave, c is the speed of light, and l is the wavelength (nm). Blue light is therefore more expensive to produce than red light. The use of blue and red LEDs has been the prime selection for producers as these wavelengths are efficiently absorbed by the primary plant pigments (chlorophylls), with red light being the most energy efficient in LED production. Both blue (420–450 nm) and red (600–700 nm) lights are absorbed by chlorophyll a (Chl a) which has its absorption peaks at 430 and 665 nm and chlorophyll b (Chl b) at 453 nm and 642 nm (Sager and McFarlane, 1997) (as shown in Fig. 1A). Although chlorophylls are the primary photosynthetic pigments, other accessory plant pigments such as carotenoids (Fig. 1A) and anthocyanins are capable of harvesting light. These accessory pigments work in conjunction with chlorophylls, which transfer light to the photosystems, dissipate Received for publication 17 Mar. 2015. Accepted for publication 1 June 2015. Corresponding author. E-mail: coo@food.au.dk. 1128 HORTSCIENCE VOL. 50(8) AUGUST 2015 excess light energy, or work as antioxidants. As the chlorophyll and nonchlorophyll pigments have different absorption spectra, the result is a composite absorption spectrum that is broadened such that a wider range of radiation is absorbed by plants (Davies, 2004) (Fig. 1B). The use of LED luminaries has the potential of passing significant energy savings to greenhouse growers. If this leads to economical savings, taking the investments in new lamps into consideration will depend on the energy prices in individual countries. The LED technology has yet to be fully integrated within the greenhouse control system and should be optimized in terms of light output and distribution, whereas LED luminaire cost should be reduced to reach a sustainable and economically viable production (Morrow, 2008). In addition, LEDs emit less heat compared with the HPS lamps and often not in a downward direction. Consequently, the temperature inside the greenhouse might be affected depending on the design of the heating system and how much the heat radiation from the HPS lamps contributes to the canopy temperature of the crop. Based on unpublished data (C-O. Ottosen, personal communication), this varies between greenhouse and heating systems. Adjustments of air temperature may be needed to keep leaf temperature at the same degree and moving away from the fluctuation in heat radiation when the HPS lamps are turned on and off might require a change in screen use and ventilation strategies. For crops with lower leaf temperature requirements, LED lighting might actually be an advantage due to less heat load from the lamps and better humidity management due to reduced leaf temperature fluctuation. Light quality, quantity, and duration are three parameters that concurrently influence plant growth (Casal and Yanovsky, 2005; Chen et al., 2004; Folta and Childers, 2008). The photosynthetic photon flux (PPF) density (PPFD) represents the amount of photons (mmol·m·s) that is used for photosynthesis within the 400–700 nm waveband of PAR, where the most important wavelengths for photosynthesis are in the blue and red region. The ‘‘gap’’ with very low light absorption in the range 500–600 nm in isolated photosynthetic pigments (as shown in Fig. 1A) has disappeared in the intact leaf (as shown in Fig. 1B). This is because of light scattering in the leaves caused by the water–air interfaces between the water saturated cell walls and the air of the intracellular spaces. The light scattering increases the probability of absorption drastically, which is demonstrated if a leaf is vacuum infiltrated by, e.g., water (as shown in Fig. 1C) (DeLucia et al., 1996). The consequence is that green light absorbed more by intact leaves than by chlorophyll and carotenoids in solution. The light absorption only show a dip in the green part of the spectrum in fresh leaves (transmitting 13% in the range 500–600 nm in Chrysanthemum morifolium), which would be more pronounced (transmitting 36%) if light scattering did not take place (as shown in Fig. 1C). The absorption spectrum in chlorophyll also means that the colors penetrate differently into the leaf. Blue and red are efficiently absorbed close to the surface, whereas green light contributes more to photosynthesis in deeper leaf layers (Brodersen and Vogelmann, 2010; Sun et al., 1998), which will decrease the potentially negative effect of the internal light gradient within the leaf. The light absorption in leaves represents absorption in all pigmen

Subfields: Plant Science

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