Light is one of the limiting factors for plant growth. To increase the PPF for plant growth, greenhouse growers must supplement solar light with electric-powered light. The most common lighting technology used to increase PPF in the greenhouse is HPS. HPS are well accepted as a result of their relatively high fixture PPF efficiency. For example, single-ended and double-ended magnetic and electronic HPS PPF efficiencies range between 0.93 to 1.85 μmol·J−1 (Nelson and Bugbee, 2013, 2014; Philips-Electronics, 2012). An alternative to HPS is the high-intensity LEDs, which currently have reportedly a PPF efficiency ranging between 0.84 and 2.3 μmol·J−1 (Nelson and Bugbee, 2013, 2014; Philips, 2014) and are projected to have a 20-fold increase on their flux per lamp output over the next decade (Haitz and Tsao, 2011).
In addition to their higher efficiencies, LED fixtures can also be built with a customized spectrum. By using different color diodes, growers have the opportunity to optimize spectra for specific growing purposes. In research using LEDs as the sole source light, plants such as peppers, wheat, lettuce, potato plantlets, Arabidopsis thaliana, soybeans, spinach, and radish grown under red light (600 to 700 nm) supplemented with blue light (400 to 500 nm) had greater growth rate and better plant development than plants grown under red light alone (Brown et al., 1995; Goins et al., 1997; Kim et al., 2005; Massa et al., 2008). Among limited information, recent studies testing LEDs as supplemental lighting have shown that the optimal electrical light spectrum for plant growth is different under sole-source lighting than for supplemental lighting For example, Hernández and Kubota (2014a, 2014b) found that plant responses to red and blue photon flux (PF) ratios of LED supplemental lighting were species-specific and dependent on background solar daily light integral (DLI). They concluded that monochromatic red supplemental lighting was preferred for the production of vegetable transplants because cucumber growth rate decreased with the increased of blue PF under low solar DLI (5.2 ± 1.2 mol·m−2·d−1) (Hernández and Kubota, 2014a).
To advance the use of LEDs as a supplemental lighting technology in greenhouses, they have to be compared with the current HPS technology in terms of plant responses and energy consumption. Limited information is available comparing HPS supplemental lighting with LED supplemental lighting in terms of plant growth and development. Currey and Lopez (2013) showed greater leaf and root dry mass on Petunia cuttings grown under LED supplemental lighting with 70:30 red:blue PF ratios compared with the cuttings grown under HPS supplemental lighting. Bergstrand and Schussler (2013) showed that Chrysanthemum plant biomass production was greater under HPS supplemental lighting than those under supplemental red:blue and white LED supplemental lighting. Limited and conflicting research reports are available comparing energy consumption between LED and HPS supplemental lighting. For example, for fresh head lettuce, Martineau et al. (2012) showed greater lettuce dry mass per electric energy input in plants grown under LED supplemental lighting than those under supplemental HPS lighting and reported a 33.8% greater electricity consumption by the HPS supplemental lighting. In tomato, Gomez et al. (2013) showed no increase in yield under supplemental LED lighting compared with the yield under supplemental HPS lighting, but reported 76% greater electrical consumption by the HPS supplemental lighting treatment compared with the LED supplemental lighting treatment. Pinho et al. (2012) reported that a small-scale experiment of supplemental lighting (1-m2 plant growing area) consumed a 40% greater electricity by the HPS lighting than the LED lighting to achieve the same PPF over the canopy. However, when simulated for a commercial greenhouse with 800 m2, HPS lighting was shown to be 44% energy-saving than LED lighting (Pinho et al., 2012).
Plant growth rate per fixture’s electric power consumption is highly correlated to the fixture’s PPF electrical efficiency (μmol·s−1·W−1 or μmol·J−1). More PPF per watt (W) often translates to greater growth rate per kWh. If a LED fixture produces greater growth rate than a HPS fixture and both have similar PPF efficiency (LED: 0.84 to 2.3 μmol·J−1, HPS: 0.93 to 1.85 μmol·J−1), two explanations are possible: 1) plant growth rate is much more enhanced by spectral optimization under the LEDs; and 2) the experimental design causes a disproportionate amount of supplemental PPF of the HPS fixture to fall outside the growing area as a result of a higher fixture density and consequently higher energy consumption than supplemental LED fixtures. In the latter case, interpretation of energy consumption should be carefully done because it presents misleading information and is biased toward LEDs.
To our knowledge, no literature is available on the comparison of HPS to LED supplemental lighting for the production of greenhouse vegetable transplants. The objective of this study is to compare supplemental LED lighting with supplemental HPS lighting in terms of plant growth and development as well as the energy consumption of the fixtures. We have selected greenhouse cucumber transplants as the model crop because cucumber is the second most produced vegetable in hydroponic greenhouses in the United States (Nanfelt, 2011). In addition, cucumber is known to be sensitive to PPF and light quality variations (Hernández and Kubota, 2014a, 2014b; Trouwborst et al., 2010a). Furthermore, cucumber transplants are commonly grown under HPS supplemental lighting in North America during the fall and winter months when solar DLI is the limiting factor for production.
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