As a result of an increasing interest in local produce, the demand for fresh fruits and vegetables will likely increase and create opportunities for the produce industry to grow more food throughout the entire year. Already-lucrative fruit crops, such as berries, are even more highly valued when consumers can purchase produce from a local grower or market (Conner et al., 2009). California, Florida, Oregon, and Washington present the largest amount of strawberry (Fragaria ×ananassa) production in the United States, accounting for 96% of the land area (20,437 ha), with California and Florida alone making up roughly 92% (National Agricultural Statistics Service, 2017). For the population of U.S. citizens that does not live in those few states, finding high-quality, local produce can prove to be challenging, especially during the winter.
The use of heated greenhouse structures, coupled with SL, may be helpful in filling the evident demand for fresh, local produce during the coldest months of the year. Producing an edible horticulture crop using this combination of factors is called controlled environmental agriculture, or CEA (Bradford et al., 2010; Hamano et al., 2016). Although moving production of high-value crops indoors, whether greenhouse or warehouse growing, presents its own challenges (e.g., structural and electrical costs, insect and disease pressures, supplying nutrients), it also allows growers more control over climate variables and can help to close the gap of seasonally unavailable produce for local consumers.
When growing in greenhouses, extra lighting is usually necessary to supplement natural sunlight, which is typically less intense in the winter than in the summer months. Day lengths are also shorter during the cooler winter months. Traditionally, high-intensity discharge forms of SL (e.g., high-pressure sodium) have been used in greenhouses to allow for winter production, but LEDs have quickly become an alternative that growers are adopting (Park et al., 2014; Singh et al., 2015). LEDs typically produce less heat and are energy efficient, and wavelengths can be adjusted for each individual crop’s growing requirements. LEDs present greenhouse growers with ways to explore expanded production and sustainability, as well as opportunities for off-season production (Massa et al., 2008; Morrow, 2008). Individual diodes that make up LED light fixtures range in their color spectra (i.e., wavelengths), which are typically comprised of a combination of blue, red, far-red, and/or white. Specifically, the combination of red/blue and red/blue/white LEDs has been shown to increase overall photosynthetic pigments as well as the net photosynthetic rate in cherry tomatoes (Liu et al., 2011), and increased leaf area (Son et al., 2018). Sole-source (SS), single-wavelength blue LEDs (475 nm) have also been reported to increase individual strawberry size by two times on average when compared with SS, single-wavelength red LEDs alone (Magar et al., 2018).
Many cultivars of strawberry only flower and bear fruit during specific times of the year as a result of corresponding daylength. This phenomenon is referred to as the photoperiod response. However, certain strawberry cultivars produce flowers and fruit independent of daylength. Day-neutral strawberry cultivars, unlike June-bearing (i.e., short-day) cultivars, produce continuously if conditions are conducive to growth. In addition to being day neutral, these cultivars are also remontant (i.e., blooming or producing a crop more than once in a given season). This is a highly desirable trait for growers because it ensures continuous harvests throughout the entire crop growth cycle. CEA strawberry production with supplemental lighting, coupled with optimum daytime growing temperatures (20 to 24 °C) (Kimura, 2008), has been shown to increase flower production in day-neutral strawberry cultivars (Nishiyama and Kanahama, 2000). It has also been demonstrated that the use of LEDs as supplemental overhead lighting improves overall berry fruit quality [e.g., degrees Brix (°Bx), flavor, and vitamin C] and yield when applied directly to the leaf canopy and fruit within a greenhouse environment (Hanenberg et al., 2016).
Studies investigating the optimum wavelengths and ratios of supplemental LED lighting for the production of high-value crops, especially strawberries, have not been well documented (Hemming, 2011). Access to this information could present a large advantage to current and future strawberry producers, particularly with the increasing amounts of greenhouse and warehouse (i.e., SS lighting) growing that has been seen during the past few years (Cherney, 2018). Commercially available fixtures currently on the market include different ratios and wavelengths of diodes that may have impacts on strawberry growth, fruit quality, and yield.
The objective of our study was to evaluate the efficacy of three commercially available LED SL top light bars with different blue, red, and far-red wavelengths at differing PPFDs for strawberry production in a greenhouse during two off-season periods (October−December and January−March) in northern Colorado. Furthermore, we evaluated two day-neutral cultivars of strawberries under these conditions. Crop parameters measured included overall berry quality, soluble solids content (°Bx), a U.S. Department of Agriculture (USDA) strawberry fruit grade rating, fruit marketability, total yield, individual fruit size, fruit number, crown number, vegetative biomass, and stolon production. We hypothesized that with the addition of LED SL, an increase in all measured parameters would be observed. It was also hypothesized that LED light bars with greater intensities of blue and red wavelengths would increase yield and SSC of the fruit. Last, we expected the two day-neutral strawberry cultivars to respond similarly to the three supplemental LED lighting treatments.
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