Hydroponics is a common method of growing crops in controlled environment agriculture systems (Jensen, 1999). Leafy greens like lettuce are a popular choice for hydroponic production (Curran, 2018). Lettuce can potentially fetch a higher price in winter than in other seasons due to decreased field acreage and production. However, maintaining an optimal environment for lettuce in a greenhouse can be challenging with prevailing light and temperature conditions during winter in the northern regions of the Unites States. Optimal light intensity for lettuce is 12 to 15 mol·m−2·d−1 (Both et al., 1997; Kang et al., 2013; Zhang et al., 2018). The normal sunlight levels received inside a greenhouse are well below the optimal in the northern regions of the United States during winter (Korczynski et al., 2002). Although lettuce is a cool-season crop, the optimal temperature for lettuce is between 20 to 24 °C (Brechner and Both, 2013; Gent, 2016). The average outside temperature during winter in many northern regions of the United States is close to freezing (NCEI, 2020). Greenhouse temperature can only be a few degrees warmer than the outside air temperature under natural conditions.
Lettuce grows slowly in the absence of optimal light (Gent, 2014) and temperature (Dufault et al., 2009; Grahn et al., 2015) conditions. Suitable environmental conditions to grow lettuce can be maintained during the winter by the addition of the SL and artificial heating inside a greenhouse in the northern regions of the United States. However, the addition of SL and artificial heating in greenhouses can significantly increase energy costs (Aldrich and Bartok, 1989; Brumfield, 2007; Marsh and Singh, 1994). Thus, in spite of a higher sale price for lettuce, profits during the winter can be lower due to high operational costs. Therefore, productivity during the winter should increase to offset higher operational costs and to maintain profits in hydroponic lettuce production. While it is possible to increase productivity by using energy-efficient SL with optimal light composition, efficient heating methods that reduce fuel wastage, best production systems, and high-yielding cultivars, there is limited research-based information available on these factors for growers to make informed decisions.
Light emitting diode (LED)-based SL are energy-efficient and highly durable (Nelson and Bugbee, 2014), making them an ideal choice for greenhouse lighting. Recently, Kong et al. (2019) surmised that lettuce yields could increase by 17% using optimal LED composition in indoor hydroponics. They further concluded that a red-to-blue ratio of 4.5 was optimal for several lettuce cultivars in indoor production. The SL composition provided during the daytime may have a smaller effect on lettuce yield compared with that observed in indoor production. This is because sunlight composition can mask the effects of SL composition provided during the daytime (Ouzounis et al., 2015). Research indicated that SL “intensity” provided at nighttime is as effective as that provided during the daytime in increasing crop growth (Tewolde et al., 2016). However, limited information is available on the effects of SL “composition” provided at nighttime on lettuce growth.
Compared with conventional greenhouse heating during winter, methods that supply heat directly to the roots can be more energy efficient. Root temperature affects plant morphology, respiration, transpiration, water movement, and nutrient uptake in lettuce (Gent, 2016). Gerovac and Lopez (2014) showed that heat supplied to the root zone, while maintaining lower-than-optimal air temperature, can maintain plant growth and reduce the energy costs of heating in bedding plant production. It is possible to increase the temperature of roots by recirculating heated nutrient solution in liquid-based hydroponic systems. Warmer temperature of the root zone likely increases the temperature of the air surrounding shoots, even when air temperature is suboptimal. Butterhead lettuce yields were comparable between air temperatures of 17 and 24 °C when water temperature was maintained at 24 °C (Thompson et al., 1998). However, the positive effects of supplying heated nutrient solution on shoot growth may depend on the proportion of roots exposed to the solution. The extent of roots in contact with nutrient solution can vary among different PS. For example, root contact with nutrient solution is partial in the NFT, whereas roots are completely submerged inside the solution in the CFT. Therefore, the positive effects of using a heated nutrient solution on lettuce growth may vary among different PS. However, studies that have compared the effect of heated hydroponic solution on crop growth in different PS are limited.
Using a suitable PS and fast-growing cultivar can increase productivity in greenhouse hydroponics. A study that compared NFT and deep-water culture (DWC) systems in aquaponics, found that lettuce yield was higher in DWC than NFT (Lennard and Leonard, 2006). In another study, spinach grown under NFT had a higher yield than DWC, but only in the summer (Ikeda et al., 1995). For basil, DWC produced a slightly higher yield than NFT, but differences were not significant (Walters and Currey, 2015). Thus, findings from research on the best production systems for leafy greens are variable and unclear. Lettuce cultivars have shown growth differences between hydroponics and substrate culture (Assimakopoulou et al., 2013). However, available information on the growth responses of lettuce cultivars to different hydroponic PS is limited. Further, the information on the effects of solution temperature and composition of nighttime SL on cultivar productivity is limited.
The purpose of our research was to develop research-based information on the environment (light and temperature) and production (type of production system and cultivar) -related factors (described above) so that greenhouse growers could make informed decisions about their investment. Our objective was to study the interactive effects of nighttime SL spectral composition, nutrient solution temperature, PS, and cultivar on lettuce productivity in a greenhouse maintained at the suboptimal conditions that are generally observed in the northern regions of the United States during winter.
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