In the New England region, high tunnels are typically used to lengthen the growing season for warm-season crops such as tomato (Solanum lycopersicum L.) (Carey et al., 2009; Fitzgerald and Hutton, 2012; Hunter et al., 2012; Knewtson et al., 2010; Lamont, 2009; O’Connell et al., 2012). However, 38% of Maine growers surveyed by Fitzgerald and Hutton (2012) reported using high tunnels for crop production during the winter months and a 2016 survey of 110 high tunnel producers in northern New England revealed that 41% used their tunnels for production of greens during the winter months (R.G. Sideman, unpublished data). Although there are few reports of this production practice in the scientific literature, fall planting of cold-tolerant vegetable crops in unheated high tunnels for harvest and sales throughout the fall and winter months is increasingly commonplace in the Northeast. Thus, maximizing tunnel production during fall, winter, and early spring, when consumer demand for local produce is high (Carey et al., 2009; Wien and Pritts, 2009), can assist growers in obtaining premium pricing (Bjelić and Moravčević, 2006; Gent, 1991) and maintaining their customer base during the off-season.
Spinach (S. oleracea L.) is one of the most popular crops for winter high tunnel cultivation (Carey et al., 2009; Fitzgerald and Hutton, 2012; Knewtson et al., 2010; Lamont, 2009; Lamont et al., 2003). Although the optimal temperature range for spinach growth is 12 to 15 °C (Bjelić and Moravčević, 2006), spinach has a high tolerance to chilling damage (Guy and Haskell, 1987; Nobel, 1974; Tamura, 2002) and has the ability to gain freezing tolerance through acclimation to low temperatures (Fennell and Li, 1985). Fennell and Li (1985) showed that the LT50 of four spinach cultivars was about −6.5 °C (LT50) before acclimation but decreased to −12 to −17 °C (LT50) (depending on cultivar) after 2 weeks of low temperature acclimation.
Regional producers report winter-grown spinach to be sweeter than summer-grown spinach (Blomgren and Frisch, 2007), and controlled experiments show that SSC and soluble sugar levels in leaves increase with exposure to low temperatures (Gent, 2016; Guy et al., 1992; Ito et al., 2013, 2015; Proietti et al., 2009). In the temperature range of 16.1 to 23.5 °C, Gent (2016) showed that sugar content within petioles increased when temperatures dropped below 20 °C and Proietti et al. (2009) report that after 7 d at 10 °C, soluble sugar content (glucose, fructose, and sucrose) of spinach leaves increased by 51%. Furthermore, Ito et al. (2013, 2015) demonstrated that SSC increases faster with exposure to colder temperatures and showed that levels significantly increased in only 4 d of root exposure to 4 °C, vs. 7 d of exposure to 14 °C. In addition, Guy et al. (1992) report that free sugar content (glucose, fructose, and sucrose) increased by 10 to 20-fold after 14 d at 5 °C and that sucrose phosphate synthase, the primary biosynthetic enzyme of sucrose, was more abundant at 5 °C than 10 °C.
A primary driver in such increases in SSC is osmotic adjustment. Under cold conditions, plants enter a state of low temperature–induced dehydration and through the synthesis of soluble solids, osmotically adjust to ensure turgor pressure and normal plant processes are maintained (Taiz and Zeiger, 2002). Sugars, especially, are a primary compatible solute used for the protection of protein structure and function (Bohnert and Shen, 1999) and chloroplast membranes during freezing conditions (Levitt, 1980). Thus, the production of sugars and other compatible solutes have a cryoprotective effect on plants (Guy et al., 1992; Levitt, 1980; Taiz and Zeiger, 2002).
Studies of off-season spinach production in high tunnels are limited and are geographically restricted to the midwestern, western, and Pacific northwestern United States (Borrelli et al., 2013; Ernst et al., 2012; Knewtson, 2008). Other studies have focused on the practice of direct seeding before hard frost for early spring production (Bjelić and Moravčević, 2006), a strategy that does not address winter-long production. We are not aware of any published research on the effects of PD, cultivar, and temperature on yield and SSC of spinach grown in this production system in the northeastern United States.
Our objectives in this study were to understand the effect of fall PD and cultivar on yield and SSC of spinach leaves. Planting dates and cultivars with greater yields may provide added revenue for producers, and practices that increase SSC and sweetness will likely add to the consumer perception of exceptional eating quality.
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