Managing the microclimate immediately surrounding plants and products—composed of temperature, light, humidity, and other conditions—is fundamental to horticultural production, postharvest handling, and research. Microclimate management often involves the use of plastics, for example, increasingly deployed as the cover of low and high tunnels (Lamont, 2005; Wells and Loy, 1985, 1993; Wittwer and Castilla, 1995). Low- and high-tunnel use are popular because they can raise farm productivity and profit potential through direct and indirect effects on crops and crop management (Carey et al., 2009; Lamont, 2005; Waterer, 2003; Wells and Loy, 1993). Low and high tunnels tend to reduce crop stress and often increase yield relative to the uncovered condition. Low and high tunnels differ in size and scope of application but share operational principles of heat and humidity retention, shading and light dispersion, and wind mitigation, especially of aerial environments.
Active root-zone or floor heating is common in year-round greenhouse systems (Elwell et al., 1985; Sachs et al., 1992; Shedlosky and White, 1987; Wai and Newman, 1992) but less common in low- and high-tunnel systems. In greenhouse systems, root-zone heating is thought to maintain or improve ornamental and vegetable plant growth and quality and lower aerial heating costs (Janes and McAvoy, 1983; Sachs et al., 1992; Shedlosky and White, 1987; Trudel and Gosselin, 1982). Relative to shoot-zone heating and regardless of setting, root-zone heating may also alter broad aspects of crop physiology including root growth, growth regulator production, transport and/or activity, water and nutrient uptake, and photosynthate allocation (Bowen, 1991; Cooper, 1973; Li et al., 1994; Macduff, 1989). As in greenhouse systems, integrating root- and shoot-zone heating in low- and high-tunnel systems may create microclimates that promote specific relationships between primary and secondary metabolism that influence yield, quality, profitability, and sustainability.
Unlike in many greenhouses, temperature modification in low and high tunnels is often a comparatively crude, localized, passive, and sunlight-dependent process unaccompanied by supplemental lighting (Nair and Ngouajio, 2010; Soltani et al., 1995; Waterer, 2003; Wells and Loy, 1985, 1993). Active heating within high tunnels, if used at all, typically is applied intermittently to elevate shoot-zone temperatures, especially for low-temperature protection (Lamont, 2005; Lamont et al., 2003). Moreover, low and high tunnels are routinely deployed in areas characterized by dynamic within- and across-season fluctuations in sunlight and temperature levels. Therefore, it is reasonable to suspect that greenhouse studies, especially those featuring root-zone heating, may have yielded an understanding of temperature effects upon which low- and high-tunnel users cannot rely exclusively, particularly if their system involves a short-statured, quick-cycling crop grown during fall and spring seasons. We set out to strengthen the record of temperature effects in low- and high-tunnel systems by employing low tunnels, a high tunnel, and root-zone-heating cables (alone and in combination) and multipronged data collection during spring- and fall-time production of leaf lettuce in 2008–10.
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