In many plant species, stem elongation is influenced by the difference between the DIF (Myster and Moe, 1995). Stem elongation is promoted when the day temperature is higher than the night temperature (+DIF) and suppressed when the day temperature is lower than the night temperature (−DIF). The effect of DIF on plant height has been studied in many common greenhouse crops such as Easter lily (Lilium longiflorum Thunb.; Erwin et al., 1989), pansy (Viola ×wittrockiana Gams.; Niu et al., 2000), and poinsettia (Euphorbia pulcherrima Willd. ex Klotz; Berghage and Heins, 1991). For example, Erwin et al. (1989) reported that plant height of Easter lily increased by 129% as DIF increased from −16 to +16 °C. During the production of floriculture crops, a −DIF is sometimes used by greenhouse growers to control height (Myster and Moe, 1995).
In temperate climates, high-energy inputs can be required to maintain a desirable greenhouse temperature, making fuel for heating one of the largest floriculture production expenses (Bartok, 2001). Greenhouse growers can reduce energy consumption by managing the greenhouse environment with dynamic temperature control (DTC) strategies (Körner et al., 2007; Lund et al., 2006). In DTC, in contrast to static temperature control, heating set points are lowered during periods when the greenhouse energy loss factor is high (e.g., outside temperature and incoming solar radiation are low) and increased when the energy loss factor is low (Körner et al., 2004). This environmental control strategy integrates temperature and maintains a target MDT over a 1- to 7-d interval (Körner et al., 2004; Körner and Challa, 2003). Lund et al. (2006) reported that a greenhouse in Denmark using DTC had 32% to 79% and 75% to 89% lower energy consumption for heating during winter and spring months, respectively, compared with a greenhouse using static temperature set points.
To achieve the greatest potential energy savings with temperature integration, a greenhouse environmental control computer with sophisticated software (e.g., DTC) is required (Aaslyng et al., 2005). However, not all greenhouses use environmental control computers, and of those that do, relatively few use DTC strategies. An alternative and simple energy-saving approach is to use a +DIF with static day and night heating and ventilation set points. With a +DIF, the heating set point is lowered during the night when energy consumption for heating is highest (Bartok, 2001). A low night temperature is compensated for by increasing the day temperature so that the target MDT is achieved.
A DTC or DIF strategy to reduce energy consumption assumes that plant developmental rate is controlled by the integrated MDT (Summerfield et al., 1991) and crop time is similar at different day and night temperatures (within limits) that deliver the same MDT. However, studies with bedding plants that compared flowering times at DIF and constant temperatures regimens with the same MDT have reported different responses among species. For example, geranium (Pelargonium ×hortorum Bailey) grown at an MDT of 18 °C flowered similarly at day/night (12 h/12 h) set points of 18/18 or 27/9 °C (White and Warrington, 1988). In contrast, Mortensen and Moe (1992) reported that petunia ‘Ultra Red’ (Petunia ×hybrida Vilm.-Andr.) flowered 5 d earlier at a day/night (16 h/8 h) of 21/15 °C compared with 19/19 and 17/23 °C, whereas red salvia (Salvia splendens F. Sello ex Roem & Schult.) flowered 5 d later at 17/23 °C compared with 19/19 and 21/15 °C. Therefore, the benefits of using DIF to reduce energy inputs or to suppress stem elongation may not be practical for all bedding plant species if crop time is delayed. The objectives of this research were to 1) quantify the effects of constant and fluctuating temperatures on growth and flowering during the finish stage of three bedding plant species; and 2) predict greenhouse heating costs for different crop finish dates, at different locations in the United States, with different DIF regimens.
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