The commercial production of ornamental greenhouse crops demands that plants be accurately scheduled for predetermined market dates (Heins et al., 2000). The ability to schedule crops requires knowledge of how environmental factors (e.g., temperature and light) influence plant growth and development. The rate of plant development is controlled by the mean daily temperature (MDT) of the apical meristem (e.g., shoot tip) (Faust and Heins, 1993; Niu et al., 2001; Roberts and Summerfield, 1987). For example, as shoot-tip MDT increased from 16 to 24 °C, flower developmental rate (1/days to flower) in campanula (Campanula carpatica Jacq.) increased from 0.018 to 0.024 (Niu et al., 2001). Although plant temperature controls developmental rate, many growers use greenhouse air temperature (dry bulb) to predict crop timing. However, depending on environmental conditions, plant temperature can differ considerably from air temperature. For example, Faust and Heins (1998) reported that shoot-tip MDT in vinca (Catharanthus roseus L.) grown at 15 and 20 °C was within 2 °C of the air MDT, whereas plants grown at 35 °C had a shoot-tip MDT 4 to 6 °C below the air MDT.
Plant temperature is determined by transpiration and the convective and radiative transfer of energy between the plant and the surrounding environment and can be calculated with an energy balance equation (Nobel, 2005). Factors that determine plant temperature include the total radiation absorbed by the plant, emitted LWR (3,000 to 100,000 nm), convection, and transpiration (Nobel, 2005). Plant energy balance models have been developed that predict shoot-tip temperature under different greenhouse conditions (Faust and Heins, 1998; Shimizu et al., 2004). During the day, SWR (300 to 3000 nm) and transpiration have the largest influence on energy transfer between the plant and the surrounding environment (Faust and Heins, 1998). During the night, convection and the transfer of LWR between the plant and the greenhouse structure as well as the radiative sky conditions (e.g., clear or cloudy sky) often have the greatest influence on plant temperature.
The exchange of LWR between plants and the surrounding environment is influenced by the temperature and emissivity of radiating surfaces. In temperate climates during winter nights, outside temperatures are low and the greenhouse glazing temperature can be considerably lower than the inside air temperature. As glazing temperature decreases, LWR emitted by the glazing material decreases, and LWR emitted by the plant can become greater than the incoming LWR. This net loss of LWR can cause plant temperature to drop below the air temperature. For example, as glazing temperature at night decreased from 2 to 16 °C below air temperature, vinca shoot-tip temperature decreased from 1 to 5 °C below air temperature (Faust and Heins, 1998).
In response to volatile fuel costs, some commercial growers have made greenhouse structural improvements to reduce energy losses such as the installation of energy curtains (Dieleman and Kempkes, 2006; Lund et al., 2006). Energy curtains are typically extended over a greenhouse crop from sunset to sunrise and retracted during the day (Bailey, 1988; Lund et al., 2006). An energy curtain that has a closed-weave construction (low air permeability) and seals tightly with the greenhouse sidewalls creates a barrier between the heated and unheated space above and below the curtain, respectively (Öztürk and Basçetinçelik, 1997; Zabeltitz and Meyer, 1984). Energy requirements for greenhouse heating can decrease with an energy curtain because the curtain material reduces heat transfer and reduces the air volume that must be heated.
Another potential benefit of energy curtains is an increase in LWR absorbed by the crop (Kittas et al., 2003). During the night, the interior surface temperature of an energy curtain is often higher than the glazing temperature. Therefore, when a curtain is extended over a crop, plants receive more LWR than without a curtain. The higher LWRnet exchange can increase plant temperature. For example, a rose (Rosa hybrida L.) crop grown under a greenhouse energy curtain with 75% SWR transmission had 100% higher absorbed LWR and 1 to 3 °C higher canopy temperature at night than a crop grown without a curtain (Kittas et al., 2003). Studies with other crops such as African violet (Saintpaulia ionantha Wendl.), tomato (Lycopersicon esculentum Mill.), and vinca have also reported higher canopy, leaf, or shoot-tip temperatures under curtains compared with no curtains (Bailey, 1977, 1981a; Faust, 1994).
The objective of this study was to quantify the effect of different curtain materials on shoot-tip temperature of New Guinea impatiens (Impatiens hawkeri Bull.) during cold nights when outside temperatures were near or below freezing. New Guinea impatiens was selected because it is among the top 10 bedding plants produced in the United States with a reported wholesale value of $51 million in 2010 (U.S. Department of Agriculture, 2011). In addition, because plant development of New Guinea impatiens is delayed considerably at a low temperature (Erwin, 1995), quantification of how curtains influence shoot-tip temperature could improve production scheduling, energy efficiency, and management of the greenhouse environment.
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