Greenhouse heating currently accounts for up to 30% of total operating costs for growers in northern latitudes, which is subject to increase with increasing energy costs for fuels such as propane and heating oil (Lopez and Runkle, 2014). Growers are therefore continually seeking ways to produce high-quality bedding plants through late winter and early spring with reduced energy inputs for heating, by installing thermal energy curtains, purchasing more efficient heaters or boilers, increasing insulation, or lowering air temperature set points for production (Brumfield, 2007; Runkle and Both, 2011). When MDT set points are lowered, flowering is often delayed (Blanchard and Runkle, 2011). Although growers correlate lower air temperatures to energy cost savings, these delays can often increase production costs, or lead to missed market dates (Blanchard et al., 2011b). Recent studies investigating the effects of low temperature production in greenhouses and high tunnels indicate that flowering of cold-tolerant crops, such as petunia (Petunia ×hybrida), is delayed by 19 to 32 d when air temperature is lowered by 9 to 12 °C (Blanchard et al., 2011a, 2011b; Currey et al., 2014; Gerovac et al., 2015). However, Gerovac (2014) reported that the flowering of seed propagated petunia ‘Dreams Midnight’ was only delayed by 4 d when grown at an MDT of 16 °C in combination with bench-top RZH of 27 °C compared with an MDT of 19 °C without RZH.
Petunias are one of the most prominent and popular bedding plants sold in the United States. Total wholesale value of this crop was over $262 million in 2014, with ≈25 million potted petunias sold in the top 15 producing states (USDA, 2015a, 2015b). Petunia has a reported optimum temperature (Topt) of ≈25 °C, where the rate of development is most rapid, and a calculated base temperature (Tb) of 1.5 °C, where development ceases (Kaczperski et al., 1991; Warner, 2010). Plants are generally categorized based on Tb as cold tolerant, cold intermediate, or cold sensitive, when Tb ≤ 4 °C, 4 °C < Tb < 7 °C, or Tb ≥ 7 °C, respectively (Blanchard and Runkle, 2011). A linear increase in the rate of plant development is generally observed in the range between Tb and Topt (Adams et al., 1998). Growers therefore select temperature set points within this range and adjust for desired market dates and finished plant quality. Although growing at the Topt may produce a crop most rapidly, a decrease in plant quality is often observed (Blanchard et al., 2011b). Overall, as MDT increases, rate of crop development increases, up to the Topt.
Adams et al. (1998) reported that, under natural photoperiods, petunia ‘Express Blush Pink’ grown at 22 or 25 °C flowered 43 d faster than when grown at 15 °C. Similarly, Blanchard et al. (2011b) found that under a 16-h photoperiod, TTF of petunia ‘Dreams Neon Rose’ was hastened by 19 d as the MDT increased from 14 to 26 °C. Petunias are long-day plants; therefore, the rate of development is also hastened by increasing photoperiod up to a critical photoperiod of 14.4 ± 0.6 h·d−1, although this can vary by cultivar (Adams et al., 1998).
Root-zone heating has been found to be an efficient method of increasing the rate of development for a variety of crops, including verbena (Verbena ×hybrida), petunia, poinsettia (Euphorbia pulcherrima), chrysanthemum (Dendranthema ×grandiflorum), tomato (Solanum lycopersicum), african violet (Saintpaulia ionantha), and snapdragon (Antirrhinum majus) (Brown and Ormrod, 1980; Gerovac, 2014; Janes et al., 1981; McAvoy and Janes, 1984; Olberg and Lopez, 2016; Vogelezang, 1988; Wai and Newman, 1992). For example, Gerovac (2014) reported that verbena ‘Aztec Blue Velvet’ took 18 d longer to flower in an MDT of 16 °C when grown without RZH, compared with when grown on a RZH set point of 27 °C. Bench-top RZH functions by circulating hot water through a series of rubber tubes on top of the bench. Potted crops are then placed into trays directly on the rubber tubes, allowing heat to transfer to the substrate, root system, and plant via conduction, and into the canopy by convection as heat rises from the bench, creating a microclimate with the plant canopy (Sachs et al., 1992; Vogelezang, 1988; Vogelezang and van Weel, 1989).
The effect of MDT on petunia has been extensively studied, but to our knowledge, there has been little research on the effect of increased root-zone temperature in combination with a lower air temperature on petunia cultivars and inbred lines. Commercially available, vegetatively propagated petunia cultivars were all selected based on breeder input for cold tolerance and vigorous growth. The objectives of this study were therefore to 1) quantify TTF of seven petunia cultivars and two RILs when the MDT was lowered by 5 °C and bench-top RZH was used and 2) determine if a high-quality petunia crop could be produced on RZH. Petunias, like most bedding plants, are considered high quality, when they are compact, fill the container, well branched, and in flower.
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