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Height regulation is crucial in poinsettia (Euphorbia pulcherrima) production for both aesthetics and postharvest handling. Controlled water deficit (WD) offers a potential alternative to plant growth retardants (PGRs) for poinsettia height regulation. We have previously shown that WD can be used to regulate poinsettia stem elongation. However, it is not clear how WD can be used to achieve different plant heights and how it affects aesthetic qualities such as bract size. Our objectives were to determine whether a range of plant heights can be achieved using controlled WD and to investigate possible adverse effects of WD on shoot morphology. Rooted cuttings of poinsettia ‘Classic Red’ were transplanted into 15-cm pots filled with 80% peat:20% perlite (v/v) substrate. Three target heights (43.2, 39.4, and 35.6 cm) were set at pinching (Day 27) and height tracking curves were used to monitor plants throughout the production cycle (77 days from pinching to finish). Substrate volumetric water content (u) was maintained at 0.40 m·m (a matric potential of ’–5 kPa) during well-watered conditions and reduced to 0.20 m·m (’–75 kPa) when plants were taller than desired based on the height tracking curves. Control plants were maintained at a u of 0.40 m·m throughout the study and had a final height of 51.2 cm. Plants with the 35.6-cm target height exceeded the upper limits of the height tracking curve despite being kept at a u of 0.20 m·m for 70 days and had a final height of 39.8 cm. The final plant heights in the 39.4and 43.2-cm target height treatments were 41.3 and 43.5 cm, respectively, within the 2.5-cm margin of error of their respective target heights. Relative to control plants, bract area was reduced by 53%, 47%, and 31% in the 35.6-, 39.4-, and 43.2-cm target height treatments, respectively. Our results indicate that WD can be an effective method of height control, but WD may also decrease bract size. Poinsettia height control involves careful application of height regulation without compromising plant quality. The use of PGRs is a standard practice in poinsettia height regulation. However, excessive application of PGRs can result in permanent growth suppression and subsequent stunting of poinsettias as well as reduced bract size (Faust et al., 2001; Lewis et al., 2004; Niu et al., 2002), whereas application of too little PGR may not sufficiently suppress stem elongation. Manipulating the difference between dayand nighttime temperatures (referred to as DIF) also can be an effective method of height control (Berghage and Heins, 1991; Moe et al., 1992). However, DIF is difficult to implement in warm, humid climates such as in the southeastern United States, because of the difficulty of cooling greenhouses in late summer, when shoot elongation of poinsettia is rapid. In addition, when multiple crops are grown in one greenhouse, DIF cannot be used to control growth of individual crops, because all crops are exposed to the same environmental conditions. The use of WD has been studied as an alternative means of height regulation and requires careful management to achieve desired results. Improperly regulated or excessive WD can result in poor-quality plants (Liptay et al., 1998). Barrett and Nell (1982) showed that increasing the irrigation interval reduced the height of poinsettia, but also decreased bract, leaf, and total shoot dry weight. Just like PGR application, timing of WD application is important for height regulation (Niu et al., 2002). Preferably, height regulation through WD should be used during vigorous vegetative growth, when the stem elongates most rapidly. We have previously shown that a controlled WD, reducing q to 0.20 m·m as needed, can be an effective method of regulating elongation of poinsettia (Alem, 2014). The target height of greenhouse crops, including poinsettia, is often determined by market demands or grower preferences (Clifford et al., 2004; Currey and Lopez, 2011; Fisher and Heins, 1995). The desired target height influences how much growth suppression is required during the poinsettia production cycle. Although there is a lot of information about the effects of PGR application rate, concentration, and frequency (e.g., Hammond et al., 2007; Latimer et al., 1999), there is little information about WD as a means of plant height regulation. Drought severity and frequency are known to result in different levels of growth suppression in many species such as salvia (Salvia splendens), Big bend bluebonnet (Lupinus havardii), petunia (Petunia ·hybrida) (Burnett et al., 2005; Niu et al., 2007; van Iersel et al., 2010), and petunia (Barrett and Nell, 1982). Alem (2014) recently reported that controlled WD can effectively regulate poinsettia height without compromising plant quality. In that study, a total of 15 d of WD reduced plant height by 10% (5 cm) compared with that of control plants. However, application of WD as a means of poinsettia height regulation has not been popular in the past as a result of the risk of excessive stress and plant loss. The use of soil moisture sensor-based precision irrigation systems (Nemali and van Iersel, 2006) can give growers much better control of the severity and duration of the WD, eliminating the risk of excessive drought stress. Such irrigation systems have been tested in greenhouses and nurseries (Chappell et al., 2013) and provide growers with the needed tools to control WD. We hypothesize that a range of poinsettia heights can be achieved by applying different durations of WD. To test the use of WD as a means of height regulation, the objectives of this study were to determine 1) whether different plant heights can be achieved by application of WD; and 2) how this affects plant quality. Materials and Methods Plant material and growing conditions. One hundred twenty-eight poinsettia ‘Classic Red’ rooted cuttings were obtained from a commercial greenhouse (Davis Floral, Dewy Rose, GA) on 7 Aug. 2012 and transplanted into 15-cm pots filled with an 80% peat, 20% perlite substrate (v/v) (Fafard 1P; Fafard, Agawam, MA). This cultivar was chosen because of its vigorous growth habit and the resulting need for height control. The plants were grown in a glass-covered greenhouse in Athens, GA, without supplemental lighting or black cloth application. The plants were fertigated with a water-soluble fertilizer (Peters Miracle Gro Excel 15-5-15 Cal-Mag; 15N–2.2P–12.5K; Scotts, Marysville, OH) with a nitrogen concentration of 200 mg·L. Fertilizer solution was injected into a drip irrigation system with 2 L·h pressure-compensating emitters (WPCJ10, Netafim USA, Fresno, CA) through a water-powered fertilizer injector (Dosatron D14MZ2-14 GPM, Clearwater, FL). Plants received fertilizer solution at every irrigation event. This fertilizer was chosen because it has high levels of magnesium and calcium, which are important for poinsettia growth (Bierman et al., 1990). Plant spacing was 6.5 plants/m. The greenhouse was cooled using a pad and fan system when the air temperature was Received for publication 10 Sept. 2014. Accepted for publication 5 Dec. 2014. This research was funded by the American Floral Endowment and USDA-NIFA-SCRI (award no. 2009-51181-05768). We thank Sue Dove for technical assistance, Davis Floral for the donation of the cuttings, Dosatron for the donation of the fertilizer injector, and Fafard for donation of the substrate. To whom reprint requests should be addressed; e-mail peteralem@yahoo.com. 234 HORTSCIENCE VOL. 50(2) FEBRUARY 2015 above 22 C, whereas the greenhouse heating was on at temperatures below 18 C. Temperature set points were the same for day and night. As a result of the large radiative heat load in late summer, actual greenhouse temperature on most days was well above the 22 C cooling set point during late summer and early fall. Relative humidity (HMP50; Vaisala, Woburn, MA) was measured at 5-min intervals and photosynthetic photon flux (PPF) (QSO-sun; Apogee Instruments, Logan, UT) was measured every 20 s. Both sensors were connected to a data logger (CR10X; Campbell Scientific, Logan, UT). The PPF data were used to determine the daily light integral (DLI). The DLI on sunny days decreased from 23 to 14 mol·m·d over the course of the experiment and was as low as 11 to 12 mol·m·d on cloudy days. Relative humidity averaged 68%. The plants were pinched 27 d after transplanting to a height of 22 cm (measured from the bench), leaving five to seven nodes, typical of commercial poinsettia production protocols (Faust and Heins, 1996). Plant height was monitored every 2 to 3 d on the same two plants in each experimental unit, starting immediately after pinching. Height tracking curves were developed with software from the University of Florida, Department of Environmental Horticulture (Gainesville, FL). These tracking curves are developed by entering the pinching date, plant height at pinching, target height, and the expected production duration from the time of pinching (77 d) into the spreadsheet. The software then fitted a sigmoidal curve describing target heights throughout the production period. Measured heights were plotted along with the height tracking curves to compare actual height with the target height at a particular date. When the average height of plants in a particular treatment was greater than the target height, plants were exposed to WD until their height was within the target range again. A fungicide drench, with a mixture of Subdue maxx (Syngenta, Switzerland) and 3336 (Cleary Chemical Corporation, Dayton, NJ), was applied on 5 Sept. 2012 to control soilborne pathogens. Height control. Substrate water content was maintained at 0.40 m·m when plant height was within the target range and reduced to 0.20 m·m to apply WD when height regulation was needed. A WD level of 0.20 m·m was chosen based on our previous work showing that this level of WD effectively reduces height without unacceptable effects on plant growth and quality

Use of Controlled Water Deficit to Regulate Poinsettia Stem Elongation