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 day- and 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 θ to 0.20 m3·m−3 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.
Alem, P. 2014 Irrigation, fertilization and non-chemical plant growth regulation in greenhouse production. PhD diss., Univ. of Georgia, Athens, GA
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