Poinsettias are one of the most economically valuable pot plants in the United States and around the world (Trejo et al., 2012; U.S. Department of Agriculture, 2014). In 2013, poinsettias had a wholesale value of over $146 million in the United States (U.S. Department of Agriculture, 2014). Height control is important for the production of marketable, compact poinsettias (Fisher and Heins, 1995; Heins et al., 1999). Poinsettia height control is also important for transportation and postharvest handling (Karlović et al., 2004; Niu et al., 2002). Optimal poinsettia height may vary depending on cultivar, intended use, and grower or consumer preference. To control poinsettia height, growers typically use PGRs to suppress stem elongation. PGRs reduce stem elongation by antagonizing or inhibiting biosynthesis of gibberellins (Brown et al., 1997; Lodeta et al., 2010).
Although widely used in poinsettia production and effective at suppressing elongation (Ecke et al., 2004), use of PGRs also has disadvantages. Apart from adding to the cost of production (Mata and Botto, 2009), PGRs are among the agrochemicals that can contribute to environmental pollution (Berghage and Heins, 1991). Because of their pollution potential, the use of PGRs has restrictions in some countries (Moe et al., 1992a). Also, if applied in excess, PGRs negatively affect plant quality and growth through phytotoxicity (Gibson et al., 2003) and stunting (Hamid and Williams, 1997).
Several nonchemical methods of height control have been described. Previous work has shown the possibility of controlling plant height by reducing temperature or by manipulating night and daytime temperatures (Berghage and Heins, 1991; Moe et al., 1992a). However, lowering temperature also reduces photosynthesis and metabolic processes, including growth rate, which can delay the crop maturity (Moe et al., 1992b). In addition, it may be difficult to cool the greenhouse enough during the day to reduce the temperature below nighttime temperatures, which may be needed to get effective height control. This is especially difficult in warm climates and in mid to late summer, when the radiative heat load is large. Other studies have shown that manipulation of light quality can be used to control poinsettia growth (Cockshull et al., 1994; Mata and Botto, 2009). However, manipulating light quality in greenhouses is not yet common practice. Low amounts of phosphate fertilizer also can promote more compact growth (Nelson et al., 2012), but there are no guidelines on how to manipulate fertilization practices to produce plants with a specific height.
The use of WD to control plant growth is not new (Hendriks and Ueber, 1995). However, it has been difficult for growers to control the severity of WD, and thus the impact on growth. If the WD is not severe enough, there may be little impact on stem elongation, while severe levels of WD can negatively affect plant quality. With the advent of precision irrigation systems such as those controlled by soil moisture sensors (Kohanbash et al., 2013), there is a potential for successful use of controlled WD to control stem elongation. Such irrigation systems can maintain specific θ levels to impose a controlled WD. We have previously shown that the combination of height tracking with regulated WD can be used to control poinsettia elongation (Alem et al., 2015).
The use of WD to control stem elongation is based on the role of water in cell expansion and growth. Water is needed for turgor pressure, which drives cell expansion and stem elongation. Suppressions of growth due to drought stress may also occur as a result of changes in cell wall properties, such as cell wall extensibility and the minimum turgor required for cell expansion (van Volkenburgh, 1999). Hence, regulated WD can be used to control plant height (Cameron et al., 2006). This technique is inexpensive and not likely to cause plant damage if managed carefully. In addition, using WD for plant height control is environmentally friendly and eliminates potential pollution caused by PGRs. Plants grown under controlled WD may also be more acclimated to survive stressful postharvest handling and conditions (Cameron et al., 2008).
We chose poinsettias as the model species because graphical tracking curves can be used to determine when a crop requires height control (Fisher and Heins, 2002; Harwood and Hadley, 2004). Graphical tracking requires regular measurement of plant height and comparing these with the expected height range at that date. When plants are taller than desired, height control is needed. The objectives of this experiment were to 1) test whether controlled WD can be used to control poinsettias height, 2) determine the effect of WD on quality characteristics such as bract color and size, and 3) compare the effects of WD on plant quality to those of PGRs.
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