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.
Baille, M., Baille, A. & Laury, J.C. 1994 A simplified model for predicting evapotranspiration rate of nine ornamental species vs. climate factors and leaf area Sci. Hort. 59 217 232
Berghage, R.D., Heins, R.D., Karlsson, M., Erwin, J.E. & Carlson, W. 1989 Pinching technique influences lateral shoot development in poinsettia J. Amer. Soc. Hort. Sci. 114 909 914
Brown, R.G.S., Kawaide, H., Yang, Y., Rademacher, W. & Kamiya, Y. 1997 Daminozide and prohexadione have similar modes of action as inhibitors of the late stages of gibberellin metabolism Physiol. Plant. 101 309 313
Cameron, R.W.F., Harrison-Murray, R.S., Atkinson, C.J. & Judd, H.L. 2006 Regulated deficit irrigation: A means to control growth in woody ornamentals J. Hort. Sci. Biotechnol. 81 435 443
Cameron, R., Harrison-Murray, R., Fordham, M., Wilkinson, S., Davies, W., Atkinson, C. & Else, M. 2008 Regulated irrigation of woody ornamentals to improve plant quality and precondition against drought stress Ann. Appl. Biol. 153 49 61
Cockshull, K.E., Langton, F.A. & Cave, C.R.J. 1994 Differential effects of different DIF treatments on chrysanthemum and poinsettia Acta Hort. 378 15 25
Currey, C.J. & Lopez, R.G. 2011 Early flurprimidol drench applications suppress final height of four poinsettia cultivars HortTechnology 21 35 40
Ecke,, Faust, P.J., Higgins, A. & Williams, J.E. 2004 The Ecke poinsettia manual. Ball Publishing, West Chicago, IL
Faust, J.E. & Heins, R.D. 1996 Axillary bud development of poinsettia ‘Eckespoint Lilo’ and ‘Eckespoint Red Sails’ (Euphorbia pulcherrima Willd.) is inhibited by high temperatures J. Amer. Soc. Hort. Sci. 121 920 926
Fisher, P.R. & Heins, R.D. 2002 UNH FloraTrack for poinsettia: Graphical tracking plant height on computer. Univ. New Hampshire Coop. Ext., Durham, NH
Gibson, J.L., Cavins, T.J., Greer, L., Whipker, B.E. & Dole, J.M. 2003 Efficacy of plant growth regulators on the growth of Argyranthemum frutescens ‘Comet Pink’ Acta Hort. 624 213 216
Hamid, M.M. & Williams, R.R. 1997 Effect of different types and concentrations of plant growth retardants on Sturt’s desert pea (Swainsona formosa) Sci. Hort. 71 79 85
Harwood, T.D. & Hadley, P. 2004 Graphical tracking systems revisited: A practical approach to computer scheduling in horticulture Acta Hort. 654 179 185
Hartley, D.E. 1992 Poinsettias, p. 305–331. In: R. Larson (ed.). Introduction to floriculture. 2nd ed. Academic Press, San Diego, CA
Heins, R.D., Runkle, E.S., Cameron, A. & Carlson, W. 1999 Forcing perennials: Follow these strategies to regulate perennial plant height Greenhouse Grower 16 129 137
Hendriks, L. & Ueber, E. 1995 Alternative methods of regulating the elongation growth of ornamental plants: A current assessment Acta Hort. 378 159 167
Karlović, K., Vršek, I., Šindrak, Z. & Židovec, V. 2004 Influence of growth regulators on the height and number of inflorescence shoots in the Chrysanthemum cultivar ‘Revert’ Agr. Conspectus Sci. 69 63 66
Kim, J., van Iersel, M.W. & Burnett, S. 2011 Estimating daily water use of two petunia cultivars based on plant and environmental factors HortScience 46 1287 1293
Kohanbash, D., Kantor, G., Martin, T. & Crawford, L. 2013 Wireless sensor network design for monitoring and irrigation control: User-centric hardware and software development HortTechnology 23 725 734
Lodeta, K.B., Ban, S.G., Perica, S., Dumičić, G. & Bućan, L. 2010 Response of poinsettia to drench application of growth regulators J. Food Agr. Environ. 8 297 301
Löfkvist, K., Larsen, R., Englund, J.E. & Alsanius, B.W. 2009 Light integral as an indicator of water use in commercial greenhouse nurseries Acta Agr. Scand. B Soil Plant Sci. 59 326 334
Moe, R., Glomsrud, N., Bratberg, I. & Valsø, S. 1992a Control of plant height in poinsettia by temperature drop and graphical tracking Acta Hort. 327 41 48
Moe, R., Fjeld, T. & Mortensen, L.M. 1992b Stem elongation and keeping quality in poinsettia (Euphorbia pulcherrima Willd.) as affected by temperature and supplementary lighting Sci. Hort. 50 127 136
Nelson, P.V., Song, C.-T., Huang, J., Niedziela, C.E. & Swallow, W.H. 2012 Relative effects of fertilizer nitrogen form and phosphate level on control of bedding plant seedling growth HortScience 47 249 253
Nowak, J.S. & Strojny, Z. 2001 Effect of different soil water potential on summer grown poinsettia (Euphorbia pulcherrima Wild. ‘Lilo’) Acta Hort. 548 425 430
Trejo, L., Feria Arroyo, T.P., Olsen, K.M., Eguiarte, L.E., Arroyo, B., Gruhn, J.A. & Olson, M.E. 2012 Poinsettia’s wild ancestor in the Mexican dry tropics: Historical, genetic, and environmental evidence Amer. J. Bot. 99 1146 1157
U.S. Department of Agriculture 2014 Floriculture crops, 2013 summary. National Agricultural Statistics Service, Washington, DC. 12 Nov. 2014. <http://usda.mannlib.cornell.edu/usda/current/FlorCrop/FlorCrop-06-19-2014.pdf>