Controlling plant stem elongation is common in ornamental plant production. Height control is necessary to meet industry standards for target plant height (Fisher and Heins, 1995), to increase plant aesthetics by producing more compact plants (van Iersel and Nemali, 2004) and because compact plants are less expensive to ship (Burnett and van Iersel, 2008). Plant growth regulators are commonly used (Berghage and Heins, 1991; Currey and Lopez, 2011), but are not always desirable as there is growing concern about the use of agrochemicals in production and their presence in runoff (Kaufmann et al., 2000). The selection of cultivars with shorter internodes and smaller growth habits can be used to produce smaller plants (Ecke et al., 2004), but such cultivars are not available for many taxa.
Environmental conditions can also be altered to manipulate plant growth and subsequent height, including alteration of day and night temperatures (Kaufmann et al., 2000), changing the daily light integral, and adjusting plant spacing (Liu and Heins, 2002). Alteration of temperature and light conditions is not always possible depending on what other crops are growing in the greenhouse or when plants are grown outdoors. Plant spacing may not be able to be increased if there is not enough space available and can increase overall production costs. Deficit irrigation or drought stress can also limit elongation; however, many growers are reluctant to expose their plants to drought stress because they do not want it to negatively affect overall plant quality (Bailey and Whipker, 1998). Sensor-controlled irrigation has been used to precisely control the timing, severity, and duration of drought stress to control elongation of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) (Alem et al., 2015). Alem et al. (2015) controlled the height of poinsettia with deficit irrigation by lowering substrate water content to 0.20 m3·m−3 until plant height was within the desired range to produce a target final height after which substrate water content was increased to 0.40 m3·m−3. This effectively lowered plant height without negatively impacting plant quality. With the development of wireless sensor networks for irrigation control in commercial greenhouses and nurseries, this technology will be available soon to growers (Kohanbash et al., 2013; Lea-Cox et al., 2013).
Diurnal patterns of elongation have been examined in many plant taxa. Circadian rhythms interact with environmental conditions to determine elongation rates. Stomatal conductance and transpiration are also controlled partly by the plant circadian clock (Farré, 2012). Stem elongation has diurnal patterns (Nozue and Maloof, 2006) and leaf growth is maximal during the day or night, depending on the taxa. Environmental factors can influence the rate of growth, but not the diurnal pattern (Ruts et al., 2012).
Knowing how and when stem elongation is affected by water availability will increase our understanding of how elongation can be controlled through drought stress without reducing plant quality. Our first objective was to quantify diurnal patterns of elongation of H. acetosella in response to well-watered and drought-stressed conditions. H. acetosella is a fast-growing herbaceous species with clear growth responses to irrigation volume and substrate water content (Bayer et al., 2013). Understanding the time of day when elongation occurs can be useful in using drought stress as a means of plant elongation control. Our second objective was to quantify the effect of rewatering on the elongation rate of previously drought-stressed plants. The results of this study can be used to determine the optimal time for applying drought stress for elongation control.
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