Ethephon [(2-chloroethyl) phosphonic acid] has been widely used as a foliar spray in the commercial greenhouse industry for decades to abort flowers, promote branching, and restrict plant growth (Kays and Beaudry, 1987). Growers have reported success with ethephon as a growth regulator (Styer, 2002), resulting in continued research on the product and specific recommendations for ethephon use. For maximum efficacy, growers are advised to spray ethephon to runoff and to ensure the leaves remain wet for 3 to 4 h (Styer, 2002). In addition, temperature must be considered when the spray applications are to be made. At low temperatures, the rate of ethylene generation is very slow, limiting the amount of uptake and reducing the efficacy of the chemical (Lougheed and Franklin, 1972). At temperatures above 33 °C, exogenous ethephon breaks down at a high rate, which limits the amount of ethephon available for plant uptake and can potentially cause phytotoxicity in the form of leaf senescence because of the rapid increase of ethylene in the air surrounding the plant (Lougheed and Franklin, 1972; Olien and Bukovac, 1978).
Another factor influencing efficacy is pH. Maintaining a solution pH between 4.0 and 4.5 when applying an ethephon spray to plants is critical. As solution pH increases, the rate at which ethephon evolves to ethylene also increases (Warner and Leopold, 1969). Consequently, as the speed at which ethephon evolution increases while in solution, ethephon availability for plant uptake decreases, therein reducing the chemical efficacy (Smith, 2010). Ethephon is a relatively strong acid, which will reduce the solution pH; however, in regions with water sources with high alkalinity, the buffering capacity of the bicarbonates in the water may prevent the solution pH from lowering to the recommended range (Camberato et al., 2014). In these circumstances, it is necessary to reduce the pH of the solution by adding an acidic buffer solution to the tank before adding ethephon to prevent ethephon degradation (Yates et al., 2011). There is no doubt that ethephon’s dependence on environmental and physical factors such as temperature, pH, and leaf wetness duration contribute to the challenges growers face in achieving optimum efficacy and consistency when applying ethephon as a PGR.
Conflicting research results exist as to the mechanism of ethylene evolution within the plant. According to Warner and Leopold (1969), ethephon is absorbed into the plant tissue and subsequently ethylene generation occurs intercellularly because of the higher pH within the plant cells. However, studies conducted by Mudge and Swanson (1978) suggest that the generation of ethylene from ethephon takes place largely extracellularly. Regardless of how the ethylene evolves, once within the plant tissue, the cells respond with a reduction in cell elongation and a reduction in apical dominance, which in turn, can cause an increase in branching (Burg, 1973; Haver and Schuch, 2001).
The extent to which ethephon or ethylene translocates within the plant is also largely unknown. In studies conducted by Edgerton and Hatch (1972), ethephon was applied to sweet cherry leaves and fruit but more ethephon was recovered in the fruit 48 to 72 h after application than what was recovered immediately after the ethephon application, which led the authors to conclude that the majority of ethephon recovered in the fruit had translocated from the leaves. This suggests that ethephon may not need to be applied to target tissue to elicit a growth response. However, it is unknown if, and to what extent, ethephon can translocate from the roots to the shoots when applied to the substrate as a drench (Miller et al., 2012).
Currently, ethephon is not EPA labeled for commercial floriculture use as a drench application. In fact, it was widely believed in commercial horticulture that ethephon had no root activity and could only be absorbed through the leaf and stem tissue (Styer, 2002; Whipker et al., 2003). However, conflicting research results indicates that this theory is incorrect. Johnson et al. (1982) reported that ethephon drench applications reduced the intercellular spaces in Ficusbenjamina L., resulting in reduced leaf area. More recently, ethephon substrate drenches reduced stem elongation of Narcissus pseudonarcissus L. and reduced plant growth and delayed flowering in a wide variety of bedding plants (Miller et al., 2012). Miller et al. (2012) also found that substrate pH affected the rate and duration of ethylene evolution from drench applications of ethephon in a closed system, in the absence of plants.
Drench applications of PGRs have many benefits to growers including a more uniform growth regulation effect across crops. In addition, generally less total chemical is used as the concentrations tend to be lower for drench applications (Currey and Lopez, 2010). Considering the slow uptake of ethephon as a spray application and the resulting logistical issues with plant irrigation and handling, ethephon drenches could have a practical application in the commercial greenhouse industry should they prove effective.
The objective of this study was 2-fold: 1) to determine if ornamental perennial plant species could respond to ethephon substrate drenches and the subsequent effect on growth and development of the shoots; and 2) to determine how differences in substrate pH affect ethephon efficacy by analyzing the shoot responses to the substrate drench application over a range of starting substrate pH.
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