Ethephon [(2-chloroethyl) phosphonic acid] is a plant growth regulator (PGR) belonging to the phosphonate family (Abeles et al., 1992). It is absorbed rapidly by aboveground plant parts, and releases ethylene readily at the pH of most plant tissues. Worldwide, ethephon is the most widely used PGR and is used to inhibit stem elongation, induce abscission of flower buds and leaves, promote branching, promote flowering of pineapple [Ananas comosus (L.) Merr.], and accelerate fruit ripening (Ben-Tal, 1992; Cooke and Randall, 1968; Glady et al., 2007; Leatherwood et al., 2009; Miller and Olberg, 2016; Miller et al., 2012; Starman et al., 2004; Turnbull et al., 1999; Yamaguchi et al., 1971).
The plant growth regulating activity of ethephon is a result of the release of ethylene in plant tissues. Ethephon breakdown products are ethylene, chloride ion, and phosphate ion, and the yield is quantitative (Abeles et al., 1992; Biddle et al., 1976). At a pH greater than 5, ethephon release occurs rapidly, and is more rapid at warmer temperatures (Cooke and Randall, 1968; Maynard and Swan, 1963; Olien and Bukovac, 1978). Most commonly applied as a foliar spray, ethephon’s effect is the result of conversion to ethylene in tissues contacted directly by the spray. However, in several systems, at least some ethephon is phloem mobile and moves in a classical source-to-sink manner (Edgerton and Hatch, 1972; Foster et al., 1992; Martin et al., 1972; Puech and Crane, 1975; Weaver et al., 1972; Yamaguchi et al., 1971). In some plants, little if any ethephon movement is seen—for example, in citrus [Citrus sp. (Young and Jahn, 1975)] and mature walnut (Juglans regia L.) leaves (Martin et al., 1972), both of which might be related to difficulty in epidermal passage of mature leaves.
There has been inconsistent interest in the efficacy of ethephon as a root zone-applied PGR (Briggs, 1975; Johnson et al., 1982; Miller and Olberg, 2016; Miller et al., 2012; Tompsett, 1973, 1974). Possibly because of its predominant use as a foliar spray, very few studies on root-zone ethephon uptake and movement in plants have been published, and little is known about xylem mobility of ethephon. Puech and Crane (1975, p. 446) stated that “significant amounts of ethephon are not transported in the xylem,” but this statement is based on a lack of xylem translocation resulting from ethephon application to source leaves, not from root-zone uptake.
Kwong and Lagerstedt (1977) demonstrated abscission of bean (Phaseolus vulgaris L.) leaves and stem terminals after root-zone ethephon application, suggesting root absorption and upward movement. Kuo and Chen (1980) found similar effects to flooding (reduced stem elongation, epinasty, adventitious root development) from root-zone ethephon applications in tomato (Solanum lycopersicum) cultivars. Unpublished data of W.B.M. indicate floral abortion (a symptom of ethylene injury) after ethephon root-zone applications in easter lily (Lilium longiflorum Thunb.), again suggesting xylem movement. Drennan and Norton (1972) observed that ethephon applied to pea (Pisum sativum L.) split-root systems inhibited nodulation on the treated side, but not on the untreated side. Kawase (1974) found soil-applied ethephon exerted effects on aboveground plant parts of sunflower (Helianthus annuus L.). Although the suggestion for xylem movement is strong, direct evidence is so far lacking.
The purpose of this study was to provide insight into ethephon root uptake, upward movement, and partitioning in plants. To assess ethephon localization, ethephon within harvested tissues or collected xylem sap was converted to ethylene in sealed containers, and the resulting ethylene was measured by gas chromatography.
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