Silver thiosulfate prevents ethylene-induced abscission in holly and mistletoe

Low concentrations of ethylene induced abscission of leaves and berries from cut branchlets of English holly (Ilex aquifolium L.) and American mistletoe [Phoradendron tomentosum (DC.) Engelm. ex Gray ssp. macrophyllum (Engelm.) Wiens]. Application of 1 μmol of Ag per branchlet (as the anionic silver thiosulfate complex, STS) via the transpiration stream was found to retard this abscission. A higher application rate (4 μmol Ag per branchlet) stimulated leaf abscission in mistletoe. There were marked differences in sensitivity to ethylene among various types of holly. Holly and mistletoe are used traditionally for decoration during the Christmas season. Both are, however, prone to leaf and berry abscission during transport and marketing. This renders them unattractive and limits full realization of their aesthetic and commercial potential. Commercial holly producers presently treat the harvested branches with naphthalene acetic acid (NAA) to reduce abscission of leaves and berries during marketing (Roberts and Ticknor, 1970), but this procedure has been observed to be only partially effective (P. Schmidt, personal communication). Abscission of plant organs is a frequent response to the presence of elevated levels of C2H4 (Reid, 1985) that may accumulate in the atmosphere surrounding harvested produce as a result of endogenous C2H4 production or as” a consequence of air pollution (Sherman, 1985). In holly, infection of branches with Phytophthora ilici aggravates organ abscission, apparently because the pathogen induces C2H4 production by the plant (D. Coyier, personal communication). The inhibition of C2H4 action by silver ion (Ag+) (Beyer, 1976a, 1976b) has been used as a practical tool to increase the longevity of some cut-flower species (Mor et al., 1984a, 1984b; Veen and van de Geijn, 1978) and Received for publication 26 May 1988. This work was supported in part by Binational Agricultural Research and Development Award no. 1-250-82. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 90 to prevent organ abscission in potted plants (Cameron and Reid, 1981; Cameron and Reid, 1983). This study was carried out to examine the role of C2H4 in abscission of leaves and berries from holly and mistletoe branchlets and to test the possible usefulness of silver thiosulfate (STS). Plant materials were collected on the Davis campus of the Univ. of California and from garden trees in the San Francisco area. A single species of mistletoe was studied [Phoradendron tomentosum (DC.) Engelm. ex Gray ssp. macrophyllum (Engelm.) Wiens]. The hollies investigated were hex Fig. 1. Leaf abscission from mistletoe branchlets, as a after commencement of a 2-day exposure to 2 ppm depict the best-fit regression functions after the 3 aquifolium L. (English holly), I. cornuta Lindl. (Chinese holly), I. cornuta cv. Burfordii (Burford holly), I. × altaclarensis Wilsonii, (Wilson holly), and I. vomitorii Ait. cv. Nana (dwarf Yaupon holly). In one experiment, we compared branchlets from spiny and spineless plants of Burford holly. The mean numbers of leaves on the 15to 20-cm branchlets were seven for mistletoe and 11 for holly. The experiments examined the ethylene response of the different Christmas greens, and the effect on the quality of control and ethylene-treated branchlets of pulsing with STS or treatment with a cut-flower preservative. In all but one experiment, branchlets were placed in 10-ml glass test tubes containing deionized water (DI) or 4% (w/v) sucrose plus 200 ppm Physan 20 (a mixture of quaternary NH4 + compounds) in DI. In the other experiment, mistletoe branchlets were placed in the 10 × 20-cm perforated plastic bags used commercially for marketing mistletoe branchlets. Each bag had twenty-four 6-mm-diameter holes. Three branchlets were placed in each bag, and solutions were supplied to the cut stem ends in presoaked cotton balls. The solutions were 1% (w/v) Floralife (a commercial flower preservative) and 0.4 or 4 mM STS in 1% (w/v) Floralife. STS was prepared as described by Reid and Farnham (1980). For experiments when STS was supplied as a pulse taken up via the transpiration stream, the uptake of Ag was measured by weighing the uptake vial (which contained 10 mM STS). A 0.1-g decrease in weight was taken to indicate uptake of 1 μmol Ag. function of STS uptake, 3 ( ❍ ) and 13 ( ● ) days C2H4. Points are means of three replicates. Curves rd (–) and 13th (---) days. HORTSCIENCE, VOL. 25(l), JANUARY 1990 Table 1. Analysis of variance for effects of ethylene on leaf retention by English, Chinese, and two selections of Burford hollies. Nonsignificant single-degree-of-freedom contrasts are not shown. *,NSSignificant at P = 0.01 or nonsignificant, respectively. Fig. 2. Effect of STS pretreatment (3 μmol/branchlet) on the response of English holly to a 5-day exposure to 2 ppm C2H4. When the branchlets had taken up the desired amount of STS, their bases were rinsed with DI and they were transferred to test tubes containing DI. When used, C2H4 was supplied to holly or mistletoe branchlets held in glass tanks (one tank, 50 cm long x 25 cm wide x 30 cm high, per concentration). The gas mixture was passed through the tank at a rate of ≈35 liter·hr. Control branchlets were placed in a tank through which air alone was passed. Before STS-treated material was exposed to C2H4, the branchlets were held overnight in DI to allow internal redistribution of Ag. HORTSCIENCE , VOL. 25(1), JANUARY 1990 All experiments were conducted in an airconditioned (≈ 22C) laboratory. A randomized complete block design with time as a subtreatment was used for each experiment. There were three replications, with one sprig per replicate, in each experiment. Percentage data were transformed (X2 = arcsin X1) for analysis. Mistletoe branchlets held in air lost 60% of their leaves after standing for 13 days in DI water (Fig. 1). Treatment with 2 ppm C2H4 caused this much abscission in 3 days; by day 13, C2H4-treated branchlets had lost > 80% of their leaves. Application of from 0.2 to 2 μmol STS per branchlet effectively prevented C2H4-induced leaf abscission in mistletoe, even 13 days after exposure to C2H4 (Fig. 1). Four micromoles of Ag per branchlet induced leaf abscission, presumably as a result of Ag toxicity, or perhaps in response to the stimulated production of ethylene that has been observed in other vegetative tissues treated with Ag (Gavinlertvatana et al., 1980). Application of STS via the transpiration stream (1 μmol) or in a soaked cotton ball (0.4 mM) prevented C2H4-stimulated leaf abscission from mistletoe packaged in plastic bags, whether or not a cut-flower preservative was included in the ball. A higher concentration of STS (4 mM) stimulated leaf shedding (to 40% leaf loss in 3 days). Chinese and English hollies were very sensitive to C2H4 (Tables 1 and 2); 0.6 ppm was sufficient to cause almost complete defoliation of English holly following a 3-day exposure. Burford holly was much less-sensitive to C2H4 (Tables 1 and 2); the spineless form of this species was less affected by C2H4 than was either English or Chinese holly, while the spiny form was unaffected by any of the C2H4 concentrations tested. Differential responses were also found among other hollies tested. Whereas all the leaves of dwarf : Yaupon holly abscised following a 3-day exposure to 35 ppm C2H4, Wilson holly branchlets were unaffected (data not shown). Silver thiosulfate treatment of English holly substantially reduced the leaf and berry abscission caused by a 5-day exposure to 2 ppm C2H4 (Fig. 2). Similar results were obtained with other C2H4-sensitive cultivars (data not shown). The studies described here indicate the potential importance of C2H4 in the postharvest life of cut holly and mistletoe. Relatively short exposures to air containing concentrations of C2H4 known to occur in supermarkets and other retail outlets caused unsightly loss of leaves and berries (unpublished data). The dramatic reduction in C2H4-induced leaf and berry abscission resulting from STS pretreatment indicates a means of controlling