Ethylene emission and abscission of immature ‘Montmorency’ sour cherry fruitlets vary with ethephon concentration, phenology stage and ambient temperatures following application

Ethylene is an endogenous plant hormone associated with natural senescence and abscission of plant organs. Ethylene release from the degradation of 2-chloroethylphosphonic acid, the a.i. of the plant growth regulator ethephon, facilitates the mechanized harvest of sour cherry fruit. Methods to abscise immature sour cherry fruit, however, have not been explored but recent carryover inventories, inexpensive sour cherry imported products, and outbreaks of the invasive insect, spotted wing drosophila, have combined to reduce crop value below the cost of production thereby necessitating crop elimination. We assessed the ethylene emission and abscission rates of untreated, ethephon-treated and AVG-treated ‘Montmorency’ sour cherry flowers and fruitlets at several phenology stages. The natural ethylene emission from untreated flowers was < 1 μL kg−1 h−1 with a small peak between full bloom (FB) and petal fall (PF). AVG reduced, but did not eliminate, ethylene biosynthesis. Ethylene emission was linearly related to ethephon concentration and remained elevated for ~ 5 days following application. Ethephon application at FB induced the highest ethylene production (10–100 μL kg−1 h−1) depending on year, timing, and rate. Application at the shuck split (SS) stage resulted in very low ethylene production (~ 1 μL kg−1 h−1). Abscission rates were unaffected by AVG suggesting that natural levels of endogenous ethylene biosynthesis do not limit fruit set. Abscission rates increased, however, with increasing ethephon dose and were also highest at FB timing compared to advanced phenology stages; however, at the highest ethephon concentration (800 ppm), ~ 90% abscission occurred for all developmental stages, despite the comparatively low ethylene production at SS. Thus, ethylene emission of ‘Montmorency’ sour cherry cannot be used to predict abscission. Because temperature affects ethylene production and the maximum ambient temperatures were variable following ethephon applications, we incubated ethephon-treated flowers and immature fruitlets over several temperatures (10, 21, 30 and 35 °C). Ethylene emission was, again, highest at full bloom and increased with ethephon dose and temperature, but tthe activation energy (Ea) for ethylene emission over the temperature range was similar among ethephon rates or phenology stages (~ 76 kJ mol−1 in all cases). Together the data suggest that ethephon can eliminate sour cherry crops, but ambient temperature, phenology stage and rate interact to regulate efficacy.