Effects of temperature on fruit thinning with ethephon in ‘Golden Delicious’ apples

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Abstract

The effect of temperature on the ability of 2-chloroethylphosphonic acid (ethephon), when applied at 20 mm stage of fruit development, to induce ethylene evolution of fruit and leaves and abscission of fruit and leaves was determined using 9-year-old root-bagged ‘Golden Delicious’/M.27 apple (Malus domestica Borkh.) trees in environment-controlled growth rooms. Ethephon at 400 μL L−1 effectively thinned apples, and its thinning effect was not affected by increasing day/night temperature from 21.1/10 to 32.2/21.1 °C. Fruit ethylene evolution was enhanced by application of ethephon. Peak fruit ethylene evolution occurred 1 day after application of ethephon when day/night temperature was 32.2/21.1 °C whereas it occurred 2 days after application of ethephon at a day/night temperature of 21.1/10 or 26.7/15.6 °C. Ethephon increased leaf ethylene evolution drastically but it did not induce leaf abscission regardless of temperature.

Introduction

Fruit thinning is to remove excessive fruitlets from apple (Malus domestica Borkh.) trees and is one of the most effective measures to improve fruit size, color and quality at harvest, and increases return bloom the following year, thereby reducing alternate bearing. Chemical thinning is superior to hand thinning because it is less expensive and it can be done earlier, thus having a greater effect on increasing return flowering (Childers et al., 1995). Chemical thinners thin fruit by accentuating the naturally occurring waves of fruit abscission (Yuan and Greene, 2000). Chemical thinners, such as naphthaleneacetic acid (NAA), carbaryl and benzyladenine (BA), alone or in combination, can effectively thin apples when applied at a fruit diameter of ≈5–16 mm (Byers, 2003, Marini, 1996). However, ‘Golden Delicious’ and spur strains of Delicious’ apples may have heavy initial fruit set, thus they are difficult to adequately thin with naphthaleneacetic acid (NAA) and carbaryl (Jones et al., 1983, Marini, 2004). In some years, naphthaleneacetic acid, carbaryl, and benzyladenine, alone or in combination, fail to thin apples when applied at a fruit diameter of ≈5–16 mm due to low temperature after applications of chemical thinners. Furthermore, these chemical thinners have little or no effect on apple fruit thinning when fruit were greater than 16 mm in diameter, and 2-chloroethyl phosphonic acid (ethephon) is the only compound that can effectively thin apples when fruit were 18–30 mm in diameter (Byers, 2003, Jones et al., 1983, Marini, 1996, Veinbrants and Hutchinson, 1976). Therefore, ethephon has been suggested as a follow up thinner when previous chemical thinning sprays have failed to thin adequately (Byers, 2003).

It has been reported that temperature during and following application is the most important environmental factor affecting fruit abscission response to ethephon in cherries (Prunus cerasus L.) (Flore and Bukovac, 1982, Olien and Bukovac, 1978, Wittenbach and Bukovac, 1973), apples (Jones and Koen, 1985), and oranges (Citrus sinensis (L.) Osb.) (Yuan and Burns, 2004). Temperature affects both absorption and degradation of ethephon to ethylene (Biddle et al., 1976, Flore and Bukovac, 1982, Olien and Bukovac, 1978). Low temperature following application results in a poor fruit abscission response whereas high temperature may lead to over-thinning or excessive leaf abscission (Jones and Koen, 1985, Olien and Bukovac, 1978, Yuan and Burns, 2004). Little information is available about the effects of temperature on the efficacy of ethephon in thinning ‘Golden Delicious’ apples or other difficult to thin cultivars when applied at a fruit diameter of 18–30 mm (Byers, 2003). Understanding the optimum temperature range for ethephon will assist in effectively managing application under a wide range of temperatures during the chemical thinning season. The purpose of this study was to determine the effects of temperature on ethylene production of ‘Golden Delicious’ apple fruit and leaves, and fruit abscission after application of ethephon.

Section snippets

Plant materials

Nine-year-old ‘Golden Delicious’ apple trees grafted on M.27 rootstock growing in 19 L root bags (Lacebark Inc., Stillwater, OK, USA) were used for this experiment. Apple trees had an average of 1.1 m in canopy height and 1.0 m in canopy diameter.

Treatment setting

Eighteen ‘Golden Delicious’ trees were selected and randomly assigned into environment-controlled growth rooms of six trees each on 22 May 2006 when fruit averaged 19.7 mm in diameter. The environment-controlled growth rooms were set at day (07:30 to 18:30

Results

Ethephon at 400 μL L−1 effectively thinned ‘Golden Delicious’ apples when applied at 20 mm stage of fruit development (Fig. 1). Ethephon-induced fruit thinning was not affected by an increase in the day/night temperature from 21.1/10 to 32.2/21.1 °C. Fruit began to abscise 3 days after treatment regardless of ethephon treatment due to the ‘June drop’ (Fig. 2). Ethephon enhanced fruit abscission, and ethephon-induced fruit abscission peaked 4 days after treatment regardless of air temperature. No

Discussion

It has been reported that the fruit abscission response of cherries (Olien and Bukovac, 1978), oranges (Yuan and Burns, 2004), and apples (Jones and Koen, 1985) to ethephon is temperature-dependant. Using Cox's Orange Pippin × Democrat apples in temperature-controlled rooms (day and night temperature was constant), Jones and Koen (1985) reported that ethephon applied at petal fall had no effect on fruit thinning when the air temperature was 8 °C or lower, whereas fruit thinning increased linearly

Conclusion

In summary, ethephon thinned ‘Golden Delicious’ apples when applied at 20 mm stage of fruit development, and fruit thinning with ethephon was not influenced by increasing day/night temperature from 21.1/10 to 32.2/15.6 °C.

Acknowledgements

The author gratefully acknowledge the financial support of the Virginia Apple Research Program and Virginia Agricultural Council, and the assistance of Chelsea Trant, Natalie Kujala, Grace Engelman and David H. Carbaugh.

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