Chemical stimulation of young fruit abscission during the first 3 weeks after bloom is a key management consideration in modern apple (Malus ×domestica Borkh.) production systems. Sequential applications of chemical thinners with different modes of action are normally made during this period to reduce fruit set to commercially acceptable levels that eliminate the need for hand thinning, increase fruit size at harvest, and increase the probability of adequate return bloom in the next year. Young fruit generally become insensitive to thinning chemicals after they reach a diameter of ≈16 mm, coincident with increasing carbohydrate reserves within the tree (Lakso et al., 1999). Studies describing the effects of shade treatments on fruit set in apple (Byers et al., 1985, 1990, 1991; McArtney et al., 2004; Zibordi et al., 2009), together with studies of altered gene expression after the imposition of abscission stimuli including shade (Zhou et al,, 2008; Zhu et al., 2011), 6-benzyladenine (Botton et al., 2011), and naphthaleneacetic acid (Zhu et al., 2011), provide evidence in support of the hypothesis that a carbohydrate deficit in the fruit is one of the earliest responses to chemical or environmental stimuli that trigger abscission in apple fruit.
Commercial recommendations for delayed or rescue thinning (Cornell Pest Management Guidelines for Commercial Tree Fruit Production, 2012; Integrated Orchard Management Guide for Commercial Apples in the Southeast, 2012; Pennsylvania 2012–2013 Tree Fruit Production Guide, 2012) rely almost exclusively on 2-chloroethylphosphonic acid (ethephon) and 1-naphthyl methylcarbamate (carbaryl), which are both coming under increasingly stringent regulatory pressures worldwide (Anon, 2006, 2009). Thinning responses after application of ethephon can be more erratic than other thinning materials and high ambient temperatures after application of ethephon at bloom may result in excessive thinning (Jones and Koen, 1985). In addition, ethephon may cause the fruit of some cultivars to flatten (Basak, 2006; Williams and Fallahi, 1999). The availability of an effective chemical strategy for delayed thinning, when fruit range from 18 to 30 mm in diameter, would provide apple growers with increased options for thinning in years when primary thinning sprays have not reduced fruit set to a commercially acceptable level.
Two new chemicals have been reported to have thinning activity in apple. The PSII inhibitor metamitron exhibited thinning activity when applied to apple fruitlets at the 10 to 12 mm diameter stage (Basak, 2011; Clever, 2007; Deckers et al., 2010; Dorigoni and Lexxer, 2007; Lafer, 2010) and to ‘SunCrisp’ apple when mean fruit diameter was ≈20 mm (McArtney and Obermiller, 2012). Metamitron disrupts the photosynthetic apparatus for 7 to 10 d after application, reducing electron transport rates by up to 60% (McArtney and Obermiller, 2012). These data suggest that thinning activity of metamitron is a response to the creation of a transient carbohydrate stress within the fruit. If a carbohydrate surplus is responsible, at least in part, for lack of activity of conventional chemical thinning agents when applied later than 3 weeks after bloom, then application of a PSII inhibitor such as metamitron may increase sensitivity of fruit to abscission agents during periods of natural carbohydrate surplus at this time or even earlier.
The ethylene precursor ACC was recently shown to have thinning activity in apple (McArtney, 2011; Schupp et al., 2012). Ethylene evolution from detached spurs was highest 1 d after application of ACC and gradually declined to control levels over the next 8 to 10 d. Lack of an autocatalytic ethylene response to exogenous application of ACC suggests that system 2 ethylene was not operating at this time. Ethylene evolution from fruiting spurs of ‘Cripps Pink’ was greatly reduced after application of ACC 31 d after bloom, when the mean fruit diameter was 20 mm compared with applications made at full bloom or 16 d after bloom (McArtney, 2011). The thinning activity of ACC is thought to be directly related to its rapid metabolism to ethylene during periods when the abscission zone cells are sensitive to this hormone. Loss of the capacity to convert exogenously applied ACC to ethylene might reflect a reduction in activity of ACC oxidase (ACO). Loss of ACO activity beginning 3 weeks after bloom might also be partly responsible for the decreased sensitivity of immature fruit to chemical thinners at this time.
Botton et al. (2011) proposed a hypothetical model for immature fruit abscission in apple in response to 6-benzylaminopurine, in which sugar starvation in the fruit cortex ultimately triggered ethylene signaling pathways, specifically upregulation of ACC synthase and Ethylene Responsive Factor genes. It was proposed that ethylene generated in the fruit cortex in response to a nutritional stress (synonym carbohydrate deficit) diffused to the seed, ultimately halting embryogenesis and polar auxin transport from the fruit. Zhu et al. (2011) also found that shading or NAA induced expression of genes involved in ethylene biosynthesis and perception and repressed the expression of genes involved in auxin transport in the fruit abscission zone. This decrease in polar auxin transport was associated with increases in ethylene production and expression of ethylene biosynthesis and signaling related genes (Zhu et al., 2011). Botton et al. (2011) proposed that generation of a carbohydrate deficit in the fruit cortex provides the primary stimulus for fruit abscission, and ethylene is not only an integral part of downstream signaling pathways, but increased ethylene levels provide the stimulus that ultimately activates the abscission zone in the fruit pedicel. According to this hypothesis, the PSII inhibitor metamitron and the ethylene precursor ACC may provide useful chemical tools with which to investigate the roles of carbohydrates and ethylene in abscission of apple fruit later than 3 weeks after bloom. The objectives of the present study were to evaluate the potential for delayed applications of the ethylene precursor ACC and the PSII inhibitor metamitron to thin apple fruit.
Anon 2006 Review report for the active substance carbaryl. 26 Oct. 2011. <http://ec.europa.eu/food/plant/protection/evaluation/existactive/list-carbaryl_en.pdf>
Basak, A. 2006 The effect of fruitlet thinning on fruit quality parameters in the apple cultivar ‘Gala’ J. Fruit and Ornamental Plant Research. 14 143 150
Basak, A. 2011 Efficiency of fruitlet thinning in apple ‘Gala Must’ by use of metamitron and artificial shading J. Fruit and Ornamental Plant Research. 19 51 62
Botton, A., Eccher, G., Forcato, C., Ferrarini, A., Begheldo, M., Zermiani, M., Moscatello, S., Battistelli, A., Velasco, R., Ruperti, B. & Ramina, A. 2011 Signaling pathways mediating the induction of apple fruitlet abscission Plant Physiol. 155 185 208
Byers, R.E., Barden, J.A., Polomski, R.F., Young, R.W. & Carbaugh, D.H. 1990 Apple fruit abscission by photosynthetic inhibition J. Amer. Soc. Hort. Sci. 115 14 19
Clever, M. 2007 A comparison of different thinning products applied to the apple variety ‘Elstar Elshof’ in the lower Elbe region Erwerbs-Obstbau 49 107 109
Cornell Pest Management Guidelines for Commercial Tree Fruit Production 2012 Cornell Univ. Coop. Ext. 25 June 2012. <http://ipmguidelines.org/TreeFruits/Chapters/CH11/default-6.aspx>
Greene, D.W. & Autio, W.R. 1994 Combination sprays with benzyladenine to chemically thin spur-type ‘Delicious’ apples HortScience 29 887 890
Integrated Orchard Management Guide for Commercial Apples in the Southeast 2012 North Carolina State Univ. North Carolina Coop. Ext. Service AG-572. 25 June 2012. <http://www.ces.ncsu.edu/fletcher/programs/apple/2012orchard-management.pdf>
Jones, K.M., Koen, T.B., Bound, S.A. & Oakford, M.J. 1991 Some reservations in thinning ‘Fuji’ apples with naphthalene acetic acid (NAA) and ethephon N. Z. J. Crop Hort. Sci. 19 225 228
Lafer, G. 2010 Effects of chemical thinning with metamitron on fruit set, yield and fruit quality of ‘Elstar’ Acta Hort. 884 531 536
Lakso, A.N., Wunsche, J.N., Palmer, J.W. & Corelli-Grappadelli, L. 1999 Measurement and modeling of carbon balance of the apple tree HortScience 34 1040 1047
McArtney, S. 2011 Effects of 1-aminocyclopropane carboxylic acid on the rate of ethylene release from detached fruiting spurs and on fruit abscission in apple J. Hort. Sci. Biotechnol. 86 640 644
McArtney, S., White, M., Latter, I. & Campbell, J. 2004 Individual and combined effects of shading and thinning chemicals on abscission and dry-matter accumulation of ‘Royal Gala’ apple fruit J. Hort. Sci. Biotechnol. 79 441 448
McArtney, S.J. & Obermiller, J.D. 2012 Comparison of the effects of metamitron on chlorophyll fluorescence and fruit set in apple and peach HortScience 47 509 514
Pennsylvania 2012–2013 Tree Fruit Production Guide 2012 Penn. State Univ. AGRS-045. 25 June 2012. <http://pubs.cas.psu.edu/FreePubs/PDFs/agrs045.pdf>
Schupp, J.R., Kon, T.M. & Winzeler, H. E. 2012 1-aminocyclopropane carboxylic acid shows promise as a chemical thinner for apple HortScience 47 1308 1311
Williams, K.M. & Fallahi, E. 1999 The effects of exogenous bioregulators and environment on regular cropping of apple HortTechnology 9 323 327
Zhou, C., Lakso, A.N., Robinson, T.L. & Gan, S. 2008 Isolation and characterization of genes associated with shade-induced apple abscission Mol. Genet. Genomics 280 83 92
Zhu, H., Dardick, C.D., Beers, E.P., Callanhan, A.M, Xia, R. & Yuan, R. 2011 Transcriptomics of shading-induced and NAA-induced abscission in apple (Malus domestica) reveals a shared pathway involving reduced photosynthesis, alterations in carbohydrate transport and signaling and hormone crosstalk. BMC Plant Biology 11:138. <http://www.biomedcentral.com/1471-2229/11/138>
Zibordi, M., Domingos, S. & Corelli Grappadelli, L. 2009 Thinning apples via shading: An appraisal under field conditions J. Hort. Sci. Biotechnol. ISAFRUIT Special Issue 138 144