The presence of russet on ‘Golden Delicious' (Malus ×domestica Borkh.) apples has long been a concern to producers and marketers of fresh fruit throughout the world because it detracts from the smooth, uniform finish of the fruit and results in economic loss resulting from grade reduction (Faust and Shear, 1972a). Physical russetting is the result of irreversible injury to the peel by impact, abrasion, or repeated agitation by another material such as might occur with hail or limb and leaf rubbing. Physiological russeting, on the other hand, although foundationally genetic (Eccher et al., 2008), may be induced or aggravated by various factors including: 1) xenobiotics (Creasy and Swartz, 1981; Hatch, 1975; Lallu et al., 2010; McCormick and Streif, 2008; Sanchez et al., 1992); 2) changes in ambient temperature, humidity, and aridity (Creasy, 1980; Faust and Shear, 1972b; Fogelman et al., 2009; Meyer, 1944; Tukey, 1959); and 3) insects and pathogens such as rust mite, ring virus, and various bacteria, yeasts, and fungi (Daines et al., 1984; Duso et al., 2010; Easterbrook and Fuller, 1986; Heidenreich et al., 1997; Lindow et al., 1998; Wood, 2010). In the orchard, these factors often are related and/or codependent.
Many methods have been evaluated to alleviate physiological russeting in apple. Like other environmentally based peel disorders such as superficial scald, sunburn, scarfskin, or flecking, a consistent response to treatment is often difficult to achieve or predict. One of the more successful methods of reducing physiological russeting is use of multiple topical applications of gibberellins during early fruit development. Since the first published reports over 35 years ago (Eccher, 1975; Eccher and Boffelli, 1978; Edgerton and Veinbrants, 1979; Taylor, 1975), researchers have conducted numerous studies to better understand the mechanisms underlying the beneficial effects of gibberellins on apple peel finish.
Eccher (1975) first observed less irregularity among epidermal cells and reduced cuticle cracking in fruit treated with GA4+7 than in untreated fruit. Skene and Greene (1982) recognized the relationship between microcracking in the cuticle and regions of russeting in ‘Cox's Orange Pippin’. [Also, see the review regarding fruit skin splitting and cracking (Opara et al., 1997).] Microcracking in ‘Golden Delicious’ peel tissue was reported to increase as a result of surface wetting (Knoche and Grimm, 2008). More recently, Knoche et al. (2011) found water-induced russeting and microcracking of ‘Golden Delicious’ apples decreased on fruit pretreated with multiple applications of GA4+7. They also reasoned the effect of GA4+7 on microcracking and, therefore, russeting must reside with the epidermal and hypodermal cell layers.
Bukovac and Nakagawa (1968) first reported increases in size and number of cortex cells in apples treated with high rates of GA4+7 in lanolin paste applied 2 weeks after anthesis. To my knowledge, the effect on apple epidermal cells, of multiple applications of GA4+7, has not been quantitatively examined. Thus, the objective of this work was to determine whether the decrease in russetting of ‘Golden Delicious’ apples from multiple applications of GA4+7 was accompanied by an increase in epidermal planar cell density (number of epidermal cells per unit peel surface area).
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Knoche, M., Khanal, B. & Stopar, M. 2011 Russeting and microcracking of ‘Golden Delicious’ apple fruit concomitantly decline due to gibberellin A4+7 application J. Amer. Soc. Hort. Sci. 136 159 164
Lallu, N., Haynes, G., Pidakala, P., Billing, D., Johnston, J., Francis, K. & McDermott, K. 2010 Factors affecting the incidence of ‘russet browning' disorder in’ Cox's Orange Pippin' apples Acta Hort. 877 499 506
Lindow, S.E., Desurmont, C., Elkins, R., McGourty, G., Clark, E. & Brandl, M.T. 1998 Occurrence of indole-3-acetic acid-producing bacteria on pear trees and their association with fruit russet Phytopathology 88 1149 1157
McCormick, R. & Streif, J. 2008 Stem end russet browning of ‘Cox's orange pippin’ apples, an undesired side effect from the application of 1-MCP Acta Hort. 796 125 128
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