Prevention of apple fruit cracking by strengthening skin resistance to growth strain
|Fruit cracking is one of the main disorders limiting fruit quality and yield|
|Idit Ginzberg1,* and Raphael A. Stern2,3|
|October 18, 2018|
The phenomenon of fruit cracking
Fruit cracking is one of the main disorders limiting fruit quality and yield. The phenomenon occurs mainly in the pre-harvest stage and initiates at the surface of the fruit, where cracks traverse the cuticle and penetrate the inner tissues; extreme cracking can result in fruit splitting. Cracks mar fruit appearance, allow water loss and pathogen invasion, decrease storage and shelf life, and therefore reduce fruit marketability. The cracking phenomenon affects many types of fruits: apple, sweet cherry, grape, pomegranate, persimmon, litchi, citrus and tomato, among others.
Apart from agricultural considerations, the cracking phenomenon, its causes and treatments enable a unique examination of peel characteristics at the cellular level. For simplicity, the peel is made up of one or two epidermal cell layers covered on the external side by hydrophobic cuticular matrix made up of cutin and wax, and underlain by two to three layers of hypodermal cells. Lack of coordination between cuticle deposition and growth expansion of the fruit surface has been implicated in a number of surface disorders, including fruit russeting and cracking.
Studies on berry fruit genotypes that are tolerant or susceptible to skin cracking have indicated that cuticle thickness and epidermal cell density are positively correlated with cracking resistance. These observations imply that manipulations of epidermal cell density and cuticle thickness and composition might enable strengthening of the peel and controlling cracking incidence.
Apple skin as a model for studying peel cracking in orchard Fruit
Recently, we applied a mixture of the plant growth regulators (PGRs) cytokinin 6-benzyladenine (BA) and gibberellins 4 and 7 (GA4+7) in order to reduce the incidence of apple cracking (Figure 3). Cytokinin and gibberellin are known to induce cell division and expansion in planta. Multiple applications of BA+GA4+7 to 'Pink Lady' fruit at early phenological stages resulted in an immediate increase in epidermal cell density that was maintained in the ripened fruit (Figure 4), and an associated reduction in calyx-end cracking disorder at harvest. Interestingly, some of the epidermal cells appeared as cell clusters within the cuticular matrix and were detached from the native epidermal layer located at the bottom of the cuticle – this phenomenon was not detected in the untreated control fruits (Figure 5).
Increased epidermal cell density reinforces the peel by adding more cell-wall components, thickening the cuticle layer and possibly enhancing crack repair. Although spraying is performed at an early stage of fruit development, its effect is maintained in the mature fruit, allowing the skin to resist high tension due to intensive fruit expansion or stressful environmental conditions. Following the positive results of the research, a protocol of BA+GA4+7 application in commercial orchards was established with similar success.
3 Department of Biotechnology, Faculty of Life Sciences, Tel-Hai College
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