S17 - Session O1 - Strengthening fruit-skin resistance to cracking by application of plant growth regulators.

Authors: Idit Ginzberg *, Raphae Stern, Yulia Kaplan

Fruit cracking is caused by failure of the skin to resist surface tensions due to fruit expansion, and is a major disorder that limits fruit quality and marketability. Simply, fruit skin is made of epidermis cells and cuticular matrix. Increased cuticle thickness, higher epidermal cell density and cell morphology that support strong adhesion between neighboring cells are characteristic of fruits tolerant to cracking compared to susceptible genotypes. Calyx-end cracking disorder in 'Cripps Pink' apple is treated with a mixture of gibberellic acids 4 and 7 (GA 4+7 ) and the cytokinin 6-benzyladenine (BA) early in fruit development. The treatment not only significantly reduces cracking incidence in the orchard, it also provides information on the cellular and molecular factors determining fruit-skin resistance to growth strain. BA+GA 4+7 application results in an immediate increase in epidermal cell density that is maintained until fruit maturation. Skin anatomical modifications are complemented by the expression of genes associated with epidermal cell patterning and cuticle formation. Gene-networking analysis supports the interaction between cell-wall synthesis, cuticle-formation and GA-signaling gene clusters. Overall, data suggestthat BA+GA 4+7 treatment does not modify developmental cues, but promotes or enhances the innate developmental program. High temperatures at the phenological stage of early fruit cell division are associated with calyx-end cracking incidence. Interestingly, covering the orchard with shade nets (shading level of 15%) enhances the effect of the bioregulators, and cracking incidence is further reduced to minimum values. It is hypothesized that GA signaling is involved with the netting effect as well. The data on BA+GA 4+7 -induced skin modifications complement previously suggested models for cracking resistance in fruit. Knowledge gained on apple fruit skin traits may be applied to control cracking in other fruit as well.