ReviewStrengthening fruit-skin resistance to growth strain by application of plant growth regulators

- Cracking of fruit skin is a major disorder that limit fruit quality.
- Apple was used to study fruit-skin strengthening by plant growth regulators.
- BA + GA4+7 treatment reduced apple skin cracking.
- Increased epidermal cell density improved skin mechanical strength.
- BA + GA4+7 treatments in other orchard fruits may reduce cracking incidence similarly.

Russeting and cracking of fruit skin are major disorders that limit fruit quality and marketability. The causes suggested to be environmental condition, orchard management and failure of the skin to resist surface tensions due to fruit expansion. Basically, 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.Apple is being increasingly considered as a model for fruit development studies. Recently, spraying a mixture of gibberellin A4 plus A7 (GA4+7) and the cytokinin 6-benzyl adenine (BA) at cell division stage of apple fruit development was shown to result with reduced incidence of skin cracking by maintaining a higher number of epidermal cells compared to untreated fruit.Various treatments with plant growth regulators (PGR) were tested for controlling cracking incidence in other fruits, including tomato, pear, persimmon, apricot, grape, mandarin and kiwi. We hypothesize a common mechanism for BA + GA4+7 effect on fruit skin, and propose to view the skin as a tissue whose characteristics may be manipulated to improve its resistance to environmental and growth strains.The review paper links practical approaches in the orchard to control costly yield losses with (limited) knowledge on fruit skin anatomy and development, and discussed the hypothesis that similar treatments may be applied with various agricultural important fruits.