Maintenance of high epidermal cell density and reduced calyx-end cracking in developing 'Pink Lady' apples treated with a combination of cytokinin 6-benzyladenine and gibberellins A4+A7

•Pink Lady apple is susceptible to pre-harvest physiological disorder of calyx cracking.•BA + GA4+7 spray increased epidermal cell density at the calyx-end and reduced the damage.•Low PGR concentration (5 mg l−1) was potent at cell division stage of fruit development.•High PGR concentration (40 mg l−1) was potent at cell expansion stage of fruit development.•The incidence of the disorder was associated with the duration of seasonal high temperatures.

Calyx-end cracking in 'Pink Lady' apple develops as concentric cracks located around the stylar-end. The factors that induce the cracking are unknown, however we previously demonstrated that the intensity of the disorder can be controlled by application of a mixture of synthetic gibberellins (GA4 plus GA7) and cytokinin (6-benzyladenine; BA) at different phenological stages of fruit development, and suggested that the plant growth regulators (PGRs) affect the elasticity of the epidermal layer at the calyx-end by increasing cell density. The present study focused on histological analyses of the peel at the calyx-end following the application of BA + GA4+7. The experiments were conducted in Northern Israel during 2011 and 2012. In 2011 the PGRs were applied as 0.2% (v/v) Superlon™ (i.e., 40 mg l−1 BA plus 40 mg l−1 GA4+7) once, twice or three times every 14 days since 60 days after full bloom (DAFB). In 2012, to test the efficacy of a lower concentration of the PGRs, 0.025% (v/v) Superlon™ was used. Spraying was applied at 60 to 90 DAFB (cell expansion phase) as before, in addition to earlier spraying at 7 to 35 DAFB (cell division phase). In all treatments given at cell expansion phase the BA + GA4+7 mixture increased the epidermal cell density at the calyx-end, compared to control. Multiple spraying maintained the high cell density during fruit expansion compared to one-time application, and the higher concentration of the PGRs was more potent than the lower. Assessment of calyx-end cracking suggested that low concentration of the PGRs (i.e., 5 mg l−1 BA plus 5 mg l−1 GA4+7) is most efficient when applied three times at cell division phase.Interestingly, in 2011 the incidence of calyx-end cracking was significantly lower relative to 2012 and 2010 that were characterized by extended daily hours of high temperatures (>34 °C). This association can be used by growers as a criterion in applying the PGR treatment early in the season when temperatures are higher than the average and high incidence of calyx-end cracking might be expected.