Benzyladenine and gibberellin treatment of developing "Pink Lady" apples results in mature fruits with a thicker cuticle comprising clusters of epidermal cells

A mixture of 6-benzyladenine (BA) and gibberellins GA(4) plus GA(7) applied to "Pink Lady" apple at early phenological stages was previously shown to result in an immediate increase in epidermal cell density and associated reduction in calyx-end cracking disorder in the mature fruit, implying a long-term effect of the BA + GA(4+7) mixture. Here, we analyzed the anatomical changes in the mature peel at the calyx end 210 days after full bloom (DAFB), following application of the plant growth regulators (PGRs) at the cell-division phase of fruit development, 21-50 DAFB. Experiments were conducted in northern Israel, and the PGRs were applied as the commercial formulation Superlon (TM) (Fine Agrochemicals Ltd.), composed of 19 g l(-1) BA and 19 g l(-1) GA(4+7). Trees were sprayed with 0.025, 0.1, or 0.2 % (v/v) Superlon (TM). The most obvious phenomenon was the presence of epidermal cell clusters within the cuticular matrix that were detached from the native epidermal layer located at the bottom of the cuticle and which could not be detected in the untreated control fruits. Treatment with 20 mg l(-1) BA + GA(4+7) (0.1 % Superlon (TM)) resulted in a markedly thicker cuticle, a higher percentage of detached epidermal cells within the cuticular membrane and a significant reduction in calyx-end cracking at harvest. The presence of cuticle-embedded epidermal cell clusters may have contributed to strengthening the peel by adding more cell-wall components, thickening the cuticle layer and possibly enhancing crack repair.