Effects of Promalin on Fruit Growth and Cuticle Properties of ‘Pinova’ Apple

Abstract

Promalin (Valent BioSciences, Libertyville, IL, USA) is a proprietary mixture of gibberellin A4 + 7 and 6-benzyladenine that is widely used in apple production to improve the fruit shape, size, and skin quality. Promalin typically increases fruit size and length. However, the increased growth likely increases the strain in the fruit skin, which may exacerbate microcracking of the cuticle and, consequently, russeting. This study aimed to monitor the growth-stimulating effect of Promalin in three different regions of fruits and investigate whether Promalin affects cuticular microcracking via effects on the deposition of cuticular components or via the accumulation of strain in the cuticle. Four Promalin sprays (20 mg⋅L−1) were applied to runoff; the first was applied at full bloom, and the remaining sprays were applied at approximately weekly intervals thereafter. Fruit surface areas and fruit surface area growth rates of the Promalin-treated fruits were higher than those of the untreated control fruits. Promalin increased the fruit length, but it had no effect on the fruit equatorial diameter. In Promalin-treated fruits, the base of each sepal extended, thickened, and became fleshy as early as 15 days after full bloom (DAFB). Allometric growth analyses revealed higher constant differential growth ratios of the pedicel and calyx length (before 36 DAFB) in Promalin-treated fruits than in control fruits. After 36 DAFB, the difference in constant differential growth ratios between Promalin-treated fruits and control fruits decreased. Cuticle mass per unit area increased with time in all regions of the fruit surface and was slightly (+3.3%) but significantly higher in fruits treated with Promalin than in control fruits. Additionally, the biaxial strain release was slightly and significantly lower in Promalin-treated fruits than in control fruits. When the isolated, cuticle was ablated from the inner surface and dewaxed, strain relaxation in the control fruits was higher than that in the Promalin-treated fruits. It was concluded that Promalin treatment increases the length of the fruit by increasing the lengths of the pedicel and calyx regions early during fruit development. Promalin only slightly increased cuticle deposition and fixation of cuticular strain. Promalin had no effects on microcracking or russeting.