Defoliation Timing of Fruit Cluster Leaves Influences Fruit Quality Attributes, Skin Coloration and Anthocyanin Contents in ‘Hongro’ Apples

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Apple skin color is an important quality trait affecting marketability.The anthocyanin accumulation in 'Hongro' apple skins was analyzed by evaluating different defoliation timing treatments of fruit cluster leaves applied two, three, and four weeks before harvest (WBH).Non-defoliated plants served as controls.Defoliation did not affect the fruit weight, size, flesh firmness, soluble solid content, titratable acidity, or the starch pattern index at harvest.However, the fruit sunburn rate was higher for defoliation at 3 and 4 WBH compared to defoliation at 2 WBH and non-defoliation treatments.The red-colored area and a* value for both the sun-exposed and sun-shaded sides of apple skin were highest in the defoliation treatments at 3 and 4 WBH, intermediate at 2 WBH, and lowest in the non-defoliation treatment case.On the sun-exposed side, the L* value was lower after the defoliation treatments compared to the non-defoliation treatment, but the b* value was not affected by any of the defoliation treatments.On the sun-shaded side, the L* and b* values were lower under defoliation at 4 WBH compared to the other treatments.The anthocyanin content on the sun-exposed side was highest after defoliation at 4 WBH, followed by 3 and 2 WBH, and was lowest in the non-defoliation treatment.Similarly, the anthocyanin content on the sun-shaded side was highest in defoliated plants at 4 and 3 WBH, intermediate at 2 WBH, and lowest in the nondefoliated plants.Additionally, the expression levels of major anthocyanin biosynthetic genes (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT, and MdMYB10) were upregulated in all defoliation treatments on both the sun-exposed and sun-shaded sides, particularly during early defoliation at 4 and 3 WBH.Therefore, this study showed that the defoliation timing influences the development of red skin coloration in 'Hongro' apples and that defoliation at 4 and 3 WBH achieves the optimum coloration for this cultivar.