Early-season Tree Shading Decreases Phenolic Acids and Quercetin Glycosides, but Not Proanthocyanidin Concentrations in Cider Apples at Harvest

Polyphenols are critical for hard cider quality, determining flavor, color, and shelf life, yet their biosynthesis in cider apples remains poorly understood. We provide a comprehensive analysis of how sunlight affects individual polyphenol development throughout fruit development, distinguishing between direct photosynthetic effects on fruit vs. indirect effects through whole-tree carbohydrate supply. For two field seasons, high-tannin cider apple trees were covered wholly or in part (just fruitlets) with shadecloth to reduce photosynthetically active radiation from 1 to 5 weeks after full bloom (WAFB). The experiment was conducted on ‘Porter’s Perfection’ trees in 2020 and 2021, and ‘Binet Rouge’ trees in 2021. Treatments included a control (no shadecloth) and low and high shading levels applied to either the whole tree or to individual fruitlets. Although most polyphenols, including procyanidins, from the full-tree shade treatments had reduced concentrations in comparison with the unshaded controls during the shading period of 1 to 5 WAFB, they did not differ in polyphenol concentration from the control at harvest. In addition, polyphenol classes responded differently to light reduction; whole-tree shading decreased phenolic acids and quercetin glycosides—compounds that contribute to cider bitterness, antioxidant activity, and flavor complexity—by 15% to 27% at harvest, whereas proanthocyanidin concentrations, the primary tannins responsible for cider astringency, remained unaffected. Direct fruit shading had minimal impact on flesh polyphenol concentrations, indicating that fruit photosynthesis and light-induced polyphenol biosynthesis in the fruit contributes minimally to polyphenol concentrations at harvest compared with whole-tree carbohydrate supply. This research advances the fundamental understanding of polyphenol regulation in apples and provides practical guidance for cider apple production. Specifically, growers should prioritize orchard designs and canopy management practices that maximize whole-tree sunlight interception to enhance photosynthetic capacity.