Impact of Post-Fruit Set Leaf Removal on Marquette Phenolic Compounds during Berry Development and Ripening

Background and goals Red wines made from interspecific cold-hardy grape cultivars tend to be low in tannin and high in acidity due to their genetic background of various Vitis species and the climatic environment in which they are grown. This study focused on whether leaf removal can improve the quality of Marquette (Vitis spp.) grapes by reducing acidity and increasing the biosynthesis of phenolic compounds.

Methods and key findings Marquette grapevine leaves (six to eight) were removed or not removed (control) from the shoot base at one week post-fruit set in 2020 and 2021. Precipitation and daily temperature were recorded, and growing degree days (base 10°C) were calculated. Basic chemical analyses, total iron-reactive phenolics, tannins, and monomeric anthocyanin concentration in juices were determined throughout berry ripening. The tannin content in grape skins and seeds throughout berry ripening was also evaluated. Early leaf removal (ELR) resulted in a reduction in malic acid concentration of up to 13% in grape juice. At harvest in 2021, the level of color intensity, total iron-reactive phenolics, and anthocyanins were higher by 21, 26, and 51%, respectively, in grape juice with ELR treatment, which could be associated with increased sunlight exposure.

Conclusions and significance The concentration of tannin in juices dramatically decreased from veraison to harvest, but no effect of leaf removal was observed. Further studies are necessary to elucidate the possible factors that lead to decline in tannin content and extractability. A noticeable effect of the growing season on the basic chemistry of Marquette grape juice was observed.