Alex Gapinski, Suzanne Slack, and Aude Watrelot*
*Iowa State University, 536 Farm House Ln, Ames, IA, 50010 (watrelot@iastate.edu)

Tannin concentration is related to red wine quality, but many factors influence it, including low extractability and/or low initial content in skins and seeds. This is a real challenge in non-Vitis vinifera grape cultivars. This study aimed to evaluate the effect of plant growth regulators on tannin content in Marquette grape skins and seeds. A complete block design was used to assign 24 Marquette vines as three-vine panels to eight treatments with three replicates. Prohexadione calcium was applied to change the cell wall thickness and structure that may reduce extractability of tannin. Phenylalanine is a precursor of tannin and anthocyanin biosynthesis. Those plant growth regulators were applied as foliar sprays five times and two times, respectively, from one week postbloom to one week postveraison. Berries were sampled five times, from one week post-fruit set to harvest. Grapes were crushed to make juice and berries were peeled and separated into skins and seeds; wine was also made at harvest maturity. Basic chemical parameters (pH, titratable acidity, color intensity, etc.) were measured on juice and wine. Tannin and iron-reactive phenolic concentrations were quantified on juice, wine, and extracts from skins and seeds by HPLC-DAD and UV-vis spectrophotometry, respectively. Preliminary results from juice showed no statistical differences among treatments for any parameters measured. This may be because plant growth regulator rates were too low to see drastic effects immediately in juice. Analysis of results from skins, seeds, and wine is ongoing. This study will be replicated in 2023 to investigate the effects of plant growth regulators over two growing seasons.

Funding Support: No funding source.