Alexander Gapinski, Suzanne Slack, and Aude Watrelot*
*Iowa State University, 536 Farm House Ln, Ames, IA, 50011 (watrelot@iastate.edu)

Cold-hardy hybrid grape cultivars have less tannin than Vitis vinifera cultivars, which leads to low-quality, unbalanced red wines. Foliar sprays of phenylalanine (Phe; 100 mg/L) and prohexadione calcium (ProCa; 50 mg/L) were applied separately and in combination (ProCa + Phe) to Marquette grown in Iowa throughout the 2022 and 2023 growing seasons. ProCa was applied to change the plant cell wall structure and modify tannin extractability. Phe is a precursor of flavanoid biosynthesis. Treatments were assigned following a split-plot design to three-vine panels with six replicates. Samples from treated vines were compared to untreated control samples using an additive effects linear regression model with random intercepts. Basic chemical properties, tannin content (high-performance liquid chromatography-diode array detection), and iron-reactive phenolic (UV-vis spectrophotometry) content of juice, wine, and skin and seed extracts were analyzed at five phenological time points. In 2022, the ProCa treatment had lower titratable acidity at three weeks post-fruit set, but Brix, pH, hue, and color intensity were unaffected at any time point. At the mid-ripening time point, Phe showed a positive effect on juice tannin concentration. However, the model showed that when Phe and ProCa + Phe treatments were applied, skin tannin and iron-reactive phenolic contents were negatively affected. Seed phenolic content was not affected by foliar-applied Phe or ProCa. Overall, limited effects on tannin and IRP contents were observed in different grape tissues, which may be explained by the low doses of Phe and ProCa used in this study. However, analysis of 2023 data is ongoing to investigate the effectiveness of Phe and ProCa foliar sprays at managing phenolic content in Marquette grapes over two growing seasons.

Funding Support: No external funding.