Foliar applications of phenylalanine and prohexadione calcium for managing tannin content in cold-hardy hybrid grape cultivars

Cold-hardy hybrid grape cultivars have low tannin content compared to Vitis vinifera cultivars, which leads to unbalanced red wines and therefore low-quality wines. Viticultural practices, such as nitrogen fertilisation and the use of plant growth regulators, have shown to be effective interventions to modify tannin content in V. vinifera cultivars and may be effective with cold-hardy hybrids. Foliar applications of phenylalanine (Phe, 100 mg/L) and prohexadione calcium (ProCa, 50 mg/L) were applied separately and in combination (ProCa+Phe) to Marquette grapevines grown in Iowa, USA in 2022 and 2023. Treatments were assigned following a split-plot design. Berry weight, diameter, seeds per berry, tannin content, and iron-reactive phenolics content of skins and seeds were analysed at five phenological time points. Samples from treated vines were compared to untreated control samples using a random-intercepts linear regression model. Overall, the effect of year was more impactful than the individual effects of treatments due to differences in temperature and rainfall during berry development. However, skin and seed tannin and IRP were significantly affected by the Year2023:ProCa interaction at 3 weeks post fruit set and véraison, respectively, suggesting ProCa treatment may be more effective with multiple applications in consecutive years. Low application rate may be a reason for limited treatment effectiveness and further work on dose response may demonstrate foliar sprays of Phe and ProCa to be effective tools at managing phenolics content in Marquette grapes.