Josh VanderWeide,* Zhongli Ma, Paolo Sabbatini, Tommaso Frioni, and Patrick Murad
*Michigan State University, 2355 Club Meridian Dr. Apt. D3, Okemos, MI 48864 (vande732@msu.edu)

Removal of basal leaves early in grapevine vegetative development limits assimilation of carbohydrates to florets, reducing fertilization and the number of berries per cluster. This subsequently controls vine yield, improving fruit quality. Mechanization of this practice can save time and money for growers, but has not yet been compared with manual application in cool climate growing regions, where adequate seasonal temperature accumulation is a major limitation on yield and fruit technological maturity at harvest. The goal of this study was to compare mechanization of leaf removal with manual removal of six leaves at the prebloom (E-L 17) or after-bloom (E-L 27) phenological stages against a control (removal at veraison) over two seasons. Results indicate that mechanical treatments removed less leaf area than manual ones at each timing, leading to lower lateral compensation and poorer fruit-zone microclimate conditions. Despite this, fruit set decreased more in mechanical treatments at each timing. Photosynthesis data showed a strong compensation in Phi2 (quantum yield of Photosystem II) in apical leaves of vines subjected to prebloom treatments. Additionally, the NPQt (non-photochemical quenching) parameter was significantly pronounced in the medial and apical leaves of both manual treatments, indicating severe stress conditions in the leaves unrelated to weather conditions. Compared to manual treatments, berry sugar concentration was higher due to mechanization, while total acidity decreased only with manual treatments. Furthermore, metabolomics analysis revealed a significant increase in anthocyanin and flavonol compounds with prebloom mechanical treatment in both experimental years (2016 and 2017). Enhanced fruit quality with the mechanical treatments proved to be the result of a fruit-zone that received adequate light and temperature exposure early in development, but that retained significantly more leaf area to influence ripening after veraison. This information provides an important strategy to ripen red vinifera cultivars in cool climates.

Funding Support: Michigan Grape and Wine Industry Council Project GREEEN