Effect of Interior Canopy Defoliation on Berry Composition and Potassium Distribution in Thompson Seedless Grapevines

¹ Department of Viticulture and Enology, University of California, Davis, Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648.

The interior portion of the Thompson Seedless grapevine canopy was defoliated approximately 950 `growing degree days' (GDDs) after budbreak in 1983 and 1984. Leaf area removed at that time was 9 m² (1983) and 7 m² (1984), which represented 35% and 27% of the total vine leaf area, respectively. Solar radiation penetration, when measured directly beneath the vine's canopy at soil level, increased from 40 µmol/m²/s for the control to 100 µmol/m²/s for the defoliated treatment. The amount of leaf dry matter lost subsequent to 1000 GDDs after budbreak was greater for the control than for the defoliated treatment. Leaf removal from the interior of the vine had no significant effect on fruit maturation characteristics in either year. In addition, there was no treatment effect on berry potassium concentration. A net loss of K from the stems of the vine prior to fruit harvest was measured, while there was no loss of K from the leaves throughout the season that could not be accounted for due to leaf fall or shoot trimming. The results indicate that the majority of K in fruit of Thompson Seedless grapevines comes either from the soil or the permanent structures of the vines during the course of berry growth with little redistribution from the current season's growth of leaves or stems under the conditions of this study.

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