Effect of Time and Severity of Defoliation on Growth of Roots, Trunk, and Shoots of 'Thompson Seedless' Grapevines

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Abstract
The effect of leaf removal on accumulation of dry matter in grapevine parts was evaluated in a pot experiment on nonfruiting rooted cuttings of Vitis vinifera L., 'Thompson Seedless'. Defoliation was 0 (control), 25, 50, and 75% (herein called OD, 25D, 50D, and 75D vines), performed at three different times; simulated fruit-set (SET), veraison (VER) and fruit maturity (MAT), over a period of 2 years. The 25D and OD vines did not differ significantly in dry weights, regardless of time of defoliation. Dry weights (DW) of canes, trunk, and roots were significantly lower in vines defoliated 50 and 75% at fruit-set than in OD vines. With defoliation at veraison, however, only the trunk and roots of 50D and 75D vines had lower DW than OD vines. At harvest, only root DW from 50D and 75D vines was significantly less than in OD vines. Total leaf area was related positively to total root DW. Trunk DW was affected most by defoliation, followed by DW of roots and canes. Total cane length and average internode length were significantly less in 75D-SET vines than in OD vines and in all other defoliated vines at all defoliation times except for 50D-SET and 75D- VER vines. All levels of defoliation at fruit-set significantly increased the number of laterals, though defoliation at later dates did not. DW per unit length of cane and number of mature nodes per vine were not affected significantly by time or amount of defoliation. There were no significant interactions between time and amount of defoliation on any of the parameters measured.

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