The Effect of Partial Defoliation on Development and Distribution of Roots of Vitis vinifera L. cv. Cabernet Sauvignon Grafted onto Rootstock 99 Richter

¹ Viticultural and Oenological Research Institute (VORI), Private Bag X5026, 7600 Stellenbosch, Republic of South Africa.

The effect of partial defoliation (33%) over the whole canopy from different developmental stages of the vine on development and distribution of roots of Vitis vinifera L. cv. Cabernet Sauvignon was investigated under field conditions. Partial defoliation significantly stimulated root density, commencing from berry set. Root density apparently increased the later during the growth season defoliation commenced. Yield and cane mass of partially defoliated vines tended to coincide with root densities. This relationship did not hold for non-defoliated vines. Root development in fine to medium diameter classes was increased by partial defoliation. However, the development of thick roots apparently decreased. Partial defoliation from pea size seemed to create the most effective root system. Early defoliation (from bud break and berry set) reduced root development compared to later defoliations (from pea size and veraison). Partially defoliated vines had higher total root numbers in all soil layers, although differences were not significant. Regardless of treatments, roots were mainly located in 0 to 800 mm soil layers, with deeper layers containing much less roots. Higher root densities and generally larger numbers of particularly fine roots in all soil layers for partially defoliated vines suggested a more efficient nutrient absorption capacity and utilization of soil for these vines. Partially defoliated vines responded to the loss in leaf area by forming new roots, which may improve vine performance.

Key words: Cabernet Sauvignon, defoliation, root system

Submitted on August 6, 1991