To: Virginia Vintners

From: Bruce Zoecklein

Subject: Fruit Maturity and Leaf Fall

Fruit maturity and leaf fall. Many vineyards around the state have experienced either a frost or hard freeze, which has in many cases defoliated vines. How does partial leaf fall influence fruit maturation? Are there desirable changes which can occur in the absence of a full leaf canopy?

The influence of frost on fruit maturation may depend largely on the maturity at the time of frost.

Aroma/flavor and color ripening are not necessarily synchronous with other ripening processes. It is generally recognized that aroma/flavor and anthocyanins are synthesized from sugars surplus to those required for respiration and tissue growth. As indicated in a previous edition of Enology Notes, at some point late in the season the phloem tissue begins to close, preventing the grape berry from importing water and photosynthates. When this occurs the berry has reached its maximum weight. Sometime following phloem shutdown and maximum berry weight, maximum aroma/flavor occurs. This is significant because it means that the compounds responsible for aroma/flavor are already in the fruit in some form. At this stage, translocation from the leaves may not be an important issue.

The influence of leaf area on skin phenol polymerization and seed oxidation (maturation) has not been thoroughly studied, but also likely depends on the extent of fruit maturity. Respiration of malic acid is governed by the malic enzyme (ME), located in the fruit. The enzyme activity is highest in the center of the berry during seed development, and in peripheral tissues during ripening (Prossner et al., 1983). The acid concentration this time of year is a function of the extent of malic respiration and changes in berry volume.

As suggested in Enology Notes # 27, careful evaluation of aroma/flavor and phenol maturity should be conducted. Additional maturity can occur following some frost damage.

Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head, Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html

Phone: (540) 231-5325
Fax: (540) 231-9293
E-mail: bzoeckle@vt.edu