Cabernet Franc vines with full trellis fill providing good vine capacity, but more vigor would give excessive canopy density.

Vine “capacity” was defined byA. J. Winkler as the total growth of the vine and crop; the total dry matter production of a vine.15 This is a function of 1) seasonal sunlight intercepted by a vine (varies with leaf area, canopy display, and climate), 2) conversion of the sunlight energy to dry matter (initially sugars from photosynthesis), and 3) the use of some sugars burned for the energy to build vine components.

Without sacrificing entire vines, it is not easy to measure vine capacity in the vineyard. Instead, several indirect measures have been developed to estimate vine capacity.

Winter pruning weight is commonly used as an indirect estimate of sunlight interception and thus vine capacity, as it is generally related to vine size late in the season. However, these relationships do not always hold as stresses during the season may give more or less vine growth and reduce leaf function. Additionally, winter pruning weights do not account for shoot growth removed by summer hedging or shoot thinning.

For a given pruning weight the actual vine sunlight interception will depend on spacing, training system, canopy display, and uniformity of trellis fill. For example, divided canopies such as the Lyre system might have similar leaf areas and pruning weights to undivided canopies, but considerably more light interception.

Additionally, the pruning weight to light interception relationship is not linear; changes in pruning weight at low pruning weights will have greater effects than at high pruning weights where additional growth may primarily create more shade rather than increased light interception. Finally, there is only one value given per year, thus any seasonal dynamics are ignored.

Actual leaf area attained during the season is a more valid estimate of vine light interception than pruning weight, and thus the leaf area is

ine balance, often discussed and felt to be a key goal of grapegrowing, refers to the relative balance of vegetative to reproductive growth. The term crop load is conceptually similar, but is more restricted to the balance of vine capacity or supply to crop demand while not directly considering vegetative demands.

The effects of vine balance or crop load on wine quality are frequently discussed in the literature and tasting room. While there is

little debate that there is an upper limit to how much fruit a vine can mature before wine quality suffers, there is also increasing evidence that wine quality is diminished when grapes are obtained from vines that are under-cropped. In other words, low quality wines are expected from vines that are greatly out of balance, either over-cropped or excessively under-cropped.

Vine balance and crop load are described in various ways that are discussed in excellent reviews by P. Dry, and E. Archer and J. J. Hunter.2,7 Here, we will primarily emphasize a few points on our current approaches, focusing on the seasonal dynamics of vine carbohydrate supply in relation to crop demand, and on the relation of vine balance to winegrape-derived aroma compounds.