Commercial grapegrowers strive to produce the maximum profitable yield of grapes at the composition required to produce wine of a target style that meets quality expectations of consumers. However, this must be done consistently. Since grapevines are perennial plants, the yield must be achieved without negatively influencing the vine’s capability of producing a crop in subsequent years. To achieve the correct balance between fruit yield and root and shoot growth, viticulturists need to enable the current season’s crop to ripen without “stealing” from vine reserves and negatively affecting next year’s crop.

When contemplating vine balance, the factors that determine the capacity of the vine to fix carbohydrates (sugars) through the process of photosynthesis and to accumulate inorganic nutrients for growth need to be considered.

The rate of photosynthesis and the ability of vines to fix carbohydrates largely reflects the environment in which the vine is growing (warmer sites with increased sunlight and a longer growing season will generally have greater capacity to accumulate carbohydrates). However, vine photosynthesis can be manipulated by growers with the selection of vine-training systems, vineyard design and spacing.

For example, the development of divided canopy-training systems has been shown to simultaneously increase potential yield and improve fruit composition.²⁴ The systems increase the intercepted solar energy and enhance the ability of vines to fix photosynthates, thereby increasing their capacity.

At the leaf level, light saturation (the point at which increasing light intensity does not further increase the rate of photosynthesis) reaches its maximum at about 1,000 µmol m^-2 s^-1.¹⁴ However, in the field, higher light intensities may continue to increase the rate of vine photosynthesis as light penetrates farther into the canopy, increasing the photosynthetic rate of leaves underneath the outer layer of leaves.²²

In addition to site effects, the available energy changes both between and within a growing season. Early in the season, temperatures are generally lower than mid-season, and day length is shorter. At the same time, the canopy is still developing. Early, rapid shoot and leaf development at this time often reflects the carbohydrate and nutrient reserves accumulated in the trunk and root system the previous year. Similar incoming energy reductions are observed in the autumn, although by this time, the canopy should be full size.

Fixed photosynthates now are accumulated in the fruit as it ripens. However, to ensure adequate over-wintering, reserves of carbohydrates and nutrients have to build in the roots and trunk at the same time. In many cool climates, leaf senescence coincides with harvest, thus it is important to build up reserves in the trunk and roots by this time.1 In warmer climates, where leaves remain functional post-harvest, the reserves can accumulate during this period. For this reason, the timing of leaf senescence in relation to the date of harvest is a key difference between warm and cool-climate viticulture.

While this review largely considers the role of photosynthates in vine balance, other environmental and biotic factors must be well managed. The professional grower must minimize damage to vines caused by pests and diseases, waterlogging, frost etc. While inadequate inorganic nutrition (nitrogen) and/or water stress will slow growth, excessive water stress will slow photosynthesis. A degree of mild water stress and a slowing of growth may be an advantage by redirecting photosynthates from vegetative growth to fruit development later in the growing season.

Additional aspects in determining vine balance include the mechanisms that control the distribution of photosynthates to the different sinks (predominantly shoots, roots and fruit, etc.) within the vine.

Distribution of the sugars from sources (generally the leaves, except in early spring when remobilization from overwintering reserves are the predominant source) depends on the demand from various sinks in the vine. Photosynthates are conducted to the various sinks through phloem vessels.
The majority view is that transport is through osmotically powered pressure-driven flow, as proposed by Ernst Munch.^17,23 The direction and rate of flow and relative contribution that a source provides to a sink depends on the proximity of the source to that sink and the concentration difference between the two.

In practice, sinks will be supplied from sources close to them, although this may be modified by the relative demand by the sink (e.g. stronger sinks will accumulate photosynthates from more distal leaves), and the direction of flow may change during the growing season. For example, fruit becomes the predominant sink during ripening, while reserves in the trunk and root system are the main sources until an adequate leaf area has developed in the spring.

Grapevine triangle
The grapevine triangle integrates two important concepts of balance common to all plants. The triangle represents the functional equilibrium between the three predominant sinks (shoot growth, root growth and fruit yield), and the strength of each sink is represented by the shape of the triangle. The capacity of the plant to accumulate photosynthates is reflected in the area of the triangle. The larger the triangle, the greater the capacity. (See “Concept of Vine Capacity and Carbohydrate Partitioning.”)

In 1927, W.H. Pearsall showed that plants, like animals, exhibit allometric growth.²⁰ He harvested plants at regular intervals and demonstrated that shoot and root dry matter accumulation were related. Regulation of the shoot-to-root ratio is not imposed by morphology. There are, in general, an excess of potential meristems in both the shoot and root that are not activated and, in practice, the control of plant form is achieved by selected suppression of meristem growth.

In 1983, R. Brouwer suggested that the relative growth rate of various organs is a reflection of the environment in which the plant is growing. In a specific environment, this results in a “functional equilibrium,” in which root growth is limited by the supply of assimilates from the shoot, while shoot growth is limited by the supply of nutrients from the roots.²

The mutual dependence of shoot and root growth results in a balance in their development with a result that, under low fertility conditions and/or sites with limited water availability, root growth will be favored at the expense of shoot growth (see “Influence of Soil Fertility on Root and Shoot Distribution”).Conversely, in shade, shoot growth will be favored over root growth.

However, unlike the herbaceous plants used by R. Brouwer, fruit is a third important sink of grapevines and the most important component to the viticulturist. Approximately 20% of the final bunch dry weight has been accumulated by véraison.²⁵ The remaining 80% is accumulated from véraison to harvest, and during this period fruit is the dominant sink for carbohydrates.

An inadequate leaf area or excessive yield (depending on the capacity of the site, variety and target soluble solids) will slow sugar accumulation by the fruit¹⁹ and deprive roots and shoots (in particular trunks) of the reserves necessary for spring growth. (See “Thinning and Trimming Impact on Soluble Solids and Titratable Acidity.”)

The conservative nature of carbohydrate distribution between the various sinks was demonstrated by Stan Howell and colleagues.⁶ In their experiments, a range of clusters from none to six were retained on pot-grown Seyval Blanc vines. Total cluster dry weight increased with the greater cluster number, but the shoot, leaf, wood and root dry weight decreased, with the result that the total vine dry weight remained the same regardless of the number of clusters on the vine. Their experiments demonstrated the conservative nature of dry matter accumulation by vines and the effect that that over-cropping may reduce reserves in the shoot and root systems necessary for shoot development the following spring. (The figure “Influence of Site Capacity and Yield on Equilibrium” conceptualizes this effect using the grapevine triangle.)

However, achieving consistency in vine balance in a vineyard is not easy, and several measures have been used to define a balanced vine. These are usually comprised of a ratio between vegetative and reproductive growth. Of these, the Ravaz Index, (the fruit yield/dormant pruning weight following harvest) is probably the most widely used. The ratio was developed by Louis Ravaz in the early 1900s, when he concluded that higher crop loads resulted in sick vines.¹⁵ The development of the Ravaz Index (elegantly recently summarized by Mark Matthews)¹⁵ remains widely used when quantifying vine balance.

However, the optimum Ravaz Index described in the literature can range from 5 to 10, with the different ratios reflecting site, variety and production goals. Other indices have been used to quantify vine balance. These indices have been summarized by Peter Dry et al.⁵ and include total leaf area (m²)/fruit weight (kg) per vine, pruning weight (kg) per unit canopy length (m) and mean cane weight (grams). However, like the Ravaz Index, each of these indices has a range, and one can conclude that there is no universal value, but recording an index provides the viticulturist with useful data about consistency between seasons and a target index measure for a particular site.

Experiments undertaken on Sauvignon Blanc in New Zealand investigated the extent to which vines pruned to a fixed node number for several years accommodated differences in retained nodes. Vines were pruned to retain between 24 and 72 nodes each year for four years.9 In the first year, yield reflected the number of nodes retained, and vines with 72 nodes produced a yield 76% greater than those pruned to 36 nodes. (See “Sauvignon Blanc’s Response to Pruning Treatments.”) However, after four years, the difference was 29%, as the increased bud number led to reduced bud burst and thinner shoots (and thus canes), fewer clusters per shoot and smaller clusters.

Ensuring vines have canes of an appropriate diameter at pruning is important for yield consistency. The diameter of canes influences the inflorescence number per shoot and size the following year.⁷ In Marlborough, New Zealand, for example, 8-mm diameter Sauvignon Blanc canes have approximately 30% fewer bunches per shoot than those of 11-mm canes,⁷ although the absolute number of bunches per shoot is affected by other factors during the inflorescence initiation period,⁷ including light and temperature exposure to the bud¹⁶ and/or water and nitrogen stress.¹⁰

Grapevine triangle and fruit ripening
Grapevine capacity (or leaf area : fruit weight ratio) may influence ripening in two ways. For example, reducing the capacity by trimming Sauvignon Blanc and Pinot Noir shoots from 12 to six leaves shortly after fruit set increased the time from flowering to fruit véraison, delaying the onset of ripening.^18,21 Similarly, the reduction in leaf area in this manner either at or before véraison slows the rate of sugar accumulation, increasing the time it takes to attain the soluble solids desirable for harvest.¹⁹ A reduction in leaf area can be reversed by thinning fruit. The rate of soluble solids accumulation on vines trimmed to six leaves with half the fruit removed was similar to that of 12-leaf vines with a full crop.²¹

Unlike soluble solids, titratable acidity is little affected by trimming. As a result, the leaf area : fruit weight ratio will change the sugar : acid balance of the fruit, and the extended ripening resulting from the lower leaf area resulted in fruit with a lower acidity at any given soluble solids content.¹⁵

How many nodes to retain?
Understanding the capacity of a site and the influence of site and vine management on balance helps viticulturists make informed decisions at pruning. The concept of balanced pruning was introduced by Walter J. Clore in the 1960s as a way to use the productivity measured in the current season to estimate the number of nodes to retain to achieve an adequate yield of mature fruit in the following season.³

Since no two growing seasons are alike, it can only be an estimate, but developing an understanding of how vines behave in a given vineyard environment will help viticulturists achieve maximum yield without compromising future productivity. It also assumes that yields are unamended by thinning.

This method has been suggested for juice grapes, in which pruning weight is used to determine vine capacity and from that the number of nodes to retain. For example, Markus Keller suggested that to balance-prune Concord vines, growers should retain 55 nodes for the first 500 grams of pruning weight and add 22 nodes for each subsequent 1 kg of pruning weight.¹³ Like the Ravaz Index, the nodes retained to pruning weight ratio will depend on the grape variety and likely will vary with site and management.

An alternate approach we have promoted in New Zealand for Sauvignon Blanc is to count the effective shoot number (ESN). This involves counting and grading during dormancy all shoots on a vine using the following grading system: vigorous shoots (larger than 15 mm diameter) = 2; average shoots (10 to 15 mm) = 1; small shoots (7 to 10 mm) = 0.5; and shoots less than 7 mm are ignored. The total plus 5 (to account for any damaged buds that may be in the canopy) gives the number of nodes to retain. The appropriate diameter classes may vary with variety and vineyard site and needs to be evaluated at each vineyard.

Counting shoots also allows an estimate of historic vine performance. If the bud burst (ESN / count nodes retained after pruning) is less than 0.90, this suggests blind budding; a value greater than 1.10 implies a significant number of non-count shoots, while 0.9 to 1.1 suggests that an appropriate number of buds were retained in the previous growing season.

Final thoughts
When pruning vines to achieve balance, do not be greedy. Only retain as many nodes to enable vines to ripen the crop adequately. Using a method to quantify vine performance (Ravaz Index, ESN, etc.) will provide a historic record of the vineyard, enabling the viticulturist to assess the inter-seasonal and long-term changes in vigor and productivity. Counting shoots and grading shoots using the ESN system provides a simple way of estimating vine potential, but this system needs to be assessed on each property and for each variety.

The leaf area: fruit weight ratio largely determines the timing and rate of soluble solids accumulation, and it is likely that the optimum ratio will depend on capacity of the site, variety, wine style, vine-training system and canopy management. There is no universal value.

When managing vines, the crop load needs to be adjusted to enable sufficient reserves to be accumulated in the roots and trunks to enable vines to produce good, uniform spring growth. Good viticulture includes managing to an appropriate yield, maintaining good pest and disease control and optimizing nutrition and irrigation. Keeping good records of vine performance will help achieve this.

Mike Trought is a principal scientist with Plant and Food Research at the Marlborough Research Centre in Blenheim, New Zealand. He is an adjunct associate professor at Lincoln University and a fellow of New Zealand Winegrowers. His research includes management of carbohydrate physiology of vines, flowering, fruit set and yield prediction and understanding the influence of soils and environment on fruit composition.

References
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