Vine Physiology and Function
Ian Merwin
presented by Tim Martinson

Main topics:

1. Basic forms and function of grape vines
2. Balance between vegetative and fruit allocation in the vine
3. Management practices that influence vine health, fruit yields, and wind quality

Trunk, head zone, cordon, shoots and canes, arms, spurs, suckers, internodes, renewal spur, cluster, leaf, tendril, primary and lateral shoots.

Annual growth cycle of vine

1. Dormancy, budburst, active growth, leaf fall
2. After budburst in spring, then shoot and flower expansion from preformed initials, in lateral buds on canes from the previous year
3. Fruit set after about 4-6 weeks of bud and shoot growth. Late June in NY
4. Next years flowers are formed during early summer, in lateral buds near base of current season shoots. Need to perform and survive winter for next yearıs crop

1. Flowers and fruit clusters on basal 2-5 nodes of current season shoots
2. Whole system from one (primary or secondary) multiple bud
3. Shoots of vinifera type grapes fairly upright, which can shade fruit clusters
4. No fixed number of nodes on shoot. Usually keep growing until intravine competition, nutrient shortages or cold damage stop growth

1. Fundamental limitation on growth and yield is sunlight interception by leaves
2. Since fruit in lower nodes, can be shaded by shoots growing above it
3. Berry clusters need abundant light (at least 50‰ of ambient) to produce full flavored berries and minimize fungal disease infections
4. Need for sufficient light in vine bearing surface means that optimum yields for wine quality are usually far less than maximum gross fruit yields of the vine

1. All living parts of vine including leaves require carbohydrates for cellular respiration
2. Below a critical amount of light, carbohydrates produced in photosynthesis (Pn) are not sufficient to meet needs of leaf itself. When Pn equals Respiration, leaf or vine is said to be at light compensation point. For leaves this occurs at low light levels, around 10% of full sunlight for plants like grapevines.
3. Pn saturates at about 75‰ of full sunlight on a leaf basis, but does not saturate even in full light for the vine as a whole, because of internal shading.
4. Shaded leaves contribute little or nothing to vine carbohydrate supply, so it is essential to maintain grape canopies open enough to allow light to penetrate into most of the leaves, as well as the fruiting zone.
5. The is the underlying principle of all vine training and canopy management systems

1. New leaves and growing shoot tips consume carbohydrates and other nutrients from other parts of the vine, until they are several weeks old
2. Actively growing shoot tips compete with basal clusters for nutrients
3. It is important for new shoot growth to slow or stop around veraison, in order for berries to accumulate sugars, develop seeds and skin coloration, and other flavor components such as polyphenolics and organic acids
4. When vigorous shoot growth continues past veraison, grape berry quality suffers
5. Vine cold acclimation can also be impaired by excessive late season vigor, because polysaccharides and proteins are also involved in hardening off of perennial tissues

1. Wild grapes can cover very large areas, and climb more than 30 meters up trees or walls.
2. Managed vines can also attain great size and age, as shown by the "Winkler Vine" at UC Davis, which is trained in massive cordons across an arbor, and covers about a thousand square meters
3. The bearing surface of a managed vine like the Winkler specimen comprises a much greater proportion of total vine area or biomass than in a wild forest vine, because regular pruning maintains sufficient light throughout the vine to ensure fruitful spurs on the cordon
4. In contrast, only the outer and upper parts of the wild grape canopy are productive

1. Grape vines integrate divers fixed and variable factors in response to soils, climate, and human interventions
2. The main site factors are soil type and depth, and nutrient and water supply
3. Critical climate factors are duration and intensity of sunlight, temperature means and extremes, humidity and rainfall, prevailing wind speed and direction, and evapo-transpiration or vine water usage.
4. Cultural practices are determined by vine and rootstock genetic traits, planting density, fertilization, irrigation, pest and disease pressure and management, and soil or groundcover vegetation management
5. All of these factors interact to determine vine vigor, foliage condition and function, canopy microenvironment, crop load, and ultimately grape quality for wine-making

1. New York: Deep fertile soils in a humid climate with cold winters demand special practices such as leaf removal, shoot thinning or positioning, and crop load adjustment to limit vine growth and prevent excessive shading or cropping
2. California: Deep fertile soils in regions with Mediterranean climates (mild wet winters and long dry summers) require competitive cover cropping and/or deficit irrigation methods to manage vine vigor. These are potentially the most productive vineyards, and can sustain high quality with high yields.
3. Southern Spain: Poor soils in hot dry regions. Here both soil and climate severely limit vine vigor. Yields are very low per unit land, but the fruit quality of adapted winegrape varieties can be excellent, without intensive management intervention

1. For each site and vineyard, there is a fundamental limitation to the cropload that a vine can carry and ripen at adequate levels of quality.
2. For example, an experiment with Zinfandel vines in California showed that wine flavor and aroma rating dropped off sharply as yields exceeded about 40 lbs/vine
3. It is worth noting that 40 lbs/vine equates to a total yield around 18 tons/acre. This is a very large crop, and wine-making quality and vine survival of subsequent winter cold would suffer at yields much lower than this is a cool-climate region.
4. Grape yield quotas are enforced in many European countries, partly to discourage overproduction of grapes, and partly to improve potential winemaking quality.
5. These quotas can be as low as 8MT/ ha of vineyard, which is far below the yields typical of irrigated vineyards in California and elsewhere

1. As nutrients and water supply are increased, berry size usually increases also.
2. The surface to volume ratio decreases rapidly as berry size increases
3. Since much of the color and flavor in red grapes are in their skins, a part of the trade-off between wine-making quality and yields involves the relative proportions of skin and flesh/juice in the larger berries that more vigorous vines may produce
4. California winemakers often blend grapes from non-irrigated hillside vineyards that produce highly colored and intensely flavored berries in with must or wine from more productive vineyards. Higher prices can compensate for lower yields in these situations

1. Total acidity decreases, pH increases, sugar concentrates, and color intensifies in most red grapes as they approach physiological ripeness
2. In regions with long, dry, warm autumns, the winemaker can delay harvest until the optimal balance between these attributes is attained
3. In cool climate vineyards, a killing frost may damage the grape canopy before these ripening processes are completed. Hence the importance of lake-effect and maritime influenced viticultural regions.
4. Grape sugar content may actually decrease approaching harvest time in cool rainy weather.

1. Grapes will ripen more fully if vine crop load is balanced with carbohydrate supply
2. Balanced pruning is a system developed by Nelson Shaulis and others at Cornell, which aims to adjust the bearing potential of the vine to its intrinsic vigor and canopy ability to supply nutrients to the crop.
3. The basic concept is to assess vine vigor by weighing its total pruning weights during the dormant season. The number of fruitful nodes or buds is then adjusted to balance the vines capacity to carry and ripen the crop.
4. The basic rule is to allow 20 fruitful nodes per vine (nodes 2 to 5 at the base of last yearıs canes are fruitful in most vinifera cultivars)
5. Another 10 fruitful nodes are left for each additional pound of prunings, up to a maximum of 40 to 60 nodes, depending upon the cultivar
6. These fruitful or "count buds" can be left on spurs or canes (more on this later)
7. Sometimes after a harsh winter the vines are "double" (partially) pruned, or not pruned at all, until the extent of winter bud mortality can be assessed

1. The simplest method for pruning vines is to cut them back to a short trunk with several side arms bearing spurs. This system is still common in Spain, and is called head training with spur pruning. It is an appropriate system for low-vigor vines.
2. Cordons are horizontal semi-perennial trunks trained along walls or trellis wires. They can be pruned to either spurs (as shown in the UC-Davis picture) or canes
3. Head trained vines can be pruned to horizontal or fan arrayed canes that can be positioned either side to side, or one over another. This is a good system in regions where frequent winter damage makes cordons a risky vine training system.

1. There are more than 3500 known cultivars of vinifera grapes, each of which evolved or was domesticated in a particular environmental and ecological niche
2. We have not taken sufficient advantage of the available genetic diversity in winegrapes, because the world wine market it largely driven by about a dozen grape cultivars that originated in Western Europe

California Viticultural Regions classified according to seasonal heat units or GDD

1. Research at UC Davis determined which regions in California were suitable for wine, juice, table grapes and raisin grape varieties, classified local regions according to the number of days each growing season with temperatures above 50 °F
2. Because of maritime effects and prevailing winds interacting with mountain ranges and other topographical features, there is a two-fold difference in the number of GDD within viticultural regions of California.
3. For example, the lower Napa and Sonoma valley regions (e.g. Los Carneros) is much cooler (GDD <2500) than the upper valleys, and is more suitable for Pinot Noir, Chardonnay, and sparkling wines
4. The upper valleys have less maritime cooling and can produce superb Cabernet Sauvignons, Merlots, Zinfandels, etc.
5. The inner Central valley regions (GDD > 3500) produce low acid high sugar grapes that make good raisins but flabby wines, unless one climbs into the foothills of the Sierra Nevada mountains, where some of the finest Zinfandels are produced.
6. The GDD classification of growing regions is widely applicable to other viticultural regions around the world.

Divided canopy systems for high-vigor sites and vines

1. Dividing the canopy enables one to have more bearing surface per unit of soil and root system, without too much shading in the fruit zone. Ideal for high vigor sites.
2. The Geneva Double Curtain system developed by Nelson Shaulis was arguably the original divided canopy system. Richard Smart was Shaulis’ student, and has greatly influenced the world of viticulture with his canopy management systems, especially useful for high vigor sites.