The Grapevine

by Tim Martinson

The history of grapevine cultivation goes back almost to the dawn of agriculture. Archaeological evidence shows evidence of wine production from around 4000 BC and almost simultaneous evidence of agricultural cultivation and movement of grapevines from the Caucasus region to Israel, Egypt, and other Mediterranean locations. What did early wine producers start out with, and how did they change grapevines in the course of domesticating them? How does the evolutionary history of grapevines affect grape growers today?

Origin and Distribution. All grapes are members of the genus Vitis , one of 15 genera in the family Vitaceae. Worldwide, there are about 65 species, distributed in temperate zones throughout the northern hemisphere. V. vinifera is the only species native to Europe. The remainder are mostly in North America and China. Wild species important to viticulture include V. riparia, V. labrusca, V. aestivalis, V. rupestris, V. cinerea, V. berlandieri and V. rotundifolia. All except V. rotundifolia hybridize freely with each other, and are important as components of rootstocks, native grape cultivars (such as Concord and Norton) grown in the east and northeast, and interspecific hybrid varieties (more on this in a future lecture). The muscadine grape, V. rotundifolia, is distributed throughout the southeast, and used to produce scuppernog¹ type wines.

Wild Vines. Wild Vitis evolved in association with trees upon which they depend for support. With their vinous growth habit, its likely that they existed among gaps along the edges of forests and that their growth occurred in tandem with the supporting tree's growth. The fruit is highly attractive to birds, which are a principal means of facilitating dispersal of seeds and establishment of new vines in the wild.

Unlike many trees that produce one spurt of foliage and a terminal bud each spring, then stop growing, storing up reserves for the following year's growth, grapevines have no terminal bud and no natural check on vegetative growth. Bud burst in grapevines is often delayed compared to trees, so shoots and leaves can position themselves on the outside of the tree's canopy to capture sunlight. Unlike many fruit trees, which produce specialized fruiting buds and vegetative buds, grapevines produce both fruit and foliage from the same buds. Buds are particularly numerous and complex. Many latent buds remain dormant for several years, before some event activates them. This gives the wild grapevine great flexibility in its form and the ability to respond to changing environmental conditions. As perennial plants, grapevines annually store and remobilize carbohydrates and mineral nutrients in response to changing conditions.

A major consequence of grapevine evolution is this fact, which is of great importance to vineyardists: During their formation, buds are directly affected by the amount of sunlight to which they are exposed. Buds that develop in sunlight initiate more flowers than buds that develop in the shade. They have higher bud fruitfulness. If fruit on wild vines exists to be consumed by birds, it makes sense to have fruit clusters exposed on top of the tree rather than buried somewhere in the shade.

Wild vines are dioecious, that is male and female flowers are produced on different vines.

Finally, in ecological terms, wild vines are classic 'K strategists' rather than 'R strategists'. K strategists are persistent, competitive over the long haul, and have a low reproductive rate. R strategists, on the other hand, grow and reproduce fast, and often don't persist in one location for a long time. Wild grapevines put relatively fewer resources into producing fruit, and more into vegetative growth. As we shall see, vegetative growth and fruit production compete for resources produced by the grapevine.

Domestication. In domesticating wild vines, humans faced several challenges. Their goal was to harvest fruit, not to maximize long term growth (as in the wild). Moreover, having vines with only male or female flowers was a problem - half of the vines would not produce ANY fruit. The problem of male and female plants was solved by selecting rare mutant vines with perfect flowers i.e. with functional stamens and ovaries. Maximizing fruit production while limiting vegetative growth was accomplished by providing support and arranging the canopy to maximize sunlight interception (trellis and training systems), and by selectively pruning to retain fruitful buds and limit vegetative growth. Practices growers use to accomplish these goals are much of what viticulture is all about.

Vine Terminology. Grapevine shoots arise from compound buds, bearing both leaf and floral primordia. Each bud has a primary, secondary, and tertiary bud, and a lateral latent bud. Shoots are comprised of nodes, from which leaves, flowers and tendrils grow, and internodes, which are simply the shoot section between nodes. Leaves are borne alternately at each node. Flower clusters and tendrils arise from similar tissues and are located opposite leaves. From 0 to 4 fruit clusters are produced on each shoot. Buds for the following year are produced in the leaf axil, distal to where the leaf petiole is attached to the cane. Canes contain vascular bundles composed of xylem and phloem. Xylem is dead tissue, and passively conducts water and some mineral nutrients from the root system through the trunk. Phloem is comprised of live tissue through which are transported carbohydrates produced through photosynthesis. In between the inner xylem and outer phloem is the vascular cambium, which annually produces a new xylem and phloem layer in older wood. Flower clusters have stamens, ovaries, and a fused calyptra (petals fused together), which separates from the individual florets at bloom. Berries are attached to the rachis via the short peduncle. Cultivated grapevines are generally cane-pruned (long canes retained) or spur pruned (short canes with 2-5 nodes retained). The permanent wood (>2 years old) is retained in one to several trunks. Cane-pruned vines have a head or renewal zone from which retained canes are selected. Spur-pruned vines generally have a permanent cordon, with permanent wood attached horizontally in one or two arms. Suckers are shoots at the base of the vine that arise from latent buds. They are generally removed, but growers often retain one or two for replacing trunks.

Vine Growth and Development. The grapevine has to accomplish several tasks during the growing season. It must produce a canopy of leaves, positioned to capture sunlight early in the season. It must complete floral development, set and mature fruit. It must produce buds for the next year's growth cycle. Finally, it must store up reserves and gain winter hardiness to survive the cold weather during the dormant season. Many of these processes occur simultaneously, and compete for reserves and photosynthate produced by the vine. Early growth is supported by reserves in canes and trunks, and crop potential is determined by the status of retained buds. Root growth and uptake of soil nutrients only becomes significant around bloom. After bloom, canopy growth slows and more of the photosynthate is directed at developing berries and clusters. After veraison (start of the final ripening period when grapes change color), photosynthate is largely directed at producing sugars for grape clusters, and starting the process of laying down carbohydrate reserves in canes, trunks, and roots for the following year. After grapes reach maturity and are harvested, carbohydrates and mineral nutrients are translocated to canes, trunks and roots.

Dormant buds and canes. Grapevines start the season with dormant buds and carbohydrate reserves stored in canes, trunk and roots. As vines enter dormancy, buds gain cold-hardiness through desiccation and physical isolation of bud tissue from the vascular system. This prevents cells from being disrupted by ice crystal formation. The living phloem tissue is plugged when callose, accumulates in the sieve tubes. Inside the dormant bud, the crop potential for the coming season is already determined. Primary buds are the most developed, and most fruitful. Secondary and tertiary buds are less fruitful, and do not develop further unless something happens to the primary bud or shoot. They carry much lower crop potential than the primary buds. Much of viticulture is about deciding how many and which buds to retain. Buds vary in quality according to accumulated reserves, position on the vine (and last year's cane), exposure to sunlight and weather conditions the previous year. 'How many' and 'which' buds are retained affects both the quantity and quality of grapes.

Early Vine Growth Ð Budburst to bloom. As the vine wakes up in the springtime, it relies on reserves mobilized locally from canes for initial growth. Water uptake proceeds through the dead xylem tissue as temperatures rise, but the rest of the vascular system - including the phloem and vascular cambium, is not hooked up. Hormonal signals from the shoot tip travel downward through the canes and eventually the trunk to reactivate the vascular cambium and produce a new phloem and xylem layer. This only gets fully activated around bloom. Before leaves are fully expanded, they are net 'importers' of carbohydrates. As the oldest leaves mature, carbohydrates are exported towards the actively growing shoot tip to support canopy development.

Bloom. Bloom is an important time in grapevine development. Shoot growth starts to slow around this time, and the 3 or 4 weeks around and after bloom are when next year's buds start to set. Poor weather at bloom can directly affect pollination and how much fruit is set, as well as influencing how fruitful next year's buds are. Root growth is generally occurring rapidly at this time (late June) as soil becomes warm. With all these processes occurring simultaneously, there can be great competition among the growing shoots, clusters and new buds for photosynthate produced by the leaves.

Fruit set to veraison. After bloom, fertilized flowers set fruit, but about 2/3 of the individual florets in the cluster drop off - a process called “shatter.” As berries develop, the fruit starts to be a greater 'sink' for photosynthate, and the growth rate of shoots declines. Berry growth occurs in two stages. - The first stage is the cell division phase (through mid July). Following the cell division phase, there is a pause in berry growth called the “lag phase”, during which seed formation progresses. After the lag phase, the number of cells in each berry is set, and further berry growth occurs through cell enlargement. At the end of this phase, the canopy is fully developed, and further vegetative growth is drastically slowed or stopped.

Veraison to harvest. At veraison, berries start to soften, develop a waxy cuticle which makes them resistant to evapotranspiration, and accumulate sugars until harvest. Simultaneously, acid levels in the berries drop, and varying proportions of the malic acid is converted to tartaric acid. At harvest, from 15% to 25% of the berry contents are stored sugars (soluble solids). During this period, most of the photosynthate produced by leaves is directed at the fruit. Simultaneously, canes and buds start turning brown from the base of the shoot outward, forming periderm from the cork cambium that develops on the outer edge of phloem tissue.

Harvest to leaf fall. Harvest removes a huge 'sink' for photosynthate from the vine. During the post-harvest period, vines mobilize nutrients and carbohydrates from leaves into the canes, shoots and root system for the following year. Leaves senesce and the vine becomes dormant. In warm climates, growers can count on several weeks of leaf activity following harvest; in cool regions such as New York, the length of post-harvest leaf function varies tremendously from year to year, and has a large impact on cold hardiness and the amount of accumulated reserves to support early growth the following season.

Implications for management. The level of soil fertility, weather-related advances and delays in development, and the ratio of exposed leaf area to fruit all influence the quantity and quality of grapes produced. Excess (through shading) or inadequate (through vine stress) vegetative growth can reduce crop. Excess crop can reduce vegetative growth, reducing vine size for the following year and can delay ripening, directly affecting fruit quality. Skilled vineyardists attempt to match crop level with leaf area to produce a balance between vegetative and reproductive growth. This is a central task for vineyardists, and not always an easy goal to accomplish. We'll talk about how growers manage and control growth processes in a future lecture.

Selected Web References on Vitis botany and anatomy:
L. Watson and M. J. Dallwitz (1992 onwards).
The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval. Version: 14th December 2000.
Vitaceae
Grape Pages

Grape Growth stages:
Concord
Chardonnay

Cornell References on Viticulture and Grape production
Grapes