Grapevines are woody perennials. The shoots they produce each growing season turn woody in the fall and, if retained after pruning, serve as supporting structures in the following year.
Three dormant buds (primary, secondary and tertiary) and a lateral bud form in every leaf axil during the growing season. Each dormant bud contains a compact shoot with up to three cluster primordia (undeveloped flower clusters). The primordia are initiated over a short period in the summer, and the numbers that form in each bud is influenced by the light microclimate of the bud and subtending leaf and by the nutrient status of the vine at that time. Lateral buds develop into lateral shoots in the same season they form and may bear fruit in small clusters, which are developmentally behind the clusters developing on shoots arising from dormant buds.
Dormant buds require exposure to cold and then warm temperatures (as in winter followed by spring) to break and develop into full shoots. In the spring, after the temperature requirements for budbreak have been met, the primary bud at each node will develop into a shoot if there are sufficient carbohydrate reserves in the bud and nearby vine wood to support the bud’s development and if the bud is not suppressed by chemical signals from other nearby developing buds or shoots.
Often, if the vine has abundant reserves, the secondary and sometimes tertiary buds will develop into shoots. If the primary bud has been damaged over winter, the secondary bud will often be undamaged and will develop into a shoot. However secondary buds most often bear fewer clusters than do primary buds.
Initially, shoot growth is supported by stored reserves of carbohydrate and mineral nutrients, but soon after the shoot elongates and its leaves become fully functional, its growth begins to be supported by sugars produced by its own leaves and by minerals newly taken up by the roots.
Sugars move through the shoot and woody parts of the vine via phloem. The direction of flow in phloem is dependent on demand, known as sink strength, exerted by developing tissues. Tissues and organs with strong sink strengths include growing shoot tips and developing clusters. When clusters are undergoing a high rate of sugar accumulation, their sink strength is particularly strong and will suppress growth of shoots.
Actively growing shoot tips can draw sugars away from ripening bunches so that sugar accumulation is slowed, flavour development reduced and pH is increased in the berries.
The nutritional status of the shoot and the light microclimate surrounding its developing buds determine not only the number of primordia in dormant buds but the size of cluster primordia. In the following year, primordium size is correlated with the size and number of flowers borne by the clusters that develop. Thus the yield components cluster number and flowers per cluster are influenced significantly by conditions in the previous year.
During the year the clusters develop, the stage most vulnerable to environmental conditions is the fruit-set period. Low temperatures during bloom can greatly reduce the number of berries that set.
Grapes are subject to a fairly common accident in fruit setting called “millerandage” in French, which, as a result of poor flowering conditions, leads to the development of normal sized and very small, seedless berries within the same bunch of grapes. These very small berries are sweeter than normal sized berries and failure of these berries to size causes yield reductions.
Berry growth follows a double-sigmoidal pattern common to other fruit crops. In the first rapid phase of growth, cell division and expansion both contribute to the growth of the skin (exocarp), flesh (mesocarp) and seeds. Further development and maturation of seeds dominate in the lag (slow-growth) phase that follows. In the final, rapid growth phase commencing with veraison, growth is mainly due to cell expansion in skin and flesh tissues. A high rate of dry matter accumulation is typical in the final growth phase while sugars are accumulating.
Heat drives all growth and developmental processes in grapevines, and thus growing season temperatures can substantially influence the timing of fruit maturation. The basal temperature for growth and development of grapevines is considered to be 10°C which is commonly used in the calculation of growing degree days for predicting phenological dates and fruit maturation.
However, from work conducted in the Okanagan Valley on Merlot vines, it appears that the basal temperature for vegetative growth is near 0°C and hear 10°C for cluster development. Thus, cool temperatures would favour vegetative growth.
Grapevine varieties differ in phenological timing, and this is likely due to differences in temperature response including basal and maximum temperatures for growth and development.
In addition to the strong influence of temperature, the supply of water and mineral nutrients also affects the progress of vine development. Low availability of water and nutrients hinders growth and physiological processes and leads to delays in the development and maturation of fruit. It also leads to poor fruit quality. However, excess supplies of water and nutrients, especially nitrogen, can lead to a combination of excess vigour, shady canopies, large berries, high yields, delayed fruit maturation and poor quality fruit. Thus there are optimum levels at which resources should be supplied to prevent excesses in water and nutrient stress, vine vigour and berry growth.
The concept of vine balance refers to the achievement of a balance between vegetative growth and crop level that results in full maturation of the crop by the end of the growing season and in fruit of a particular quality goal. Vine balance is achieved mainly through a combination of vine training and pruning, shoot and/or cluster thinning and the supply of optimum levels of water and nutrients. Appropriate vine spacing during vineyard establishment can also contribute to the achievement of a balanced vineyard.
A commonly accepted way to measure vine balance is to determine the weight ratio of crop to prunings (C/P). Depending on viticultural goals, C/P of a balanced vineyard can range from 4 to 11. For example, optimum C/P for a white variety growing in a warm region might be 10, whereas for a red variety growing in a cool climate it might be 4.
Vines out of balance produce fruit of inferior quality. If C/P is too low, vines can develop excess vigour and shady canopies that are detrimental to fruit quality and result in reduced bud fruitfulness (number of cluster primordia in dormant buds). If C/P is too high, vines are over-cropped and fruit may not mature. And if the canopy is sparse, excess fruit exposure can result in sunburn damage. Supply of water and nutrient resources can influence fruit quality at a specific C/P level.
Providing more abundant supplies of water and nutrients can increase both vegetative vigour and yield without affecting C/P, but fruit quality will likely be different. In either case (low or high resource supply), fruit quality can be affected further by canopy management.
Important to achieving vine balance in most vineyards is shoot and/or cluster thinning. The timing of these practices can influence canopy microclimate and the partitioning of vine resources among shoots and clusters at critical developmental stages that ultimately affect bud fruitfulness and fruit quality. To achieve timely increases in light and vine resources provided to shoots and clusters, shoot thinning should be done early in the growing season when shoots are small and can be easily removed.
Similarly, to increase vine resources and light received by developing clusters (when clusters are stacked), cluster thinning should also be done in the early stages of berry development.
The practice of “bleeding off” an overabundance of vine resources (i.e. carbohydrate, water and nitrogen) that leads to rank growth and large berries by retaining excess numbers of shoots and/or clusters until late in the season achieves little benefit to fruit quality other than the late cluster thinning to cull immature or diseased clusters. Late removal of sound clusters of the same quality as those retained does little other than reduce yield.
Fruit compositional development is influenced by many environmental and viticultural factors. There is abundant literature indicating that fruit microenvironment, including ambient temperature and light, can affect the colour, aroma, tannin profile and other fruit compositional components that influence winemaking quality. Thus canopy management practices that influence fruit microenvironment are essential for attaining desirable fruit quality. Sunlight exposure and ambient temperature both influence berry temperature, which affects the rate of berry development, and relative rates of metabolic processes involved in the degradation of acid and synthesis of phenolics and aroma compounds.
Another important contributor to berry quality is berry size, which is influenced heavily by vine water status and by the supply of other vine resources. Berry size affects the relative amount of skin, flesh and seed tissues in berries, and each contributes different compositional components to must.
Larger berries have more seeds and a lower skin to pulp ratio by weight, whereas smaller berries have fewer seeds and a greater ratio of skins to pulp. Grape skins are the principle source of aromatic compounds and flavour precursors. Seeds are the principle source of phenolic compounds. Pulp is the primary source of sugars, acids, mineral cations, nitrogenous compounds and pectic substances. Within a grape berry, the pulp will mature first, the skin second and the seeds last.
Much of the benefit of deficit irrigation to must and wine quality is likely due to small berry size and the higher amounts of berry skin and seeds relative to flesh. However, there is good reason to believe that water stress affects berry quality characteristics other than size and relative tissue amounts.
ABA produced in roots in response to moisture deficits is translocated to leaves, where is causes partial closure of stomates to conserve water. Recent research findings indicate that ABA directly enhances the production of berry anthocyanins.
At dehydration, the average berry weight decreases while sugar increases. Dehydration can d-crease average berry weight by 5-10%. Wines produced from grapes harvested at this dehydrated stage usually have an increased concentration of colour, aroma and flavour.
Reduced irrigation in the Okanagan Valley can affect fruit quality by advancing fruit maturation. Moisture deficits resulting from drip irrigation, as compared with sprinkler irrigation, were found to increase vineyard temperatures by reducing cover crop growth, vine canopy size and transpiration rates. Despite having substantially lower leaf areas and photosynthesis rates, vines under drip irrigation produced fruit that matured earlier. This effect could be beneficial in vineyards where fruit does not always reach full maturity.
However, fruit that matures too early in the growing season under warm temperatures tends to be lower in acidity and may lack some of the aroma and other quality components that develop in fruit ripening over a longer cooler period late in the season.
