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Status of grapevine canopy management and future prospects
J.J. (Kobus) Hunter and Eben Archer
ARC Institute for Fruit, Vine and Wine,
ARC Institute for Soil, Climate and Water and
Department of Viticulture and Oenology,
University of Stellenbosch,
Stellenbosch, South Africa

Introduction

In order to meet challenges of ever increasing national and international market competition and requirements, Wine Industries all over the world are committed to increase grape and wine quality. A question that is often asked is : Why is canopy management important in this regard? The answer is simply that everything affecting the performance of the grapevine is eventually reflected its canopy. This situation, however, poses a difficulty to viticulturists and the like in that an understanding of effects of different aspects must be attained in order to successfully manage the grapevine and per implication the canopy. This also means that investigators are willingly or unwillingly involved in the study of canopy management even though their studies are not directly focused on the canopy. In this view, canopy management cannot be considered to be restricted to the canopy per se, but to every direct or indirect aspect having an influence on eventual physical appearance as well as physiological performance of the canopy. Canopy management must therefore be discussed within the framework of a total management approach to optimize the grapevine for production of high quality grapes.

Over the years, the status of canopy management has grown from being a practice that was originally primarily used for the purpose of growth accommodation, obtaining sustainable yields and disease control until its current status of being an integral, pivotal practice in viticulture and oenology, absolutely essential for obtaining and improvement of grape and wine quality (Kliewer, 1982; Kliewer et al., 1988; Koblet, 1988; Candolfi-Vasconcelos & Koblet, 1990; Hunter & Visser, 1990a, 1990b; Smart et al., 1990; Hunter et al., 1995; Hunter, 2000; Hunter & Archer, 2001b). Currently, disease control obtained with canopy management can be considered a secondary, but nevertheless, important spin-off of canopy management, definitely contributing to grape quality (Savage & Sall, 1983; Thomas et al., 1988; Stapleton & Grant, 1992; Volschenk & Hunter, 2001a, 2001b).

This article focuses on seasonal canopy management with discussion of some physiological and practical considerations dictating current application. From research conducted over the years, a clear picture emerged that seasonal canopy management should be done from the beginning of the growth season and that timing and the way in which it is applied, are of critical importance for it to be successful (Hunter, 1999; Hunter, 2000; Hunter & Archer, 2001a, 2001b).

Objective of canopy management

The ultimate objective of canopy management is to obtain a photosynthetic efficient, homogeneous canopy with uniformly and well-distributed shoots of similar vigour, producing healthy, high quality grapes of similar bunch and berry size and with a uniform level of ripeness. In addition, in order to maintain longevity, growth and development of other parts of the vine must not be impaired.

Some physiological and practical considerations

In order to meaningful discuss the philosophy behind seasonal canopy management, the role of at least vine spacing, trellising and water management should be mentioned briefly. To accommodate growth in such a way that shoot crowding is prevented and optimal water consumption and soil utilisation are obtained by roots, narrower vine spacing and smaller trellises should be used on low to medium potential soil, whereas wider spacing and bigger trellises should be used on medium to high potential soils (Archer & Strauss, 1985; Hunter, 1998a, 1998b; Hunter & Archer, 2001a). The water table level and availability of water for irrigation will affect spacing in both scenarios. Selection of a trellising system is dependent on soil potential, vigour of the scion-rootstock combination, climate, ease of mechanical practices and maintenance requirements. Although numerous trellising systems are used (Carbonneau & Cargnello, 1999; Carbonneau et al., 2001), focus should always be on establishing a well-balanced vine with a photosynthetic efficient canopy. In short, growth must be accommodated in such a way that shoots are not crowded, interior-canopy shade is restricted and sufficient room is available for development of at least 1.4 m shoot length or approximately 16 primary leaves (Hunter, 2000; Nadal et al., 2001). To increase grape quality and decrease production costs, trellising systems must therefore conform to basic canopy management principles (Archer et al., 1988; Hunter & Archer, 2001a; Hunter & Volschenk, 2001). Some of the reasons why vertical (espalier-like) systems are to a large extent still preferred, are: ease of management and mechanical harvesting; more balanced exposure of leaves and the bunch zone to sunlight, which would favour uniform berry ripening (Volschenk & Hunter, 2001b); 27% lower photosynthesis of high trained cordon pruned vines with shoots hanging down than that of low-trained cane or cordon-pruned vines with shoots trained upright (Koblet et al., 1996); lower ribulose 1,5 bisphosphate carboxylase/oxygenase activity and net photosynthesis in leaves of downward trained shoots; and smaller xylem transectional area and lower hydraulic conductance found in downward trained shoots compared to horizontal and upward shoots (Schubert et al., 1995).

When normal summer day temperatures are outside the temperature range (15–250C) for optimum red grape colouring (Kliewer, 1977) and a greater possibility for a rise in pH exists, berry size becomes a very important potential quality parameter because of the increase in skin:pulp ratio and greater extractability of phenolic compounds (particularly anthocyanins) obtained with smaller berries. Under such conditions, irrigation management during the berry cell division stage should be focused on berry size reduction. Although berry size displayed a marked resistance during the ripening period (Smart et al., 1974; Van Zyl, 1984; Greenspan et al., 1994, 1996), it is sensitive for water stress, improved light conditions, and competition with vegetative growth during the pre-véraison period. All of these will decrease berry size, but only improved light conditions will decrease pH. Imposition of water stress during the berry set to pea berry size period must therefore be done with great care and in such a manner that excessive vine stress, inducing negative effects on canopy capacity, i.e. size and efficiency, and other concurrent quality determining factors such as sugar production, acid development and maintenance, pH, berry colouring and flavour, is prevented. A photosynthetic efficient canopy and thus sucrose production, must be maintained in order not to nullify positive effects of a higher skin:pulp ratio of a smaller berry on colour extraction. A lower pH will increase extraction as well as colour intensity. The canopy must still be well exposed to restrict excessive potassium loading into the berry and thus tartaric acid salt formation and pH increase.

Vegetative-reproductive growth interrelationships

The accumulation of organic acids is dependent on sucrose supply from leaves (Hunter & Ruffner, 2001), primarily during the pre-véraison period (Ruffner, 1982a, 1982b; Saito & Kasai, 1984). Furthermore, except for dilution, and respiration in the case of malic acid, salt formation due to potassium influx decreases tartaric acid levels in the berry. Since tartaric acid has a higher buffering capacity than malic acid, tartaric acid is of primary importance for establishing and maintaining a low berry and must pH under warm environmental conditions. Potassium, like sucrose, has an osmoregulatory function in the phloem and berry (Giaquinta, 1983). This characteristic of potassium has serious implications for tartaric acid maintenance and pH. The potassium accumulation in the berry is slow pre-véraison, but increases strongly during berry ripening, in parallel with increased sucrose and phloem water flow (Possner & Kliewer, 1985; Düring et al., 1987; Findley et al., 1987; Greenspan et al., 1994, 1996; Ollat & Gaudillére, 1996). To prevent an excessive, preferential osmoregulatory loading of potassium into the phloem, sucrose availability should be sustained to maintain phloem turgor. In this way, sucrose instead of potassium is loaded into the phloem. Any factor inducing vine stress will reduce sucrose production by leaves, reduce organic acid formation, stimulate potassium loading and transport to berries and increase pH (along with delayed ripening). General stress factors include canopy shade (leading to e.g. a reduction in sucrose availability), too high or too low ambient temperature, wind stress, water stress, water logging, nutrient deficiencies, over-fertilisation (N), leaf and berry infection, as well as imbalances in the root system (e.g. primary versus secondary roots), vegetative growth (e.g. older versus younger leaves, shorter versus longer shoots) and yield (e.g. smaller versus larger bunches). It is evident from the above that development of young leaves in the canopy should be stimulated during the berry set to pea berry size period by canopy management, in order to increase photosynthetic and therefore sucrose production capacity during the ripening period. It is imperative to still have a well-exposed canopy during both pre- and post-véraison periods to obtain maximum photosynthesis of younger leaves and to stimulate activity of older leaves.

Although the berries are dependent on primary precursors (e.g. sucrose and amino acids) from leaves, they are also metabolically active (Gholami et al., 1995; Hardie et al., 1996) in forming e.g. secondary compounds, such as flavoursome isoprenoids (e.g. monoterpenes) and nitrogen containing 2-methoxy-3-isobutylpyrazine (grassy/green pepper typical aroma in Sauvignon blanc, Cabernet Sauvignon and Sémillon - Lacey et al., 1991), as well as anthocyanins, through respiratory processes (Hunter, 1999; Hunter & Archer, 2001b). They certainly contain a relevant enzyme composition (Seymour et al., 1993; Mohr & Schopfer, 1995; Famiani et al., 2000). Like nitrate reductase, activity of many enzymes, including phenylalanine ammonia lyase, involved in anthocyanin synthesis, is controlled by light (Roubelakis-Angelakis & Kliewer, 1986; Hunter & Ruffner, 1997). Gene expression for flavonoid biosynthesis is promoted by light (Sparvoli et al., 1994). Anthocyanidin formation in the berry skin is dependent on supply of the amino acid phenylalanine as precursor and also glucose to form anthocyanin (Pirie & Mullins, 1980; Hunter et al., 1991). Colour expression is pH dependent (Hrazdina & Moskowitz, 1982; Singleton, 1982; Somers, 1982). The ability to control red grape pH and to create conditions that would increase the glucose- and amino acid-sink and metabolic activity of the skin, is therefore a very important prerequisite to increase colour intensity. Colour development on particularly abnormal short/non-lignified shoots must be monitored during the ripening period. If colour development is delayed, such grapes should be removed in order to obtain more uniform ripening of the rest of the crop. It should be clear that it is equally important that the bunch zone is well exposed during both pre- and post-véraison periods in order to obtain maximum sink metabolism [for sucrose attraction, respiration, organic acid synthesis, low pH and anthocyanin (colour) and flavour compound formation in the berry]. Preferentially, the ripening period should be entered at higher phenolic and flavour levels, higher acid, lower pH, and higher levels of precursors for anthocyanin formation.

Manipulation of the canopy

Different parts of the season are important because of varying physiological events and it is for this reason that the way in which short-term practices are applied and timing of application are critical in obtaining the required result. To accommodate these events within a canopy management strategy, the growth cycle can be divided into a dormancy period, budding to flowering period, a flowering/berry set to pea berry size/véraison period, a ripening period, a harvesting period and a post-harvest period.

A uniformly distributed canopy will to a large extent be dependent on shoot density (Smart, 1988) and canopy management therefore already starts during the winter dormancy period by applying the correct pruning practices and pruning system. In countries where temperatures during dormancy are not low enough or variable, resulting in unpredictable bud burst, spur pruning is mostly applied. Focus should be on creating sufficient room for shoots to develop in summer. In practice, this would require approximately 14 cm between two-bud spurs. This norm will be affected by the extent of bud burst and growth of the cultivar-rootstock combination and is best accommodated by pruning according to cane mass. The performance of shoots will also to a large extent be affected by the thoroughness of a practice such as shoot positioning in summer. Depending on judiciousness of application of the latter, the distance between spurs is somewhat variable.

During the period budding to flowering, leaves in the bunch zone and just above the bunch zone still display high chlorophyll content (Hunter & Visser, 1989) and photosynthetic activity (Hunter et al., 1994). As leaves are progressively situated towards apical parts of the shoots, highest chlorophyll content occurs later in the season, e.g. leaves in the bunch zone will have highest concentrations at berry set, whereas for apical leaves this will only occur during ripening. The budding to flowering period is also very important for the formation of inflorescence primordia and their initiation and differentiation (Swanepoel & Archer, 1988). It has been found that light exposure of the canopy during this time favours bud fruitfulness (May, 1965; Smart et al., 1982; Hunter & Visser, 1990b). Infertile and/or excessive shoots that contribute to non-uniformity of growth and shade in the canopy should therefore be removed during this period. This comprises a judicious removal of infertile shoots not located on spurs before and/or at approximately 30 cm shoot length. This practice (called suckering) restricts the use of reserve nutrients after bud burst and ensures bud fertility through a well-exposed canopy. The practice also contributes to obtaining sustained, predictable yields of good quality. To obtain maximum benefit from the suckering practice and to decide on further canopy management, shoots must be positioned. During this practice, shoots on the vertical trellis are picked up to a vertical position by means of movable wires. The shoots are then positioned by hand or machine in line with their corresponding spurs. This is a continuous and extremely important practice that ensures sunlight penetration to interior leaves and bunches. Early positioning ensures tendril attachment to foliage wires in the correct position and in this way also prevents the breaking of tendrils when positioning is done later. It also ensures that every individual shoot is exposed to a microclimate that would allow optimal functioning of all leaves and bunches. Damage by wind and cultivation practices as well as hampering of the latter in the vineyard row are also prevented. Well-distributed shoots will also contribute to better disease control and uniform ripening of grapes. Harvesting as well as pruning will be facilitated. Early sun exposure of bunches (especially to UV-rays) results in bunches being less susceptible to sunburn and infection during ripening. Increased airflow through shoot-positioned canopies leads to lower berry temperatures and further contributes to grape quality, especially when warm summer temperatures are experienced.

Translocation studies showed that grape berries are major sinks from after berry set (Hunter & Visser, 1988a). Redistribution of nutrients is evident at ripeness and after harvest. Leaves in the lower part of the canopy contribute to bunches throughout the growth season. Photosynthetic activity of these leaves is highest before berry softening, but after that younger leaves in the canopy (those in the top part of the canopy and those on lateral shoots) contribute the most to carbohydrate production (Hunter et al., 1994). A similar pattern is found for leaf nitrate reductase enzyme activity (involved in ammonium and amino acid synthesis) (Hunter & Ruffner, 1997). Tipping and topping are done during the flowering to pea berry size period and reduce competition between vegetative growth, the inflorescence and the developing berry. Tipping is normally applied just before bloom, but is also applied as replacement for topping. Tipping is particularly useful for cultivars that set poorly, meaning also that cultivars that normally set well, should not be tipped during bloom in order to prevent too compact bunches prone to rot during ripening. Tipping increases the percentage young leaves in the canopy through lateral shoot development. Topping of up to 30 cm of the canopy should only be applied to apical (top) parts of the canopy. Topping of primary shoots along the sides of the canopy will result in short shoots with insufficient leaf area to ripen their grapes. Such shoots “parasitise” on the rest of the shoots and contribute to non-uniform ripening and colouring and depletion of reserves. Together with non-suckered short shoots carrying bunches, they also make it impossible to define optimum harvesting time of the vineyard. Correct and timely topping will also stimulate development of younger, active leaves in the canopy. These leaves increase photosynthetic capacity of the canopy during the ripening period and can be considered equal to active, apical leaves during this time.

If necessary (when the canopy is still dense after removal of e.g. infertile shoots and shoot positioning or if it is anticipated that the canopy will increase in density/shade), leaf thinning can also be done during the berry set to pea size/véraison period. This can be done randomly in two zones, depending on the vigour of the canopy (Hunter et al., 1995; Hunter, 1999). Approximately one third of leaves are removed. Random, even removal is critical and serves to increase exposure of bunches to sunlight, while at the same time prevents excessive exposure. Leaves in the bunch zone are also at this stage still very active and excessive removal must be avoided in order not to hamper the current and next season’s yield (Hunter & Visser, 1990b). It also serves to control early Botrytis infection. The first leaf thinning is done in the bunch zone during berry set (or at any other stage from this point up to véraison). The berry set thinning can be followed by a second leaf thinning in the zone up to half of the canopy (in the lower half) at pea berry size stage. This practice increases photosynthetic activity of all leaves and metabolic activity of bunches for the rest of the season (Hunter et al., 1995). It facilitates uniform, filtered (diffused) sunlight exposure of grapes and homogeneous ripening. It allows sunlight to penetrate bunches from both side and top (and bottom through reflection), reaching all the berries. These are important criteria in order to predict ripeness and for quality determination of the yield. Pest and disease control is improved and the use of chemicals restricted.

The photosynthetic activity of leaves along the whole shoot as well as export of photo-assimilates can be increased by an improved canopy microclimate and a lower source:sink ratio brought about by canopy management (Hunter & Visser, 1988b) (also Hofäcker, 1978; Johnson et al., 1982; Candolfi-Vasconcelos & Koblet, 1990). The canopy should, however, always be manipulated in such a way that sufficient leaf area is still available to support grape development and that sunlight penetrates into the interior of the canopy, but at the same time is optimally intercepted, without a loss in potentially utilisable energy. It was found that a vigorous (control) and a too open canopy [severely (66%) defoliated] both resulted in inefficient utilisation of energy (Hunter & Visser, 1990a) and a reduction in total CO2 assimilation rates (Hunter & Visser, 1989).

A stable, ongoing photosynthetic activity and nitrate reductase activity of basal leaves up to harvest and thereafter indicate a continued competence to support bunches and contribute to maintenance metabolism and the carbohydrate and nitrogen compound reserve pool of the vine (Hunter et al., 1994; Hunter & Ruffner, 1997). Nitrogen absorbed during the post harvest period is preferentially utilised for new spring growth (Conradie, 1991). It has been found that starch contents of basal parts of canes increased when proper canopy management was applied during summer (Hunter, 1999). This will favour bud burst and initial shoot growth pre-bloom.

Well-performed pruning practices as well as suckering of infertile shoots and shoot positioning can be considered base practices which are needed for any grape quality standard. Additional practices such as tipping, topping and leaf thinning will be needed on the basis of further (undesired) canopy development and for obtaining the highest grape quality under the particular conditions of the vineyard.

Practical criteria

It is evident that proper canopy management procedures should be integrated with the growth cycle and physiological demands of the grapevine. The physical structure, microclimate and physiological functioning of the canopy affect grapevine performance and management as a whole. The development of the canopy has a physical and physiological bearing (also via microclimate) on the potential of the canopy for producing high quality grapes. From research devoted to the functioning of and seasonal changes in the canopy and the berry (and grapevine as a whole), some practical criteria emerged that are needed for establishment of highly efficient canopies that are able to support requirements for the longevity of the vine and the development of grapes that would eventually result in high quality wines. Some of the most critical criteria that were established by considering vine physiology, viticulture and oenology aspects and that are easily assessable in the vineyard, are the following :

A well-balanced permanent structure

· A vertically positioned canopy (or slightly inclined, depending on the trellising system)
· A balance between older and younger leaves at berry softening stage: ratio of 0.7
· Well-exposed leaves (small sun flecks in the shade pattern of the canopy noticeable on soil surface between rows)
· Chlorophyll-rich interior-canopy leaves showing no sign of early senescence
· Approximately four leaf layers from side to side in the canopy (from bottom to top)
· Approximately 16 primary leaves on every shoot
· Homogeneous shoot lengths (and vigour) of approximately 1.4 m
· No active primary shoot lengthening in the post-berry softening period
· Grapes exposed to filtered sunlight (20-30%)
· Pests and diseases on grapes and leaves are easily controllable

Depending on prevailing environmental conditions, a canopy that conforms to the above criteria will support high photosynthetic activity of leaves, predictable and continued budding, bud fertility, and yields, high grape quality, and limited pest and disease occurrence, while vines can be easily mechanically harvested. Achieving such a canopy and to manifest the full potential of the vine into yield and grape quality without impairing longevity, will require a total strategy involving well-selected and well-performed long-term practices (selection of site and soil type, soil mapping of planting site, establishment techniques, rootstock-scion combination, trellising and training system, vine spacing and row direction) as well as seasonal canopy manipulation practices (pruning, suckering of infertile and/or excessive and/or too short shoots, shoot positioning, tipping/topping and leaf thinning). This should be accompanied by a comprehensive seasonal management strategy with focus on timing and way of irrigation, canopy manipulation and harvesting.

Challenges for the future

A few challenges that are both practically recognizable and assessable and which are considered necessary to be a future role player in the practice of canopy management as well as viticulture and oenology in general, include the following : 1) determination of sites on which climatic and soil conditions are best suited to physiological requirements of the grapevine (Hunter & Bonnardot, 2002), creation of a well-balanced grapevine with homogeneous vigour (Hunter & Archer, 2001a, 2001b), elucidation of the importance of the pre-véraison period in the determination of the quality of the final product, further quantification and identification of grape quality parameters in conjunction with molecular physiology, and establishment of optimal ripeness for harvesting for a specific product objective.

Selection of sites best suited to grapevine physiology

Climatic requirements favouring the optimal physiological functioning of the vine are mostly not considered in the context of “terroir” and zoning studies. However, the extent to which physiological requirements of a grapevine subjected to a specific “terroir” is met, is paramount for optimal functioning and achievement of maximum berry quality. The ultimate goal would be to facilitate “terroir” selection and zoning. Macro-, meso-, micro- and even nano-(e.g. inside the bunches and at soil-root interface level)climatic conditions and the physiological requirements of the grapevine cultivar-rootstock combination in a particular region and at a particular site, must be married to each other. In order to understand grapevine behaviour, to optimally manage the grapevine within a particular “terroir”, and to facilitate future “terroir” selection and zoning, these concepts should be studied together. This will allow us to manage the grapevine for the best expression of “terroir” potential in grape and wine quality. Preliminary results showed that climatic profiles in different regions might have serious implications for the physiological functioning of grapevines (Hunter & Bonnardot, 2002). Minimum/maximum data and frequency of occurrence would seem to be suitable parameters for climatic profile quantification aimed at accommodating grapevine physiological requirements. The impact of potential climatic stress (direct and indirect) on grapevine physiological processes should be further quantified in order to facilitate selection of a “terroir” and in zoning for grapevine cultivation.

Well-balanced grapevine with homogeneous vigour

The objective should be to create homogeneous vines within a vineyard that are characterised by a balance between thick and thin roots, a balance between root volume and top growth, a balance between left and right cordon arms, a balance between crop load and efficient leaf area, and a balance between younger and older leaves in the canopy. For example, a well-branched and well-balanced root system composed of fine and thick (also deep-penetrating) roots, that would allow efficient water and nutrient absorption and sufficient accumulation of reserves and production of hormones, should be established and maintained. This would be dependent on soil conditions, planting practices and canopy sufficiency and efficiency. Only trellising systems that conform to canopy management principles for sustained, high quality yields should be used (and further promoted in future). Furthermore, soil preparation before planting to alleviate chemical and physical barriers to root development is of paramount importance. Focus should be on the creation of a well structured root system that can buffer the vine against any unfavourable climatic conditions, such as heat waves and dry periods, and thus to ensure continued, homogeneous ripening without major grape quality fluctuations from year to year. Indications exist that a well-buffered vineyard will provide a larger window during which a ripeness level is maintained. It is very important that sunlight be uniformly distributed in the canopy and that grapes are protected from excessive exposure, but still receive enough sunlight for optimal functioning. Along with the striving for a homogeneous canopy structure, bunches and berries must also be homogeneous. In addition, bunches and berries must receive sunlight to such an extent that not only the microclimate, but even the nano-climate (inside the bunches) is also to a large extent homogeneous.

Importance of the pre-véraison period in grape quality

Indications are that this period is much more important than previously realised (Carbonneau & Deloire, 2001; Hunter & Archer, 2001b). Much more attention should be devoted to quantifying the role of both leaf and grape composition and performance during this period in the determination of eventual grape and wine quality.

Quantification and identification of grape quality parameters

Further quantification and identification of parameters significant to the establishment of grapes that would lead to an improvement of wine quality, remain a real challenge. This should be done in conjunction with studies on the grapevine genome, particularly functional genomics, with consideration of whole-vine physiology and environmental effects (“molecular ecophysiology” – Carbonneau & Deloire, 2001). Particular attention should be given to the identification, characterization and expression of genes involved during different developmental stages of the berry, in order for them to be accommodated in the management of grapevines for a specific product objective.

Optimal ripeness for harvesting

Theoretically, optimal ripeness within a crop can only be achieved when homogeneous canopies are created. This can be the only way that the level of ripeness can be accurately monitored and grapes be harvested at the same ripeness level. Naturally, this is a very difficult objective to reach in practice and would require an awareness of environmental influences. Along with further identification and quantification of the composition of grapes needed for high quality wine, establishing parameters reflecting the optimal grape ripeness that will result in the best possible wine from a particular vineyard, is a major challenge to be pursued.

Conclusions

Progress on the application of existing cultivation criteria and on the above-mentioned future challenges will have a continuous effect on management of the grapevine. It will necessitate the regular re-visiting of management programs applied by producers and also results obtained under “sub-optimal” conditions during research. The latter aspect is very important and will require that the very best “standard” conditions be created for grapevines on which experiments are to be done. Naturally, this will also require a thorough knowledge of aspects other than those that the particular experiment is being focused on. Researchers and producers face constant, immense challenges to improve grape and wine quality. The extent of success will be determined by solid, basic research (multi-disciplinary) on well-defined aspects as well as the ability to visualize the physiological reaction of the grapevine to environmental influences to be experienced under commercial conditions.

Acknowledgements

Financial support by the South African Vine and Wine Industry (through Winetech) and contributions by C.G. Volschenk, V. Bonnardot, D.J. Le Roux, G.W. Fouché, L.F. Adams, L.M. Paulse and W. Hendricks in the obtaining of research results are gratefully acknowledged.

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