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A Technical Guide for Wine Producers
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RECENT ARTICLES | WYNBOER HOME
Vine Balance: Its Importance To Successful Cultivation
INTRODUCTION
The vine is a climber and as such it is genetically inclined to favour shoot growth. As long as conditions favour the vine (sufficient water and nutrients and high temperature) it will be inclined to grow vegetatively - often at the expense of reproductive growth. Therein lies the challenge for viticulturists: to manage the vine in such a way as to achieve controlled growth. The only natural way for the vine to propagate is through its seed. If the vine receives the impulses that times are going to be tough, thus threatening its survival, its natural reaction is to enable its metabolism to favour seed development in an attempt to foster high quality seed for propagation.
In the process the development of the mesocarp (around the seed) benefits and high quality berry development occurs at the same time. By so doing good grape composition is achieved. The challenge for viticulturists therefore lies in managing the vine in such a way that it will promote seed development in order to propagate, without going into a real stress situation where normal physiological processes are compromised. In human terms, management actions were therefore established to "cheat" the vine into "believing" that it would perish, without the danger of this actually happening. This level of vineyard management is relatively simple to achieve in instances where controlled growth occurs vis á vis instances where vegetative growth is overly vigorous. Controlled growth occurs where the following balances occur in the vine: (i) Balance between subterranean and aboveground growth, (ii) Balance between fine and thick (superficial and deep) roots, (iii) Balance between left and right cordon, (iv) Balance between shoot growth and yield and (v) Balance between young and old leaves in the canopy.
BALANCE BETWEEN SUBTERRANEAN GROWTH AND GROWTH ABOVE THE SURFACE
The vine strives towards a 2,5:1 ratio between aboveground and subterranean growth (Hunter, 1998). An improvement in root growth therefore means a 2,5 times improvement in growth above the surface. There is always a balance between the number of root points (as an indication of root distribution) and the amount of growth above the surface (Table 1). Vines with more roots per allocated space have a bigger shoot growth capacity than those with fewer roots. This relationship is clearly illustrated by Figs 1 to 3. Colonisation of the soil volume available to a vine is therefore of primary importance.
TABLE 1. The influence of the number of roots on the shoot
mass as influenced by various rootstocks, Nietvoorbij, 1974-1981
(Southey & Archer, 1988)
Rootstock Number of roots
per m² profile wall Average shoot mass
(t/ha) Ramsey 595 4,22 99
Richter 402 3,06 101-14
Mgt 343 2,95 143-B
Mgt 340 2,41 Jacquez 293 1,96 3306
Couderc 266 1,84 Teleki
5BB 136 1,24
In most instances the subterranean:aboveground growth balance is dictated by the size of the root system, which is why the effectiveness of soil preparation is decisive in the establishment of the balance. A root volume that enables colonisation of both a fine root as well as a deeper tap root system is a prerequisite for a sustainable balance between growth above and below the surface. It should be borne in mind, however, that with an ineffective trellis system, which results in canopy crowding and consequently in interior canopy shade, this balance as well as the balance between fine and tap roots may be disturbed unilaterally, thus causing an imbalance in the vine.
Root growth points are important production organs for certain growth hormones. The hormone balance that exists within the vine dictates the growth and ripening processes and if this balance is disturbed (e.g. by a poorly developed and functioning root system), the internal plant processes which are necessary for a balanced vine, are also disturbed. Poor pruning and/or canopy management practices, on the other hand, are also responsible for a disturbance of the balance, thus contributing to unbalanced vineyards. If a balance is created between the rootstock and scion space, it must be maintained by the correct cultivation practices (Fig 4).
BALANCE BETWEEN FINE AND THICK ROOTS
Balanced growth above the surface brings about balanced distribution of nutrients resulting in the optimal functioning of all vine parts. So the correct trellis system, plant spacing, pruning practices and canopy management as well as judicious fertilisation and irrigation will also impact on the composition and distribution of the root system. In Figure 5 the effect of correct canopy management (resulting in optimal canopy microclimate and functioning) on the root diameter balance is shown. Canopy management stimulates the development of roots especially in the smaller diameter classes (Hunter and Le Roux, 1992; Hunter et al., 1995). A higher incidence of such roots increases the uptake ability of the root system and results in more efficient utilisation of minerals and water. A further very important consequence is the production of growth hormones that regulate aboveground growth, bunch development and grape composition.
Shoot
length (cm) Sugar
concentration (°B) Acid
concentration(g/l) pH Skin
colour (520 nm) ±
60 23,4 5,2 3,8 1,203 ±
120 24,5 7,4 3,3 2,761 >
200 21,9 8,9 3,2 1,078 Shoot
length (cm) Sugar
concentration (°B) Acid
concentration (g/l) pH Skin
colour (520 nm) ±
60 23,6 4,2 4,1 1,341 ±
120 24,9 7,1 3,4 2,043 >
200 21,3 10,3 3,9 0,981 Shoot
length (cm) Sugar
concentration (°B) Acid
concentration (g/l) pH Fruitiness
on taste ± 60 20,5 4,0 3,8 None ± 120 24,3 8,6 3,3 Outspoken > 200 19,1 14,0 3,7 Traces
only
BALANCE BETWEEN RIGHT AND LEFT CORDONS
Although this balance is applicable to a split cordon vine shape, the principles involved are also of vital importance to single cordon, as well as bush vines. Vines with an unbalanced permanent structure such as e.g. a short cordon on the one side and a long cordon on the other side, or an unbalanced circle of bearer arms in the case of bush vines, result in heterogeneous shoot lengths with corresponding heterogeneous levels of ripeness in the grapes. Tables 2, 3 and 4 indicate that heterogeneous shoot length deriving from uneven cordons in the case of Sauvignon blanc, Merlot and Cabernet Sauvignon result in uneven ripening. The occurrence of green and overripe grapes in the same vine play a role in the dilution of the potential quality and leads to yields with unpredictable and poor wine quality. Heterogeneous shoot lengths also result in heterogeneous bunch sizes (Fig 6).
The only way to obtain balanced cordons is by ensuring that upright trunks are formed during the vine development procedures. In this regard a uniform cordon split with cordons that fully cover the cordon wire, is a prerequisite (Fig 7).
A balanced cordon is obtained by allowing the same number of buds on the left and right cordon shoots. The balancing is based not only on length or thickness. If the balance has been created in the adult vines it is vitally important that it be maintained by means of effective pruning practices. This is achieved by allocating the same number of buds left and right annually so that the same number of sinks on both sides may exercise a balanced demand for the division of the water and nutrient pool.
BALANCE BETWEEN SHOOT GROWTH AND YIELD
When primary shoot growth is naturally arrested within three weeks after the beginning of véraison, it is a sign that there is a good balance between shoot growth and crop. Such a balance ensures good ripening with balanced grape composition and eventual good wine quality, especially once the shoots have reached a length of 1,2 m to 1,6 m. This shoot length is indicative of sufficient leaves per shoot to ripen two bunches completely, but the prerequisite is that the leaves must be efficiently exposed to sunlight.
Cardinal prerequisites for this balance are the following:
(i) A spur spacing which provides each fertile shoot with enough space in the canopy so that it may assume its rightful place - in an upright position - in the trellis without too much overlapping (Fig. 8).
(ii) An effective spur length which prevents the building up of spurs so that each fertile shoot may reach its rightful length for a correct leaf surface: fruit mass ratio. This ensures complete ripening (Fig 9).
(iii) A trellis system that can accommodate the shoot growth and does not limit it. This prevents excessive and untimely topping and provides sufficient space for shoots to stop growing spontaneously after véraison. On such a trellis system it is also easier to achieve even shoot length (Fig 10).
Vines with shoots that stop growing before véraison are just as unbalanced as vines with shoots that continue to grow after ripening. The occurrence of both these kinds of shoots results in grapes with heterogeneous composition and play a role in inferior wine quality. Shoots that stop growing before véraison have insufficient leaves to ripen their bunches and shoots that grow actively during ripening have growth points that compete too much with ripening bunches for photosynthetic products. In both instances optimal grape composition is compromised.
Fig 11 shows a trellis system that will accommodate effective shoot growth under most conditions. The spacing of the moveable canopy wires in particular is of cardinal importance. For cultivars with shoots that grow upright (e.g. Sauvignon blanc, Cabernet Sauvignon, Cabernet franc, Pinotage, etc.) it is recommended that the first set of moveable canopy wires be attached no higher than 20 cm above the cordon wire. Further moveable canopy wires must be spaced not more than 30 cm apart and the entire pole must be used right to the top. For cultivars with floppy shoots (e.g. Merlot, Shiraz) it is recommended that the first set of moveable canopy wires be attached no higher than 15 cm above the cordon wire. Further moveable canopy wires must not be spaced more than 20 cm apart and the whole pole must be used right to the top.
It is important for the bottom set of canopy wires not to be moved once it has been hooked up and it is often necessary, in instances where these wires are not rigid enough, to draw them closer with wire clamps. The only set of wires that changes in position is the top set, which is moved to the top of the pole as soon as the shoots have grown about 15 cm past the pole height. By so doing an extra 30 cm in the height of the canopy is ensured. The purpose of this is to achieve upright shoots in the canopy, all of which require timely tipping to the same height in order to obtain uniform shoot length. This will prevent slanted shoots with unhindered and uncontrollable growth in the canopy, while simultaneously ensuring that sufficient lateral shoots develop to carry younger leaves (Fig. 12).
BALANCE BETWEEN YOUNG AND OLD LEAVES IN THE CANOPY
It is a well-known fact that the contribution made to bunch feeding of leaves aged 80-90 days plus starts to taper off. This means that the contribution made to sustained ripening by leaves which unfolded in the second half of September and the first half of October, starts to decrease as from the second half of December and the first half of January. These are the leaves in the bunch zone of the canopy and this means that the leaves situated higher in the canopy (younger leaves) play a cardinal role during ripening. Obviously the time of full ripeness plays an important role and this means that cultivars which ripen later (e.g. Cabernet Sauvignon, Cabernet franc, Shiraz, etc.) must have more young leaves, percentage-wise, than early ripening cultivars. These young leaves are situated higher up on the primary shoot (for this to occur, the trellis system must be higher) and also on the lateral shoots. For effective ripening the main shoot leaf surface:lateral shoot leaf surface must be approximately 0,7 (Hunter, 2000). This means that at the time of final ripening the lateral shoot surface must be approximately 30% bigger than the main shoot leaf surface.
Economics and mechanisation prohibit excessively high trellis systems and therefore it is very important for optimal ripening to obtain and maintain lateral shoots (especially in late cultivars); particularly in trellis systems that are too short to achieve efficient shoot length. For the same reason very late topping is detrimental to optimal ripening because in most cases it removes the most productive leaves. Therefore timely and also more regular tipping of primary shoots is beneficial because it results in the formation of lateral shoots (more young leaves) earlier in the season (Fig 13).
CONCLUSION
To obtain a balanced vineyard, a strategic approach is required in which all practices are directed at inducing moderate growth. This entails all long and short term practices, but it is the former especially which requires careful planning to limit short term input. Three of the long term practices in particular deserve special mention in this regard, viz: root stock choice, the choice of plant spacing and the choice of the trellis system. The choice of a less vigorous rootstock, narrower inter-row spacing with a wider vine spacing in the row, together with a trellis system that will accommodate and not limit the vigour, play a big role in the creation of balanced vines with homogeneous shoot length. If the nutrition and irrigation programmes of such vineyards are attuned to obtaining moderate growth, the battle to obtain balanced vines is halfway won.
Soil preparation has a direct impact on the balance between rootstock and scion growth and the trellis system affects the way in which this balance can be accommodated and maintained. Vine spacing and structure effect the balance between the left and right cordon arms. The trellis system and pruning influence the balance between shoot growth and yield, while canopy management practices, together with the trellis system, have a direct impact on the balance between young and old leaves in the canopy, as well as root diameter composition of the root system. This is why a total strategy is required to obtain balanced vines. Balanced vines are the key to success, irrespective of the production goal. The only difference between vineyards that are managed for icon and/or ultra premium wines on the one hand, and standard and/or distilling wine on the other hand, is that the relevant balances lie on different levels.
LITERATURE
Archer E, 2001. Die verband tussen stokvormingspraktyke en druifgehalte. Wynboer Tegnies. 145, 19-20.
Hunter JJ, 1998. Plant spacing implications for grafted grapevine I. Soil characteristics, root growth, dry matter partitioning, dry matter composition and soil utilisation. S. Afr. J Enol Vitic., 19, 25-34.
Hunter JJ, 2000. Implications of seasonal canopy management and growth compensation. S. Afr. J Enol Vitic., 21, 81-91.
Hunter JJ & Le Roux DJ, 1992. The effect of partial defoliation on development and distribution of roots of Vitis vinifera L. cv. Cabernet Sauvignon grafted onto rootstock 99 Richter. Am.. J Enol. Vitic., 43, 71-78.
Hunter JJ, Ruffner HP, Volschenk CG & Le Roux DJ, 1995. Partial defoliation of Vitis vinifera L. cv. Cabernet Sauvignon/99 Richter: Effect on root growth, canopy efficiency, grape composition and wine quality. Am. J Enol. Vitic., 46, 306-314.
Southey JM & Archer E, 1988. The effect of rootstock cultivar on grapevine root distribution and density. In: JL van Zyl (compiler) The grapevine root and its environment. Tech. Comm 215. Dept Agric Water Suppl.
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E Archer1) and JJ Hunter2)
Fig 1. Influence of root growth on the growth of a vine above the surface. Left: Small root volume due to insufficient soil preparation. Right: Large root volume as a result of effective soil preparation.
Fig 2. Influence of root growth on the growth of a vine above the surface. Left: Small root volume due to ineffective establishment. Right: Large root volume as a result of effective establishment.
Fig 3. Influence of root growth on the growth of a vine above the surface. Left: Small root growth supports a small volume above the surface. Right: Large root volume supports a large volume above the surface.
Fig 4. A balanced volume above the surface is maintained by the correct pruning practices to achieve balanced shoot growth.
Fig 6. Heterogeneous shoots result in heterogeneous bunch development.
Fig 7. Poorly formed cordon arms (A, B and C) result in imbalance. Cordon shoots cross over for balanced vine development (D). The correct vine development process results in balanced cordon splits (E).
Fig 8. Upright shoots ensure effective use of the trellis.
Correct short spur
Wrong short spur
Fig. 10 Even shoot length obtained by means of an effective trellis system
Fig. 13 Lateral shoots ensure sufficient young leaves to support ripening
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