Maintaining the balance between vegetative and reproductive growth in wine grape varietals is one of the most testing problems in modern viticulture. Grapevines which exhibit excessive vine vigour are likely to produce less fruit of reduced quality, and vines with inadequate vigour may be compromised in terms of their potential yield. The need for better management of vine vigour has become more acute in recent years with the increased use of irrigation, adoption of vigour-imparting rootstocks and the expansion of vineyards into cooler geographic regions.
A number of strategies can be used to control vine vigour. Chemical growth regulators, although capable of reducing vine vigour, have never received acceptance due to undesirable side effects and concerns over chemical residues.
De-vigourating rootstocks, likewise, may have the potential to control vigour but none are in wide commercial use. Achieved through manipulation of planting densities, competition by cover crops, regulation of water drip irrigation or regulation of water availability can all achieve a degree of de-vigouration but often at the expense of fruit yield and quality.
Manipulation of vines through pruning and trellis design are probably the most commonly methods used for the control of vine vigour. A high number of nodes retained at pruning combined with trellises which allow open canopies have proved very successful. Advances in the understanding of the physiological factors influencing shoot growth and transpiration have allowed the development of innovative irrigation methods for the control of vine vigour.
These techniques exploit the fact that chemical signals originating in the roots are primarily responsible for the control of shoot growth and transpiration. Stimulation of these signals through partial drying of the root system - results in a significant reduction in shoot growth and water-use while maintaining crop yield and improving fruit quality. These techniques, in combination with appropriate pruning and trellising methods, are providing ideal viticultural tools for controlling vine vigour and water-use efficiency.
Vine vigour can be primarily dependent on the varietal of the grape vine, the fertility of the soil and the local climate. It has also observed that young vines planted in fertile soil - can produce extremely compact bunches with very large berries that, once harvested, had a deficient level of even ripeness (berries still green with others over-ripe).
Older vines are known to produce a quality grape for several reasons: The regular damage to the vine caused by yearly pruning prevents sap from passing and reduces its vigour, thus leading to grapes with small, loose berries. Vines planted on steep slopes without terracing also reduces vigour of the plant, given that any fertile soil is washed away by the rain. In this case, erosion is the case of lower, yet higher quality production.
So in new terraced plantations with fertile soil with no erosion and water retention, the vine expresses all its vigour and tends to produce very large, compact grapes. Its berries no longer ripen at the same time and, as a result, the vine loses quality. To date, the conventional solution to this problem of quality consisted of making the plant suffer, to reduce its vigour: no irrigation, no tilling, less fertilisation etc. In other words, vigour is dealt with like a fault that must be corrected to obtain lower yields of a higher quality, similar to that of old vines.
Long period and varied experiments have proven that the morphology of the grape is related to the diameter of its vine-shoot. Shoots exceeding 10mm in diameter produce compact grapes, but if the diameter is between 6 - 8mm, the grape berries are smaller and loose. So to ensure that the shoots are thin enough, a larger number of shoots are left on the vine so that competition between them would lead to the required reduction in size. Where the vigour of the plant, which is inherent to its characteristics and growing conditions, is distributed among few shoots, these would be thick and long. However, where the same vigour or amount of wood is to be distributed among a larger number of shoots, these will be thinner and shorter.