VINE (VEGETATIVE) DEVELOPMENT
grow many sinker roots, whereas one with a shallow angle tends to grow spreader roots. In terms of determining the suitability of rootstock vines to areas with or without a high water table, this information can be very useful.
Like the above-ground parts of the vine, the roots also have a period of quiescence in the dormant season. As the soil warms in the spring, root tips begin to explore the soil, but their development is relatively slow (in terms of dry matter accumulation) compared with the growth and development of the shoots and flower clusters. After fruit set, the full canopy of the vine is able to support more root development but, as the berries ripen toward the end of the season, root growth again slows due to the shift in partitioning of carbo- hydrates towards the fruit. In areas where there is sufficient warmth to allow photosynthesis after harvest, there may be another flush of root growth before the vine becomes dormant, which appears to be related to the excess photosynthetic capacity on the vine after the fruit is removed (Williams, 1996;
Conradie, 2005). In cooler climate areas where there is little or no time between harvest and leaf-fall this cannot, of course, happen.
Tendrils
Most, if not all, vinous plants produce tendrils as a means to support themselves on another structure. It is through this specialized lateral branch (Pratt, 1974) that the vine can invest less energy in developing a solid trunk and more into growing in length, which is in keeping with the vine’s evolutionary origins.
To facilitate grasping on to a support, the tendrils grow away from light and curl around or within objects, eventually becoming lignified and woody (see Plate 9). If a tendril does not latch onto something, then it will wither and fall from the shoot. Unlike shoots, tendrils have determinant growth, i.e. they grow to a finite length.
Cane maturation
Similarly to tendrils, shoots will lignify, become more woody and change colour from green to brown (sometimes distinctly red) as they age (see Plate 10). It is this formation of a periderm that is associated with the shoot becoming more resistant to drought and cold ⫺an adaptation that allows the plant to survive below freezing temperatures that are found in many areas where Vitisspecies have developed. The compound buds at each node on the hardened shoot ⫺ now called a cane ⫺also go through a similar process, with the bud scales becoming lignified and resistant to drought, temperature and mechanical damage. As shoots mature, there is a build-up of starch and/or soluble sugars within, which is associated with the vine being able to survive below-freezing temperatures (Hamman et al., 1996; Chapter 4) and which provides energy for growth in the following spring.
Leaf-fall and abscission
Towards the end of the growing season environmental signals trigger the vine to accelerate its physiological preparations for the dormant season. Leaves may begin to yellow (or become red if a red-skinned grape cultivar, see Plate 11) and fall from the vines. However, if the vines still have their crop, senescence may be delayed due to the demand for photosynthates from the fruit (Petrie et al., 2000a). The ability of vines to photosynthesize after harvest, and therefore augment storage of photosynthates before the dormant period, is significant in terms of vine performance. Increased carbohydrate status in vines at the end of the season is correlated with increased winter hardiness and fruitfulness in the following season (Howell et al., 1978; Bennett et al., 2005). The coincidence of harvest and leaf-fall in cool climate vineyard areas, and thus limitation on photosynthesis, may be a reason why vine yields are lower in cool climates as opposed to warmer ones (Howell, 2001).
Dormancy
The period of dormancy is integral to the survival of the vine in areas where winter temperatures fall below 0°C, although dormancy is also an important part of the vine cycle in warmer areas, too. Grapevines are grown in tropical areas, and as shoots are not determinant they will grow indefinitely. This can create some production and management problems, however, so vines in these areas are often forced into and back out of dormancy through the use of severe pruning or chemicals such as hydrogen cyanamide (Lavee, 1974; Shulman et al., 1983). It has also been proposed that cyanamide could be used to force shoots to grow even before leaves have been removed or senesce, resulting in evergreen production of grapes (Lin and Wang, 1985).
Naturally occurring dormancy is brought about by low temperatures in the late growing season as well as by shortening day length (Schnabel and Wample, 1987). Non-V. viniferaspecies, such as V. labruscaandV. riparia, seem to be more sensitive to day length than the V. viniferavines (Fennell and Hoover, 1991;
Wake and Fennell, 2000). Once the leaves have fallen the vine is in a state of ecodormancy, which is controlled by the environment (e.g. cold temperatures) as well as in a state of endodormancy (through physiological factors within the plant). Therefore, buds will not develop immediately even if environmental conditions improve. While vines do not have a chilling requirement (i.e. needing a certain amount of time below 10°C in order to emerge from endodormancy) per se, cold temperatures do facilitate the process, decreasing the length of time needed for the plant to start growing again when environmental conditions improve (Schnabel and Wample, 1987).
The major task of pruning occurs during the dormant season (Chapter 6).