|dc.description.abstract||ABSTRACT: The grapevine must constantly find a balance between two continually changing environments,
the rhizosphere (i.e. soil) and the troposphere (i.e. macroclimate). The adaptations are
extremely complex because they encompass complicated and interrelated processes that are
not yet fully understood.
In terms of water-use behaviour, differences between cultivars have been described in the
literature. In this study, the water status and stomatal conductance of four cultivars (Shiraz,
Grenache, Pinot noir and Chardonnay) grafted onto R99 were studied. Diurnal cycles of water
status and stomatal conductance, from 07:00 to 19:00, were followed for a single day at the end
of the 2009 season. The results showed that, at the end of the season, Shiraz was subjected to
water stress conditions, losing leaves and showing symptoms of berry shrivelling. The other
three cultivars had a much better canopy status and no symptoms of berry shrivelling were
observed. Based on the canopy observations and a comparison of the curves of stem water
potential (Ψs) and stomatal conductance (gs), it seems that Pinot noir and Chardonnay are
closer to the water-use behaviour of Grenache noir, which is known as a “pessimistic” cultivar,
than to Shiraz, which is an “optimistic” cultivar.
A study of four plots each of Chardonnay/101.14 Mgt and Shiraz/101.14 Mgt was carried out in
eight commercial vineyards in the Robertson region in order to investigate the relationship
between soil and root morphology, and the influence thereof on canopy development and berry
growth. These plots had different soil types. Important soil properties are reported to limit root
growth, individually or as a combination of restrictions. It was found that the size of the root
system of 101.14 Mgt is defined by soil physical and chemical properties. The roots of 101.14
Mgt under irrigation can grow to a depth of 100 cm or beyond if the soil physical and chemical
properties allow it.
Because the soil properties define the root system and the water storage/drainage, they greatly
influence the plant water status, even under irrigation. In an arid zone like Robertson, irrigation
is an important management tool. The balance between canopy growth before véraison and the
ability of the root-soil system to maintain that canopy size during the ripening process is crucial
in an area with a high evaporative demand. In this regard, not all the soil properties-root system
combinations showed satisfactory performance in maintaining the canopy functioning, which
affected berry sugar loading and berry volume.
In another study that is presented, forty soil profiles were characterised in the Robertson valley.
The root systems were considered as a product of the soil properties, and thus the morphology
of the root systems was used as a starting point to group soils together. The importance of soil
depth has been described well, thus the root systems were first classified according to rooting
depth – into shallow and deep root systems. The deep root systems were then subdivided,
creating two subgroups of high root density and low root density. The two extreme groups (i.e.
shallow roots, and deep roots with high root density) have particularly different soil properties.
The soil characteristics found in these extremes are represented up to certain point by families
of the South African soil taxonomy, mainly due to the restrictive function of the B horizon. This
restrictive function is related to soil properties that are taken into consideration in the South
African soil classification and that are important for grapevine root growth, as well as the
thickness of the described horizons and the physical and chemical differences between the
Soil properties have an important influence on root morphology. Due to the fundamental role
played by the root system in the overall plant functioning, soil properties are of critical
importance. In an arid area, the low water pressure in the atmosphere and the high temperature
greatly affect the plant water status. The soil-root system combination plays an important role in
the ability of the root system to supply the plant with water during times of high evaporative
demand. Different cultivars will react differently due to differences in transpiration control. The
maintenance of an adequate water status will have an immense influence on canopy
development and maintenance, and on normal and steady berry ripening. In this study it was
found that not all the soil-root combinations can fulfil this satisfactorily. Thus, the grapevine
balance determined by the combination of the soil-root-canopy complex and the influence of
management techniques is extremely important for the favouring of a good canopy:root system
ratio, a functional canopy throughout the season and a steady berry ripening curve.||en