Identification of terroirs in the Robertson valley for Chardonnay and Shiraz : a focus on soil and roots
Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2011.
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 horizons. 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.