Identification of terroirs in the Robertson valley for Chardonnay and Shiraz : a focus on soil and roots

[摘要] 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 areextremely complex because they encompass complicated and interrelated processes that arenot yet fully understood.In terms of water-use behaviour, differences between cultivars have been described in theliterature. 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 waterstatus and stomatal conductance, from 07:00 to 19:00, were followed for a single day at the endof the 2009 season. The results showed that, at the end of the season, Shiraz was subjected towater stress conditions, losing leaves and showing symptoms of berry shrivelling. The otherthree cultivars had a much better canopy status and no symptoms of berry shrivelling wereobserved. Based on the canopy observations and a comparison of the curves of stem waterpotential (Ψs) and stomatal conductance (gs), it seems that Pinot noir and Chardonnay arecloser 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 ineight commercial vineyards in the Robertson region in order to investigate the relationshipbetween soil and root morphology, and the influence thereof on canopy development and berrygrowth. These plots had different soil types. Important soil properties are reported to limit rootgrowth, individually or as a combination of restrictions. It was found that the size of the rootsystem of 101.14 Mgt is defined by soil physical and chemical properties. The roots of 101.14Mgt under irrigation can grow to a depth of 100 cm or beyond if the soil physical and chemicalproperties allow it.Because the soil properties define the root system and the water storage/drainage, they greatlyinfluence the plant water status, even under irrigation. In an arid zone like Robertson, irrigationis an important management tool. The balance between canopy growth before véraison and theability of the root-soil system to maintain that canopy size during the ripening process is crucialin an area with a high evaporative demand. In this regard, not all the soil properties-root systemcombinations showed satisfactory performance in maintaining the canopy functioning, whichaffected 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 morphologyof the root systems was used as a starting point to group soils together. The importance of soildepth has been described well, thus the root systems were first classified according to rootingdepth – 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 familiesof the South African soil taxonomy, mainly due to the restrictive function of the B horizon. Thisrestrictive function is related to soil properties that are taken into consideration in the SouthAfrican soil classification and that are important for grapevine root growth, as well as thethickness of the described horizons and the physical and chemical differences between thehorizons.Soil properties have an important influence on root morphology. Due to the fundamental roleplayed by the root system in the overall plant functioning, soil properties are of criticalimportance. In an arid area, the low water pressure in the atmosphere and the high temperaturegreatly affect the plant water status. The soil-root system combination plays an important role inthe ability of the root system to supply the plant with water during times of high evaporativedemand. Different cultivars will react differently due to differences in transpiration control. Themaintenance of an adequate water status will have an immense influence on canopydevelopment and maintenance, and on normal and steady berry ripening. In this study it wasfound that not all the soil-root combinations can fulfil this satisfactorily. Thus, the grapevinebalance determined by the combination of the soil-root-canopy complex and the influence ofmanagement techniques is extremely important for the favouring of a good canopy:root systemratio, a functional canopy throughout the season and a steady berry ripening curve.

[发布机构] Stellenbosch University