Thesis (PhD(Agric))--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Water scarcity is a key limiting factor for viticulture in dry regions. Traditionally drought sensitive varieties have the potential to grow in dry areas, however in most situations, through the use of rootstocks. Drought-tolerant rootstocks are expected to improve grapevine response to water deficit by improving the water uptake and transport and by reducing the water loss in leaves by root-to-shoot signalling. The mechanisms of rootstocks’ tolerance to drought are not yet fully understood. The main aim of this study was to improve the understanding of the rootstock/scion-cultivar interaction in the regulation of grapevine water use and leaf stomatal behaviour. Irrigated field vines without any water constraint were compared to rain-fed grapevines subjected to moderate water constraint. To better manage vine water status, reduce variability, and compare more rootstocks, greenhouse trials were also conducted where plants were well watered or subjected to severe water constraints. Pinotage grapevines (Vitis vinifera L.) grafted onto 110 Richter, 140 Ruggeri and 1103 Paulsen rootstocks were used for field experiments whereas Pinotage grapevines grafted onto 99 Richter, 110 Richter, 140 Ruggeri, 1103 Paulsen and Ramsey were used for greenhouse experiments. Our study suggested the influence of rootstocks on scion-cultivar water status and leaf stomatal size and density and gas exchange of the scion, implying an influence on water uptake and transport and a tight regulation of the stomatal conductance. Our data supported the hypothesis that the influence of rootstock in response to drought seemed to be higher under increasing water deficit up to a point where the plant water status is the main driver of the stomatal conductance and therefore photosynthesis regulation, considering the plant water status thresholds. In addition, the results suggested that stomatal development is affected by light, drought and possibly by rootstocks. Nevertheless, it is still not clear how the rootstock affects stomatal development and the link with scion-cultivar water use. It seems that the transpiration rate of leaves is more related to stomatal size than density. Thus one possible mechanism of Pinotage leaf adaptation to water constraints was structural during leaf growth, with a reduction in pore size to reduce plant water loss. The results showed that the rootstock is regulating the cultivar's stomatal size (anatomical changes during leaf growth) and functioning (stomatal regulation) through a complex signalling process. The effect of light on stomatal development is interesting in the context of canopy microclimate and canopy manipulation (choice of the vine architecture vs canopy size, in the context of climate change versus the possible increase in drought and water scarcity). The use of rootstocks is a long term investment which aims to provide resistance to soil pests and pathogens and to confer to the scion-cultivar drought and salt tolerance. The use of drought tolerant rootstocks is actually one of the most relevant practical solutions in dry terroir – units and in situations where water availability is limited. The understanding of the physiological and genetic mechanisms which govern scion-cultivar drought tolerance/behaviour induced by rootstocks is critical in terms of rootstocks choice in interaction with the scion-cultivar and is critical to assist breeding programs to create/select drought tolerant rootstocks.