This project investigates the feasibility of using rootstocks to lower berry potassium concentrations ([K]) in Cabernet Sauvignon grapevines grown in the Coonawarra wine region. The overall aim is to achieve lower pH and higher titratable acidity in grape juice so as to maintain wine stability and bring down the cost of acid adjustment during winemaking. The objective is to provide new insights into the potential of particular rootstocks to modify K uptake by Cabernet Sauvignon grapevines grown in ‘Terra Rossa’ soil and their partitioning and accumulation into grape berries. To achieve this objective, the soils of a replicated rootstock trial located in the Limestone Coast of South Australia were characterised and plant tissue, grape and juice nutrient content were assessed at oenological maturity for 8 different rootstocks. Rootstock had a significant impact on cations of the vegetative tissue of Cabernet Sauvignon, with M5512 having the lowest petiole [K]. The concentrations of major cations in the berry were, however, not significantly altered by rootstock. While no particular rootstock stood out in its ability to limit Cabernet Sauvignon berry K accumulation, Börner berries tended to have slightly lower concentrations (< 10%) relative to vines on their own roots. Across the rootstocks, juice pH tended to increase with greater juice [K], while juice TA tended to decrease with greater juice [K]. It was found that juice titratable acidity was higher for the rootstocks 140RU and 110R, and juice pH tended to be lower for the rootstocks 110R, 140RU, M5512 and M5489. There was no effect of rootstock on total soluble solids.
Excess [K] in grapes reduces the free tartaric acid concentration in the juice and can modify the pH of both the juice and wine. A common amelioration strategy is tartaric acid addition during winemaking, which can be costly. High [K] can lead to precipitation of natural, as well as tartaric acid added during winemaking, into potassium bitartrate which further increases cost. Industry observation, in the Limestone Coast, found that the [K] level in the juice varies amongst vintages; excess [K] in the juice appears to be exacerbated by stressful growing conditions such as heatwaves.
In this project, we characterised the concentrations of soil, petiole, rachis and grape K, calcium (Ca), and magnesium (Mg) cations of an established rootstock trial in the Coonawarra. The trial is composed of own rooted Cabernet Sauvignon and Cabernet Sauvignon on 7 rootstocks. This new database provides links between soil, vine, berry and juice nutrient and composition at flowering and harvest. This project characterised the three major cations temporally and spatially in the soil-vine complex. It provided a better understanding of K uptake and partitioning in Cabernet Sauvignon grapevines as affected by rootstocks to allow vineyard managers to make informed decisions on rootstock selection for Cabernet Sauvignon wine production in this region.
The characterisation of the soil established that the cation profile and other soil compositional parameters were consistent, across the trial site at the sampled depths. Notably, rootstock was able to modify petiole [K] and [Mg] at flowering and [K] and [Ca] at harvest. Although concentrations of K, Ca and Mg in the whole berries were not affected by rootstocks, change in juice pH at harvest was evident. Importantly, no significant difference was observable for juice [K] between any rootstocks or own roots.