Regional impact on rootstock/scion mediated methoxypyrazine accumulation in rachis

Aim: To investigate the impact of Geographical Indications (GI) of South Australia on the rootstock/scion-mediated methoxypyrazine accumulation within the rachis of Shiraz and Cabernet Sauvignon. 

Methods and Results: Cabernet Sauvignon and Shiraz bunches were sampled at maturity from two South Australian GIs over the 2019 and 2020 harvest periods. From each region, a minimum of 18 bunches per rootstock/scion combination were sampled from across the vineyard and their rachis material was assessed for 3-isobutyl-2-methoxypyrazine (IBMP). Results indicated that region and rootstock choice significantly affect the concentrations of methoxypyrazines within the rachis material of both Shiraz and Cabernet Sauvignon varieties at harvest. 

Conclusion: 

This research highlights the effect of regionality on the concentration of methoxypyrazines within the rachis material of Cabernet and Shiraz vines grown on common rootstock varieties. The outcomes will conceivably inform viticulturalists and winemakers of how methoxypyrazine characteristics of Shiraz and Cabernet Sauvignon rachis are impacted by common rootstock/scion combinations permitting informed rootstock selection and assisting in production of a target wine style.

Significance and Impact of the Study: The presence of rachis material during red must fermentation can confer methoxypyrazines to the wine. The presence of methoxypyrazines, and predominately 3-isobutyl-2-methoxypyrazine (IBMP), in red wine can impact the flavour and aroma profile due to their ‘green’ and ‘earthy’ characteristics. Interestingly, this phenomenon has been shown to impact the aroma profile of Shiraz wines, a variety that has not been shown to naturally produce methoxypyrazines within the berries. Furthermore, it appears that the concentration of methoxypyrazines within the rachis is mediated by rootstock/scion combination and the region in which the vines are grown. As rootstock uptake increases across Australia in response to biological threats and abiotic stresses, an understanding of the viticultural and regional influences on rootstock/scion mediated rachis composition is essential to facilitate the production of high-quality Australian wines under increasingly challenging conditions.