Methoxypyrazine concentrations in grape-bunch rachis are influenced by rootstock, region, light, and scion.

Abstract

Methoxypyrazines (MPs) are readily extracted from grape berry and rachis during fermentation and can impart “green” and “herbaceous” sensory attributes to wine. Irrespective of whether MPs, including 3-isobutyl-2-methoxypyrazine (IBMP), 3-isopropyl-2-methoxypyrazine (IPMP), and 3-sec-butyl-2-methoxypyrazine (SBMP), are extracted from berry or other vine material, techniques for remediation of wine with overpowering sensory characters attributable to MPs suffer from poor specificity or produce undesirable sensory outcomes, meaning that alternative control approaches are needed. Although often less considered than grape material, rachis contains comparatively higher concentrations of MPs, and Cabernet Sauvignon and Shiraz scions grafted onto rootstocks may influence the concentration of these compounds in the rachis. This work investigated the impact of region, scion, light, and rootstock on the concentration of MPs in the rachis of Shiraz and Cabernet Sauvignon at harvest. Grape bunches from Cabernet Sauvignon and Shiraz vines grown on common rootstocks within a number of Australian Geographical Indications (GIs) were
sampled at maturity across multiple vintages. Berries were removed and rachis material was segmented, extracted, and analysed for IBMP, IPMP, and SBMP by GC-MS/MS using a stable isotope dilution assay. Pruning weights were recorded as indicative measures of vegetative growth over the previous season for a single GI for both Cabernet Sauvignon and Shiraz vines and light exclusion boxes were applied to bunches to investigate the effect of shading. Data analysis was achieved with linear mixed models, one-way analysis of variance, and linear regression. The research showed that MP concentrations in Shiraz rachis at harvest were significantly impacted by GI and an interaction effect was observed between growing region and rootstock, with IBMP in the rachis ranging from an average of 21 ng/kg to 690 ng/kg across different regions and rootstocks. It was hypothesised that rootstock-mediated vine vigour influenced MP concentrations due to changes in canopy porosity and size, altering light interception by the grape bunches. This was supported by a statistically significant positive linear relationship with vine vigour independent of scion variety. The importance of light interception on MP concentrations was seen with the application of light exclusion boxes to bunches at veraison, resulting in an increase in mean IBMP concentration in rachis by up to 8 times compared to the unboxed controls. Vine vigour and rootstock significantly impacted the concentration of MPs in Cabernet Sauvignon and Shiraz rachis at harvest, with Shiraz rachis being significantly affected by GI. These findings provide grapegrowers with knowledge to assist in the selection of new rootstocks for plantings or management of existing ones and equip winemakers with decision-making tools for achieving wines of targeted style and quality with respect to MP concentrations.