Project 4.4.4

Rotundone and its role in defining terroir in iconic Australian cool climate ‘peppery’ Shiraz

Project summary

Focusing on premium cool-climate Shiraz, and targeting vineyards with old planting material which may be unique to Australia, this project extends the collaborative research between AWRI, CSIRO and Mount Langi Ghiran. It seeks to define genetic features in grapevine planting material (which may be transferable through propagation and between vineyards) and/or environmental features (which are site-specific and might be influenced by management practices), that are contributing to distinctive aroma attributes in wine.

The project builds on research at the Mt Langi ‘Old Block’ in the Grampians, but expands that study to other selected premium Shiraz sites across South Australia and Victoria. Research to date has demonstrated that the Grampians and Pyrenees regions can produce wines with substantially higher concentrations of the ‘spicy’ flavour compound rotundone than other notable Shiraz-producing regions (e.g. Barossa and McLaren Vale). Patterns of rotundone variation appear to be temporally stable within an individual vineyard across different growing seasons, with large differences in concentrations occurring across individual vineyards (~15-fold differences observed) and across different growing seasons (~30 to 40-fold differences). In general, cooler ripening periods and limited bunch exposure appear to favour the synthesis/accumulation of rotundone in Shiraz grapes. However, it appears unlikely that seasonal conditions, temperature and bunch exposure alone, can explain the magnitude of the observed site-specific differences in rotundone. Hence this project aims to identify genetic and/or environmental factors which alone, or in combination, may shape grape composition and wine flavour in Shiraz.

Building on previous research between the AWRI, CSIRO and Mount Langi Ghiran, this project will:
• generate rotundone and α-guaiene maps for Shiraz vineyards in the Grampians, Pyrenees, and/or Adelaide Hills, in addition to the Mount Langi Ghiran ‘Old Block’, and characterise their variation in vine performance and the underlying land via EM38, elevation and vigour mapping
• investigate potential genetic determinants for rotundone concentration using reciprocal bud grafts and/or potted vines in greenhouses
• investigate unique key aroma compounds in Shiraz wine from cool-climate terroirs, the role of photochemical oxidation in the formation of aroma compounds from sesquiterpenes, and the kinetics of guaiene and rotundone accumulation/degradation
• investigate the potential relationship between soil microbes and inherent spatial variation in grape composition and rotundone concentration.

Latest information

Background
Rotundone is the potent, grape-derived compound responsible for ‘black pepper’ aroma in wine. Previous research demonstrated that the Grampians and Pyrenees regions in Victoria can produce wines with substantially higher levels of rotundone than other Shiraz-producing regions such as Barossa and McLaren Vale. Patterns of rotundone variation appear to be stable within an individual vineyard across different growing seasons. A new collaborative project with CSIRO commenced in 2017, focusing on premium cool climate Shiraz, that aims to define features at the within-vineyard scale that contribute to rotundone formation. The research seeks to identify genetic and biophysical factors responsible for large differences in rotundone concentrations at harvest, and will provide insights into how the rotundone element of terroir, and grape aroma in general, may be influenced and managed at a range of scales.

Grape aroma compounds in cool climate Shiraz
GC-MS analysis of rotundone and α-guaiene in samples collected in 2018 in the Adelaide Hills showed a similar range of concentrations to those observed in the same vineyard in 2017. Mapping of α-guaiene in samples collected in 2017 and 2018 from this vineyard showed that the structure and patterns of spatial variability in α-guaiene were, for all practical purposes, the same as for rotundone. This is consistent with the view that α-guaiene is the precursor to rotundone in grapes. Again, rotundone formation in grapes occurred only relatively late in the ripening season, as had been observed previously in the Grampians. This suggests that grape rotundone concentrations and ‘peppery’ attributes in wine can be influenced by harvest timing, with earlier harvest reducing the likelihood of obtaining ‘peppery’ aromas and late harvest increasing the likelihood of such characters.
In addition, existing GC-MS data sets were re-analysed using bioinformatics tools for non-targeted profiling of grape volatiles. Spatial analysis and mapping of metabolites identified through this metabolomics approach showed that the structure and pattern of spatial variability of a range of sesquiterpenes is, for all practical purposes, the same as for rotundone. However, the 2018 rotundone and sesquiterpene maps from the Adelaide Hills vineyard were substantially different to the 2017 maps. This suggests that both the synchronised induction of sesquiterpene biosynthesis (by environmental trigger(s) which may vary from year to year) and availability of precursor α-guaiene are key to explaining grape rotundone concentrations.

The role of site characteristics in defining grape aroma compounds
A ripening experiment was conducted in 2019 to assess the impacts of harvest timing and extended hang-time on grape rotundone concentrations. In addition, vineyard experiments in the Grampians and Adelaide Hills regions were repeated in the Eden Valley to corroborate earlier observations.
Analysis was conducted to assess the role of microbial populations in vineyard soils, soil composition and soil attributes in shaping grape rotundone concentrations. Specifically, soil samples underwent chemical analysis, DNA samples were prepared and submitted to amplicon sequencing for bacteria (16S) and fungi (ITS) community composition, and metagenomic analysis of sequencing was completed for a vineyard in the Grampians region. The results were summarised in an article published in Frontiers in Microbiology (Gupta et al. 2019). In short, metagenomics analysis of surface soil samples collected from the previously identified high- and low-rotundone zones showed marked differences in the genetic diversity and composition of the soil bacterial and fungal microbiomes; a few specific taxa/groups of microorganisms were associated with the rotundone-based variation; and bacterial communities in soil from the high-rotundone zone appeared to form a much more complex and connected network than those in the low-rotundone zone soils. Also, short-term mulching effects did not seem to mask this rotundone zone-based variation.

Reference
Gupta, V.V.S.R., Bramley, R.G.V., Greenfield, P., Yu, J., Herderich, M.J. 2019. Vineyard soil microbiome composition related to rotundone concentration in Australian cool climate ‘peppery’ Shiraz grapes. Front. Microbiol. 10(1607): 1-19.


Project Contact

Markus Herderich
Mark Krstic

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