Factors affecting the levels of 3-isobutyl-2-methoxypyrazine and C6 compounds in Vitis vinifera L. Merlot.

Two field trials and a regional survey were conducted in Vitis vinifera L.Merlot vineyards located in California over several seasons to investigate the effects of weather variables such as rainfall and growing degree days and vineyard management practices, such as irrigation and fertilization, on the main compounds responsible for fruit and wine green aromas, 3- isobutyl -2 -methoxypyrazine (IBMP) and C 6 compounds. In the first study, the concentration of IBMP and C6 compounds in fruit harvested from 69 commercial vineyards located in three distinctly different wine grape growing regions were measured at commercial maturity from 2007 to 2010. Seasonal variation of green aromas was found to be more important than regional variation, with similar trends among regions observed between for each season. The temperature during the spring, i.e. a period of active vine growth, was found to be the main driver of fruit green aromas at harvest, likely due to interactions with vine vigour and fruit shading. A first field trial was conducted in 2009 and 2010 to investigate the influence of irrigation and N fertilization on green aromas in fruit and wine. Experimental treatments comprised two irrigation levels (the standard irrigation rate of 70% ETc, and 100% of ETc) and a higher than standard nitrogen fertilisation (60 kg/ha, also at 100% of ETc). Irrigation and nitrogen fertilization promoted canopy growth and the decreased fruit exposure resulted in elevated levels of IBMP through fruit maturation compared to the standard irrigation. However, C6 compounds were not affected by either factor. Fruit colour and aroma precursors were higher under deficit irrigation conditions, while vine yield decreased significantly. Wines made with fruit derived from the deficit irrigation treatment were perceived as having less intense vegetal notes, and enhanced fruit characters compared to those from the non deficit treatment. The additional application of nitrogen fertilizer also had a significant and positive effect on the vegetal perception of wines. A second field trial involving exclusion of winter rainfall was also conducted during the 2009 and 2010 growing seasons. Control vines were exposed to normal rainfall conditions, while winter rainfall was excluded from experimental vines by covering the ground with a plastic tarpaulin during the entire dormant season. During the growing season, irrigation in both treatments was maintained at a rate of 70%; of crop evapotranspiration (ETc) until commercial harvest. Canopy growth, berry size and vine yield were significantly reduced on the vines from the rainfall exclusion treatment, and a significant increase in the fruit to pruning weight ratio was recorded over time. IBMP concentration was greater under normal rainfall conditions, and fruit levels remained significantly higher during the season compared to fruit from vines that did not receive winter rainfall. Significantly different concentrations of C6 compounds were only observed in rainfall exclusion fruit towards the end of the second season. Fruit and wine quality parameters, mainly colour and mouth feel compounds, were positively affected by the absence of rainfall in both years. Descriptive analysis indicated that wines from the rainfall exclusion treatment were perceived as being less green and more fruity in the first season. However, as a consequence of the reduction in vine growth, the same treatment produced less fruity wines, of inferior tannin quality in the subsequent season. These findings demonstrate for the first time that vineyard management practices affect fruit and wine IBMP levels, but have no significant impact on the concentration of C6 compounds. As such, grape growers seeking to minimise IBMP fruit levels at harvest would benefit from reduced water and nitrogen application in the vineyard. Furthermore, these results show the importance of achieving adequate canopy growth early in the season, especially under low or no rainfall conditions. Otherwise, the positive effects on fruit and wine quality achieved through reduced berry size may be offset by an even more significant reduction in total yield and a severe lack of balance between vegetative and reproductive growth.

Author dc:creator

• Mendez-Costabel, Martin Pablo

Advisors dc:contributor.advisor

• Wilkinson, Kerry Leigh
• Ford, Christopher Michael
• Bastian, Sue
• McCarthy, Michael George
• Dokoozlian, Nick

• green aromas; yield; rainfall; methoxypyrazine; irrigation; wine

Handle dc:identifier.uri

http://hdl.handle.net/2440/84752

OAI identifier oai:identifier

oai:digital.library.adelaide.edu.au:2440/84752