Understanding and managing the timing of berry ripening and the flavour-ripe/sugar-ripe nexus

Abstract

The primary aim of this project was to further our understanding of berry ripening and how its onset and progression are controlled by endogenous plant growth regulators (PGRs). The knowledge gained was used to develop and test methods for manipulating the timing of veraison/harvest using exogenous PGRs in both laboratory and field settings. We have demonstrated the ability to delay ripening and harvest time and increase ripening synchronicity through PGR application while maintaining similar wine sensory properties. In addition, experiments were conducted to get a better understanding of the influence of harvest time versus sugar levels on wine flavour/aroma volatiles.

Summary

Changes in market requirements and climate have generated new challenges for the grape and wine industries. Warmer, drier growing conditions and higher CO2 levels appear to be the causes of altered grapevine growth and berry ripening. Grapes of all varieties are ripening earlier in the season and one consequence of this is ‘seasonal compression’ where the length of the harvesting season is considerably shortened. A consequence of this is that bottlenecks occur in grape intake and processing as the ability to process the fruit in a timely manner is swamped by large intakes during a restricted period. This can result in wastage, increased costs and reduced wine quality. These issues have been exacerbated by the apparent disconnect between sugar accumulation and the accumulation of flavour and aroma compounds and their precursors in the berry. More rapid sugar accumulation has led to fruit with adequate sugar levels but under-ripe flavour potential. In an effort to reduce ‘green characters’ in grapes they are frequently left on the vine to reach higher sugar levels which has resulted in a steady increase in wine alcohol levels. A better understanding of ripening and tools to alter it are required to alleviate some of the challenges discussed above. Plant growth regulators (PGRs) are key signalling molecules that control and coordinate plant development, including grape berry ripening. This makes them ideal targets for altering ripening in a global manner. This is not without challenges as their effects are wide ranging and the interactions between pathways are complex. In this project considerable progress has been made in understanding the metabolism and action of a number of key PGRs. Improved and detailed knowledge of the control of the initiation of grape berry ripening, which is a major determining factor in harvest timing, has been developed, which is vital to any attempts to usefully manipulate ripening. This project concentrated efforts on PGRs that are thought to be involved in, and can alter, the timing of ripening initiation. As many significant issues are due to earlier ripening much of the work has focused on auxins. Endogenous auxins delay ripening and can also be applied to fruit to delay its ripening. Although the pathways targeted by auxins are yet to be determined a detailed knowledge of auxin metabolism, both biosynthesis and breakdown, has been developed. We now know much about how auxin levels are controlled and why some auxins are more effective than others in controlling the timing of veraison. New tools have been developed to study the effects of auxins, which have led to the possibility of novel fit for purpose synthetic versions. Another important PGR is ethylene, which is involved in the control of ripening in a range of fruit. Our understanding of its role in grape ripening has been greatly advanced and we have shown that it can be used to delay ripening if applied at the appropriate time. Ethylene also interacts with auxins and so knowledge of its action is required to better tailor any manipulation of ripening. PGRs that advance ripening have also been studied. Aminoethoxyvinylglycine (AVG) inhibits both ethylene and auxin production and has the potential to significantly advance ripening. Other PGRs that do not affect the timing of veraison, but affect other aspects of berry development, have also been investigated. At least one of these appears to be involved in controlling some aspect of the mid/late stages of berry ripening with the potential to be important to wine flavour and aroma. Scientific evidence regarding the relative influence of berry harvest date and sugar content on wine volatile composition has been produced. This is important to our understanding of the effects of higher sugar content and delayed harvest times on wine composition. The results indicate that, while many flavour/aroma metabolites with a positive influence on wine style are related to sugar content, some with negative effects require time to decrease. Therefore, extending hang time may not improve the positive aspects of wine flavour but will reduce the concentration of negative flavours. The scientific outcomes described above have been put to use under commercial vineyard conditions to develop new ways of managing berry growth and ripening to alleviate the problems associated with altered berry development. Substantial and predictable delays in ripening, and therefore harvest, have been demonstrated in both white and red wine cultivars in different vineyards, under different climates during a number of seasons using a range of different auxins. Much knowledge on the timing and manner of application has been gained. Apart from delaying harvest other benefits have included an increase in the synchronicity of berry ripening. Wine metabolite levels, and in some cases sensory properties, have been significantly altered but these changes were not dramatic and may simply reflect the delayed fruit ripening later in the year under slightly different climatic conditions. This work has considerably enhanced our knowledge of berry ripening, has demonstrated some of the potential for PGRs in controlling ripening and has made progress towards a practical application of new techniques to usefully manipulate it. Copies of scientific papers arising from this project and referenced in this report can be obtained from the John Fornachon Memorial Library (http://www.awri.com.au/information_services/library-services-to-levy-payers/)

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