Flavonoids are a large and diverse group of compounds that, by their presence or absence, contribute greatly to wine quality. While the flavonoid content and composition of a wine reflects the vinification process to some extent, the primary determinant is the composition of the grapes at harvest. Thus, considerable research has been directed toward understanding the nature of flavonoids in grapevines, the factors that influence their biosynthesis, and how this knowledge might be used to manage and manipulate the flavonoid composition of berries at harvest. This review examines the flavonoids as a class of compounds, the role these compounds play in the plant, their contributions to wine quality, and recent research on the impacts of environmental factors and cultural practices on the flavonoid content and composition of grape berries.

Cultural Practice and Environmental Impacts on the Flavonoid Composition of Grapes and Wine: A Review of Recent Research

Opaque boxes were applied to bunches of Shiraz grapesprior to flowering to determine the effect of sunlight on berry developmentand accumulation of flavonoids. The boxes were designed to maintain airflowwhile excluding light and thus to minimise changes in temperature andhumidity. There was no significant effect of shading on sugar accumulationand in two of the three seasons studied there was no effect on berryweight. Chlorophyll concentration was much lower in the shaded fruit, whichappeared pale yellow until veraison. The fruit coloured normally in theshaded bunches and in two of the three seasons there was no significantchange in anthocyanin content. Expression of the gene encoding UDP-glucoseflavonoid-3-O-glucosyl transferase (UFGT), a key gene in anthocyaninsynthesis, increased after veraison and was similar in both shaded andexposed fruit. Anthocyanin composition was altered in the shaded fruit,which had a greater proportion of the dioxygenated anthocyanins, theglucosides of cyanidin and peonidin. Shading had no significant effect onthe levels of condensed tannins in the skin or seeds of ripe fruit. Shadingsignificantly reduced the levels of flavonols in the grape skin. In theexposed fruit, flavonol concentration was highest around flowering thendeclined as the berries grew, but there was an increase in flavonols perberry during ripening. When the boxes were applied before flowering, shadedfruit had much lower levels of flavonols throughout berry development andat harvest the level of flavonols were less than 10% of that in exposedfruit. A gene encoding flavonol synthase (FLS) was expressed at floweringand during ripening in exposed grapes but its expression was greatlyreduced in shaded fruit. The results indicate that shading had littleeffect on berry development and ripening, including accumulation ofanthocyanins and tannins, but significantly decreased flavonol synthesis.

The effect of bunch shading on berry development and flavonoid accumulation in Shiraz grapes

Plant phenolics have been for many years a theme of major scientific and applied interest. Grape berry phenolics contribute to organoleptic properties, color and protection against environmental challenges. Climate change has already caused significant warming in most grape-growing areas of the world, and the climatic conditions determine, to a large degree, the grape varieties that can be cultivated as well as wine quality. In particular, heat, drought and light/UV intensity severely affect phenolic metabolism and, thus, grape composition and development. In the variety Chardonnay, water stress increases the content of flavonols and decreases the expression of genes involved in biosynthesis of stilbene precursors. Also, polyphenolic profile is greatly dependent on genotype and environmental interactions. This review deals with the diversity and biosynthesis of phenolic compounds in the grape berry, from a general overview to a more detailed level, where the influence of environmental challenges on key phenolic metabolism pathways is approached. The full understanding of how and when specific phenolic compounds accumulate in the berry, and how the varietal grape berry metabolism responds to the environment is of utmost importance to adjust agricultural practices and thus, modify wine profile.

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Berry Phenolics of Grapevine under Challenging Environments

The properties and expression patterns of the six isoforms of sucrose synthase in Arabidopsis are described, and their functions are explored through analysis of T-DNA insertion mutants. The isoforms have generally similar kinetic properties. Although there is variation in sensitivity to substrate inhibition by fructose this is unlikely to be of major physiological significance. No two isoforms have the same spatial and temporal expression patterns. Some are highly expressed in specific locations, whereas others are more generally expressed. More than one isoform is expressed in all organs examined. Mutant plants lacking individual isoforms have no obvious growth phenotypes, and are not significantly different from wild-type plants in starch, sugar and cellulose content, seed weight or seed composition under the growth conditions employed. Double mutants lacking the pairs of similar isoforms sus2 and sus3, and sus5 and sus6, are also not significantly different in these respects from wild-type plants. These results are surprising in the light of the marked phenotypes observed when individual isoforms are eliminated in crop plants including pea, maize, potato and cotton. A sus1/sus4 double mutant grows normally in well-aerated conditions, but shows marked growth retardation and accumulation of sugars when roots are subjected to hypoxia. The sucrose synthase activity in roots of this mutant is 3% or less of wild-type activity. Thus under well-aerated conditions sucrose mobilization in the root can proceed almost entirely via invertases without obvious detriment to the plant, but under hypoxia there is a specific requirement for sucrose synthase activity.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2006.03011.x

Analysis of the sucrose synthase gene family in Arabidopsis.

Nutrient Exchange and Regulation in Arbuscular Mycorrhizal Symbiosis

Cluster thinning (removal of 50% of the cluster one month after bloom) was tested for three consecutive years in a vineyard located in southern Piedmont (Italy). At harvest, concentrations of total anthocyanins and flavonoids were assessed by spectrophotometry and concentrations of individual anthocyanins were determined by high-performance liquid chromatography. Berry weight and berry skin weight slightly increased following thinning. Soluble solids and berry skin anthocyanins and flavonoids were more concentrated in berries from cluster-thinned plants. Cluster thinning increased the concentrations of cyanidin-3-glucoside, peonidin-3-glucoside, and, to a lesser extent, petunidin-3-glucoside. Concentrations of malvidin-3-glucoside and of acylated anthocyanins were not affected by cluster thinning.

Effect of Cluster Thinning on Berry Skin Anthocyanin Composition of Vitis vinifera cv. Nebbiolo

Summary
• Here we tested the hypothesis that the activity of sucrose-cleaving enzymes is modified in roots colonized by arbuscular fungi.
• The activities of soluble and cell-wall bound invertases and of sucrose synthase were assessed in nonnodulated roots of soybean (Glycine max), either inoculated with the arbuscular mycorrhizal fungus Glomus mosseae, or in the absence of inoculation. Furthermore, we assessed the activity of sucrose-cleaving enzymes in cultured extraradical mycelium of the arbuscular fungus Glomus intraradices.
• Both soluble and cell wall-bound acid invertase activities decreased during the course of the experiment; there were no differences between mycorrhizal and nonmycorrhizal plants. Alkaline invertase activity was significantly higher in inoculated roots from the beginning of root colonization. Sucrose synthase activity showed no significant differences between treatments for 35 d, thereafter it became significantly higher in mycorrhizal roots. No sucrose cleaving activities were detected in extraradical mycelium.
• Fungal colonization of soybean roots induces an increase in alkaline invertase activity, which could provide hexoses for the fungal symbiont and for development of colonized cells.

Pierpaolo Allara

Papers

Effect of Cluster Thinning on Berry Skin Anthocyanin Composition of Vitis vinifera cv. Nebbiolo

Cleavage of sucrose in roots of soybean (Glycine max) colonized by an arbuscular mycorrhizal fungus