Phenolic compounds include a diverse group of substances that perform a variety of functions in vascular plants. Phenolics are commonly divided into flavonoid (tannins, anthocyanins, flavan-3-ols, and flavonols) and nonflavonoid (hydroxycinnamates and stilbenes) compounds (Cheynier et al., 1999). Flavonoids are differentiated from nonflavonoid phenolics by their carbon skeleton (Kennedy et al., 2006). Most phenolics are purported to protect plants from ultraviolet radiation damage; some help defend against bacterial and fungal infection (Downey et al., 2006). Anthocyanins and other pigments attract animals that perform such services as pollination and seed dispersal (Downey et al., 2006). Tannins and flavan-3-ols discourage herbivory because of their astringent and/or bitter flavors (Santos-Buelga and Scalbert, 2000).
In addition to their many functions within plants, polyphenols contribute greatly to the sensory attributes of wine; increased concentration has generally been associated with high quality (Cheynier et al., 2006; Reynolds et al., 1995; Ristic et al., 2007). According to Downey et al. (2006), wine flavonoid content is mostly determined by grape composition and only partly by winemaking procedures; however, Holt et al. (2008a, 2008b) demonstrated that increased total phenol, tannin, and anthocyanin concentrations in must did not have an effect on wine quality scores.
A majority of the research on phenolic compounds has focused on their positive effects on human health. The capacity to neutralize free radicals (reactive molecules that cause mutations, heart disease, skin problems, and aging), also called antioxidant activity (AOA), is common to phenolics as a group (Iacopini et al., 2008). The AOA of many polyphenols is much greater than that of the essential dietary vitamins, and grapes and wine contain some of the highest phenol concentrations in the human diet (Iacopini et al., 2008). In addition to their antioxidant properties, phenolics act as anti-inflammatory agents to improve human health. Polyphenols in wine reduce the risk of cardiovascular disease (Fang et al., 2008) in two ways: by inhibiting the aggregation of platelets and by protecting low-density lipoprotein (LDL) cholesterol against oxidation (Santos-Buelga and Scalbert, 2000). Phenolics also protect against lung cancer, act as general antitumoral and anticarcinogenic agents, reduce systolic blood pressure, and lower plasma cholesterol levels (Fang et al., 2008; Santos-Buelga and Scalbert, 2000).
The light environment within the canopy is the most important factor influencing grapevine yield and quality (Dokoozlian and Kliewer, 1995; Smart and Robinson, 1991). Trellises or training systems determine the spacing and orientation of shoots, thereby controlling light interception in the canopy (Dokoozlian and Kliewer, 1995; Howell et al., 1991). It is generally agreed that shading of berries reduces total phenol concentrations (Cortell and Kennedy, 2006; Morrison and Noble, 1990; Wolf et al., 2003), and that fruit shading reduces wine phenolic content (Joscelyne et al., 2007; Macaulay and Morris, 1993; Ristic et al., 2007), but results have not always been consistent.
As well as affecting the total phenol concentration in grapes, shading also changes the relative abundance of various phenolic components. Shade treatments increased seed tannins (low molecular weight) relative to skin tannins (high molecular weight) (Ristic et al., 2007). When Downey et al. (2004) applied artificial shading to ‘Shiraz’ clusters, flavonol synthesis was decreased although there was no significant effect on anthocyanin or tannin concentrations. In another study, sunlight-exposed clusters contained nearly 10 times the amount of flavonol of fruit grown in the shade (Spayd et al., 2002). Some authors observed that fruit shading decreased total anthocyanin concentration and altered its composition (Koyama and Goto-Yamamoto, 2008; Price et al., 1995; Smart et al., 1988). Artificial shading did not change the amount of anthocyanins in fruit although it did change the composition; however, wines made from the shaded grapes had lower anthocyanin concentration than wines made from sunlit grapes (Ristic et al., 2007). In a study of the effects of artificial shade and increased sun exposure on wine quality, wines made from shaded fruit contained lower concentrations of anthocyanins and were less astringent than control wines. Yet anthocyanin concentration and astringency of wines made from experimentally exposed fruit did not differ significantly from control wines (Joscelyne et al., 2007). Morrison and Noble (1990) compared wines made from differently shaded ‘Cabernet Sauvignon’ fruit. Cluster shading decreased the anthocyanin content of wines, but sensory analysis yielded no corresponding differences in flavor or perceived aromas (Morrison and Noble, 1990). Total phenolic content of ‘Shiraz’ fruit was correlated with the percentage of ambient PAR (wavelength 400–700 nm) available in the fruit zone, but anthocyanin concentration was not related to PAR (Wolf et al., 2003).
The influence of fruit-zone light environment on phenolic compounds in classical red wine grapes has been extensively studied, but similar research has not been performed on cold climate hybrid grapes in general and on ‘Frontenac’ in particular. This is particularly significant because the genetic background of ‘Frontenac’ consists largely of Vitis riparia, whereas cited studies involve Vitis vinifera cultivars (Minnesota Agricultural Experiment Station, 2008).
Because previous investigations have yielded such varied and complex results, it is problematic to draw conclusions, especially for a relatively new cultivar. Therefore, this study was designed to determine if chemical composition of ‘Frontenac’ fruit is influenced by light intensity within the canopy. In addition to soluble solids, pH, and titratable acidity, we measured total phenolic and flavonoid concentrations of the fruit to provide a broader perspective.
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