Shading of the fruit zone to reduce grape yield and quality losses caused by sunburn

摘要

Aim: To test the hypothesis that shading of the fruiting zone of the plants might reduce yield losses caused by excessive exposure to sun while avoiding the most damaging effects associated with reduced radiation.Methods and results: A number of grapevine rows were shaded with a double layered white plastic netting on their south-facing side, from the ground to about 20 cm above the cluster zone. Data on meteorological conditions, plant growth, plant water availability, yield components and must characteristics were recorded during three growing seasons. Shading significantly increased yield but did not alter significantly the must characteristics.Conclusion: Partial shading of the grapevine canopy reduced yield losses attributable to excessive radiation. The must obtained from shaded berries had a lower concentration of anthocyanins, and the wines made from these musts had a lighter colour which may be detrimental to their quality.Significance and impact of the study: The study highlights the effects of solar radiation on the composition of grape musts and suggests a potentially cost-effective method to control excessive radiation in vineyards. IntroductionThe incidence of large numbers of shriveled berries on ripening grape clusters is very high in the Douro wine-producing region of Portugal. Fruits exposed to high radiation and heat are damaged by the sun; many berries desiccate completely (raisining), making the fruit inappropriate for winemaking, thereby reducing yield. This is a common phenomenon in regions with high light intensities and temperatures during the growing period (Cuevas et al., 2006; Krasnow et al., 2010; Greer and Weedon, 2012). Wine grape growers address this problem by opting for a north-south orientation of rows when slope permits so that fruits on both sides of the canopy achieve a balance in photosynthetic efficiency and exposure to solar radiation (Tarara et al., 2005; Cuevas et al., 2006). When east-west row orientation is unavoidable, yield loss due to sunburn can be substantial.Exposure to radiation and heat can be reduced by shading the plants, but this creates an imbalance in the carbon budget, reducing vine biomass and consequently its reproductive allocation (Greer et al., 2011). Furthermore, canopy photosynthesis may also be negatively affected (Morandi et al., 2011). Faced with elevated doses of solar radiation, higher plants have evolved a number of effective protective mechanisms, one of which is the accumulation of phenolic compounds (Merzlyak et al., 2002; Solovchenko and Schmitz-Eiberger, 2003). Phenolic compounds have been the subject of considerable interest in the medical community due to their antioxidant, antimicrobial, antiviral and anticarcinogenic effects found in wine grapes, they are also important contributors to the organoleptic qualities of wine (Jackson, 2000; Cheynier, 2005). The skin of shaded berries displays lower concentrations of flavonol and proanthocyanidins compared to fully exposed berries (Koyama et al., 2012). The synthesis of anthocyanins and proanthocyanidins in the berry skin requires a particular combination of light and temperature (Tarara et al., 2008), the former being the most abundant class of phenols in grape berries and the latter being responsible for the bitter and astringent properties of red wine (Vidal et al., 2003).Though shading may also influence vine development and must composition, there is no research-based consensus on these effects. Some authors report that shading has little effect on berry ripening and sugar accumulation but increases the pH and titratable acidity of the must (Ristic et al., 2007; Matus et al., 2009). Others have found that shaded berries ripen later and have a lower total soluble content (Bertamini et al., 2007; Marta et al., 2008; Abd El-Razek et al., 2010). Greer and Weedon (2012) found that irradiance, irrespective of seasonal temperatures, has no effect on the timing of budbreak nor on shoot phenology, stem growth or yield. While shade may slow vine development it may also promote increased leaf size (Greer et al., 2010). Nevertheless, the precise response of grapevines to shading is dependent on the grapevine genotype and also on the berry cluster microclimate, something that is influenced by viticultural practices such as vine training, row orientation, leaf canopy density and position of the grape clusters (Pastore et al., 2013).It is not practical or cost-effective to shade entire vines, since it can exacerbate the imbalance between the supply of and demand for carbon and result in greatly reduced vine biomass (Greer et al., 2011). However, shading only the fruiting zone of the vines may reduce sunburn-induced yield losses and still avoid the adverse effects of an overall reduction of radiation. To test this hypothesis, a number of east-west oriented rows of a commercial vineyard were shaded either from the onset of fruit setting to maturation or from veraison to maturation over three su