Submitted to: American Journal of Enology and Viticulture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/3/2002
Publication Date: 10/15/2002
Citation: SPAYD, S.E., TARARA, J.M., MEE, D.L., FERGUSON, J.C. SEPARATION OF SUNLIGHT AND TEMPERATURE EFFECTS ON THE COMPOSITION OF VITIS VINIFERA CV. MERLOT BERRIES. AMERICAN JOURNAL OF ENOLOGY AND VITICULTURE. 2002. Interpretive Summary: Red wine grapes require sunlight to develop color and some of the flavors and aromas that make high quality wine, which brings a premium price for the producer. However, excessive exposure to sunlight leads to 'sunburn', or the browning of the berry skins and shrivelling of the berries. Sunburn degrades the chemicals responsible for good color and aroma in the finished wine. It was not known whether sunburn in grapes was caused by ultraviolet radiation, too much total solar radiation, or excessively high temperatures. By controlling both the exposure of berries to sunlight and UV, and by manipulating the temperatures of the grapes while on the vine, we determined that sunburn is due to the berries becoming too hot: the pigments and other compounds break down, leading to browning and undesirable aromas. In vineyards with rows oriented north-south, sunburn is more prevalent on the west side of the canopy. On any clear day, fruit on the west side of the canopy receives the same amount of sunlight as fruit on the east side of the canopy, but the west-facing fruit becomes hotter because it is exposed to sunlight during the hottest part of the day. As grape growers try to manage their vines with less water, they reduce the amount of foliage that could partially shade fruit to keep it from becoming too hot. It is important for these growers to understand why sunburn occurs and to manage the west side of their vines in a way that allows some sunlight onto the fruit but does not allow excessively high temperatures to occur.
Technical Abstract: Anthocyanin and phenolic profiles of berry skins from Vitis vinifera cv. Merlot in the Yakima Valley of Washington were influenced by sun exposure and temperature in 1999 and 2000. Total skin monomeric anthocyanin (TSMA) concentrations were higher in 2000 than in 1999 in any given treatment. Berry temperature was increased as much as 13 C above ambient and shaded cluster temperatures when clusters were exposed to sunlight, regardless of aspect for north-south oriented rows. However, maximum fruit temperatures (sometimes exceeding 40 C) were higher for clusters on the west side of the canopy because ambient temperatures were higher in the afternoon. East-exposed clusters had higher TSMA concentrations than west-exposed or shaded clusters. To separate light and temperature effects, west-exposed clusters were cooled to the temperature of shaded clusters and shaded clusters were heated to the temperature of west-exposed clusters. Exposure to sunlight increased TSMA concentrations regardless of temperature in both years. In 1999 and 2000, cooling sun-exposed clusters increased TSMA concentrations. Heating shaded clusters decreased TSMA concentrations in 1999, but had no effect during the cooler ripening period of 2000. Decreased TSMA concentrations in berry skins from west-exposed clusters were due to temperature and not due to UV radiation. Exposure to solar radiation increased concentrations of the 3-glycosides of quercetin, kaempferol, and myricetin. In 2000, sun-exposed clusters, regardless of aspect, had almost ten times greater concentrations of total flavonols than shaded clusters. Blocking UV significantly reduced individual and total flavonol concentrations, while temperature had little to no effect on their concentrations.