High Altitude UV Radiation and Polyphenol/Color Development

UV-B radiation, spanning wavelengths of roughly 280 to 315 nanometers, is the portion of the solar spectrum most effectively absorbed by stratospheric ozone and atmospheric gases. At sea level, relatively little UV-B reaches the vine. At high-altitude vineyard sites, the thinner atmosphere filters out less incoming radiation, and UV levels increase by approximately 10% for every 1,000 meters of elevation gain. Grapevines perceive this increased UV-B dose as an environmental stressor and mount a measurable biochemical defense, producing secondary metabolites, particularly flavonoids, stilbenes, and other polyphenolic compounds, in their berry skins. This is the foundation of the altitude-phenolic link that winemakers and viticulturists in the Andes and elsewhere have long observed.

• UV-B (280–315 nm) is selectively filtered by atmospheric ozone; at altitude, less atmosphere means more UV-B reaches the canopy and fruit
• UV radiation represents roughly 5–9% of total solar radiation reaching the Earth's surface, yet drives outsized phenolic responses in exposed grape berries
• High UV-B also reduces berry size and yields in some trials, concentrating phenolics by reducing dilution from water and pulp

The grapevine, like all flowering plants, detects UV-B through a dedicated photoreceptor protein called UV RESISTANCE LOCUS 8 (UVR8). In the absence of UV-B, UVR8 exists as an inactive homodimer. Upon absorbing UV-B photons, it monomerizes and interacts with the downstream signaling partner COP1, which in turn stabilizes the transcription factor HY5. HY5 then directly activates genes in the phenylpropanoid pathway, including chalcone synthase (CHS), phenylalanine ammonia-lyase (PAL), and flavonol synthase (FLS). In grapevine berries specifically, UV radiation consistently induces the expression of flavonol biosynthetic genes such as VvFLS1, VvGT5, and VvGT6, with flavonol accumulation being the most reliably documented response. Anthocyanin and tannin responses are also observed but are more variable across cultivar, UV dose, and winemaking conditions.

• UVR8 monomerizes upon UV-B absorption and interacts with COP1 to stabilize the HY5 transcription factor, upregulating phenolic pathway genes
• In grapevine berries, UV radiation has been shown to significantly alter the expression of 121 genes, with secondary metabolism pathways most prominently induced
• Flavonols (quercetin, kaempferol, myricetin glycosides) are the phenolic compounds most consistently elevated by UV exposure and most reliably carried through into finished wine

High-altitude UV exposure translates into wines with measurable phenolic differences, though the picture is more nuanced than simple linear amplification. The most consistent effect documented across multiple studies is elevated flavonol concentration, which contributes to UV-absorbing compound levels and color co-pigmentation. Color intensity can be enhanced through greater anthocyanin accumulation, though this response varies significantly by cultivar and UV dose. Resveratrol and other stilbenes are notably elevated in high-altitude sites; wines from Bolivia's Tarija have been measured with trans-resveratrol levels of 7.7 mg/L, compared to 1.0–3.2 mg/L in lower-altitude comparators. Research at Argentina's Catena Institute demonstrates that high-altitude Malbec consistently shows more intense color, a denser palate, and greater age-worthiness, alongside better-retained acidity.

• Flavonols are the phenolic compounds most reliably conserved from UV-exposed grapes to wine, contributing to color stability through co-pigmentation with anthocyanins
• Bolivian Tarija wines document trans-resveratrol at 7.7 mg/L versus 1.0–3.2 mg/L in wines from lower-altitude countries, linked directly to high UV-B exposure
• Catena Institute studies show high-altitude Malbec displays more intense color, a denser palate, lower alcohol, higher acidity, and greater aging potential than lower-altitude equivalents

Argentina provides the world's most studied and commercially prominent high-altitude wine terroirs. In Salta, the Calchaquí Valleys host vineyards planted between 1,700 and 3,100 meters above sea level, with Bodega Colomé, founded in 1831 and one of Argentina's oldest wineries, growing Malbec at four sites ranging from 1,750 to 3,111 meters. At these extreme heights, grapes develop thicker skins and higher levels of antioxidants and polyphenols. In Mendoza, the Uco Valley (including Tupungato, with vineyards up to approximately 1,650 meters) and Luján de Cuyo represent the premium zone for high-altitude Malbec. Bolivia's Tarija, at approximately 1,900 to 2,000 meters, is an emerging region where intense UV-B has been directly linked to elevated resveratrol and phenolic content in grapes and wines.

• Bodega Colomé (Salta, Argentina): founded 1831, vineyards at 1,750–3,111 m, producing Malbec with documented thick-skin development and elevated polyphenol content
• Mendoza's Uco Valley (Tupungato, Gualtallary): premium sites at 1,100–1,650 m with calcareous soils producing structured, age-worthy Malbec with high acidity
• Bolivia's Tarija: approximately 1,900–2,000 m altitude, with documented UV Index peaks of up to 16 and trans-resveratrol levels dramatically exceeding lower-altitude benchmarks
• Europe's CERVIM organization defines heroic viticulture starting at 500 m; Argentina's winemakers generally consider 1,200 m the practical threshold for meaningful altitude-driven phenolic effects

The UV-B response in plants is now well characterized at the molecular level. UVR8 uses tryptophan residues rather than a separate chromophore to absorb UV-B photons, triggering structural disruption of its homodimer into active monomers. These monomers interact with the E3 ubiquitin ligase COP1, preventing the degradation of HY5 and allowing it to accumulate and activate downstream genes. In grapevines, whole-genome expression analysis has shown that UV radiation alters the expression of more than 100 genes related to secondary metabolism, including multiple PAL, stilbene synthase (STS), chalcone synthase, and flavonoid glycosyltransferase genes. Abscisic acid (ABA) also plays a downstream role, amplifying the phenolic accumulation response. Importantly, research confirms that the altitude-phenolic effect is cultivar-dependent: some varieties show increased acylated anthocyanins at altitude while others do not, underscoring that UV dose, latitude, canopy management, and variety all interact.

• UVR8 uses intrinsic tryptophan residues as its chromophore for UV-B absorption, monomerizing and initiating COP1-HY5 signaling to activate phenolic pathway transcription
• ABA acts downstream of UV-B signaling in grapevines, amplifying berry skin polyphenol accumulation, particularly anthocyanins and total phenolics
• The altitude-phenolic effect is cultivar-dependent: some varieties increase acylated anthocyanins at elevation while others, such as certain Merlot and Cabernet Sauvignon clones, show less consistent responses

UV radiation does not act alone at high altitude. The full terroir package includes large diurnal temperature swings, often around 20 degrees Celsius at sites above 1,200 meters, which slow sugar accumulation while preserving acidity and extending the phenolic ripening window. Reduced atmospheric pressure and low humidity accelerate evapotranspiration, potentially concentrating berry solutes. Intense daytime solar radiation drives photosynthesis and ripening while cool nights slow malic acid degradation. The synergy of these factors, not UV alone, produces the distinctive high-altitude wine profile: structured phenolics, lively acidity, and concentrated dark-fruit character at lower alcohol levels than equivalent ripeness would achieve at sea level. As Fernando Buscema of the Catena Institute notes, altitude's effect on wine quality is meaningful only when considered alongside latitude, geography, and proximity to moderating influences.

• Diurnal temperature swings of approximately 20°C at sites above 1,200 m extend the ripening window and help preserve malic acidity alongside phenolic development
• Cool nighttime temperatures slow sugar accumulation relative to phenolic maturation, often yielding wines with lower alcohol and higher acidity compared to warmer lowland equivalents
• The UV-B effect is inseparable from complementary altitude factors: reduced pressure, low humidity, high radiation dose, and large thermal amplitude all shape the final polyphenolic outcome