Several regional climate models have been proposed in order to forecast the overall effects of individual or combined climate change-related variables [53
]. Some models take into account air temperature and other variables, including precipitation, humidity, radiation, and historical viticultural records [54
]. Spatial modeling research has indicated potential geographical shifts and/or expansion of viticultural regions with parts of southern Europe becoming too hot to produce high-quality wines and northern regions becoming viable [17
]. For the Northern hemisphere, Jones et al.
] predicted that temperatures at regions producing high-quality wine between 2000 and 2049 are going to warmby 0.42 °C per decade and 2.04 °C overall. In the Bordeaux region, the predicted increase temperature overall trend would be 2.3 °C in the same period (Figure 4
For vineyards, the increase in the number of days with high temperatures is particularly relevant. Grape production and quality are sensitive to heat waves, especially at certain growth stages, such as flowering and ripening. At high temperatures, replacement of starch by lipids in leaf chloroplasts has been reported for grapevines [57
]. Prolonged periods with temperatures above 30 °C cause a reduction in photosynthesis, with consequent berry size and weight reduction [58
]. High temperature conditions may have implications in premature veraison
, berry abscission and reducing flavour development.
In the viticultural French region of Languedoc, the climacteric evolution over the period 1950–2006 obeyed to two distinct climate periods, according to Laget et al.
]. Observing the evolution of mean annual and seasonal temperatures, total solar radiation, night freshness index, the distribution and efficiency of rainfall and potential evapotranspiration (pET), it was reported an increase in mean annual temperatures of +1.3 °C between 1980 and 2006 and an increase in the mean pET of 900 mm/year since 1999. It was also concluded that the harvest dates advanced by up to three weeks and sugar concentrations at harvest increased by up to 1.5% potential alcohol. In the Bordeaux region, from 1952 to 1997 changes in the dates of all the phenological events and in the length of the growing season were reported for Cabernet Sauvignon and Merlot [64
]. Similar results were found in the southern hemisphere. In Australia, the date of designated maturity of Chardonnay, Cabernet Sauvignon and Shiraz advanced at rates of between 0.5 and 3.1 days per year between 1993 and 2006 [65
]. A trend towards earlier maturity of several varieties was observed in 12 different Australian winegrape growing regions form 1993–2009 [66
]. For most of the cases, the rate of change in the date of designated maturity was correlated with the rate of change in temperature.
3.1. Temperature and Radiation
Of environmental factors including all external stimuli, the most influential of which for phenolic synthesis are light/radiation and temperature, as well as water and nutritional status. Phenolic synthesis and accumulation in grape berry is also determined by genetic factors and the interaction between genotype and environment [3
]. The role of phenolics as photo-protectants explains their dependency on sun exposure [53
]. In warmer climates, high light exposure can increase the concentration of phenolics and anthocyanins because of the higher activity of PAL [67
]. Sun exposure is generally considered to be of primary importance for high quality wine production. However, it is not clear whether the effect on fruit composition is due to visible light or ultraviolet light or both [68
It has been shown that UV-B provoke several morphological, physiological and biochemical changes in higher plants, depending on the intensity, total dosage, plant species and the balance between UV-B and photosynthetically active radiation (PAR, 400–700 nm) [69
]. On the other hand, UV-A and visible light may induce both protective and repair mechanisms, thus decreasing the negative impact of UV-B light [71
]. However, relatively high levels of solar UV-B were reported to enhance the accumulation of UV-absorbing compounds, including flavonoids and related phenolics [72
]. UV-B is also known to upregulate genes encoding PAL and CHS [70
]. Phenolics transform short-wave, high-energy and highly destructive radiation into longer wavelength light, less destructive to the cellular leaf structures, including the photosynthetic apparatus [69
]. Very few studies have attempted to separate the effects of visible light from those of UV light [59
]. As discussed by Keller [74
], this is surprising given that phenolic compounds are absorbed predominantly in the UV range of the spectrum and form an important part of fruit quality in grapes.
Stilbene synthesis is enhanced in response to several abiotic factors. These factors include UV-radiation, wounding, ozone, anoxia and metal ions. Exposure to UV light induces the accumulation of stilbenes in grape berry through the induction of STS expression [75
]. In berries, this is dependent on the development stage, since unripe berries respond to UV irradiation to a greater extent. A study on grape plantlets proved the existence of a positive correlation between resveratrol synthesis in leaves (induced by UV) and field resistance [76
Flavonols are thought to protect plant tissue to UV radiation whereas anthocyanins are thought to provide some protection to UV radiation and high extreme temperatures [6
]. Synthesis of flavonols is a light-dependent process. Sealing grape bunches in light-excluding boxes from before flowering until harvest completely inhibits flavonol synthesis. If shading is applied later in fruit development, flavonol content is reduced and no further accumulation is detected after the initiation of light deprivation [3
]. In Pinot Noir, Shiraz, and Merlot varieties, the amount of these compounds has been shown to be highly dependent on light exposure of the tissues in which they accumulate [78
]. Light modulates the expression of flavonol synthase
), a key flavonol structural gene,
and of VvMYBF1
, a transcriptional regulator of flavonoid synthesis [79
]. In Cabernet Sauvignon and Chardonnay, flavonols are the only phenolic components in both grape leaves and berries that are consistently and severely increased by UV radiation [68
]. It was suggested that flavonols, but not anthocyanins or hydroxycinnamic acids, are important for UV protection in grapevine tissues. Similar results were recently confirmed by Koyama et al.
] who showed that UV light specifically induced flavonols while not affecting other flavonoid components. However, the relatively high concentrations of flavonols found even in the absence of UV radiation suggest that flavonols may also have a protective function against excess visible radiation [68
]. In the vineyard, any cultural practices that favor the exposure of grape brunches to sunlight boost flavonol accumulation. This occurs equally in white and red grapes.
Flavan-3-ols and proanthocyanidins are the most stable phenolics under diverse growing conditions. This is also true for accumulation of these compounds in seeds. However, some studies have shown a positive association between temperature and the number of seeds and total proanthocyanidin levels per berry at harvest [82
]. Shading treatments increased the amount of seed proanthocyanidins and affected their composition in Pinot Noir [84
], while had no effects in Shiraz [78
], reiterating the importance to discriminate between irradiation and temperature effects [53
Skin flavan-3-ols and proanthocyanidins are more sensitive than seed ones to environmental cues; sunlight has been shown to affect their relative content [78
], as well as their mean degree of polymerization [81
]. Sunlight exposure consistently increased the relative abundance of the tri-hydroxylated gallocatechins at the expense of the di-hydroxylated catechins and increased the mean degree of polimeryzation.
When the effect of cluster temperature on proanthocyanidins biosynthesis was studied it was shown that there is no consistent relationship between temperature and total proanthocyanidins accumulation across three seasons [16
]. In this field, experiment grape bunches were cooled during the day and heated at night (±8 °C). However, composition of proanthocyanidins was affected in the experiment because decreasing thermal time in degree-days favored a shift towards tri-hydroxylated forms.
Although anthocyanins and proanthocyanidins share several steps in the biosynthetic pathway, there are many differences in their regulation and reactivity. In fact, in contrast with proanthocyanidins, several authors reported that light, temperature, and their interactive effects, highly influence anthocyanin accumulation in berry skins [85
]. Exposure to sunlight is associated with an increase in anthocyanin accumulation, until the point when excessive heat causes berry temperature to become detrimental [3
]. In growth chambers, optimal conditions for anthocyanin accumulation occurred when grapes were exposed to cool nights (15 °C) and mild, temperate days (25 °C) during ripening [88
]. Higher temperatures (30–35 °C) promote the degradation of the existing anthocyanins [89
]. In the Merlot variety, attenuation of the diurnal temperature fluctuations led to increased ripening rates and higher anthocyanin concentrations at harvest [90
]. Moreover, absolute anthocyanin levels and chemical composition changes have also been related with warmer seasons, as indicated by the increased formation of malvidin, petunidin, and delphinidin coumaroyl derivatives [85
]. In another study [87
], the association of high temperatures with the increase of delphinidin, petunidin and peonidin-based anthocyanins in sun-exposed Merlot berries were observed, while malvidin derivatives remained unaffected. The complexity of combined solar radiation and temperature effects on flavonoid composition further expands the understanding of the effect of such environmental factors on anthocyanin biosynthesis [53
3.2. Agricultural Practices and the Levels of Synthesized Metabolites
In a vineyard, the environment varied due to the natural soil heterogeneity and the uneven light distribution. Physical characteristics of the vineyard can also affect flavonoid accumulation. These include altitude of the cultivation site, heat stress, defoliation, mineral supply or soil type, all of which have shown some influence. Nitrogen, potassium and phosphate are the nutrients commonly applied as fertilizers, although only nitrogen and potassium have thus far attracted viticultural research. Both low and excessively high levels of nitrogen have been shown to decrease color in grape berries, while high potassium has been reported to decrease color in grapes [85
]. Despite the age of the soil, which largely determine the micronutrient pool, structure and texture, and significantly affects plant growth [93
], the major consequence of soil type is the capacity of the soil to hold water while remaining sufficiently well-drained to avoid waterlogging [85
Despite the relevance of these parameters, vineyard microclimate has a fundamental influence in the metabolite biosynthesis. The importance of the effect of canopy microclimate on chemical composition of berry was initially raised by Shaulis and co-workers [98
] in their investigations with Concord grapevines. The amount and the distribution of light intercepted by the vines are determined by the architecture of the vineyard, mainly row orientation, height, width, porosity of the canopy, and distance between rows [99
]. The term “microclimate” was adopted by Smart [100
] to define the environmental conditions within the immediate vicinity of the leaves and fruit [101
Cultural practice effects on berry have long been studied; among them, leaf removal and cluster thinning, which modify leaf area/yield ratio and fruit-zone microclimate, could potentially improve grape quality [86
]. The amount of intercepted light affects the whole plant photosynthetic capacity, water balance, and source to sink balance [99
]. The source to sink balance is an important parameter that controls berry sugar, organic acids, and secondary metabolites content with qualitative enological potential [105
]. In general, berries grown under open canopy conditions, compared to berries grown under shaded canopy conditions, have higher juice sugar concentration (measured as total soluble solids), improved acid balance (lower juice pH and higher titratable acidity). However, while some exposure to light may be appropriate, high temperatures resulting from full exposure of berries are likely to inhibit anthocyanin metabolism [101
Vine vigor has been reported to impact upon the proanthocyanidins content and chemical composition of grape skins in Pinot noir. In the berry skin, proanthocyanidins were higher in low-vigor vines, with an increase in the proportion of epi-gallocatechin subunits, as much in polymers as on average size, observed with decreasing vine vigor [85
]. It seems that severe canopy shade down regulate gene expression in the anthocyanin biosynthesis pathway, [107
] while photon fluxes of 100 mmol/m2
/s on the berries temperature becomes the overriding variable in anthocyanin synthesis [74
Among environmental and viticultural parameters investigated in the past decades for various grape varieties, it is known that the water status is a potential modulator of secondary metabolism during the berry development [109
]. Many scientific articles have extensively reported the effects of water deficit on the accumulation of various grape secondary metabolites (Table 2
). Grapevine irrigation can alleviate water-stress-related reductions in plant growth and development, demonstrating the importance of cultural practice at vineyard to guarantee wine quality or even plant survival in regions affected by seasonal drought [113
]. Several reports demonstrated that large fluxes of water are not essential for the optimal plant performance for agricultural purposes and that moderate water deficits might be used successfully in grapevine production through control of sink-source relationships, thereby maintaining or ameliorating fruit quality [113
]. Plant water status affects berry composition, but the effects might be contrasting according to the level and the moment in time when water is applied or deficit is imposed. Furthermore, grape response to moderate irrigation might also be cultivar-dependent as V. Vinifera
varieties have been shown to respond differently to water stress [114
]. Overall, regulation of grapevine water deficit is a powerful tool to manage the amount of secondary metabolite compounds and improve wine quality [115
The impact of water on stilbene biosynthesis in grapes has been evaluated. The water deficit increases the specific steady state transcript abundance of a STS gene and phenylpropanoid metabolism in general. The increase of STS mRNA abundance suggests an increase in resveratrol accumulation [116
]. However, conflicting results have been reported on the effects of water deficit on resveratrol synthesis. Research conducted by Vezzuli et al.
] observed little effect of drought on resveratrol concentrations in grape berry skin. In another study on Cabernet Sauvignon and Chardonnay varieties, harvested at six and eight weeks after veraison
, respectively, Deluc et al.
] demonstrated that water deficit increased the accumulation of trans
-piceid (the glycosylated form of resveratrol) by five-fold in Cabernet Sauvignon berries but not in Chardonnay. However, the abundance of two stilbene-derived compounds—trans
-piceid and trans
-resveratrol—was not significantly different between the two cultivars when well-watered. Similarly, water deficit significantly increased the transcript abundance of genes involved in the biosynthesis of stilbene precursors in Cabernet Sauvignon. In contrast, the transcript abundance of the same genes declined in Chardonnay in response to water deficit.
The increased concentration of flavonols, skin-derived proanthocyanidins and anthocyanins has also been observed in wines from grapes grown under the decreased vine water status [85
Recently, it was shown that the concentrations of flavonol increase under drought stress in a white grapevine Chardonnay, but not in a red grapevine Cabernet Sauvignon [119
]. Few studies have reported that water deficit may modify the skin proanthocyanidins [120
], but this topic still awaits further clarification. In Shiraz, the application of water stress before and after veraison
differently affects the grape berry polyphenol biosynthesis [124
]. The authors showed that pre-veraison
water deficit had no effect on total proanthocyanidin accumulation, whereas pre- and post-veraison
deficits specifically affected the flux of anthocyanin biosynthesis in stressed grape berries sampled with equivalent sugar content. However, both water deficits differently affected the anthocyanin composition. Pre-veraison
water deficit increased anthocyanin accumulation except for malvidin and p
-coumaroylated derivatives, whereas post-veraison
water deficit enhanced the overall anthocyanin biosynthesis, particularly malvidin and p
-coumaroylated derivatives. In Merlot variety under water stress, an increase of anthocyanin content between 37% and 57% for two consecutive years was reported by Castellarin et al.
Imposing water deficits from the onset of ripening until maturity in the Merlot variety reduced the berry weight and increased the concentration of anthocyanins and skin tannins [126
], and the application of water deficits also modulated chemical composition changes during berry ripening [125
When Aragonez (Syn. Tempranillo) grapevines were subjected to three irrigation regimes (conventional sustained deficit irrigation (DI), regulated deficit irrigation (RDI) and non-irrigated (NI)), the main compounds affected by water availability were proanthocyanidins and flavonols which were increased with irrigation at pea size, veraison
, mid-ripening and full maturation phenological stages [128
]. Concentrations of anthocyanin at full maturation were observed to be higher in the skin of berries belonging to DI and RDI vines than in NI ones. In general, although no differences in sugar accumulation were observed between the water treatments, a decrease in the quality parameters in grape skins in NI vines was observed, may resulting from high temperature and excessive cluster sunlight exposition.