**3. Key climatic variables affecting grapes polyphenolic compounds**

Secondary metabolites such polyphenols play significant ecological functions within the defense and signaling mechanisms in plants [40]. Different climatic variables such as air temperature, radiation, rainfall, relative humidity, wind, altitude, and topographic features play vital role in the polyphenol biosynthesis pathway in grapes. In this section, we shall review research studies focusing on key environmental variables.

#### **3.1 Temperature**

Temperature plays a significant role in vine phenology whereas increase in mean temperature prolonged the vegetative and reproductive cycle of grapevine and hence berry developmental and maturity stages are shifted in warmer months of the growing plant reproductive cycles [41]. Available historical records of harvest timings from different grape growing regions indicate an advance of 1–2 weeks during last few decades [42–47]. Although, some management practices may be the one reason for the advancement of ripening [48]. The conjugated effects of progressive phenology along with rise in temperatures during berry ripening with higher sugar contents, lesser organic acids concentration and altered berry composition of metabolites, such as phenolic compounds [41]. Research studies have encompassed the effect of wide ranging of temperature intensities; from moderate to high heat stress i.e., up to 35–45°C during day or night period at key berry development stages. The genotype, plant material and experimental constraints may affect the response of berry metabolism to temperature variations [49]. Although, difficult to fully relate with field conditions, controlled climate chamber experiments are also conducted to understand the influence of environmental variations [50–52].

#### *3.1.1 Temperature impact on phenolic compounds*

Effects of temperature on polyphenols are not always consistent as recently highlighted [49, 53]. However, there are unequivocal scientific evidence which indicate the deleterious effects of elevated temperature on the biosynthesis of anthocyanins in the grape berry. Studies of the impact of elevated temperature were validated at physiological and molecular studies [54–63]. It was noticed that heat stress repressed chief anthocyanin biosynthesis regulators, such as VviMYBA1 and downstream regulating genes such as VviCHI, VviUFGT, VviDFR, VviF3H2, and VviLDOX. However, not all of these research studies indicated unambiguous suppression or a strong correlation with lower anthocyanin accumulation. Various aspects of viticulture e.g., vines, cultivars, berry development stages, treatment intensities and sampling strategy take part in accumulation and production of anthocyanin. The effect of temperature on anthocyanin biosynthesis varies highly between different genotypes. For instance, when maximum temperature exceeds 35°C during berry ripening, it inhibits the color formation prominently e.g., in cv. Grenache than in cv. Carignan [64].

Previously, it has also been established that timings of temperature variations during day-night period have a strong influence on berry metabolites and lower temperature near berry ripening time particularly at night was related with improved coloration of grapes [55]. It was recently confirmed through experiments at molecular level that lower night temperatures increased anthocyanin accumulation and expression of related genes e.g., VviF3H1, VviUFGT, VviCHS3, and VviMYBA1 [65]. More pronouncing effects of lower night temperature were noticed near veraison stage in Corvina grapes. In a related study on Kyoho grapes, 3°C rise in temperature (27 to 30°C) during berry ripening caused less berry coloration and induced a significant decrease in transcript levels of anthocyanin regulating genes [66]. Similarly, in cv. Merlot, increase in day's temperature from 20 to 25°C during ripening caused decrease in anthocyanin levels by 37% [63]. In addition to repressing of anthocyanin regulating genes, high temperature may stimulate anthocyanin degradation due to the augmented activity of peroxidases [33]. It has been established that a peroxidase coding gene; VviPrx3 is up regulated, in berries when exposed to high temperature [67], and similar effects have been noticed in other plant species, such as Brunfelsia, litchi and strawberry [68–70]. A related increase in quantity of acylated and tri-hydroxylated anthocyanins has been observed in cvs. Merlot, Cabernet Sauvignon, Sangiovese and Malbec under higher temperature conditions [52, 61, 63, 71, 72], alongside overexpression of the acyltransferase gene Vvi3AT

#### *Effect of Climate Change on Polyphenols Accumulation in Grapevine DOI: http://dx.doi.org/10.5772/intechopen.99779*

activity. Similarly for anthocyanins, elevated temperature impeded flavanol buildup while significantly augmented methoxylated (isorhamnetin & syringetin) and 3′, 4′, 5′-substituted (myricetin & syringetin) flavonols in cv. Merlot. More interestingly, rise in temperature may cause a disconnection of sugar-anthocyanin accumulation and biosynthesis, hence leading to a lower anthocyanin-sugar ratio which might be due to delayed anthocyanin biosynthesis or lesser anthocyanin accumulation during ripening phases. The extent of this thermal decoupling is highly cultivar dependent as indicated for cv. Grenache and cv. Carignan and can vary even among the clones of same cultivar as discussed for cv. Tempranillo [63, 64, 72–75].

The effect of temperature on tannins biosynthesis is yet not fully understood. However, it may be pointed that elevated temperature can enhance the production of tannin monomers, flavan-3- ols as highlighted by [76]. However, some other studies report non-significant effects on tannin production as tannins were not much affected by heat stress in cv. Sangiovese at veraison stage. More recently, scientists came up with similar results indicating no effect on flavan-3-ol or tannin levels. Although, significantly higher galloylation of flavan-3-ols levels were noticed in consistent with earlier findings. It was further indicated that an overexpression of UDP glucose-gallic acid-glucosyltransferase genes under elevated temperature. Moreover, heat stress also reserved the expression levels of members of STS biosynthetic pathway. However, lower temperature upregulated STS transcripts hence accelerated stilbene biosynthesis [49, 61, 76–79].

#### **3.2 Radiation**

Berry exposure to sunlight is generally associated with better berry quality attributes due to more total soluble solids (TSS), anthocyanins, and phenolics. On the other hand, it also lowers acidity and pH along with lower disease incidence due to favorable improved microclimate [49, 80–82].

#### *3.2.1 Effect of radiations on phenolic compounds*

Increased levels of phenolic compounds have been noticed in cvs. Pinot Noir, Riesling, Summer Black and Cabernet Sauvignon owing to better exposure to sunlight [83–86]. It also augmented the expression level of regulatory and structural phenylpropanoid genes as highlighted by recent studies [87–90]. Flavonoids particularly flavonol glucosides are the most light-responsive phenolic compounds ones whose levels increased with better exposure to sunlight. This positive effect was in consistent with their UV radiation-screening activity and their capability to reduce oxidative damage. Flavonoids were produced upon exposure to UV-B radiation as adaptive traits to reduce the radiation damage, as there exists a strong correlation between physiology and quercetin-3-O-glucoside & kaempferol-3-O-glucoside levels in UV-B radiation stressed vines [91–97]. Recently, a more comprehensive study elucidated that shoot removal and leaf thinning in cvs. Cabernet Sauvignon & Petit Verdot improved light exposure, hence it significantly augmented the flavonols kaempferol, quercetin and myricetin levels. However, little or no change was noticed for other flavonoid compounds. Similarly, higher levels of hydroxycinnamic acids and flavonol were noticed due to increased sun light exposure in cv. Cabernet Sauvignon [90, 98].

Several transcriptomic studies indicated that flavonol genes such as VviGT5, VviGT6 and VviFLS1 were induced more than other phenylpropanoids genes when exposed to UV radiation as observed in cv. Tempranillo berry skin. In return, lower expression level of VviFLS4 gene and its transcriptional regulator i.e., MYB12 was

noticed under shade [92]. However, it has not yet been established that to what level UV light contributed to stimulate the synthesis of phenolic compounds. It can be deduced from literature that UV-B radiations are responsible for overexpression of key flavonoid genes [40, 99–104]. Recently, VviHY5 and VviHYH; the two bZIP TFs elongated hypocotyl 5 protein (HY5) orthologs were identified as the key components of UV-B reaction pathway along with mediated flavonol accumulation owing to high radiation exposure in grapevines [100, 105].

Anthocyanin accumulation increased significantly when grapes clusters were exposed to increased light, whereas shading decreased them. Recently, it was indicated that the UV-B radiation might prompted up-regulation of miR3627/4376 which facilitated anthocyanin accumulation [106, 107]. In a related *in vitro* study in which effect of berry exposure to light and temperature was studied it was inferred that elevated light increased anthocyanin levels in grapes [59]. The augmented anthocyanin levels found associated with the up regulation of correlated genes of anthocyanin biosynthesis pathway. Some other studies also endorse the stimulation of key anthocyanin genes e.g., TF VviMYBAa and VviUFGT under higher sun light exposure [66, 99]. Interestingly, UV-B radiation prompted the expression of VviMYBA1 gene while delaying the down regulation of VviMYBA6 and VviMYBA7 genes at later berry developmental stages [105]. Less light exposure modulated the quantity of di- to trihydroxylated anthocyanins more toward tri-hydroxylated anthocyanins as demonstrated through the down regulation of VviF3′ 5 ′Hs, somewhat similar but inconstant trends have been reported in cvs. Cabernet Sauvignon and Petite Verdot under warm climatic conditions [59, 90, 98, 99, 106]. However, low light conditions may increase non-acylated anthocyanins concentration as highlighted by [92, 107]. There is still need for further research to develop a better understanding.

#### **3.3 Water**

Water is an important constituent of plant structure and performs variety of functions in addition to transport of mineral nutrients from soil. It is a key component of photosynthetic pathway in plants. Moreover, water balance is necessary for quality table and wine grape production. Similarly, primary and secondary metabolite production is regulated by balanced water availability.

#### *3.3.1 Impact of vine water status on phenolic compounds*

Different primary and secondary metabolites are significantly influenced by drought stress in grapevines. Recent research has focused on probing the effects of water on berry physiology and quality attributes [40, 108, 109] and it has been noticed that drought stress may increase primary metabolites and polyphenols up to 85% and 60% respectively under different stress treatments. The impact of water deficit varies with intensity and duration of the stress conditions as well as berry developmental stage. Water deficit during the initial growth phases has more negative impact on final volume and yield at harvest as it reduces cell expansion, however rate of cell expansion is not affected much [110] while ripening phase, and it has little impact on berry size. Primary metabolites such as citric acid and glyceric acid synthesis was affected by both short and prolonged stress whereas polyphenols biosynthesis was accelerated only by the prolonged drought stress treatment.

Selective water deficit applications increased anthocyanin accumulation in grape skin along with the activation of genes of corresponding anthocyanin biosynthesis

pathway [111]. For instance, in grape cv. Chardonnay, water stress increased the content of flavonols and decreases the expression of genes involved in biosynthesis of stilbene precursors [40].

It has been observed that modest water deficit i.e., predawn leaf water potential of 0.3 to −0.5 MPa is useful for better wine quality especially for red cultivars [112, 113] These positive effects may partially be attributed to increased solute concentration owing reduced berry volume under water deficit conditions. However, a higher buildup of secondary metabolites independent of change in berry volume has been reported [114]. More elaborative research findings at molecular level highlighted an upregulation of key enzymes of the phenylpropanoid and flavonoid pathways in response to water stress [40, 115–120]. But these beneficial effects were more noticeable when water deficit occurred throughout berry ripening phase [49].

In addition to an increase in the accumulation of phenylpropanoids and flavonoids due to water stress, an altered composition of anthocyanins has also been noticed owing to increased levels of tri-hydroxylated anthocyanins i.e., petunidin, delphinidin and malvidin [111, 121–124]. However, these observed changes in the anthocyanin profile of grapes due to water stress appear to be highly varietal dependent [125, 126] due to varying genotypic response associated to environmental variables. Similarly, an increase in proanthocyanidin concentration and proanthocyanidin polymerization along with higher catechin levels in grape berry skins have also been indicated by [127–129]. The increase in phenolic levels when water deficit occurred before veraison may be due to concentration effects [130, 131] however, several other scientists discussed increase in anthocyanin content at berry level [111, 114, 123, 132]. More focused research is needed to validate ribose, glyceric acid, citric acid, kaempferol-3-O-glucoside and quercetin-3-O-glucoside interactions as indicators of drought stress [133].
