## 1 Introduction  
Soil water deficit is the main environmental constraint for viticulture in the Mediterranean basin [1; 2]. Climate projections for this area predict changes in rainfall and seasonal temperature patterns in the next decades, with a higher frequency of extreme climate events [3; 4]. These future trends due to global climate change are expected to negatively affect grapevine physiology and grape quality. Stomatal closure is one of the first responses to drought. It is widely reported that the reduction in carbon fixation due to stomatal limitations is the crucial issue for grapevine performances under mild drought, as indicated by increased water use efficiency (WUE, i.e., the ratio of photosynthesis to transpiration) [5; 6; 7; 8; 9; 10]. For more prolonged and/or more pronounced water deficits leading to decrease of stomatal conductance (\(g_{\mathrm{s}}\)) below 50 mmol H\({}_{2}\)O m\({}^{-2}\) s\({}^{-1}\), photosynthetic limitations becomes more dependent on nonstomatal processes, especially decreased mesophyll conductance to CO\({}_{2}\) and impaired photochemistry [11; 12]. Moreover, abiotic stresses, such as drought and high temperatures, influence the primary and secondary metabolism thus accelerating phenological stages (i.e., budburst, flowering, and veraison) and berry ripening [13]. In particular, the synthesis of phenolic compounds and aromatic ripeness in berry skin are delayed, while total sugars increase, thus resulting in wine with low acidity and high alcohol concentration [14; 15; 16]. In grape berries, the most abundant polyphenol subclasses are flavonols and anthocyanins [17]. These phenols are especially responsible for stability to oxidation, color, and flavor of wines [18; 19; 20]. The total amount of anthocyanins and the relative abundance of single anthocyanins are under genetic control [21]. This means that all cultivars express functional genes coding for enzymes that lead to the biosynthesis of di-hydroxylated (peonidin and cyanidin) or tri-hydroxylated (delphinidin, petunidin, and malvidin) anthocyanins as well as to the methylation of primary anthocyanins. Harsh environmental conditions, such as water shortage, high radiations, and hot temperatures, interacting with genotype, boost the shift of cyanidin and delphinidin (hydroxylated anthocyanins) into their methoxylated counterparts, namely peonidin, petunidin, and malvidin [22; 23]. Hence, the cultivation of grapevines in a climate change scenario might require a selection of sustainable management practices to maintain grape quality under the future environmental constraints, like the foliar application of biostimulants [24; 25; 26]. Many studies have reported the ability of _Ascophlylumn nodosum_-based biostimulants to promote plant growth, yield, and fruit quality [27; 28; 29] and to enhance the tolerance of grasses and crops to drought events [30; 31; 32; 33; 34; 35]. This broad effectiveness may be ascribed to the high concentration of amino acids found in the _Ascophlylumn_-based extracts, which may act as compatible solutes under osmotic stress [36; 37], as well as to the presence of bioactive secondary metabolites such as vitamins and their precursors [38; 39], that may protect plant tissues against stress-induced reactive oxygen species (ROS) [40]. If compared to the published research on the effects of _A. nodosum_ applications on the yield of table grapes [41; 42; 43; 44], only a few studies have focused on grapevines responses to abiotic stresses [45; 46] and on berry composition [47; 48; 49; 50; 46; 46; 47; 49; 50] following seaweed extract treatments. In a previous two-year field experiment, [PERSON] et al. [46] demonstrated an improvement of gas exchanges and grape maturity balance, induced through the application of _A. nodosum_ treatments on the phenylpropanol pathway. Although these results supported the potential effect of _A.