Effects of Ascophyllum nodosum extract on Vitis vinifera: Consequences on plant physiology, berry secondary metabolism and drought stress tolerance

Throughout the Mediterranean region, V. vinifera are subjected to a wide range of environmental stresses, especially during the summer, when prolonged dry periods are combined with high radiation load and high temperatures. Furthermore, given the strong influence of the atmospheric factors on this species, seasonal weather pattern can significantly affect grapevine physiology and wine quality under future climate change projections. Although grapevines have multiple acclimation strategies to environmental constrains, the evidence for significant climate change in the upcoming decades urges adaptation and mitigation measures to be taken by the whole viticulture and winemaking sectors. Short‐term adaptation measures can be considered as a first protection strategy and should be focused at specific threats, mostly changes in agronomical practices, such as the application of biostimulants to enhance nutrition efficiency, abiotic stress tolerance and quality traits of plants. Seaweed-based extracts have been recently employed as sustainable tools to improve abiotic stress tolerance and increase grape quality. However, the effect of these extracts on secondary metabolism compounds, that are fundamental for grape and wine quality, is still scarce. Under this scenario, the objectives of the present PhD project were: 1) to investigate the physiological performances of V.vinifera and the biochemical adjustments induced in grape berry skins by A. nodosum foliar treatments under field conditions; 2) to evaluate if these treatments affect grapevine water relations and berry phenylpropanoid metabolism, mitigating the effects of a post-véraison drought in potted vines. An enhancement in the biosynthesis of secondary metabolites in berry skins and in leaves in response to these treatments, effective to improve grape quality and help vines to cope with abiotic stresses, was hypothesized. In order to achieve these goals, two experiments were arranged. The comparison between vines treated with A. nodosum extract and non-treated ones was set up in a field experiment carried out in 2016 and 2017 in a commercial vineyard in the Chianti Classico area (Tuscany, Italy), on 18-year-old vines of the red cv. Sangiovese (V. vinifera). Furthermore, in order to deeply investigate if A. nodosum treatments are effective in help vines to cope with drought stress, a second study was performed in 2017 on potted vines (cv. Pinot noir), subjected to two irrigation regimes (well watered and water stressed) associated with A. nodosum foliar treatments, performed with the same protocol as the field experiment. In both experiments, gas exchanges, chlorophyll fluorescence and water potentials on leaves treated with A. nodosum extract and non-treated control leaves, were monitored at three phenological stages. In addition, at the same stages, anthocyanins, flavonols and hydroxycinnamic acids were quantified in berry skins. In general, this PhD thesis provides evidence of A. nodosum treatments-induced changes in eco-physiological traits and berry skin metabolism of V. vinifera, resulting in different physiological adjustments to counteract environmental stress. Furthermore, in both experiments, the biochemical berry skin analyses revealed that the A. nodosum extract likely acted selectively in the phenylpropanoid pathway. In particular A. nodosum extract may influence anthocyanin biosynthesis by affecting the activity of enzymes involved in the phenylpropanoid pathway, thus resulting in changes in the content of methoxylated compounds in treated vines. Moreover, the increases in the phenolic and flavonoid profiles have a significant effect on enhancing multiple stress tolerance as well as improving plant water status and photosynthetic performances. Overall, these results support previous findings on the beneficial effects of A. nodosum treatments on plant acclimation to stressful environmental condition. For the first time, this study shows the potential effect of seaweed extracts in promoting higher tolerance to stress in vines. Considering the challenges posed by climate change in the Mediterranean basin, the use of seaweed extracts might represent a sustainable tool to mitigate the negative effects caused by increasing severity of drought events, often associated to heat-waves, in the viticulture sector.