INFLUENCE OF THE INTERACTION SCION X ROOTSTOCK ON GRAPEVINE RESPONSE TO DROUGHT STRESS AS A TOOL IN NEW GRAPEVINE ROOTSTOCKS BREEDING PROGRAMS.

Viticulture is expected to face several challenges related to current and future climate changes. Particularly, the vine-growing areas experiencing severe drought stress are expected to increase in the coming years. This could affect grape quality and yield, as well as the character and style of wines, and, more generally, the viticultural potential of many regions. Many studies suggest adaptive strategies to address changing environmental conditions, and the use of drought-tolerant rootstocks is considered an effective mid-term solution to mitigate and manage these negative impacts. However, the range of available rootstocks is limited, and most of them were selected in the late 19th century, when grapevine cultivation practices vastly differed from those of today. As a result, there is a growing demand for drought-tolerant rootstocks that are suitable for modern viticulture. In response to this request, some new genotypes have been released in recent years. Among those, the M-series by University of Milano has shown promising performances in different viticultural contexts. A deep insight into the behaviour of these new rootstocks under water stress conditions, especially in comparison to traditional ones, could provide valuable information, useful at different levels. On one hand, a better understanding of rootstocks performance under drought stress would support winegrowers in making informed decisions when planning new vineyards. On the other hand, it would provide breeders with crucial data on the grapevine responses to water stress, with particular regard to the complex interaction between scion and rootstock, allowing them to better direct their research efforts. To address these questions, three experiments were conducted. The first study evaluates the scion x rootstock interaction effect on drought tolerance, considering two grapevine varieties characterized by different physiological strategies against water stress. More in detail, the study aimed to: (a) analyse the behaviour of ungrafted rootstocks (1103P, M4) and own-rooted vines (Sangiovese, Montepulciano) under increasing water deficit conditions and after rewatering; (b) evaluate the behaviour of the four grafting combinations in the same experimental conditions; and (c) assess the response of the four grafting combinations to different drought dynamics. The second experiment delves into the specific water stress tolerance strategies employed by the two grafting combinations of Pinot Blanc with 1103P and M4, focusing particularly on changes in morphology of the scion, including leaf, petiole and xylem vessels induced by rootstocks. The third experiment examines the influence of rootstock on grapevine production and quality. When selecting a rootstock, it is important not only to assess its tolerance to water stress but also to consider its impact on grape quality, taking into account environmental conditions and oenological objectives. This study analysed six different grafting combinations of Chardonnay with 1103P, SO4, M1, M2, M3, M4, all cultivated in the same vineyard over a four-year period. Data on production per plant, vine vigour, and must quality (sugar content, pH, total acidity, K⁺ concentration) were collected. Furthermore, micro-vinifications of the six combinations were produced in two years and wines sensorial characteristics were evaluated. The results generally highlight the crucial role of the interplay between rootstock, scion, and water stress intensity in shaping drought response mechanisms. Specifically, differences in leaf gas exchange are primarily driven by the variety under moderate water stress and by the rootstock under severe water stress and after rewatering. In particular, M4 outperforms 1103P, demonstrating a superior response under severe water stress conditions and a faster recovery following rehydration. This pattern is consistently observed across all three grafted combinations: Pinot Blanc, Sangiovese, and Montepulciano. The differing responses of the two rootstocks appear to be influenced by both chemical and morphological factors. In particular, variations in osmolyte content and mobilization patterns were observed. On the other hand, an interaction between rootstock genotype and water stress level was identified for Pinot Blanc. Under water stress, 1103P induced a higher density of vessels with smaller diameters, whereas M4 did not cause any notable changes in xylem anatomy. Additionally, a loss of hydraulic conductivity was observed in the stems and petioles of vines grafted onto 1103P, indicating a greater vulnerability to cavitation compared to those grafted onto M4. Finally, the results of productive and qualitative parameters collected in open field on six grafting combinations of Chardonnay, highlight a statistically significant influence of the rootstock on most of the analysed parameters. Differences were found also in the sensory characteristics of the wines, highlighting the critical role of rootstock selection in achieving the desired oenological goal. In conclusion, this work analysed various aspects related to the role of the rootstock and its close interaction with the scion in the grapevine's response to water stress. In particular, the rootstock can induce different morphological and physiological responses in the plant under water deficit conditions, which can determine a varying degree of tolerance to water stress of the different grafting combination. In this context, it is important to highlight how this knowledge could be important for farmers in making optimal rootstock x scion choices for their vineyards and for breeders for the selection of new rootstocks tailored to specific cultivars or environmental conditions