GRAPEVINE ROOTSTOCK CHARACTERIZATION FOR DROUGHT TOLERANCE

Grapevine is worldwide grafted on rootstocks to create a biological barrier to the phylloxera (Daktulosphaira vitifoliae). Despite the key role of rootstock in the adaptation to environmental conditions, a limited number of genotypes is available for winegrowers, showing a narrow genetic background. The gap between the importance of rootstocks in abiotic stress tolerance and their low genetic variability leads to consider rootstock breeding as a promising strategy to face climate change. In the last decades, new breeding programs were developed with the aim to provide new rootstocks able to cope with drought and other abiotic stresses. Nowadays, the continuous progress in genetic techniques can assist and accelerate the selection process of new tolerant genotypes. In the present PhD project, several genotypes at different stages in rootstock selection process were analyzed for drought tolerance. The first part of the thesis focused on 3 genotypes belonging to the recent M-series, the second part was about a new selection of 30 genotypes, coming from different breeding programs, and in the last part a breeding population of 141 genotypes was used for a genome wide association study (GWAS). The new M-rootstocks (M1, M3 and M4), recently placed on the market, were compared to traditional rootstocks, in order to better understand their behavior under drought. In a pot experiment under controlled conditions, M1, M3 and M4 were compared to nine rootstocks with different genetic background at decreasing levels of water availability. M-rootstock performance under water deficit was similar to the tolerant rootstocks 1103P and 110R, in both phenotypic and genetic responses to water stress. These rootstocks adopted a strategy of tolerance to face water stress, increasing the water use efficiency (WUE) under deficit conditions. To deeply investigate the behavior of tolerant rootstocks under drought, a second experiment in semi-controlled conditions was set up, comparing M4 to 1103P under progressive water deficit, in grafting combination with V. vinifera cv Pinot Blanc. Similar performances were reported by the two grafting combinations under mild to moderate water deficit, but a different response occurred under sever conditions: 1103P reduced stomatal conductance, transpiration, and carbon assimilation more than M4, which was able to preserve water use efficiency and operating efficiency of photosystem II. In the second part of the thesis 30 new selected genotypes were compared to rootstock M2 for water stress tolerance and nutritional status, in order to characterize the rootstock material before the marketing process and to identify new pre-breeding material. The experiment was carried out in un-grafted conditions for two years and in two experimental fields, characterized by different water availability. Several parameters were analyzed, such as transpiration, WUE, vigor, macronutrients and micronutrients in the leaves. Genotypes ranked for both abiotic stresses and the differences between the two sites allowed to estimate their plasticity for each trait. Finally, a GWA approach was applied on a breeding population, counting 141 genotypes, in order to identify the genomic regions involved in drought tolerance. The population was genotyped with a 18k SNP array, after the validation on non-vinifera germplasm, belonging to a rootstock core-collection of 70 genotypes. Three phenotyping cycles under increasing water deficit were performed on the breeding population under greenhouse-controlled conditions. Vigor, shoot growth rate, transpiration, stomatal conductance and leaf turgor were estimated for each genotype at different water deficit levels. A group of tolerant genotypes with high performance under water deficit condition was identified and used in GWAS approach to detect the loci associated to drought tolerance of rootstocks. In conclusion, this work enhanced the knowledge about rootstock response to water deficit, characterized the water tolerance of a large panel of rootstocks and identified potential target genes for future breeding programs.