PHENOTYPING OF GRAPEVINE ROOT SYSTEM FOR DROUGHT ADAPTATION

Drought events increasingly affect many viticultural regions worldwide, negatively influencing grape yield and quality and threatening the sustainability of the wine sector. Among the available adaptation strategies, the adoption of drought tolerant rootstocks has emerged as one of the most promising and sustainable approaches. Since the outbreak of Phylloxera in the 19th century grapevine has been predominantly propagated through grafting and rootstocks have become essential tools to regulate plant vigour, nutrient uptake and tolerance to abiotic stresses, including drought. However, currently available rootstocks derive from a relatively narrow genetic base highlighting the need to select new genotypes with enhanced adaptive potential under changing environmental conditions. Rootstock shapes root system architecture, thereby defining the plant’s capacity to explore the soil and to access and absorb water under limiting conditions. Despite their central role, knowledge of grapevine root systems and their regulation under drought remains limited, mainly due to the intrinsic difficulty of root observation. Recent advances in root phenotyping, improving the feasibility of root trait characterization, open new perspectives for the study of root system involvement in grapevine drought adaptation. In this context, the present PhD project aimed to study grapevine root system at different scales adopting distinct phenotyping approaches to characterize its role in grapevine adaptation to water limiting conditions. The first part of the project assessed how soil properties, scion genotype and rootstock influence grapevine root system architecture in vineyards within an environment prone to water scarcity. Below-ground organs of six rootstock varieties grafted with Grillo and Nero d’Avola were observed in loamy sand and clay soil through the profile wall method. Four M rootstocks, recently released by the University of Milan were compared with traditionally adopted 1103 Paulsen and 140 Ruggeri. Soil type, scion and rootstock all significantly affected root system development. M2 and M4 exhibited the overall best performance showing the highest root densities, whereas 140 Ruggeri was characterised by lower root development. The second part of the project evaluated root system regulation adopted by Vitis plants under limited water availability. A first experiment focused on whole root system architecture and included a core collection of 70 genotypes grown in rhizotrons under controlled conditions. Plants were phenotyped under progressive soil drying adopting a smartphone-based approach. Plant water consumption, and growth rates at both aerial and root levels were monitored. Four groups displaying contrasting physiological behaviours and root system architectures were identified. One group of 23 genotypes exhibited the most promising behaviour for drought prone environments combining reduced water consumption, tight stomatal control, sustained growth under water deficit, and a well-developed root system. The number of lateral roots emerged as a key trait in root system plasticity under water scarcity. A second experiment focused on root anatomy and involved two groups of genotypes identified as contrasting in their response to drought at root level. Plants grown in pots under controlled conditions were subjected to different levels of water deficit. Plants water status, above-ground growth and gas exchanges were recorded and root sections were sampled for anatomical traits observation. The two groups exhibited distinct responses to drought at both physiological and anatomical level. One group displayed a tolerance-oriented strategy maintaining root hydraulic conductivity and intrinsic water use efficiency even under severe water deficit whereas the second group displayed an avoidance-based strategy. In conclusion the integration of different phenotyping approaches and plant developmental stages enabled a multi-scale interpretation of grapevine drought adaptation mechanisms at root level. The results highlight the central role of root system architecture, plasticity, and hydraulic properties in grapevine responses to water deficit and support the selection and breeding of more drought-tolerant rootstocks.