Agronomic Performance of Tomato Rootstocks Under Mediterranean Greenhouse Organic Farming †

Vegetable grafting is increasingly adopted to stabilize tomato production under Mediterranean conditions, where water scarcity and soil-borne pressures limit crop performance. A factorial rootstock × scion trial was conducted during an organic cold greenhouse cycle in Sicily (2022–2023). Three experimental rootstocks (two interspecific and one intraspecific, developed within the H2020 BRESOV framework) were compared with the commercial rootstock Optifort, along with self-grafted and non-grafted controls. Three commercial F1 scions (Barbarela, Cherry, Vittorio) were evaluated for vegetative growth, root traits, flowering dynamics, yield components, and fruit quality. Grafting generally enhanced plant vigor compared with self- and non-grafted plants, and significant rootstock × scion interactions were observed for several traits, indicating that performance depended on partner compatibility. Root biomass and yield varied widely among combinations, while fruit soluble solids ranged from 3.63 to 7.10 °Brix, with consistently higher values in Cherry and Vittorio scions. Multivariate analyses highlighted a predominant scion effect on fruit-related traits, whereas rootstocks mainly influenced vegetative growth and root system development. Tomato performance under Mediterranean organic greenhouse conditions strongly depends on rootstock–scion compatibility, confirming grafting as an effective strategy to improve yield stability and fruit quality in sustainable production systems.