In April 2010, the research program 'Green Breeding' started in the Netherlands, which is subsidized by the Dutch Ministry of Agriculture, Nature and Food Quality (LNV) and several private breeding companies. With this program, the Dutch government stimulates breeding research, which contributes to a more sustainable agriculture and horticulture. As part of this research program, this tomato rootstock project was granted for the period April 2010 to April 2013. It is aimed at identifying the key physiological traits that are characteristic of a vigorous rootstock, thus providing breeding companies with an effective selection method to breed robust rootstocks for both the conventional and organic production of tomatoes. Breeding for vigorous rootstocks is still a matter of trial and error. Practical selection tools like genetic markers are lacking because the knowledge about the physiology behind a successful rootstock (root-shoot interaction) is limited. In addition, the desired root traits studied in this project, nutrient-uptake efficiency, salinity and suboptimal-temperature tolerance, are complex and the identified Quantitative trait loci (QTLs) have hardly any value for practical breeding purposes. This project will therefore not focus on the identification of single allele-specific markers for traits that are expressed in a particular hybrid combination. Instead, it aims to identify biomarkers (particular physiological traits) as generic tools to develop a reliable screening method which supports the selection of vigorous rootstocks. If successful, the knowledge gained from this project will enable the selection and breeding of rootstock cultivars which improve the sustainability and profitability of both conventional and organic cultivation of tomatoes.