Genetic Mapping of Traits Associated with Water Stress Tolerance from Wild Tomato (Solanum habrochaites) Under Restricted Irrigation in the Field.

Limited freshwater resources and global climate change pose significant challenges to agricultural production in California and worldwide. Cultivated tomato yields and fruit quality are negatively impacted by water stress imposed by insufficient water and exposure to temperature extremes. Tomatoes bred for tolerance to water stress and temperature stress would contribute to agricultural sustainability by decreasing crop water requirements and extending the growing season. Wild tomato, S. habrochaites, grows in the Peruvian Andes at 3300 m elevation and thrives in xeric habitats at chilling temperatures detrimental to S. lycopersicum. When exposed to rapid-onset water stress induced by root chilling (6°C),  S. habrochaites responds by closing stomata and maintaining shoot turgor, while cultivated tomato (S. lycopersicum) fails to close stomata and wilts. This trait is controlled by a major QTL (stm9) on chromosome 9, which we high-resolution mapped using 18 recombinant sub-NILs to a 0.32 cM region. To determine if this introgressed S. habrochaites chromosome 9 region controls other traits related to water stress-tolerance, we evaluated the same set of 18 sub-NILs in replicated field experiments in 2012 and 2013 under severely restricted drip irrigation (1/3 evapotranspiration rate (ETc of tomato), which causes slow-onset water stress, and normal drip irrigation (full ETc). Significant QTL were identified for specific leaf area, delta-13, shoot dry weight, total yield, and maturity traits within the 1.28 cM introgressed region represented in the 18 sub-NILs. QTL for these traits were closely linked (but not coincidental) to stm9. Evidence to date suggests that genetic elements within this region in S. habrochaites play important roles in plant responses to abiotic stress tolerance. Our research will aid tomato improvement efforts required to address the negative effects of limited fresh-water resources on tomato yields and quality.