Using lycopersicon hirsutum as a source of cold tolerance in processing tomato breeding

Chilling stress limits tomato production, particularly in early spring when field conditions might not be optimal for vegetative growth. A chilling-tolerant accession of Lycopersicon hirsutum (LA1777) was used to generate a backcross recombinant inbred line population, using L. esculentum cv. E6203 as recurrent parent. Cold tolerance was evaluated under controlled greenhouse conditions, measuring plant growth and development at 9/4°C (day/night temperature) for 20 days in plants with 5-7 leaves. Quantitative trait loci (QTL) analysis of leaf generation, photobleaching, leaf purpling, biomass accumulation, and flower development showed that chilling-tolerant alleles are present in both parents. Tolerant QTL alleles conferring higher leaf development under chilling stress were usually not coincident with tolerant QTL alleles inducing more advanced flower development. Photobleaching and purple leaves were not associated with lower growth rates or lower biomass accumulation. Dry mass and leaf generation rate were mapped to the same regions of the genome only in chromosomes 2 and 3. QTL effects were studied in near isogenic lines, and 20 of 21 QTLs were confirmed on chromosomes 2, 3, 4, 6, 7, 9, and 10. Particularly promising for breeding purposes are QTLs on chromosomes 2, 3, and 9, which can induce 25-41% higher dry mass than isogenic control during the cold treatment. We are currently studying the physiological responses to low temperature in NILs containing introgressions that include these QTLs that induce cold tolerance, and mapping QTLs for low-temperature tolerance during fruit set and fruit growth under field conditions.