Breeding for Cold Tolerance in St. Augustinegrass using Conventional and Molecular Breeding Methods

St. Augustinegrass (Stenotaphrum secundatum [Walt.] Kuntz) is a warm-season grass species commonly utilized in the turfgrass industry for its superior shade tolerance, stoloniferous growth habit, and moderately low input requirements. However, winter survival is a major limiting factor for the species. Additionally, the most cold-tolerant cultivars often lack the aesthetic characteristics, such as a semi-dwarf growth habit and fine leaf texture that are desirable in the market. Therefore, new St. Augustinegrass cultivars with improved cold tolerance and desirable turf quality are needed for the turfgrass industry, especially in the transitional climatic region of the United States.;In order to efficiently utilize sources of cold tolerance in a breeding program, an understanding of the genetic control of this trait and its relationship to important turf quality traits is required. Six diploid genotypes of St. Augustinegrass were selected as parents for a diallel mating design without reciprocals. Combining ability analysis revealed that both general and specific combining abilities were significant across years and locations. Specific combining ability was the largest source of genetic variation for winterkill, genetic color, turf density, and end-of-season cover indicating that non-additive gene effects play a key role in the inheritance of these traits. Lines identified as parental selfs generally showed lower winter survival and inferior turf quality than the original parental lines indicating that inbreeding depression can occur in St. Augustinegrass.;In addition to field studies, lab-based freeze tests mimicking field winter survivability can contribute to the selection of cold hardy lines. A whole plant freeze method was used to evaluate four freezing temperatures and two data collection systems in freeze tests of nine St. Augustinegrass genotypes. Results indicated -3°C and -4°C to be more suitable evaluation temperatures than -5°C and -6°C. Survival and regrowth were correlated with one another over a six week evaluation period post-freeze. Digital imaging techniques utilized in turfgrass field studies were shown to be useful in estimating survival and recovery in lab-based freeze tests. Additionally, the effects of cold acclimation and deacclimation on genotypes were evaluated. Accounting for all levels of acclimation provided excellent cultivar separation at -3 and -4°C freezing temperatures and supports the hypothesis that the inclusion of different acclimation responses offers the best overall assessment of freeze tolerance in St. Augustinegrass. Results also indicated that cold acclimation and deacclimation both play crucial roles in the winter survivability of St. Augustinegrass.;The first complete linkage map for St. Augustinegrass was constructed for cultivars 'Raleigh' and 'Seville' using a pseudo-F2 strategy. A total of 178 simple sequence repeat markers were mapped to nine linkage groups covering a total distance of 1299.95 cM. Quantitative trait loci (QTL) controlling winter survival, freeze tolerance, and turf quality traits were mapped. Putative QTL were identified for all traits across multiple environments with the exception of winter survival, where QTL were only identified in single environments. Specific genetic regions were found to have overlapping QTL between winterkill and spring green-up, as well as overlapping QTL between cold tolerance traits collected in the field, and survival and recovery traits collected in the laboratory post-freeze. These results provide strong support for cold tolerance-related QTL in these regions. A large region on LG5 was also identified as a possible region for a large sweep of freeze tolerance and cold acclimation QTL.