ABSTRACT

Plant response to environmental stress is complex; it is controlled by more than one gene and highly affected by other environmental variables, typical of quantitative traits. In addition, stress response is a developmentally regulated, stage specific phenomenon. Tolerance at one stage of plant development is often not correlated with tolerance at other developmental stages. Specific ontogenic stages, including seed germination and emergence, seedling survival and growth, and vegetative growth and reproduction, should be evaluated separately for assessment of tolerance and identification, characterization and genetic manipulation of tolerance components. We have investigated genetic controls of salt tolerance (ST) and cold tolerance (CT) during seed germination and vegetative growth in tomato, using conventional protocols of plant genetics and breeding and contemporary techniques of molecular markers and quantitative trait loci (QTLs) analysis. The overall results have indicated that, unlike the traditional viewpoint that quantitative traits are controlled by many small-effect genes (QTLs) whose individual effects can not be determined, ST or CT at each plant stage is generally controlled by a few QTLs with major effects and several QTLs with smaller effects. The major QTLs can be further characterized genetically for map-based cloning of stress tolerance genes and for use in marker-assisted breeding for stress tolerance. The results have also indicated that different QTLs affect stress tolerance at different developmental stages and that selection for tolerance at one stage does not affect tolerance at other stages. However, the identification of QTLs for tolerance at different stages should facilitate simultaneous or sequential introgression of QTLs to develop genotypes with enhanced tolerance at more than one stage. For the seed germination stage, two types of QTLs were identified: stress-nonspecific QTLs, which affected germination rate under both stress and nonstress conditions, and stress-specific QTLs, which affected germination only under salt-stress or cold-stress but not nonstress conditions. In comparison, no genetic relationship was observed between plant ST and CT during vegetative growth. Despite the apparent complexities of genetic controls of ST and CT in tomato, the current knowledge indicates a good prospect for improving these traits in commercial tomato cultivars via marker-assisted breeding.