A Simple in Vitro Seed Germination Assay to Classify Switchgrass Genotypes for Thermotolerance.

Cardinal temperatures for plant processes have been used for thermotolerance screening of genotypes, geoclimatic adaptability determination and phenological prediction in crops.  Current simulation models for switchgrass utilize blanket cardinal temperatures across genotypes for both vegetative and reproductive processes although intraspecific variation exists among genotypes.  An experiment was conducted to estimate the cardinal temperatures of 14 diverse switchgrass genotypes and to classify genotypes for temperature tolerance.  Stratified seeds of each genotype were germinated at seven constant temperatures from 15 to 45 °C and under a constant light intensity of 35 µmol m^-2 s^-1 for 12 h d^-1.  Germination was recorded at 6-h interval in all treatments.  Maximum seed germination (MSG) and germination rate (GR), estimated by fitting sigmoid function to germination-time series data, varied among genotypes. Quadratic and bilinear models best described the MSG and GR responses to temperature, respectively. The mean cardinal temperatures, T_min, T_opt and T_max, were 8.1, 26.6 and 45.1°C for MSG and 11.1, 33.1 and 46.0°C for GR, respectively. Cardinal temperatures for MSG and GR; however, varied significantly among genotypes. Genotypes were classified as sensitive (Cave-in-Rock, Dacotah, Expresso, Forestburg, Kanlow, Sunburst, Trailblazer and Warrior), intermediate (Alamo, Blackwell, Carthage, Shawnee and Shelter) and tolerant (Summer) to high temperature based on cumulative temperature response index (CTRI) estimated by summing the individual response indices estimated from the MSG and GR cardinal temperatures. Similarly, genotypes were also classified as sensitive (Alamo, Blackwell, Carthage, Dacotah, Shawnee, Shelter and Summer), moderately sensitive (Cave-in-Rock, Forestburg, Kanlow, Sunburst and Warrior), moderately tolerant (Trailblazer) and tolerant (Expresso) to low temperatures.  The cardinal temperature estimates would be useful to improve switchgrass models for field applications.  Additionally, the identified cold- and heat-tolerant genotypes can be selected for niche environments and in switchgrass breeding programs to develop new genotypes for cold and hot environments.