Physiological and Genetic Characterization of Sorghum Association Panel for Chilling Tolerance during Germination and Seedling Vigor.

Early planting of sorghum is a promising strategy to avoid negative impact of terminal heat and drought stress coinciding with flowering and grain-filling stages. The major obstacle to this effort has been low soil temperature (<15^oC) with early planting, which negatively impacts seed germination, seedling emergence and vigor in sorghum. To quantify the genetic diversity for early seedling vigor and chilling stress tolerance, Sorghum Association Panel (SAP) consisting of 361 accessions were screened by challenging them to cold (20/10^oC; day and night), and optimum (30/20^oC) temperatures during germination (Experiment 1) and seedling emergence (Experiment 2), using growth chambers. In Experiment 1, two replicates of five seeds for each SAP accession were arranged on a standard germination paper in Petri plates and exposed to the same chilling conditions mentioned above to assess the germination rate. Germination experiment confirmed the viability of all accessions both under control and chilling stress conditions. In Experiment 2, three seeds sown per pot were monitored on a daily basis to document the rate of emergence, and 7 d after first sign of emergence, the plants were harvested to record seedling vigor related traits - leaf number, seedling height, root length, root biomass and total biomass. Under chilling stress, in many genotypes emergence was less than 50%, and in some shoot growth was completely restricted without affecting root growth. As expected the rate of seedling emergence was significantly lower in the chilling treatment compared to control. However, even under chilling conditions some accessions had 100% emergence, indicating large genetic diversity. Sorghum accessions differed for all the traits measured in response to chilling stress including emergence and germination index, shoot and root biomass. Several novel accessions with better tolerance to chilling stress have been identified and further physiological validation and genetic mapping exercises are currently ongoing.