Cold-tolerance screening for cotton cultivars using germination chill protocols Charles A Tuck 1, 4 , Daniel KY Tan 1,3 , Michael P Bange 1,2,3 , Warwick N Stiller 2 1 Faculty of Agriculture, Food and Natural Resources, The University of Sydney, Sydney, NSW 2006, Australia. www.usyd.edu.au Email daniel.tan@sydney.edu.au 2 CSIRO Plant Industry, Locked Bag 59, Narrabri, NSW 2390. www.csiro.au 3 Cotton Catchment Communities Cooperative Research Centre, Australian Cotton Research Institute, Locked Bag 1001, Narrabri, NSW 2390, Australia. http://www.cottoncrc.org.au/content/Communities/Home.aspx 4 Current address: Commonwealth Bank of Australia, Level 1, 139 Macquarie Street, Dubbo, NSW 2830, Australia. http://www.commbank.com.au/ Email: Charles.Tuck@cba.com.au Abstract Cotton (Gossypium hirsutum) is sensitive to cold conditions during germination and establishment. Identifying cultivars with cold tolerance may provide growers with increased flexibility for sowing date options and allow for better establishment under cool conditions, reducing the costs associated with possible replanting. This study utilised germination chill protocols, and investigated their applicability in determining genetic variation in seedling chilling tolerance. Seeds of ten cultivars (DP16, Namcala, Pima A-8, Sicala 350B, Sicot 289RR, Sicot 71, Sicot 75, Sicot 81, Siokra V-18 and TL) were germinated at four temperatures (14, 18, 22 and 30?C) and germination probability was determined after 4, 7 and 10 d. Ten seedlings were selected randomly and the length from the hook of the hypocotyl to the tip of the radicle (seedling length) was measured. An electrolyte leakage test was also conducted after seed imbibition in 5?C water for 24 h. These laboratory tests were correlated to an early planted (August) field experiment conducted 40 km west of Narromine, central west NSW. Seedling length provided a better indication of cold tolerance than germination probability. The best correlation with field emergence was with the Cool Warm Seedling Length (SWSL) test (an average of seedling length between cool temperature 14?C at Day 7 and warm temperature 30?C at Day 4) which provided an R 2 of 0.73. The electrolyte leakage test provided a negative correlation with field emergence (R 2 = 0.43). There was no relationship between the weight of 200 seeds and field emergence. The laboratory and field experiments indicated that there was genetic variation in the cotton cultivars tested, with Namcala, DP16, Sicot 75, Sicala 350B and Siokra V- 18 showing some degree of cold tolerance. Further research is needed to test more cultivars and to validate these tests. Key Words Cotton, cold tolerance, germination, establishment, cold germination assays, electrolyte leakage Introduction Poor germination of cotton (Gossypium hirsutum L.) can limit yield potential either through delays in crop development or patchy stands causing non-uniform growth. Cotton is sensitive to cold during germination and establishment (Wanjura et al. 1969). To germinate and establish successfully, cotton needs soil temperatures of 14?C or greater for three consecutive days (Constable and Shaw 1988). Chilling injury can occur in cotton seedlings when the temperature drops below 15 to 20?C during germination (Cole and Wheeler 1974). Prolonged exposure to temperatures below 15?C will slow seed metabolic activity and make the seed susceptible to plant pathogens and other stresses when soil temperatures start to rise (Buxton and Sprenger 1976). Identifying cultivars with chilling tolerance will allow cultivars to withstand the cold shocks during early cotton growth (15 th September to 30 th November in south-eastern Australia), allowing increased flexibility for sowing date options and allow for better establishment under cool conditions, reducing the costs associated with possible replanting. While some research has been conducted on American cultivars (Duesterhaus 2000; Schulze et al. 1996), few germination chill experiments have been carried out on modern Australian cultivars to assess their potential for cold tolerance. This study utilised germination chill protocols, and investigated their applicability in determining genetic variation in seedling chilling tolerance.