DNA extraction
The DNA of each of the 63 genotypes listed in Table 1 was extracted from young leaves of plants aged 30 d grown in a glasshouse of the Department of Agronomy, Faculty of Agriculture, Bangkhen Campus, Kasetsart University, Thailand following the modified CTAB method (Haque et al., 2004). The quality and concentration of DNA were checked on 1% agarose gel and compared with standard lambda DNA (Fermentas; Burlington, Ontario Canada). All samples were stored at -20 °C before analysis.