A Comparison of U.S. and Chinese Sorghum Germplasm for Early Season Cold Tolerance

Early season cold tolerance in grain sorghum [Sorghum bicolor (L.) Moench] is a desirable trait for extending its production range and minimizing risks associated with early spring plantings. Ten Chinese Kaoliang accessions were compared with 10 U.S. inbred parental lines and 10 U.S. commercial hybrids for a range of cold tolerance traits under laboratory, growth chamber, and field settings. Chinese lines were superior to both the inbred and hybrid classes in laboratory germination rates and field-based rates of emergence. In the growth chamber assays, Kaoliangs were not significantly different than U.S. hybrids for most traits measured at either of the two temperature treatments (12 and 24C), with the exception of shoot length, for which the Chinese germplasm was higher. At the cooler temperature, Kaoliangs were significantly greater than U.S. inbreds for only fresh shoot weight; when tested at the warmer temperature, Kaoliangs had higher dry root weight, fresh and dry shoot weights, and fresh and dry whole plant weights, relative to the U.S. inbred class. The U.S. hybrids had greater total plot weight and final stand counts in the field than Kaoliangs, which were likewise higher than U.S. inbreds for both of these traits. Chinese accessions from this working group would serve as a source of favorable genes primarily for tolerance to low temperatures during the germination and emergence phase of growth in the breeding of cold tolerance sorghum lines. I N RECENT YEARS, there has been increased interest in the development of grain sorghum germplasm adapted to U.S. production systems with heightened levels of tolerance to early season cold temperatures. This germplasm could serve both to expand the geographical range of grain sorghum cultivation and minimize the inherent risks involved in early season planting of grain sorghum within sorghum production regions. Additionally, an earlier sowing date could offer growers the option of capitalizing on higher levels of available soil moisture in the early spring and lower evapotranspirative demands, and thus, could potentially serve as a drought avoidance strategy. The initial phase of any plant improvement program is the identification of superior germplasm for the trait of interest (Stoskopf, 1993). With respect to early season cold tolerance in sorghum, Chinese sorghum germplasm of the working group Nervosum-Kaoliang has historically been reported as being superior in this regard. There is ample anecdotal evidence of this class of germplasm possessing heightened levels of early season cold tolerance, but specific measures of the germination and seedling vigor qualities of this germplasm as a class are somewhat limited. Stickler et al. (1962), in comparing a small set of Chinese and U.S. germplasm, found that the Kaoliangs were typically superior in terms of germination and emergence under suboptimal temperatures and indicated that these lines might also possess some advantage in terms of their seedling growth rate across all temperatures tested. Qingshan and Dahlberg (2001) examined a large number of Chinese accessions and noted that several accessions within the Chinese germplasm collection showed high levels of both germination under cold temperatures and seedling phase cold tolerance. It has been postulated, in both sorghum and maize (Zea mays L.), that the germination–emergence and early growth stage phases of seedling development are each under the control of entirely different suites of genes (Cisse and Ejeta, 2003; Hodges et al., 1997; Nordquist, 1971). In breeding for overall early season cold tolerance, therefore, it may be necessary to evaluate germplasm sources for tolerance in each phase. This study was undertaken to examine a set of Chinese sorghum lines in laboratory, growth chamber, and field settings for both cold temperature germination and seedling vigor traits and to compare the response of these lines with a representative set of U.S. parental lines and commercial hybrids.