Analysis of cold tolerance in sorghum under controlled environment conditions
Introduction
Sorghum originated in the semi-arid tropics and generally is sensitive to low-temperature stress in the range of 20 °C to about 0 °C (Peacock, 1982). Sorghum suffers chilling injury when subjected to nonfreezing temperatures below 10–15 °C (Peacock, 1982). Low-temperature stress at planting time usually results in poor seedling establishment of sorghum because of slow emergence rate, reduced emergence percentage, and reduced growth rate after emergence (Pinthus and Rosenblum, 1961, Singh, 1985).
Early planting, use of minimum tillage, and attempts to expand sorghum into higher elevations and more temperate latitudes necessitate the identification of genotypes that are tolerant to low temperatures during seed germination, seedling emergence, and early plant growth (Singh, 1985, Bacon et al., 1986). In maize (Zea mays L.), much effort has been devoted to improving seedling cold tolerance and numerous studies have been reported (Mock and Eberhart, 1972, Mock and Bakri, 1976, McConnell and Gardner, 1979a, Menkir and Larter, 1985). Progress made in cold tolerance research partially explained a broader range of adaptation for maize. Such parallel improvement of cold tolerance in sorghum is needed (Bacon et al., 1986).
Although large numbers of cultivars or advanced breeding lines can be evaluated for cold tolerance under field conditions by early spring planting, unpredictable climate conditions from year to year make field selection unreliable (McConnell and Gardner, 1979a, Haskell and Singleton, 1949). The presence of genotype×environment interactions also complicates field evaluation for cold tolerance, and proper characterization requires multi-environment testing (Mock and Eberhart, 1972, Mock and McNeill, 1979, McConnell and Gardner, 1979a). A growth chamber assay could substitute for early field sowings as a controlled selection method or at least as a preliminary test to discriminate weak from vigorous lines before spring planting (Pinnell, 1949). Few studies, however, have been done to evaluate the usefulness of growth chamber assays for quantifying cold tolerance in sorghum.
Genetic variability for cold tolerance exists in sorghum and has been detected both by early planting in the field and under controlled temperature conditions (Pinthus and Rosenblum, 1961, Stickler et al., 1962, Thomas and Miller, 1979, Miller, 1982, Brar and Stewart, 1994). Empirically, cold tolerance in sorghum was measured by different components such as germination, emergence, and seedling growth under low-temperature stress (Soujeole and Miller, 1984). Mesocotyl elongation was also suggested as an indicator of cold tolerance in sorghum (Maiti, 1996). Little research in sorghum, however, has been reported in simultaneous evaluation of cold tolerance during germination, emergence, and early seedling growth under low-temperature stress.
The objectives of this study were to quantify the variability of cold tolerance in commercial hybrid seed lots under growth chamber conditions, to examine different components of cold tolerance and their relationships, and to evaluate the consistency of laboratory results with data from field tests.
Section snippets
Soil-based assay
Thirty commercial grain sorghum hybrids with uniform seed quality (>90% germination potential at 25 °C) were evaluated for cold tolerance in growth chamber assays at four temperature regimes, 15/10, 13/10, 11/8, and 25/20 °C, using a 13/11 h day/night cycle. Low-temperature regimes were chosen for this study based on preliminary testing. Seed lots for this assay were the same as those used in field studies. Relative humidity and irradiance were controlled as 90–95% and 350 μmol m−2 s−1, respectively.
Results
Combined analysis of the three low-temperature soil-based experiments indicated that the temperature effect was significant for emergence index, seedling height, and shoot dry weight (Table 1). The lack of significant effect of temperature on emergence and root dry weight were partly caused by the relatively large effects of replication within run by temperature as denoted by term Error (a) in Table 1. The relative narrow range of temperature regimes (15/10, 13/10, 11/8 °C) used in this study
Discussion
The absence of entry by temperature interactions indicated that the seed lots evaluated in the soil-based assay performed similarly under the three cold-temperature stress conditions. These results are consistent with reports in maize. Eagles (1982) tested emergence of four maize inbreds and their 12 F1 hybrids under 11/11, 15/10, and 15/5 °C and concluded that genotype×temperature interactions were not significant. Pesev (1970) also described a similar reaction: the tolerance rankings of maize
Conclusions
Significant variation of cold tolerance existed in commercial sorghum hybrid seed lots studied under controlled low temperature. Cold-tolerance traits measured under controlled low-temperature conditions were correlated significantly, indicating that simultaneous improvement of these components could be captured in new hybrid cultivars. Significant correlations among traits measured in laboratory and field assays for cold tolerance indicated that a soil-based laboratory assay could be used as
Acknowledgements
Thanks to the Director of the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, for approving the publication of this paper.
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