The nuclear DNA content was determined in two separate experiments for 19 maize inbred lines and 26 maize F1 hybrids. Ten inbred lines were initially screened in the first experiment. An 11 per cent difference in nuclear DNA amount was observed between the two lines with the lowest amounts of DNA and two lines with the largest genome sizes. All possible hybrid combinations were made among these four lines. In all cases, the genome sizes of the F1 hybrids were not significantly different from their expected parental means. In several cases, however, F1 plants of specific crosses were not uniform in genome size. In these crosses, genome sizes observed ranged from the genome size of the low parental genome to the largest parental genome. These results indicate an instability in F1 genome sizes in certain maize crosses. In order to corroborate this hypothesis, a second experiment was performed. To remove any biases with respect to genome size, 14 F1 maize hybrids were selected solely on the basis of their heterotic response with no regard for their genome sizes or the genome sizes of their parental inbred lines. The nuclear DNA content of the nine parental lines and 14 hybrids was determined. In most of the crosses, the nuclear DNA content of the F1 hybrids was not significantly different from their respective parental means. However, in five parental combinations, the mean nuclear DNA content of the F1 hybrids was significantly higher than their respective parental means. The combined results of this study support the hypothesis of instability in nuclear DNA content in F1 hybrids of maize. This instability appears not to be universal in all maize hybrids but is restricted to specific parental combinations.

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Previous studies have indicated a relationship between genome size and cold tolerance in plants. Many species adapted to growth in cool environments have large genome sizes. These studies are based on interspecific DNA content variation. In this study, the nuclear DNA content of eight maize populations was determined. These populations were obtained from the University of Nebraska and represent populations selected for cold tolerance and their respective unselected original populations. Intraspecific DNA content variation was observed between the selected and unselected popolations. Upon assessing the data based solely on cold tolerance, no clear relationship between genome size and cold tolerance was apparent. When both freeze tolerance and cold tolerance were considered, populations which were cold tolerant and exhibited a certain degree of freeze tolerance were observed to have significantly larger genomes relative to the unselected populations. Thus, it appears that the relationship between intraspecific genome size variation and cold tolerance is similar to the relationship between interspecific genome size variation and growth at cooler temperatures in plants.