Several studies have indicated a possible link between genome size and earliness in maize. In this study, an original maize population, South African photo‐period insensitive maize composite II (C0) and several selected generations were analyzed for days to flowering, plant height, ear height, and yield. The selection criterion was earliness. Over six cycles of selection a 14‐day decrease in days to flowering was obtained with no significant reduction in grain yield. In the initial population (C0) and the most advanced selection (C6) 101 plants were analyzed for nuclear DNA content. A significant decrease in the mean nuclear DNA content of the C0 population (102 AU) was observed with respect to the C0 population (105 AU). This reduction was the result of a decrease in frequency of plants in the C6 population with large genome sizes. Therefore it was concluded that selection for earliness resulted in selection against plants with large genome sizes. Hence, the nucleotype of a maize plant can be modified by selection such as early flowering time.

Herbicide contamination of natural resources, specifically groundwater, has drawn considerable public attention in recent years. Although deleterious effects of herbicides at large dosages have been well documented, information is lacking about the possible effects of herbicide levels that are found in public water supplies. Chromosomal damage is examined by flow cytometry on Chinese hamster ovary (CHO) cells that are exposed to three herbicides (atrazine, simazine, and bentazon) at concentrations deemed safe by the U.S. Environmental Protection Agency (USEPA) for drinking water. A known clastogen (ara-C) is also included as a reference for comparing the magnitude of chromosomal damage caused by herbicides. Chromosomal damage was assessed by measuring the coefficient of variation (CV) and percent chromosomes present in the larger chromosome distribution peaks. Exposure to atrazine (6-chloro-A L ethyl-W-(l-methylethyl)-l,3,5triazine-2,4-diamine) increased the CV of the largest chromosome distribution peak at concentrations considered safe by the USEPA. Chromosomes exposed to both simazine (fi-chloro-A^W-diethyl-l^Striazine-2,4-diamine) and .bentazon (3-(l-methylethyl)-(lff)-2,l,3benzothiadiazin-4(3/?)-one 2,2-dioxide) did not exhibit chromosomal damage. Further analyses carried out on atrazine concentrations about equal to levels found in public water supplies revealed the potential of atrazine to induce chromosome breakages in CHO cells. Atrazine concentrations that were manyfold greater than the contamination limits exhibited a true clastogenic nature like ara-C. The results provide evidence for further investigations as to the potential health risk of consuming water contaminated with atrazine.

Photosynthetic rates reportedly increase with increasing ploidy levels among lowland and upland ecotypes of switchgrass (Panicum virgatum L.). Such reports are based on data derived from native plant materials and no investigation has tested whether a similar relationship exists among released or experimental populations. A series of studies were conducted to address this question, with rates of photosynthesis measured with an infrared gas analyzer and ploidy levels estimated with flow cytometric analysis of nuclear DNA contents. Analyses of nuclei isolated from young stems and stained with propidium iodide indicated that of the 14 populations examined, eight are tetraploids (‘Alamo’1, ‘Kanlow’, ‘NC‐116’, ‘NC‐216’, ‘PMT‐279’, ‘PMT‐785’, ‘Stuart’, and ‘Wabasso’) and six are hexaploids (‘Blackwell’, ‘Caddo’, ‘Cave‐in‐Rock’, ‘Pathfinder’, ‘Shelter’, and ‘Trailblazer’). Twelve of these populations (not including NC‐116 and NC‐216) were grown in the greenhouse and photosynthetic rates measured on the youngest fully expanded leaf blade 40 to 45 d after germination. Rates of photosynthesis did not differ among populations or between tetraploids and hexaploids. Field studies at Knoxville, TN, Stephenville, TX, and Blacksburg, VA, did indicate, however, that tetraploids often had mean photosynthetic rates 12 to 18% higher than those of hexaploids. These differences were observed at many field sites during May, June, and July, yet based on repeated sampling at Knoxville, rates of leaf photosynthesis measured in September were up to 6.5 μmol CO2m−2 s−1 higher in hexaploid compared with tetraploid populations. Contrary to earlier reports that photosynthetic rates increase with increasing ploidy level, our results indicate that such a relationship is not consistent for tetraploid and hexaploid populations of switchgrass but rather may change with season or environmental stress.

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.

The nuclear DNA content for a group of 40 Malus species and hybrids has been estimated using flow cytometry. Estimates of nuclear DNA content for this germplasm collection range from 1.45 pg for Malus fusca (diploid) to 2.57 pg for Malus ioensis (triploid). Among diploids, the nuclear (2C) DNA ranges from 1.45 pg for M. fusca to 1.68 pg for Malus transitoria. Among triploids, the nuclear (3C) DNA content ranges from 2.37 pg / 3C for Malus sikkimensis to 2.57 pg / 3C for M. ioensis. Given the complexity of the apple genome and its suggested allopolyploid origin, the results obtained in this study confirm earlier reports that polyploids can easily withstand the loss of a certain amount of DNA, and that there is a slight tendency towards diminished haploid nuclear DNA content with increased polyploidy.