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Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways ¹

VII. CHROMOSOME NUMBERS, METAPHASE I CHROMOSOME BEHAVIOR, AND MODE OF REPRODUCTION OF PHOTOSYNTHETICALLY DISTINCT PANICUM SPECIES

Department of Agronomy, University of Georgia, Athens, Georgia 30602

Panicum species of the Laxa group were investigated in a series of published reports and were found to possess C₄, C₃, and intermediate photosynthetic characteristics. Taxonomic and other relationships among these plants, however, are not clear. It was the objective of this investigation to document chromosome number, metaphase I chromosome behavior, and mode of reproduction, including abnormalities in the embryo sac, for these species.

Chromosome counts showed a basic number (x) of 10 and ploidy levels of diploid (2n = 2x = 20), tetraploid (2n = 4x = 40), and hexaploid (2n = 6x = 60) in this group of Panicum. One diploid and one tetraploid accession of the C₄ species, Panicum prionitis Griseb., were obtained. Of the intermediate species, Panicum milioides Nees ex Trin. was diploid, Panicum schenckii Hack. was hexaploid, and Panicum decipiens Nees, ex Trin. was found to possess two ploidy levels, one accession being diploid and the other accession being hexaploid. All the C₃ species, which included two accessions of Panicum laxum Sw., three accessions of Panicum hylaeicum Mez., and one accession of Panicum rivulare Trin., were tetraploid.

Meiosis was regular with primarily bivalent pairing at metaphase I in all species except the tetraploid accession of P. prionitis which possessed from 4 to 10 tetravalents. Stainable pollen was high in all species, ranging from 70 to 99%. Embryo sac analyses showed a single sac in all plants except the tetraploid accession of P. prionitis, which was found to possess an additional sac at anthesis. An additional sac was also observed in some ovaries of the P. schenckii accession. Self-pollinated seed set was high in all accessions except the diploid accession of P. prionitis and one accession of P. laxum where no seed was set under bagged conditions.

This information establishes, within the limits of this collection, a base for future studies on genetic, taxonomic, photosynthetic, and evolutionary relationships among these plants. Possession of the same basic chromosome number, regular meiotic pairing, a high degree of stainable pollen, and good seed set in most of the plants studied indicate possible success in making hybrids for a genetic study of photosynthetic pathways in Panicum.

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