CO2 Exchange, Cytogenetics, and Leaf Anatomy of Hybrids between Photosynthetically Distinct Flaveria Species1

Hybrids between the C4-like species, Flaveria brownii, A. M. Powell and the C3-C4 intermediate species Flaveria linearis Lag., Flaveria floridana Johnston, and Flaveria opposithfoIIa (DC.) Rydb. exhibited bivalent chromosome pairing during meiosis and stainability of pollen was high, ranging from 51 to 95%. An F2 population produced from an F. brownil x F. linearls F1 hybrid, exhibited bivalent chromosome pairing and high pollen stainability indicating a high degree of fertility in the hybrid. Oxygen inhibition of apparent photosynthesis averaged 6.8% for F. brownll and 22.2% for the C3-C4 species (in two experiments), and F1 hybrids exhibited inhibitions which were intermediate to their parents. Values of carbon dioxide compensation concentration determined at low irradiance were 4.0, 34.0, and 6.5 microliters per liter for F. brownH, F. Iinearis and their F, hybrid, respectively. The mean value at low irradiance for 33 F1 plants was 6.8 microliters per liter, and individual values ranged only from 3.7 to 11.7 microliters per liter. Anatomical characteristics for the F1 hybrid leaves were intermediate to those of the parents, and there was considerable variation among F2 plants derived from F. brownH x F. linearis. In the F2 population 613C values ranged from -270/ to -200/. The expression of more C4-like characteristics by the F1 hybrids in this study and their apparent high fertility make them promising specimens for producing segregating populations for use in C4 inheritance studies. Physiological and anatomical traits associated with C4 photosynthesis have been well characterized and are known to provide certain physiological and environmental advantages to plants possessing them. Hybridization between plants which exhibit C4 photosynthesis and those which do not is a means of assessing the feasibility of incorporating C4 traits into C3 species through plant breeding. Segregating populations from fertile C3 X C4 hybrids would be useful for studying inheritance of C4 traits and their linkage. The first artificial C3 x C4 hybrids were between the C4 species Atriplex rosea and the C3 species Atriplex patula (2-4). The F, hybrid was intermediate to the parents in several anatomical and biochemical characteristics, but was similar to the C3 parent in 'Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Stations, by the U.S. Department of Agriculture Competitive Research Grants Office Agreement 87-CRCR-12408 and by the University of Georgia Program in Biological Resources and Biotechnology. 02 inhibition of Ap2 and r, and was below both parents in AP. Chromosome pairing was erratic at meiosis and seed set was poor, but a small population of 20 F2 individuals was produced and segregation occurred for several traits including leaf anatomy, phosphoenolpyruvate carboxylase activity, and r. Hybridization among photosynthetic types has been more successful in the Flaveria genus (5, 8, 9, 14, 17). Hybrids between the C4 species Flaveria trinervia and three C3-C4 species exhibited photosynthetic traits between those of the parents (5). However, the F1 hybrids were sterile making evaluation of advanced generations impossible. Holaday et aL (13, 14) reported hybrids between the C4-like species Flaveria brownii and Flaveria pringlei (C3). The F, hybrids had reduced r relative to the C3 parent, but 02 inhibition of AP, the initial slope of the CO2 response of AP and Ci/Ca were similar to the C3 parent. Most C4 enzymes in the hybrids had activities slightly higher than the C3 parent except that NADP-malate dehydrogenase was similar in the hybrid and F. brownii. Powell (17) reported a high degree of fertility in hybrids between the C4 species F. brownii and the C3-C4 species F. floridana, F. linearis, and F. oppositifolia, including moderate pollen stainability (36-64%), bivalent chromosome pairing at meiosis, and production of F2 plants. There has, however, been very little biochemical, anatomical or physiological characterization of these fertile hybrids. Cheng et al. (8) have found F, hybrids between F. brownii and F. floridana to exhibit 613C values similar to the mean of the parents, while backcrosses of the hybrid to F. brownii had values near those of the C4 parent. In order to further characterize hybrids between these apparently closely related species we have made the hybrids and studied their leaf anatomy, CO2 exchange, and cytological characteristics. MATERIALS AND METHODS Plant Material and Growth The two accessories of Flaveria brownii A. M. Powell used in these experiments (85-212 and 85-250) were vegetatively propagated from plants collected on the Texas Gulf Coast by R. H. Brown. Flaveria linearis Lag. (85-26), Flaveriafloridana J. R. Johnston (85-10), and Flaveria oppositifolia (DC.) Rydb. 'Abbreviations: AP, apparent photosynthesis; r, C02 compensation concentration; Ca, CO2 concentration external to the leaf; Ci, intercellular CO2 concentration; BSC, bundle sheath cells.