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Senecio L. es el género que presenta mayor número de especies dentro de la familia Asteraceae. Exhibe importantes conflictos sistemáticos, sus límites no están claramente definidos y la identificación de sus especies se ve dificultada por el solapamiento que presentan los caracteres exomorfológicos diagnósticos; que sumado a la gran variabilidad que presentan algunos taxones, lleva a descripciones y claves poco precisas y ambiguas. Los estudios anatómicos foliares y palinológicos son escasos para las especies argentinas del género en tanto que se cuenta con mayor conocimiento de su citogenética, en particular de la serie Corymbosi. En esta tesis se evaluó la utilidad taxonómica de la anatomía foliar, micromorfología de estambres y estilos, morfología polínica y número cromosómico (microcaracteres) en el género Senecio, en particular, de especies argentinas que presentan conflictos taxonómicos. Parte de la información obtenida permite profundizar el conocimiento del género tanto a nivel local como general y contribuye a esclarecer problemas taxonómicos. Los estudios se realizaron, principalmente en Senecio sect. Senecio serie Xerosencio y los resultados fueron comparados con miembros de Senecio sect. Senecio ser. Corymbosi; Senecio sect. Senecio serie Culcitium y Senecio sect. Senecio serie Otopteri. Se efectuó una recopilación y revisión exhaustiva del material bibliográfico relacionado con la diversidad y taxonomía del género, se examinaron especímenes Resumen ii provenientes de tres herbarios de la Argentina y se estudiaron plantas vivas en su ambiente natural. Se observaron caracteres vegetativos y reproductivos de los ejemplares de herbario y de plantas provenientes de su entorno natural. Se elaboraron descripciones ampliadas y detalladas de cada una de las especies consideradas, incluyendo aspectos micromorfológicos vegetativos (anatomía foliar) y reproductivos (micromorfología de estambres y estilos, palinología y número cromosómico). Para todas las especies, se señalaron sus respectivas áreas de distribución geográfica con comentarios sobre su hábitat. Los microcaracteres que se enuncian a continuación mostraron importancia taxonómica: ornamentación de la cutícula, tipos de tricomas, contorno de las células epidérmicas, características de las células oclusivas de los estomas, espesor de la pared tangencial externa de las células epidérmicas, presencia de células epidérmicas papilosas y estructura del mesofilo (caracteres anatómicos foliares), presencia de margo y de mesoapertura en los granos de polen, forma del collar del filamento, de la base y del apéndice estéril de la antera. Se realizaron estudios citogenéticos de las especies estudiadas, estableciéndose por primera vez el número cromosómico para cuatro taxones argentinos de Senecio, se confirma el contabilizado previamente por otros autores para S. vira-vira y se registra un nuevo nivel de ploidía para S. filaginoides. Se presentan claves dicotómicas que permiten diferenciar especies afines y variedades considerando el uso de estos microcaracteres. Se presenta un análisis sobre la validez de las series y subseries en que tradicionalmente se agruparon las especies estudiadas mediante la realización de un Análisis de componentes principales. Resumen iii La nueva información obtenida complementa la previamente existente y permite apoyar la sinonimia de las entidades infraespecíficas de S. perezii y los tratamientos infraespecíficos propuestos para S. subumbellatus y S. microcephalus. Por otra parte, no se apoya la sinonimia propuesta para S. subulatus var. erectus y S. subulatus var. salsus. Se proveen nuevos caracteres que justifican la exclusión de S. otites de Senecio sensu lato. / Senecio L., with more than 1250 species, is the largest genus of the family Asteraceae. It exhibits important systematic conflicts, its boundaries are not clearly defined and species identification is hampered by the overlapping of diagnostic exomorphological characters, which added to the large variability of some taxa, leads to ambiguous descriptions and makes keys difficult to use. Leaf anatomy and palynological studies are scarce for the Argentinean species of the genus, while there is a better knowledge of its cytogenetics, in particular for the series Corymbosi. The taxonomic utility of leaf anatomy, micromorphology of stamens and styles, pollen morphology and chromosome number (microcharacters) in the genus Senecio, particularly, of Argentinean species that present taxonomic conflicts, were evaluated. These studies were carried out mainly in Senecio sect. Senecio series Xerosenecio and their results were compared with members of Senecio sect. Senecio ser. Corymbosi; Senecio sect. Senecio series Culcitium and Senecio sect. Senecio series Otopteri. A comprehensive compilation and revision of bibliography related to the diversity and taxonomy of the genus was carried out. Vegetative (leaf anatomy) and reproductive (micromorphology of stamens and styles, palynology and chromosome number) characteristics of herbarium specimens and plants in their natural environments were observed and detailed descriptions were made of each species. The following microcharacters showed taxonomic importance: ornamentation of the cuticle, type of trichomes, type of contour of the epidermal cells and characteristics of the stomatal occlusive cells, thickness of the external wall of the epidermal cells, presence of papillary epidermal cells, and mesophyll structure, the presence of margo and mesoaperture in pollen grains, and shape of the filament collar, anthers base and sterile anther appendix. Chromosome numbers for four Argentinean taxa of Senecio are established for the first time, previous counts for S. vira-vira are confirmed, and a new level of ploidy for S. filaginoides is reported. However, this character has little value in the clustering of Senecio species. Dichotomous keys to differentiate related species and varieties considering microcharacters are presented. The validity of the series and subseries in which the species studied were traditionally grouped is tested using Principal Component Analysis. New morphological support is provided for the synonymy of the infraspecific entities of S. perezii which complements the infraspecific treatment given to S. subumbellatus and S. microcephalus. On the other hand, the synonymy of S. subulatus var. erectus and S. subulatus var. salsus, is not supported by the new data. The consideration of these microcharacters supports the exclusion of S. otites from Senecio sensu lato. Some of the information obtained throughout this thesis contributes to the knowledge of the genus Senecio and are an aid for the resolution of its internal systematics at the local and global levels.

Ozone is known to be the most widespread phytotoxic air pollutant. Ozone causes visible injury, reductions in photosynthesis, growth, and yield. Plant response to ozone may vary with species, varieties, and physiological age. Comparison between sensitive and tolerant cultivars has a key role in assessing ozone damage, investigating the sites of cellular injury, and identifying ozone tolerance mechanism. The objectives of this study were to investigate the effects of high ozone concentration (200 ppb) as well as ambient ozone concentrations (under field conditions) on net CO2 assimilation and PSII function in plants with different sensitivities to ozone. Two species of plants, tobacco (Nicotiana tabacum L.) and black cherry (Prunus serotina) were studied. Gas exchange analysis and chlorophyll fluorometry were utilized to characterize physiological function. Two tobacco cultivars, Bel-B and Bel-W3, tolerant and sensitive to ozone, respectively, were grown in a greenhouse supplied with charcoal filtered air and then exposed to 200 ppb ozone for 4hr. Effects on chlorophyll fluorescence, net photosynthesis, and stomatal conductance are described. Quantum yield was calculated from chlorophyll fluorescence and the initial slope of the assimilation-light curve measured by the gas exchange method. Only the sensitive cultivar, Bel-W3, developed visible injury symptoms involving up to 50% of the 5th leaf. The maximum net photosynthetic rate of ozone-treated plants of the tolerant cultivar was reduced 40% compared to control plants immediately after ozone fumigation; however, photosynthesis recovered by 24 hr post fumigation and remained at the same level as control plants. In the sensitive cultivar, on the other hand, ozone exposure reduced maximum net photosynthesis up to 50%, with no recovery, apparently causing permanent damage to the photosystem. Reductions in apparent quantum efficiency, calculated from the assimilation-light curve, differed between cultivars. Bel-B showed an immediate depression of 14% compared to controls, whereas Bel-W3 showed a 27% decline. Electron transport rate (ETR), at saturating light intensity, decreased 58% and 80% immediately after ozone treatment in Bel-B and Bel-W3, respectively. Quantum yield decreased 28% and 36% in Bel-B and Bel-W3, respectively. It can be concluded that ozone caused a greater relative decrease in linear electron transport than maximum net photosynthesis, suggesting greater damage to PSII than the carbon reduction cycle. Two different sensitivity classes of black cherry, tolerant and sensitive, growing under natural environmental conditions in Giles County, VA were assessed for physiological responses to ambient ozone concentrations. Additional measurements were made at two other sites near Blacksburg. Leaf gas exchange rates and visible foliar injury were determined monthly during the growing seasons of 2000, 2001, and 2002 to characterize the relationship of injury to altered photosynthetic function. Ambient ozone levels were sufficient to induce visible symptoms which were highly correlated with a reduction in PnMAX (maximum net photosynthetic rate under saturating light conditions) and à CO2 (quantum yield for carbon reduction) only in sensitive black cherry. Electron transport rate (ETR) and quantum yield of PSII (à PSII) were also reduced in sensitive black cherry. Maximum photochemical efficiency (Fv/Fm) in sensitive trees was severely damaged by ambient ozone. There were positive correlations between increasing cumulative ozone concentration and substantial reductions in PnMAX and in à CO2 of sensitive trees compared to tolerant trees. There was a negative correlation between chlorophyll content and percent leaf injury in sensitive black cherry / Ph. D.

The position of newly expanded shoot leaves in the canopy of apple trees had significant and similar effects on specific leaf weight (SLW), leaf thickness, length of palisade tissue, and number of cell layers in the palisade tissue. Inside canopy leaves had lower SLW, leaf thickness, palisade length, and number of palisade cell layers than middle and outside leaves at each of six sample dates ranging from late May to early October. Outside canopy leaves had the highest values at all sample dates. Differences in SLW, leaf thickness, and palisade length between inside and outside leaves increased as the season progressed, primarily due to a general increasing trend in outside leaves that developed later in the season. Effects of canopy position on spur leaves were not as great as with shoot leaves. Regression analysis showed SLW to be significantly correlated with leaf thickness and palisade length. / M.S.