Mariel G. Perreta

The Flowering Unit in the Synflorescences of Amaranthaceae

The structure of the synflorescence and the flowering units in Amaranthaceae are characterized. The synflorescence is polytelic. In the flowering unit we recognize the main florescence and the enrichment zone. The florescences may consist of: (1) Fully developed partial florescences bearing three or more flowers; (2) Partial florescences reduced to one or a few fertile flowers having prophylls with more or less modified axillary productions; or (3) No partial florescences but solitary flowers having prophylls with no axillary productions. We described the flowering unit in species with florescences bearing a solitary flower and the flowering unit in species with florescences bearing partial florescences. Hypothesized developmental processes are described, with a view to finding relationships among different models characterized in the family as well as defining characters for cladistic studies, which may be useful to depict all the variations observed.

Structure and development of the branching system in Melica macra (Poaceae)

Summary Melica macra Nees is a widespread grass of Central and Northern Argentina. It is considered an invasive weed in natural grasslands. The aim of the present paper is to describe the branching system of M. macra on the base of both the analysis of living plants obtained from seeds and maintained under cultivation, and the observation of plants growing in their natural habitat. The plants are cespitose and able to produce tillers profusely and bear a rhizomatous basal structure ensuring perenniality. Branching along the main axis occurs up to the sixth node, whereas branches of first-order axes are axillary of leaves 1–3, and those of higher-order axes mainly of the first and second leaves. Such higher-order axes are responsible for the formation of the basal rhizomatous structure, which arise as a result of sympodially arranged rhizome links, each of them consisting of four proximal short internodes (including the short hypopodium) of each tiller. The morphology of tillers is largely influenced by the phenological stage, so that tillers developed before and after flowering can be clearly distinguished.

CARACTERÍSTICAS ESTRUCTURALES Y FISIOLÓGICAS DE Petunia axillaris RELACIONADAS CON SU BAJA SENSIBILIDAD A GLIFOSATO

Repeated use of glyphosate as herbicide in the same site, among other things, increases the frequency of individuals with low sensitivity. Petunia axillaris is among the many reported cases of weed species that have presented problems for proper management. This study evaluates their sensitivity and discusses the possible mechanisms involved in its differential sensitivity. For this purpose, we analyzed the retention, absorption and translocation of radiolabeled herbicide, and the morphological characteristics related to the herbicide response. Studies confirms that Petunia axillaris is a glyphosate tolerant specie and the shikimic acid metabolic pathway is not altered. The low sensitivity to glyphosate in Petunia axillaris could be associated to low foliar retention and increased time of absorption, which delayed the entrance and movement of the herbicide. Regardless the altered structure with glyphosate, seed production is high enough which ensures the perpetuation of the less sensitive genotypes. Petunia axillaris showed a combination of morphological, structural, and physiological characteristics that determined their adaptive advantage to perpetuate and increase its population in systems with intensive use of glyphosate.

Trophotagma Enrichment Axes in Poaceae

This review is intended: (1) to interpret and characterize morphological variations observed in the structure of the enrichment axes, located below the terminal inflorescence in Poaceae; and (2) to study the relationship between the intensity of development of such axes and the size of terminal inflorescence. An important reduction in the development of the terminal inflorescence is generally accompanied by a significant development of enrichment axes. It is necessary to adequately characterize these enrichment axes, differentiating them from the terminal inflorescence. Since the intensive development of enrichment axes in synflorescences of many grass genera has caused misinterpretations of the inflorescence structure, to include them as parts of the terminal inflorescence.

Spatial structure and development of Paspalum vaginatum (Poaceae): an architectural approach

Paspalum vaginatum Sw. is a perennial grass originating from tropical and subtropical regions worldwide that plays important ecological and utilitarian roles such as dune stabilisation and erosion control, and is used as a lawn and forage plant. The vegetative morphology of shoots was studied using an architectural approach with the aim of identifying constant features that characterise the architectural unit of this species, describing its developmental dynamics, and exploring the morphological basis of its extraordinary plasticity and adaptability to multiple ecological conditions. Plants of two ecotypes were cultivated in pots outdoors during two consecutive summers, and axes of different branching order were marked for periodical observations. Leaf typology, orientation and position of branches and occurrence and position of reproductive axes were considered. The basic growth pattern arises as a result of extensive plagiotropic growth of axes up to fourth branching order, which initially behave as stolons but can become rhizomes insofar the soil cover increases. Orthotropic floriferous axes up to fifth order develop at the axils of stolons, which can form ‘daughter’ clumps by basal tillering. Despite quantitative differences, both ecotypes share similar architectural features. Architectural analysis provides the basis to predict space occupation dynamics under different environmental conditions of growth.