Megasporogenesis and megagametogenesis in several Tripsacum species (Poaceae)

Le complexe agamique des #Tripsacum (x=18) représente une source de caractères intéressants pour l'amélioration génétique du maïs, sous réserve que l'apomixie puisse être manipulée. L'apomixie chez #Tripsacum a été décrite pour la première fois il y a plus de 40 ans ; cependant sa présence dans l'ensemble du genre n'a pas été établie. Les modes de reproduction chez huit espèces mexicaines et deux espèces sud-américaines ont été déterminés d'après l'analyse d'ovaires éclaircis dans une solution de benzyl benzoate-dibutyl phthalate et observés par contraste d'interférence. La présence de callose lors de la mégasporogenèse a été étudiée en microscopie à fluorescence après éclaircissement des ovaires dans une solution aqueuse de sucrose contenant du bleu d'aniline. Les génotypes diploïdes sont sexués, et les formes polyploïdes se reproduisent apomictiquement selon le type Antennaria de la diplosporie (avortement complet de la méiose). Le type Taraxacum de la diplosporie (production de mégaspores non réduites après restitution de noyaux) a aussi été observé, mais rarement. Aucune synthèse de callose n'a été observée au niveau des parois cellulaires des mégasporocytes diplosporiques, alors qu'elle est caractéristique des méiocytes. L'absence de callose suggère que les formes diplosporiques sont le résultat de mutations affectant le processus méiotique. L'apomixie dans le genre #Tripsacum$ est facultative ; la production de nouveaux génotypes polyploïdes grâce à des échanges génétiques impliquant aussi bien des géniteurs apomictiques et sexués est donc possible. (Résumé d'auteur

Morphogenetic responses of embryo culture of wheat related to environment culture conditions of the explant donor plant

Availability of immature embryos as explants to establish wheat (Triticum aestivum L.) by tissue culture can be limited by climatic factors and the lack of high quality embryos frequently hampers experimentation. This study evaluates the effects of rainfall, various temperature-based variables and sunshine duration on tissue culture response (TCR) traits including callus formation (CF), regenerating calli (RC), and number of plants per embryo (PPE) for 96 wheat genotypes of worldwide origin. The objectives of this study were to evaluate the significance of a particular climatic factor on TCR traits and to determine the period of wheat growth during which these factors were the most effective. The genotypes were grown in an experimental field during three seasons differing in meteorological conditions. The relationships between TCR traits and climatic factors within three time periods of wheat growth: 2, 6 and 10 weeks prior to embryo sampling were analysed by biplot analysis. The tissue culture traits were influenced at very different degrees by climatic factors: from 16.8% (RC) to 69.8% (CF). Donor plant environment with high temperatures and low rainfalls reduced (p lt 0.05) the tissue culture performance of wheat genotypes. Callus formation was most sensitive to the temperature based factors. The environmental conditions between flowering and the medium milk stage were the most important for CF, while RC and PPE were not particularly related to any period

Learning Objects, Learning Objectives and Learning Design.

Educational research and development into e-learning mainly focuses on the inclusion of new technological features without taking into account psycho-pedagogical concerns that are likely to improve a learner's cognitive process in this new educational category. This paper presents an instructional model that combines objectivist and constructivist learning theories. The model is based on the concept of a learning objective which is composed of a set of learning objects. A software tool, called the Instruction Aid System (IAS), has been developed to guide instructors through the development of learning objectives and the execution of the analysis and design phases of the proposed instructional model. Additionally, a blended approach to the learning process in Web-based distance education is also presented. This approach combines various event-based activities: self-paced learning, live e-learning and the use of face-to-face contact in classrooms

Solving the Phosphorus Pollution Puzzle: Synthesis and Directions for Future Research

Despite the success of efforts to reduce phosphorus (P) pollution from point sources, P from non-point agricultural sources remains a vexing problem with many U.S. water bodies having impairments. Key to solving the P pollution puzzle is to take stock of progress to date, the puzzle pieces available, and the gaps to be filled. In this paper, we synthesize the state of knowledge on P pollution, discuss the state of existing public programs, and review economists’ contributions to informing P pollution policies. We review the water quality valuation literature, identifying limitations in the linkages to policy-relevant environmental quality metrics. We examine how and why P is used agriculturally, along with recent advances in market-based policy design and field testing. We survey new knowledge in biology and engineering, including improved understanding of the fate and transport of P. In light of recent learning and persistent knowledge gaps, we recommend directions for economic research to add needed pieces to the puzzle of how to protect our water bodies. Puzzle gaps meriting attention include mechanisms to target public funds more effectively in voluntary abatement programs, policy design for emerging mitigation technologies, new ways to implement performance-based policies, means to leverage social norms and behavioral cues, changes in the “pay-the-polluter” paradigm, and application of state-of-the-art evaluation methods to conservation programs. Beyond the realm of public policy lies that of private supply chains, where establishment of environmental standards holds additional promise. Rich research opportunities exist for economists in tandem with biologists, engineers, and other