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Essential oil (EO) yield and composition of five accessions of Mentha longifolia (L.) L. (leaves) were determined throughout their vegetative cycle by GC-MS and GC-FID analysis. These accessions were selected from individuals of a wild population located in Teruel (Spain). This selection was carried out based on the individual TLC profiles of dichloromethane extracts from leaves in such a way that each accession showed a homogeneous and characteristic composition. The accession rich in -terpineol acetate and carvone acetate has not been previously reported.As for EO yield, all the accessions showed the maximum value in advanced flowering stage. With regard to composition, significant changes were noted in the accession A respect to the relative amounts of its main compounds, piperitone and piperitenone oxides. Similar behaviour was also detected in accession B, in which piperitone oxide was the major compound together with pulegone and piperitenone oxide. As for accession C, carvone acetate showed a considerable increase (6.0 to 20.1 %) during the vegetative growing stage, while the amount of the other major compound (-terpineol acetate) remained stable. Slight or no significant changes were observed in accession D (Z-dihydrocarvone as major compound, 68.3 -73.7 %). Significant changes in the major compounds of accession E (pulegone and isomenthone) were also observed during the initial vegetative growing period. In general, the most noticeable changes were detected at the beginning of the vegetative cycle, whereas the flowering stage showed more stable EO composition. The knowledge of seasonal changes for selected EO chemical profiles should be emphasized because the biological activities are linked with them.

Two commercial self-heating food products have been used to apply chemical concepts such as stoichiometry, enthalpies of reactions and solutions, and heat transfer in a classroom activity. These products are the self-heating beverages sold in Europe and the Meals, Ready to Eat or MREs used primarily by the military in the United States. The main goal of the activity is to propose a real-life chemistry problem for which students calculate the heat produced by the chemical reaction or the dissolution process, the accompanying theoretical change in temperature, and finally compare the theoretical change to the temperature observed. The activity is designed to use five minutes of class time and allow students to work in groups, outside the classroom, to solve the posted problems. We have used two different methodologies with this activity: problem-based learning and inquiry-guided instruction. The context of these activities connects a variety of chemistry topics and provides the opportunity to practice common operations such as unit conversion and algorithmic problem solving in the context of a real-world scenario. The key element is the evaluation process that students must conduct at every stage, which includes examining food products promotion information.

Recebido em 5/5/09; aceito em 30/11/09; publicado na web em 26/3/10 PROBLEM BASED LEARNING AS ESTRATEGY FOR METHODOLOGICAL CHANGE IN LABORATORY WORK. The Problem Based Learning (PBL) can be used as a strategy for methodological change in conventional learning environments. In this paper, the integration of laboratory work in PBL grounded activities during an introductory organic chemistry course is described. The most decisive issues of their implementation are discussed. The results show how this methodology favours the laboratory work contextualization in subject-matter and promotes the Science-Technology-Society-Environment relationships. Besides, it contributes to competence development like planning and organization skills, information search and selection, cooperative work, etc., the same way as the tutorial action improvement.Keywords: problem based learning; laboratory work; competences. INTRODUCCIÓN Y OBJETIVOS El aprendizaje basado en problemasEl aprendizaje basado en problemas (ABP) es una metodología educativa ampliamente extendida actualmente a numerosos campos de conocimiento.1 Como estrategia curricular institucional puede ser concebido como eje vertebrador de todo el proceso de aprendizaje, 2 pero también puede ser útil, desde la innovación docente, para la transformación de actividades convencionales como los clásicos ejercicios, problemas o trabajos de laboratorio en tareas que favorezcan la adquisición de competencias de carácter metodológico y comunicativo, 3 así como la incorporación de las tecnologías de la información y la comunicación. 4,5 Esencialmente, el ABP trata de introducir al alumnado en el contexto de un problema vinculado a la realidad social y profesional de modo que deba planificar, mediante un trabajo cooperativo y bajo la supervisión del profesorado, un proceso de resolución que implica las siguientes etapas: la exploración del problema y la generación de hipótesis; la identificación de los conocimientos necesarios para abordarlos; la búsqueda de información y estudio independiente del tema; el análisis crítico y la discusión sobre los conocimientos adquiridos; la aplicación de éstos a la resolución del problema y la reflexión sobre el proceso, llevando a cabo el feed-back necesario.Desde un punto de vista teórico, el ABP se fundamenta en las tendencias educativas más relevantes de las últimas décadas, tales como las diferentes aportaciones de la psicología cognitiva. Así, se considera esencial el papel de los conocimientos previos ya que éstos determinan el volumen y la naturaleza de los que el alumno va integrando posteriormente en su proceso de aprendizaje, siendo continuamente activados y reestructurados a medida que se avanza en el análisis y resolución del problema. De este modo, el aprendizaje puede contemplarse como la construcción de un entramado conceptual progresivamente más rico y complejo que exige la incorporación de nuevas herramientas metodológicas como las características de la actividad científica: formulación de hipótesis, elaboración de diseños experiment...

In the search of sustainable and environmentally friendly methods for weed control, there is increasing interest in essential oils (EOs) as an approach to reduce synthetic herbicide use. The phytotoxicity of Thymbra capitata, Mentha piperita, Eucalyptus camaldulensis, and Santolina chamaecyparissus EOs against the noxious weed Erigeron bonariensis were evaluated in pre- and post-emergence assays in greenhouse conditions. The EOs were applied at 2, 4, and 8 µL/mL, with Fitoil used as emulsifier. In post-emergence, two ways of application were tested, irrigation and spraying. Several germination parameters (germination %, mean germination time, and synchrony of the germination process) were evaluated in pre-emergence tests, and the phytotoxicity level was assessed in post-emergence. In pre-emergence, all EOs significantly reduced seed germination as compared to the controls, ranking: T. capitata > E. camaldulensis > S. chamaecyparissus > M. piperita. The effectiveness of all EOs varied with the tested dose, always following the rank 2 μL < 4 μL < 8 μL, with T. capitata EO showing full effectiveness even at the lowest dose. In post-emergence, T. capitata was the most effective EO, inducing a rather complete inhibition of plantlet growth at the highest two doses. These EOs demonstrated to have good potential for the formulation of natural herbicides.

1,8-cineole defines a typical chemotype of Thymus vulgaris L. in Iberian Peninsula. This compound has a wide range of potentially useful bioactive properties. In order to study the influence of drought stress in the essential oil (EO) composition of this chemotype, sixty plants from six wild populations of Eastern Iberian Peninsula were distilled and analyzed by GC and GC/MS. The harvest dates (May and August) were selected in such a way that the typical summer drought in Mediterranean climates was the critical factor affecting EO composition. Despite the high intrapopulational variability, significant increases of 1,8-cineole were found after the drought period (21-8 %-43.2%, in May, up to 42.6 %-68.5 % in August). On the other hand, individuals from one of the populations showed different profiles rich in linalool and camphor or sesquiterpenoid compounds as -cadinol.

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<p>La implicación del alumnado universitario en su evaluación puede contribuir notablemente a su formación académica. Por contra, profesorado y alumnado pueden mostrarse reticentes a esta práctica, por lo que consideramos interesante realizar experiencias que testen el acuerdo de las calificaciones dadas por alumnos y profesores, así como las causas de desacuerdo. En este trabajo se realizó una experiencia de coevaluación en un curso de Química Orgánica, para comparar las calificaciones otorgadas por alumnos y profesor. Los alumnos debían realizar un póster sobre una molécula de interés agroalimentario. Se diseñó una <em>Rating Scale</em> para establecer claramente los criterios de evaluación. En la mayoría de casos no se encontraron diferencias significativas en función del evaluador (p&gt;0.05), y solamente en dos ítems de la <em>Rating Scale</em>, los alumnos otorgaron calificaciones significativamente inferiores al profesor (p&lt;0.05). En ningún caso se observaron diferencias significativas en el sentido contrario. Los grados de acuerdo entre evaluadores fueron muy dependientes del ítem de la <em>Rating Scale</em>. Nuestros resultados demuestran cómo la coevaluación puede ser una excelente herramienta para suscitar reflexiones de gran relevancia educativa, de tal modo que podríamos plantearnos en función del aspecto a evaluar, la conveniencia de asignarlo a un único evaluador (profesor o alumnado).</p>