Weed seed dormancy as a survival mechanism: brief review

Um dos principais mecanismos de sobrevivência das plantas daninhas em ambientes constantemente perturbados é a alta produção de sementes. Essas possuem geralmente algum mecanismo de dormência, o qual contribui para a perpetuação de espécies interferentes nos cultivos agrícolas. A dormência pode ser caracterizada pela ausência temporária da germinação, mesmo quando em condições adequadas de sua ocorrência. Isso permite que inúmeras espécies vegetais sobrevivam às adversidades, sobretudo aquelas que dificultam ou impeçam o seu crescimento vegetativo e reprodutivo. As causas da dormência são provenientes de dois mecanismos básicos, sendo o primeiro relacionado a eventos internos das sementes (embrião) e o segundo, às características externas (tegumento, endosperma ou as barreiras impostas pelo fruto). Conceitualmente, a dormência pode ser distinguida em dois tipos: dormência primária (quando os mecanismos de dormência ocorrem ainda na planta-mãe) e secundária (quando os mecanismos de estabelecimento da dormência ocorrem após a dispersão das sementes). A ocorrência desses dois tipos de dormência é comum em plantas daninhas. A sua alternância ou ciclagem garante o fluxo de germinação destas espécies, o qual depende das características iniciais durante a formação das sementes (dormência primária) e, posteriormente, das condições ambientais (dormência secundária). Todavia, muitos são os mecanismos que coordenam a dormência, sendo a distinção destes ainda controversos. Nesse sentido, este estudo tem por objetivo abordar alguns dos principais conceitos e mecanismos de dormência em plantas daninhas, com intuito de contribuir e estimular as pesquisas, ainda escassas, nessa área.The high production of seeds in constantly disturbed environments is one of the main mechanisms of weeds survival. These seeds have usually some dormancy mechanism which constitutes weed species perpetuation in the crops. Seed dormancy can be characterized by temporally absence of the germination capacity, even though the seeds have satisfactorily conditions to germinate, thus allowing species survival under adversities, mainly those that make it difficult or hinder vegetative and reproductive growth. The causes of dormancy stem from two basic mechanisms: the first is related to inner seed events (embryo) and the second to outer characteristics in the seeds (tegument, endosperm or fruit barriers). Conceptually, dormancy can be classified as primary dormancy (when the mechanisms occur in plants-mother) and secondary dormancy (when the mechanisms causing dormancy occur after seed dispersion). These types of dormancy occur normally in weeds. Their alternation or cycling ensures germination flow these species, which depends on the characteristics occurring at the initial stages of seed formation (primary dormancy), and later, on the environmental conditions (secondary dormancy). However, many mechanisms coordinate dormancy, with the differences among them being still controversial. Thus, this study aimed to approach some of the main concepts and mechanisms in weed dormancy, in order to contribute and stimulate research which is still scarce in this area

Association Between Ventilation Index And Time On Mechanical Ventilation In Infants With Acute Viral Bronchiolitis

Objective: To evaluate the association between time on mechanical ventilation and anthropometric, clinical and pulmonary function variables, measured early, in infants on invasive mechanical ventilation with acute respiratory failure due to viral bronchiolitis, and the temporal progression of variables with significant correlations. Methods: Twenty-nine infants admitted to the pediatric intensive care unit of UNICAMP university hospital were studied. Acute viral bronchiolitis was defined according to clinical and radiological criteria. Children with chronic diseases and those that were hemodynamically unstable were excluded. All measurements were taken after 24 to 72 hours' mechanical ventilation, using volumetric capnography and blood gas analysis. Mechanical ventilation time was divided into: ≤ 7 days and > 7 days. Association between time on mechanical ventilation and the variables analyzed was determined by Spearman's Correlation Coefficient (r s). Results: Time on mechanical ventilation showed a significant positive correlation with PaCO 2 (r s = 0.45, p = 0.01) and ventilation index (r s = 0.51, p = 0.005), and a negative correlation with pH (r s = -0.40, p = 0.03). Ventilation indices of 37, measured between day one and day five, was associated with a progressively increased risk of more than 7 days on mechanical ventilation (OR = 4.2 on the first day to 15.71 on the fourth day). Conclusions: Ventilation index, PaCO 2 and pH, measured early, were associated with prolonged mechanical ventilation, reflecting the severity of ventilatory disturbance and the need for support. Copyright © 2005 by Sociedade Brasileira de Pediatria.816466470Shay, D.K., Holman, R.C., Newman, R.D., Liu, L.L., Stout, J.W., Anderson, L.J., Bronchiolitis-associated hospitalizations among US children, 1980-1996 (1999) JAMA, 282, pp. 1440-1446Torres, A., Gatell, J.M., Aznar, E., El-Ebiary, M., Puig De La Bellacasa, J., Gonzalez, J., Re-intubation increases the risk for nosocomial pneumonia in patients needing mechanical ventilation (1995) Am J Respir Crit Care Med, 152, pp. 137-141Esteban, A., Alia, I., Gordo, F., Fernandez, R., Solsona, J.F., Vallverdu, I., Extubation outcome after spontaneous breathing trials with T-tube or pressure support ventilation (1997) Am J Respir Crit Care Med, 156, pp. 459-465. , The Spanish Lung Failure Collaborative GroupBont, L., Kavelaars, A., Heijnen, C.J., Van Vught, A.J., Kimpen, J.L., Monocyte interleukin-12 production is inversely related to duration of respiratory failure in respiratory syncytial virus bronchiolitis (2000) J Infect Dis, 181, pp. 1772-1775Tasker, R.C., Gordon, I., Kiff, K., Time course of severe respiratory syncytial virus infection in mechanically ventilated infants (2000) Acta Paediatr, 89, pp. 938-941Arnold, J.H., Thompson, J.E., Arnold, L.W., Single breath CO 2 analysis: Description and validation of a method (1996) Crit Care Med, 24, pp. 96-102Riou, Y., Leclerc, F., Neve, V., Dupuy, L., Noizet, O., Leteurtre, S., Reproducibility of the respiratory dead space measurements in mechanically ventilated children using the CO 2SMO monitor (2004) Intensive Care Med, 30, pp. 1461-1467Hubble, C.L., Gentile, M.A., Tripp, D.S., Craig, D.M., Meliones, J.N., Cheifetz, I.M., Deadspace to tidal volume ratio predicts successful extubation in infants and children (2000) Crit Care Med, 28, pp. 2034-2040Main, E., Stocks, J., The influence of physiotherapy and suction on respiratory deadspace in ventilated children (2004) Intensive Care Med, 30, pp. 1152-1159Law, B.J., Carbonell-Estrany, X., Simoes, E.A., An update on respiratory syncytial virus epidemiology: A developed country perspective (2002) Respir Med, 96 (SUPPL. B), pp. S1-7Davison, C., Ventre, K.M., Luchetti, M., Randolph, A.G., Efficacy of interventions for bronchiolitis in critically ill infants: A systematic review and meta-analysis (2004) Pediatr Crit Care Med, 5, pp. 482-489Frankel, L.R., Lewiston, N.J., Smith, D.W., Stevenson, D.K., Clinical observations on mechanical ventilation for respiratory failure in bronchiolitis (1986) Pediatr Pulmonol, 2, pp. 307-311Wang, E.E., Law, B.J., Boucher, F.D., Stephens, D., Robinson, J.L., Dobson, S., Pediatric Investigators Collaborative Network on Infections in Canada (PICNIC) study of admission and management variation in patients hospitalized with respiratory syncytial viral lower respiratory tract infection (1996) J Pediatr, 129, pp. 390-395Paret, G., Ziv, T., Barzilai, A., Bem-Abraham, R., Vardi, A., Manisterski, Y., Ventilation index and outcome in children with acute respiratory distress syndrome (1998) Pediatr Pulmonol, 26, pp. 125-12

Constant Curvature Coefficients and Exact Solutions in Fractional Gravity and Geometric Mechanics

We study fractional configurations in gravity theories and Lagrange mechanics. The approach is based on Caputo fractional derivative which gives zero for actions on constants. We elaborate fractional geometric models of physical interactions and we formulate a method of nonholonomic deformations to other types of fractional derivatives. The main result of this paper consists in a proof that for corresponding classes of nonholonomic distributions a large class of physical theories are modelled as nonholonomic manifolds with constant matrix curvature. This allows us to encode the fractional dynamics of interactions and constraints into the geometry of curve flows and solitonic hierarchies.Comment: latex2e, 11pt, 27 pages, the variant accepted to CEJP; added and up-dated reference