RESUMEN
Declining net primary production (NPP) with forest age is often attributed to a corresponding decline in gross primary production (GPP). We tested two hypotheses explaining the decline of GPP in ageing stands (14-115 years old) of Pinus taeda L.: (1) increasing N limitation limits photosynthetic capacity and thus decreases GPP with increasing age; and (2) hydraulic limitations increasingly induce stomatal closure, reducing GPP with increasing age. We tested these hypotheses using measurements of foliar nitrogen, photosynthesis, sap-flow and dendroclimatological techniques. Hypothesis (1) was not supported; foliar N retranslocation did not increase and declines were not observed in foliar N, leaf area per tree or photosynthetic capacity. Hypothesis (2) was supported; declines were observed in light-saturated photosynthesis, leaf- and canopy-level stomatal conductance, concentration of CO(2) inside leaf air-spaces (corroborated by an increase in wood δ(13) C) and specific leaf area (SLA), while stomatal limitation and the ratio of sapwood area (SA) to leaf area increased. The sensitivity of radial growth to inter-annual variation in temperature and drought decreased with age, suggesting that tree water use becomes increasingly conservative with age. We conclude that hydraulic limitation increasingly limits the photosynthetic rates of ageing loblolly pine trees, possibly explaining the observed reduction of NPP.
Asunto(s)
Fotosíntesis , Pinus taeda/fisiología , Estrés Fisiológico , Agua/fisiología , Dióxido de Carbono , Modelos Teóricos , Nitrógeno , Transpiración de Plantas , Factores de Tiempo , Estados UnidosRESUMEN
The prevalence of rather negative, or at best mixed, attitudes of doctors towards participation in management has been reported in several previous research studies. This paper adds to that growing empirical literature, but complements most of the previous studies by focusing on general medical practitioners in primary care rather than on doctors in secondary care settings. The focus for the study were doctors who had agreed to undertake the lead partner role in the now-defunct general practitioner fundholding initiative within the UK National Health Service. The findings indicate widespread reluctance but the presence of a minority of enthusiasts, which resonates with previous studies of secondary care settings. A finding of potential significance for future research and practice was that some doctors who were initially reluctant became more keen on undertaking a management role once they had experienced it.
Asunto(s)
Medicina Familiar y Comunitaria , Ejecutivos Médicos , Administración de la Práctica Médica , Medicina Estatal , Toma de Decisiones , Humanos , Pautas de la Práctica en Medicina , Reino UnidoRESUMEN
Sixty-six elderly patients with massive upper gastrointestinal bleeding were retrospectively analyzed. This study supports the concept of vigorous resuscitation, early diagnosis with fiberoptic endoscopy, and prompt surgical intervention in patients with continued bleeding. When this approach was utilized in a community hospital, operative mortality was reduced to 5 per cent.
Asunto(s)
Hemorragia Gastrointestinal/cirugía , Anciano , Úlcera Duodenal/complicaciones , Úlcera Duodenal/diagnóstico , Úlcera Duodenal/cirugía , Endoscopía , Femenino , Tecnología de Fibra Óptica , Hemorragia Gastrointestinal/diagnóstico , Humanos , Masculino , Persona de Mediana Edad , Úlcera Péptica/diagnóstico , Úlcera Péptica Hemorrágica/diagnóstico , Úlcera Péptica Hemorrágica/cirugía , Resucitación , Estudios Retrospectivos , Úlcera Gástrica/complicaciones , Úlcera Gástrica/cirugíaRESUMEN
We assessed optimal conditions for assay of porphobillinogen synthase (EC 4.2.1.24) activity in human blood containing abnormally high concentrations of lead. Zn2+and -SH, both required for complete activation of the enzyme, had additive effects. Using a modified method based on these studies, we found blood lead concentration to be strictly proportional to ln(activated/nonactivated) enzyme activity. One brand of commercially available "lead-free" tubes contained a substance that interfered with this relationship. In vitro studies, with the modified assay, showed ALAD to be activated by low concentrations but inactivated by high concentrations of Hg2+, Cd2+, and ethylenediaminetetraacetate. We fouund no genetically influenced differences among unexposed individuals when in(activated/nonactivated) enzyme activities were compared. The technique is suitable for use in screening for lead poisoning in humans.