Sleep-wake pattern of medical students: early versus late class starting time

The sleep-wake cycle of students is characterized by delayed onset, partial sleep deprivation and poor sleep quality. Like other circadian rhythms, the sleep-wake cycle is influenced by endogenous and environmental factors. The aim of the present study was to determine the effects of different class starting times on the sleep-wake pattern of 27 medical students. The data were collected during two medical school semesters having different class starting times. All subjects answered the Portuguese version of the Horne and Östberg Morningness/Eveningness Questionnaire, the Pittsburgh Sleep Quality Index (PSQI) and kept a sleep diary for two weeks during each semester. Better sleep quality (PSQI = 5.3 vs 3.4), delayed sleep onset (23:59 vs 0:54 h) and longer sleep duration (6 h and 55 min vs 7 h and 25 min) were observed with the late schedule. We also found reduced sleep durations during weekdays and extended sleep durations during weekends. This pattern was more pronounced during the semester with the early class schedule, indicating that the students were more sleep deprived when their classes began earlier in the morning. These results require further investigation regarding the temporal organization of our institutions.

Relationship between the prevalence of anti-glutamic acid decarboxylase autoantibodies and duration of type 1 diabetes mellitus in Brazilian patients

The objective of the present study was to determine whether the duration of disease has any influence on the prevalence of glutamic acid decarboxylase autoantibodies (GADA) in Brazilian patients with type 1 diabetes (T1D) and variable disease duration. We evaluated 83 patients with T1D. All participants were interviewed and blood was obtained for GADA measurement by a commercial radioimmunoassay (RSR Limited, Cardiff, UK). Four groups of patients were established according to disease duration: A) 1-5 years of disease (N = 24), B) 6-10 years of disease (N = 19), C) 11-15 years of disease (N = 25), and D) >15 years of disease (N = 15). GADA prevalence and its titers were determined in each group. GADA was positive in 38 patients (45.8%) and its frequency did not differ between the groups. The prevalence was 11/24 (45.8%), 8/19 (42.1%), 13/25 (52%), and 6/15 (40%) in groups A, B, C, and D, respectively (P = 0.874). Mean GADA titer was 12.54 ± 11.33 U/ml for the sample as a whole and 11.95 ± 11.8, 12.85 ± 12.07, 10.57 ± 8.35, and 17.45 ± 16.1 U/ml for groups A, B, C, and D, respectively (P = 0.686). Sex, age at diagnosis or ethnic background had no significant effect on GADA (+) frequency. In conclusion, in this transversal study, duration of disease did not affect significantly the prevalence of GADA or its titers in patients with T1D after one year of diagnosis. This was the first study to report this finding in the Brazilian population.

Correlations of circulating peptide YY and ghrelin with body weight, rate of weight gain, and time required to achieve the recommended daily intake in preterm infants

The objective was to elucidate the relationships between serum concentrations of the gut hormone peptide YY (PYY) and ghrelin and growth development in infants for potential application to the clinical observation index. Serum concentrations of PYY and ghrelin were measured using radioimmunoassay from samples collected at the clinic. For each patient, gestational age, birth weight, time required to return to birth weight, rate of weight gain, time required to achieve recommended daily intake (RDI) standards, time required for full-gastric feeding, duration of hospitalization, and time of administration of total parenteral nutrition were recorded. Serum PYY and ghrelin concentrations were significantly higher in the preterm group (N = 20) than in the full-term group (N = 20; P < 0.01). Within the preterm infant group, the serum concentrations of PYY and ghrelin on postnatal day (PND) 7 (ghrelin = 1485.38 ± 409.24; PYY = 812.37 ± 153.77 ng/L) were significantly higher than on PND 1 (ghrelin = 956.85 ± 223.09; PYY = 545.27 ± 204.51 ng/L) or PND 3 (ghrelin = 1108.44 ± 351.36; PYY = 628.96 ± 235.63 ng/L; P < 0.01). Both serum PYY and ghrelin concentrations were negatively correlated with body weight, and the degree of correlation varied with age. Serum ghrelin concentration correlated negatively with birth weight and positively with the time required to achieve RDI (P < 0.05). In conclusion, serum PYY and ghrelin concentrations reflect a negative energy balance, predict postnatal growth, and enable compensation. Further studies are required to elucidate the precise concentration and roles of PYY and ghrelin in newborns and to determine the usefulness of measuring these hormones in clinical practice.

Relative performance of the two hands in simple and choice reaction time tasks

There is evidence that the left hemisphere is more competent for motor control than the right hemisphere. This study investigated whether this hemispheric asymmetry is expressed in the latency/duration of sequential responses performed by the left and/or right hands. Thirty-two right-handed young adults (16 males, 16 females; 18-25 years old) were tested in a simple or choice reaction time task. They responded to a left and/or right visual target by moving their left and/or right middle fingers between two keys on each side of the midline. Right hand reaction time did not differ from left hand reaction time. Submovement times were longer for the right hand than the left hand when the response was bilateral. Pause times were shorter for the right hand than the left hand, both when the responses were unilateral or bilateral. Reaction time results indicate that the putatively more efficient response preparation by the left hemisphere motor mechanisms is not expressed behaviorally. Submovement time and pause time results indicate that the putatively more efficient response execution by the left hemisphere motor mechanisms is expressed behaviorally. In the case of the submovements, the less efficient motor control of the left hand would be compensated by a more intense attention to this hand.

Effects of disturbed blood flow during exercise on endothelial function: a time course analysis

This study aimed to examine the time course of endothelial function after a single handgrip exercise session combined with blood flow restriction in healthy young men. Nine participants (28±5.8 years) completed a single session of bilateral dynamic handgrip exercise (20 min with 60% of the maximum voluntary contraction). To induce blood flow restriction, a cuff was placed 2 cm below the antecubital fossa in the experimental arm. This cuff was inflated to 80 mmHg before initiation of exercise and maintained through the duration of the protocol. The experimental arm and control arm were randomly selected for all subjects. Brachial artery flow-mediated dilation (FMD) and blood flow velocity profiles were assessed using Doppler ultrasonography before initiation of the exercise, and at 15 and 60 min after its cessation. Blood flow velocity profiles were also assessed during exercise. There was a significant increase in FMD 15 min after exercise in the control arm compared with before exercise (64.09%±16.59%, P=0.001), but there was no change in the experimental arm (-12.48%±12.64%, P=0.252). FMD values at 15 min post-exercise were significantly higher for the control arm in comparison to the experimental arm (P=0.004). FMD returned to near baseline values at 60 min after exercise, with no significant difference between arms (P=0.424). A single handgrip exercise bout provoked an acute increase in FMD 15 min after exercise, returning to near baseline values at 60 min. This response was blunted by the addition of an inflated pneumatic cuff to the exercising arm.

Subjective, behavioural and electrophysiological measurements of the time-course and intensity of sleep inertia after a full night of sleep /

Several recent studies have described the period of impaired alertness and performance known as sleep inertia that occurs upon awakening from a full night of sleep. They report that sleep inertia dissipates in a saturating exponential manner, the exact time course being task dependent, but generally persisting for one to two hours. A number of factors, including sleep architecture, sleep depth and circadian variables are also thought to affect the duration and intensity. The present study sought to replicate their findings for subjective alertness and reaction time and also to examine electrophysiological changes through the use of event-related potentials (ERPs). Secondly, several sleep parameters were examined for potential effects on the initial intensity of sleep inertia. Ten participants spent two consecutive nights and subsequent mornings in the sleep lab. Sleep architecture was recorded for a fiiU nocturnal episode of sleep based on participants' habitual sleep patterns. Subjective alertness and performance was measured for a 90-minute period after awakening. Alertness was measured every five minutes using the Stanford Sleepiness Scale (SSS) and a visual analogue scale (VAS) of sleepiness. An auditory tone also served as the target stimulus for an oddball task designed to examine the NlOO and P300 components ofthe ERP waveform. The five-minute oddball task was presented at 15-minute intervals over the initial 90-minutes after awakening to obtain six measures of average RT and amplitude and latency for NlOO and P300. Standard polysomnographic recording were used to obtain digital EEG and describe the night of sleep. Power spectral analyses (FFT) were used to calculate slow wave activity (SWA) as a measure of sleep depth for the whole night, 90-minutes before awakening and five minutes before awakening.

Determinants of bankruptcy protection duration for Canadian firms

The present thesis examines the determinants of the bankruptcy protection duration for Canadian firms. Using a sample of Canadian firms that filed for bankruptcy protection between the calendar years 1992 and 2009, we fmd that the firm age, the industry adjusted operating margin, the default spread, the industrial production growth rate or the interest rate are influential factors on determining the length of the protection period. Older firms tend to stay longer under protection from creditors. As older firms have more complicated structures and issues to settle, the risk of exiting soon the protection (the hazard rate) is small. We also find that firms that perform better than their benchmark as measured by the industry they belong to, tend to leave quickly the bankruptcy protection state. We conclude that the fate of relatively successful companies is determined faster. Moreover, we report that it takes less time to achieve a final solution to firms under bankrupt~y when the default spread is low or when the appetite for risk is high. Conversely, during periods of high default spreads and flight for quality, it takes longer time to resolve the bankruptcy issue. This last finding may suggest that troubled firms should place themselves under protection when spreads are low. However, this ignores the endogeneity issue: high default spread may cause and incidentally reflect higher bankruptcy rates in the economy. Indeed, we find that bankruptcy protection is longer during economic downturns. We explain this relation by the natural increase in default rate among firms (and individuals) during economically troubled times. Default spreads are usually larger during these harsh periods as investors become more risk averse since their wealth shrinks. Using a Log-logistic hazard model, we also fmd that firms that file under the Companies' Creditors Arrangement Act (CCAA) protection spend longer time restructuring than firms that filed under the Bankruptcy and Insolvency Act (BIA). As BIA is more statutory and less flexible, solutions can be reached faster by court orders.

Investigation of the Time Dependent Influence of Extracellular Osmotic Stress on Protein Turnover in Skeletal Muscle Cells

Acute alterations in cell volume can substantively modulate subsequent metabolism of substrates. However, how such alterations in skeletal muscle modulate protein metabolism is limited. The purpose of this study was to determine the time dependent influence of extracellular osmotic stress on protein turnover in skeletal muscle cells. L6 cells were incubated in hyperosmotic (HYPER; 425.3 ± 1.8mmol/kg), hypo-osmotic (HYPO; 235.4 ± 1.0mmol/kg) or control (CON; 333.5 ± 1.4mmol/kg) media for 4, 8, 12, or 24hrs. During the final 4hrs, incorporation of L-[ring-3,5-3H]-tyrosine was measured to estimate protein synthesis. Western blotting measured markers of protein synthesis and degradation. No differences were observed in any outcomes except p70S6K phosphorylation whereby HYPO was lower (p<0.05) than CON and HYPER; which remained similar except for a large increase at 8hrs for HYPER. These findings suggest that regardless of duration, extracellular osmotic stress does not significantly affect protein metabolism in L6 cells.

New statistical methods to assess the effect of time-dependent exposures in case-control studies

Contexte. Les études cas-témoins sont très fréquemment utilisées par les épidémiologistes pour évaluer l’impact de certaines expositions sur une maladie particulière. Ces expositions peuvent être représentées par plusieurs variables dépendant du temps, et de nouvelles méthodes sont nécessaires pour estimer de manière précise leurs effets. En effet, la régression logistique qui est la méthode conventionnelle pour analyser les données cas-témoins ne tient pas directement compte des changements de valeurs des covariables au cours du temps. Par opposition, les méthodes d’analyse des données de survie telles que le modèle de Cox à risques instantanés proportionnels peuvent directement incorporer des covariables dépendant du temps représentant les histoires individuelles d’exposition. Cependant, cela nécessite de manipuler les ensembles de sujets à risque avec précaution à cause du sur-échantillonnage des cas, en comparaison avec les témoins, dans les études cas-témoins. Comme montré dans une étude de simulation précédente, la définition optimale des ensembles de sujets à risque pour l’analyse des données cas-témoins reste encore à être élucidée, et à être étudiée dans le cas des variables dépendant du temps. Objectif: L’objectif général est de proposer et d’étudier de nouvelles versions du modèle de Cox pour estimer l’impact d’expositions variant dans le temps dans les études cas-témoins, et de les appliquer à des données réelles cas-témoins sur le cancer du poumon et le tabac. Méthodes. J’ai identifié de nouvelles définitions d’ensemble de sujets à risque, potentiellement optimales (le Weighted Cox model and le Simple weighted Cox model), dans lesquelles différentes pondérations ont été affectées aux cas et aux témoins, afin de refléter les proportions de cas et de non cas dans la population source. Les propriétés des estimateurs des effets d’exposition ont été étudiées par simulation. Différents aspects d’exposition ont été générés (intensité, durée, valeur cumulée d’exposition). Les données cas-témoins générées ont été ensuite analysées avec différentes versions du modèle de Cox, incluant les définitions anciennes et nouvelles des ensembles de sujets à risque, ainsi qu’avec la régression logistique conventionnelle, à des fins de comparaison. Les différents modèles de régression ont ensuite été appliqués sur des données réelles cas-témoins sur le cancer du poumon. Les estimations des effets de différentes variables de tabac, obtenues avec les différentes méthodes, ont été comparées entre elles, et comparées aux résultats des simulations. Résultats. Les résultats des simulations montrent que les estimations des nouveaux modèles de Cox pondérés proposés, surtout celles du Weighted Cox model, sont bien moins biaisées que les estimations des modèles de Cox existants qui incluent ou excluent simplement les futurs cas de chaque ensemble de sujets à risque. De plus, les estimations du Weighted Cox model étaient légèrement, mais systématiquement, moins biaisées que celles de la régression logistique. L’application aux données réelles montre de plus grandes différences entre les estimations de la régression logistique et des modèles de Cox pondérés, pour quelques variables de tabac dépendant du temps. Conclusions. Les résultats suggèrent que le nouveau modèle de Cox pondéré propose pourrait être une alternative intéressante au modèle de régression logistique, pour estimer les effets d’expositions dépendant du temps dans les études cas-témoins

Limited sampling strategies for estimation of cyclosporine exposure in pediatric hematopoietic stem cell transplant recipients : methodological improvement and introduction of sampling time deviation analysis.

Le suivi thérapeutique est recommandé pour l’ajustement de la dose des agents immunosuppresseurs. La pertinence de l’utilisation de la surface sous la courbe (SSC) comme biomarqueur dans l’exercice du suivi thérapeutique de la cyclosporine (CsA) dans la transplantation des cellules souches hématopoïétiques est soutenue par un nombre croissant d’études. Cependant, pour des raisons intrinsèques à la méthode de calcul de la SSC, son utilisation en milieu clinique n’est pas pratique. Les stratégies d’échantillonnage limitées, basées sur des approches de régression (R-LSS) ou des approches Bayésiennes (B-LSS), représentent des alternatives pratiques pour une estimation satisfaisante de la SSC. Cependant, pour une application efficace de ces méthodologies, leur conception doit accommoder la réalité clinique, notamment en requérant un nombre minimal de concentrations échelonnées sur une courte durée d’échantillonnage. De plus, une attention particulière devrait être accordée à assurer leur développement et validation adéquates. Il est aussi important de mentionner que l’irrégularité dans le temps de la collecte des échantillons sanguins peut avoir un impact non-négligeable sur la performance prédictive des R-LSS. Or, à ce jour, cet impact n’a fait l’objet d’aucune étude. Cette thèse de doctorat se penche sur ces problématiques afin de permettre une estimation précise et pratique de la SSC. Ces études ont été effectuées dans le cadre de l’utilisation de la CsA chez des patients pédiatriques ayant subi une greffe de cellules souches hématopoïétiques. D’abord, des approches de régression multiple ainsi que d’analyse pharmacocinétique de population (Pop-PK) ont été utilisées de façon constructive afin de développer et de valider adéquatement des LSS. Ensuite, plusieurs modèles Pop-PK ont été évalués, tout en gardant à l’esprit leur utilisation prévue dans le contexte de l’estimation de la SSC. Aussi, la performance des B-LSS ciblant différentes versions de SSC a également été étudiée. Enfin, l’impact des écarts entre les temps d’échantillonnage sanguins réels et les temps nominaux planifiés, sur la performance de prédiction des R-LSS a été quantifié en utilisant une approche de simulation qui considère des scénarios diversifiés et réalistes représentant des erreurs potentielles dans la cédule des échantillons sanguins. Ainsi, cette étude a d’abord conduit au développement de R-LSS et B-LSS ayant une performance clinique satisfaisante, et qui sont pratiques puisqu’elles impliquent 4 points d’échantillonnage ou moins obtenus dans les 4 heures post-dose. Une fois l’analyse Pop-PK effectuée, un modèle structural à deux compartiments avec un temps de délai a été retenu. Cependant, le modèle final - notamment avec covariables - n’a pas amélioré la performance des B-LSS comparativement aux modèles structuraux (sans covariables). En outre, nous avons démontré que les B-LSS exhibent une meilleure performance pour la SSC dérivée des concentrations simulées qui excluent les erreurs résiduelles, que nous avons nommée « underlying AUC », comparée à la SSC observée qui est directement calculée à partir des concentrations mesurées. Enfin, nos résultats ont prouvé que l’irrégularité des temps de la collecte des échantillons sanguins a un impact important sur la performance prédictive des R-LSS; cet impact est en fonction du nombre des échantillons requis, mais encore davantage en fonction de la durée du processus d’échantillonnage impliqué. Nous avons aussi mis en évidence que les erreurs d’échantillonnage commises aux moments où la concentration change rapidement sont celles qui affectent le plus le pouvoir prédictif des R-LSS. Plus intéressant, nous avons mis en exergue que même si différentes R-LSS peuvent avoir des performances similaires lorsque basées sur des temps nominaux, leurs tolérances aux erreurs des temps d’échantillonnage peuvent largement différer. En fait, une considération adéquate de l'impact de ces erreurs peut conduire à une sélection et une utilisation plus fiables des R-LSS. Par une investigation approfondie de différents aspects sous-jacents aux stratégies d’échantillonnages limités, cette thèse a pu fournir des améliorations méthodologiques notables, et proposer de nouvelles voies pour assurer leur utilisation de façon fiable et informée, tout en favorisant leur adéquation à la pratique clinique.

Space and time characterization of laser-induced plasmas for applications in chemical analysis and thin film deposition

Après des décennies de développement, l'ablation laser est devenue une technique importante pour un grand nombre d'applications telles que le dépôt de couches minces, la synthèse de nanoparticules, le micro-usinage, l’analyse chimique, etc. Des études expérimentales ainsi que théoriques ont été menées pour comprendre les mécanismes physiques fondamentaux mis en jeu pendant l'ablation et pour déterminer l’effet de la longueur d'onde, de la durée d'impulsion, de la nature de gaz ambiant et du matériau de la cible. La présente thèse décrit et examine l'importance relative des mécanismes physiques qui influencent les caractéristiques des plasmas d’aluminium induits par laser. Le cadre général de cette recherche forme une étude approfondie de l'interaction entre la dynamique de la plume-plasma et l’atmosphère gazeuse dans laquelle elle se développe. Ceci a été réalisé par imagerie résolue temporellement et spatialement de la plume du plasma en termes d'intensité spectrale, de densité électronique et de température d'excitation dans différentes atmosphères de gaz inertes tel que l’Ar et l’He et réactifs tel que le N2 et ce à des pressions s’étendant de 10‾7 Torr (vide) jusqu’à 760 Torr (pression atmosphérique). Nos résultats montrent que l'intensité d'émission de plasma dépend généralement de la nature de gaz et qu’elle est fortement affectée par sa pression. En outre, pour un délai temporel donné par rapport à l'impulsion laser, la densité électronique ainsi que la température augmentent avec la pression de gaz, ce qui peut être attribué au confinement inertiel du plasma. De plus, on observe que la densité électronique est maximale à proximité de la surface de la cible où le laser est focalisé et qu’elle diminue en s’éloignant (axialement et radialement) de cette position. Malgré la variation axiale importante de la température le long du plasma, on trouve que sa variation radiale est négligeable. La densité électronique et la température ont été trouvées maximales lorsque le gaz est de l’argon et minimales pour l’hélium, tandis que les valeurs sont intermédiaires dans le cas de l’azote. Ceci tient surtout aux propriétés physiques et chimiques du gaz telles que la masse des espèces, leur énergie d'excitation et d'ionisation, la conductivité thermique et la réactivité chimique. L'expansion de la plume du plasma a été étudiée par imagerie résolue spatio-temporellement. Les résultats montrent que la nature de gaz n’affecte pas la dynamique de la plume pour des pressions inférieures à 20 Torr et pour un délai temporel inférieur à 200 ns. Cependant, pour des pressions supérieures à 20 Torr, l'effet de la nature du gaz devient important et la plume la plus courte est obtenue lorsque la masse des espèces du gaz est élevée et lorsque sa conductivité thermique est relativement faible. Ces résultats sont confirmés par la mesure de temps de vol de l’ion Al+ émettant à 281,6 nm. D’autre part, on trouve que la vitesse de propagation des ions d’aluminium est bien définie juste après l’ablation et près de la surface de la cible. Toutefois, pour un délai temporel important, les ions, en traversant la plume, se thermalisent grâce aux collisions avec les espèces du plasma et du gaz.

The persistent colombian conflict bubnational analysis of the duration of violence

The growing empirical literature on the analysis of civil war has recently included the study of conflict duration at the cross-country level. This paper presents, for the first time, a within-country analysis of the determinants of violence duration. I focus on the experience of the Colombian armed conflict. While the conflict has been active for about five decades, local violence ebbs and flows and areas experiencing continuous conflict coexist with places that have been able to resile and where violence is mostly absent. I examine a wide range of factors potentially associated with violence duration at the municipal level, including scale variables, geographical conditions, economic and social variables, institutions and state presence, inequality, government intervention, and victimization variables. I characterize a few variables robustly correlated with the persistence of localized conflict, both across specifications and using different econometric models of duration analysis.

The influence of time on lead toxicity and bioaccumulation determined by the OECD earthworm toxicity test

Internationally agreed standard protocols for assessing chemical toxicity of contaminants in soil to worms assume that the test soil does not need to equilibrate with the chemical to be tested prior to the addition of the test organisms and that the chemical will exert any toxic effect upon the test organism within 28 days. Three experiments were carried out to investigate these assumptions. The first experiment was a standard toxicity test where lead nitrate was added to a soil in solution to give a range of concentrations. The mortality of the worms and the concentration of lead in the survivors were determined. The LC(50)s for 14 and 28 days were 5311 and 5395 mug(Pb) g(soil)(-1) respectively. The second experiment was a timed lead accumulation study with worms cultivated in soil containing either 3000 or 5000 mug(Pb) g(soil)(-1). The concentration of lead in the worms was determined at various sampling times. Uptake at so' Sol both concentrations was linear with time. Worms in the 5000 mug g(-1) soil accumulated lead at a faster rate (3.16 mug Pb g(tissue)(-1) day(-1)) tiss than those in the 3000 mug g(-1) soil (2.21 mug Pb-tissue g(-1) day(-1)). The third experiment was a timed experiment with worms cultivated in tiss soil containing 7000 mugPb g(soil)(-1). Soil and lead nitrate solution were mixed and stored at 20 degreesC. Worms were added at various times over a 35-day period. The time to death increased from 23 h, when worms were added directly after the lead was added to the soil, to 67 It when worms were added after the soil had equilibrated with the lead for 35 days. In artificially Pb-amended soils the worms accumulate Pb over the duration of their exposure to the Pb. Thus time limited toxicity tests may be terminated before worm body load has reached a toxic level. This could result in under-estimates of the toxicity of Pb to worms. As the equilibration time of artificially amended Pb-bearing soils increases the bioavailability of Pb decreases. Thus addition of worms shortly after addition of Pb to soils may result in the over-estimate of Pb toxicity to worms. The current OECD acute worm toxicity test fails to take these two phenomena into account thereby reducing the environmental relevance of the contaminant toxicities it is used to calculate. (C) 2002 Elsevier Science Ltd. All rights reserved.

Climate change and the flowering time of annual crops

Crop production is inherently sensitive to variability in climate. Temperature is a major determinant of the rate of plant development and, under climate change, warmer temperatures that shorten development stages of determinate crops will most probably reduce the yield of a given variety. Earlier crop flowering and maturity have been observed and documented in recent decades, and these are often associated with warmer (spring) temperatures. However, farm management practices have also changed and the attribution of observed changes in phenology to climate change per se is difficult. Increases in atmospheric [CO2] often advance the time of flowering by a few days, but measurements in FACE (free air CO2 enrichment) field-based experiments suggest that elevated [CO2] has little or no effect on the rate of development other than small advances in development associated with a warmer canopy temperature. The rate of development (inverse of the duration from sowing to flowering) is largely determined by responses to temperature and photoperiod, and the effects of temperature and of photoperiod at optimum and suboptimum temperatures can be quantified and predicted. However, responses to temperature, and more particularly photoperiod, at supraoptimal temperature are not well understood. Analysis of a comprehensive data set of time to tassel initiation in maize (Zea mays) with a wide range of photoperiods above and below the optimum suggests that photoperiod modulates the negative effects of temperature above the optimum. A simulation analysis of the effects of prescribed increases in temperature (0-6 degrees C in + 1 degrees C steps) and temperature variability (0% and + 50%) on days to tassel initiation showed that tassel initiation occurs later, and variability was increased, as the temperature exceeds the optimum in models both with and without photoperiod sensitivity. However, the inclusion of photoperiod sensitivity above the optimum temperature resulted in a higher apparent optimum temperature and less variability in the time of tassel initiation. Given the importance of changes in plant development for crop yield under climate change, the effects of photoperiod and temperature on development rates above the optimum temperature clearly merit further research, and some of the knowledge gaps are identified herein.

Duration from vine emergence to flowering suggests a long-day or rate of change of photoperiod response in white yam (Dioscorea rotundata Poir.)

The objective of this study was to quantify the effect of photoperiod on the duration from vine (shoot) emergence to flowering in white or Guinea yam (Dioscorea rotundata). The duration from vine emergence to flowering in two clonal varieties of yam (TDr 131 and TDr 99-9) was recorded at 10 different sowing dates/locations in Nigeria. Durations to flowering varied from 40 to > 88 days. Mean daily temperature and photoperiod between vine emergence and flowering varied from 25 to 27 degrees C and 13.1 to 13.4 h day(-1), respectively. Both clones had similar responses to temperature, with base and optimum temperatures of 12 and 25-27 degrees C, respectively. Thermal durations to flowering were strongly related (r(2) > 0.75-0.83) to absolute photoperiod (h) at vine emergence as well as to rate of change of photoperiod (s day(-1)) at vine emergence. The response to absolute photoperiod suggests that white yams are quantitative LDPs, flowering sooner in long than short days. Yams also flowered earlier when the rate of change of photoperiod was positive but small, or was negative. It is suggested that yams may use a combination of photoperiod and rate of change in order to fine tune flowering time. (c) 2006 Elsevier B.V. All rights reserved.