Possible interference of fertilization in the natural recovery of a declining sugar maple stand in southern Quebec

A five year study was conducted in a 100–120 year old even-aged sugar maple stand in southern Quebec (46°07N 73° 56W; 305 m altitude) to explore the effect of different fertilization formulations aimed at 1) correcting the most common nutrient deficiencies observed in declining maple stands (K and Mg), 2) decreasing soil acidity, and 3) simulating enrichment with atmospheric N. Seven fertilizer mixtures were applied in the spring of 1987: 400 kg ha^-1 of K₂SO₄, CaCO₃, CaMg(CO₃)₂, (NH₄)₂SO₄, complete fertilizer (Maplegro) and 800 kg ha^-1 of an equal mixture of K₂SO₄+CaCO₃ or K₂SO₄+CaMg(CO₃)₂. The site was divided into twenty-four 25×25 m plots and treatments including control were replicated three times. Leaves and soils (organic and mineral) were sampled in 1987, 1988 and 1991. Trees were cored at 1.2 m to measure their response in diameter growth. The application of K₂SO₄+CaMg(CO₃)₂ was the only treatment that significantly increased (+13%) the average growth rate over the five year period after fertilization. The application of (NH₄)₂SO₄, Maplegro, CaMg(CO₃)₂ and K₂SO₄ reduced growth relative to the control for the five year period by 29, 24, 20 and 12 %, respectively. Positive and negative effects on growth can be explained mainly in terms of changes in leaf K. Both the application of Maplegro and (NH₄)₂SO₄ increased soil P availability. Overall, the rate of growth showed a cubic pattern of change over the 5 year period with peaks in 1988 and 1991. Trees in control plots went from a limiting foliar status of Ca and Mg, and surplus N in 1987 to a surplus of Ca and Mg, and lower N concentration in 1991. Our results suggest that nutrient deficiencies observed at our site were associated with a disturbance of the biogeochemical cycle of nutrients rather than soil nutrient depletion.Abbreviations BS base saturation - CEC cation exchange capacity - DRIS diagnosis and recommendation integrated system     

Sex determination and population biology in the hymenoptera

The Hymenoptera (ants, bees, wasps and sawflies) display a great variety of social systems and sex ratios and have played a key role in the development and testing of many evolutionary models. Traditionally, considerable emphasis was placed on the fact that hymenopterans have haploid males and diploid females but it is now clear that many species also regularly produce sterile, diploid males. Recent studies explore the diverse ways in which production of these diploid males influences selection on mating systems, sex ratios and social behaviour.     

Identification of Trans-Acting Genes Necessary for Centromere Function in Drosophila Melanogaster Using Centromere-Defective Minichromosomes

Deletions in the Drosophila minichromosome Dp1187 were used to investigate the genetic interactions of trans-acting genes with the centromere. Mutations in several genes known to have a role in chromosome inheritance were shown to have dominant effects on the stability of minichromosomes with partially defective centromeres. Heterozygous mutations in the ncd and klp3A kinesin-like protein genes strongly reduced the transmission of minichromosomes missing portions of the genetically defined centromere, but had little effect on the transmission of minichromosomes with intact centromeres. Using this approach, ncd and klp3A were shown to require only the centromeric region of the chromosome for their roles in chromosome segregation. Increased gene dosage also affected minichromosome transmission and was used to demonstrate that the nod kinesin-like protein gene interacts genetically with the centromere, in addition to interacting with extracentromeric regions as demonstrated previously. The results presented in this study strongly suggest that dominant genetic interactions between mutations and centromere-defective minichromosomes could be used effectively to identify novel genes necessary for centromere function.     

Novel β-Secretase Cleavage of β-Amyloid Precursor Protein in the Endoplasmic Reticulum/Intermediate Compartment of NT2N Cells

Previous studies have demonstrated that NT2N neurons derived from a human embryonal carcinoma cell line (NT2) constitutively process the endogenous wild-type β-amyloid precursor protein (APP) to amyloid β peptide in an intracellular compartment. These studies indicate that other proteolytic fragments generated by intracellular processing must also be present in these cells. Here we show that the NH₂-terminal fragment of APP generated by β-secretase cleavage (APPβ) is indeed produced from the endogenous full length APP (APP_FL). Pulse–chase studies demonstrated a precursor–product relationship between APP_FL and APPβ as well as intracellular and secreted APPβ fragments. In addition, trypsin digestion of intact NT2N cells at 4°C did not abolish APPβ recovered from the cell lysates. Furthermore, the production of intracellular APPβ from wild-type APP appears to be a unique characteristic of postmitotic neurons, since intracellular APPβ was not detected in several non-neuronal cell lines. Significantly, production of APPβ occurred even when APP was retained in the ER/ intermediate compartment by inhibition with brefeldin A, incubation at 15°C, or by expression of exogenous APP bearing the dilysine ER retrieval motif.     

Ditylenchus dipsaci Infestation of Trifolium repens. I. Temperature Effects,Seedling Invasion,and a Field Survey

Rates of development of stem nematode (Ditylenchus dipsaci) in white clover (Trifolium repens) seedlings were found to be linearly related to temperature. Basal developmental temperature (T_b) was 3 °C, and the thermal constant (S) for development of gravid adult females from freshly laid eggs was 270 accumulated day-degrees above the T_b. Only 12% at 20 °C and 4% at 4 °C of the gravid female nematodes inoculated into seedling axils successfully penetrated seedling epidermis. These nematodes slowly migrated within the seedling and after a lag of 5 days at 20 °C started to lay eggs. The maximal rate of egg production was temperature-dependent, being 0.8 and 3.1 eggs female⁻¹ day⁻¹ at 10 and 20 °C, respectively. Nematodes emigrated rapidly from infested stolons when they were immersed in water, with rates being highest at 25 °C and lowest at 4 °C. The sensitivity to temperature of many of the parameters that govern nematode population dynamics indicates that climatic changes will have a marked effect upon this host-parasite system. A study of infested stolons from the field indicated that nematode numbers increased up to 3,000 or more before tissue senesence, triggered by nematode damage, caused a mass emigration of nematodes from the stolon.     

Alterations in Glutamate Transporter Protein Levels in Kindling-Induced Epilepsy

Abstract: There is increasing evidence that levels of glutamate are elevated in certain brain regions immediately prior to and during induction and propagation of seizures. Modulation of high-affinity glutamate uptake is a potential mechanism responsible for the elevated levels observed with seizures. To date, three distinct Na⁺-dependent glutamate transporters have been cloned from rat and rabbit: GLT-1, GLAST, and EAAC-1. We performed a series of experiments to determine whether levels of these transporters are altered in amygdala-kindled rats. Levels of GLT-1, GLAST, and EAAC-1 were examined in three brain regions (hippocampus, piriform cortex/amygdala, and limbic forebrain) by quantitative immunoblotting using subtype-specific antibodies. GLAST protein was down-regulated in the piriform cortex/amygdala region of kindled rats as early as 24 h after one stage 3 seizure and persisting through multiple stage 5 seizures. In contrast, kindling induced an increase in EAAC-1 levels in piriform cortex/amygdala and hippocampus once the animals had reached the stage 5 level. No changes in GLT-1 were observed in any region examined. Changes in transporter levels could contribute to the changes in glutamate levels seen with kindling.     

Relationship between heterogeneous changes in airway morphometry and lung resistance and elastance

Lutchen, Kenneth R., and Heather Gillis. Relationshipbetween heterogeneous changes in airway morphometry and lung resistanceand elastance. J. Appl. Physiol.83(4): 1192-1201, 1997.We present a dog lung model to predictthe relation between inhomogeneous changes in airway morphometry andlung resistance (RL) andelastance (EL) for frequenciessurrounding typical breathing rates. TheRL andEL were sensitive in distinctways to two forms of peripheral constriction. First, when there is alarge and homogeneous constriction, theRL increases uniformly over thefrequency range. The EL israther unaffected below 1 Hz but then increases with frequencies up to5 Hz. This increase is caused by central airway wallshunting. Second, the RL andEL are extremely sensitive to mild inhomogeneous constriction in which a few highly constricted ornearly closed airways occur randomly throughout theperiphery. This results in extreme increases in the levelsand frequency dependence of RLand EL but predominantly attypical breathing rates (<1 Hz). Conversely, theRL andEL are insensitive to highly inhomogeneous airway constriction that does not produce any nearly closed airways. Similarly, alterations in theRL andEL due to central airway wallshunting are not likely until the preponderance of the peripheryconstricts substantially. The RLand EL spectra are far moresensitive to these two forms of peripheral constriction than toconstriction conditions known to occur in the central airways. On thebasis of these simulations, we derived a set of qualitative criteria toinfer airway constriction conditions from RL andEL spectra.

     

Thermoregulatory control during pregnancy and lactation in rats

Eliason, Heather L., and James E. Fewell.Thermoregulatory control during pregnancy and lactation in rats.J. Appl. Physiol. 83(3): 837-844, 1997.Although the mechanisms remain unknown, maternal coretemperature (T_c) decreases nearterm of pregnancy and is increased throughout lactation in rats. Thepurpose of our present experiments was to determine whether pregnancy and lactation shift the thermoneutral zone of rats and to investigate whether the changes in maternal T_cduring pregnancy and lactation result from "forced" or"regulated" thermoregulatory responses. Conscious, chronicallyinstrumented nonpregnant and pregnant and lactating rats were studiedboth in a thermocline (a chamber with a linear temperature gradientfrom 12 to 36°C) and in a metabolic chamber to determine theinfluence of pregnancy and lactation on selected ambient temperature aswell as the thermoregulatory response to changes in ambienttemperature. We found that selected ambient temperature, oxygenconsumption, and thermal conductance did not change in rats studied ina thermocline as T_c decreased nearterm of pregnancy. There was, however, a downward shift in thethermoneutral zone of rats studied in a metabolic chamber near term ofpregnancy. During lactation, selected ambient temperature decreased inrats studied in a thermocline as oxygen consumption andT_c increased. The thermoneutralzone of lactating rats was not different from that of nonpregnantanimals. Thus our data provide evidence that the decrease inT_c near term of pregnancy in ratsresults from a regulated thermoregulatory response,whereas the increase in T_c duringlactation results from a forced thermoregulatory response.