β-Amyloid 25-35 Peptide Reduces the Expression of Glutamine Transporter SAT1 in Cultured Cortical Neurons

β-Amyloid (Aβ) peptides may cause malfunction and death of neurons in Alzheimer’s disease. We investigated the effect of Aβ on key transporters of amino acid neurotransmission in cells cultured from rat cerebral cortex. The cultures were treated with Aβ(25-35) at 3 and 10 μM for 12 and 24 h followed by quantitative analysis of immunofluorescence intensity. In mixed neuronal–glial cell cultures (from P1 rats), Aβ reduced the concentration of system A glutamine transporter 1 (SAT1), by up to 50% expressed relative to the neuronal marker microtubule-associated protein 2 (MAP2) in the same cell. No significant effects were detected on vesicular glutamate transporters VGLUT1 or VGLUT2 in neurons, or on glial system N glutamine transporter 1 (SN1). In neuronal cell cultures (from E18 rats), Aβ(25-35) did not reduce SAT1 immunoreactivity, suggesting that the observed effect depends on the presence of astroglia. The results indicate that Aβ may impair neuronal function and transmitter synthesis, and perhaps reduce excitotoxicity, through a reduction in neuronal glutamine uptake. Special issue article in honor of Dr. Frode Fonnum.     

PARTITIONING GROWTH AND PHOTOSYNTHESIS BETWEEN LEAVES AND STEMS DURING NITROGEN LIMITATION IN SPARTIUM JUNCEUM

A clone of Spartium junceum (Spanish broom) was used to evaluate the consequence of different soil nitrogen regimes on growth and assimilation of leaves and stems. Nitrogen limitation caused a general reduction in aboveground growth, and NO₃^- limitation resulted in a greater reduction in leaf area compared to stem area. Supplemental soil nitrogen, from NH₄⁺, caused increased growth and a greater increase in stem area compared to leaf area. Excess NH₄⁺ caused decreased growth of leaves and stems. Under nitrogen-limiting conditions, a greater amount of nitrogen was in stems than in leaves, particularly on a surface area basis. Both stem and leaf assimilation were reduced under limiting nitrogen, but the net effect was an increase in the stem contribution to daily carbon gain compared to a decrease for leaves. Stem and leaf assimilation had similar linear relationships with tissue nitrogen concentration. Nitrogen use efficiency increased under nitrogen limitation. Thus, as the proportion of whole plant nitrogen in the stems increased compared to that in leaves at low nitrogen availability, the nitrogen use efficiency increased.     

Seasonal and diurnal leaf movements of Rhododendron maximum L. in contrasting irradiance environments

Summary Leaf orientation (azimuth and angle) and leaf curling were measured seasonally and diurnally on Rhododendron maximum L. under an evergreen and a deciduous canopy. The microclimatic conditions under the evergreen canopy (mixed pine and hemlock) were characterized by lower irradiance but similar temperature, and vapor pressure deficit (vpd) to that under the deciduous canopy (mixed oak and maple). Under both canopies irradiance was more intense during winter months.On a seasonal basis leaf angle was closer to horizontal under the evergreen canopy but there was no difference between leaf curling in the two sites. Stomatal conductance was higher under the deciduous canopy but stomata were closed in the winter (following canopy abscission) under the evergreen and deciduous canopies even during warm winter days. Leaf water potentials were lower in the winter and Rhododendron maximum had higher leaf water potentials under the evergreen canopy.Significant association between mean leaf angle and curling index were found above a mean leaf angle of 70°. Leaf curling was highly associated with leaf temperature where 0° C was a critical value stimulating leaf curling. Leaf angle was linearly related to leaf temperatures above 0° C although this relationship was different under the two canopy types as a result of differing irradiance or differing water potential.     

QUANTITATIVE PHENOLOGY AND LEAF SURVIVORSHIP OF RHODODENDRON MAXIMUM IN CONTRASTING IRRADIANCE ENVIRONMENTS OF THE SOUTHERN APPALACHIAN MOUNTAINS

Seasonal plant growth patterns were compared for Rhododendron maximum L. in two contrasting subcanopy environments. The two subcanopy, above ground environments differed only in their quantity of irradiance by virtue of the relative dominance of evergreen or deciduous trees in the canopy. A third site had no canopy influence. Overall growth (shoot elongation, woody increment, leaf production) was maximized under the open (BMO) and deciduous dominated canopy (PCD). The leaf pool was significantly smaller under the evergreen dominated canopy (PCE) but average leaf area per leaf was slightly larger at PCE. Individual age-specific leaf cohorts, identified from shoot morphology, indicated increased leaf survivorship with a decreased irradiance environment. Leaf production was synchronous and rapid (1 week), followed by three weeks of leaf expansion, which created the even-aged leaf cohorts. Wood growth (diameter increment), in contrast, continued through the beginning of the winter. Reproductive effort increased with increasing irradiance environment. Significant variation in growth was observed between canopy shoot types at all three research sites. The significance of these phenological patterns is discussed in view of the variable subcanopy environment of southwestern Virginia.     

THE INFLUENCE OF LOW PLANT WATER POTENTIAL ON THE GROWTH AND NITROGEN METABOLISM OF THE NATIVE CALIFORNIA SHRUB LOTUS SCOPARIUS (NUTT. IN T & G) OTTLEY

Reduced plant water potential, induced by polyethylene glycol in hydroponics, inhibited growth and decreased the number of leaves per branch in the southern California drought-deciduous species Lotus scoparius (Nutt. in T & G) Ottley. Decreasing plant water potential diminished the proportion of large leaves per branch and therefore reduced the leaf area. Nitrate uptake rate decreased with decreasing water potential, although the nitrate ion concentration increased in the roots and the leaves. Ammonium ion concentration increased significantly in the roots at −5 bars and lower osmotic potentials in the root medium. Kjeldahl nitrogen remained the same in all treatments and tissues over the experimental period. It is suggested that the increase in ammonium ion in the roots was due to a decreased rate of ammonium transport caused by low plant water potential. The slight increase in nitrate ion in the roots may correspond to a decrease in nitrate reductase activity. This study indicates that some of the changes in nitrogen metabolism associated with low water potentials in agricultural plants occur also in a plant which experiences frequent droughts in its native habitat.     

A computational study of ANTA and NTO derivatives

This work is a study of 5-amino-3-nitro-1,2,4-triazole (ANTA), 3-nitro-1,2,4-triazol-5-one (NTO), and nitrated derivatives of ANTA and NTO. RDX and TNT were studied for comparison. ANTA and NTO are low-sensitive high explosives with detonation properties comparable to 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). We showed previously that nitrated NTO and ANTA compounds, when used in a glycidyl azide polymer (GAP) matrix in rocket propellants, could give impulses above 2600 m/s and that the oxygen balance is positive. If used in aluminized explosives, the heat of detonation may be increased to a practical level significantly above RDX/aluminum compositions. Here, we use two different methods for sensitivity and two density functional theory functionals, B3LYP and M06-2X with the 6-31G(d) basis set, together with the complete basis set method CBS-4M. Calculations indicate that most of the nitrated derivatives have nearly equal sensitivity to RDX. Significantly different bond dissociation energies in the nitrimino functional group are predicted, although most models give much the same result.     

Phenology,productivity, and nutrient accumulation in the post-fire chaparral shrub Lotus scoparius

Summary Lotus scoparius is a drought-deciduous shrub which is an early and abundant colonizer of sites following fire in southern California chaparral. Productivity, seasonal nutrient concentrations, nutrient accumulations and phenology were studied in a 4-year-old burn site in Adenostoma chaparral in which L. scoparius had established 49% cover. Net aboveground primary production for L. scoparius was 105 g m^-2 y^-1; leaves accounted for 40% of the annual production. The true increment to biomass was only 17 g m^-2 y^-1; 83% of the net production entered the litter layer or standing dead components. In response to the Mediterranean climatic regime, most of the annual net production and plant activity occurred from May through June when photoperiod and temperatures were favorable and moisture was available. In July leaf abscission occurred in response to the summer drought conditions. Correlation and principal component analysis suggested consistent seasonal behavior in the foliar concentrations of N, P, Zn, and Mn. Nitrogen, P, K, and Zn were strongly reabsorbed from leaf tissues before abscission. Calcium, Mg, and Fe formed a second functional group of elements which increased in concentration throughout leaf maturation and which were not reabsorbed from senescing foliage. The seasonal pattern of nitrogen-containing organic compounds (chlorophylls and proteins) was most associated with the leaf phenology and water stress. The rapid growth of Lotus scoparius plays a role in conserving nutrients that might be lost through runoff and erosion after fire in the chaparral.     

The relationship between freezing tolerance and thermotropic leaf movement in five Rhododendron species

Summary Leaf movement kinetics in five species of Rhododendron were studied in response to leaf temperature, leaf freezing point, and leaf water deficit. There was a gradient in the degree of leaf curling among species in the following order from the greatest curling to the least curling: Rhododendron catawbiense, R. maximum, R. minus, R. macrophyllum, R. ponticum. Those species found to be tolerant of winter conditions had the most intense leaf movements (both curling and angle) while those species with minimal cold tolerance had limited or no leaf movements. Leaf curling occurred at leaf temperatures above the tissue freezing points in all species. Athough leaf angle was influenced by leaf turgor, general tissue desiccation was not the ultimate cause for thermotropic leaf curling in any species tested. Those species with the greatest leaf curling and angle movements had the highest osmotic potential, the lowest water deficit at the turgor loss point, and the lowest symplastic water fraction. These data suggest that there is a trade off in Rhododendron leaf physiology between cold tolerance (due to leaf movements) and water stress tolerance (due to turgor maintenance mechanisms).