Seasonal changes in the concentrations of four taxoids in Taxus baccata L. during the autumn-spring period.

The concentrations of four common taxoids: baccatin III, paclitaxel, cephalomannine and 10-deacetylbaccatin III (10-DAB III) were measured in fresh needles and stems of Taxus baccata L. during the late autumn-spring period (November'96-April'97) which has not been investigated to date in this species. Baccatin III, paclitaxel and 10-DAB III were present on the surface of the twigs in concentrations of 8-26 micrograms/1000 g (fresh weight). Changes in the levels of baccatin III and paclitaxel inside the needles and stems showed similar trends over the investigated period. From November to March the total level of taxoids differed between the needles and stems, and were the same only in April. Total levels in fresh needles were stable from December to March. The highest concentrations of 10-DAB III in the whole analysed period in fresh stems were measured, as well as in the fresh needles except for samples collected in November and December when the levels of cephalomannine were higher. The concentrations of paclitaxel were usually the lowest. These results confirm that epigenetic factors--date of collection (and thus phyllogenesis) and kind of plant tissue--determine taxoid levels during the late autumn-spring period in T. baccata. The opposite patterns of changes for 10-DAB III and cephalomannine, especially in the fresh needles, suggest a possible role for 10-DAB III in the biosynthetic pathway to cephalomannine, a less polar taxoid with a side-chain at position C-13. As well, owing to the thermolability of taxoids, the influence of low temperatures in December and January could explain the highest observed concentrations of 10-DAB III in the fresh stems and needles, respectively.     

Immunofluorescence and inhibitor studies on creatine kinase and mitosis

Immunofluorescence microscopy of mitotiv PtKl cells with antibody against M-line creatine kinase from chicken breast muscle revealed spindle staining largely identical with that of antibody against tubulin. Treatment with epoxycreatine, a creatine analog and covalent creatine kinase inhibitor, allowed the study of creatine kinase function in these non-muscle cells. Cells fixed at interphase following epoxycreatine treatment maintained actin stress fibers and creatine kinase localization on intermediate filaments. Treated prometaphase cells lacked spindles as observed by immunofluorescence microscopy, but displayed instead a central region of diffuse immunofluorescent staining surrounded by condensed chromosomes. Those observed in metaphase or anaphase appeared normal by phase-contrast microscopy, but lacked immunofluorescent staining. Living epoxycreatine-treated cells were observed by phase-contrast microscopy to form spindles and progress through mitosis following an extended prometaphase.     

Autofluorescence Reveals Different Histological Localizations of Furanocoumarins in Fruit of Some Umbelliferae and Leguminosae

Differences were shown in histological localization of psoralensof Angelica archangelica, Daucus carota, Sium suave and Psoraleabituminosa. All the fruits examined contained furanocoumarinson the surface. Fruit tissues contained very low (Sium), medium(Angelica) or high (Psoralea) concentrations of these compounds.Seed covers showed low autofluorescence in Angelica but highin Sium . Not all the glands in fruit or seed tissue exhibitedautofluorescence. Embryos always contained furanocoumarins ontheir surface and in their tissues in varying proportions. Autofluorescencecan be homogeneously distributed among all cells of the embryo(Sium) or can be concentrated in certain cells (Psoralea).Copyright1993, 1999 Academic Press Autofluorescence, furanocoumarins     

Do different competitive abilities of three fern species explain their different regional abundances?

Question: Do different competitive abilities of three fern species explain their different regional abundances? Location: Estonia, Europe. Methods: The factorial pot experiment, in which single individuals of three fern species were grown in natural soil with 0, 2,4, and 8 neighbouring individuals of Deschampsia flexuosa. Results: The response patterns of different biomass fractions and morphological parameters of fern species were similar to each other. The diploid D. expansa was the most vulnerable to competition, while tetraploid D. carthusiana and D. dilatata were more tolerant. D. carthusiana allocated relatively more to below‐ground parts than the other two species and allocation to roots increased when neighbour density increased. For D. expansa and D. dilatata, allocation to below‐ground parts decreased at high neighbour density, while in D. dilatata also the relative length of the stipe increased. Thus, the response of D. carthusiana corresponds to a ‘persistence type’ and that of D. dilatata to a ‘foraging type’. Conclusions: Lower vulnerability of D. carthusiana to competition, compared to D. expansa, may explain the higher regional and local abundance of the former. The rarity of D. dilatata in Estonia, however, could rather be explained by the impact of climatic factors, since this species is near its northeastern distribution limit in Estonia.     

Ecological aspects of nitrogen fixation by Purshia tridentata

Summary This study examines several aspects of nitrogen fixation by Purshia tridentata (Pursh) D.C., a rosaceous shrub widespread in the Central Oregon pumice region, especially as an understory species in Pinus ponderosa and Pinus contorta forests. Acetylene reduction was used to assay nodule activity in both field and greenhouse plants. The maximum rates were observed at 20°C, although summer soil temperatures were frequently around 15°C, at which a much lower rate was observed. Acetylene reduction by excised nodules was linear for 5 h and then slowly declined, finally ceasing after 19 h. Nodule activity declined in water stressed plants, essentially ceasing in plants with xylem pressure potentials below –25 bars.Field studies at five sites revealed that nodule activity began in mid-May or early June when soil temperature at 20 cm increased to above 10°C. Activity began later and remained lower until July 20 in plants located under Pinus contorta, probably because of the cooler temperatures at this site. Nodule activity at all sites was maximum in June and July. In late July, nodule activity declined sharply, corresponding with moisture stress readings in the –25 bar range. Acetylene reduction rates declined sharply during the night; this decline was even more severe late in the season.Only 46 per cent of Purshia plants were nodulated. Several possible explanations for this low nodulation are discussed, but the primary reasons appear to be low soil temperature and unfavorable moisture conditions. Previous speculations that Purshia may contribute significant amounts of nitrogen to the ecosystems in which it occurs are disputed using estimates based on seasonal acetylene reduction rates and a determination of nodule biomass at one site. The estimated nitrogen accretion rate was only 0.057 kg N/ha.yr. re]19760310     

Sequence learning recodes cortical representations instead of strengthening initial ones

We contrast two computational models of sequence learning. The associative learner posits that learning proceeds by strengthening existing association weights. Alternatively, recoding posits that learning creates new and more efficient representations of the learned sequences. Importantly, both models propose that humans act as optimal learners but capture different statistics of the stimuli in their internal model. Furthermore, these models make dissociable predictions as to how learning changes the neural representation of sequences. We tested these predictions by using fMRI to extract neural activity patterns from the dorsal visual processing stream during a sequence recall task. We observed that only the recoding account can explain the similarity of neural activity patterns, suggesting that participants recode the learned sequences using chunks. We show that associative learning can theoretically store only very limited number of overlapping sequences, such as common in ecological working memory tasks, and hence an efficient learner should recode initial sequence representations.