Nutrient conservation strategies in native Andean‐Patagonian forests

Abstract. Nutrient conservation in vegetation affects rates of litter decomposition and soil nutrient availability. Although resorption has been traditionally considered one of the most important plant strategies to conserve nutrients in temperate forests, long leaf life‐span and low nutrient requirements have been postulated as better indicators. We aimed at identifying nutrient conservation strategies within characteristic functional groups of NW Patagonian forests on Andisols. We analysed C‐, N‐, P‐, K‐ and lignin‐concentrations in mature and senescent leaves of ten native woody species within the functional groups: broad‐leaved deciduous species, broad‐leaved evergreens and conifers. We also examined mycorrhizal associations in all species. Nutrient concentration in mature leaves and N‐ resorption were higher in broad‐leaved deciduous species than in the other two functional groups. Conifers had low mature leaf nutrient concentrations, low N‐resorption and high lignin/N ratios in senescent leaves. P‐ and K‐resorptions did not differ among functional groups. Broad‐leaved evergreens exhibited a species‐dependent response. Nitrogen in mature leaves was positively correlated with both N resorption and soil N‐fertility. Despite the high P‐retention capacity of Andisols, N appeared to be the more limiting nutrient, with most species being proficient in resorbing N but not P. The presence of endomycorrhizae in all conifers and the broad‐leaved evergreen Maytenus boaria, ectomycorrhizae in all Nothofagus species (four deciduous, one evergreen), and cluster roots in the broad‐leaved evergreen Lomatia hirsuta, would be possibly explaining why P is less limiting than N in these forests.     

T-cell responsiveness in Mycobacterium lepraemurium infections in a "resistant" (CBA) and a "susceptible" (BALB/c) mouse strain

Antigen-specific and mitogen-nonspecific T-lymphocyte proliferation and lymphokine release (interleukin 2 and macrophage activation factor) were studied in BALB/c and CBA mice infected intravenously with 10(8) Mycobacterium lepraemurium organisms. The responsiveness of spleen cells from infected animals to Con A and specific MLM antigen declined as the infection progressed. Thus, the decreased responsiveness appeared earlier and was more profound in the relatively susceptible BALB/c strain than in the relatively resistant CBA strain. Nylon-wool-purified, T-cell-enriched spleen cells from both strains, however, responded to both M. lepraemurium antigen and Con A until the later stages of infection (17 weeks postinfection). The relevance of nonspecific immunodepression mediated by nylon-wool-adherent spleen cells to the progressive nature of this infection is discussed.     

Evidence for regulation of actin synthesis in cytochalasin D-treated HEp-2 cells

In HEp-2 cells treated with 0.2 or 2.0 μM cytochalasin D (CD), the relative rate of actin synthesis increased for about 12 h and then reached a plateau; this increase was suppressed by actinomycin D (AD). When CD was washed from cells which had been treated for 20 h, the elevated rate of actin synthesis declined to the control value within ca 4 h, as the actin-containing cytoskeletal components rearranged by CD recovered their normal morphology. Subsequently, actin synthesis was depressed below control values for a prolonged period; during recovery from 2 h treatment with CD, this depression was of much shorter duration. Re-addition of CD to cells after a 3 h recovery period again induced the cytoskeletal alterations characteristic of CD treatment but did not reverse the prior decline in the rate of actin synthesis. In HEp-2 cells treated with cycloheximide during exposure to CD for 20 h, the relative rate of actin synthesis measured after removal of cycloheximide was twofold higher than with CD alone and such cells exhibited a twofold slower decline in the rate of actin synthesis during recovery from CD in the continued presence of cycloheximide. These effects of cycloheximide, which resemble observations on “super-induction”, suggest that actin synthesis in CD-treated and recovering HEp-2 cells may be regulated by a repressor protein. The possibility that the proposed repressor protein is actin and that actin may thus be a feedback inhibitor of its own synthesis is discussed.     

Termination of inspiration by phase-dependent respiratory vagal feedback in awake normal humans.

Imperceptible levels of proportional assist ventilation applied throughout inspiration reduced inspiratory time (TI) in awake humans. More recently, the reduction in TI was associated with flow assist, but flow assist also reaches a maximum value early during inspiration. To test the separate effects of flow assist and timing of assist, we applied a pseudorandom binary sequence of flow-assisted breaths during early, late, or throughout inspiration in eight normal subjects. We hypothesized that imperceptible flow assist would shorten TI most effectively when applied during early inspiration. Tidal volume, integrated respiratory muscle pressure per breath, TI, and TE were recorded. All stimuli (early, late, or flow assist applied throughout inspiration) resulted in a significant increase in inspiratory flow; however, only when the flow assist was applied during early inspiration was there a significant reduction in TI and the integrated respiratory muscle pressure per breath. These results provide further evidence that vagal feedback modulates breathing on a breath-by-breath basis in conscious humans within a physiological range of breath sizes.     

HISTORICAL BIOGEOGRAPHY IN NORTH AMERICAN ARID REGIONS: AN APPROACH USING MITOCHONDRIAL-DNA PHYLOGENY IN GRASSHOPPER MICE (GENUS ONYCHOMYS)

Restriction-endonuclease-site variation of mitochondrial DNA (mtDNA) was used to investigate patterns of geographic and phylogenetic divergence within the rodent genus Onychomys. Onychomys has occupied arid habitats in the western North American deserts, shrub-steppes, and grasslands since the late Tertiary. A phylogenetic analysis of the total mtDNA restriction-site variation throughout the range of Onychomys suggests that the distribution of this genus has been affected by the same Quaternary pluvial-interpluvial climatic fluctuations that have resulted in the periodic fragmentation of arid habitats in western North America. Onychomys mtDNA haplotypes define at least five discrete geographical subsets, suggesting that there are five areas of endemism for biota restricted to arid and semiarid habitats in North America. The mtDNA-haplotype phylogeny can be used to infer an hypothesis of historical relationships among the five areas of endemism as follows: ([{(Wyoming Basin + Interior Plains + Colorado Plateaus) + (Columbia Basin + Great Basin)} + Gulf Coastal Plain] + Chihuahuan) + Western Deserts. The results of this study point to the potential use of mtDNA-haplotype phylogenies to reconstruct historical biogeographic events in Quaternary time. The utility of mtDNA variation depends in part on the ecology and distribution of the species being examined. Therefore, our hypothesized area cladogram can be tested by investigating regional relationships in other western North American taxa with distributions similar to Onychomys.     

Metabolism of ecdysteroids in the female tickAmblyomma hebraeum (Ixodoidea,Ixodidae): accumulation of free ecdysone and 20-hydroxyecdysone in the eggs

Summary [³H]-20-hydroxyecdysone ([³H]-20E) injected intoAmblyomma hebraeum females 7 days before the beginning of oviposition,viz. at the beginning of vitellogenesis, was converted to 3 polar peaks of unknown nature called 1, 2 and 3, and to apolar conjugates AP1, AP2 and AP3. AP2 have the same retention times as the esters of 20E with long chain fatty acids described inOrnithodoros moubata (Diehl et al. 1985). However, principally unmetabolized 20E was incorporated into the ovaries, and 16% of the injected labelling was recovered in the eggs, 3/4 being free 20E. When 20E was injected during oviposition, it was not converted to the polar products but only to the apolar products. At this time, 76% of the total radiolabel injected accumulated in the egg-batch, principally in the form of free unmetabolized 20E.After injection of [³H]-ecdysone (³H]-E), the three polar metabolites 1, 2 and 3, probably 20-deoxy homologues of 1, 2 and 3 described above were always produced irrespective of the time of injection. In addition, E was metabolized to 20E and to the apolar conjugates AP1, AP2, and AP3. E, 20E and peak 2 were incorporated into the ovary within the first day after injection. These 3 compounds were found in freshly laid eggs in variable proportions, the quantity of E decreasing with time while 20E and peak 2 increased. At the end of oviposition, ca. 60% of the injected radiolabel had been incorporated into the eggs. Apolar products and polar metabolites accumulating in the body were apparently not used as a source of free hormone for the eggs.Our results with tritiated ecdysteroids confirm our data concerning endogenous ecdysteroids of the eggs ofA. hebraeum (Connat et al. 1985). This species, in contrast to 2 other female ticks,Ornithodoros moubata andBoophilus microplus, incorporates free E and 20E instead of ecdysteroid conjugates into its eggs. The role of these free ecdysteroids remains to be elucidated.Abbreviations ES ecdysteroids - E ecdysone - 20E 20-hydroxyecdysone - HPLC high-performance liquid chromatography - RP-18 reyerse phase with C18 aliphatic chains grafted on the silica - RIA radio-immunoassay Part of this work was realized for the PhD thesis of E.M. Dotson     

Determination of “active” cytochrome P-450 from relaxation kinetics of product formation

We estimate the active part of cytochrome P-450, which is involved in a special substrate transformation, by measuring the initial change of the production rate as a function of the relaxation transitions between two different steady states of the reaction cycle of cytochrome P-450 using the light-reversibility of the carbon monoxide inhibition. The kinetic data of such relaxations are interpreted within a model cycle, which reduces the reaction cycle to three steps. The estimation of the rate constant of the first reduction step, derived from model simulation of the production rate, is confirmed by independent experimental study of the reduction kinetics.An application of our model to the O-deethylation of 7-ethoxycoumarin reveals that — in a time average — 10%–15% of the spectroscopically detectable cytochrome P-450 is involved in that transformation.Abbreviations Cyt. P-450 microsomal cytochrome P-450 - 7-EC 7-ethoxycoumarin