The nitrogen cycle in lodgepole pine forests,southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m^–2), with aboveground detritus (37.5–48.8g · m^–2) and living biomass (19.5–24.0 g · m^–2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m^–2 · yr^–1), and biological fixation of atmospheric N (<0.03 g · m^–2 · yr^–1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m^–2 · yr^–1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m^–2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m^–2 · yr^–1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L^–1) in 02 leachate. A critical transport process was solubilization of organic N, mostly fulvic acids. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m^–2 · yr^–1 with estimated root turnover of 0.37-gN · m^–2 · yr^–1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m^–2 · yr^–1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.     

Seasonal changes in photosynthetic activity of snow tussocks (Chionochloa spp.) along an altitudinal gradient in Otago,New Zealand

Summary Photosynthetic characteristics were studied in Chionochloa rigida, an altitudinally widespread snow tussock and a closely related high-altitude species C. macra. Along a gradient from near sea level to 1600 m there were no consistent differences in maximum photosynthetic capacity which averaged 4.5 mol CO₂ m^-2s^-1. The photosynthetic temperature optimum ranged between 15 and 18°C and there was only a limited capacity for seasonal adjustment. Net photosynthesis was light-saturated at about 500 mol photons m^-2s^-1. In winter, the photosynthetic capacity decreased significantly with increasing altitude of origin of the snow tussocks. A transplant experiment indicated this was partly genetically controlled.     

Knob heterochromatin homology in maize and its relatives

Summary We have characterised the major DNA sequence component of knob heterochromatin in maize, teosinte andTripsacum. Sequence analysis of this DNA gives strong support to the proposal that maize originated by selection of variants in teosinte. In situ hybridization has confirmed that this repeating DNA sequence, which is the major component of maize knob heterochromatin, is also the major component of knobs in teosinte,Zea diploperennis andTripsacum. In Southern blot hybridizations the repeat has a similar basic organization in all taxa;Tripsacum, however, is differentiated from maize and teosinte by a number of sequence features. Maize and teosinte knob heterochromatin are indistinguishable with regard to the distribution of mutations in the 180-bp repeat and the presence and organization of a 202-bp variant sequence. The knob DNA sequence was not detectable in three species ofCoix, an Old World genus of the Maydeae.Within the repeat unit is a 27-bp region that shows no sequence changes in maize, teosinte orTripsacum. The remainder of the repeat unit has randomly distributed nucleotide changes. The presence of the conserved sequence region suggests that knob DNA may have a functional role in the nucleus.     

Leukotriene inhibitors do not block hypoxic pulmonary vasoconstriction in dogs

We studied whether intravenously administered inhibitors of leukotriene synthesis (diethylcarbamazine, DEC) or end-organ effect (FPL-55712) would change the distribution of regional pulmonary blood flow (rPBF) caused by left lower lobe (LLL) alveolar hypoxia in dogs. Both drugs failed to alter rPBF. In addition, the pressor response to whole-lung hypoxia was not blocked by an FPL-55712 infusion. On the other hand, nitroprusside, as a nonspecific vasodilator also administered intravenously, was able to partially reverse the effects of LLL hypoxia on rPBF. Thus our data do not support a role for leukotriene mediation of hypoxic pulmonary vasoconstriction in dogs.     

Hyperthermic sensitivity and growth stage in Escherichia coli

Hyperthermic sensitivities of Escherichia coli B/r and Bs-1 were determined for lag-, midlog-, and stationary-phase cells at 47, 48, and 49 degrees C. In both strains midlog-phase cells were strikingly more heat sensitive (100-fold greater killing after 4 h at 48 degrees C) than stationary-phase cells, with intermediate sensitivity for lag-phase cells. In contrast to the reported difference in the radiation sensitivity between these two strains, very little difference in heat sensitivity was seen. Patterns of fatty acid composition of both strains were very similar at each phase of growth. From midlog to stationary phase, 16:1 and 18:1 unsaturated fatty acids decrease from 16 and 30% to 0.5 and 3%, respectively, while the C17 and C19 cyclopropane fatty acids increase from 7 and 3% to 22 and 25%, respectively. Concomitant with these changes in fatty acid composition, substantially higher membrane microviscosity values were recorded for stationary-phase cells. Total membrane microviscosity was positively associated with the C17 and C19 cyclopropane fatty acid composition and with cell survival following hyperthermia. In contrast to hyperthermic sensitivity, radiation survival differences between B/r and Bs-1 are little affected by growth stage. We propose that these results are consistent with a critical influence of membrane lipids on cellular hyperthermic sensitivity and further that the target sites for radiation and hyperthermia are different in these cells.     

Cloning and orientation of the gene encoding polynucleotide phosphorylase in Escherichia coli.

Mutations which affect the activity of polynucleotide phosphorylase (PNPase) map near 69 min on the bacterial chromosome. This region of the chromosome has been cloned by inserting the kanamycin-resistant transposon Tn5 near the argG and mtr loci at 68.5 min. Large SalI fragments of chromosomal DNA containing the Tn5 element were inserted into pBR322, and selection was made for kanamycin-resistant recombinant plasmids. Two of these plasmids were found to produce high levels of PNPase activity in both wild-type and host strains lacking PNPase activity. The pnp gene was further localized and subcloned on a 4.8 kilobase HindIII-EcoRI fragment. This fragment was shown to encode an 84,000-molecular weight protein which comigrated with purified PNPase during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The orientation of the pnp gene was determined by insertion of Tn5 into the 4.8 kilobase fragment cloned in pBR322. Some of the insertions had lost the ability to elevate the level of PNPase activity in the host bacterium. Restriction mapping of the positions of the Tn5 insertions and analysis of plasmid-encoded polypeptides in UV-irradiated maxi-cells indicated that the pnp gene is oriented in the counterclockwise direction on the bacterial chromosome.     

Skeletal growth in a medieval population from Sudanese Nubia

Long bone growth is analyzed for 180 children from a Medieval population at Kulubnarti in Sudanese Nubia (550–1450 A.D.). A regional interpopulation comparison is made with growth data from Wadi Halfa in Lower Nubia, and an intrapopulation analysis is undertaken to assess diachronic changes in growth at Kulubnarti. Changes in growth patterns are interpreted in the context of mortality and morbidity profiles and relationships between the three variables are discussed. It is suggested that changes in the sociopolitical environment may have been responsible in part for altering levels of biological stress and impacting growth.