Distribution and recovery of15N after fertilization of Douglas-fir saplings with different nitrogen sources

Summary Four-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) saplings planted in pots with a sand and peat mix (11) were fertilized at the rate of 200 kg N/ha of (¹⁵NH₂)₂CO (U-15),¹⁵NH₄NO₃ (A-15) and NH₄ ¹⁵NO₃(An-15). They were placed in a shadehouse and watered regularly to maintain soil moisture at field capacity over periods of one and two years. Quantity of¹⁵N in foliage generally increased from old to current growth, irrespective of the nitrogen source. Utilization of¹⁵N fertilizers by saplings after the first and second growing seasons following fertilization was greatest with nitrate labelled ammonium nitrate AN-15, and nearly equal for urea U-15 and ammonium labelled ammonium nitrate A-15. The soil immobilized more fertilizer nitrogen-15 from U-15 and A-15 than from AN-15. Data from the present study, in which leaching losses of fertilizer were minimized, demonstrated that in terms of nitrogen uptake by the saplings the nitrate fertilizer was superior to ammonium fertilizer.     

The exchange hypothesis for the formation of chromatid aberrations: an experimental test using bleomycin

The association between bleomycin-induced chromatid aberrations and BUdR-label exchange between sister chromatids was investigated in order to evaluate Revell's exchange hypothesis for the formation of chromatid aberrations. The results of this study indicate that a larger than expected proportion of chromatid breaks can be accounted for by the exchange hypothesis though not all breaks are the result of incomplete exchange.     

A prevalent persistent global nonrandomness that distinguishes coding and non-coding eucaryotic nuclear DNA sequences

Summary Coding sequences of eucaryotic nuclear DNA were characterized by an excess of short runs and a deficit of long runs of weak and of strong hydrogen bonding bases; non-coding sequences by a deficit of short runs and an excess of long runs, in the same of purines and of pyrimidines. The conservation of these attributes across DNA sequences coding for proteins of widely different function, across widely different eucaryotic species for the same protein and across related genes that diverged a long time ago and that now show large differences in base and, if coding, amino acid sequence suggested that these attributes have survival value. It was concluded that these attributes constitute probalistic constraints on th primary structure (base sequence) of both coding and non-coding DNA.     

载脂蛋白多基因家族分子进化的研究

与脂质运输有关的载脂蛋白基因构成一个复杂的多基因家族。为探讨这种演化时间长的基因家族的进化规律,本文首先建立了一种在非均衡进化速率条件下计算系统发生树中任意分支长度的简易方法,并可在此基础上算出无根分支系统树中分歧年代的期望值。进一步对本文科１０个种属共２６种载脂蛋白的系统演作作了实际分析,结果提示：①ＡｐｏＡ－Ｉ＇ＡｐｏＡ－ＩＶ,ＡｐｏＥ及ＡｐｏＡ－ＩＩ的共同祖先可能在奥陶纪水生脊椎动物中就已存     

Mutual support and selection between branches of damaged plants

Snow (1931) showed that the fate of a branch of a plant that had suffered local herbivory could be determined by correlative effects of other branches. This neglected work was continued in the current study, herbivory being simulated by the removal of leaves of different ages from pea plants with two branches. A damaged branch was suppressed when an undamaged alternative branch was present; otherwise the damage never prevented continued development. The removal of mature leaves had a smaller effect than the removal of immature, expanding leaves. When leaves were removed from both branches it was the branches that suffered less damage to their immature leaves that continued shoot development. Branches from which all photosynthetic leaves were repeatedly removed developed only when they retained their immature leaves and remained dominant, inhibiting the development — but not the photosynthesis — of the other branch on the same plant. Accounting for these results requires mechanisms that compare the different branches of the plant and select for development the ones that have the greatest potential for future, rather than present, photosynthesis. It is concluded that a compartmentalized or sectorial distribution of essential substrates can be modified by correlative relations that are probably mediated by hormones.     

Correspondence of environmental tolerances with leaf and branch attributes for six co-occurring species of broadleaf evergreen trees in northern California

 For the angiosperm dominants of northern California’s mixed evergreen forests, this study compares the display of photosynthetic tissue within leaves and along branches, and examines the correspondence between these morphological attributes and the known environmental tolerances of these species. Measurements were made on both sun and shade saplings of six species: Arbutus m e n z i e s i i (Ericaceae), C h r y s o l e p i s c h r y s o p h y l l a (Fagaceae), L i t h o c a r p u s d e n s i f l o r u s (Fagaceae), Quercus c h r y s o l e p i s (Fagaceae), Quercus w i s l i z e n i i (Fagaceae), and Umbellularia c a l i f o r n i c a (Lauraceae). All species had sclerophyllous leaves with thick epidermal walls, but species differed in leaf specific weight, thickness of mesophyll tissues and in the presence of a hypodermis, crystals, secretory idioblasts, epicuticular deposits, and trichomes. The leaves of Arbutus were 2 – 5 times larger than those of C h r y s o l e p i s, L i t h o c a r p u s and Umbellularia and 4 – 10 times larger than those of both Quercus species. Together with differences in branch architecture, these leaf traits divide the species into groups corresponding to environmental tolerances. Shade-tolerant C h r y s o l e p i s, L i t h o c a r p u s, and Umbellularia had longer leaf lifespans and less palisade tissue, leaf area, and crown mass per volume than the intermediate to intolerant Arbutus and Quercus. Having smaller leaves, Quercus branches had more branch mass per leaf area and per palisade volume than other species, whereas Arbutus had less than other species. These differences in display of photosynthetic tissue should contribute to greater growth for Quercus relative to the other species under high light and limited water, for Arbutus under high light and water availability, and for C h r y s o l e p i s, L i t h o c a r p u s, and Umbellularia under limiting light levels. Accepted: 22 March 1996     

Allometric analysis of Sitka spruce branches: mechanical versus hydraulic design principles

The geometry of tree branches can have considerable effect on their efficiency in terms of carbon export per unit carbon investment in structure. The purpose of this study was to evaluate different design criteria using data describing the form of Picea sitchensis branches. Allometric analysis of the data suggests that resources are distributed to favour shoots with the greatest opportunity for extension into new space, with priority to the extension of the leader. The distribution of allometric relations of links (branch elements) was tested against two models: the pipe model, based on hydraulic transport requirements, and a static load model based on the requirement of shoots to provide mechanical resistance to static loads. Static load resistance required the load parameter to be proportional to the link radius raised to the power of 4. This was shown to be true within a 95% statistical confidence limit. The pipe model would require total distal length to be proportional to link radius squared but the measured branches did not conform well to this model. The comparison suggests that the diameters of branch elements were more related to the requirements for mechanical load. The cost of following a hydraulic design principle (the pipe model) in terms of mechanical efficiency was estimated and suggested that the pipe model branch would not be mechanically compromised but would use structural resources inefficiently. Resource allocation among branch elements was found to be consistent with mechanical stability criteria but also indicated the possibility of allocation based on other criteria, such as potential light interception by shoots. The evidence suggests that whilst branch topology increments by reiteration of units of morphogenesis, the geometry follows a functional design pattern.     

Osteometry and autoradiography of the long bones of sleletons of women exposed to internal radiation at 13–19 years of age

Lengths of long bones of skeletons were examined in 25 women first exposed to large skeletal doses of radiation (alpha particles from radium) at the age of 13–19 years. Meanlengths did not differ significantly between two subgroups based on age at first exposure toradiation (i.e., 13–16 vs. 17–19 years). Autoradiographs of femora of some women who ingested radium at 13–15 years of age showed evidence for bone growth when blood levels of radium were low (i.e., after ingestion of radium). These findings indicate no detectable effect of large skeletal doses of radiation on growth in adolescent and post-adolescent periods.     

Identification of mouse chromosomes required for murine leukemia virus replication.

The replication patterns of five ecotropic and two amphotropic strains of murine leukemia virus (MuLV) were studied by infecting 41 Chinese hamster x mounse hybrid primary clones segregating mouse (Mus musculus) chromosomes. Ecotropic and amphotropic strains replicated in mouse and some hybrid cells, but not in hamster cells, indicating that replication of exogenous virus requires dominantly expressed mouse cellular genes. The patterns of replication of the five ecotropic strains in hybrid clones were similar; the patterns of replication of the two amphotropic strains were also similar. When compared to each other, however, the replication patterns of ecotropic and amphotropic viruses were dissimilar, indicating that these two classes of MuLV require different mouse chromosomes for replication. Chromosome and isozyme analyses assigned a gene, Rec-1 (replication of ecotropic virus), to mouse chromosome 5 that is necessary and may be sufficient for ecotropic virus replication. Because of preferential retention of mouse chromosomes 15 and 17 in the hybrid clones, however, the possibility that these chromosomes carry genes that are necessary but not sufficient for ecotropic virus replication cannot be excluded. Similarly, the data indicate that mouse chromosome 8 (or possibly 19) carried a gene we have designated Ram-1 (replication of amphotropic virus) which is necessary and may be sufficient for amphotropic virus replication. Because chromosomes 8 and 19 tended to segregate together and two of the three clones excluding 19 have chromosome reaggrangements, we cannot exclude 19 as being independent of amphotropic virus replication. In addition, because of preferential retention, chromosomes 7, 12, 15, 16 and 17 cannot be excluded as being necessary but not sufficient. Hybrid cell genetic studies confirm the assignment of the Fv-1 locus to chromosome 4 previously made by sexual genetics. In addition, our results demonstrate that hybrid cells which have segregated mouse chromosome 4 but have retained 5 become permissive for replication of both N and B tropic strains of MuLV.     

Purification and properties of the RuvA and RuvB proteins of Escherichia coli

Summary The RuvA and RuvB proteins of Escherichia coli play important roles in the post-replicational repair of damaged DNA, genetic recombination and cell division. In this paper, we describe the construction of over expression vectors for RuvA and RuvB and detail simple purification schemes for each protein. The purified 22 kDa RuvA polypeptide forms a tetrameric protein (M_r ca. 100000) as observed by gel filtration. The tetramer is stabilised by strong disulphide bridges that resist denaturation during SDS-PAGE (in the absence of boiling and -mercaptoethanol). In contrast, purified RuvB polypeptides (37 kDa) weakly associate to form a dimeric protein (M_r ca. 85000). At low protein concentrations, the RuvB dimer dissociates into monomers. The multimeric forms of each protein may be covalently linked by the bifunctional cross-linking reagent dimethyl suberimidate. Addition of purified RuvA and RuvB to a RecA-mediated recombination reaction was found to stimulate the rate of strand exchange leading to the rapid formation of heteroduplex DNA.