四川缙云山森林群落林窗边缘效应的研究

 本文运用Shannon—Wiener物种多样性指数、Simpson生态优势度指标以及边缘效应强度指数,对缙云山亚热带森林群落林窗的边缘效应现象进行了初步研究。29个林窗样地不同部位上述指标的测定表明用物种多样性指数拟合边缘效应测度通式,缙云山亚热带森林群落林窗的边缘效应强度值为1—3左右;而用生态优势度值拟合边缘效应强度测式,则上述边缘效应强度值为0.1一1.2左右。研究表明林窗边缘区由于边缘效应的作用有增大物种多样性的趋势。而林窗面积、所处坡向以及林窗所在的森林群落类型,都对处于不同发育阶段的林窗的边缘效应强度具有影响。在此基础上,本文对林窗边缘效应强度的变化规律、效应性质、测定意义以及对森林营造和优化管理上的作用进行了一定探讨。     

Environmental and auxin regulation of wood formation involves members of the Aux/IAA gene family in hybrid aspen

Indole acetic acid (IAA/auxin) profoundly affects wood formation but the molecular mechanism of auxin action in this process remains poorly understood. We have cloned cDNAs for eight members of the Aux/IAA gene family from hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) that encode potential mediators of the auxin signal transduction pathway. These genes designated as PttIAA1-PttIAA8 are auxin inducible but differ in their requirement of de novo protein synthesis for auxin induction. The auxin induction of the PttIAA genes is also developmentally controlled as evidenced by the loss of their auxin inducibility during leaf maturation. The PttIAA genes are differentially expressed in the cell types of a developmental gradient comprising the wood-forming tissues. Interestingly, the expression of the PttIAA genes is downregulated during transition of the active cambium into dormancy, a process in which meristematic cells of the cambium lose their sensitivity to auxin. Auxin-regulated developmental reprogramming of wood formation during the induction of tension wood is accompanied by changes in the expression of PttIAA genes. The distinct tissue-specific expression patterns of the auxin inducible PttIAA genes in the cambial region together with the change in expression during dormancy transition and tension wood formation suggest a role for these genes in mediating cambial responses to auxin and xylem development.     

A cohesion/tension model for the gating of aquaporins allows estimation of water channel pore volumes in Chara

The water permeability (hydraulic conductivity; Lp) of turgid, intact internodes of Chara corallina decreased exponentially as the concentration of osmolytes applied in the medium increased. Membranes were permeable to osmolytes and therefore they could be applied on both sides of the plasma membrane at concentrations of up to 2.0 m (5.0 MPa of osmotic pressure). Organic solutes of different molecular size (molecular weight, MW) and reflection coefficients (σ_s) were used [heavy water HDO, MW: 19, σ_s: 0.004; acetone, MW: 58, σ_s: 0.15; dimethyl formamide (DMF), MW: 73, σ_s: 0.76; ethylene glycol monomethyl ether (EGMME), MW: 76, σ_s: 0.59; diethylene glycol monomethyl ether (DEGMME), MW: 120, σ_s: 0.78 and triethylene glycol monoethyl ether (TEGMEE), MW: 178, σ_s: 0.80]. The larger the molecular size of the osmolyte, the more efficient it was in reducing cell Lp at a given concentration. The residual cell Lp decreased with increasing size of osmolytes. The findings are in agreement with a cohesion/tension model of the osmotic dehydration of water channels (aquaporins; AQPs), which predicts both reversible exponential dehydration curves and the dependence on the size of osmolytes which are more or less excluded from AQPs (Ye, Wiera & Steudle, Journal of Experimental Botany 55, 449–461, 2004). In the presence of big osmolytes, dehydration curves were best described by the sum of two exponentials (as predicted from the theory in the presence of two different types of AQPs with differing pore diameters and volumes). AQPs with big diameters could not be closed in the presence of osmolytes of small molecular size, even at very high concentrations. The cohesion/tension theory allowed pore volumes of AQPs to be evaluated, which was 2.3 ± 0.2 nm³ for the narrow pore and between 5.5 ± 0.8 and 6.1 ± 0.8 nm³ for the wider pores. The existence of different types of pores was also evident from differences in the residual Lp. Alternatively, pore volumes were estimated from ratios between osmotic (P_f) and diffusional (P_d) water flow, yielding the number of water molecules (N) in the pores. N-values ranged between 35 and 60, which referred to volumes of 0.51 and 0.88 nm³/pore. Values of pore volumes obtained by either method were bigger than those reported in the literature for other AQPs. Absolute values of pore volumes and differences obtained by the two methods are discussed in terms of an inclusion of mouth parts of AQPs during osmotic dehydration. It is concluded that the mouth part contributed to the absolute values of pore volumes depending on the size of osmolytes. However, this can not explain the finding of the existence of two different types or groups of AQPs in the plasma membrane of Chara.     

Validation of sensitivity to praziquantel using Schistosoma mansoni worm muscle tension and Ca2+-uptake as possible in vitro correlates to in vivo ED50 determination

Schistosome worm muscle tension and [45Ca2+]-uptake were tested as possible correlates of susceptibility to praziquantel (PZQ) assessed by estimating the drug ED50. Schistosoma mansoni cercariae of PZQ sensitive (S-CD, S-MOC and S-GP) and insensitive S. mansoni isolates (I-EE2, I-BANL and I-Senegal 47) were used to infect batches of CD-1 Swiss albino mice. Seven weeks after infection, animals of each batch were divided into six groups. Five of them received PZQ in doses of 12.5, 25, 50, 100 or 200 mg/kg PZQ, respectively, for five consecutive days, while the sixth was left as untreated controls. Two weeks after treatment mice were sacrificed, perfused and PZQ ED50's were estimated. Male worms recovered from infected untreated controls were examined for their muscle tension increase in response to PZQ using a physiological recorder coupled to a photooptic transducer. [45Ca2+]-uptake of male worms in the presence and absence of PZQ was determined using a liquid scintillation beta counter. Data revealed that PZQ insensitive isolates had significantly higher drug ED50 (>130 mg/kg) than PZQ sensitive isolates with ED50's <100 mg/kg. Moreover, in response to PZQ they were found to possess significant reductions in their worm muscle tension and their [45Ca2+]-uptake were <100%. Both parameters showed a significant negative correlation to PZQ ED50 in vivo and a significant positive correlation to each other.     

Habitat use of nightjar (Caprimulgus europaeus) in an Austrian pine forest

The Steinfeld in Lower Austria supports a population of European Nightjar (Caprimulgus europaeus) which was extensively studied during 1997 and 1998. The study area encompassed a pine forest of 20 km². The population densities of 1.05 and 1.25 territories/km², respectively, lies within the range found in central European populations. Annual monitoring until 2001 has shown the population to be stable. To gain an insight into habitat use of the species, various habitat-related parameters were measured inside and outside the territories, namely structure of trees, density of trees, structure of undergrowth vegetation and structure of clearings. Discriminant analysis was applied to assess the factors responsible for habitat choice of the Nightjar population. The findings showed that the Nightjars territories were frequently centered on a large clearing with an area of at least 0.7 ha. Clearings less than 50 m wide were not colonized. The requirement for a minimum width of a clearing in addition to a minimum area probably relates to better hunting conditions. Nightjars prefer trees where the lower edge of the crown is on average 4.38 m higher than at control points so that males can churr from dead branches immediately below the canopy. Such trees were found on the edge of clearings in the forest, and the edge of a clearing thus had a pronounced effect on the quality of a territory. In contrast to reports in the literature, neither the proportion of bare patches of ground nor the average height of undergrowth vegetation was found to be decisive for territory selection.     

Responses of skilletfish Gobiesox strumosus to high temperature and low oxygen stress

This study quantified physiological responses of skilletfish Gobiesox strumosus exposed to thermal and oxic stress. Fish acclimated at 12, 22 and 32° C had low oxygen tolerance values (mean ±s.d .) of 0·40 ± 0·09, 0·40 ± 0·08 and 0·35 ± 0·03, and critical thermal maxima (mean ±s.d .) of 33·2 ± 0·5, 38·1 ± 0·0 and 39·5 ± 0·3° C, respectively. Furthermore, G. strumosus were oxygen conformers at all acclimation temperatures, i.e. the fish allowed oxygen consumption rates to decrease with ambient oxygen concentration. High temperature tolerance, low oxygen tolerance and decreasing metabolic rates during hypoxic events allow the fish to survive harsh environmental conditions encountered in their natural environment.     

Laurdan fluorescence senses mechanical strain in the lipid bilayer membrane

The precise molecular mechanisms by which cells transduce a mechanical stimulus into an intracellular biochemical response have not yet been established. Here, we show for the first time that the fluorescence emission of an environment-sensitive membrane probe Laurdan is modulated by mechanical strain of the lipid bilayer membrane. We have measured fluorescence emission of Laurdan in phospholipid vesicles of 30, 50, and 100 nm diameter to show that osmotically induced membrane tension leads to an increase in polarity (hydration depth) of the phospholipid bilayer interior. Our data indicate that the general polarization of Laurdan emission is linearly dependent on membrane tension. We also show that higher membrane curvature leads to higher hydration levels. We anticipate that the proposed method will facilitate future studies of mechanically induced changes in physical properties of lipid bilayer environment both in vitro and in vivo.     

Development of periodic tension in the contractile vacuole complex membrane of paramecium governs its membrane dynamics

The contractile vacuole complex is a membrane-bound osmoregulatory organelle of fresh water protozoa such as Paramecium. In Paramecium it consists of a central vacuole (the contractile vacuole) and 5-10 arms that radially extend from the vacuole into the cytosol (the radial arms). Excess cytosolic water, acquired osmotically, is segregated by the radial arms and enters the vacuole, so that the vacuole swells (the fluid-filling phase). The vacuole then rounds (the rounding phase) and the radial arms sever from the vacuole. The vacuole membrane then fuses with the plasma membrane at the pore region and the pore opens. The vacuole shrinks as its fluid is discharged through the pore (the fluid-discharging phase). The pore closes when the fluid has been discharged. The radial arms then reattach to the vacuole, so that the vacuole swells again as the fluid enters from the arms (the next fluid-filling phase). We found that the vacuole continued to show rounding and slackening even after it together with a small amount of cytosol had been isolated from the cell. Using a microcantilever placed on the surface of the vacuole the tension of the in vitro vacuole increased to 5 x 10(-3)N m(-1) as the vacuole rounds, and its lowest value was 1 x 10(-4)N m(-1) during slackening. We propose a hypothesis that an increase in the spontaneous curvature of the organelle's membrane leads to an increase in membrane tension and thus to the vacuole's rounding, severing of the radial arms from the vacuole, and opening of the pore. Conversely, a decrease in the spontaneous curvature accompanied by a decrease in membrane tension could lead to the closing of the pore and reattachment of the radial arm at the start of the fluid-filling phase.     

The gating mechanism of the bacterial mechanosensitive channel MscL revealed by molecular dynamics simulations: From tension sensing to channel opening

One of the ultimate goals of the study on mechanosensitive (MS) channels is to understand the biophysical mechanisms of how the MS channel protein senses forces and how the sensed force induces channel gating. The bacterial MS channel MscL is an ideal subject to reach this goal owing to its resolved 3D protein structure in the closed state on the atomic scale and large amounts of electrophysiological data on its gating kinetics. However, the structural basis of the dynamic process from the closed to open states in MscL is not fully understood. In this study, we performed molecular dynamics (MD) simulations on the initial process of MscL opening in response to a tension increase in the lipid bilayer. To identify the tension-sensing site(s) in the channel protein, we calculated interaction energy between membrane lipids and candidate amino acids (AAs) facing the lipids. We found that Phe78 has a conspicuous interaction with the lipids, suggesting that Phe78 is the primary tension sensor of MscL. Increased membrane tension by membrane stretch dragged radially the inner (TM1) and outer (TM2) helices of MscL at Phe78, and the force was transmitted to the pentagon-shaped gate that is formed by the crossing of the neighboring TM1 helices in the inner leaflet of the bilayer. The radial dragging force induced radial sliding of the crossing portions, leading to a gate expansion. Calculated energy for this expansion is comparable to an experimentally estimated energy difference between the closed and the first subconductance state, suggesting that our model simulates the initial step toward the full opening of MscL. The model also successfully mimicked the behaviors of a gain of function mutant (G22N) and a loss of function mutant (F78N), strongly supporting that our MD model did simulate some essential biophysical aspects of the mechano-gating in MscL.