Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.     

立地条件差异对天然东北红豆杉(Taxus cuspidata)种间竞争的影响

立地因子空间梯度特征是影响种间竞争的重要因素。通过研究主要竞争树种及竞争强度随海拔、坡向、坡位和坡度等立地因子变化的规律,揭示不同立地条件对天然东北红豆杉种间竞争的影响,为濒危东北红豆杉种群的保护与恢复提供理论指导。研究结果表明：东北红豆杉的种间竞争压力主要来自紫椴、臭冷杉、色木槭、红松、青楷槭、枫桦、花楷槭等7个树种,主要竞争树种以小径级个体居多。立地因子差异对种间竞争强度和主要竞争树种的排序有明显影响。东北红豆杉在阴坡受到的种间竞争压力是阳坡的3.27倍,主要树种的竞争强度显著高于阳坡,竞争树种的排序发生更迭;随着坡位的提升,东北红豆杉受到的种间竞争压力明显增加,主要竞争种类和强度也发生变化;坡度增加,种间竞争强度增加,主要竞争树种排序发生变化;海拔对种间竞争的影响总体不大,但影响主要竞争树种的竞争强度和排序。     

Shifts from competition to facilitation between a foundation tree and a pioneer shrub across spatial and temporal scales in a semiarid woodland

* Theoretical and empirical research has supported the hypothesis that plant-plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems. * During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon pine, Pinus edulis) and a common shrub (Apache plume, Fallugia paradoxa) in replicated areas of high and low stress. * The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the interactions depended on temporal variation in climate and age of P. edulis. Both above- and below-ground factors contributed to competition, while only above-ground factors contributed to facilitation. * These results support the hypothesis that interactions can change from competition to facilitation as abiotic stress increases in semiarid environments. A shift from competition to facilitation may be important for the recovery of P. edulis and other foundation species that have experienced large-scale mortality during recent droughts.     

Competition and facilitation in synthetic communities of arbuscular mycorrhizal fungi

Interactions between arbuscular mycorrhizal fungal (AMF) species cocolonizing the same host plant are still little understood in spite of major ecological significance of mycorrhizal symbiosis and widespread occurrence of these fungi in communities rather than alone. Furthermore, shifting the composition of AMF communities has demonstrated consequences for the provision of symbiotic benefits to the host as well as for the qualities of ecosystem services. Therefore, here we addressed the nature and strength of interactions between three different AMF species in all possible two‐species combinations on a gradient of inoculation densities. Fungal communities were established in pots with Medicago truncatula plants, and their composition was assessed with taxon‐specific real‐time PCR markers. Nature of interactions between the fungi was varying from competition to facilitation and was influenced by both the identity and relative abundance of the coinoculated fungi. Plants coinoculated with Claroideoglomus and Rhizophagus grew bigger and contained more phosphorus than with any of these two fungi separately, although these fungi obviously competed for root colonization. On the other hand, plants coinoculated with Gigaspora and Rhizophagus, which facilitated each other's root colonization, grew smaller than with any of these fungi separately. Our results point to as yet little understood complexity of interactions in plant‐associated symbiotic fungal communities, which, depending on their composition, can induce significant changes in plant host growth and/or phosphorus acquisition in either direction.     

Distribution and species richness of woody dryland legumes in Baja California,Mexico

Abstract. We analysed the biogeographic patterns of woody legumes in the Baja California peninsula, NW Mexico. From the specimen labels of eight herbaria, we digitized 4205 records from 78 species, and projected them onto a grid of 205 cartographic cells (20’ longitude × 15’ latitude). Most species followed distribution patterns that coincide with floristic subdivisions of the peninsula. Endemism is high, reaching 60–70% in the centre of the peninsula, where the driest deserts are found and where significant floristic changes took place during Pleistocene glacial events. The number of cartographic cells (i.e. their geographic ranges) were log‐normally distributed, as has been reported for many other taxa. Floristic richness was found to be clumped around some cells where the observed richness is significantly higher than could be expected from chance variation. We tested the hypothesis that these ‘hotspots’ could be attributable to great collection efforts or to large land surfaces, but we still found 16 cells where richness is significantly high once these two factors are accounted for. Species richness and micro‐endemism increase towards the south, conforming to Rapoport's rule that predicts that species ranges become smaller towards the equator while richness increases. The floristic hotspots for woody legumes in Baja California occur in the Cape Region and along the Sierra de la Giganta in the southern Gulf Coast, where 77% of the total peninsular legume flora can be found. These hotspots are mostly unprotected, and should be considered priority areas for future conservation efforts.     

Spatial relationships between uninfected and infected cells in root nodules of soybean

In soybean (Glycine max (L.) Merr.) the uninfected cells of the root nodule are responsible for the final steps in ureide production from recently fixed nitrogen. Stereological methods and an original quantitative method were used to investigate the organization of these cells and their spatial relationships to infected cells in the central region of nodules of soybean inoculated with Rhizobium japonicum strain USDA 3I1B110 and grown with and without nitrogen (as nitrate) in the nutrient medium. The volume occupied by the uninfected tissue was 21% of the total volume of the central infected region for nodules of plants grown without nitrate, and 31% for nodules of plants grown with nitrate. Despite their low relative volume, the uninfected cells outnumbered the much larger infected cells in nodules of plants grown both without and with nitrate. The surface density of the interface between the ininfected and infected tissue in the infected region was similar for nodules in both cases also, the total range being from 24 to 26 mm²/mm³. In nodules of plants grown without nitrate, all sampled infected cells were found to be in contact with at least one uninfected cell. The study demonstrates that although the uninfected tissue in soybean nodules occupies a relatively small volume, it is organized so as to produce a large surface area for interaction with the infected tissue.     

Competition drives trait evolution and character displacement between Mimulus species along an environmental gradient

Closely related species may evolve to coexist stably in sympatry through niche differentiation driven by in situ competition, a process termed character displacement. Alternatively, past evolution in allopatry may have already sufficiently reduced niche overlap to permit establishment in sympatry, a process called ecological sorting. The relative importance of each process to niche differentiation is contentious even though they are not mutually exclusive and are both mediated via multivariate trait evolution. We explore how competition has impacted niche differentiation in two monkeyflowers, Mimulus alsinoides and M. guttatus, which often co‐occur. Through field observations, common gardens, and competition experiments, we demonstrate that M. alsinoides is restricted to marginal habitats in sympatry and that the impacts of character displacement on niche differentiation are complex. Competition with M. guttatus alters selection gradients and has favored taller M. alsinoides with earlier seasonal flowering at low elevation and floral shape divergence at high elevation. However, no trait exhibits the pattern typically associated with character displacement, higher divergence between species in sympatry than allopatry. Thus, although character displacement was unlikely the process driving initial divergence along niche axes necessary for coexistence, we conclude that competition in sympatry has likely driven trait evolution along additional niche axes.     

Predation risk alters interactions among species: competition and facilitation between ants and nesting birds in a boreal forest

Although interactions between species are often assumed to be fixed, theory and empirical evidence suggest that they may be quite variable, changing in the presence of other species or environmental conditions. The interaction between ants and nesting birds exhibits such variability, ants sometimes being predators of bird nests and other times protectors of them. Hypothesizing that predation risk might be a critical factor in altering the interaction of ants with birds, I investigated the interaction of wood ants Formica aquilonia with nesting birds under different levels of predation risk. In a controlled field experiment, I allowed tits ( Parus major, P. caeruleus, P. ater ) and pied flycatchers ( Ficedula hypoleuca ) to select nest boxes in trees with ants (ant trees) or trees without ants. I found that birds usually nested in trees without ants, apparently to avoid the danger of injury from encounters with ants. Nesting in ant trees occurred mainly in the habitat where risk of predation was highest (along the forest edge), and with the bird taxa that lost nests most frequently in trees without ants (tits). Tits nesting on the forest edge achieved significantly greater nesting success, and fledged significantly more young, in ant trees compared with trees without ants. As the season progressed, ant traffic increased in trees without nesting birds, but decreased in trees with nesting birds, indicating that the outcome of interference competition between ants and nesting birds was reversed under increased predation risk. These results support the idea that predation risk can shift species interactions from predominately competitive processes to predominately facilitative processes.     

Competition in Kenyan soils between Rhizobium leguminosarum biovar phaseoli strain Kim5 and R. tropici strain CIAT899 using the gusA marker gene

The contribution of appropriate inoculum strains to more efficient nitrogen fixation by legumes has been difficult to assess due to the laborious nature of the assays involved in assessing establishment of inoculum strains in the field. The use of marker genes, in particular the GUS system, changes this, making it possible to assess occupancy by the inoculum strain in large numbers of nodules on whole root systems. Here we used the GUS system to evaluate the competitive ability of two rhizobial strains, Rhizobium leguminosarum bv. phaseoli strain Kim5 and R. tropici strain CIAT899 in two soil types from Kenya. The results confirm that Kim5 is a highly competitive strain, forming 86% of the nodules in a near-neutral pH soil. Although the competitiveness of CIAT899 is enhanced in an acid (pH 4.5) soil it still only formed 35% of the nodules. There were no differences between inoculum strains in their efficiency of nitrogen fixation in either soil type, and virtually no N2-fixation occurred in the acid soil due to the lack of tolerance of the Phaseolus genotype to soil acidity.     

基于视觉搜索的外源易化和返回抑制脑机制研究

采用事件相关电位技术研究了在视觉搜索过程中的外源易化和返回抑制（inhibition of return,IOR）的相互关系。当外源注意保持在序列搜索过的位置上时,有一个延时反应(即IOR）,伴随其产生的相关脑电成分有：分布在后顶的潜伏期为200 ms 的正差异、分布在前额叶内侧靠左的潜伏期为240 毫秒的负差异,以及分布在两侧颞顶联合区的潜伏期为280 ms 的负差异。而当外源注意保持在平行搜索的位置上时,则出现了明显的易化效应,伴随其产生的脑电成分仅为分布在枕顶区域的潜伏期为280 ms 的负差异。这些结果表明,外源易化和IOR 涉及了不同的脑区和神经过程,从而支持两者在机制上是可分离性的观点。     

南亚热带典型地区农业土壤中多环芳烃和有机氯农药研究

参考美国EPA标准方法对南亚热带潮汕地区农业土壤中持久性有机污染物多环芳烃（PAHs）和有机氯农药（OCPs）进行分析,并对其分布和来源进行研究,最后探讨了不同种植方式下污染物的污染特征。结果表明：该区农业土壤中PAHs含量范围从22．1ng／g到1256．9ng／g,与其它地区的污染程度和一些土壤环境质量标准相比,该区处于中等污染水平。高温裂解是土壤PAHs的主要来源。OCPs在被禁止20a后在土壤中仍有残留,残留水平从4．6ng／g到1021．5ng／g,其中HCHs和DDTs的残留未超过国家土壤环境质量标准。早期使用残留是其主要污染源,但新近仍有DDT和硫丹的输入。PAHs和OCPs的污染主要集中在土壤表层和亚表层,且随着深度的加深,污染逐渐减少,但40cm以下,污染变化很小。菜地土壤PAHs污染水平低于稻田土壤,而OCPs污染水平高于稻田土壤。     

Repeatability for Gaussian and non‐Gaussian data: a practical guide for biologists

Repeatability (more precisely the common measure of repeatability, the intra‐class correlation coefficient, ICC) is an important index for quantifying the accuracy of measurements and the constancy of phenotypes. It is the proportion of phenotypic variation that can be attributed to between‐subject (or between‐group) variation. As a consequence, the non‐repeatable fraction of phenotypic variation is the sum of measurement error and phenotypic flexibility. There are several ways to estimate repeatability for Gaussian data, but there are no formal agreements on how repeatability should be calculated for non‐Gaussian data (e.g. binary, proportion and count data). In addition to point estimates, appropriate uncertainty estimates (standard errors and confidence intervals) and statistical significance for repeatability estimates are required regardless of the types of data. We review the methods for calculating repeatability and the associated statistics for Gaussian and non‐Gaussian data. For Gaussian data, we present three common approaches for estimating repeatability: correlation‐based, analysis of variance (ANOVA)‐based and linear mixed‐effects model (LMM)‐based methods, while for non‐Gaussian data, we focus on generalised linear mixed‐effects models (GLMM) that allow the estimation of repeatability on the original and on the underlying latent scale. We also address a number of methods for calculating standard errors, confidence intervals and statistical significance; the most accurate and recommended methods are parametric bootstrapping, randomisation tests and Bayesian approaches. We advocate the use of LMM‐ and GLMM‐based approaches mainly because of the ease with which confounding variables can be controlled for. Furthermore, we compare two types of repeatability (ordinary repeatability and extrapolated repeatability) in relation to narrow‐sense heritability. This review serves as a collection of guidelines and recommendations for biologists to calculate repeatability and heritability from both Gaussian and non‐Gaussian data.     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

河北省侧蒴藓类植物的地理成分及其与邻近地区的比较

通过对河北省侧蒴藓类植物较详尽的野外考察、标本采集、鉴定和系统研究,已知河北省侧蒴藓类植物17科,66属,196种(包括种下单位),其地理成分可划分为9种类型,北温带分布型占主导地位,东亚分布型次之,热带分布型不起主要作用。运用物种丰富度、优势科比较、属相似性系数等统计方法,对河北与邻近地区的区系地理成分进行了比较、分析和讨论。从河北省与我国东北、内蒙古、山东、秦岭、西藏和横断山等6个地区侧蒴藓类区系组成的比较分析得出,本省该类群物种丰富度位居第4,横断山居于第1位,河北与其它6个地区的共有优势科前3位为青藓科、柳叶藓科、灰藓科;河北与内蒙古、山东相似程度最高,与东北和秦岭相似程度次之,与西藏、横断山相差最大。同时对这7个地区的具体地理成分也作了比较。这些结果有力地说明了河北省侧蒴藓类植物区系具有明显的温带性质,同时兼有浓厚的东亚区系色彩,也表明了中国华北与日本、欧洲和北美东北部在苔藓植物区系物种起源上有深厚的历史渊源。     

Water relations,photosynthesis, growth and water-use efficiency in tomato plants subjected to partial rootzone drying and regulated deficit irrigation

Abstract

Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) are water-saving irrigation systems that have been developed to increase water-use efficiency (WUE) without significant yield reduction. In order to investigate whether a high-value horticultural crop such as tomato responded differently to RDI and PRD, we compared the physiological and growth responses of tomato plants using a split-root system. Plants were grown in a greenhouse under controlled conditions with their roots separated equally between two soil compartments. Three irrigation treatments were imposed: (i) Control, receiving an amount of water equivalent to 100% of plant transpiration; (ii) PRD, in which one compartment was watered with 50% of the amount of water supplied to the controls, allowing one-half of the root system to be exposed to dry soil and switching irrigation between sides weekly; and (iii) RDI, in which 50% of the amount of water given to the controls was supplied, half to each side of the root system. Leaf RWC and midday leaf Ψ decreased substantially in RDI-treated plants, while the PRD plants exhibited relatively higher Ψ and RWC values. Both PRD and RDI treatments reduced by about 30% the total plant dry mass compared with the control. However, plant transpiration was reduced by about 50% in both PRD and RDI, allowing a significant improvement in whole-plant WUE. Stomatal conductance (Gs) and leaf growth were also significantly reduced by PRD and RDI. These results may be related to a significant increase in xylem sap pH and leaf apoplastic pH. Generally, the photosynthetic apparatus of tomato leaves had a high resistance to restricted water availability. In fact, the decreased Gs had no major negative impact on carbon assimilation. However, V _cmax, i.e. Rubisco efficiency, was significantly decreased in RDI plants with respect to control ones. This may imply that, although the differences between the PRD and RDI treatments in our study were subtle, they may become more marked with a more prolonged and severe water deficit.     

Population fluctuations and interspecific competition between Tephritid flies attacking fruit crops in the New Valley oases,Egypt

Population fluctuations of the Mediterranean fruit fly Ceratitis capitata (Wiedemann) and the peach fly Bactrocera zonata (Saunders) were monitored with lure trap collections in three provinces in the New Valley oases, Western Desert, Egypt. Results showed marked temporal differences in peak trap catches of the two flies in the selected sites all over the entire studied areas. One annual peak of C. capitata was recorded during both October 2005 and February 2006 and coincided with the ripening period of citrus trees in Kharga oases. However, two annual peaks were recorded during June and September 2005 in Bodkholow province and coincided with the ripening period of apple and mango. On the other hand, two annual peaks of B. zonata were recorded in Kharga oases throughout May and September and coincided with the ripening periods of apricot, mango and guava. One annual peak only was recorded in the round up of September and/or October in both Moot and Bodkholow in Dakhla oases and coincided with the ripening period of mango, guava and citrus. The occurrence of C. capitata was very limited in comparison with B. zonata. Population fluctuations of the two pests in the studied sites were significantly different. The ability of the traps used in capturing both C. capitata and B. zonata indicated that the yellow sticky trap was more effective in capturing C. capitata. However, the Abdel-Kawi trap was significantly efficient at trapping B. zonata. The occurrence of B. zontata in high numbers all over the study period compared to C. capitata is considered as good proof that this invading fly may be considered as a vigorous competitive tephritid fly to the native fly C. capitata.     

Competition between two similar plant varieties,green shrunk perilla and red shrunk perilla,in mixed cultures

Influences of plant density and time after seeding on the growth of two horticultural forms of perilla (Perilla frutescens var.crispa), green shrunk perilla (f.viridi-crispa) and red shrunk perilla (f.crispa), were examined in a mixed culture experiment. Relationships between mean individual plant weight and plant density in mixed populations were approximated by Ogawa's non-interaction type (NI-type) reciprocal equation. The density conversion factors in the equation for green and red perillas were always, respectively, smaller and larger than unity, suggesting that effects of a green perilla on the other individuals were always stronger than those of a red one in a mixed population. All coefficients in the NI-type reciprocal equation were expressed as functions of time after seeding. As a result, time trends of mean individual plant weights for both species in mixed populations could be reasonably estimated for different plant densities and mixed proportions. The results were also applied to Lotka-Volterra's equation. Time trends of Lotka-Volterra's competition coefficients for both plants could be calculated and were compared with those of density conversion factors.     

Model of gas exchange and diffusion in legume nodules

A mathematical model is described which allows the estimation of rates of O₂, CO₂, N₂, and H₂ exchange from legume nodules under steady state conditions of N₂ fixation. Calculated rates of gas exchange under defined conditions of nodule size, relative growth rate (RGR), specific total nitrogenase activity (TNA), nitrogenase electron allocation coefficient (EAC), uptake-hydrogenase activity (HUP) and nature of the N export product compared favorably with experimentally-obtained rates reported in the literature. Therefore the model was used to predict the effects of varying each of these nodule characteristics on the rates of gas exchange, and on the apparent respiratory cost (CO₂/NH₃) and sucrose cost (sucrose consumed/NH₃) of N₂ fixation.The model predicted that, all other characters being equal, ureide-producing nodules would consume 8% less sucrose per N fixed than asparagine-producing nodules, but would display an apparent respiratory cost which would be 5% higher than that in asparagine-producing nodules. In both ureide-producing and asparagine-producing nodules, the major factor affecting the apparent respiratory cost of N₂ fixation was predicted to be EAC, followed by TNA, nodule RGR and nodule size. The relative importance of HUP in improving the apparent respiratory cost of N₂ fixation was predicted to be largely dependent upon its potential role in the regulation of EAC. Abbreviations: See Appendix 1.     

The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (permeability coefficient, P_s), and reflection coefficients (σ_s) were measured on internodes of Chara corallina, Klein ex Willd., em R.D.W.. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 °C which provided evidence about pathways of water and solutes as they move across the plasma membrane (water channel and bilayer arrays). Test solutes used to check for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accuracy, Q₁₀ values for Lp and for the diffusive water permeability (P_d) were identical (Q₁₀ for Lp = 1.29 ± 0.17 (± SD; n = 15 cells) and Q₁₀ for P_d = 1.25 ± 0.16 (n = 5 cells)). The Q₁₀ values were equivalent to activation energies of E_a = 16.8 ± 6.4 and 16.6 ± 10.0 kJ · mol⁻¹, respectively, which is similar to that of self-diffusion or of viscous flow of water. The Q₁₀ values and activation energies for P_s of the alcohols were significantly larger (ethanol: Q₁₀ = 1.68 ± 0.16, E_a = 37.1 ± 5.9 kJ · mol⁻¹; n-propanol: Q₁₀ =  1.75 ± 0.40, E_a = 43.1 ± 15.3 kJ · mol⁻¹; iso-propanol: Q₁₀ = 2.12 ± 0.42, E_a =  52.2 ± 14.6 kJ · mol⁻¹; tert-butanol: Q₁₀ = 2.13 ± 0.56, E_a = 51.6 ± 17.1 kJ · mol⁻¹; ±SD; n = 5 to 6 cells). Effects of temperature on reflection coefficients were most pronounced. With increasing temperature, σ_s values of the alcohols decreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary and isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in σ_s values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of σ_s with temperature was theoretically predicted from the temperature dependence of P_s and Lp. Differences between predicted and measured values of σ_s allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of test solutes. It is concluded that water channels are much less selective than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of σ_s. The σ_s of HDO was extremely low. Its response to temperature was opposite to that for the σ_s of the alcohols. This suggested a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a differential sensitivity of different mechanisms to temperature. Received: 10 October 1996 / Accepted: 2 December 1996