Nonequilibrium thermodynamic model of the rat proximal tubule epithelium.

The rat proximal tubule epithelium is represented as well-stirred, compliant cellular and paracellular compartments bounded by mucosal and serosal bathing solutions. With a uniform pCO2 throughout the epithelium, the model variables include the concentrations of Na, K, Cl, HCO3, H2PO4, HPO4, and H, as well as hydrostatic pressure and electrical potential. Except for a metabolically driven Na-K exchanger at the basolateral cell membrane, all membrane transport within the epithelium is passive and is represented by the linear equations of nonequilibrium thermodynamics. In particular, this includes the cotransport of Na-Cl and Na-H2PO4 and countertransport of Na-H at the apical cell membrane. Experimental constraints on the choice of ionic conductivities are satisfied by allowing K-Cl cotransport at the basolateral membrane. The model equations include those for mass balance of the nonreacting species, as well as chemical equilibrium for the acidification reactions. Time-dependent terms are retained to permit the study of transient phenomena. In the steady state the energy dissipation is computed and verified equal to the sum of input from the Na-K exchanger plus the Gibbs free energy of mass addition to the system. The parameter dependence of coupled water transport is studied and shown to be consistent with the predictions of previous analytical models of the lateral intercellular space. Water transport in the presence of an end-proximal (HCO3-depleted) luminal solution is investigated. Here the lower permeability and higher reflection coefficient of HCO3 enhance net sodium and water transport. Due to enhanced flux across the tight junction, this process may permit proximal tubule Na transport to proceed with diminished energy dissipation.     

Does a type specimen necessarily or contingently belong to its species?

In a recent article, Alex Levine raises a paradox. It appears that, given some relatively uncontroversial premises about how a species term comes to refer to its species, a type specimen belongs necessarily and contingently to its species. According to Levine, this problem arises if species are individuals rather than natural kinds. I argue that the problem can be generalized: the problem also arises if species are kinds and type specimens are paradigmatic members used to baptize names for species. Indeed, the same problem arises with respect to kinds like gold and the samples used to ground names for them. After arguing that the paradox arises whether or not species are individuals, I attempt to show how the paradox can be resolved. Levine's argument that a type specimen belongs necessarily to its species is specious. The appeal of the argument stems from a failure to distinguish between two different modal statements concerning type specimens, one de dicto and the other de re. Type specimens belong contingently to their respective species. Even so, they can be known a priori to belong to them: hence, that a particular type specimen belongs to its species is an example of contingent a priori knowledge.     

Thermodynamic Analysis of Energy Transfer in Acidogenic Cultures

A global thermodynamic analysis, normally used for pure cultures, has been performed for steady‐state data sets from acidogenic mixed cultures. This analysis is a combination of two different thermodynamic approaches, based on tabulated standard Gibbs energy of formation, global stoichiometry and medium compositions. It takes into account the energy transfer efficiency, ?, together with the Gibbs free energy dissipation, ΔG_o, analysis of the different data. The objective is to describe these systems thermodynamically without any heat measurement. The results show that ? is influenced by environmental conditions, where increasing hydraulic retention time increases its value all cases. The pH effect on ? is related to metabolic shifts and osmoregulation. Within the environmental conditions analyzed, ? ranges from 0.23 for a hydraulic retention time of 20 h and pH 4, to 0.42 for a hydraulic retention time of 8 h and a pH ranging from 7–8.5. The estimated values of ΔG_o are comparable to standard Gibbs energy of dissipation reported in the literature. For the data sets analyzed, ΔG_o ranges from –1210 kJ/mol_x, corresponding to a stirring velocity of 300 rpm, pH 6 and a hydraulic retention time of 6 h, to –20744 kJ/mol_x for pH 4 and a hydraulic retention time of 20 h. For average conclusions, the combined approach based on standard Gibbs energy of formation and global stoichiometry, used in this thermodynamic analysis, allows for the estimation of Gibbs energy dissipation values from the extracellular medium compositions in acidogenic mixed cultures. Such estimated values are comparable to the standard Gibbs energy dissipation values reported in the literature. It is demonstrated that ? is affected by the environmental conditions, i.e., stirring velocity, hydraulic retention time and pH. However, a relationship that relates this parameter to environmental conditions was not found and will be the focus of further research.     

In search of a thermodynamic description of biomass yields for the chemotrophic growth of microorganisms

Correlations for the prediction of biomass yields are valuable, and many proposals based on a number of parameters (Y(ATP), Y(Ave), eta(o), Y(c), Gibbs energy efficiencies, and enthalpy efficiencies) have been published. This article critically examines the properties of the proposed parameters with respect to the general applicability to chemotrophic growth systems, a clear relation to the Second Law of Thermodynamics, the absence of intrinsic problems, and a requirement of only black box information. It appears that none of the proposed parameters satisfies all these requirements. Particularly, the various energetic efficiency parameters suffer from major intrinsic problems. However, this article will show that the Gibbs energy dissipation per amount of produced biomass (kJ/C-mod) is a parameter which satisfies the requirements without having intrinsic problems. A simple correlation is found which provides the Gibbs energy dissipation/C-mol biomass as a function of the nature of the C-source (expressed as the carbon chain length and the degree of reduction). This dissipation appears to be nearly independent of the nature of the electron acceptor (e.g., O(2), No(3) (-), fermentation). Hence, a single correlation can describe a very wide range of microbial growth systems. In this respect, Gibbs energy dissipation is much more useful than heat production/C-mol biomass, which is strongly dependent on the electron acceptor used. Evidence is presented that even a net heat-uptake can occur in certain growth systems.The correlation of Gibbs energy dissipation thus obtained shows that dissipation/C-mol biomass increases for C-sources with smaller chain length (C(6) --> C(1)), and increases for both higher and lower degrees of reduction than 4. It appears that the dissipation/C-mol biomass can be regarded as a simple thermodynamic measure of the amount of biochemical "work" required to convert the carbon source into biomass by the proper irreversible carbon-carbon coupling and oxidation/reduction reactions. This is supported by the good correlation between the theoretical ATP requirement for biomass formation on different C-sources and the dissipation values (kJ/C-mol biomass) found. The established correlation for the Gibbs energy dissipation allows the prediction of the chemotrophic biomass yield on substrate with an error of 13% in the yield range 0.01 to 0.80 C-mol biomass/(C)-mol substrate for aerobic/anaerobic/denitrifying growth systems.     

Properties of membrane stationary states

Summary The flux of permeant species through a membrane is examined using discrete state stochastic models for the transport process within the membrane. While a membrane flux is maintained due to a concentration gradient between bathing solutions, the distribution of species within the membrane evolves to a time invariant configuration which can differ significantly from the equilibrium configuration. Some special properties of these stationary states are examined using linear, microcanonical models for the membrane transport process. Analysis of these models reveals properties which are masked by the phenomenological analysis of irreversible thermodynamics. For example, the models can be used to study the nature of multi-state relaxation within the membrane by observation of the time dependence of the net membrane flux when the membrane is perturbed from its stationary state distribution. Under some conditions, multi-state models will produce relaxation similar to that observed for single-state processes. The symmetry within the membrane is a critical factor for monitoring relaxation processes within the membrane. Because of the stationary nature of the membrane configuration, statistical thermodynamic variables can be defined for the membrane configuration. The total system is not in equilibrium since the baths must still be described by dissipation functions. In the stationary state, the configurational entropy of the membrane is lowered relative to equilibrium and is shown to depend quadratically on the time independent parameter (j/p) wherej is the membrane flux andp is a characteristic transition probability for intra-membrane transitions. The basic membrane system serves as a quantitative example of the entropy reduction possible in a stationary state system. An allosteric transition mediated by the stationary state configuration is examined as a means of utilizing this negentropy production.     

Allocation to defense or growth in dipterocarp forest seedlings in Borneo

We quantified the allocation of net production to plant secondary metabolites (especially condensed tannins and lignins) to evaluate the investment into defense mechanisms of three tropical forest canopy species in Borneo vis-à-vis the resource availability hypothesis. In particular, we focused on Borneo ironwood (Eusideroxylon zwageri, Lauraceae), which seemed to employ an extreme defensive strategy. The wood of this species is extremely durable and has a high specific gravity with a very slow growth rate. The allocation to defense by Borneo ironwood was compared to two emergent species of Dipterocarpaceae, the dominant family in this forest community. We conducted shade-house experiments on seedlings under four controlled conditions (two light levels × two nutrient levels) and showed that the growth rate of E. zwageri was much lower than those of the other two species, and it allocated more of its net production to leaves and roots than to stems. The concentrations of condensed tannins and lignins were very high in the leaves and stems of this species, at about 20 and 30%, respectively. In total, E. zwageri allocated a maximum of about 35% of its net production to defensive substances (i.e., condensed tannins + lignins). In contrast, the two dipterocarp species allocated about 20–25% of their net production to defensive substances. The condensed tannins in E. zwageri help to prolong the lifespan of the leaves, and the lignins in the stems enhance the durability of the wood. Thus, although E. zwageri grows very slowly, the allocation to defensive substances seems to be an advantageous strategy for survival under dark conditions.An erratum to this article can be found at     

Does microbial life always feed on negative entropy? Thermodynamic analysis of microbial growth.

Schr?dinger stated in his landmark book, What is Life?, that life feeds on negative entropy. In this contribution, the validity of this statement is discussed through a careful thermodynamic analysis of microbial growth processes. In principle, both feeding on negative entropy, i.e. yielding products of higher entropy than the substrates, and generating heat can be used by microorganisms to rid themselves of internal entropy production resulting from maintenance and growth processes. Literature data are reviewed in order to compare these two mechanisms. It is shown that entropy-neutral, entropy-driven, and entropy-retarded growth exist. The analysis of some particularly interesting microorganisms shows that enthalpy-retarded microbial growth may also exist, which would signify a net uptake of heat during growth. However, the existence of endothermic life has never been demonstrated in a calorimeter. The internal entropy production in live cells also reflects itself in the Gibbs energy dissipation accompanying growth, which is related quantitatively to the biomass yield. An empirical correlation of the Gibbs energy dissipation in terms of the physico-chemical nature of the growth substrate has been proposed in the literature and can be used to predict the biomass yield approximately. The ratio of enthalpy change and Gibbs energy change can also be predicted since it is shown to be approximately equal to the same ratio of the relevant catabolic process alone.     

Photosynthetic responses to temperature in tropical lotic macroalgae

A comparative analysis of the photosynthetic responses to temperature (10–30°C) was carried out under short‐term laboratory conditions by chlorophyll fluorescence and oxygen (0₂) evolution. Ten lotic macroalgal species from southeastern Brazil (20°11–20°48′S, 49°18–49°41′W) were tested, including Bacillariophyta, Chlorophyta, Cyanophyta, Rhodophyta and Xanthophyta. Temperature had significant effects on electron transport rate (ETR) only for three species (Terpsinoe musica, Bacillariophyta; Cladophora glomerata, Chlorophyta; and C. coeruleus, Rhodophyta), with highest values at 25–30°C, whereas the remaining species had no significant responses. It also had similar effects on non‐photochemical quenching and ETR. Differences in net photosynthesisldark respiration ratios at distinct temperatures were found, with an increasing trend of respiration with higher temperatures. This implies in a decreasing balance between net primary production and temperature, representing more critical conditions toward higher temperatures for most species. In contrast, high net photosynthesis and photosynthesisldark respiration ratios at high and wide ranges of temperature were found in three species of green algae, suggesting that these algae can be important primary producers in lotic ecosystems, particularly in tropical regions. Optimal photosynthetic rates were observed under similar environmental temperatures for five species (two rhodophytes, two chlorophytes and one diatom) considering both techniques, suggesting acclimation to their respective ambient temperatures. C. coeruleus was the only species with peaks of ETR and 0₂ evolution under similar field‐measured temperatures. All species kept values of ETR and net photosynthesis close to the optimum under a broad range of temperatures. Increased non‐photochemical quenching, as a measure of thermal dissipation of excess energy, toward higher temperatures was observed in some species, as well as positive correlation of non‐photochemical quenching with ETR, and were interpreted as two mechanisms of adaptation of the photosynthetic apparatus to temperature changes. Different optimal temperatures were found for individual species by each technique, generally under lower temperatures by 0₂ evolution, indicating dependence on distinct factors: increases in temperature generally induced higher ETR due to increased enzymatic activity, whereas increments of enzymatic activity were compensated by increased respiration and photorespiration leading to decreases in net photosynthesis.     

Water relations of indigenous versus exotic tree species,growing at the same site in a tropical montane forest in southern Ethiopia

The objective of the study was to compare the water relations of two indigenous [Podocarpus falcatus (Thunb.) Endl., Croton macrostachys Hochst. ex. Del.] and two exotic tree species (Eucalyptus globulus Labille., Cupressus lusitanica Miller) growing in the same location in the montane Munessa State Forest, southern Ethiopia. Stem flow was measured with Granier type thermal dissipation probes. Sap flux, normalized per unit sapwood area, and the total sapwood areas of the particular trees were used to estimate daily transpiration. Maximum daily transpiration values (60 kg water) were recorded for Croton when at full foliage. After shedding most of its leaves in the dry season transpiration was reduced to 8 kg per day. Eucalyptus had the next highest transpiration (55 kg), in this case at the peak of the dry season. It transpired 4–5 times more than Podocarpus and Cupressus trees of similar size. Maximum stem flux density was tree-size dependent only in Croton. Diurnal patterns of stem flux indicated that Croton, Eucalyptus and Podocarpus, in contrast to Cupressus, responded more directly to light than to atmospheric water pressure deficit. At high VPD (>1.0 kPa) stem flux reached a plateau in Croton and Podocarpus indicating stomatal limitation. Per unit leaf area Croton had the highest and Podocarpus and Cupressus the lowest daily transpiration rates. In summary, the pioneer tree Croton had the lowest and Podocarpus the highest water use efficiency. The contribution of the study to the understanding of the role of each tree species in the hydrology of the natural forest and the plantations is discussed.     

黄土丘陵区辽东栎群落优势种和主要伴生种树干液流动态特征

运用Granier热扩散探针法(TDP),于2011年5月对黄土丘陵区延安市南郊公路山辽东栎群落优势种(辽东栎)和3个伴生种(山杏、细裂槭、桃叶卫矛)树干液流进行连续测定,并同步监测气象环境因子(太阳辐射、空气温度和相对湿度)和土壤水分动态,比较分析辽东栎、山杏、细裂槭、桃叶卫矛的树干液流通量密度日变化动态及其对环境因子的响应特征。结果表明,辽东栎、山杏、细裂槭和桃叶卫矛液流通量密度日变化动态特征总体上反映了气象环境因子变化的昼夜规律性,呈单峰或双峰型曲线,但辽东栎与山杏、细裂槭、桃叶卫矛液流通量密度日变化规律存在一定的差异。辽东栎液流启动时间较早,通常在5:00左右,而山杏、细裂槭和桃叶卫矛液流启动时间均晚于辽东栎一个小时以上。辽东栎液流达到峰值的时间也较其它树种早,通常出现在9:00前后;细裂槭、桃叶卫矛和山杏液流通量密度达到峰值的时间分别在10:00、11:00和13:00前后。细裂槭和桃叶卫矛液流通量密度日变化曲线峰值较窄。除此之外,土壤水分状况对伴生种的影响程度要比优势种的大。相关分析表明,树干液流通量密度与太阳辐射和空气水汽压亏缺均呈极显著正相关。优势种与伴生种的液流动态差异可能与树木本身特性以及群落对光能的限制性再分配有关。     

pH affects ammonium,nitrate and proton fluxes in the apical region of conifer and soybean roots

The effect of pH on nitrate and ammonium uptake in the high‐affinity transport system and low‐affinity transport system ranges was compared in two conifers and one crop species. Many conifers grow on acidic soils, thus their preference for ammonium vs nitrate uptake can differ from that of crop plants, and the effect of pH on nitrogen (N) uptake may differ. Proton, ammonium and nitrate net fluxes were measured at seedling root tips and 5, 10, 20 and 30 mm from the tips using a non‐invasive microelectrode ion flux measurement system in solutions of 50 or 1500 µM NH₄NO₃ at pH 4 and 7. In Glycine max and Pinus contorta, efflux of protons was observed at pH 7 while pH 4 resulted in net proton uptake in some root regions. Pseudotsuga menziesii roots consistently showed proton efflux behind the root tip, and thus appear better adapted to maintain proton efflux in acid soils. P. menziesii's ability to maintain ammonium uptake at low pH may relate to its ability to maintain proton efflux. In all three species, net nitrate uptake was greatest at neutral pH. Net ammonium uptake in G. max and net nitrate uptake in P. menziesii were greatly reduced at pH 4, particularly at high N concentration, thus N concentration should be considered when determining optimum pH for N uptake. In P. menziesii and G. max, net N uptake was greater in 1500 than 50 µM NH₄NO₃ solution, but flux profiles of all ions varied among species.     

The accuracy of the thermal dissipation technique for estimating sap flow is affected by the radial distribution of conduit diameter and density

There are conflicting reports on the accuracy of the thermal dissipation probe (TDP, the Granier method) measurement using the original formula, which is widely used to estimate the transpiration of individual trees and forest stands. In this article, six woody species of three wood types were used to study a possible association between TDP measurement accuracy and wood anatomical characteristics, including the vessel diameter and density, as well as sapwood depth. We found that TDP technique with Granier’s original equation underestimated the sap flux density in six species to various degrees, dependent on conduit size and sap flux. Our calibration using two conifers with small diameters and a high density of tracheids was relatively consistent with Granier’s calibration; however, because there were larger diameters and lower densities of vessels in the two diffuse-porous species, the original calibration significantly underestimated sap flow. Two ring-porous species had the largest diameters and lowest densities of vessels. In particular, Robinia pseudoacacia possessed the shallowest sap wood depth, less than a probe length. Our calibration for the ring-porous species, especially R. pseudoacacia, deviated far from the original calibration, which mostly underestimated the sap flow. The degree of underestimation was well associated with sap wood depth and the radial diameter and density distribution of conduits. Our results demonstrated that a new calibration must be operated for each species together with the sapwood depth determination and more probes may be applied for one stem in the field to obtain the more accurate sap flux. In addition, we investigated the effects of different environmental temperature and perfusing fluid composition on the TDP-based sap flux measurement. We found that an environmental temperature reduction from 25 to 0 °C did not alter the values of the maximum temperature difference (ΔT_m) between a heated probe and a reference probe when there was no sap flow, verifying that ΔT_m measured at night can be used as a reference in daytime.     

Absorption by a moving spherical organelle in a heterogeneous cytoplasm: implications for the role of trafficking in a symplast

An organelle which absorbs (or secretes) a particular factor will find its mass transfer rate diffusion-limited if it is stationary with respect to its ambient cytoplasm; but organellar motion will raise that limit as a non-decreasing function of the Peclet number P. It is shown analytically that (i) no Whitehead paradox need be encountered in the creeping flow regime and (ii) the flux of the factor will be an even function of the Peclet number, P. By a novel analytic solution method, the flux is shown numerically to increase as P2 for P < or = 1. For P > or = 10, a quasi-planar approximating geometry yields analytically a flux which increases as P1/3. These two solutions overlap smoothly in the range 1 < or = P > or = 10. For P approximately 1, convection should increase the mass flux by roughly 100%.     

The thermodynamic meaning of metabolic exchange fluxes

Metabolic flux analysis (MFA) deals with the experimental determination of steady-state fluxes in metabolic networks. An important feature of the ¹³C MFA method is its capability to generate information on both directions of bidirectional reaction steps given by exchange fluxes. The biological interpretation of these exchange fluxes and their relation to thermodynamic properties of the respective reaction steps has never been systematically investigated. As a central result, it is shown here that for a general class of enzyme reaction mechanisms the quotients of net and exchange fluxes measured by ¹³C MFA are coupled to Gibbs energies of the reaction steps. To establish this relation the concept of apparent flux ratios of enzymatic isotope-labeling networks is introduced and some computing rules for these flux ratios are given. Application of these rules reveals a conceptional pitfall of ¹³C MFA, which is the inherent dependency of measured exchange fluxes on the chosen tracer atom. However, it is shown that this effect can be neglected for typical biochemical reaction steps under physiological conditions. In this situation, the central result can be formulated as a two-sided inequality relating fluxes, pool sizes, and standard Gibbs energies. This relation has far-reaching consequences for metabolic flux analysis, quantitative metabolomics, and network thermodynamics.     

Antioxidants and xanthophyll cycle-dependent energy dissipation in Cucurbita pepo L. and Vinca major L. acclimated to four growth PPFDs in the field

The acclimation of photochemistry, xanthophyll cycle-dependent energy dissipation, and antioxidants was characterized in leaves of Cucurbita pepo L. and Vinca major L. that developed under photosynthetic photon flux densities (PPFDs) ranging from deep shade to full sunlight in the field. The predominant acclimatory response of leaf pigment composition was an increase in the xanthophyll cycle pool size with increasing growth PPFD. In both species, the estimated rate of thermal energy dissipation at midday increased with increasing PPFD and midday levels of zeaxanthin and antheraxanthin per chlorophyll were closely correlated with the levels of non-photochemical fluorescence quenching under all growth PPFD regimes. However, at full sunlight there appeared to be considerably higher levels of xanthophyll cycle dependent energy dissipation in V. major compared with pumpkin while estimated rates of photochemistry exhibited the reverse trend. Leaf activities of the antioxidant enzymes ascorbate peroxidase and superoxide dismutase, as well as ascorbate content, increased with increasing growth PPFD in both plant species. Activities/contents were higher under 100% full sunlight and increased more strongly from intermediate growth PPFDs to 100% full sunlight in V. major than in C. pepo. These patterns of acclimation are similar to those exhibited by xanthophyll cycle-dependent energy dissipation. The patterns of acclimation of glutathione reductase are discussed in the context of the multiple roles for reduced glutathione. Catalase acclimated in a manner consistent with its role in scavenging H2O2 generated via photorespiration and/or mitochondrial respiration. Leaf -tocopherol did not exhibit growth PPFD-dependent trends.     

Using chlorophyll fluorescence to assess the fraction of absorbed light allocated to thermal dissipation of excess excitation

In the present study we explored the possibility of assessing the allocation of photons absorbed by photosystem II (PSII) antennae to thermal energy dissipation and photosynthetic electron transport in leaves of several plant species under field conditions. Changes in chlorophyll fluorescence parameters were determined in situ over the course of an entire day in the field in sun-exposed leaves of two species with different maximal rates of photosynthesis, Helianthus annuus (sunflower) and Vinca major. Leaves of Vinca minor (periwinkle) growing in a deeply shaded location were also monitored. We propose using diurnal changes in the efficiency of open PSII centers (F′_v/F′_m) in these sun and shade leaves to (a) assess diurnal changes in the allocation of absorbed light to photochemistry and thermal energy dissipation and, furthermore, (b) make an estimate of changes in the rate of thermal energy dissipation, an analogous expression to the rate of photochemistry. The fraction of light absorbed in PSII antennae that is dissipated thermally (D) is proposed to be estimated from D = 1-F′_v/F′_m, in analogy to the widely used estimation of the fraction of light absorbed in PSII antennae (P) that is utilized in PSII photochemistry from P = F′_v/F′_m× q_P (where q_P is the coefficient for photochemical quenching; Genty, B., Briantais, J.-M. & Baker, N. R. 1989. Biochim. Biophys. Acta 990: 87-92). The rate of thermal dissipation is consequently given by D × PFD (photon flux density), again in analogy to the rate of photochemistry P × PFD, both assuming a matching behavior of photosystems I and II. Characterization of energy dissipation from the efficiency of open PSII centers allows an assessment from a single set of measurements at any time of day; this is particularly useful under field conditions where the fully relaxed reference values of variable or maximal fluorescence needed for the computation of nonphotochemical quenching may not be available. The usefulness of the assessment described above is compared with other currently used parameters to quantify nonphotochemical and photochemical chlorophyll fluorescence quenching.     

The relationship between CO2 assimilation, photosynthetic electron transport and water–water cycle in chill-exposed cucumber leaves under low light and subsequent recovery

The effects of chilling under low light (9/7 °C, 100 µmol m^?2 s^?1) on the photosynthetic and antioxidant capacities and subsequent recovery were examined in two (one tolerant and one sensitive) cucumber genotypes. Chilling resulted in an irreversible inhibition of net CO₂ assimilation and growth for the sensitive genotype, which was accompanied by decreases in the maximum velocity of RuBP carboxylation by Rubisco (V_cmax), the capacity for ribulose‐1,5‐bisphosphate regeneration (J_max), Rubisco content and activity, and the quantum efficiency of photosystem II, in the absence of any stomatal limitation of CO₂ supply or inorganic phosphate limitation. In contrast, CO₂ assimilation for the tolerant genotype fully recovered after chill. The chill‐induced decrease in the proportion of electron flux for photosynthetic carbon reduction was mostly compensated by an O₂‐dependent alternative electron flux driven by the water–water cycle, especially in the sensitive genotype. Compared with the tolerant genotype, the sensitive genotype after chill showed reduced capacity for scavenging reactive oxygen species and increased accumulation of reactive oxygen species. The balance between O₂‐dependent alternative electron flux and the capacity for scavenging reactive oxygen species in response to chill plays a major role in determining the tolerance of cucumber leaves to this stress factor. It is concluded that the water–water cycle operates at high rates when CO₂ assimilation is restricted in cucumber leaves subjected to chill and low light conditions.     

Wilderness Restoration: The Paradox of Public Participation

Recent trends in ecological restoration complicate the job of wilderness managers. An emphasis on volunteer participation in restoration designed to foster human/nature relationships often conflicts with the mandate to leave land untrammeled. We frame this conflict as the “participation paradox.” Higgs’ (2003) Nature by Design contains a response to the paradox that includes a strong defense of participatory focal restoration and a related critique of wilderness. After identifying the limitations of Higgs’ arguments, we address the paradox by showing how an appeal to the moral virtues of humility, self‐restraint, and altruism supports a restrictive conception of wilderness and a healing metaphor for wilderness restoration. The virtue‐informed healing metaphor provides an argument for restricting volunteer participation and long‐term restoration projects in wilderness areas. It also identifies the general conditions in which damaged wilderness should be allowed to “heal itself.” The upshot of our approach to the paradox is that some standards for good restoration should be contextualized to land use designations. In particular, the emphasis on participatory restoration is appropriate in humanized landscapes but not in wilderness.     

Gender, season and habitat: Patterns of variation in photosynthetic activity, growth and fecundity in Thymelaea velutina

Changes in the ecophysiological performance of a plant species due to different environmental conditions generally reflect adaptations to the habitat where the plant grows and are often related to its survival capacity in a particular place. We examined this with the dioecious shrub Thymelaea velutina, in two contrasting populations representing the extremes of the altitudinal gradient where the species lives (coastal dunes and mountain habitats over 1000 m). We measured net photosynthetic rates and stomatal conductance, estimated the level of plant stress by chlorophyll fluorescence, and assessed their correlations with growth rate, plant size, flower production and fruit set. We hypothesized that plants at high altitude were more photosynthetically stressed than at sea level and expected a gender × habitat interaction in performance as females need more resources than males. Plants in the mountain experienced chronic photoinhibition during winter and a reduced photosynthetic performance both in winter and spring compared to plants in coastal dunes. However, there was no association between any of the fluorescence variables and either plant growth or fecundity, suggesting that other factors are involved determining performance. Mountain plants showed also an apparent lower capacity of heat dissipation to excessive radiation than dune plants. In the dunes, the greater leaf area and mass can lead to a higher photosynthetic carbon gain by whole individuals compared to plants in the mountain. No effect of gender was detected on the ecophysiological performance of this species, which we partly attribute to the small size of fruits of the female plants.     

内蒙古典型草原建群种羊草基因型多样性抑制群落物种多样性的生态功能

植物群落的物种多样性以及群落建群种的基因型多样性对群落生态功能是否存在交互影响已成为群落生态学研究的热点内容。以内蒙古典型草原群落内常见物种为研究对象,研究了群落物种多样性与建群种羊草(Leymus chinensis)基因型多样性及其交互作用对群落生物量生态功能特性的影响。结果表明:(1)羊草基因型多样性、物种多样性及其交互作用对群落地上、地下和总生物量无显著影响(P0.05);(2)羊草基因型多样性、物种多样性及其交互作用对多样性效应(净多样性效应、互补效应和选择效应)有显著影响(P0.05)。羊草基因型多样性抑制多样性净效应的发挥,且主要抑制互补效应;而物种多样性则促进多样性净效应的发挥,主要表现为选择效应对地上生物量的正效应;(3)互补效应对群落生物量多样性净效应起主要贡献。实验所得结果不仅为探讨多样性效应在物种水平以及群落水平上对群落生物量的影响因素提供了重要启示,而且为内蒙古草原种质资源的保护及合理利用,乃至生态系统的恢复和重建提供理论指导。