Temperature responses of dark respiration in relation to leaf sugar concentration

Changes in leaf sugar concentrations are a possible mechanism of short‐term adaptation to temperature changes, with natural fluctuations in sugar concentrations in the field expected to modify the heat sensitivity of respiration. We studied temperature‐response curves of leaf dark respiration in the temperate tree Populus tremula (L.) in relation to leaf sugar concentration (1) under natural conditions or (2) leaves with artificially enhanced sugar concentration. Temperature‐response curves were obtained by increasing the leaf temperature at a rate of 1°C min^?1. We demonstrate that respiration, similarly to chlorophyll fluorescence, has a break‐point at high temperature, where respiration starts to increase with a faster rate. The average break‐point temperature (T_RD) was 48.6 ± 0.7°C at natural sugar concentration. Pulse‐chase experiments with ¹⁴CO₂ demonstrated that substrates of respiration were derived mainly from the products of starch degradation. Starch degradation exhibited a similar temperature‐response curve as respiration with a break‐point at high temperatures. Acceleration of starch breakdown may be one of the reasons for the observed high‐temperature rise in respiration. We also demonstrate that enhanced leaf sugar concentrations or enhanced osmotic potential may protect leaf cells from heat stress, i.e. higher sugar concentrations significantly modify the temperature‐response curve of respiration, abolishing the fast increase of respiration. Sugars or enhanced osmotic potential may non‐specifically protect respiratory membranes or may block the high‐temperature increase in starch degradation and consumption in respiratory processes, thus eliminating the break‐points in temperature curves of respiration in sugar‐fed leaves.     

Coupling of respiration, nitrogen, and sugars underlies convergent temperature acclimation in Pinus banksiana across wide-ranging sites and populations

Patterns and mechanisms of short‐term temperature acclimation and long‐term climatic adaptation of respiration among intraspecific populations are poorly understood, but both are potentially important in constraining respiratory carbon flux to climate warming across large geographic scales, as well as influencing the metabolic fitness of populations. Herein we report on leaf dark respiration of 33‐year‐old trees of jack pine (Pinus banksiana Lamb.) grown in three contrasting North American common gardens (0.9, 4.6, and 7.9 °C, mean annual temperature) comprised of identical populations of wide‐ranging geographic origins. We tested whether respiration rates in this evergreen conifer acclimate to prevailing ambient air temperatures and differ among populations. At each of the common gardens, observed population differences in respiration rates measured at a standard temperature (20 °C) were comparatively small and largely unrelated to climate of seed‐source origin. In contrast, respiration in all populations exhibited seasonal acclimation at all sites. Specific respiration rates at 20 °C inversely tracked seasonal variation in ambient air temperature, increasing with cooler temperatures in fall and declining with warmer temperatures in spring and summer. Such responses were similar among populations and sites, thus providing a general predictive equation regarding temperature acclimation of respiration for the species. Temperature acclimation was associated with variation in nitrogen (N) and soluble carbohydrate concentrations, supporting a joint enzyme and substrate‐based model of respiratory acclimation. Regression analyses revealed convergent relationships between respiration and the combination of needle N and soluble carbohydrate concentrations and between N‐based respiration (R_N, μmol mol N^{? 1} s^{? 1}) and soluble carbohydrate concentrations, providing evidence for general predictive relationships across geographically diverse populations, seasons, and sites. Overall, these findings demonstrate that seasonal acclimation of respiration modulates rates of foliar respiratory carbon flux in a widely distributed evergreen species, and does so in a predictable way. Genetic differences in specific respiration rate appear less important than temperature acclimation in downregulating respiratory carbon fluxes with climate warming across wide‐ranging sites.     

Macromolecular rate theory (MMRT) provides a thermodynamics rationale to underpin the convergent temperature response in plant leaf respiration

Temperature is a crucial factor in determining the rates of ecosystem processes, for example, leaf respiration (R) – the flux of plant respired CO₂ from leaves to the atmosphere. Generally, R increases exponentially with temperature and formulations such as the Arrhenius equation are widely used in earth system models. However, experimental observations have shown a consequential and consistent departure from an exponential increase in R. What are the principles that underlie these observed patterns? Here, we demonstrate that macromolecular rate theory (MMRT), based on transition state theory (TST) for enzyme‐catalyzed kinetics, provides a thermodynamic explanation for the observed departure and the convergent temperature response of R using a global database. Three meaningful parameters emerge from MMRT analysis: the temperature at which the rate of respiration would theoretically reach a maximum (the optimum temperature, T_opt), the temperature at which the respiration rate is most sensitive to changes in temperature (the inflection temperature, T_inf) and the overall curvature of the log(rate) versus temperature plot (the change in heat capacity for the system, ). On average, the highest potential enzyme‐catalyzed rates of respiratory enzymes for R are predicted to occur at 67.0 ± 1.2°C and the maximum temperature sensitivity at 41.4 ± 0.7°C from MMRT. The average curvature (average negative ) was ?1.2 ± 0.1 kJ mol^?1 K^?1. Interestingly, T_opt, T_inf and appear insignificantly different across biomes and plant functional types, suggesting that thermal response of respiratory enzymes in leaves could be conserved. The derived parameters from MMRT can serve as thermal traits for plant leaves that represent the collective temperature response of metabolic respiratory enzymes and could be useful to understand regulations of R under a warmer climate. MMRT extends the classic TST to enzyme‐catalyzed reactions and provides an accurate and mechanistic model for the short‐term temperature response of R around the globe.     

Carbon fluxes acclimate more strongly to elevated growth temperatures than to elevated CO2 concentrations in a northern conifer

Increasing temperatures and atmospheric CO₂ concentrations will affect tree carbon fluxes, generating potential feedbacks between forests and the global climate system. We studied how elevated temperatures and CO₂ impacted leaf carbon dynamics in Norway spruce (Picea abies), a dominant northern forest species, to improve predictions of future photosynthetic and respiratory fluxes from high‐latitude conifers. Seedlings were grown under ambient (AC, c. 435 μmol mol^?1) or elevated (EC, 750 μmol mol^?1) CO₂ concentrations at ambient, +4 °C, or +8 °C growing temperatures. Photosynthetic rates (A_sat) were high in +4 °C/EC seedlings and lowest in +8 °C spruce, implying that moderate, but not extreme, climate change may stimulate carbon uptake. A_sat, dark respiration (R_dark), and light respiration (R_light) rates acclimated to temperature, but not CO₂: the thermal optimum of A_sat increased, and R_dark and R_light were suppressed under warming. In all treatments, the Q₁₀ of R_light (the relative increase in respiration for a 10 °C increase in leaf temperature) was 35% higher than the Q₁₀ of R_dark, so the ratio of R_light to R_dark increased with rising leaf temperature. However, across all treatments and a range of 10–40 °C leaf temperatures, a consistent relationship between R_light and R_dark was found, which could be used to model R_light in future climates. Acclimation reduced daily modeled respiratory losses from warm‐grown seedlings by 22–56%. When R_light was modeled as a constant fraction of R_dark, modeled daily respiratory losses were 11–65% greater than when using measured values of R_light. Our findings highlight the impact of acclimation to future climates on predictions of carbon uptake and losses in northern trees, in particular the need to model daytime respiratory losses from direct measurements of R_light or appropriate relationships with R_dark.     

Temperature dependence of Fe(III) and sulfate reduction rates and its effect on growth and composition of bacterial enrichments from an acidic pit lake neutralization experiment

Microbial Fe(III) and sulfate reduction are important electron transport processes in acidic pit lakes and stimulation by the addition of organic substrates is a strategy to remove acidity, iron and sulfate. This principle was applied in a pilot‐scale enclosure in pit lake 111 (Brandenburg, Germany). Because seasonal and spatial variation of temperature may affect the performance of in situ experiments considerably, the influence of temperature on Fe(III) and sulfate reduction was investigated in surface sediments from the enclosure in the range of 4–28 °C. Potential Fe(III) reduction and sulfate reduction rates increased exponentially with temperature, and the effect was quantified in terms of the apparent activation energy E_a measuring 42–46 kJ mol^?1 and 52 kJ mol^?1, respectively. Relatively high respiration rates at 4 °C and relatively low Q₁₀ values (～2) indicated that microbial communities were well adapted to low temperatures. In order to evaluate the effect of temperature on growth and enrichment of iron and sulfate‐reducing bacterial populations, MPN (Most Probable Number) dilution series were performed in media selecting for the different bacterial groups. While the temperature response of specific growth rates of acidophilic iron reducers showed mesophilic characteristics, the relatively high specific growth rates of sulfate reducers at the lowest incubation temperature indicated the presence of moderate psychrophilic bacteria. In contrast, the low cell numbers and low specific growth rates of neutrophilic iron reducers obtained in dilution cultures suggest that these populations play a less significant role in Fe and S cycling in these sediments. SSCP (Single‐Strand Conformation Polymorphism) or DGGE (Denaturing Gradient Gel Electrophoresis) fingerprinting based on 16S rRNA genes of Bacteria indicated different bacterial populations in the MPN dilution series exhibiting different temperature ranges for growth.     

The effect of heat waves,elevated [CO2] and low soil water availability on northern red oak (Quercus rubra L.) seedlings

The frequency and intensity of heat waves are predicted to increase. This study investigates whether heat waves would have the same impact as a constant increase in temperature with the same heat sum, and whether there would be any interactive effects of elevated [CO₂] and soil moisture content. We grew Quercus rubra seedlings in treatment chambers maintained at either ambient or elevated [CO₂] (380 or 700 μmol CO₂ mol^?1) with temperature treatments of ambient, ambient +3 °C, moderate heat wave (+6 °C every other week) or severe heat wave (+12 °C every fourth week) temperatures. Averaged over a 4‐week period, and the entire growing season, the three elevated temperature treatments had the same average temperature and heat sum. Half the seedlings were watered to a soil water content near field capacity, half to about 50% of this value. Foliar gas exchange measurements were performed morning and afternoon (9:00 and 15:00 hours) before, during and after an applied heat wave in August 2010. Biomass accumulation was measured after five heat wave cycles. Under ambient [CO₂] and well‐watered conditions, biomass accumulation was highest in the +3 °C treatment, intermediate in the +6 °C heat wave and lowest in the +12 °C heat wave treatment. This response was mitigated by elevated [CO₂]. Low soil moisture significantly decreased net photosynthesis (A_net) and biomass in all [CO₂] and temperature treatments. The +12 °C heat wave reduced afternoon A_net by 23% in ambient [CO₂]. Although this reduction was relatively greater under elevated [CO₂], A_net values during this heat wave were still 34% higher than under ambient [CO₂]. We concluded that heat waves affected biomass growth differently than the same amount of heat applied uniformly over the growing season, and that the plant response to heat waves also depends on [CO₂] and soil moisture conditions.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Acclimation of snow gum (Eucalyptus pauciflora) leaf respiration to seasonal and diurnal variations in temperature: the importance of changes in the capacity and temperature sensitivity of respiration

We investigated the relationship between daily and seasonal temperature variation and dark respiratory CO₂ release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled‐environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2–3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent ‘respiratory capacity’) was greater in seedlings grown under low average daily temperatures (i.e. acclimation occurred), both in the field and under controlled‐environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e. the Q₁₀ of leaf respiration) exhibited little seasonal variation over much of the year. However, Q₁₀ values were significantly greater on cold winter days (i.e. when daily average and minimum air temperatures were below 6° and –1 °C, respectively). These differences in Q₁₀ values were not due to bias arizing from the contrasting daily temperature amplitudes in winter and summer, as the Q₁₀ of leaf respiration was constant over a wide temperature range in short‐term experiments. Due to the higher Q₁₀ values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 °C than at 25 °C. The net result of these changes was that there was relatively little difference in total daily leaf respiratory CO₂ release per unit leaf dry mass in winter and summer. Under controlled‐environment conditions, acclimation of respiration to growth temperature occurred in as little as 1–3 d. Acclimation was associated with a change in the concentration of soluble sugars under controlled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold‐induced photo‐inhibition. We conclude that cold‐acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent ‘capacity’ of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.     

Regulation of root respiration in two species of Plantago that differ in relative growth rate: the effect of short- and long-term changes in temperature

This study investigates the effect of short‐ and long‐term changesin temperature on the regulation of root respiratory O₂ uptakeby substrate supply, adenylate restriction and/or the capacityof the respiratory system. The species investigated were the lowland Plantagolanceolata L. and alpine Plantago euryphylla Briggs, Carolin& Pulley, which are inherently fast‐ and slow‐growing, respectively. Theplants were grown hydroponically in a controlled environment (constant23 °C). The effect of long‐term exposure to lowtemperature on regulation of respiration was also assessed in P.lanceolata using plants transferred to 15/10 °C(day/night) for 7 d. Exogenous glucose and uncoupler (CCCP)were used to assess the extent to which respiration rates were limitedby substrate supply and adenylates. The results suggest that adenylatesand/or substrate supply exert the greatest control overrespiration at moderate temperatures (e.g. 15–30 °C)in both species. At low temperatures (5–15 °C),CCCP and glucose had little effect on respiration, suggesting thatrespiration was limited by enzyme capacity alone. The Q₁₀ (proportionalincrease of respiration per 10 °C) of respirationwas increased following the addition of CCCP and/or exogenousglucose. The degree of stimulation by CCCP was considerably lowerin P. euryphylla than P. lanceolata. This suggeststhat respiration rates operate much closer to the maximum capacity in P.euryphylla than P. lanceolata. When P. lanceolata wastransferred to 15 °C for 7 d, respirationacclimated to the lower growth temperature (as demonstrated by an increasein respiration rates measured at 25 °C). In addition,the Q₁₀ was higher, and the stimulatory effectof exogenous glucose and CCCP lower, in the cold‐acclimated rootsin comparison with their warm‐grown counterparts. Acclimation of P.lanceolata to different day/night‐time temperatureregimes was also investigated. The low night‐time temperature wasfound to be the most important factor influencing acclimation. The Q₁₀ valueswere also higher in plants exposed to the lowest night‐time temperature.The results demonstrate that short‐ and long‐term changes in temperaturealter the importance of substrate supply, adenylates and capacityof respiratory enzymes in regulating respiratory flux.     

Temperature dependence and ontogenetic changes of metabolic rate of an endemic earthworm <Emphasis Type="Italic">Dendrobaena mrazeki</Emphasis>

Ontogenetic changes and temperature dependency of respiration rate were studied in Dendrobaena mrazeki, an earthworm species inhabiting relatively warm and dry habitats in Central Europe. D. mrazeki showed respiration rate lower than in other earthworm species, < 70 μl O₂ g⁻¹ h⁻¹, within the temperature range of 5–35°C. The difference of respiration rate between juveniles and adults was insignificant at 20°C. The response of oxygen consumption to sudden temperature changes was compared with the temperature dependence of respiratory activity in animals pre-acclimated to temperature of measurement. No significant impact of acclimation on the temperature response of oxygen consumption was found. The body mass-adjusted respiration rate increased slowly with increasing temperature from 5 to 25°C (Q₁₀ from 1.2 to 1.7) independently on acclimation history of earthworms. Oxygen consumption decreased above 25°C up to upper lethal limit (about 35°C). Temperature dependence of metabolic rate is smaller than in other earthworm species. The relationships between low metabolic sensitivity to temperature, slow locomotion and reactivity to touching as observed in this species are discussed.     

Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature‐dependent susceptibility to photoinhibition

To determine the effects of elevated CO₂ concentration ([CO₂]) on the temperature‐dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1‐year‐old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO₂] levels (ambient: 400 µmol mol^?1 and elevated: 800 µmol mol^?1) and limited N availability (90 mg N plant^?1). Plants grown under elevated [CO₂] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated‐CO₂‐grown plants exceeded those grown under ambient [CO₂] when compared at their growth [CO₂]. Electron transport rates were significantly lower in elevated‐CO₂‐grown plants than ambient‐CO₂‐grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 ? q_P)× F_v′/F_m′] between CO₂ treatments across the temperature range. The quantum yield of regulated non‐photochemical energy loss was significantly higher in elevated‐CO₂‐grown plants than ambient, when compared at their respective growth [CO₂] below 25°C. These results suggest that elevated‐CO₂‐induced downregulation might not exacerbate the temperature‐dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.     

Consistent temperature dependence of respiration across ecosystems contrasting in thermal history

Ecosystem respiration is a primary component of the carbon cycle and understanding the mechanisms that determine its temperature dependence will be important for predicting how rates of carbon efflux might respond to global warming. We used a rare model system, comprising a network of geothermally heated streams ranging in temperature from 5 °C to 25 °C, to explore the nature of the relationship between respiration and temperature. Using this ‘natural experiment’, we tested whether the natal thermal regime of stream communities influenced the temperature dependence of respiration in the absence of other potentially confounding variables. An empirical survey of 13 streams across the thermal gradient revealed that the temperature dependence of whole‐stream respiration was equivalent to the average activation energy of the respiratory complex (0.6–0.7 eV). This observation was also consistent for in‐situ benthic respiration. Laboratory experiments, incubating biofilms from four streams across the thermal gradient at a range of temperatures, revealed that the activation energy and Q₁₀ of respiration were remarkably consistent across streams, despite marked differences in their thermal history and significant turnover in species composition. Furthermore, absolute rates of respiration at standardised temperature were also unrelated to ambient stream temperature, but strongly reflected differences in biofilm biomass. Together, our results suggest that the core biochemistry, which drives the kinetics of oxidative respiratory metabolism, may be well conserved among diverse taxa and environments, and that the intrinsic sensitivity of respiration to temperature is not influenced by ambient environmental temperature.     

Rapid laboratory measurement of the temperature dependence of soil respiration and application to changes in three diverse soils through the year

Determining the temperature dependence of soil respiration is needed to test predictive models such as Arrhenius-like functions and macro-molecular rate theory (MMRT). We tested a method for rapid measurement of respiration using a temperature gradient block, cooled at one end (~2 °C) and heated at the other (~50 °C) that accommodated 44 tubes containing soil incubated at roughly 1 °C increments. Gas samples were taken after 5 h incubation and analysed for CO₂. The temperature gradient block allowed rapid assessment of temperature dependence of soil respiration with the precision needed to test models and explore existing theories of how temperature and moisture interact to control biochemical processes. Temperature response curves were well fitted by MMRT and allowed calculation of the temperature at which absolute temperature sensitivity was maximal (T_inf). We measured temperature response of three soils at seven moisture contents and showed that the absolute rate and sensitivity of respiration was partly dependent on adjusted moisture content. This result implied that comparisons between soils need to be made at a common moisture content. We also measured potential changes in the temperature dependence (and sensitivity) of respiration for three different soils collected at one site throughout a year. T_inf ranged from 43 to 51 °C for the three soils. T_inf and temperature sensitivity were not dependent on soil type collected but was partly dependent on time of year of collection. Temporal changes in temperature response suggested that the microbial communities may tune their metabolisms in response to changes in soil temperatures.     

Composition of the early Oligocene ocean from coral stable isotope and elemental chemistry

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation (～30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth‐band‐parallel microsamples of original aragonite exhibit well‐constrained fluctuations that range between ?2.0 and ?4.8. Variation in δ¹⁸O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 °C about a mean of 18 °C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental‐ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 °C while those from Sr/Ca are 5.8 ± 2.2 °C. Assuming that δ¹⁸O‐derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today's ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ～81% (4.2 mol mol^?1) and Sr/Ca ～109% (9.9 mmol mol^?1) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea‐floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today.     

Direct and indirect inhibition of single leaf respiration by elevated CO2 concentrations: Interaction with temperature

Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 μmol mol⁻¹) and elevated (729 μmol mol⁻¹) CO₂ concentrations at constant temperatures of 15, 20, 25 and 30°C in order to examine direct and indirect changes in nighttime CO₂ efflux rate (respiration) of single leaves. Direct (biochemical) effects of CO₂ on nighttime respiration were determined for each growth condition by brief (<30 min) exposure to each CO₂ concentration. If no direct inhibition of respiration was observed, then long-term reductions in CO₂ efflux between CO₂ treatments were presumed to be due to indirect inhibition, probably related to long-term changes in leaf composition. By this criterion, indirect effects of CO₂ on leaf respiration were observed at 15 and 20°C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25°C in D. glomerata; therefore the observed reductions in respiration for leaves grown and measured at elevated relative to ambient CO₂ concentrations could not be distinguished as indirect inhibition. No inhibition of respiration at elevated CO₂ was observed at the highest growth temperature (30°C) in either species. CO₂ efflux increased with measurement and growth temperature for M. sativa at both CO₂ concentrations; however, CO₂ efflux in D. glomerata showed complete acclimation to growth temperature. Stimulation of leaf area and weight by elevated CO₂ levels declined with growth temperature in both species. Data from the present study suggest that both direct and indirect inhibition of respiration are possible with future increases in atmospheric CO₂, and that the degree of each type of respiratory inhibition is a function of growth temperature.     

Use of a mammalian gonadotropin‐releasing hormone (GnRH) agonist to characterize pituitary GnRH receptors in white sturgeon (Acipenser transmontanus Richardson)

Aim of the present study was to characterize the pituitary GnRH receptor in white sturgeon, Acipenser transmontanus, using a superagonist analog of mammalian GnRH [D‐Ala⁶, des‐Gly¹⁰–Pro⁹]‐ethylamide and to investigate the possible effect of estradiol‐17β treatment on the concentration and affinity of the GnRH receptor in immature white sturgeon. The binding of ¹²⁵I‐GnRH‐A to sturgeon pituitary receptors was rapid and saturable at 4°C and 20°C. However, maximal binding at 20°C was almost two‐fold greater than the highest binding noted at 4°C. Specific binding of radioligand was directly related to the amount of tissue included in the assay system over the range of 5–20 mg fresh tissue equivalents per ml. The binding capacity of ¹²⁵I‐GnRH‐A with sturgeon pituitary tissue was much greater than radiolabeled GnRH. Administration of E₂ to immature sturgeon caused an almost two‐fold increase in GnRH‐A binding capacity (E₂ treated: Bmax = 2.87 fmoles 3 mg^?1 FTE; control: Bmax = 1.70 fmoles 3 mg^?1), and did not affect GnRH‐A binding affinity (E₂ treated: Ka = 0.13 × 10¹¹ m ^?1; control: Ka = 0.15 × 10¹¹ m ^?1). Overall, the study provides evidence that the GnRH analog is effective for characterizing the GnRH receptor in white sturgeon; however, more experimentation is necessary to determine whether E₂ administration to immature white sturgeon can increase the GnRH receptor capacity.     

Respiration of thermogenic inflorescences of skunk cabbage Symplocarpus renifolius in heliox

The respiration rate of the thermogenic inflorescences of Japanese skunk cabbage Symplocarpus renifolius can reach 300 nmol s^?1 g^?1, which is sufficient to raise spadix temperature (T_s) up to 15 ° C above ambient air temperature (T_a). Respiration rate is inversely related to T_a, such that the T_s achieves a degree of independence from T_a, an effect known as temperature regulation. Here, we measure oxygen consumption rate (?o ₂) in air (21% O₂ in mainly N₂) and in heliox (21% O₂ in He) to investigate the diffusive conductance of the network of gas‐filled spaces and the thermoregulatory response. When T_s was clamped at 15 ° C, the temperature that produces maximal ?o ₂ in this species, exposure to high diffusivity heliox increased mean ?o ₂ significantly from 137 ± 17 to 202 ± 43 nmol s^?1 g^?1 FW, indicating that respiration in air is normally limited by diffusion in the gas phase and some mitochondria are unsaturated. When T_a was clamped at 15 ° C and T_s was allowed to vary, exposure to heliox reduced T_s 1 ° C and increased ?o ₂ significantly from 116 ± 10 to 137 ± 19 nmol s^?1 g^?1, indicating that enhanced heat loss by conduction and convection can elicit the thermoregulatory response.     

Thermal Acclimation of Respiration and Photosynthesis in the Marine Macroalga Gracilaria lemaneiformis (Gracilariales,Rhodophyta)

The responses of respiration and photosynthesis to temperature fluctuations in marine macroalgae have the potential to significantly affect coastal carbon fluxes and sequestration. In this study, the marine red macroalga Gracilaria lemaneiformis was cultured at three different temperatures (12, 19, and 26°C) and at high‐ and low‐nitrogen (N) availability, to investigate the acclimation potential of respiration and photosynthesis to temperature change. Measurements of respiratory and photosynthetic rates were made at five temperatures (7°C–33°C). An instantaneous change in temperature resulted in a change in the rates of respiration and photosynthesis, and the temperature sensitivities (i.e., the Q₁₀ value) for both the metabolic processes were lower in 26°C‐grown algae than 12°C‐ or 19°C‐grown algae. Both respiration and photosynthesis acclimated to long‐term changes in temperature, irrespective of the N availability under which the algae were grown; respiration displayed strong acclimation, whereas photosynthesis only exhibited a partial acclimation response to changing growth temperatures. The ratio of respiration to gross photosynthesis was higher in 12°C‐grown algae, but displayed little difference between the algae grown at 19°C and 26°C. We propose that it is unlikely that respiration in G. lemaneiformis would increase significantly with global warming, although photosynthesis would increase at moderately elevated temperatures.     

Effects of clove oil,essential oil of Lippia alba and 2‐phe anaesthesia on juvenile meagre,Argyrosomus regius (Asso, 1801)

The objectives of this experiment were to (i) determine the efficacy of essential oils of clove (CO) and Lippia alba (EOLA) to induce deep anaesthesia in juvenile specimens (49.0 ± 6.2 g body mass, 16.6 ± 0.8 cm; n = 8 per treatment) of meagre (Argyrosomus regius); and (ii) study the feasibility of these substances, together with 2‐phenoxyethanol (2‐PHE), as potential sedatives [low concentration: (i) EOLA: 12 mg L^?1; (ii) CO: 1 mg L^?1; and (iii) 2‐PHE: 33 mg·L ^?1; n = 8 per treatment] for live fish transport of this species. All test were performed at a constant temperature (18°C). Thus, the main primary stress indicator (plasma cortisol) and secondary factors (plasma metabolites) were evaluated. In addition, growth hormone (GH) mRNA expression was also evaluated in the pituitary gland. The results indicated that EOLA is considered to be effective for deep anaesthesia when the concentration is close to 160 mg L^?1, while CO produces the same effect when lower concentrations are added (40–50 mg L^?1). Regarding sedative concentrations, a significant ~3‐fold increase in plasma cortisol levels was detected in the EOLA group when compared to control specimens. In addition, glucose levels were not reduced and significantly increased (~1.6‐fold) for 2‐PHE in relation to the control fish. None of the anaesthetics promoted a significant difference for GH expression with respect to the control group, but a significant ~2‐fold increase for 2‐PHE treatment with respect to the EOLA exposition was found in this gene expression. Results show that none of the anaesthetics analysed, at least in the ranges of concentrations used in this study (EOLA 12 mg L^?1, CO 1 mg L^?1, 2‐PHE 33 mg L^?1), are recommended for live fish transport, as shown by the absence of inhibition on the stress parameters assessed.     

Effects of different protein and carbohydrate levels on growth performance and feed utilisation of brook trout,Salvelinus fontinalis (Mitchill, 1814), at two temperatures

A 12‐week feeding trial was conducted to determine the optimum dietary protein requirement of brook trout, Salvelinus fontinalis, at 15 and 19°C. Twelve iso‐energetic (22 MJ · kg^?1) and iso‐lipidic (23%) diets (36–58% protein at 2% increments) were prepared. Fish (29.45 ± 3.25 g · fish^?1) were fed 2% of body weight per day, divided into two equal rations. The specific growth rate (SGR, % · day^?1), feed efficiency ratio (FER), productive protein value (PPV), productive lipid value (PLV) and productive energy value (PEV), apparent digestibility of diet (AD_DM) and protein (AD_CP) were significantly higher at optimum temperature (15°C). Increasing PPV with increasing dietary carbohydrate and with decreasing dietary protein content was due to the protein‐sparing effect of carbohydrates. A piecewise regression (broken line) model between the SGR and digestible dietary protein level revealed that the digestible dietary protein requirement of brook trout was 44 and 40% at 15 and 19°C, respectively. When PPV (digestible protein retention basis) was modelled with a broken line, the digestible protein requirement of brook trout was 39 and 35% at 15 and 19°C, respectively. A reduction in dietary protein content balanced by increased gelatinised carbohydrate might be useful for improving the protein utilization efficiency for growth at 15 and 19°C; however, the growth and feed efficiency was lower at the elevated temperature.