Thermal limits of leaf metabolism across biomes

High‐temperature tolerance in plants is important in a warming world, with extreme heat waves predicted to increase in frequency and duration, potentially leading to lethal heating of leaves. Global patterns of high‐temperature tolerance are documented in animals, but generally not in plants, limiting our ability to assess risks associated with climate warming. To assess whether there are global patterns in high‐temperature tolerance of leaf metabolism, we quantified T_crit (high temperature where minimal chlorophyll a fluorescence rises rapidly and thus photosystem II is disrupted) and T_max (temperature where leaf respiration in darkness is maximal, beyond which respiratory function rapidly declines) in upper canopy leaves of 218 plant species spanning seven biomes. Mean site‐based T_crit values ranged from 41.5 °C in the Alaskan arctic to 50.8 °C in lowland tropical rainforests of Peruvian Amazon. For T_max, the equivalent values were 51.0 and 60.6 °C in the Arctic and Amazon, respectively. T_crit and T_max followed similar biogeographic patterns, increasing linearly (?8 °C) from polar to equatorial regions. Such increases in high‐temperature tolerance are much less than expected based on the 20 °C span in high‐temperature extremes across the globe. Moreover, with only modest high‐temperature tolerance despite high summer temperature extremes, species in mid‐latitude (~20–50°) regions have the narrowest thermal safety margins in upper canopy leaves; these regions are at the greatest risk of damage due to extreme heat‐wave events, especially under conditions when leaf temperatures are further elevated by a lack of transpirational cooling. Using predicted heat‐wave events for 2050 and accounting for possible thermal acclimation of T_crit and T_max, we also found that these safety margins could shrink in a warmer world, as rising temperatures are likely to exceed thermal tolerance limits. Thus, increasing numbers of species in many biomes may be at risk as heat‐wave events become more severe with climate change.     

Shape of leaf photosynthetic electron transport versus temperature response curve is not constant along canopy light gradients in temperate deciduous trees

Responses of foliar light-saturated net assimilation rate (A_max), capacity for photosynthetic electron transport (J_max) and mitochondrial respiration rate (R_d) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17–28 m) and of Tilia cordata Mill. in the lower canopy layer (4–17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (T_max) was 5·5 °C (range 0·7–10·5 °C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between T_max and seasonal average integrated quantum flux density (Q_int), as well as between seasonal average daily mean temperature and Q_int. Because of changes in leaf dry mass and nitrogen per unit area, A_max, J_max, and R_d scaled positively with Q_int in both species at a common leaf temperature (T). According to J_max versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of J_max, T_opt, of 33·5 ± 0·6 °C than in T. cordata with T_opt of 40·7 ± 0·6 °C. This difference was suggested to manifest evolutionary adaptation of photosynthetic electron transport to cooler environments in P. tremula, the range of which extends farther north than that in T. cordata. Possibly because of acclimation to long-term canopy temperature environment, T_opt was positively related to Q_int in P. tremula, foliage of which was also exposed to higher irradiances and temperatures, but not in T. cordata, in the canopy of which quantum flux densities and temperatures were lower, and gradients in the environmental factors less pronounced. Parallel to changes in T_opt, the activation energy for photosynthetic electron transport decreased with increasing Q_int in P. tremula, indicating that J_max of leaves acclimated to colder environment was more responsive to T in lower temperatures than that of high T acclimated leaves. Similar alterations in the activation energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves long-term changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of J_max versus T, and R_d versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.     

Ground cavity nest temperatures and their relevance to Blue Swallow Hirundo atrocaerulea conservation

Blue Swallows Hirundo atrocaerulea are Critically Endangered within South Africa. They nest in natural underground holes in mist-belt grasslands. Temperature dataloggers were used to record ground cavity nest (T_n) and ambient temperature (T_a) for one artificial and 11 natural Blue Swallow nests. Mean ground cavity T_n was significantly different to mean T_a. T_n ranged from 17.0 ± 0.1 °C to 28.5 ± 0.3 °C and varied less than T_a (14.0 ± 0.2 to 47.7 ± 0.4 °C). Mean ground cavity T_n averaged 3.3 ± 0.9 °C warmer than mean T_a for 58% of nests, and mean T_a averaged 2.6 ± 0.5 °C warmer than mean ground cavity T_n for 42% of nests. There was no significant difference in mean ground cavity T_n for the aardvark-excavated holes (22.7 ± 1.6 °C) and sinkholes (21.5 ± 1.2 °C). Blue Swallows also nest in man-made holes, potentially a way to increase nesting sites. Mean aardvark-excavated T_n (19.2 ± 0.1 °C) was significantly warmer than mean artificial cavity T_n (18.5 ± 0.2 °C). Further investigation of breeding success of Blue Swallows in relation to T_n, incubation strategies and predation risk needs to be addressed in future studies for a better understanding of their reproductive ecology.     

Temperature regulation in the echidna (Tachyglossus aculeatus)

The echidna (Tachyglossus aculeatus) maintained a body temperature of 30.7°C ± 1.03 s.d. (N = 23) at ambient temperatures (T_A) between 0 and 25°C. It may, however, also become hypothermic at low T_A. At T_A = 30°C or above the echidna became hyperthermic. The thermoneutral range was about 20–30°C. At low T_A the metabolic rate might be increased several fold. The thermal conductance was at a minimum at T_A = 20°C, and was not further reduced at lower T_A. At higher T_A the thermal conductance increased up to five-fold. The evaporation showed little change with increasing T_A. At the highest T_A we used (33°C) the evaporation on the average accounted for the dissipation of only about one-third of the metabolic heat produced. These findings suggest that the echidna, although it can maintain its body temperature at low ambient temperature, cannot rely upon evaporation as the major avenue for heat loss at high ambient temperature.     

Stomatal response to vapour pressure deficit and the effect of plant water stress

Abstract The dynamic response of stomata to changes in atmospheric humidity was investigated in Fragaria × ananassa Duch., Picea engelmannii Parry, and Pseudotsuga menziesii (Mirb.) Franco; and the effect of water stress on this response was determined in Pseudotsuga menziesii. The plants were rotated through three regimes of ambient temperature and vapour pressure deficit: 35°C–3. 5kPa, 35°C–0. 5 kPa, and 20°C–1. 5kPa. Branch and leaflet conductance were measured with a steady-state porometer, first at ambient vapour pressure deficit and then at one of four treatment conditions achieved by increasing or decreasing vapour pressure within the porometer cuvette. All three species showed similar stomatal response: enhanced conductance at low vapour pressure deficit and depressed conductance at high vapour pressure deficit. Engelmann spruce was more sensitive than Douglas fir and strawberry. Plant water status significantly altered stomatal response to vapour pressure deficit. The relationship of conductance of xylem water potential was linear under ambient conditions but became curvilinear when conductance was measured above and below ambient vapour pressure deficit. Between ?0. 5 MPa and ?2. 0 MPa xylem water potential, the stomata were sensitive to vapour pressure deficit, but below ? 2. 0 MPa, the sensitivity decreased.     

The effect of flutter on the temperature of poplar leaves and its implications for carbon gain

The leaf temperatures of two poplar species (Populus tremuloides Michx. and P. fremontii Wats.) were characterized by attaching thermocouples to leaves that were either constrained to a fixed position or allowed to flutter naturally. There were no observed temperature differences between fluttering and constrained leaves in the lower canopy, but fluttering leaves at the top of the canopy were as much as 2–4°C cooler than constrained leaves. An increase in heat transfer, a decrease in light interception or both could account for these observed differences in the temperature of fluttering versus constrained leaves. Fluttering can increase the boundary-layer conductance to convective heat exchange by as much as 50 and 20% for laminar and turbulent flow, respectively. The benefit that these leaf temperature differences may provide to the carbon economy of a poplar canopy was dependent on the ambient temperature. Populus fremontii, which is frequently exposed to daytime temperatures exceeding 35°C during summer months in the central valley of California, USA, could show an increase in carbon gain as a result of lower upper canopy leaf temperatures. For aspen, the benefit would be much smaller and often negative because of much lower air temperatures. Lower leaf temperatures may also increase the water use efficiency of poplars. However, the maintenance of lower leaf temperatures may not be the primary adaptive significance of leaf flutter.     

Temperature dependence of two parameters in a photosynthesis model

The temperature dependence of the photosynthetic parameters V_cmax, the maximum catalytic rate of the enzyme Rubisco, and J_max, the maximum electron transport rate, were examined using published datasets. An Arrehenius equation, modified to account for decreases in each parameter at high temperatures, satisfactorily described the temperature response for both parameters. There was remarkable conformity in V_cmax and J_max between all plants at T_leaf < 25 °C, when each parameter was normalized by their respective values at 25 °C (V_cmax0 and J_max0), but showed a high degree of variability between and within species at T_leaf > 30 °C. For both normalized V_cmax and J_max, the maximum fractional error introduced by assuming a common temperature response function is < ± 0·1 for most plants and < ± 0·22 for all plants when T_leaf < 25 °C. Fractional errors are typically < ± 0·45 in the temperature range 25–30 °C, but very large errors occur when a common function is used to estimate the photosynthetic parameters at temperatures > 30 °C. The ratio J_max/V_cmax varies with temperature, but analysis of the ratio at T_leaf = 25 °C using the fitted mean temperature response functions results in J_max0/V_cmax0 = 2·00 ± 0·60 (SD, n = 43).     

The effect of calcium on the thermotropic properties of bovine blood coagulation factors IX and X and their activation intermediates and products

The thermotropic properties of bovine blood coagulation Factors IX and X, as well as the activation intermediates and products of these proteins, have been investigated by differential scanning microcalorimetry in the presence and absence of Ca²⁺. Bovine Factor IX displays a single thermal-denaturation transition characterized by a temperature midpoint (T_M) of 54.5 ± 0.5 °C and a calorimetric enthalpy (ΔH_c) of 105 ± 15 kcal/mol, in the absence of Ca²⁺. In the presence of Ca²⁺ concentrations sufficient to saturate its sites on Factor IX, the T_m value is increased to 57.0 ± 0.5 °C and the ΔH_c is virtually unchanged. When the activation intermediate, Factor IXα, is similarly analyzed in the absence of Ca²⁺, a broad, diffuse thermogram was obtained which did not lend itself to calculation of thermodynamic parameters. In the presence of Ca²⁺, Factor IXα displayed thermograms characterized by a T_M of 51.0 ± 0.5 °C and a ΔH_c of 109 ± 10 kcal/mol. The activated product, Factor IXaα, in the absence of Ca²⁺ (the values in the presence of saturating Ca²⁺ are given in parentheses), undergoes thermal denaturation with a T_M of 54.5 ± 0.5 °C (57.0 ± 0.5 °C) and a ΔH_c of 158 ±10 kcal/mol (156 ± 10 kcal/mol). Similarly, the terminal-activation product, Factor IXaβ, displays a T_M of 51.5 ± 0.5 °C (54.0 ± 0.5 °C) and a ΔH_c of 85 ± 5 kcal/mol (126 ± 10 kcal/mol). Bovine blood coagulation Factor X has been analyzed in this same fashion, and shows very similar thermal properties to Factor IX. The thermal denaturation of Factor X is represented by a T_M of 54.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔH_c of 102 ± 10 kcal/mol (118 ± 10 kcal/mol), whereas its activated form, Factor Xaβ, possesses a T_M of 55.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔH_c of 92.0 ± 5 kcal/mol (136 ± 10 kcal/mol). These studies indicate that, for many of these proteins, Ca²⁺ induces a conformational alteration to a more thermally stable form, which also requires the absorption of greater amounts of heat for thermal denaturation.     

Spatial distribution of leaf water-use efficiency and carbon isotope discrimination within an isolated tree crown

The spatial variations in the stable carbon isotope composition (δ¹³C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well‐watered, 20‐year‐old walnut tree growing in a low‐density orchard. The observed leaf carbon isotope discrimination (Δ) was compared with that computed by a three‐dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO₂ gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long‐term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in Δ. The main conclusions of this study are: (i) leaf Δ can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf Δ, and (iii) the spatial variations in Δ and water‐use efficiency resulted from the short‐term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long‐term acclimation of leaf characteristics to the local light regime.     

Thermal ecology of the Australian agamid Pogona barbata

This study compares the thermal ecology of male bearded dragon lizards (Pogona barbata) from south-east Queensland across two seasons: summer (1994–1995) and autumn (1995). Seasonal patterns of body temperature (T _b) were explored in terms of changes in the physical properties of the thermal environment and thermoregulatory effort. To quantify thermoregulatory effort, we compared behavioral and physiological variables recorded for observed lizards with those estimated for a thermoconforming lizard. The study lizards' field T _bs varied seasonally (summer: grand daily mean (GDM) 34.6 ± 0.6°C, autumn: GDM 27.5 ± 0.3°C) as did maximum and minimum available operative temperatures (summer: GDM T _max 42.1 ± 1.7°C, T _min 32.2 ± 1.0°C, autumn: GDM T _max 31.7 ± 1.2°C, T _min 26.4 ± 0.5°C). Interestingly, the range of temperatures that lizards selected in a gradient (selected range) did not change seasonally. However, P. barbata thermoregulated more extensively and more accurately in summer than in autumn; lizards generally displayed behaviors affecting heat load nonrandomly in summer and randomly in autumn, leading to the GDM of the mean deviations of lizards' field T _bs from their selected ranges being only 2.1 ± 0.5°C in summer, compared to 4.4 ± 0.5°C in autumn. This seasonal difference was not a consequence of different heat availability in the two seasons, because the seasonally available ranges of operative temperatures rarely precluded lizards from attaining field T _bs within their selected range, should that have been the goal. Rather, thermal microhabitat distribution and social behavior appear to have had an important influence on seasonal levels of thermoregulatory effort. Received: 28 April 1997 / Accepted: 29 December 1997     

Temperature dependence of embryonic cardiac gap junction conductance and channel kinetics

We have investigated the effects of temperature on the conductance and voltage-dependent kinetics of cardiac gap junction channels between pairs of seven-day embryonic chick ventricle myocytes over the range of 14–26°C. Records of junctional conductance (G _j ) and steady-state unit junctional channel activity were made using the whole-cell double patch-clamp technique while the bath temperature was steadily changed at a rate of about 4°C/min. The decrease inG _j upon cooling was biphasic with a distinct break at 21°C. In 12 cell pairs,Q ₁₀ was 2.2 from 26 to 21°C, while between 21 and 14°C it was 6.5. The meanG _j at 22°C (G _j22 ) was 3.0±2.1 nS, ranging in different preparations from 0.24 to 6.4 nS. At room temperature, embryonic cardiac gap junctions contain channels with conductance states near 240, 200, 160, 120, 80 and 40 pS. In the present study, we demonstrate that cooling decreases the frequency of channel openings at all conductance levels, and at temperatures below 20°C shifts the prevalence of openings from higher to lower conductance states: all 240 pS openings disappear below 20°C; 200 pS openings are suppressed at 17°C; below 16°C 160 and 120 pS events disappear and only 80 and 40 pS states are seen. Temperature also affected the voltage-dependent kinetics of the channels. Application of a 6 sec, 80 mV voltage step across the junction (V _j80 ) caused a biexponential decay in junctional conductance. Decay was faster at lower temperatures, whereas the rate of recovery ofG _j after returning toV _j0 was slowed. Cooling reduced the fast decay time constant, increased both recovery time constants, and decreased the magnitude of GitG_j decay, thus leaving a 10–16% larger residual conductance (G _ss/G _init,±80 mVV _j ) at 18 than at 22°C. From these results we propose that embryonic chick cardiac gap junctions contain at least two classes of channels with different conductances and temperature sensitivities.     

Responses of canopy duration to temperature changes in four temperate tree species: relative contributions of spring and autumn leaf phenology

While changes in spring phenological events due to global warming have been widely documented, changes in autumn phenology, and therefore in growing season length, are less studied and poorly understood. However, it may be helpful to assess the potential lengthening of the growing season under climate warming in order to determine its further impact on forest productivity and C balance. The present study aimed to: (1) characterise the sensitivity of leaf phenological events to temperature, and (2) quantify the relative contributions of leaf unfolding and senescence to the extension of canopy duration with increasing temperature, in four deciduous tree species (Acer pseudoplatanus, Fagus sylvatica, Fraxinus excelsior and Quercus petraea). For 3 consecutive years, we monitored the spring and autumn phenology of 41 populations at elevations ranging from 100 to 1,600 m. Overall, we found significant altitudinal trends in leaf phenology and species-specific differences in temperature sensitivity. With increasing temperature, we recorded an advance in flushing from 1.9 ± 0.3 to 6.6 ± 0.4 days °C⁻¹ (mean ± SD) and a 0 to 5.6 ± 0.6 days °C⁻¹ delay in leaf senescence. Together both changes resulted in a 6.9 ± 1.0 to 13.0 ± 0.7 days °C⁻¹ lengthening of canopy duration depending on species. For three of the four studied species, advances in flushing were the main factor responsible for lengthening canopy duration with increasing temperature, leading to a potentially larger gain in solar radiation than delays in leaf senescence. In contrast, for beech, we found a higher sensitivity to temperature in leaf senescence than in flushing, resulting in an equivalent contribution in solar radiation gain. These results suggest that climate warming will alter the C uptake period and forest productivity by lengthening canopy duration. Moreover, the between-species differences in phenological responses to temperature evidenced here could affect biotic interactions under climate warming.     

Temperature response of carbon isotope discrimination and mesophyll conductance in tobacco

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated the temperature response of g_m in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO₂ in either 2 or 21% O₂ with carbon isotope measurements using tuneable diode laser spectroscopy. The g_m increased linearly with temperature in 2 or 21% O₂. In 21% O₂, isotope discrimination associated with g_m decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO₂ partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in g_m with increasing leaf temperature resulted in the drawdown in CO₂ partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.     

Evaluation of mean skin temperature formulas by infrared thermography

 To study the reliabiliity of formulas for calculating mean skin temperature (T _sk), values were computed by 18 different techniques and were compared with the mean of 10,841 skin temperatures measured by infrared thermography. One hundred whole-body infrared thermograms were scanned in ten resting males while changing the air temperature from 40° C to 4° C. Local, regional average and mean skin temperatures were obtained using an image processing system. The agreement frequency, defined as the percentage of the calculated T _sk values which agreed with the corresponding infrared thermographic T _sk within ±0.2° C, ranged for with the various formulas from 7% to 80%. In many sites, the local skin temperature did not coincide with the regional average skin temperature. When the local skin temperatures which showed the highest percentage similarity to the regional average skin temperature within ±0.4° C were applied to the formula, the agreement frequency was markedly improved for all formulas. However, the agreement frequency was not affected by changing the weighting factors from specific constants to individually measured values of regional surface area. By applying the physiologically reliable accuracy range of ±0.2° C in the moderate and ±0.4° C in the cool condition, agreement frequencies of at least 95% were observed in formulas involving seven or more skin temperature measurement sites, including the hand and foot. We conclude that calculation of a reliable mean skin temperature must involve more than seven skin temperature measurement sites regardless of ambient temperature. Optimal sites for skin temperature measurement are proposed for various formulas. Received: 2 December 1996 / Accepted: 25 June 1997     

Long-term drought effects on the thermal sensitivity of Amazon forest trees

The continued functioning of tropical forests under climate change depends on their resilience to drought and heat. However, there is little understanding of how tropical forests will respond to combinations of these stresses, and no field studies to date have explicitly evaluated whether sustained drought alters sensitivity to temperature. We measured the temperature response of net photosynthesis, foliar respiration and the maximum quantum efficiency of photosystem II (F_v/F_m) of eight hyper-dominant Amazonian tree species at the world's longest-running tropical forest drought experiment, to investigate the effect of drought on forest thermal sensitivity. Despite a 0.6°C–2°C increase in canopy air temperatures following long-term drought, no change in overall thermal sensitivity of net photosynthesis or respiration was observed. However, photosystem II tolerance to extreme-heat damage (T₅₀) was reduced from 50.0 ± 0.3°C to 48.5 ± 0.3°C under drought. Our results suggest that long-term reductions in precipitation, as projected across much of Amazonia by climate models, are unlikely to greatly alter the response of tropical forests to rising mean temperatures but may increase the risk of leaf thermal damage during heatwaves.     

Stomatal behaviour in a beech canopy: an analysis of Bowen ratio measurements compared with porometer data

On the basis of measurements or stand transpiration and microclimate, the bulk stomatal or bulk leaf conductance (g_L) of a beech forest in northern Germany was calculated for periods in which leaves were fully expanded and the canopy was dry. This conductance depends strongly on light and humidity conditions above the forest. During periods with photosynthetic photon flux densities Q > 1200 μmol m^?2s^?1, g_L was reduced from 1500mmol m^?2s^?1 at a vapour pressure deficit D= 0.5kPa to 500 mmol m^?2s^?1 at D= 2kPa. Light saturation of g_L was not reached until Q= 1200 μmol m^?2s^?1 at low D, or until even higher Q at higher D. The dependence of g_L, on Q and D was described mathematically by a non-linear equation that requires two empirical parameters. Values for g_L as simulated by this equation provided a satisfactory agreement with independent porometer data collected on single leaves and scaled up to the canopy. A comparison of stomatal and aerodynamic conductances showed a strong coupling between the forest canopy and the atmosphere, indicating that transpiration of the beech forest is controlled mainly by the stomata.     

Pressure-induced elevation of phase transition temperature in dipalmitoylphosphatidylcholine bilayers: An electron spin resonance measurement of the enthalpy of phase transition

The application of 136 atm of helium pressure to an aqueous dispersion of dipalmitoylphosphatidylcholine increased the temperature of the primary phase transition at 40.4 ± 0.2 °C by 3.0 °C. The lower temperature pretransition at 30.5 ± 0.5 °C, thought to be due to phosphate headgroup reorganization, was increased by 1.7 °C. Addition of 4% dipalmitoylphosphatidic acid to the dipalmitoylphosphatidylcholine affected the phase transition in the head group region more than in the hydrocarbon chain region. The pressure and temperature data obtained, taken together with the literature value for the bilayer volume expansion during solid-fluid phase transition, and inserted into the Clausius-Clapeyron equation yield a ΔH value of 8.8 kcal/mole for this phase transition. This value is within experimental error of the ΔH value obtained from differential scanning calorimetry and serves to support the validity of the data and the experimental technique. Phase transition was observed by electron spin resonance measurement of the exclusion of the small spin label Tempo (2,2,6,6-tetramethylpiperidine-N-oxyl) from the solid domains of the bilayer. This result offers a possible explanation for the direct antagonism by high pressure of the effects of the inhalation anesthetics.     

Torpor and thermal energetics in a tiny Australian vespertilionid,the little forest bat (<Emphasis Type="Italic">Vespadelus vulturnus</Emphasis>)

Data on thermal energetics for vespertilionid bats are under-represented in the literature relative to their abundance, as are data for bats of very small body mass. Therefore, we studied torpor use and thermal energetics in one of the smallest (4 g) Australian vespertilionids, Vespadelus vulturnus. We used open-flow respirometry to quantify temporal patterns of torpor use, upper and lower critical temperatures (T _uc and T _lc) of the thermoneutral zone (TNZ), basal metabolic rate (BMR), resting metabolic rate (RMR), torpid metabolic rate (TMR), and wet thermal conductance (C _wet) over a range of ambient temperatures (T _a). We also measured body temperature (T _b) during torpor and normothermia. Bats showed a high proclivity for torpor and typically aroused only for brief periods. The TNZ ranged from 27.6°C to 33.3°C. Within the TNZ T _b was 33.3±0.4°C and BMR was 1.02±0.29 mlO₂ g⁻¹ h⁻¹ (5.60±1.65 mW g⁻¹) at a mean body mass of 4.0±0.69 g, which is 55 % of that predicted for a 4 g bat. Minimum TMR of torpid bats was 0.014±0.006 mlO₂ g⁻¹ h⁻¹ (0.079±0.032 mW g⁻¹) at T _a=4.6±0.4°C and T _b=7.5±1.9. T _lc and C _wet of normothermic bats were both lower than that predicted for a 4 g bat, which indicates that V. vulturnus is adapted to minimising heat loss at low T _a. Our findings support the hypothesis that vespertilionid bats have evolved energy-conserving physiological traits, such as low BMR and proclivity for torpor.     

Stomatal activity within the crowns of tall deciduous trees under forest conditions

The variation in stomatal activity within the crowns ofAcer campestre, Carpinus betulus andQuercus cerris was measured by vapour exchange porometer on several summer days in an oak-hornbeam forest, in SW Slovakia, Czechoslovakia. Variation resulted from crown position in the forest stand and from leaf position within the canopy. The highest stomatal conductance was in sunlit sun leaves in the upper part of the canopy. Stomatal conductance decreased with increasing depth in the canopy. The steepest decrease was in the upper canopy, in the intermediate zone between fully sunlit and fully shaded leaves, and was caused by the decline in leaf irradiance and in stomatal density. In codominant trees, the conductance in shade leaves at the base of the crown was significantly lower than in the sun leaves at the top of the crown. In a dominant tree,Q. cerris, the differences in stomatal conductance were small and most frequently insignificant. Variation in incident light also determined the diurnal variation of stomatal conductance with respect to crown aspect. Differences between sun leaves on the east and west facing aspects of the overstory crown ofQ. cerris were demonstrated for several days.     

Neglecting acclimation of photosynthesis under drought can cause significant errors in predicting leaf photosynthesis in wheat

Extreme climatic events, such as heat waves, cold snaps and drought spells, related to global climate change, have become more frequent and intense in recent years. Acclimation of plant physiological processes to changes in environmental conditions is a key component of plant adaptation to climate change. We assessed the temperature response of leaf photosynthetic parameters in wheat grown under contrasting water regimes and growth temperatures (T_growth). Two independent experiments were conducted under controlled conditions. In Experiment 1, two wheat genotypes were subjected to well-watered or drought-stressed treatments; in Experiment 2, the two water regimes combined with high, medium and low T_growth were imposed on one genotype. Parameters of a biochemical C₃-photosynthesis model were estimated at six leaf temperatures for each factor combination. Photosynthesis acclimated more to drought than to T_growth. Drought affected photosynthesis by lowering its optimum temperature (T_opt) and the values at T_opt of light-saturated net photosynthesis, stomatal conductance, mesophyll conductance, the maximum rate of electron transport (J_max) and the maximum rate of carboxylation by Rubisco (V_cmax). T_opt for V_cmax was up to 40°C under well-watered conditions but 24–34°C under drought. The decrease in photosynthesis under drought varied among T_growth but was similar between genotypes. The temperature response of photosynthetic quantum yield under drought was partly attributed to photorespiration but more to alternative electron transport. All these changes in biochemical parameters could not be fully explained by the changed leaf nitrogen content. Further model analysis showed that both diffusional and biochemical parameters of photosynthesis and their thermal sensitivity acclimate little to T_growth, but acclimate considerably to drought and the combination of drought and T_growth. The commonly used modelling approaches, which typically consider the response of diffusional parameters, but ignore acclimation responses of biochemical parameters to drought and T_growth, strongly overestimate leaf photosynthesis under variable temperature and drought.