Evidence for NO-dependent vasodilation in the trout (Oncorhynchus mykiss ) coronary system

The effects of l-arginine, and its analogues N ^ω-nitro-l-arginine methyl ester and N ^ω-nitro-l-arginine on vascular resistance were investigated in the intact coronary system of an isolated non-working trout heart preparation. l-Arginine, at 10^–8 mol · l^–1induced a slight vasodilatory effect (max 10%). N ^ω-nitro-l-arginine methyl ester and N ^ω-Nitro-l-arginine in the range 10^–8–10^–4 mol · l^–1 caused dose-dependent increases in coronary resistance. The vasodilatory action of l-arginine was abolished when the preparation was pretreated with 10^–4 mol · l^–1 N ^ω-nitro-l-arginine or N ^ω-nitro-l-arginine methyl ester. Nitroprusside alone at 1 mmol · l^–1 induced a maximum vasodilation (30%) of the coronary system. Methylene blue a known inhibitor of guanylate cyclase, induced a strong vasoconstriction (already significant at 10^–5 mol · l^–1) and was able to overcome the vasodilative effect of nitroprusside. The endothelial nitric oxide agonists acetylcholine and serotonin, established in mammalian vessels, also mediate vasodilation in trout coronary system. In 50% of preparations, acetylcholine induced a biphasic response with vasodilation at low concentration (max 15% at 10^–8 mol · l^–1). Serotonin displayed a dose-response vasodilation in the range 10^–8–10^–4 mol · l^–1 (max 20%). These vasodilative effects were reduced or abolished by 10^–4 mol · l^–1 l-NA. These data support the existence of NO-mediated vasodilation mechanisms in the trout coronary system. Accepted: 1 July 1996     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995     

In vitro studies on calcium absorption from the gastrointestinal tract in␣small ruminants

Unidirectional flux rates of Ca²⁺ across gastrointestinal tissues from sheep and goats were measured in vitro by applying the Ussing-chamber technique. Except for the sheep duodenum, mucosal to serosal Ca²⁺ flux rates (J _ms) exceeded respective flux rates in the opposite direction (J _sm) in both species and in all segments of the intestinal tract. This resulted in net Ca²⁺ flux rates␣(J _net = J _ms − J _sm) ranging between −2 and 9 nmol · cm⁻² · h⁻¹ in sheep and between 10 and 15 nmol cm⁻² · h⁻¹ in goats. In sheep, only J _net in jejunum, and in goats, J _netin duodenum and jejunum were significantly different from zero. Using sheep rumen wall epithelia, significant J _net of Ca²⁺ of around 5 nmol · cm⁻² · h⁻¹ could be detected. Since the experiments were carried out in the absence of an electrochemical gradient, significant net Ca²⁺ absorption clearly indicates the presence of active mechanisms for Ca²⁺ transport. Dietary Ca depletion caused increased calcitriol plasma concentrations and induced significant stimulations of net Ca²⁺ absorption in goat rumen. J _net of Ca²⁺ across goat rumen epithelia was significantly reduced by 1 mmol · l ⁻¹ verapamil in the mucosal buffer solution. In conclusion, there is clear evidence for the rumen as a main site for active Ca²⁺ absorption in small ruminants. Stimulation of active Ca²⁺ absorption by increased plasma calcitriol levels and inhibition by mucosal verapamil suggest mechanistic and regulatory similarities to active Ca²⁺ transport as described for the upper small intestines of monogastric species. Accepted: 31 July 1996     

Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?

Though field data for naturally senesced leaf litter are rare, it is commonly assumed that rising atmospheric CO₂ concentrations will reduce leaf litter quality and decomposition rates in terrestrial ecosystems and that this will lead to decreased rates of nutrient cycling and increased carbon sequestration in native ecosystems. We generally found that the quality of␣naturally senesced leaf litter (i.e. concentrations of C, N and lignin; C:N, lignin:N) of a variety of native plant species produced in alpine, temperate and tropical communities maintained at elevated CO₂ (600–680 μl l⁻¹) was not significantly different from that produced in similar communities maintained at current ambient CO₂ concentrations (340–355 μl l⁻¹). When this litter was allowed to decompose in situ in a humid tropical forest in Panama (Cecropia peltata, Elettaria cardamomum, and Ficus benjamina, 130 days exposure) and in a lowland temperate calcareous grassland in Switzerland (Carex flacca and a graminoid species mixture; 261 days exposure), decomposition rates of litter produced under ambient and elevated CO₂ did not differ significantly. The one exception to this pattern occurred in the high alpine sedge, Carex curvula, growing in the Swiss Alps. Decomposition of litter produced in situ under elevated CO₂ was significantly slower than that of litter produced under ambient CO₂ (14% vs. 21% of the initial litter mass had decomposed over a 61-day exposure period, respectively). Overall, our results indicate that relatively little or no change in leaf litter quality can be expected in plant communities growing under soil fertilities common in many native ecosystems as atmospheric CO₂ concentrations continue to rise. Even in situations where small reductions in litter quality do occur, these may not necessarily lead to significantly slower rates of decomposition. Hence in many native species in situ litter decomposition rates, and the time course of decomposition, may remain relatively unaffected by rising CO₂. Received: 12 September 1996 / Accepted: 30 November 1996     

Cold induced vasodilatation and cardiovascular responses in humans during cold water immersion of various upper limb areas

To study the physiological responses induced by immersing in cold water various areas of the upper limb, 20 subjects immersed either the index finger (T1), hand (T2) or forearm and hand (T3) for 30 min in 5°C water followed by a 15-min recovery period. Skin temperature of the index finger, skin blood flow (Q_sk) measured by laser Doppler flowmetry, as well as heart rate (HR) and mean arterial blood pressure (ˉBP_a) were all monitored during the test. Cutaneous vascular conductance (CVC) was calculated as Q_sk / ˉBP_a. Cold induced vasodilatation (CIVD) indices were calculated from index finger skin temperature and CVC time courses. The results showed that no differences in temperature, CVC or cardiovascular changes were observed between T2 and T3. During T1, CIVD appeared earlier compared to T2 and T3 [5.90 (SEM 0.32) min in T1 vs 7.95 (SEM 0.86) min in T2 and 9.26 (SEM 0.78) min in T3, P < 0.01]. The HR was unchanged in T1 whereas it increased significantly at the beginning of T2 and T3 [+13 (SEM 2) beats · min⁻¹ in T2 and +15 (SEM 3) beats · min⁻¹ in T3, P < 0.01] and then decreased at the end of the immersion [−12 (SEM 3) beats · min⁻¹ in T2, and −15 (SEM 3) beats · min⁻¹ in T3, P < 0.01]. Moreover, ˉBP_aincreased at the beginning of T1 but was lower than in T2 and T3 [+9.3 (SEM 2.5) mmHg in T1, P < 0.05;  +20.6 (SEM 2.6) mmHg and 26.5 (SEM 2.8) mmHg in T2 and T3, respectively, P < 0.01]. The rewarming during recovery was faster and higher in T1 compared to T2 and T3. These results showed that general and local physiological responses observed during an upper limb cold water test differed according to the area immersed. Index finger cooling led to earlier and faster CIVD without significant cardiovascular changes, whereas hand or forearm immersion led to a delayed and slower CIVD with a bradycardia at the end of the test. Accepted: 26 November 1996     

Differences in renal-cloacal function between Crocodylus porosus and Alligator mississippiensis have implications for crocodilian evolution

Major electrolytes and nitrogenous excretory products were analysed in the blood plasma, ureteral urine and cloacal urine of juvenile Alligator mississippiensis and Crocodylus porosus in fresh and hypoosmotic salt water (206 mosmol · l⁻¹). Both species coped well with saline water, showing little (Alligator) or no (Crocodylus) change in plasma composition. Comparisons of renal-cloacal function point to major differences in their osmoregulatory physiology. The cloaca of C. porosus is a very active osmoregulatory organ in salt and fresh water, contributing to water conservation and NaCl excretion through the lingual salt glands. In contrast, the cloaca of Alligator has little impact on the composition of excreted urine. It seems likely that A.␣mississippiensis is largely constrained to a renal response to osmotic and ionic stress while C. porosus is able to call on a more complex mix of renal response, post-renal modification of urine in the cloaca, and excretion of excess NaCl through the salt glands. The results support the idea that there are deep-seated differences in the osmoregulatory physiology of alligatorids and crocodylids (Eusuchia), an understanding of which should provide valuable insights into their evolution and zoogeography. Accepted: 7 September 1996     

Between-tree variations in leaf δ13C of Quercus pubescens and Quercus ilex among Mediterranean habitats with different water availability

In this study, sun leaf carbon isotope composition (δ¹³C) of two co-occurring woody Mediterranean species (Quercus pubescens Willd., a deciduous oak, and Q. ilex L., an evergreen one) was investigated on four sites with different water availability. The total range of δ¹³C values was 4.4 and 3.1‰ for Q. pubescens and Q. ilex respectively. The intra-site variability was about 3‰. Total mean per species was equal. There were significant differences among sites, but at each site means of δ¹³C were not significantly different between species. A simple physiological model predicts no difference in intrinsic water-use efficiency (WUE_i) between evergreen and deciduous oaks. The relationship between site means of δ¹³C and water parameters suggests that there is a leaf functional adjustment with respect to available water resource. No correlation was found between δ¹³C and the contents of any mass-based biochemical constituent. Nevertheless there was a significant correlation between δ¹³C and leaf mass per area of Q. ilex. For both species, there is also a positive correlation between leaf δ¹³C and individual crown area, i.e. a structural characteristic at tree level. Causal relations between δ¹³C and plant-environment interactions are discussed. Received: 25 October 1996 / Accepted: 19 January 1997     

Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States

Carbon isotope ratios (δ¹³C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The δ¹³C_leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5–2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5–4.5) exhibited strongly decreasing δ¹³C_leaf values (increasing carbon isotope discrimination) with increasing LAI. The δ¹³C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, δ¹³C_leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c _i/c _a (> 60%) and to a minor extent due to δ¹³C of canopy air (< 40%). This intra-canopy depletion of δ¹³C_leaf was lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ¹³C_leaf did not change with canopy LAI, δ¹³C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ¹³C of decomposing organic carbon became increasingly enriched over time (by 1.7–2.9‰) for all deciduous and evergreen dry temperate forests. The δ¹³C_canopy of CO₂ in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ¹³C_canopy, and [CO₂]_canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Δ_e) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ¹³C_canopy and [CO₂]_canopy were larger in denser Acer spp. stands than those in the open stand. However, ¹³C enrichment above and photosynthetic draw-down of [CO₂]_canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship δ¹³C_canopy versus 1/[CO₂]_canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Δ_e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δ_e varied seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰). Received: 15 April 1996 / Accepted: 19 October 1996     

A phospholipase inhibitor, manoalide, and a G-protein activator, Mas-7, both affect the turnover of phototransductive membranes by crab retinas in darkness With a model for the regulation of rhabdomeral membrane turnover

(1) In vitro retinas of a crab, Leptograpsus, were treated with a phospholipase inhibitor, manoalide, or a G-protein activator, Mas-7. Both drugs address early stages of the phototransduction cascade. (2) Manoalide inhibited the light-dependent reduction of rhabdoms during the `day' phase of the light cycle, but did not induce rhabdom overgrowth. Following a period of darkness manoalide failed to affect the diminution of illuminated rhabdoms. (3) The diminution of rhabdoms that follows photoreceptor depolarisation induced by 100 mmol · l<sup>−1</sup> K<sup>+</sup> in darkness was not affected by 2␣μmol · l<sup>−1</sup> manoalide. (4) When retinas in the `night' phase were treated with Mas-7 in darkness, rhabdom diameters were augmented, concurrently with endocytosis of photoreceptor plasma membranes. (5) The results of combining manoalide and Mas-7 with actinomycin D, U-57908 or okadaic acid, drugs used in previous studies to manipulate steps notionally lower in the transduction cascade, lead to a hypothetical model for the regulation of phototransductive membrane turnover by arthropods. Accepted: 3 October 1996     

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

We tested the hypothesis that elevated CO₂ would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO₂. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO₂ and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g⁻¹ day⁻¹ net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO₂ resulted in significantly higher maximum leaf photosynthesis (A _max) at both soil N levels. A _max was higher at high N than at low N soil in elevated, but not ambient CO₂. Photosynthetic N use efficiency was higher at elevated than ambient CO₂ in both soil types. Elevated CO₂ resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO₂ and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V _cmax/J _max) than at ambient CO₂ and/or low N availability. From the top to the bottom of the tree crowns, V _cmax/J _max increased in ambient CO₂, but it decreased in elevated CO₂ indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO₂. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A _leaf for each segment. This analysis indicated that increased A _leaf at elevated CO₂ did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO₂. Received: 12 August 1996 / Accepted: 12 November 1996     

Effect of exercise-induced hyperventilation on airway resistance and cycling endurance

The purpose of the present study was to investigate the effect of exercise induced hyperventilation and hypocapnia on airway resistance (R _aw), and to try to answer the question whether a reduction of R _aw is a mechanism contributing to the increase of endurance time associated with a reduction of exercise induced hyperventilation as for example has been observed after respiratory training. Eight healthy volunteers of both sexes participated in the study. Cycling endurance tests (CET) at 223 (SD 47) W, i.e. at 74 (SD 5)% of the subject's peak exercise intensity, breathing endurance tests and body plethysmograph measurements of pre- and postexercise R _aw were carried out before and after a 4-week period of respiratory training. In one of the two CET before the respiratory training CO₂ was added to the inspired air to keep its end-tidal concentration at 5.4% to avoid hyperventilatory hypocapnia (CO₂-test); the other test was the control. The pre-exercise values of specific expiratory R _aw were 8.1 (SD 2.8), 6.8 (SD 2.6) and 8.0 (SD 2.1) cm H₂O · s and the postexercise values were 8.5 (SD 2.6), 7.4 (SD 1.9) and 8.0 (SD 2.7) cm H₂O · s for control CET, CO₂-CET and CET after respiratory training, respectively, all differences between these tests being nonsignificant. The respiratory training significantly increased the respiratory endurance time during breathing of 70% of maximal voluntary ventilation from 5.8 (SD 2.9) min to 26.7 (SD 12.5) min. Mean values of the cycling endurance time (t _cend) were 22.7 (SD 6.5) min in the control, 19.4 (SD 5.4) min in the CO₂-test and 18.4 (SD 6.0) min after respiratory training. Mean values of ventilation ( _E) during the last 3␣min of CET were 123 (SD 35.8) l · min⁻¹ in the control, 133.5 (SD 35.1) l · min⁻¹ in the CO₂-test and 130.9 (SD 29.1) l · min⁻¹ after respiratory training. In fact, six subjects ventilated more and cycled for a shorter time, whereas two subjects ventilated less and cycled for a longer time after the respiratory training than in the control CET. In general, the subjects cycled longer the lower the _E, if all three CET are compared. It is concluded that R _aw measured immediately after exercise is independent of exercise-induced hyperventilation and hypocapnia and is probably not involved in limiting t _cend, and that t _cend at a given exercise intensity is shorter when _E is higher, no matter whether the higher _E occurs before or after respiratory training or after CO₂ inhalation. Accepted: 11 September 1996     

Leaf gas exchange in a clonal eucalypt plantation as related to soil moisture, leaf water potential and microclimate variables

In order to determine how environmental and physiological factors affect leaf gas exchange in a 9-year-old clonal eucalypt plantation (Eucalyptus grandis Hill ex. Maiden hybrids) in the State of Espirito Santo, Brazil, the diurnal patterns of predawn leaf water potential (Ψ_pd), and leaf gas exchange were monitored from November 1995 to August 1996. Soil water content (Θ) and microclimatic variables were also recorded. Most of the rainfall during the experimental period occurred from October to December 1995 and from March to April 1996, causing a significant variation in Θ and Ψ_pd. A high positive correlation (r ²=0.92) was observed between Ψ_pd and Θ measured at 0.3 m depth from the soil surface. During conditions of high soil water availability, the maximum values of stomatal conductance for water vapor (g _s) and net photosynthetic rate (A) were over 0.4 mol m^–2 s^–2 and l5 μmol m^–2 s^–1, respectively. The results showed that Ψ_pd and leaf gas exchange of the examined trees were susceptible to changes in the water content of the upper soil layers, where the major concentration of active roots occur. Multiple linear regression analysis indicated that photosynthetic active radiation (Q), vapor pressure deficit (VPD), atmospheric CO₂ molar fraction (C _a), and Ψ_pd were the most important factors controlling g _s whereas Q and VPD were the main microclimatic variables controlling A. Received: 5 November 1998 / Accepted: 10 November 1999     

In vivo expression of a full-length cDNA copy of potato leafroll virus (PLRV) in protoplasts and transgenic plants

A full-length cDNA copy (PLRV_fl) of potato leafroll virus (PLRV) was constructed and examined in vivo for its biological activities by transient expression experiments with plasmid DNA or in vitro transcribed RNA. In addition, PLRV_fl cDNA was stably introduced into the genome of potato plants by Agrobacterium-mediated leaf disc transformation. Both transient and stable expression of PLRV_fl resulted in the synthesis of genomic and subgenomic PLRV RNAs. Transgenic plants accumulated the 17-kDa movement protein and displayed the typical symptoms of PLRV infection. This is the first example of the constitutive expression of a phloem-limited virus in planta. Received: 20 August 1996 / Accepted: 24 September 1996     

Walking performance and economy in chronic heart failure patients pre and post exercise training

The effect of a 3-week exercise programme on performance and economy of walking was analysed in 16 male patients with chronic heart failure [mean age 51.8 (SD 6.9) years, height 174.9 (SD 6.3) cm, body mass 75.3 (SD 11.5) kg, ejection fraction 20.8 (SD 5.0)%]. They were submitted to a cardiopulmonary exercise test on a cycle ergometer and a 6-min walking test on a treadmill before and after the period of exercise training. The training programme consisted of interval cycle (five times a week for 15 min), and treadmill ergometer training (three times a week for 10 min) at approximately 70% cycling peak oxygen uptake (O_2peak) and supplementary exercises (three times a week for 20 min). Compared to the pre values cycling O_2peak [11.9 (SD 2.9) vs 14.0 (SD 2.3) ml ·  kg^–1 · min^–1], maximal self paced walking speed [0.68 (SD 0.33) vs 1.16 (SD 0.30) m · s^–1], and net walking power [2.16 (SD 0.89) vs 2.73 (SD 0.91) W · kg^–1] had increased (P < 0.01) while net energy cost [3.31 (SD 0.66) vs 2.33 (SD 0.38) J · kg^–1 ·  m^–1] had decreased (P < 0.001) after the training period. Approximately 42% of the increase of walking speed resulted from a higher walking power output, whereas approximately 58% corresponded to a positive effect on walking economy. The improvement in walking economy was a function of an increase in walking velocity itself and a result of a more efficient walking technique. These results would indicate that in patients with marked exercise intolerance, adequate exercise training programmes could contribute to favourable metabolic changes with positive effects on the economy of motion. Accepted: 29 August 1996     

Seasonal changes in fatty acid composition associated with cold-hardening in third instar larvae of Eurosta solidaginis

Third-instar larvae of the goldenrod gall fly Eurosta solidaginis (Diptera: Tephritidae) survive extended periods in winter during which tissue water is frozen. Both low temperature and reduced water activity during freezing present challenges for the structural integrity of cellular lipids. Fatty acids of both phospholipids and triacylglycerols from fat body cells of E. solidaginis were analyzed throughout fall and early winter, a period that encompasses the acquisition of freeze-tolerance, to determine if adaptations to freezing include changes in fatty acid unsaturation. The five most abundant fatty acids from both fractions were (in decreasing order) oleic (40–65%), palmitoleic (18–20%), palmitic (12–17%), linoleic (5–10%), and stearic acids (4 –7%). This represents a typical complement of Dipteran fatty acids, although oleic acid levels were higher in E. solidaginis than those reported from other Dipterans (˜28%; Downer 1985). From September to November, monounsaturates increased from 59 to 70% in phospholipids at the expense of saturated fatty acids (25% –20%) suggesting activation of a Δ⁹-desaturase enzyme. These changes resulted in an increase in the ratio of unsaturated to saturated fatty acids (U/S) from 3.0 to 4.2, although there was no change in the average number of double bonds per fatty acid (unsaturation index, UI ≈ 1.2 in phospholipids and 0.9 in triacylglycerols throughout the season). These changes were temporally correlated to decreasing ambient temperatures and increasing larval and fat body cell freeze-tolerance. Accepted: 31 October 1996     

Effects of aerobic exercise on the torque-velocity relationship in cycling

The kinetics of the torque-velocity (T-ω) relationship after aerobic exercise was studied to assess the effect of fatigue on the contractile properties of muscle. A group of 13 subjects exercised until fatigued on a cycle ergometer, at an intensity which corresponded to 60% of their maximal aerobic power for 50 min (MAP60%); ten subjects exercised until fatigued at 80% of their maximal aerobic power for 15 min (MAP80%). Of the subjects 7 exercised at both intensities with at least a 1-week interval between sessions. Pedalling rate was set at 60 rpm. The T-ω relationship was determined from the velocity data collected during all-out sprints against a 19 N · m braking torque on the same ergometer, according to a method proposed previously. Maximal theoretical velocity (ω₀) and maximal theoretical torque (T ₀) were estimated by extrapolation of the linear T-ω relationship. Maximal power (P _max) was calculated from the values of T ₀ and ω₀ (P _max = 0.25 ω₀ T ₀). The T-ω relationships were determined before, immediately after and 5 and 10 min after the aerobic exercise. The kinetics of ω₀, T ₀ and P _max was assumed to express the effects of fatigue on the muscle contractile properties (maximal shortening velocity, maximal muscle strength and maximal power). Immediately after exercise at MAP60% a 7.8% decrease in T ₀ and 8.8% decrease in P _max was seen while the decrease in ω₀ was nonsignificant, which suggested that P _max decreased in the main because of a loss in maximal muscle strength. In contrast, MAP80% induced a 8.1% decrease in ω₀ and 12.8% decrease in P _max while the decrease in T ₀ was nonsignificant, which suggested that the main cause of the decrease in P _max was probably a slowing of maximal shortening velocity. The short recovery time of the T-ω relationship suggests that the causes of the decrease of torque and velocity are processes which recover rapidly. Accepted: 25 November 1996     

Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area. Received: 8 August 1996 / Accepted: 29 January 1997     

Stem respiration of ponderosa pines grown in contrasting climates: implications for global climate change

We examined the effects of climate and allocation patterns on stem respiration in ponderosa pine (Pinus ponderosa) growing on identical substrate in the cool, moist Sierra Nevada mountains and the warm, dry, Great Basin Desert. These environments are representative of current climatic conditions and those predicted to accompany a doubling of atmospheric CO₂, respectively, throughout the range of many western north American conifers. A previous study found that trees growing in the desert allocate proportionally more biomass to sapwood and less to leaf area than montane trees. We tested the hypothesis that respiration rates of sapwood are lower in desert trees than in montane trees due to reduced stem maintenance respiration (physiological acclimation) or reduced construction cost of stem tissue (structural acclimation). Maintenance respiration per unit sapwood volume at 15°C did not differ between populations (desert: 6.39 ± 1.14 SE μmol m⁻³ s⁻¹, montane: 6.54 ± 1.13 SE μmol m⁻³ s⁻¹, P = 0.71) and declined with increasing stem diameter (P = 0.001). The temperature coefficient of respiration (Q ₁₀) varied seasonally within both environments (P = 0.05). Construction cost of stem sapwood was the same in both environments (desert: 1.46 ± 0.009 SE g glucose g⁻¹ sapwood, montane: 1.48 ± 0.009 SE glucose g⁻¹ sapwood, P = 0.14). Annual construction respiration calculated from construction cost, percent carbon and relative growth rate was greater in montane populations due to higher growth rates. These data provide no evidence of respiratory acclimation by desert trees. Estimated yearly stem maintenance respiration was greater in large desert trees than in large montane trees because of higher temperatures in the desert and because of increased allocation of biomass to sapwood. By analogy, these data suggest that under predicted increases in temperature and aridity, potential increases in aboveground carbon gain due to enhanced photosynthetic rates may be partially offset by increases in maintenance respiration in large trees growing in CO₂-enriched atmospheres. Received: 4 November 1996 / Accepted: 23 January 1997     

The effect of a thiamin derivative on exercise performance

The purpose of this study was to investigate the effect of a thiamin derivative, thiamin tetrahydrofurfuryl disulfide (TTFD), on oxygen uptake (˙VO₂), lactate accumulation and cycling performance during exercise to exhaustion. Using a randomized, double-blind, cross-over design with a 10-day washout between trials, 14 subjects ingested either 1 g · day⁻¹ of TTFD or a placebo (PL) for 4 days. On day 3, subjects performed a progressive exercise test to exhaustion on a cycle ergometer for the determination of ˙VO_2submax, ˙VO_2peak, lactate concentration ([La⁻ ]), lactate threshold (Th_La) and heart rate ( f _c). On day 4, subjects performed a maximal 2000-m time trial on a cycle ergometer. A one-way analysis of variance (ANOVA) with repeated measures was used to determine significant differences between trials. There were no significant differences detected between trials for serial measures of ˙VO_2submax, [La⁻] or f _c. Likewise, ˙VO_2peak [PL 4.06 (0.19) TTFD 4.12 (0.19) l · min⁻¹, P = 0.83], Th_La [PL 2.47 (0.17), TTFD 2.43 (0.16) l · min⁻¹, P = 0.86] and 2000-m performance time [PL 204.5 (5.5), TTFD 200.9 (4.3) s, P = 0.61] were not significantly different between trials. The results of this study suggest that thiamin derivative supplementation does not influence high-intensity exercise performance. Accepted: 19 December 1996