Seasonal variation in the relationship between growth rate and phlorotannin production in the kelp Ecklonia radiata

Most theories for the evolution of plant chemical defences assume that defences are costly. In order to investigate the potential cost of phlorotannin production in the brown alga (kelp) Ecklonia radiata, I measured the correlation between changes in phlorotannin levels and growth rates for E. radiata in two seasons, spring and autumn. In spring, when both phlorotannin levels and growth rates in E. radiata were highest, there was a significant negative relationship between changes in phlorotannin levels and growth, consistent with a cost of phlorotannin production. No relationship was evident in autumn, indicating that moderate levels of phlorotannins and growth could be maintained simultaneously. These and related results suggest that the cost of defence will vary as a function of (1) endogenous patterns of growth or metabolite production and (2) variation in available resources. Evidence for a cost of phlorotannins in E. radiata adds to the paradox of the maintenance of high levels of phlorotannins in Australasian brown algae.     

Diversity of phlorotannin profiles among sargassasacean species affecting variation and abundance of epiphytes

In general, epiphytes have detrimental effects on the growth of their basiphytes due to competition for light and nutrients. Therefore, basiphyte species must expend energy suppressing epiphytes. Some studies suggest that phlorotannins, i.e. brown algal polyphenols, prevent colonization by epiphytes, whereas others question their allelopathic function because there is not necessarily a negative correlation between epiphyte abundance and the phlorotannin content of the basiphyte algae. Various phlorotannin components are found in brown algal species, thus we hypothesized that the antifouling activities of polyphenolic compounds may differ and that the analysis of phlorotannin profiles could be useful for estimating their ecological functions. We surveyed the epiphyte richness in the apical portions of 373 thalli from 15 sargassacean species, demonstrating that the variation and abundance of epiphyte species differed remarkably among the basiphyte species. However, there was a weak negative correlation between the density and total phlorotannin content of the basiphyte algae in only one of the 18 epiphyte species. The interspecific differences in the phlorotannin profile were characterized by quantitative ¹H nuclear magnetic resonance spectroscopy (qNMR), and four major groups were categorized based on cluster and principal component analyses of polyphenolic signals in the qNMR spectra. The epiphyte Neosiphonia harveyi was more abundant on Sargassum hemiphyllum, S. patens and S. piluliferum than on other basiphyte species, and these three species were similar according to the cluster analysis. These results suggest that some phlorotannin components may be more effective for antifouling; thus interspecific differences in the phlorotannin profile could affect the variation and abundance of epiphytes.     

Harvesting of the Kelp Ecklonia Maxima in South Africa Affects Its Three Obligate, Red Algal Epiphytes

In South Africa, more than 7000 t (f wt) of kelp (Ecklonia maxima) fronds are harvested annually to feed cultured abalone. Carpoblepharis flaccida, Gelidium vittatum and Polysiphonia virgata are conspicuous red algal epiphytes on older kelps and provide habitat and food for numerous animals. Over 4.5 y, we examined the effects of one destructive harvest of E. maxima on these 3 epiphytes. Two 20 × 20 m plots of kelp with similar epiphyte loads were demarcated. In one, all E. maxima sporophytes with stipes longer than 50 cm were harvested. The other plot served as a control. After 2.5 y the biomass of E. maxima in the harvested plot had recovered to control levels, but the epiphyte load (g epiphytes. Kg kelp⁻¹) was statistically lower in the harvested plot after 2.5 and 3.5 y, and only recovered after 4.5 y. While most commercial harvesters cut through the “heads” (primary blades) of the kelp, effectively killing them, a new, non-lethal method removes secondary blades 20–30 cm from their bases, leaving the meristems and primary blades intact. At 5 sites studied, G. vittatum and P. virgata were found almost entirely on stipes and primary blades, and harvesting only distal parts of secondary blades limited losses to about 50% of C. flaccida biomass. To protect epiphytes, non-lethal harvesting is recommended and permanent non-harvest zones have been established in addition to limiting kelp yields and disallowing harvesting in Marine Protected Areas.     

Insulin and glucagon immunoreactivity during high-intensity exercise under opiate blockade

Eight fit men [maximum oxygen consumption (O_2max) 64.6 (1.9) ml · kg⁻¹ · min⁻¹, aged 28.3 (1.7) years (SE in parentheses) were studied during two treadmill exercise trials to determine the effect of endogenous opioids on insulin and glucagon immunoreactivity during intense exercise (80% O_2max). A double-blind experimental design was used with subjects undertaking the two exercise trials in counterbalanced order. Exercise trials were 20 min in duration and were conducted 7 days apart. One exercise trial was undertaken following administration of naloxone (N; 1.2 mg; 3 ml) and the other after receiving a placebo (P; 0.9% NaCl saline; 3 ml). Prior to each experimental trial a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Immediately after, each subject received either a N or P bolus injection. Blood samples were also collected after 20 min of continuous exercise (running). Glucagon was higher (P < 0.05), while insulin was lower (P < 0.05), during exercise compared with pre-exercise values in both trials. However, glucagon was higher (P < 0.05) in the P than in the N exercise trial [141.4 (8.3) ng · l⁻¹ vs 127.2 (7.6) ng · l⁻¹]. There were no differences in insulin during exercise between the P and N trials [50.2 (4.3) pmol · l⁻¹ vs 43.8 (5) pmol · l⁻¹]. These data suggest that endogenous opioids may augment the glucagon response during intense exercise. Accepted: 15 June 1996     

Differences in muscle contractile characteristics among bodybuilders, endurance trainers and control subjects

The purpose of this investigation was to compare the myosin heavy chain (MHC) isoform expression of the triceps brachii muscle and isoinertial, isometric and isokinetic strength indices in competitive bodybuilders (CB, n = 5), recreational resistance trainers (RT, n = 5), endurance-trained rowers (ER, n = 5) and control (C, n = 5) subjects. Muscle tissue samples were analysed for MHC isoform content using 6% sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The CB possessed significantly smaller (P < 0.05) percentage of MHC type IIb proteins [12.92 (SD 7.08)%] than RT [30.08 (SD 6.58)%] ER [31.20 (SD 2.74)%] and C [38.22 (SD 2.95)%] groups (i.e. CB < RT ≈ ER < C). While the content of MHC type IIa isoforms did not differ significantly between the two resistance-trained groups [CB = 55.76 (SD 5.38)%; RT = 45.72 (SD 7.8)%], CB presented significantly more type IIa MHC isoforms than ER [42.84 (SD 2.98)%] and C [34.72 (SD 1.57)%] subjects (i.e. CB ≈ RT > ER ≈ C). The MHC type I protein content did not differ significantly among RT [24.20 (SD 4.89)%] ER [25.38 (SD 1.67)%] and C [27.06 (SD 1.81)%] groups. The CB [31.32 (SD 2.67)%] presented significantly more type I MHC isoforms only in comparison with RT. However, when changes in the percentage of MHC type I isoforms were converted to effect sizes (ES), it appeared that low statistical power rather than the absence of an effect accounted for the nonsignificant differences between CB and other groups (i.e. CB > RT ≈ ER ≈ C). Significant differences existed in isoinertial strength among the trained athletes (i.e. CB > RT > ER ≈ C), while isometric and isokinetic strength were not significantly different among any of the trained groups. However, the ES transformation of data demonstrated that large differences existed between resistance-trained groups and ER for isometric and isokinetic strength (i.e. CB ≈ RT > ER ≈ C). A statistically significant negative correlation (P < 0.001) was found between MHC type IIb isoforms and isoinertial strength index (r = − 0.68). The MHC type IIa proteins were positively related to all the strength measures considered (r = 0.51 – 0.61; P < 0.001). These data demonstrated different patterns of MHC isoform expression among the different groups of athletes and it is suggested that these differences on occasion may affect the expression of strength. Accepted: 24 September 1996     

The relationship between preferred and optimal positioning during submaximal cycle ergometry

This study was designed to determine how changes in oxygen uptake (O₂) and heart rate (HR) during submaximal cycle ergometry were determined by changes in cycle geometry and/or lower-limb kinematics. Fourteen trained cyclists [Mean (SD): age, 25.5 (6.4) years; body mass 74.4 (8.8) kg; peak O₂, 4.76 (0.79) l. min⁻¹ peak] were tested at three seat-tube angles (70°, 80°, 90°) at each of three trunk angles (10°, 20°, 30°) using a modified Monark cycle ergometer. All conditions were tested at a power output corresponding to 95% of the O₂ at each subject's ventilatory threshold while pedalling at 90 rpm and using aerodynamic handlebars. Sagittal-view kinematics for the hip, knee, and ankle joints were also recorded for all conditions and for the subjects' preferred positioning on their own bicycles. No combination of seat-tube and trunk angle could be considered optimal since many of the nine conditions elicited statistically similar mean O₂ and HR values. Mean hip angle (HA) was the only kinematic variable that changed consistently across conditions. A regression relationship was not observed between mean O₂ or HR and mean hip angle values (P > 0.45). Significant curvilinear relationships were observed, however, between ΔO₂ (O₂ − minimum O₂) and ΔHA (mean HA − preferred HA) using the data from all subjects (R = 0.45, SEE = 0.13 l . min⁻¹) and using group mean values (R = 0.93, SEE = 0.03 l . min⁻¹). In both cases ΔO₂ minimized at ΔHA = 0, which corresponded to the subjects' preferred HA from their own bicycles. Thus, subjects optimized their O₂ cost at cycle geometries that elicited similar lower-limb kinematics as the preferred geometries from their own bicycles. Accepted: 3 July 1996     

Variations in leaf β13C along a vertical profile of irradiance in a temperate Japanese forest

The vertical profile of stable carbon isotope ratios (δ¹³C) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of long-term averaged δ¹³C in atmospheric CO₂ (δ_a) was estimated from δ¹³C of dry matter from NADP-malic enzyme type C₄ plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δ_a value obtained from δ¹³C in Z.␣mays was lowest at the forest floor (−9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (−7.14 ± 0.14‰). Then leaf Δ values for the tree species were calculated from tree leaf δ¹³ C andδ_a. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰ in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly due to decreasing intercellular CO₂ (C _i) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height, since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing the degree of stomatal limitation in photosynthesis with height. Received: 30 September 1995 / Accepted: 25 October 1996     

Cloning of the cDNA for glutaredoxin, an abundant sieve-tube exudate protein from Ricinus communis L. and characterisation of the glutathione-dependent thiol-reduction system in sieve tubes

Sieve-tube exudate protein (STEP) from Ricinus communis L. seedlings consists of a characteristic set of more than 100 different polypeptides, against which a complex antiserum was raised. This antiserum cross-reacted with dominant protein species (molecular weights 10–30 kDa) present in the sieve-tube exudate and, to a lesser extent, with proteins in tissue extracts of Ricinus and a wide range of other plant species. For further elucidation of the nature of individual STEPs in the sieve tubes the anti-STEP serum was used to screen a cDNA expression library constructed from Ricinus cotyledon mRNA. Two clones that differed in the 3′ untranslated region encoded a protein of 11 kDa which showed striking homology to bacterial and eucaryotic glutaredoxin sequences. Glutaredoxin activity was confirmed for the recombinant protein after overexpression in Escherichia coli and characterised in detail in sieve-tube exudate. Michaelis Menten constants (K _m) for reduced glutathione and cysteine were 2 mM and 50 μM, respectively. Besides l-cysteine, dehydroascorbate and protein disulphides were also reduced by the activity present in the sieve-tube exudate. Glutathione, which is the obligate donor of reduced thiols for glutaredoxin, was present in sieve-tube sap in millimolar concentrations (up to 3 mM) with a ratio of total to oxidised glutathione of 3:1. It is suggested that glutaredoxin and glutathione in sieve tubes prevent oxidative damage and may be involved in redox regulation of sieve-tube proteins. Received: 13 December 1996 / Accepted: 28 December 1996     

Exercise-induced oxyhemoglobin desaturation and pulmonary diffusing capacity during high-intensity exercise

The purpose of this investigation was to examine if exercise-induced arterial oxyhemoglobin desaturation selectively observed in highly trained endurance athletes could be related to differences in the pulmonary diffusing capacity (D _L) measured during exercise. The D _L of 24 male endurance athletes was measured using a 3-s breath-hold carbon monoxide procedure (to give D _LCO) at rest as well as during cycling at 60% and 90% of these previously determined O_2max. Oxyhemoglobin saturation (S _aO₂%) was monitored throughout both exercise protocols using an Ohmeda Biox II oximeter. Exercise-induced oxyhemoglobin desaturation (DS) (S _aO₂% < 91% at O_2max) was observed in 13 subjects [88.2 (0.6)%] but not in the other 11 nondesaturation subjects [NDS: 92.9 (0.4)%] (P ≤ 0.05), although O_2max was not significantly different between the groups [DS: 4.34 (0.65) l / min vs NDS: 4.1 (0.49) l / min]. At rest, no differences in either D _LCO [m1 CO · mmHg⁻¹ · min⁻¹: 41.7 (1.7) (DS) vs 41.1 (1.8) (NDS)], D _LCO / _A [8.2 (0.4) (DS) vs 7.3 (0.9) (NDS)], MVV [l / min: 196.0 (10.4) (DS) vs 182.0 (9.9) (NDS)] or FEV₁/FVC [86.3 (2.2) (DS) vs 82.9 (4.7) (NDS)] were found between groups (P ≥ 0.05). However, _E /O₂ at O_2max was lower in the DS group [33.0 (1.1)] compared to the NDS group [36.8 (1.5)] (P ≤ 0.05). Exercise D _LCO (m1 CO · mmHg⁻¹ · min⁻¹) was not different between groups at either 60% O_2max [DS: 55.1 (1.4) vs NDS: 57.2 (2.1)] or at 90% O_2max [DS: 61.0 (1.8) vs NDS: 61.4 (2.9)]. A significant relationship (r = 0.698) was calculated to occur between S _aO₂% and _E /O₂ during maximal exercise. The present findings indicate that the exercise-induced oxyhemoglobin desaturation seen during submaximal and near-maximal exercise is not related to differences in D _L, although during maximal exercise S _aO₂ may be limited by a relatively lower exercise ventilation. Accepted: 25 September 1996     

Effects of above-ground browsing by mammals on mycorrhizal infection in an early successional taiga ecosystem

Using an exclosure experiment in the willow stage of primary succession on the floodplain of the Tanana River, we tested the hypothesis that browsing can reduce mycorrhizal infection. We measured the effects winter browsing by moose (Alcesalces) and snowshoe hare (Lepusamericanus) had on mycorrhizal infection and fine root biomass of willow (Salix spp.) and balsam poplar (Populusbalsamifera). We found that protection from winter browsing increased ectomycorrhizal infection by 10% in the top 5 cm of the soil profile, by 23% at 5–10 cm, and by 42% at the 10–15 cm depth. Mammal browsing in taiga forests is now recognized as a major cause of the shift from palatable deciduous species such as willow and balsam poplar to less palatable species such as alder and spruce. We suggest that browsing-induced reduction in ectomycorrhizal infection of salicaceous species plays a central role in this shift in plant community composition. Received: 26 March 1996 / Accepted: 26 September 1996     

Specificity of resistance training responses in neck muscle size and strength

This study examined hypertrophy after head extension resistance training to assess which muscles of the complicated cervical neuromuscular system were used in this activity. We also determined if conventional resistance exercises, which are likely to evoke isometric action of the neck, induce generalized hypertrophy of the cervical muscle. Twenty-two active college students were studied. [mean (SE) age, weight and height: 21 (1) years, 71 (4) kg and 173 (3) cm, respectively]. Subjects were assigned to one of three groups: RESX (head extension exercise and other resistance exercises), RES (resistance exercises without specific neck exercise), or CON (no training). Groups RESX (n = 8) and RES (n = 6) trained 3 days/week for 12 weeks with large-muscle mass exercises (squat, deadlift, push press, bent row and mid-thigh pull). Group RESX also performed three sets of ten repetitions of a head extension exercise 3 days/week with a load equal to the 3 × 10 repetition maximum (RM). Group CON (n = 8) was a control group. The cross-sectional area (CSA) of nine individual muscles or muscle groups was determined by magnetic resonance imaging (MRI) of the cervical region. The CSA data were averaged over four contiguous transaxial slices in which all muscles of interest were visible. The 3 × 10 RM for the head extension exercise increased for RESX after training [from 17.9 (1.0) to 23.9 (1.4) kg, P < 0.05] but not for RES [from 17.6 (1.4) to 17.7 (1.9)␣kg] or CON [from 10.1 (2.2) to 10.3 (2.1) kg]. RESX showed an increase in total neck muscle CSA after training [from 19.5 (3.0) to 22.0 (3.6) cm², P < 0.05], but RES and CON did not [from 19.6 (2.9) to 19.7 (2.9)␣cm² and 17.0 (2.5) to 17.0 (2.4) cm², respectively]. This hypertrophy for RESX was due mainly to increases in CSA of 23.9 (3.2), 24.0 (5.8), and 24.9 (5.3)% for the splenius capitis, and semispinalis capitis and cervicis muscles, respectively. The lack of generalized neck muscle hypertrophy in RES was not due to insufficient training. For example, the CSA of their quadriceps femoris muscle group, as assessed by MRI, increased by 7 (1)% after this short-term training (P < 0.05). The results suggest that: (1) the splenius capitis, and semispinalis capitis and cervicis muscles are mainly responsible for head extension; (2) short-term resistance training does not provide a sufficient stimulus to evoke neck muscle hypertrophy unless specific neck exercises are performed; and (3) the postural role of head extensors provides modest loading in bipeds. Accepted: 15 October 1996     

The effect of endurance exercise at moderate altitude on serum lipid peroxidation and antioxidative functions in humans

We investigated the effect of training and racing at moderate altitude (MA) on oxidative stress by assessment of serum diene conjugation (DC) and serum antioxidant potential (TRAP). Nine male top level skiers were studied during a national race (20–30 km) at sea level (SL). Thereafter, the athletes trained for 2 weeks at MA, after which they participated in a 20–30 km race at MA. Venous blood samples were taken before and after the race. The DC, indicating early events of lipid peroxi dation, did not change during the race at SL (16 850 vs 15 900 ΔAbsorbance · l⁻¹) or at MA (19 870 vs. 20 630 ΔAbs · l⁻¹). At MA serum DC was higher than at SL both before (25%) and after (30%) the race, the postrace difference being statistically significant (P < 0.05). The TRAP increased during the race at MA (from 1387 to 1943 μmol · 1⁻¹, P  =  0.016), but not at SL (1713 vs 1582 μmol · l⁻¹). These observations would suggest that the level of oxidative stress might be greater during living, training and racing at MA (higher DC levels). Increased TRAP during the race at MA may indicate that the physiological adaptation to extreme acute oxidative stress was altered. The physiological significance of this observation remains to be investigated. Accepted: 18 October 1996     

Transpiration from a mature Eucalyptus globulus plantation in Portugal during a spring-summer period of progressively higher water deficit

The rates of transpiration from a mature Eucalyptus globulus Labill. stand in Portugal were evaluated during a drying period of the spring-summer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental period daily transpiration varied between 3.64 and 0.50 mm day⁻¹. For high-transpiration days, a good agreement was observed between Penman-Monteith estimates and sap flow measurements, both on a daily and on an hourly basis. However, for low-transpiration days, the Penman-Monteith model overestimated transpiration in comparison with the sap flow method. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods through a progressive stomatal closure. As soil moisture deficit increased, the daily maximum stomatal conductance decreased from 0.46 to 0.14 cm s⁻¹. The results also show that, on a seasonal basis, stomatal conductance and daily transpiration were mainly related to predawn leaf water potential and, thus, to soil moisture content. Received: 26 January 1996 / Accepted: 20 October 1996     

Sequential functioning of Sym-13 and Sym-31 , two genes affecting symbiosome development in root nodules of pea ( Pisum sativum L.)

Two Fix⁻ mutants of pea (Pisum sativum L.) which are unable to fix molecular nitrogen, E135f (sym-13) and Sprint-2Fix⁻ (sym-31), were crossed to create the doubly homozygous recessive line, named RBT (sym-13, sym-31). The ultrastructural organization of nodules of the RBT line was compared with that of each of the two parental mutant lines and with the original wild-type genotypes of the cultivars Sparkle and Sprint-2. It was shown that the RBT line is similar to the mutant line Sprint-2Fix⁻ in having abnormal symbiosome composition and bacteroids with relatively undifferentiated morphology. Because the phenotypic manifestation of the sym-31 mutant allele suppresses the phenotypic manifestation of the sym-13 mutant allele, it is concluded that the function of the gene Sym-31 (which is mutated in the Sprint-2Fix⁻ line) is necessary at an earlier stage of symbiosome development than the gene Sym-13 (which is mutant in the E135f line). Received: 28 October 1996 / Accepted: 22 January 1997     

Quantitative assessment of degenerative changes in soleus muscle after hindlimb suspension and recovery

The aim of this study was to quantify the degenerative and regenerative changes in rat soleus muscle resulting from 3-week hindlimb suspension at 45° tilt (HS group, n = 8) and 4-week normal cage recovery (HS-R group, n = 7). Degenerative changes were quantified by microscope examination of muscle cross sections, and the myosin heavy chain (MHC) composition of soleus muscles was studied by sodium dodecyl sulphate polyacrylamide gel electrophoresis. At the end of 3-week hindlimb suspension, histological signs of muscle degenerative changes were detected in soleus muscles. There was a significant variability in the percentage of fibres referred to as degenerating (%dg) in individual animals in the HS group [%dg = 8.41 (SEM 0.5)%, range 4.66%–14.08%]. Moreover, %dg varied significantly along the length of the soleus muscle. The percentage of fibres with internal nuclei was less than %dg in HS-soleus muscles [4.12 (SEM 0.3)%, range 1.24%–8.86%]. In 4-week recovery rats, the greater part of the fibres that were not referred to as normal, retained central nuclei [15.8 (SEM 2.2)%, range 6.2%–21.1%]. A significant increase in the slow isoform of MHC was recorded in the HS-R rats, compared to muscles from age-matched rats (P < 0.01). These results would suggest that a cycle of myofibre degeneration-regeneration occurred during HS and passive recovery, and that the increased accumulation of slow MHC observed in soleus muscles after recovery from HS could be related to the prevalence of newly formed fibres. Accepted: 14 October 1996     

Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

Canopy CO₂ concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO₂]) throughout the canopy (0.02–38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO₂] in the upper and middle canopy decreased on average 7–10 μmol mol⁻¹ below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO₂] did not differ between the wet and dry seasons. In contrast, [CO₂] below 0.7 m were generally higher during the dry season, resulting in larger [CO₂] gradients. Supporting this observation, soil CO₂ efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO₂] immediately above the forest floor of about 50␣μmol mol⁻¹. Temporal and spatial variations in [CO₂]_canopy were reflected in changes of δ¹³C_canopy and δ¹⁸O_canopy values. Tight relationships were observed between δ¹³C and δ¹⁸O of canopy CO₂ during both seasons (r ² > 0.86). The most depleted δ¹³C_canopy and δ¹⁸O_canopy values were measured immediately above the forest floor (δ¹³C = −16.4‰; δ¹⁸O = 39.1‰ SMOW). Gradients in the isotope ratios of CO₂ between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ¹³C, and between 1.0‰ and 3.5‰ for δ¹⁸O. The δ¹³C_leaf and calculated c _i/c _a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ¹³C_leaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c _i/c _a. Plotting 1/[CO₂] vs. the corresponding δ¹³C ratios resulted in very tight, linear relationships (r ² = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ¹³C of respired sources were close to the measured δ¹³C of soil respired CO₂ and to the δ¹³C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ_e, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season. Received: 3 March 1996 / Accepted: 19 October 1996     

The relationship between plasma potassium concentration and muscle torque during recovery following intense exercise

The present study investigated the relationship between plasma potassium ion concentration ([K⁺]) and skeletal muscle torque during three different 15-min recovery periods after fatigue induced by four 30-s sprints. Four males and one female completed the multiple sprint exercise on three separate days; recovery was passive, i.e. no cycling exercise (PRec), active cycling at 30% peak oxygen consumption O_2peak (30% Rec) and active cycling at 60% O_2peak (60% Rec). Plasma [K⁺] was measured from blood sampled from an antecubital vein of subjects at rest and at 0, 3, 5, 10 and 15 min into each recovery. Isokinetic leg strength was measured at rest and at 1, 6, 11 and 16 min during each recovery. Following the exhaustive sprints, [K⁺] increased significantly from an average mean (SEM) resting value of 3.81 (0.07) mmol · l⁻¹ to 4.48 (0.19) mmol · l⁻¹ (P < 0.01). In all recovery conditions, plasma [K⁺] returned to resting levels within 3 min following the fourth sprint. However, in the two active recovery conditions plasma [K⁺] increased over the remainder of the recovery periods to 4.36 (0.12) mmol · l⁻¹ in the 30% Rec condition and 4.62 (0.12) mmol · l⁻¹ in the 60% Rec condition, the latter being significantly higher than the former (P < 0.01). The maximum torque measured following the sprints decreased significantly, on average, to 61.1 (8.36)% of peak levels (P < 0.01). After 15 min of recovery, maximum torque was highest in the 30% Rec condition at 92.13 (3.06)% of peak levels (P < 0.01), compared to 85.23 (3.64)% and 85.71 (0.82)% for the PRec and 60% Rec conditions, respectively. In contrast to the significant differences in plasma [K⁺] across all three recovery conditions, muscle torque recovery was significantly different in only the 30% Rec condition. In summary, recovery of peak levels of muscle torque following fatiguing exercise does not appear to follow changes in plasma [K⁺]. Accepted: 18 October 1996     

Muscle strength and soft tissue composition as measured by dual energy x-ray absorptiometry in women aged 18–87 years

Dual energy x-ray absorptiometry (DEXA) offers the possibility of assessing regional soft tissue composition, i.e. lean mass (LM) and fat mass : LM may be considered a measure of muscle mass. We examined age-related differences in LM, percentage fat (%fat) and muscle strength in 100 healthy non-athletic women aged 18–87 years. Relationships between muscle strength and leg LM in 20 elite female weight lifters and in 18 inactive women with previous hip fractures were also studied. The LM and %fat of the whole body, trunk, arms and legs were derived from a whole body DEXA scan. Isokinetic knee extensor strength (KES) and flexor strength (KFS) at 30° · s^–1 were assessed using an isokinetic dynamometer. The women aged 71–87 years had 35% lower KES and KFS than the women aged 18–40 years (P < 0.0001). Differences in LM were less pronounced. The LM of the legs, for instance, was 15% lower in the old than in the young women (P < 0.0001). In a multiple regression analysis with age, body mass, height and leg LM or KES as independent variables and KES or leg LM as the dependent variable, age was the most important predictor of KES (r _partial = −0.74, P < 0.0001). The same applied to KFS. Body mass, not age, was the most important predictor of leg LM (r _partial = 0.65, P < 0.0001) and of LM at all other measurement sites. The LM measured at different regions decreased equally with increasing age. The KES:leg LM ratio was negatively correlated with age (r = −0.70, P < 0.0001). The weight lifters had significantly higher KES:leg LM ratios than age-matched controls (+12%, P < 0.0001) and vice versa for the women with previous hip fractures (–36%, P < 0.0001). In conclusion, from our study it would seem that in healthy nonathletic women, age is a more important determinant of muscle strength than is LM as measured by DEXA. Muscle strengthening exercises and inactivity seem to have a considerably stronger influence on muscle strength than on LM. Accepted: 27 August 1996     

Long-term c i /c a response of trees in western North America to atmospheric CO2 concentration derived from carbon isotope chronologies

To evaluate how the land carbon reservoir has been responding to the rising CO₂ concentration of the atmosphere, it is important to study how plants in natural forests adjust physiologically to the changing atmospheric conditions. Many experimental studies have addressed this issue, but it has been difficult to scale short-term experimental observations to long-term ecosystem-level responses. This paper derives carbon-isotope-related variables for the past 100–200 years from measurements on trees from natural forests. Calculations show that the c _i/c _a ratios [c _i/c _a is the ratio of the CO₂ concentration (μmol mol⁻¹) in the intercellular space of leaves to that in the atmosphere] of the trees were constant or increased slightly before the 20th century, but changed more rapidly in the 20th century; some increased, some decreased, and some stayed constant. In contrast, the CO₂ concentration inside plant leaves increased monotonically for all trees. Received: 12 June 1997 / Accepted: 29 June 1998