Chlorella-bearing ciliates dominate in an oligotrophic North Patagonian lake (Lake Pirehueico, Chile): abundance, biomass and symbiotic photosynthesis

1. Large mixotrophic ciliates ( Stentor araucanus , S. amethystinus and Ophrydium naumanni ) were a characteristic component of a temperate, oligotrophic lake in North Patagonia. During a 1-year study, the abundance, biomass and primary production of these large Chlorella -bearing ciliates were compared with those of the total plankton community.
2. Mixotrophic ciliates peaked in spring and from late summer to autumn, accounting for 1.6–43% (annual average: 16.3%) and 67–99% (annual average: 92%) of total ciliate abundance and biomass, respectively. Their contribution to total zooplankton biomass, including flagellates, rotifers, ciliates and crustaceans, was 14–76%, or 47% as an annual average. Endosymbiotic algae accounted for up to 25% of total autotrophic biomass (annual mean: 3.9%).
3. Maximum cell-specific photosynthetic rates of S. araucanus and S. amethystinus at light saturation varied between 80 and 4400 pg C ciliate^–1 h^–1 with high values during autumn and winter, and low values during summer. The depth-integrated rates of photosynthesis (0–40 m) of algal endosymbionts contributed 1–25% to total photosynthesis (annual mean: 6.5%).
4. A comparison of calculated ingestion rates with photosynthetic rates of Stentor indicates that photosynthate produced by endosymbionts generally exceeded heterotrophic food supply of Stentor during autumn and winter, but was much lower during summer, when food supply was high.
5. The mixotrophic ciliates represent an important 'link' between nanoplankton and higher trophic levels within the plankton community because of their high heterotrophic biomass and considerable contribution to total photosynthesis.     

Light absorption and photosynthesis of a benthic moss community: importance of spectral quality of light and implications of changing light attenuation in the water column

1. The underwater light climate and benthic moss communities of Grane Langsø were investigated in May 1997 to determine the potential effects on benthic production of changing water column attenuation and spectral quality of light.
2. A reduction in water clarity in the lake since the 1960s was manifested as a marked increase in the attenuation of blue light, relative to red light, which can be attributed to increased dissolved organic carbon.
3. The biomass of the benthic moss community ranged from a maximum of 195 gDW m⁻² at a depth of 4 m to 39 g DW m⁻² at a depth of 10 m and Drepanocladus exannulatus contributed 70% of the biomass at all depths.
4. Absorption of PAR by D. exannulatus was maximal in the highly pigmented youngest parts of the plant and these correspondingly showed the highest rates of net photosynthesis. The absolute amount of chlorophyll- a per g dry weight was greater at 10 m than 2 m, but the ratio of accessory pigments to chlorophyll- a did not change. Deep growing plants did not show adaptation to changed light quality.
5. Increased attenuation of blue light and the resultant overall decrease in water clarity is likely to impact negatively on net annual production of benthic macrophytes of Grane Langsø. Any further increase in dissolved organic carbon concentration has the potential to markedly decrease the depth to which mosses grow by reducing the length of time in a year during which net photosynthesis occurs.     

Catchment land-use, macroinvertebrates and detritus processing in headwater streams: taxonomic richness versus function

1. We used a litter bag technique to assess the effect of catchment land-use (forest, wetland, agriculture, urban) on the processing of red maple ( Acer rubrum L.) litter in 17 streams in Maine, U.S.A. Litter processing by fungi was predicted to increase with nutrient concentrations along a gradient of land use, from relatively unmodified to highly modified. Litter processing by litter-shredding macroinvertebrates was predicted to decline along this gradient because of a decline in their taxonomic richness and biomass.
2. Land use was associated with the anticipated gradient in nutrient and macroinvertebrate attributes, and a significant relationship was found between land use and nitrate concentration. There was, however, no significant relationship between land use and soluble reactive phosphorus (SRP) concentration. Similarly, shredder taxonomic richness was significantly related to land use type, whereas shredder biomass showed no significant relationship to land use.
3. Attributes of the shredder assemblage structure and nutrient concentrations were both strong determinants of litter processing. Increasing biomass and taxonomic richness of shredders was significantly related to increasing rates of litter mass loss. Increasing concentrations of nitrate and SRP were significantly related to increasing rates of litter softening below threshold concentrations (approximately 0.20 mg NO₃-N L^–1 and 5 μg SRP L^–1).
4. The potentially additive effects of nitrate and SRP concentrations or shredder richness and biomass on litter processing rates were confounded by the lack of significant correlation between these pairs of variables. Consequently, rates of litter processing (as rates of softening or mass loss) did not vary systematically among different land use regimes.     

Differences in photosynthetic behaviour and leaf senescence of soybean (Glycine max [L.] Merrill) dependent on N2 fixation or nitrate supply

Biological N₂ fixation can fulfil the N demand of legumes but may cost as much as 14% of current photosynthate. This photosynthate (C) sink strength would result in loss of productivity if rates of photosynthesis did not increase to compensate for the costs. We measured rates of leaf photosynthesis, concentrations of N, ureides and protein in leaves of two soybean cultivars ( Glycine max [L.] Merrill) differing in potential shoot biomass production, either associated with Bradyrhizobium japonicum strains, or amended with nitrate. Our results show that the C costs of biological N₂ fixation can be compensated by increased photosynthesis. Nodulated plants shifted N metabolism towards ureide accumulation at the start of the reproductive stage, at which time leaf N concentration of nodulated plants was greater than that of N-fertilized plants. The C sink strength of N₂ fixation increased photosynthetic N use efficiency at the beginning of plant development. At later stages, although average protein concentrations were similar between the groups of plants, maximum leaf protein of nodulated plants occurred a few days later than in N-fertilized plants. The chlorophyll content of nodulated plants remained high until the pod-filling stage, whereas the chlorophyll content of N-fertilized plants started to decrease as early as the flowering stage. These results suggest that, due to higher C sink strength and efficient N₂ fixation, nodulated plants achieve higher rates of photosynthesis and have delayed leaf senescence.     

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).
2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12-week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day^–1) and k _d (degree day^–1).
3. There were no significant differences in processing rates ( k day^–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.
4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.     

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).
2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12-week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day^–1) and k _d (degree day^–1).
3. There were no significant differences in processing rates ( k day^–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.
4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.     

Inorganic carbon limitation of photosynthesis in lake phytoplankton

1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate.
2. The influence of total dissolved inorganic carbon (DIC) and free CO₂ on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1–120 μg l^–1), pH (5.6–9.6) and DIC concentration (0.02–2.5 m m ). Photosynthesis experiments were performed using the ¹⁴C technique in CO₂-manipulated water samples, either by changing the pH or by adding/removing CO₂.
3. Lake waters were naturally CO₂ supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO₂ concentrations declined. Similarly, photosynthesis was limited by low CO₂ concentrations during phytoplankton blooms in the hardwater alkaline lakes.     

Outdoor continuous culture of Porphyridium cruentum in a tubular photobioreactor: quantitative analysis of the daily cyclic variation of culture parameters

The present work reports on the daily cyclic variation of oxygen generation rates, carbon consumption rates, photosynthetic activities, growth rates and biochemical composition of the biomass in a pilot plant continuous outdoor culture of the microalgae Porphyridium cruentum. A linear relationship between the external irradiance and the average irradiance inside the culture was found. In addition, the oxygen generation and carbon consumption rates were found to be a function of the average irradiance inside the culture. A reduction in photosynthetic activity of the cells at noon and recovery in the afternoon was also observed. Therefore, the cells showed a short-term response of parameters such as oxygen generation rate as well as carbon consumption rate with external and average irradiance; a model of photosynthesis rate considering photoinhibition is proposed. This model is a useful tool for the operation and scaleup of tubular photobioreactors, and can be used for determining CO₂ requirements of the system. The higher the photosynthesis rates, the lower the carbon losses, ranging from 25% at noon to 100% during the night. The growth rate showed a linear relationship with the daily mean average irradiance inside the culture with a long-term response. Likewise, a linear relationship among the oxygen generation rate and the growth rate was obtained. With respect to the biochemical composition of the biomass, the cells showed a long-term response of metabolic routes to mean daily culture conditions. During the illuminated period, energy was stored as carbohydrates and synthesis of proteins was low. During the night, the stored carbohydrates were consumed. The fatty acid dry weight (DW) content decreased during the daylight period, whereas the fatty acid profile, as total fatty acids, was a function of growth rate. A short-term variation of exopolysaccharides synthesis with solar irradiance was also observed, i.e. the higher the external irradiance the higher the excretion of exopolysaccharides as a protection against adverse culture conditions.     

Antarctic stream ecosystems: physiological ecology of a blue-green algal epilithon

SUMMARY. 1. Several dozen summer meltwater streams are located in the McMurdo Sound region (c. 78°S 165°E) of southern Victoria Land. They are characterized by a highly variable flow regime at diel, seasonal and annual times caleis; wide fluctuations in temperature and nutrient content; and a very simple epilithic community of cyanophytes ( Nostoc spp., Oscillatoriaceae), bacteria, fungi and microherbivores.
2. The epilithon survives the dark Antarctic winter as dry, frozen mats which provide a large inoculum for growth the following summer. This overwintering assemblage retains a high metabolic capacity and responds rapidly to rehydration.
3. In a series of artificial substrate experiments, biomass accumulation rates were generally less than 0.1 In units d⁻¹. Colonization and growth on the substrates was inversely related to the suspended sediment load of the stream. There was also a visual correspondence between per cent algal cover of the natural streambed and the clarity of the streamwater. Sloughing losses may limit community biomass, particularly in the turbid flowing waters.
4. During running water conditions the mature communities had very low gross photosynthetic rates per unit chlorophyll (<0,1 μg C (μg chl a .h)⁻¹ and per unit carbon (<0,2 μg C (mg biomass C.h)⁻¹). Respiration was generally a high percentage (up to 92%) of gross photosynthesis, which probably reflected the high population densities of microheterotrophs in the community.
5. The floristically simple epilithic mats slowly accumulate to extreme biomass levels (>20 μg chl a cm ⁻², <20 mg C cm⁻²). Production rates per unit biomass are low, probably in response to the cold temperatures of the Antarctic stream environment, and the accumulated biomass represents several seasons of growth.     

Short-term productivity responses of algae and bacteria to zooplankton grazing in two freshwater lakes

SUMMARY. 1. The specific productivities of algae and bacteria were measured in short-term (4 day) experiments consisting of enclosures with natural or reduced zooplankton biomass. Experiments were repeated five times over a season in each of two lakes that differed in the background concentration of dissolved organic carbon (DOC).
2. Algal biomass as estimated by chlorophyll a was suppressed in enclosures with ambient grazer levels in six of ten experiments and enhanced in one experiment. Distribution of chlorophyll among net and nanoplankton was not significantly affected by grazing.
3. Relative to enclosures with reduced zooplankton, normal grazer biomass (97–466μg 1⁻¹ dry weight) enhanced specific algal productivity in only one of five experiments in the low DOC take and had no effect in all five experiments in the high DOC lake. The main effects of grazers on algae was through removal of biomass rather than through indirect changes in turnover rate.
4. Between experiments, bacterial density was either unaffected, or mildly enhanced (4–87%) in enclosures with ambient macrozooplankton compared to those with reduced levels. Bacterial productivity and turnover estimated by incorporation of [³H]thymidine into DNA showed different responses across experiments; increasing, declining or remaining the same with grazer minipulation. This variability was not related to differences in dissolved primary production or to background DOC between lakes or experiments. Comparison of bacterial productivities based on thymidine incorporation rates with changes in cell densities indicated that control of bacterial loss processes by macrozooplankton is more important than control of growth rates.     

Effect of temperature on yield and night biomass loss in Spirulina platensis grown outdoors in tubular photobioreactors

Outdoor experiments carried out in Florence, Italy (latitude 43.8° N, longitude 11.3° E), using tubular photobioreactors have shown that in summer the average net productivity of a Spirulina platensis culture grown at the optimal temperature of 35 °C was superior by 23% to that observed in a culture grown at 25 °C. The rates of night biomass loss were higher in the culture grown at 25 °C (average 7.6% of total dry weight) than in the one grown at 35 °C (average 5%). Night biomass loss depended on the temperature and light irradiance at which the cultures were grown, since these factors influenced the biomass composition. A net increase in carbohydrate synthesis occurred when the culture was grown at a low biomass concentration under high light irradiance or at the suboptimal temperature of 25 °C. Excess carbohydrate synthesized during the day was only partially utilized for night protein synthesis.     

Rubisco content and photosynthesis of leaves at different positions in transgenic rice with an overexpression of RBCS

As ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) activity limits light-saturated photosynthesis under present atmospheric condition, the effects of an overexpression of RBCS on Rubisco content and photosynthesis were examined in the leaves at different positions in rice ( Oryza sativa L.). Rubisco content in the transformant was significantly greater in the uppermost, fully expanded leaves but decreased to levels similar to those in wild-type plants in the lower leaves. The mRNA levels of total RBCS and rbcL in these leaves were much less than those in the expanding leaves, where Rubisco synthesis is active, suggesting commensurately low level of synthesis. Although the activation state of Rubisco was lower in the uppermost, fully expanded leaves of the transformant, it recovered to its full level in the lower leaves. As a result, the photosynthetic rate did not differ in leaves at the same position between the transformant and the wild type. Similarly, whole plant biomass did not differ between these genotypes. Thus, we conclude that although the overexpression of RBCS led to an enhancement of Rubisco protein content in the uppermost, fully expanded leaves, it does not result in increased photosynthetic rates or plant biomass, because of an apparent down-regulation in its activation state.     

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition.
2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem.
3. Although overall mass loss rates were similar between treatments ( k = 0.0213 day⁻¹ and 0.0206 day⁻¹), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour.
4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves.
5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching.
6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.     

Higher treefall rates on slopes and waterlogged soils result in lower stand biomass and productivity in a tropical rain forest

1.  Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability.
2.  We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls.
3.  Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher.
4.  In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops.
5.  Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands.
6.   Synthesis . Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.     

Decay rates of autumn and spring leaf litter in a stream and effects on growth of a detritivore

SUMMARY. 1. Differences in decay rates of autumn and spring balsam poplar (Populus balsamifera L.) leaf litter input to a stream and their effects on a lotic detritivore Tipula commiscibilis Diane were investigated.
2. Autumnal leaf litter decay rates were significantly greater than spring decay rates despite higher initial quality of spring leaves. Reduced spring/summer decomposition rates were the result of decreased microbial activity and biomass, and significantly lower numbers, kinds and biomass of macroinvertebrate detritivores.
3. Growth of the detritivore Tipula commiscibilis was significantly lower when fed spring leaves indicating that they were a poorer quality food source than autumn leaves.
4. Lower numbers of detritivores coupled with reduced leaf quality resulted in lower leaf litter decay rates characteristic of spring/summer.     

Effects of enhanced UV-B radiation on production-related properties of a Sphagnum fuscum dominated subarctic bog

1. The aim of the study was to investigate effects of enhanced UV-B radiation on the balance between biomass production and decay in an ombrotrophic bog which is dominated by one species of Sphagnum ( S. fuscum). This paper concerns production.
2. Enhanced UV-B radiation (simulating 15% ozone depletion under clear sky conditions) was applied by means of fluorescent tubes during two growing seasons.
3. In S. fuscum, shoot density, mass relations and length increment over time were measured and productivity was estimated. Pigment concentration, rates of dark respiration and maximum net photosynthesis were recorded.
4. Sphagnum fuscum productivity was not changed by enhanced UV-B radiation while properties determining production were highly influenced although in opposite directions.
5. Height increment was decreased by 20% in the first growing season and by 31% in the second growing season under enhanced UV-B radiation. After two growing seasons spatial shoot density was decreased by 8% by enhanced UV-B radiation. The shoots became stunted as capitulum dry mass and stem dry mass per unit length were increased by 21 and 17%, respectively, under enhanced UV-B radiation.
6. Dark respiration was significantly decreased by 31% after growth under enhanced UV-B radiation.
7. The UV-B induced change in shoot biometry together with the reduced spatial shoot density involve potential long-term effects on peat structure with possible feedback on productivity, decomposition and the strength of the system as a carbon sink.     

Growth rate correlates negatively with protein turnover in Arabidopsis accessions

Previous studies with Arabidopsis accessions revealed that biomass correlates negatively to dusk starch content and total protein, and positively to the maximum activities of enzymes in photosynthesis. We hypothesized that large accessions have lower ribosome abundance and lower rates of protein synthesis, and that this is compensated by lower rates of protein degradation. This would increase growth efficiency and allow more investment in photosynthetic machinery. We analysed ribosome abundance and polysome loading in 19 accessions, modelled the rates of protein synthesis and compared them with the observed rate of growth. Large accessions contained less ribosomes than small accessions, due mainly to cytosolic ribosome abundance falling at night in large accessions. The modelled rates of protein synthesis resembled those required for growth in large accessions, but were up to 30% in excess in small accessions. We then employed ¹³CO₂ pulse‐chase labelling to measure the rates of protein synthesis and degradation in 13 accessions. Small accessions had a slightly higher rate of protein synthesis and much higher rates of protein degradation than large accessions. Protein turnover was negligible in large accessions but equivalent to up to 30% of synthesised protein day^?1 in small accessions. We discuss to what extent the decrease in growth in small accessions can be quantitatively explained by known costs of protein turnover and what factors may lead to the altered diurnal dynamics and increase of ribosome abundance in small accessions, and propose that there is a trade‐off between protein turnover and maximisation of growth rate.     

Can a community of small-bodied grazers control phytoplankton in rivers?

1. Phytoplankton, zooplankton and grazing were monitored throughout the growing season for three years (1994–96) in the Belgian section of the River Meuse.
2. A size structure analysis of the algal community shows that there was a summer shift toward larger algal units, following a decline in phytoplankton biomass. These changes occurred after an increase in zooplankton biomass and diversity.
3. Daily filtration rates of grazers ranged from 1 to 113% day^–1 and maxima were observed during the summer period. Higher rates tended to correspond with peaks of rotifer biomass. A decline in total phytoplankton biomass within two weeks followed the increase in zooplankton biomass and filtration rate. A rapid biomass recovery was then observed, along with a shift of the algal community toward larger units. When grazing activity was not sustained, due to zooplankton fluctuations, the change in phytoplankton size structure was less marked.
4. We suggest that the composition of the phytoplankton community of large rivers may at times be controlled by grazers. However, such biotic interactions can take place only when physical constraints are reduced, i.e. when discharge is low, and when increased transfer time, high temperature and availability of grazeable algae allow high zooplankton biomass.     

Seasonal changes in canopy photosynthesis and respiration, and partitioning of photosynthate, in rice (Oryza sativa L.) grown under free-air CO2 enrichment

An increase in atmospheric CO(2) concentration ( [CO(2)]) is generally expected to enhance photosynthesis and biomass. Rice plants (Oryza sativa L.) were grown in ambient CO(2) (AMB) or free-air CO(2)-enrichment (FACE), in which the target [CO(2)] was 200 micromol mol(-1) above AMB. (13)CO(2) was fed to the plants at different stages so we could examine the partitioning of photosynthates. Furthermore, canopy photosynthesis and respiration were measured at those stages. The ratio of (13)C content in the whole plant to the amount of fixed (13)C under FACE was similar to that under AMB at the vegetative stage. However, the ratio under FACE was greater than the ratio under AMB at the grain-filling stage. At the vegetative stage, plants grown under FACE had a larger biomass than those grown under AMB owing to enhancement of canopy photosynthesis by the increased [CO(2)]. On the other hand, at the grain-filling stage, CO(2) enrichment promoted the partitioning of photosynthate to ears, and plants grown under FACE had a greater weight of ears. However, enhancement of ear weight by CO(2) enrichment was not as great as that of biomass at the vegetative stage. Plants grown under FACE did not necessarily show higher canopy photosynthetic rates at the grain-filling stage. Therefore, we concluded that the ear weight did not increase as much as biomass at the vegetative stage owing to a loss of the advantage in CO(2) gain during the grain-filling period.     

Effects of ischaemia, blood loss and reperfusion on rat muscle protein synthesis, metabolite concentrations and polyribosome profiles in vivo.

In adult rat gastrocnemius muscles, on reperfusion after 45 min of tourniquet ischaemia, protein synthetic rates were depressed by over half for 1 h compared to normal (12%/day), and were at least one-third below normal for up to 5 h afterwards. Ischaemia caused muscle concentrations of phosphocreatine to be depressed by 70%, and those of lactate to be elevated by 350%; the proportion of ribosomes as polyribosomes was decreased by half. Unlike the rates of protein synthesis, all of these variables returned to normal after 35 min of reperfusion. When 25% of the blood volume was removed (for 10-45 min), there were falls in the rate of gastrocnemius protein synthesis and in phosphocreatine concentration, and an increase in lactate concentration. On blood replacement, protein synthesis and metabolite concentrations returned to normal within 15 min. Polyribosome profiles were unaffected by blood loss or replacement. There were highly significant correlations between the rate of gastrocnemius protein synthesis and both phosphocreatine concentration and 1/(lactate concentration), during blood loss and replacement, i.e. during both the fall and rise in muscle energy status. We conclude that the effects of ischaemia and blood loss on protein synthesis are not equivalent.