Assimilation efficiencies of slugs on different food materials

Summary The relative assimilation efficiencies of two common British species of slug, Limax flavus (L.) and Arion hortensis (Fér.), were studied in the laboratory using a gravimetric method. Results are given for adult slugs feeding on carrot root and potato tuber at 5°, 10°, 15°, and 20° C, and for L. flavus juveniles of known age feeding on potato at 10° and 15° C. A few results are also provided for Agriolimax reticulatus (Müller) feeding on both potato and carrot at 15° C.High mean assimilation efficiencies of 76.94 and 76.87% were obtained for L. flavus feeding on carrot and potato respectively. The equivalent results for A. hortensis were 88.60 and 89.42%. For A. reticulatus at 15° C the figures were 76.1 and 71.9%. Ingestion rates and assimilation rates are given for the first two species at all temperatures. Assimilation efficiency was found to be independent of temperature for L. flavus but not for A. hortensis. Ingestion rate and assimilation rate were temperature dependent in all cases except for A. hortensis feeding on potato. A. hortensis was more efficient at consuming both carrot and potato per gram live weight. All species consumed more potato than carrot per gram live weight.Juvenile L. flavus were found to have higher assimilation efficiencies and ingestion rates than the adults per gram live weight.Linear regression equations were determined from the relationship between dry weight of food consumed, and dry weight of faeces produced from that food for L. flavus.It is concluded that food quality and age of slug may be the factors most strongly influencing assimilation efficiency in this group.     

Feeding kinetics of Brachionus plicatilis fed Isochrysis galbana

Clearance and ingestion rates of Brachionus plicatilis were measured using ¹⁴C-labeled Isochrysis galbana Tahiti. Experiments were conducted at 20–22 °C, 20 ppt salinity, and algal concentrations ranging from 0.13–64 mg C 1^–1. Clearance rates were constant and maximal at concentrations <2 mg C 1^–1, with maximum rates ranging from 3.4–6.9 µl ind.^–1 hr^–1. The ingestion rate varied with food concentration, and was described by a rectilinear model. The maximum ingestion rate varied considerably, and was dependent on the growth rate of the rotifers. Depending on the pre-conditions, B. plicatilis ingested about 0.5 to 2 times its body carbon per day at saturating food concentrations.     

In situ metabolic budget for the calanoid copepod Acartia clausi in a tropical brackish water lagoon (Ebrié Lagoon,Ivory Coast)

Simultaneous measurements of respiration, excretion and production rates were carried out several times over a year period at five representative stations of the Ebrié Lagoon. Assuming a constant assimilation efficiency rate of 69.4%, we derived metabolic budgets for carbon, nitrogen and phosphorus. Daily specific ingestion rates calculated were rather generally high, and ranged between 54 and 159% of body carbon, between 26 and 102% of body nitrogen and between 108 and 307% of body phosphorus. Regional and seasonal variations depended mainly upon variations in trophic conditions. Curvilinear relationships between ingestion production, or net production efficiency K2, and food concentration (as chlorophyll-a + phaeopigments) showed that food could have been a limiting factor. Furthermore, K2 were low when compared with data from the literature (mean of 21% in carbon, 39% in nitrogen and 11% in phosphorus).Complementary laboratory experiments carried out on adults fed with enriched natural particles or algal cultures (Tetraselmis sp. or Dunaliella sp.) showed similar production (egg-production) vs food concentration curvilinear relationships as in the field. However, considerably higher maximal ingestion and production rates were obtained for animals fed algal cultures suggesting that optima for food acquisition and transformation were not reached in field conditions.Consequently, A. clausi, which represents more than 50% of the zooplankton biomass, appears to be rather inefficient in transforming the abundant particulate organic matter produced in the lagoon. This results from its high level of metabolic expenditure through respiration or excretion (about 50% of ingestion in terms of carbon) and from the small size and poor trophic value of food particles (high percentage of detritus).     

Growth,growth efficiency,and assimilation efficiency of the Tahoe sucker in cyclic and constant temperature

Synopsis Tahoe sucker, Catostomus tahoensis, were fed at three ration levels (starvation, 50% of repletion, and repletion) at three constant and cyclic temperature regimes (4–12°, 8°, 8–18°, 13°, and 13°–23°, 18° C) to examine growth rate and gross growth efficiencies. Growth rates increased with increasing temperature and ration level. Growth rates were not different between cyclic temperatures and the constant temperature equivalent to the mean of the cycle. Growth efficiencies were similar for cyclic and constant temperature regimes. Maintenance rations increased from 0.9% of the initial wet weight per day at low temperatures to 2.0 and 1.7% at intermediate and high temperatures, respectively. Assimilation efficiencies (not measured at low temperatures) did not differ between constant and cyclic temperatures. Tahoe sucker growth rates and assimilation efficiencies may not be enhanced in small streams because of this species' inability to mediate temperature cycles through behavioral thermoregulation.     

Effect of algal diet and temperature on the embryonic development time of the rotifer Brachionus plicatilis in culture

The embryonic development times of two strains of Brachionus plicatilis (Bs and S-1) cultured on three different algal diets (Nannochloris oculata, N. maculata and Nannochloropsis gaditana), have been determined at 20°C, 25°C and 30°C. As expected, the embryonic development times decreased with increasing temperature in all cases. However, embryos from adults fed on N. gaditana tended to develop more slowly than those of individuals fed on the other algal species. Mean egg volume was also affected by diet, larger eggs being produced by females fed on N. gaditana. No obvious relationship between egg size and temperature was detected.Two principal factors seemed to affect the embryonic development time. The first was temperature which acts through its well known effect on metabolic rates. The second was maternal diet which probably affects development time through its effect on yolk content, as reflected in the size of the egg.     

Ingestion rate of the burrowing mayfly Hexagenia limbata as determined with14C

The effects of nymph size and temperature on the ingestion rate ofHexagenia limbata were investigated using¹⁴C labelled algae mixed into sediment. Ingestion rate increased with nymph length and temperature. Daily ingestion rates of large 19 mm nymphs burrowing in silt averaged 58 and 192% of the dry body weight at 16° and 21 °C, respectively. Ingestion of seston due to filter feeding in artificial burrows was insignificant compared to that ingested by burrowing nymphs.     

Gram-scale purification of eicosapentaenoic acid (EPA, 20:5n-3) from wetPhaeodactylum tricornutum UTEX 640 biomass

Eicosapentaenoic acid (EPA, 20:5n-3) was obtained from the microalgaPhaeodactylum tricornutum following a three-step process: fatty acid extraction by direct saponification of wet biomass, polyunsaturated fatty acid (PUFA) concentration by formation of urea inclusion compounds and EPA isolation by preparative HPLC. Direct saponification of wet biomass was carried out with KOH-ethanol (96% v:v) (1 h, 60 °C), extracting 91% of the EPA. PUFAs were concentrated by the urea method with an urea/fatty acid ratio of 4:1 at a crystallization temperature of 28 °C using methanol as the urea solvent. An EPA concentration ratio of 1.5 (55.2/36.3) and recovery of 79% were obtained. This PUFA concentrate was used to obtain 95.8% pure EPA by preparative HPLC, using a reverse-phase column (C₁₈, 4.7 cm i.d. × 30 cm) and methanol-water (1% AcH) 80:20 w/w as the mobile phase. Ninety-seven per cent of EPA loaded was recovered and 70% EPA present in theP. tricornutum biomass was recovered in a highly pure form by means of this three-step downstream processing. In each of the HPLC preparative runs, 635 mg PUFA concentrate were loaded, obtaining 326 mg of a highly concentrated EPA fraction (2.46 g d^–1). Finally, a preliminary cost statement has been calculated.     

Relative importance of the trophic and direct pathways on PCB contamination in the rotifer species Brachionus calyciflorus (Pallas)

To determine the contribution of food ingestion (trophic pathway) to PCB contamination of zooplankton in the river Meuse (Belgium), we used ¹⁴C-labelled algae (Dictyosphaerium ehrenbergianum) to measure ingestion and assimilation rates in the rotifer species Brachionus calyciflorus. When the concentration of algae in the culture medium varied from 20 10³ to 200 10³ algal cells ml^–1 (0.12 to 1.18 mg Cl^–1), the Brachionus calyciflorus ingestion rate varied from 0.25 ± 0.12 to 1.52 ± 0.43 ng C ind^–1 h^–1 at 15 °C and from 0.74 ± 0.17 to 5.93 ± 0.61 ng C ind^–1 h^–1 at 20 °C. The assimilation efficiency (ratio of the assimilation rate to the ingestion rate) measured in a culture medium containing 200 10³ algal cells ml^–1 was 55.7 ± 5.8%. Since the PCB concentration measured in the phytoplankton of the river Meuse is about 3 µg PCBs g^–1 D.W., the estimated PCB contamination of zooplankton ascribable to the trophic pathway ranges from 0.22 ± 0.17 to 1.31 ± 0.77 µg PCBs g^–1 D.W. at 15 °C and from 0.64 ± 0.34 to 5.10 ± 2.10 µg PCBs g^–1 D.W. at 20°C. The lower figure based on measurements effected at 20 °C is comparable to the actual level measured in zooplankton samples collected in the river Meuse (0.69 ± 0.20 µg PCBs g^–1 D.W.). The applicability of the formula used in our estimate was checked in a 48-hour in vitro experiment in which the rotifers were fed contaminated algae. The PCB accumulation measured in the rotifers was found to coincide with the calculated PCB contamination. Additional experiments were carried out to determine the contribution of the direct pathway to PCB contamination of zooplankton living in the river Meuse (0.02 µg PCBs l^–1 of water; average dissolved organic matter: 3 mg C 1^–1). The PCB concentration in zooplankton resulting from direct uptake of PCBs from the water was estimated at 0.19 ± 0.05 µg PCBs g^–1 D.W. These results show that in zooplankton living in polluted ecosystems, PCBs are likely to accumulate via the trophic pathway to concentrations up to 30 times higher than by direct contamination. Furthermore, our estimates of PCB contamination via the trophic pathway coincide quite well with actual concentrations measured in situ.     

Temperature and food quality influences feeding behavior,assimilation efficiency and growth rate of arctic woolly-bear caterpillars

Summary The energy budget for feeding activity and growth of larval Gynaephora groenlandica was investigated on the tundra and in the laboratory. Larvae fed only in June when the buds and young leaves of Salix arctica, its principal host plant, contained the highest concentrations of macro-nutrients and total nonstructural carbohydrates (TNC). The mid-summer hiatus in larval feeding was coincident with an abrupt decline in the TNC content of leaves and a buildup of plant secondary metabolites in the leaves of S. arctica. Following cessation of feeding, the larvae remained concealed from the sun within crevices and vegetation mats. Growth rates of larvae incubated at 15 and 30°C were similar (4.7–5.0 mg/larva/day), but the assimilation efficiency at 15°C was four times greater (40%) than at 30°C. Growth rates were lowest at 5°C (0.22mg/larva/day) as was the assimilation efficiency (6.6%), because of the extended residence time of food in the gut. The high rate of ingestion and excretion at 30°C was caused by elevated maintenance metabolism. Changes in metabolic state influenced oxygen consumption, which was highest for feeding larvae (0.29 ml/g/h) and significantly lower for each, digesting, moving, starved larvae, and lowest for inactive larvae (0.06 ml/g/h). An influence of temperature and leaf quality on digestion rate and maintenance metabolism is the most likely cause of the feeding behavior pattern in G. groenlandica. The larvae may undergo voluntary hypothermia in order to avoid an energy, deficit resulting from high maintenance metabolism during mid-season when the energy content and food quality declines. The restriction of growth and development to a very short period prior to mid-summer may have contributed, to the extended 14-year life cycle of this species.     

Optimisation of culture conditions for production of eicosapentaenoic acid byMortierella elongate NRRL 5513

Summary WhenMortierella elongata NRRL 5513 was cultured in shake flasks at 25°C, mycelial growth reached a stationary phase at 48 h but maximum eicosapentaenoic acid (EPA) production was observed at 6 days. When incubated at 11°C, EPA production also continued to rise during the stationary phase of growth, reaching a maximum after 10 days. An initial culture pH of 6.1 was found to be optimum for EPA production. The effect of temperature on EPA production was dependent on medium constituents. In glucose and linseed oil supplemented media, optimum temperature for EPA production was 11 and 15°C respectively. A maximum EPA yield of 0.61 g/l was obtained in linseed oil (2%), yeast extract (0.5%) supplemented basal medium. Maximum EPA content as a percentage of lipids (15.12%) was observed when the latter medium was supplemented with 0.25% urea.     

Temperature aspects of ecological bioenergetics in Brachionas angularis (Rotatoria)

The influence of temperature and food quality was studied on the following energy balance parameters of B. angularis: ingestion, production, growth and mortality. The ingestion rate rises to an optimum at 15 and 20 °C and decreases at 25 °C. The other rates increase continuously over the 5–25 °C range. The Q₁₀-values of production rate are higher than those of ingestion rate. Temperature also modifies the relationship between food concentration and bioenergetic rates. They react according to a Monod function (production at all temperatures, growth at 10 °C) or decrease at high concentrations (growth at 15° and 20 °C.)     

The effects of temperature and host density on the development and survival of Copidosoma koehleri

Developmental rates for Copidosoma koehleri Blanchard (Hymenoptera: Encyrtidae) and its host, Phthorimaea operculella (Zeller), were determined at 10, 15, 23, 27, 29, 32, 34 and 35°C from host egg to adult. The developmental rates determined for both species showed good fit to mathematical models of insect development. At 15°C mean emergence of adult C. koehleri was 15 days after P. operculella adults emerged. At 29°C mean emergence of C. koehleri was only 5 days after that of P. operculella. P. operculella developed at 35°C, but parasitized larvae, and therefore C. koehleri, did not tolerate temperatures above 32°C. In the presence of C. koehleri, host survival was low (3%) at low host egg densities, but greater (20%) at higher host densities. Parasitized larvae of P. operculella were less able to compete for food resources, as measured by adult emergence.     

In situ versus laboratory estimations of length–weight regression and growth rate of Daphnia magna (Branchiopoda, Anomopoda) from an aerated waste stabilization pond

The present paper questions the adequacy of using length–weight regressions and growth rates calculated in the laboratory under constant physico-chemical and food conditions for the estimation of biomass and secondary production of animals living in a variable environment from the physico-chemistry and food availability point of view. Length–weight regressions (LWR) and growth rate of Daphnia magna were determined in situat five key periods of the year. In parallel, LWR and growth rate were determined in laboratory incubators at temperature adjusted to the mean temperature measured during the in situexperiments. LWR estimated from pond daphnids collected during the in situ experiments were, on the whole, not significantly different from LWR established during laboratory experiments, indicating that the food availability was globally similar in the laboratory and in situexperiments, even though food items were substantially different between the experiments. In situ algal biomass was indeed low compared to the algal biomass in laboratory experiments, but high biomasses of bacteria, protozoa and detritus were available for daphnid feeding in the tubes incubated in situ. Growth rate of D. magnawas monitored in situusing 50-ml tubes closed with Nylon net (mesh size = 80 m) and in the laboratory using 50-ml glass flasks. The physico-chemical, bacteriological and algological variables were checked to be similar in the tubes and in the pond. Growth rates varied according to the size of the animal and according to the water temperature. The maximum growth rates were observed for juveniles at 20.2 °C. Growth rates were also determined in the laboratory at temperature corresponding to the mean temperature recorded in the pond during the in situ growth experiments. Differences between in situ and laboratory body length–growth rate regressions (LgR) were observed for the experiments conducted at 15.6 °C and 23.6 °C. Due to differences in LWR and LgR between in situ and laboratory experiments, biomass and daily production estimated from laboratory cultures were found to be significantly, but not severely, higher than biomass and daily production estimated on the basis of in situ experiments. It has been, therefore, concluded that, when the constraints linked to the realization of in situ growth experiments are too strong, the laboratory approach is fully justified.     

The spring-time abundance and feeding ofEurytemora affinis (Poppe) in Volkerak-Zoommeer,a newly-created freshwater lake system in the Rhine delta (The Netherlands)

Eurytemora affinis, a calanoid copepod, has been encountered in Volkerak-Zoommeer (Rhine delta region, S.W. Netherlands) both before this lake system was isolated in 1987 from the estuarine influence, and after. It was the main particle-feeding crustacean at all the 3 sampling stations in March–April 1990 when it reached densities of up to 215 ind.l^–1. Its decline from mid April onwards, and low densities through summer, coincided with increase in cladocerans, especiallyDaphnia spp. (D. pulex andD. galeata), a decrease in seston (<33m) and chlorophyll concentrations and in primary production rates. The clearance rates (CR) ofEurytemora measured in the spring period varied enormously (0.6–24 ml.ind^–1.d^–1) depending mainly on size (0.44–1.06 mm), food concentration (0.8–2.2 mg C.l^–1), and the water temperature which varied only narrowly (8.0–9.0°C). Mean ingestion rates of the animals measuring 0.68±0.02 mm during the study was 6.7±3.2 gC.ind^–1.d^–1; and assimilation efficiency varied between 27 and 53% (mean: 41±9%). The weight specific CR (SCR) varied between 0.96 and 6.4 litre.mg^–1 body C.d^–1. Pooled regression of SCR on the animal's body weight at the 3 study stations revealed a significant inverse relationship. Also daily ration and specific assimilation ofE. affinis varied greatly and inversely with the body weight. This calanoid contributed from about 50 to 100% to the zooplankton community grazing rates and assimilation rates, the former often exceeding the phytoplankton primary production.     

Combined Effects of Algal (<Emphasis Type="Italic">Chlorella vulgaris</Emphasis>) Food Level and Temperature on the Demography of <Emphasis Type="Italic">Brachionus havanaensis</Emphasis> (Rotifera): a Life Table Study

We evaluated the combined effects of food (0.5 × 10⁶, 1.0 × 10⁶ and 2.0 × 10⁶ cells ml⁻¹ of Chlorella vulgaris) and temperature (15, 20 and 25 °C) on life history variables of B. havanaensis. Regardless of Chlorella density there was a steep fall in the survivorship of B. havanaensis at 25 °C. Both food level and temperature affected the fecundity of B. havanaensis. At any given food level, rotifers cultured at 15 °C showed extended but low offspring production. At 25 °C, offspring production was elevated, the duration of egg laying reduced and the fecundity was higher during the latter part of the reproductive period. The effect of food level was generally additive, at any given temperature, and higher densities of Chlorella resulted in higher offspring production. Average lifespan, life expectancy at birth and generation time were 2–3 times longer at 15 °C than at 25 °C. At 20 °C, these remained at intermediate levels. The shortest generation time (about 4 days) was observed at 25 °C. Gross and net reproductive rates and the rate of population increase (r) increased with increasing temperature and generally, at any given temperature, higher algal food levels contributed to higher values in these variables. The r varied from 0.11 to 0.66. The survival patterns and lower rates of reproduction at 15 °C suggest that the winter temperatures (10–15 °C) prevailing in many waterbodies in Mexico City allow this species to sustain throughout the year under natural conditions.     

Effect of temperature on vegetative growth among isolates of Metarhizium anisopliae and M. flavoviride

Effects of temperature on vegetative growth on a semi-synthetic medium of 22 isolates of Metarhizium anisopliae and 14 isolates of M. flavoviride were determined. The majority of isolates of both species grew between 11 and 32°C; several isolates grew at 8 and 37 °C. None of the isolates grew at 40 °C. Relative growth rate, calculated from the maximum growth rate for each isolate, was significantly affected by temperature and isolate, with significant isolate * temperature interactions. The maximum absolute growth rates among the isolates ranged from 2.5 mm to 5.9 mm/day. Optimal temperatures were generally between 25 and 32 °C with several isolates exhibiting optimal growth at temperatures as high as 32 °C. Overall, relative growth rates were greater in isolates of M. anisopliae than M. flavoviride at temperatures of 25 °C or lower; conversely mean relative growth rates were greater in M. flavoviride than M. anisopliae at temperatures higher than 25 °C. However, the two most cold tolerant isolates at 8 °C were M. flavoviride and the three most heat tolerant at 35 °C were M. anisopliae. Since temperature growth responses varied considerably between isolates, strain selection according to thermal tolerance may be warranted when choosing a strain for development as a microbial control agent.This revised version was published online in October 2005 with corrections to the Cover Date.     

Maximum feeding potential of larvae and adults ofthe scale insect predator, Chilocorus nigrituswith a new method of estimating food intake

A method of estimating the weight ofindividual Abgrallaspis cyanophylli (Signoret)without the need for removal from the host plant isdescribed. Using this method, which enables accurateestimations of scale insect weight by measuring lengthand relating it to a previously determined regressionmodel, maximum feeding potential in male and femaleChilocorus nigritus (F.) adults was examined atvarious constant temperatures over the range of 13 to30 °C and at a cycling temperature of 12 h/12 hat 14/30 °C (r.h. in the range of 62 to 68%). Mean daily potential food intake varied from 0.097 mg/day at 13 °C to 1.432 mg/day at 30 °C.However, intake at the cycling temperature was significantly higher than that at constant temperatures (1.98 mg/day). At 15, 20 and 30 °C there were no significant differences between male and female potential food requirements whilst at temperatures in the mid range, there was a considerable increase in female potential voracitywhen compared to that of the males. Maximum potentiallarval food requirement for development at 26 °Cand 62% r.h. in C. nigritus was also estimatedusing the above method. A mean of 16.24 mg of Abgrallaspis cyanophylli (Signoret) was required forlarvae of both sexes to complete development. Thisstudy suggests that C. nigritus would be mostefficient as a biological control agent if used inglasshouses with a mean daily temperature above22 °C.     

Daily egg production rate of the planktonic calanoid copepod Acartia lilljeborgi Giesbrecht in the Cananéia Lagoon estuarine system,São Paulo,Brazil

Seasonal variation in daily egg production rate of the planktonic calanoid copepod Acartia lilljeborgi Giesbrecht in relation to temperature, salinity and chlorophyll a concentration was studied in the Cananéia Lagoon estuarine system, from March 1995 to January 1996. Recently captured A. lilljeborgi adult females were individually incubated in bottles filled with surface water screened through a 40-m mesh, containing a natural assemblage of phytoplankton in the laboratory, at temperatures corresponding to ambient. Daily egg production rate ranged from 13.8±3.5 to 66.8± 15.1 eggs female^–1 d^–1 (mean ± 95% CL). The mean and maximum rates of daily egg production increased with temperature from 19.5 to 25.2 °C but then decreased with further increase in temperature at 28.4 through 29.1 °C, attaining the highest rates at approximately annual mean ambient water temperature (ca. 24–25 °C). The egg production rates increased linearly with chlorophyll a <40 m fraction. Hatching success varied from 68.6 to 91.9%. Cannibalism varied from 1.4±0.7 to 7.1±3.3 nauplii female^–1 d^–1 (mean ± 95% CL). These results suggest that water temperature and phytoplankton concentration are important factors affecting the egg production rate of A. lilljeborgi in the Cananéia Lagoon estuarine.     

In situ primary production in young Antarctic sea ice

An in situ incubation technique used successfully to measure the photosynthetic carbon assimilation of internal algal assemblages within thick multiyear Arctic ice was developed and improved to measure the photosynthetic carbon assimilation within young sea ice only 50 cm thick (Eastern Weddell Sea, Antarctica). The light transmission was improved by the construction of a cylindrical frame instead of using a transparent acrylic-glass barrel. The new device enabled some of the first precise measurements of in situ photosynthetic carbon assimilation in newly formed Antarctic sea ice, which is an important component in the sea ice ecosystem of the Antarctic Ocean. The rates of carbon assimilation of the interior algal assemblage (top to 5 cm from bottom) was 0.25 mg C m^–2 d^–1 whereas the bottom algal community (lowest 5 cm) attained only 0.02 mg C m^–2 d^–1. Chl a specific production rates (P^Chl) for bottom algae (0.020 – 0.056 g C g chl a ^–1 h^–1) revealed strong light limitation, whereas the interior algae (P^Chl = 0.7 – 1.2 g C g chl a ^–1 h^–1) were probably more limited by low temperatures (< –5 °C) and high brine salinities.     

Limitation of photosynthesis by changes in temperature

The aim of this work was to examine the effect of abrupt changes in temperature in the range 5 to 30°C upon the rate of photosynthetic carbon assimilation in leaves of barley (Hordeum vulgare L.). Measurement of the CO₂-assimilation rate in relation to the intercellular partial pressure of CO₂ at different temperatures and O₂ concentrations and at saturating irradiance showed that as the temperature was decreased photosynthesis was saturated at progressively lower CO₂ partial pressures and that the transition between the CO₂-limited and ribulose-1,5-bisphosphate-regeneration-limited rate became more abrupt. Feeding of orthophosphate to leaves resulted in an increased rate of CO₂ assimilation at lower temperatures at around ambient or higher CO₂ partial pressures both in 20% O₂ and in 2% O₂ and it removed the abruptness in the transition between the CO₂-limited and ribulose-1,5-bisphosphate-regeneration-limited rates. Phosphate feeding tended to inhibit carbon assimilation at higher temperatures. The response of carbon assimilation to temperature was altered by feeding orthophosphate, by changing the concentrations of CO₂ or of O₂ or by leaving plants in the dark at 4°C for several hours. Similarly, the response of carbon assimilation to phosphate feeding or to changes in 2% O₂ was altered by leaving the plants in the dark at 4°C. The mechanism of limitation of photosynthesis by an abrupt lowering of temperature is discussed in the light of the results.Abbreviations A rate of CO₂ assimilation - P _i intercellular partial pressure of CO₂ - RuBP ribulose-1,5-bisphosphate