Animal Guts as Ideal Chemical Reactors: Maximizing Absorption Rates

I solved equations that describe coupled hydrolysis in and absorption from a continuously stirred tank reactor (CSTR), a plug flow reactor (PFR), and a batch reactor (BR) for the rate of ingestion and/or the throughput time that maximizes the rate of absorption (=gross rate of gain from digestion). Predictions are that foods requiring a single hydrolytic step (e.g., disaccharides) yield ingestion rates that vary inversely with the concentration of food substrate ingested, whereas foods that require multiple hydrolytic and absorptive reactions proceeding in parallel (e.g., proteins) yield maximal ingestion rates at intermediate substrate concentrations. Counterintuitively, then, animals acting to maximize their absorption rates should show compensatory ingestion (more rapid feeding on food of lower concentration), except for the lower range of diet quality for complex diets and except for animals that show purely linear (passive) uptake. At their respective maxima in absorption rates, the PFR and BR yield only modestly higher rates of gain than the CSTR but do so at substantially lower rates of ingestion. All three ideal reactors show milder than linear reduction in rate of absorption when throughput or holding time in the gut is increased (e.g., by scarcity or predation hazard); higher efficiency of hydrolysis and extraction offset lower intake. Hence adding feeding costs and hazards of predation is likely to slow ingestion rates and raise absorption efficiencies substantially over the cost-free optima found here.     

Test, rejection, and reformulation of a chemical reactor-based model of gut function in a fruit-eating bird

We explored modulation of retention time in cedar waxwings (Bombycilla cedrorum) by feeding them diets varying in hexose concentration. Our goals were to (1) test three predictions of a chemical reactor-based model of how guts might respond optimally to diet shifts; (2) determine whether modulation of retention time can occur quickly, thereby facilitating rapid changes in diet; (3) tease apart the relative influence of ingestion rate and nutrient concentration on retention time; and (4) examine the degree of axial mixing in the intestine and its relationship with retention time. The model's predictions were rejected: mean retention time did not decrease, ingestion rate did not increase, and glucose assimilation efficiency did not decrease with increased hexose concentration of the diet. Instead, birds displayed maximal intake rate at intermediate sugar concentration, and mouth to cloaca mean retention times increased with hexose concentration. Significant modulation of retention time occurred quickly, within 3 h of exposure to a different diet. Birds did equally well in terms of total energy assimilated on diets differing 3.3-fold in hexose concentration (from 500 mmol/L to 1660 mmol/L) but showed reduced intake when fed food with low hexose concentration (110 mmol/L). Far more variation in retention time was explained by direct effects of ingestion rate than by direct effects of hexose concentration. Finally, a gut dispersion index that measured degree of axial mixing was positively correlated with mean retention time, indicating that higher retention times are accompanied by increased axial mixing. We propose a modification of the assumptions of the original model. The resulting "osmotic constraint" model better captures the interaction between feeding rate and digestive function in fruit-eating birds.     

The effect of intracellular pH on the rate of hexose uptake in Chlorella.

The rate of hexose uptake by Chlorella is reduced by uncouplers such as carbonyl cyanide p-trifluoromethoxyphenyl hydrazone or dinitrophenol even before concentration equilibrium is reached. The addition of uncouplers changes the membrane potential and the intracellular pH. The membrane potential does not influence the initial velocity of net sugar uptake, whereas manipulation of the cell pH by means of dimethyloxazolidinedione or by butyric acid uncovered a dramatic influence of cell pH on the rate of hexose uptake: at pH values of 7.5--6.8 maximal rate of uptake is observed but at more acid pH a strong inhibition takes place with virtually total blockage of uptake at pH 6.1. The decrease of cell pH to 6.1 in the presence of carbonyl cyanide p-trifluoromethoxyphenyl hydrazone could therefore account for the decrease in hexose transport rate. It was shown that the intracellular pH as such determines the rate of uptake and not the pH difference between inside and outside; the transport rate did not correlate with delta pH.     

Effects of nectar concentration and flower depth on flower handling efficiency of bumble bees

Summary Fluid viscosity only affected ingestion rates of bumble bees (Bombus) for solutions greater than 35–40% sucrose (mass of solute per mass of solution). This contrasts with previously published models based on fluid dynamics which predicted continuous depression of ingestion rates with increasing viscosity. Individual bees maintained constant lapping rates regardless of sucrose concentration (up to at least 70%). The decline in ingestion rates at higher concentrations apparently resulted from the tongue not contacting liquid long enough to become saturated due to reduced capillary flow. Increasing flower depth similarly decreased the volume of liquid ingested per lap, and did not affect lapping rate. Morphologically dissimilar bees drank at different rates because glossa length affects lapping rate and volume ingested per lap, and body mass affects lapping rate. An additional species-specific component to lapping rate also influenced ingestion rates. Deviations from a regression model derived to explain ingestion rates as a function of glossa length, body mass, flower depth and liquid viscosity suggest mechanistic and behavioralaspects to flower probing time. Because of the relation between ingestion rate and liquid viscosity, the sucrose concentration maximizing a bee's rate of net energy uptake should lie between 50–65%, depending primarily on specific conditions of nectar volume, inflorescence size and flight time between inflorescences.     

Intestinal anion exchange in teleost water balance

Simultaneous measurements of all major electrolytes including HCO3(-) and H+ as well as water demonstrated that fluids absorbed by the anterior intestine of the marine gulf toadfish under in vivo-like conditions on an overall net basis are hypertonic at 380 mOsm and acidic ([H+] = 27 mM). This unusual composition of fluids absorbed across the intestinal epithelium is due to the unusual intestinal fluid chemistry resulting from seawater ingestion and selective ion and water absorption along the gastro-intestinal tract. Measurement under near symmetrical conditions with high NaCl concentrations and low MgSO4 concentrations revealed absorption of iso-osmotic and much less acidic fluids by the intestinal epithelium, a situation resembling that of other water absorbing leaky vertebrate epithelia. Reduced luminal NaCl concentrations seen in vivo results in lower absolute water absorption rates but higher Cl-/HCO3(-) exchange rates which are associated with higher net H+ absorption rates. It appears that apical anion exchange is important for net Cl- uptake by the marine teleost intestine especially when luminal NaCl concentrations are low and/or when MgSO4 concentrations are high. Observations indicate that fluid absorption from solutions of low NaCl but high MgSO4 concentrations is energetically more demanding than absorption from NaCl rich solutions at the level of the intestinal epithelium. Furthermore, the high luminal MgSO4 concentration which is an unavoidable consequence of seawater ingestion projects a demand for renal and branchial compensation for intestinal MgSO4 uptake and absorption of hypertonic and acidic fluid by the intestine.     

Hexose uptake by Catharanthus roseus cell suspensions is inhibited by a high medium salt content

Summary The uptake of glucose and fructose from the medium by Catharanthus roseus cell suspensions was strongly inhibited by high medium salt concentration, such as found in LS (Linsmaier and Skoog 1965) medium. After inoculation into standard LS nutrient medium with less than 5 mM hexose no uptake occurred, while in low salt medium hexose was completely depleted. At a hexose concentration of 50 mM the uptake rate was higher in low salt medium than in standard medium. The lower rate of uptake at high salt concentration was not the result of a pH or osmotic effect of the salts. Probably the affinity of the hexose carrier is affected by the ion concentration of the medium. The decrease in medium salt concentration during normal batch culture probably will have a considerable effect on hexose uptake.Abbreviations LS Linsmaier and Skoog - S sucrose - N mineral nitrogen - K K₂SO₄ - F fructose     

沉积再悬浮颗粒物对马氏珠母贝摄食生理影响的室内模拟

采用实验生态学方法室内模拟研究了不同浓度沉积再悬浮颗粒物对马氏珠母贝清滤率、摄食率、吸收率的影响。结果表明:(1)水体中总悬浮颗粒物对马氏珠母贝清滤率的影响极显著(P<0.01)。总悬浮颗粒物由低浓度(12.6 mg/L)趋高浓度(500 mg/L)时,马氏珠母贝的清滤率呈现峰值变化规律。与总悬浮颗粒物浓度50 mg/L时的最大清滤率(1.12 L.个体-1.h-1)比较,悬浮颗粒物浓度为500 mg/L时,清滤率达最小值(0.17 L.个体-1.h-1)),其清滤率降幅达85%。这表明在高浓度悬浮颗粒物的水环境下,贝类受到环境胁迫,其生理和自身摄食机制受到限制,引起摄食减少和机体损伤。马氏珠母贝类的清滤率(CR)与总悬浮颗粒物浓度(TPM)之间的关系可表达为:CR=-0.701+1.627×TPM-0.463×TPM2+0.036×TPM3(R2=0.928)。(2)水体中总悬浮颗粒物对马氏珠母贝摄食率的影响极显著(P<0.01)。马氏珠母贝的摄食率随着总悬浮颗粒物浓度的升高而增加,在50 mg/L时达最大值(38.28 mg/h),当总悬浮颗粒物浓度超过50 mg/L时,摄食率反而下降,在总悬浮颗粒物浓度为500 mg/L时,降为最小值(16.22 mg/h),摄食率降幅为58%。随着悬浮颗粒物浓度的增加,马氏珠母贝摄食率受到的影响小于清滤率受到的影响。马氏珠母贝类的摄食率(IR)与总悬浮颗粒物浓度(TPM)之间的关系可表达为:IR=-46.631+70.957×TPM-18.385×TPM2+1.367×TPM3(R2=0.907)。(3)水体中总悬浮颗粒物对马氏珠母贝吸收率影响极显著(P<0.01)。总悬浮颗粒物由低浓度(12.6 mg/L)趋高浓度(500 mg/L)时,马氏珠母贝的吸收率呈逐渐下降趋势,在总悬浮颗粒物12.6 mg/L时,马氏珠母贝的吸收率最大(48.57%),而总悬浮颗粒物500 mg/L时,马氏珠母贝的吸收率最小(8.56%)。马氏珠母贝的吸收率(AE)与总悬浮颗粒物浓度(TPM)之间的关系可表达为:AE=52.189+0.132×TPM-3.111×TPM2+0.316×TPM3(R2=0.976)。     

Animal Guts as Nonideal Chemical Reactors: Partial Mixing and Axial Variation in Absorption Kinetics

Animal guts have been idealized as axially uniform plug-flow reactors (PFRs) without significant axial mixing or as combinations in series of such PFRs with other reactor types. To relax these often unrealistic assumptions and to provide a means for relaxing others, I approximated an animal gut as a series of n continuously stirred tank reactors (CSTRs) and examined its performance as a function of n. For the digestion problem of hydrolysis and absorption in series, I suggest as a first approximation that a tubular gut of length L and diameter D comprises n=L&solm0;D tanks in series. For n>/=10, there is little difference between performance of the nCSTR model and an ideal PFR in the coupled tasks of hydrolysis and absorption. Relatively thinner and longer guts, characteristic of animals feeding on poorer forage, prove more efficient in both conversion and absorption by restricting axial mixing. In the same total volume, they also give a higher rate of absorption. I then asked how a fixed number of absorptive sites should be distributed among the n compartments. Absorption rate generally is maximized when absorbers are concentrated in the hindmost few compartments, but high food quality or suboptimal ingestion rates decrease the advantage of highly concentrated absorbers. This modeling approach connects gut function and structure at multiple scales and can be extended to include other nonideal reactor behaviors observed in real animals.     

Oral water ingestion in the pulmonate freshwater snail,Lymnaea stagnalis

InLymnaea stagnalis, oral uptake of ambient medium was studied using⁵¹Cr-ethylenediaminetetra-acetic acid. In normal snails Cr-ethylenediaminetetra-acetic acid uptake showed two components: a high uptake rate within the first hour followed by moderate uptake proportional with time. The tracer accumulated mainly in the digestive system. All animals showed initial, transient uptake. Moderate uptake proportional with time did not occur in snails in which the buccal ganglia had been extirpated, in which both the buccal ganglia had been extirpated and the oesophagus was sectioned, or in snails provided with an oesophageal fistula. These snails did not accumulate tracer in the intestinal system. This type of tracer accumulation clearly represented oral ingestion of surrounding water. The oral water ingestion rate ranged from 8 to 12 μl·h⁻¹·g⁻¹. Assuming complete absorption, this accounts for 20–30% of the urine production rate. At low external concentrations the contribution of oral water ingestion to Na⁺ balance is negligible. However, its importance will grow with increasing external concentrations and becomes a major factor at higher concentrations. The ingestion rate increased almost sixfold when starving snails were allowed to feed. It is suggested that oral water ingestion is a consequence of making bite cycles and swallowing.     

Nitrate supply affects ammonium transport in canola roots

Plants may suffer from ammonium (NH4+) toxicity when NH4+ is the sole nitrogen source. Nitrate (NO3-) is known to alleviate NH4+ toxicity, but the mechanisms are unknown. This study has evaluated possible mechanisms of NO3- alleviation of NH4+ toxicity in canola (Brassica napus L.). Dynamics of net fluxes of NH4+, H+, K+ and Ca2+ were assessed, using a non-invasive microelectrode (MIFE) technique, in plants having different NO3- supplies, after single or several subsequent increases in external NH4Cl concentration. After an increase in external NH4Cl without NO3-, NH4+ net fluxes demonstrated three distinct stages: release (tau1), return to uptake (tau2), and a decrease in uptake rate (tau3). The presence of NO3- in the bathing medium prevented the tau1 release and also resulted in slower activation of the tau3 stage. Net fluxes of Ca2+ were in the opposite direction to NH4+ net fluxes, regardless of NO3- supply. In contrast, H+ and K+ net fluxes and change in external pH were not correlated with NH4+ net fluxes. It is concluded that (i) NO3- primarily affects the NH4+ low-affinity influx system; and (ii) NH4+ transport is inversely linked to Ca2+ net flux.     

Effects of ophiobolin a on ion leakage and hexose uptake by maize roots

Ophiobolin A, a sesterterpene metabolite of Helminthosporium maydis, Nisikado and Miyake, stimulates net leakage of electrolytes and glucose from maize (Zea mays L.) seedling roots. Treatment of the roots with ophiobolin A at a concentration of 10 mug/ml (25 mum) inhibits uptake of 10 mm 2-deoxyglucose by 50% and of 0.5 mm 2-deoxyglucose by 85%. Compartmental analysis of the efflux of 3-O-methylglucose failed to show a similar effect of ophiobolin A on the rate of efflux of hexose. The inhibition of uptake is not reversible by washing. There is no difference in the effects on roots from cytoplasmic male sterile or normal cytoplasm plants, and exposure of carrot (Daucus carota L.) root discs to ophiobolin A also causes inhibition of 2-deoxyglucose uptake by this tissue.     

Electrolyte Metabolism in HeLa Cells

Methods have been developed to study cellular Na, K, and Cl concentrations in HeLa cells. Cell [Na] and [K] are functions of the age of the culture. As the culture grows [K], expressed in mmols/liter cell H₂O, rises from an initial value of 121 to a peak of 206 at about 4 days, and thereafter falls until it has almost returned to the initial value by the 9th day. [Na] falls as [K] rises, but there is no fixed relationship between the cellular concentrations of the two cations. There is, however, a correlation between generation time and cellular [K]. Measurements of net K uptake and net Na extrusion were carried out during 1 hour incubation at 37°C of low K cells. Both net K uptake and net Na extrusion took place against chemical concentration gradients, so that at least one transport system must be active; if the Cl distribution is passive both net K uptake and net Na extrusion are active. Studies with inhibitors of respiration and glycolysis lead to the conclusion that respiration is not required for these net transports, which appear to derive their energy from glycolytic sources.     

Cyclic variations in nitrogen uptake rate of soybean plants: effects of external nitrate concentration

Net uptake of NO3- by non-nodulated soybean plants [Glycine max (L.) Merr. cv. Ransom] growing in flowing hydroponic cultures containing 0.5, 1.0 and 10.0 mol m-3 NO3- was measured daily during a 24-d period of vegetative development to determine if amplitude of maximum and minimum rates of net NO3- uptake are responsive to external concentrations of NO3-. Removal of NO3- from the replenished solutions during each 24-h period was determined by ion chromatography. Neither dry matter accumulation nor the periodicity of oscillations in net uptake rate was altered by the external NO3- concentrations. The maxima of the oscillations in net uptake rate, however, increased nearly 3-fold in response to external NO3- concentrations. The maxima and minima, respectively, changed from 4.0 and 0.6 mmol NO3- per gram root dry weight per day at an external solution level of 0.5 mol m-3 NO3- to 15.2 and -2.7 mmol NO3- per gram root dry weight per day at an external solution level of 10.0 mol m-3 NO3-. The negative values for minimum net uptake rate from 10.0 mol m-3 NO3- solutions show that net efflux was occurring and indicate that the magnitude of the efflux component of net uptake was responsive to external concentration of NO3-.     

Short-term adaptation of digestive processes in the cockle Cerastoderma edule exposed to different food quantity and quality

Feeding and digestive parameters were analysed in cockles Cerastoderma edule fed for 3 days on two foods of different qualities, both foods given in two different concentrations. With low quality food, gut content was found to increase with ingestion rate. Such increased capacity of the gut to allocate food precludes negative effects upon throughput time, and so absorption efficiency remained nearly constant at the two food concentrations. With high quality food, gut content remained at high constant values and consequently enhancement of food ingestion rate with a high food ration leads to a significant reduction in throughput time, resulting in lower absorption efficiencies. Significantly higher levels of amylases and cellulases have been found within the digestive gland of cockles fed high quality diets. Coincidentally, absorption of carbohydrates is increased and absorption of lipids decreased in such diets as compared to low quality diets. Implications of the positive correlation between digestive enzyme activity and food quality are discussed in relation to the role that both digestive investments and endogenous faecal losses play in digestive processes. Results obtained in this study indicate that investments in the form of digestive enzymes are a key factor in the functional response of cockles to short-term variations in the food regime. Accepted: 13 September 1997     

Inhibition of glucose-induced insulin release by 3-O-methyl-D-glucose: Enzymatic,metabolic and cationic determinants

The analog of D-glucose, 3-O-methyl-D-glucose, is thought to delay the equilibration of D-glucose concentration across the plasma membrane of pancreatic islet B-cells, but not to exert any marked inhibitory action upon the late phase of glucose-stimulated insulin release. In this study, however, 3-O-methyl-D-glucose, when tested in high concentrations (30-80 mM) was found to cause a rapid, sustained and not rapidly reversible inhibition of glucose-induced insulin release in rat pancreatic islets. In relative terms, the inhibitory action of 3-O-methyl-D-glucose was more marked at low than high concentrations of D-glucose. It could not be attributed to hyperosmolarity and appeared specific for the insulinotropic action of D-glucose, as distinct from non-glucidic nutrient secretagogues. Although 3-O-methyl-D-glucose and D-glucose failed to exert any reciprocal effect upon the steady-state value for the net uptake of these monosaccharides by the islets, the glucose analog inhibited D-[5-3H]glucose utilization and D-[U-14C]glucose oxidation. This coincided with increased 86Rb outflow and decreased 45Ca outflow from prelabelled islets, as well as decreased 45Ca net uptake. A preferential effect of 3-O-methyl-D-glucose upon the first phase of glucose-stimulated insulin release was judged compatible with an altered initial rate of D-glucose entry into islet B-cells. The long-term inhibitory action of the glucose analog upon the metabolic and secretory response to D-glucose, however, may be due, in part at least, to an impaired rate of D-glucose phosphorylation. The phosphorylation of the hexose by beef heart hexokinase and human B-cell glucokinase, as well as by parotid and islet homogenates, was indeed inhibited by 3-O-methyl-D-glucose. The relationship between insulin release and D-glucose utilization or oxidation in the presence of 3-O-methyl-D-glucose was not different from that otherwise observed at increasing concentrations of either D-glucose or D-mannoheptulose. It is concluded, therefore, that 3-O-methyl-D-glucose adversely affects the metabolism and insulinotropic action of D-glucose by a mechanism largely unrelated to changes in the intracellular concentration of the latter hexose.     

Maintenance of cation gradients in cold-stored erythrocytes of guinea pig and ground squirrel: improvement by amiloride

Red blood cells of ground squirrel, a hibernator, gain Na at one-third the rate of guinea pig red blood cells when stored in saline medium at 5 degrees C for several days. This result correlates with the known slower loss of K during storage in ground squirrel cells. In ground squirrel cells Na gain is balanced by K loss, so that there is no net gain of solute; in guinea pig cells the total cation content rises progressively. Amiloride, a drug which inhibits Na entry, retards Na uptake in cells of both species. Surprisingly, amiloride also slowed K loss and, in guinea pig red cells, the decline of ATP content. In guinea pig cells amiloride reduced the gain of total cation by half. The results substantiate the difference in cold sensitivity of ion regulation of red blood cells of these two species and demonstrate the possible usefulness of amiloride-type drugs in nonfreezing preservation of red blood cells.     

Mechanisms of absorption of caseinophosphopeptide bound iron

Binding iron (Fe) to the 1-25 caseinophosphopeptide obtained from enzyme hydrolysis of beta casein (beta CPP) improves Fe bioavailability in the rat. To assess the mechanisms involved in its absorption, a perfused, vascularized duodenal rat loop model was used in controls and in Fe-deficient (bleeding of 25% blood volume) rats. Inhibitors of oxidative phosphorylation [2-4 dinitrophenol (DNP)] and/or of endocytosis [phenylarsine oxide (PAO)] were added to the perfusion solution containing 50 microM Fe as beta CPP bound Fe (Fe-beta CPP) or gluconate (Fe Gluc). Fe-beta CPP enhanced Fe uptake, reduced mucosal storage, and improved net absorption both in controls and in deficient animals. DNP reduced uptake, mucosal storage, and net absorption by the same percentage in Fe-beta CPP and Fe Gluc perfused rats in both control and Fe-deficient animals. PAO decreased uptake, mucosal storage, and net absorption of Fe-beta CPP but not of Fe Gluc. At the end of the experiment Fe serum levels were increased only in Fe Gluc animals. These results confirm the improved bioavailability of beta CPP bound Fe. They suggest that at least part of its absorption can occur by a different pathway than usual Fe salts. Fe-beta CPP can be taken up by endocytosis and absorbed bound to amino acids or peptides.     

Central place foraging in a human-dominated landscape: how do common cranes select feeding sites?

Human infrastructure and disturbance play an important role when animals select resources in human-modified landscapes. Theory predicts that animals trade food intake against costs of movement or disturbance to optimize net energy gain and fitness, but other necessary resources may also constrain the decisions, e.g. when animals repeatedly need to return to a central location, such as a nest, waterhole or night roost. Central place foraging theory states that the probability of occurrence of an animal decreases with the distance to the central location while selectivity for food items or foraging sites providing high net energy gain should increase with distance. We studied foraging patterns of common cranes Grus grus feeding in an agricultural landscape adjacent to a wetland to which they return for night roost. We used availability of spilled grains on harvested fields and distance to human settlement as proxy for site quality (i.e. increased likelihood of increased net energy gain with increased food availability and less disturbance). As predicted by theory, our results clearly show that cranes were more likely (more than twice as high resource selection function scores) to select foraging sites close to roosts. However, contrary to predictions, the selection of high quality sites in terms of high food availability decreased with distance to roost sites. Nevertheless, our results indicate that cranes were more likely to select sites with low risk of human disturbance far from roost sites, and were more tolerant to disturbance close to roost sites. How different species respond to the local and environmental conditions will increase the understanding of the species’ resource requirement, and also where in the landscape to prioritize conservation or management actions (e.g. mitigation of human disturbance and crop damage prevention to sustain agricultural production).     

Relatively large nitrate efflux can account for the high specific respiratory costs for nitrate transport in slow-growing grass species

In this paper we address the question why slow-growing grass species appear to take up nitrate with greater respiratory costs than do fast-growing grasses when all plants are grown with free access to nutrients. Specific costs for nitrate transport, expressed as moles of ATP per net mole of nitrate taken up, were 1.5 to 4 times higher in slow-growing grasses than in fast-growing ones (Scheurwater et al., 1998, Plant, Cell & Environ. 21, 995–1005). The net rate of nitrate uptake is determined by two opposing nitrate fluxes across the plasma membrane: influx and efflux. To test whether differences in specific costs for nitrate transport are due to differences in the ratio of nitrate influx to net rate of nitrate uptake, nitrate influx and the net rate of nitrate uptake were measured in the roots of two fast-growing ( Dactylis glomerata L. and Holcus lanatus L.) and two slow-growing (Deschampsia flexuosa L. and Festuca ovina L.) grass species at four points during the diurnal cycle, using ¹⁵NO₃ ^-. Efflux was calculated by subtraction of net uptake from influx; it was assumed that efflux of nitrogen represents the flux of nitrate. Transfer of the plants to the solution containing the labelled nitrate did not significantly affect nitrate uptake in the present grass species. The net rate of nitrate uptake was highest during the middle of the light period in all species. Diurnal variation in the net rate of nitrate uptake was mostly due to variation in nitrate influx. Variation in nitrate efflux did not occur in all species, but efflux per net mole of nitrate taken up was higher during darkness than in the light in the slow-growing grasses. The two fast-growing species, however, did not show diurnal variation in the ratio of efflux to net nitrate uptake. Integrated over 24 hours, the slow-growing grasses clearly exhibited higher ratios of influx to net uptake than the fast-growing grass species. Our results indicate that the higher ratio of nitrate influx to net nitrate uptake can account for higher specific costs for nitrate transport in slow-growing grass species compared with those in their fast-growing counterparts, possibly in combination with greater activity of the non-phosphorylating alternative respiratory path. Therefore, under our experimental conditions with plants grown at a non-limiting nitrate supply, nitrate uptake is less efficient (from the point of ATP consumption) in slow-growing grasses than in fast-growing grass species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Respiratory energy requirements of roots vary with the potential growth rate of a plant species

The rates of growth, net rate of nitrate uptake and root respiration of 24 wild species were compared under conditions of optimum nutrient supply. The relative growth rate (RGR)of the roots of these species varied between 110 and 370 mg g^-1 day^-1 and the net rate of nitrate uptake between 1 and 7 mmol (g root dry weight)^-1 day^-1. The rate of root respiration was positively correlated with the RGR of the roots. Root respiration was also calculated from the measured rate of growth and nitrate uptake, using previously determined values for the costs of maintenance, growth and ion uptake of two slow-growing species. The calculated rate of respiration was slightly lower than the measured one for slow-growing species, but twice as high as measured rates for rapid-growing species. This discrepancy was not due to a relatively smaller electron flow through the alternative pathway and, consequently, a more efficient ATP production in the fast-growing species. Neither could variation in specific costs for root growth or maintenance explain these differences. Therefore, we conclude that fast-growing species have lower specific respiratory costs for ion uptake than slow-growing ones. Due partly to these lower specific costs of nutrient uptake, the fraction of respiration that rapid-growing species spend on anion uptake is lower than that of slow-growing species, in spite of the much higher rate of ion uptake of the fast-growing ones.