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1.
Diel variations in inorganic carbon and nitrogen uptake by phytoplankton in an arctic lake 总被引:1,自引:0,他引:1
Timedepth variations in inorganic carbon and nitrogenuptake by phytoplankton in Toolik Lake were examined over 36h using isotope tracer techniques. Rates of dissolved inorganiccarbon (DIC = CO2 + + ) and maximum uptake were phased with the did high light/low light regime characteristic of the briefarctic summer with the amplitude of oscillation greater forDIC than for . Ammonium uptake was continuous at uptake-saturating concentrations. No conclusive evidencewas found for a diel periodicity in nitrogenous nutrient levelsor uptake of and ambient concentrations. A pronounced lighttemperature effecton dissolved inorganic nitrogen (DIN = + ) uptake was evident at depth when rates of uptake were maximum. Depth-integrated daily C/N uptake ratios(mol) estimated as the mean of four consecutive 6 h incubationsranged from 1.86.4 under conditions of substrate saturationand from 6.016.1 at ambient levels of DIN. The efficacyof 24 h incubations to estimate accurately day-rates of DICand substrate-saturated DIN uptake was assessed by comparingestimates obtained from 24 h exposures to those approximatedby summing results from serial short-term incubations. Experimentsof 24 h duration accurately predicted day-rates of maximum uptake but underestimated daily DIC uptake by 13 7% ( SD). Day-length incubations introduced serious errors in the estimation of day-rates of maximum uptake, effecting an underestimation of 29 5%( SD).
1Institute of Marine Science Contribution No. 538. 相似文献
2.
Light-limited photosynthetic carbon incorportion is expectedto be directly proportional to the scalar quantum irradiance.The proportionality constant is , where mis the maximum quantum yield (mol C Einstein1 absorbed)and $$\stackrel{\¯}{{\hbox{ k }}_{\hbox{ c }}}$$ isthe mean spectral absorption coefficient (m2 mg1 chla). Recent efforts to evaluate of in situphytoplankton photosynthesis are variously flawed. Lack of evidenceof proportionality and lack of correction of cosine to scalarirradiance are common deficiencies. Most data, as we interpretthem, indicate values in the range 0.0003 0.0006 mol C m2 Einstein1 abs mg1 chl a. New determinationsin lrondequoit Bay, New York, lie in this range. Most estimatesof at depth have been about 0.010 m2 mg1chl a. Similar values are being obtained for total particulatesfrom lrondequoit Bay; whether detritus contributes significantlyis not yet known. Published data, in our view, all point tovalues of m in situ in the range 0.030.07 mol C Einstein1abs. Published values >0.10 are almost certainly due to imprecisionor systematic error.
*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982. 相似文献
3.
The translocation of various anions, cations, and acids acrossthe membrane system of turnip mitchondria has been investigatedby light scattering techniques. The results imply that the membranesof turnip mitochondria are not permeable to choline, K+, Na+,, , , and . It has been established that Cl,, and SCN passively permeate the mitochondrial membrane. It is probable that permeates as NH3 and that and cross the membrane either as the corresponding acid or on ananion/ hydroxide antiporter. The alkali metals Na+ and K+ equilibrateacross the membrane via specific H+/Na+ and H+/K+ antiporters,having similar activity at neutral pH. 相似文献
4.
The effects of Cl and pretreatment on 36Clinflux and influx into Characorallina cells were examined. Both treatments reduced 36Clinflux into C. corallina cells in the acid pH range (4.57.0). pretreatment stimulated influx into C. corallina cells, but Cl pretreatment hadno effect. There was a direct inhibitory effect of CI on influx into C. corallina cells, but no apparenteffect on net NO uptake. The time course of 36Cl accumulation into cytoplasmic and vacuolarcompartments during incubation of the cells with showed that significant radioactivity appeared in the vacuolarsap after 30 min. There was a linear increase in the percentageof total 36CI which crossed the tonoplast (c-v). There was noeffect of Cl or pretreatment on accumulation of radioactivity in the vacuole. Thin layer chromatography ofthe vacuolar sap showed that after 2 h only one component waspresent which had an RF which was similar to 36CI. Therate of accumulation of 36C1 in the vacuole could beused to estimate rates of reduction. Key words: Chloride, Chlorate, Chara, Nitrate 相似文献
5.
Cl and ions interact apparently competitively during influx across the plasmalemma of carrot root cells. Cl,however, reduces influx much less than predicted from the effect of on Cl influx.Cl and plasmalemma influxes both increase with time after excision of carrot tissue. Cland may therefore be transported by a common mechanism. The effect of pH changes on the influx of malate across theplasmalemma in barley roots shows that malate crosses the plasmalemmaas the singly charged anion. Stimulations of influx by bothK2SO4 and KCl suggest that the malate anion crosses in associationwith. K+. If malate entry is passive, Pmal- is about 2?108cm s1, but it is thought that malate entry is partlyan active process. A slight, apparently competitive inhibition by Cl ofmalate flux into the vacuole of barley root cells suggests thatthe two anions may be transported by a common process at thetonoplast, but this is not thought to be physiologically significant. The accumulation of 14C from 1 mM is drastically reduced by 10 mM Cl. A quantitative analysis of the kineticsof 14C exchange shows that Cl directly inhibits the formationof malate from . The decreased influx of endogenously produced malate to the vacuole in the presenceof Cl is probably a secondary consequence of the fallin the cytoplasmic concentration. The nature of the Clinhibition of malate formation is discussed. In KCl-loaded tissue the influx of external malate and the accumulationof 14C from external are reduced. The location of these effects is not certain, but the effects suggest thatregulation of malate synthesis and accumulation may be relatedto the negative-feedback regulation of Cl and transport. 相似文献
6.
TOLLEY-HENRY LESLIE; RAPER C. DAVID JR.; GRANATO TOM C. 《Journal of experimental botany》1988,39(5):613-622
Net uptake of by non-nodulated soybean plants [Glycine max (L.) Merr.cv. Ransom] growing in flowing hydroponiccultures containing 05, 1.0 and 10-0 mol m3 was measured daily during a 24-d period of vegetativedevelopment to determine if amplitude of maximum and minimumrates of net uptake are responsive to external concentrations of . Removal of from the replenished solutions during each 24-h period was determinedby ion chromatography. Neither dry matter accumulation nor theperiodicity of oscillations in net uptakerate was altered bythe external concentrations. The maxima of the oscillations in net uptake rate, however, increased nearly3-fold in response to external concentrations. The maxima and minima, respectively, changed from 4.0 and 06mmol per gram root dry weight per day at an external solution level of 05 mol m3 to 152and -27 mmol per gram root dry weight per day at an external solution level of 100mol m3 . The negative values for minimum net uptake rate from100 mol m3 solutions show that net efflux was occurring andindicate thatthe magnitude of the efflux component of net uptake was responsiveto external concentration of . Key words: Nitrate uptake rate, carbon-nitrogen partitioning, flowing solution culture 相似文献
7.
Nitrate Uptake and Assimilation following Nitrate Deprivation 总被引:2,自引:0,他引:2
Upon first exposure to , the uptake and reduction capacities of dark-grown corn (Zea maysL.) roots are initially low, but increase markedly within 6h. The development of the accelerated uptake rate appears to be substrate induced as is reductase (NR), the first enzyme in the assimilatory pathway. However, the inductionof uptake is independent of NR induction. The effect of deprivation was studied to determine the role of endogenous on subsequent uptake and reduction. Corn roots were induced for 24 h in 05 mol m3 nutrient solution and then exposed for 0 to 32 h to -free nutrient solution. Uptake and reduction of were determined periodically by exposing sets of roots to a1 h pulse of 0.5 mol m3 . Neither uptake (4.57 µmol root1 h1)nor the percentage of absorbed reduced (27%) was changed significantly (P 0.05) by exogenous deprivation. However, the estimated induced componentof uptake decreased significantly (50% after 32 h). Concurrently, the non-induced basal componentof uptake increased. Previously accumulated decreased from 23 to 4.5 µmol root1 after 32 h of exogenous deprivation. Nearly equivalent quantities of endogenous were used for translocation and reduction during deprivation. During each 1 h pulse, the amounts of translocation and net efflux of to the uptake solution were similar. Net efflux of was strongly correlated (r = 0.991) to the amount of endogenous . The remaining endogenous and its rate of utilization were apparently sufficient to minimize a rapid declineor complete loss in both the induced uptake state and the rate of in vivo assimilation. Key words: reduction, translocation, efflux, root, Zea mays L 相似文献
8.
Expression of Characteristics of Ammonium Nutrition as Affected by pH of the Root Medium 总被引:6,自引:1,他引:5
CHAILLOU S.; VESSEY J. K.; MOROT-GAUDRY J. F.; RAPER C. D. JR; HENRY L. T.; BOUTIN J. P 《Journal of experimental botany》1991,42(2):189-196
To study the effect of root-zone pH on characteristic responsesof -fed plants, soybeans (Glycine max {L.}Merr. cv. Ransom) were grown in flowing solution culture for21 d on four sources of N (1.0 mol m3 , 0.67 mol m3 plus 0.33 mol m3, 0.33 mol m3 plus 0.67 mol m3 , and 1.0 mol m3) with nutrient solutions maintained at pH 6.0, 5.5, 5.0, and 4.5. Amino acid concentration increased inplants grown with as the sole source of N at all pH levels. Total amino acid concentration in the rootsof -fed plants was 8 to 10 times higher than in -fed plants, with asparagine accounting for more than 70% of the total in the roots of these plants.The concentration of soluble carbohydrates in the leaves of-fed plants was greater than that of -fed plants, but was lower in roots of -fed plants, regardless of pH. Starch concentration was only slightlyaffected by N source or root-zone pH. At all levels of pH tested,organic acid concentration in leaves was much lower when was the sole N source than when all or part of theN was supplied as . Plants grown with mixed plus N sources were generally intermediate between - and -fed plants. Thus, changes in tissue compositioncharacteristic of nutrition when root-zone pH was maintained at 4.5 and growth was reduced, still occurredwhen pH was maintained at 5.0 or above, where growth was notaffected. The changes were slightly greater at pH 4.5 than athigher pH levels. Key words: Ammonium, nitrogen nutrition, root-zone pH, soybean, tissue composition 相似文献
9.
Nitrate Transport Characteristics in Hordeum vulgare L. Seedlings using Three Different Tracer Techniques 总被引:2,自引:0,他引:2
Deane-Drummond, C. E. and Thayer, J.R. 1986. Nitrate transportcharacteristics in Hordeum vulgare L. seedlings using threedifferent tracer techniques.J. exp. Bot. 37: 429439. and have been used to investigate various properties of nitrate uptake and translocation intoHordeum vulgare L seedlings. Short term / influx into seed lings grown in CaSO4 was stimulated by after a lag of 2 h. The apparent kinetics of shortterm / influx over the concentration range 0?00?7mol m fitted Michaelis-Menten equations The apparent Vmax didnot change when seedlings were used that had been pretreatedin 10 or 100 mmol m3 and Vmax=3.77 and 3?56µmol g1 fr. wt. h1respectively. The apparent Michaelis constants were also similarand Km=0?47 and 0?45 mol m3 respectively. Longer term pulse chase experiments with the heavy isotope 15Nhave shown that feeding roots with resulted in the preferential appearance of 15N labelled aminoacids in the xylem sap. Pulse chase experiments with the radioisotope13N have shown that feeding shoots with resulted in a radial pattern of distnbution of labelin the leaf veins, which can be detected using autoradiography. The limitations and advantages of all three techniques are comparedby reference to other known experimental data. Key words: 36Chlorate, 13nitrate, 15nitrate, Hordeum vulgare 相似文献
10.
The rate of exponential growth of Amphidinium carterae Hulburtwas the same (0.025 h1) with either or as sole N-source. Nevertheless, in short-term experiments, cells growing exponentially with as N-source took up added 56 times faster than . accumulated in the cells. Addition of inhibited, reversibly, disappearance of from the medium; prior N-deprivation of the cells did not affect this inhibition. N-repIetecells, grown with as N-source, took up for several hours both in light and darkness, butthe uptake by such cells soon ceased in darkness although it continued in light. When uptake ceased, the cells could still take up rapidly in darkness. Ammonium taken up was assimilated rapidly into organic-N includingglutamine, other amino acids and protein. Ammonium uptake in darkness was accompanied by the utilizationof cellular polysaccharide, mainly glucose polysaccharide. Mostof this carbon was unavailable for the dark assimilation of. Addition of the analogue, methylammonium, did not initiate polysacchande catabolism. Itis suggested that a control mechanism is in operation, througha product of assimilation, which operates on one or more of the enzymes concerned with polysaccharidebreakdown, for example, -amylase or phosphorylase. Uptake of was accompanied by a high rate of dark 14CO2 fixation and with both Amphidintum klebsii Kof.et Swezy and A. carterae N-deprivation led to a marked increasein this rate following addition of ; addition of had much less effect. The possible implications of these findings for the eco-physiology of marine dinoflagellatesare discussed. Key words: Ammonium, nitrate, CO2 fixation 相似文献
11.
Effects of light flux density (LFD) during growth and uptakeassay on induction of transport system and kinetics of transport were studied using the Azolla pinnata-Anabaena azollae association (Azolla). Theinduction and uptake kinetics of the transport system were determined using an automated system that measuredthe NO3 concentration in the growth medium as a function oftime, using an on-line high performance liquid chromatograph(HPLC) with a UV-VIS detector. Full induction of the transport system required about 1.5 to 2.0 h and occurred without any apparent lag phase,regardless of the LFD provided. The level of induction of transport of Azolla grown at 600 µmol m2s1 LFD was higher than for that grown at 100 µmolm2 s1. Similarly, 600 µmol m1 s1LFD during the assay resulted in a higher level of inductionthan did 100 umol m2 s1. An increase in the LFDeither during the growth or the assay period increased the uptake rate; however, an increase in LFD duringthe latter period had greater effect. Azolla grown and assayedat 600 umol m2 s1 had the highest uptake rate. The uptake rate at 50 cm3 m3ambient CO2 concentration was initially higher than at 305 cm3m3, but the uptake rate decreased rapidly with time andeventually dropped below that at 305 cm3 m3 CO2. Thesedata suggest that the energy required for transport in Azolla may bypass the photosynthetic CO2 fixationand carbon-cycling. Key words: carbon dioxide, concentration dependence, light flux density, uptake 相似文献
12.
Low salt roots of Cucumis sativus L. cv. Burpeeana Hybrid weresubjected to iso-ionic treatments in which the external solutionconcentration of K+ was maintained at 14 mM. Solution concentrationof varied from 0 to 14 mM, other anions compensating. When Cl was the compensating ion, its concentrationin the exudate increased during the first 4 h and thereafterwas nearly the same as that of the external solution in alltreatments containing I mM or more. After 8 h of equilibration the concentration in the exudate increased almost exactly as its concentrationin the external solution. Rates of exudation and K+ transportwere almost constant between I and 14 mM KNO2. More Clwas transported from solutions of similar Cl concentrationwhen was also present. When water transport was inhibited with mannitol in treatments containing both KNO3and KCI, exudate concentrations of K+ and were increased, but exudate concentration of Cl was notsignificantly affected except at the highest Cl 相似文献
13.
Uptake of Methylammonium Ions by Hydrodictyon africanum 总被引:3,自引:0,他引:3
Methylamine influx into Hydrodictyon has been measured with[l4C]methylamine. The influx increases with rising externalpH up to about pH 8. Between pH 8 and 9 influx remains quiteconstant, with a further increase above pH 9. Influx is light-dependent,temperature-sensitive, and is decreased by . During short-term influx (less than 4 h) metabolism of methylamineappears negligible. Prolonged influx results in CH3 accumulation, and efflux of K+, Na+, and H+. There is no effecton Cl influx. Methylamine decreases the membrane electricp.d. by 60120 mV at external concentrations of 0?21?0mM. The results indicate that, below pH 9, methylamine enters thecell almost entirely as CH3. It is suggested that a passive electrogenic uniporter is involved, and thatby analogy uniport of may also be expected in Hydrodictyon. The results are discussed in relation to theevidence for uptake of CH3 and by other plant cells. 相似文献
14.
Allen, S., Thomas, G. E. and Raven, J. A. 1986. Relative uptakerates of inorganic nutrients by and grown Ricinus communis and by two Plantago species.J. exp. Bot. 37: 419428. The relative rates of uptake and assimilation of C, N, P, S,Cl, K+ , Na+ Ca2+ and Mg2+ by and grown Ricinus conimunisand by NH4NO3- grown Plantago lanceolata and P. major were calculatedfrom data presented elsewhere. Results showed that for grown Ricinus the short term relativeuptake rates, for each nutrient X did not change significantly over the steady-state periodof exponential growth. The average gave , the mean relative uptake rate during exponential growth, for each nutrient. The amountof each nutrient taken up from a nutrient solution over a periodof time could, therefore, be calculated. For and -grown R. communis,the relative uptake rate of each nutrient was a constant fractionof the relative rate of carbon assimilation. It is suggestedthat this is typical of plants of cauline habit. For both Plantago spp., the relative rates of nitrogen uptakeand assimilation fell significantly during the exponential growthphase It is suggested that this could be a characteristic ofthe growth habit of the rosette plant. Key words: Relative uptake rates, Ricinus, Plantago, ammonium, nitrate, cauline, rosette 相似文献
15.
The effect of growth temperature on the short term influx of86Rb+/K+, 36Cl/Cl, , 45Ca2+/Ca2+and into barley plants (Hordeum vulgareL. cv. Betzes) has been examined. When barley plants were grownwith a differential root: shoot temperature (15 ?C/25 ?C) therewas a marked stimulation of influx of all ions except Ca2+.Q10 measurements were close to 2.0 for all ions except Ca2+,where Q10 1.0 was found. Kinetics of ion influx showed thatthere had been almost complete compensation for the lower growingtemperature over a range of concentrations. The regulation of uptake was affected by growth temperature. On exposure to 15?C efflux/influx fell initially but was restored after 5 d. Sensitivity of net uptake to was increased by growth at a lower temperature. The importance of these observations in relationto application to field experiments and the proposed mechanismfor uptake is discussed. Key words: Growth temperature, Nitrate, Ammonium 相似文献
16.
Oilseed rape (Brassica napus L. cv. Bien venu) plants were grownin a system of flowing solution culture and pre-treated at roottemperatures of 3 ?C or 13 ?C for 7 d with or without N suppliedas 10 mmol m3 NH4NO3. Subsequently, N was re-suppliedand root temperatures were reversed for 7 d. Shoot temperatureswere common at 20/15 ?C day/night. Net uptake of , and K+, leaf area, root length, and transpirationwere measured and compared with control plants having root temperaturesof 3 ?C and 13 ?C throughout. Plants that were continuouslysupplied with N and pre-treated at 3 ?C showed a 50% increasein total uptake of , and measured at 13 ?C over 7 d compared with control plants at 3?C,but uptake of was 28% lower and uptake of was 43% higher than that shown by control plants at 13 ?C. Pre-treatment at 3 ?C did not enhance the subsequentuptake of total N or of K+ at 13 ?C relative to the 13 ?C control.Transpiration rates at 3 ?C were on average 70% of those at13 ?C. The concentration of total N in plants was halved after7 d without a supply of N, but total dry matter production wasnot significantly affected. N starvation also increased thetemperature sensitivity of subsequent uptake relative to that of uptake. After N starvation at 13 ?C the uptake of and measured at 13 ?C was 50% higher over 7 d than that measuredunder continuous N supply. In contrast, after N starvation at3 ?C the uptake of at 3 ?C was 70% less, whilst uptake was 50% more than the respective totals absorbed by plants that were continuously supplied withN at 3 ?C. After N starvation the uptake of was generally 4060% of the daily total N uptake, comparedwith 6080% in plants continuously supplied with N. Key words: Brassica napus, oilseed rape, root temperature, nitrate, ammonium, potassium, N-deficiency, ion uptake rate, transpiration 相似文献
17.
Svenning Mette M.; Junttila Olavi; Macduff James H. 《Journal of experimental botany》1996,47(6):729-738
An experimental study in flowing solution culture compared threeNorwegian ecotypes (from Saerheim, Pasvik and Bod) and a commercialcultivar (Ac51) of white clover (Trifolium repens L.) and demonstrateddifferences with respect to the rate and intensity with whichN2 fixation was affected when or were supplied at 20 mmol m3 over 14 d. Plants were nodulated and N2 fixation wasestablished over 17 d prior to supplying mineral N, with shoottemperature 25/15 C day/night and root temperature adjustedprogressively to 12C. Control plants received no mineral Nthroughout, and did not grow as rapidly as those supplied with or . Mineral N generally depressed the total N2 fixation per plantrelative to control plants, with two exceptions. The effectof on N2 fixation was moresevere than that of and, over the 14 d, -fed plants fixed more N2 than the comparable -fed plants. Interpolated daily rates of N2 fixation per plant andnodule dry wts were used to calculate specific rates of N2 fixation.These showed that (1) addition of either or initially stimulated fixation relative to control plants and to mineral N uptake;(2) this stimulation was greatest and was delayed by 1-2 d in-fed plants; and (3) nutrition sustained higher residualrates of N2 fixation after 8 d compared with nutrition, under which fixation all but ceased after10 d in three of the genotypes. Ecotype Bode showed by far themost severe induced depression of N2 fixation. Key words: N2 fixation, nitrate, ammonium, white clover, northern ecotypes 相似文献
18.
The effects of sulphite ion () and sulphate ion () on both the activation and the catalytic activities of ribulose- 1, 5-bisphosphate carboxylase (EC 4.1.1.39
[EC]
)were studied and compared to those of other effectors of theenzyme, particularly inorganic phosphate (P1). The activationby CO2 and Mg2+ of a slow activating form of the carboxylasein the presence of the two anions produced high specific activitieswith significant lower concentrations of CO2 than normally required.This was due to stabilization of the ternary complex betweenthe enzyme, CO2 and Mg2+. With a rapidly activating speciesof enzyme, and caused only a small increase in activation with subsaturating CO2. , and P1, with saturatingconcentrations of CO2 also enhanced the catalytic activity abovethat achieved with CO2 and Mg2+ alone; P1 was the most effectiveof the anions, producing a 50% increase in the specific activity,both with the slow and rapidly activating species. and were potent inhibitors of the carboxylase and oxygenase reactions of the enzyme. was a non-competitive inhibitor with respect to CO2, and competitive/mixedwith respect to ribulose-1, 5-bisphosphate. The time courseof the carboxylase and oxygenase reactions in the presence of were biphasic with inhibition apparent only in the second phase. Key words: Ribulose bisphosphate carboxylase, Activation, SO32-, SO42- 相似文献
19.
Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm Zostera muelleri 总被引:1,自引:0,他引:1
Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependentphotosynthesis in the marine angiosperm Zostera muelleri.J.exp. Bot. 37: 965976. Photosynthetic oxygen evolution in the seagrass Zostera muelleriIrmisch ex Aschers. was inhibited in iso-osmotic sucrose. Theapparent affinity of the leaves for CO2 in seawater increasesfrom pH 8?2 to 8?9 indicating that as well as CO2 may act as a substrate for photosynthesis. Theaffinity for CO2 was lower in iso-osmotic sucrose and was notaffected by pH. Under these conditions was not a substrate for photosynthesis. The differencebetween the photosynthetic rate in seawater and iso-osmoticsucrose at the same concentration of CO2 was used to estimate assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissuewas more appropriate than the Michaelis-Menten theory for calculatingthe apparent affinity of the leaf slices for CO2. The apparentKm CO2 was calculated as 116 mmol m3 at pH 8?2 by Michaelis-Mentenkinetics but only 8?10 mmol m3 by the Briggs-Maskellequation. The stimulation by various ions in Seawater of use was investigated. The cations,in decreasing order of effectiveness were Ca2+, Mg2+, K+ andNa+ Anions were ineffective. No single cation at its concentrationin seawater was capable of supporting use at the rate observed in seawater. Acetazolamide,an inhibitor of carbonic anhydrase, inhibited the use of for photosynthesis but had littleeffect on CO2 photosynthesis. Thus, carbonic anhydrase activityis required for -dependent photosynthesis. Key words: Zostera muelleri, photosynthesis, salinity 相似文献
20.
BECANA MANUEL; APARICIO-TEJO PEDRO M.; S?NCHEZ-D?AZ MANUEL 《Journal of experimental botany》1986,37(6):798-806
Three-month-old plants (vegetative stage) of alfalfa (Medicagosaliva L cv. Aragon) were supplied for one week with 1.0dm3(uniformly distributed) nutrient solutions containing 0 or 20mol m3 . One week after initiation of treatment, the plants were subjected to drought by withholding water. Bacteroidsand cytosol of nodules were obtained at different stages ofstress, and used for enzyme assays and for determination of, and . Proteins of bacteroids were more stable than cytosolic proteinswith respect to the detrimental effects of water stress and. Protein contents of bacteroids and cytosol were inversely related to proteolytic activitiesagainst azocasein in both nodule fractions. Specific nitrate reductase activity (NRA) and nitrite reductaseactivity (NiRA) of bacteroids from -treated plants were inhibited by c. 70% and 45%, respectively, as leafwater potential (w) declined from 0.5 MPa (control) to1.8 MPa. At still lower w both activities began to increase:NRA was doubled, whereas NiRA only returned to its control level.Cytosolic NRA was strongly inhibited by drought, but the correspondingNiRA remained constant. Ammonia concentration in bacteroids and nodule cytosol keptbasically constant, whereas accumulated in the cytosol at severe stress, due to the activationof bacteroid nitrate reductase. Results indicate that nitrate and nitrite reductases of thebacteroids and the nodule cytosol act in different form: assimilatory,the cytosolic enzymes; and dissimilatory, the enzymes of bacteroidsat low w The possibility that assimilation of also occurs in bacteroids at control or mild waterstress conditions is suggested. Key words: Assimilatory and dissimilatory reduction, bacteroids, Medicago saliva, nodule cytosol, water stress 相似文献