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1.
When subjected, directly (through nutritional deficiencies) or indirectly (through alkaline constraints leading to such deficiencies) to nutrient deficiencies, certain plants respond by developing special root structures called cluster roots. This phenomenon can be considered as an ecophysiological response to a specific nutrient deficiency enabling plants to enhance nutrient uptake. Experiments conducted on an alkaline and an acid soil showed that Casuarina glauca (Sieber ex Spreng.) produced cluster roots only in the alkaline soil and not in the acid soil. In addition, iron (Fe) and phosphorus (P) deficiencies were examined separately or together to determine their effect on cluster root formation in C. glauca seedlings grown hydroponically. Results from experiments carried out on three Casuarina species (C. glauca, C. cunninghamiana Miq. and C. equisetifolia L.) indicated that Fe is involved in cluster root formation. In nutrient media lacking P but containing Fe, no cluster roots formed while seedlings receiving P and lacking Fe developed cluster roots. When incubated on chrome-azurol S-agar on blue plates (CAS assay), a technique used routinely to detect the production of siderophores by micro-organisms, the root system of Fe-deficient plants exhibited orange halos around cluster roots, indicating production of a ferric-chelating agent. It is concluded that the capacity of cluster roots of C. glauca to chelate Fe allows the plant to grow normally on alkaline soils.  相似文献   

2.
Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N2 fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.  相似文献   

3.
From homogenates prepared from surface-sterilized nodules ofseedlings of Casuarina cunninghamiana grown aeroponically, astrain of Frankia designated HFPCc13 was isolated and has beengrown in pure filamentous culture in a defined synthetic nutrientmedium. Vesicle and sporangium formation can be induced by removalof combined nitrogen from the medium.Frankia strain HFPCc13nodulates young seedlings of C. cunninghamiana and C. equisetifoliawithin three weeks of inoculation with an optimum root mediumpH of 6–7 for nodulation and optimum temperature of 30–35°C. The presence of combined nitrogen in the root mediuminhibits nodulation with NH4+ more inhibitory than NO3.Frankia HFPCc13 does not nodulate Allocasuarina species withinthe same family nor several other possible actinorhizal plantstested. Thus this strain is quite precise in its host specificity.The rate of acetylene reduction was greater in C. cunninghamianathan the closely related species C. equisetifolia. In neitherof these host species were vesicles observed to occur withinthe infected root nodules which had been demonstrated to beactively fixing dinitrogen. Root nodules were shown to be activein acetylene reduction over a range of O2 concentration in thegaseous environment with an optimum at about 20 per cent O2,the ambient PO2 of the air. The mechanism(s) for oxygen protectionof nitrogenase within the filamentous form of Frankia withinthese nodules remains to be explained. Casuarina, Frankia, nodulation, nitrogen fixation  相似文献   

4.
The effects of excess salinity and oxygen deficiency on growthand solute relations in Zea mays L. cv. Pioneer 3906 were examinedin greenhouse experiments. The roots of plants 14 d old growingin nutrient solution containing additions of NaCl in the range1.0–200 mol m–3 were either exposed to a severedeficiency of O2 by bubbling with nitrogen gas (N2 treatment),or maintained with a supply of air (controls), for a periodof 1–7 d. The threshold NaCl concentration resulting inappreciable inhibition of leaf extension, and shoot f. wt gainin controls was between 10 and 25 mol m–3. At 25 mol m–3NaCl the ratio of Na+/K+ transported to shoots was about 20times greater than in plants in 1.0 mol m–3 NaCl. Theeffect of addition of NaCl to the nutrient solution was to enhanceNa+ movement but simultaneously depress the rate of K+ transportto shoots (per g f. wt roots). Interactions between NaCl levels and aeration treatment wereshown by analyses of variance to be statistically significantfor leaf extension, shoot and root f. wt gains, Na+ and K+ concentrationsin shoots and roots. When roots were N2-treated, shoot and rootgrowth were depressed, the effect of aeration treatment beinggreatest at NaCl concentrations of 50 mol m–3 or less.Additionally, N2-treatment greatly accelerated Na- transportto shoots while depressing K+ transport still further, so thatat 10 mol m–3 NaCl the ratio Na+/K+ acquired by the shootswas 230 times greater than in controls. Over the concentrationrange 1.0 to 50 mol m–3 NaCl, the ratio Na+/K+ transportedto shoots by anoxic roots increased by a factor of 860. Mechanisms controlling changes in solute flux to the shoot,and the significance in relation to plant tolerance of excesssalts or oxygen deficiency are discussed. Anaerobic, corn, flooding, maize, oxygen-deficiency, salinity  相似文献   

5.
Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N2 fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with 15N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N2 fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N2fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N2 fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of 15N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N2 fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N2 fixation, N remobilization, N2 partitioning, regrowth  相似文献   

6.
Three methods were used to study N2 fixation and effects ofwater deficit on N2 fixation: C2H2 reduction assay (ARA), 15Ndilution technique and accumulated N content. In addition, 15Ndilution was calculated both in a traditional way and in a modifiedway, which takes into consideration N and 15N content for theplants before the experiment started. The three methods wereapplied on the following Rhizobium-symbioses: Acacia albidaDel (Faidherbia albida (Del) A. Chev.) and Leucaena leucocephala(Lam) de Wit., and the Frankia-symbiosis Casuarina equisetifoliaL. The plants wereabout 4-months-old when they were harvested. Nitrogen derived from N2 fixation in control plants of Acaciaalbida was 54·2 mg as measured with ARA, while it was28·5 mg as measured with the 15N dilution technique,compared to 30·7 mg calculated as accumulated N. In comparison,L. leucocephala fixed 41·6 mg N (ARA), 53·5 mgN(15N dilution technique) and 56·3 mg N (accumulatedN). The Frankia-symbiosis had fixed 27·4 mg N as measuredby ARA, 8·1 mg N as measured by 15N dilution techniqueand 12·3 mg N as accumulated N. There were no differencesbetween the estimates based ontraditional and modified waysof calculating 15N dilution. The immediate effect of water deficit treatment on N2 fixationwas continuously measured inall species with ARA, which startedto decrease approximately 10 d after the initiation of the treatment,and declined to less than 5% of the initial level after 21–28d. The decrease in the amount of N derived from N2 fixation wasstudied in L. leucocephala during the period of treatment. Therewas a 26% decrease in amount of N derived from N2 fixation asresult of water deficit (as measured with ARA), while the decreasewas 23% when measured withboth the 15N dilution method and asaccumulated N. The three different methods for measuring N2 fixation and effectsof water deficit on N2 fixation are discussed. Key words: Acacia albida, ARA, Casuarina equisetifolia, Leucaena leucocephala, 15N dilution, N2N fixation, water deficit  相似文献   

7.
Diurnal variations in acetylene reduction and net hydrogen evolutionwere shown in five tropical and subtropical nitrogen-fixingtree symbioses. The symbioses studied in a growth chamber were:Acacia albida x TAL 1457, Leucaena leucocephala x TAL 1145,Prosopis chilensis x TAL 600, Casuarina glauca x HFP Cc13 andC. obesa x HFP Cc13. Acetylene reduction was highest at the end of the light periodin all symbioses studied. In the A. albida x TAL 1457 symbiosis,the diurnal variations in acetylene reduction and net hydrogenevolution showed a minor synchrony, while in the other symbiosesthe diurnal pattern of acetylene reduction and net hydrogenevolution clearly differed. Also, a diurnal variation in relativeefficiency of nitrogenase was shown in the A. albida x TAL 1457symbiosis. A hydrogen uptake enzyme was detected at a low substrate concentration(24.5 mmol m–3 H2) for L. leucocephala x TAL 1145, C.obesa x HFP Cc13 and has earlier been found for C. glauca xHFP CcI3. A hydrogen uptake system was also found for P. chilensisx TAL 600 and A. albida x TAL 1457 at a 17-fold higher substrateconcentration. The results show that a diurnal variation in C2H2 reductionand H2 evolution occurs, and that diurnal variation in the conversionfactor between C2H2 reduction and N2 fixation could occur. Thisfact raises criticisms regarding the use of a point estimateof this factor. Key words: Acetylene reduction, hydrogen evolution, uptake hydrogenase, nitrogen-fixing tree symbioses  相似文献   

8.
Growth and N-accumulation rates in leaves, stolons and rootsof individual white clover plants were studied in three experimentsusing two methods. In a growth chamber experiment, the relativedifferences between tissues were found to be almost constantfor a wide range of clover plant sizes. The stolon dry matter(DM) production was 56% and the root DM production 40% of theDM production in leaves. The N yield of stolons was 30% whileN yield in roots was 34% of N yield in leaves. The effect ofN application on these relations was investigated in a glasshouseexperiment. Application of N reduced the root:shoot N ratiofrom 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e.for abovevs.below cutting-height tissues) was only reduced from0.97 to 0.80. In a field trial with two contrasting N regimes,growth and N accumulation were measured on individual cloverplants. Dinitrogen fixation was estimated by15N isotope dilutionbased on analysis of leaves-only or by including stolons. Usingleaves-only did not affect the calculation of percentage ofclover N derived from N2fixation (% Ndfa) since the15N enrichmentwas found to be uniform in all parts of the clover. A correctionfactor of 1.7 to account for N in below cutting-height tissueis suggested when N2fixation in white clover is estimated byharvesting the leaves only.Copyright 1997 Annals of Botany Company Leaves; N accumulation; N2fixation; 15N isotope dilution; pastures; roots; root/shoot ratio; stolons; Trifolium repensL.; white clover  相似文献   

9.
Growth of two actinorhizal species was studied in relation tothe form of N supply in water culture. Non-nodulated bog myrtle(Myrica gale) and grey alder (Alnus incana) were grown withNH4+, NH4NO3 or NO3 (4 mol m–3 N). A nodulatedseries of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/rootratio were highest for the two species with the N completelysupplied as NH4+. In both species, nitrate was largely reducedin the roots and the presence of NO3 in combined-N supplyalways affected the RGR and N utilization rate negatively. BothN2 fixation and complete NO3 nutrition represented conditionsof relative N-deficiency resulting in relatively low tissue-Nconcentrations and a greater allocation of dry mass to the roots.The physiological N status of nodulated M. gale plants was comparativelygood, as indicated by a normal nodule weight ratio and a relativelyhigh N2-fixing rate per unit nodule mass. However, whole-plantN2-fixing capacity remained relatively low in comparison withacquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directlyexcreted as an OH efflux. H + /N ratios generally agreedwith the theory. In comparison with NH4+ nutrition, carboxylateconcentrations were higher in N2-fixing M. gale plants and theH + /N ratio in nodulated plants was less than unity below thevalue for ammonium plants as previously found for other actinorhizalspecies. Therefore, NH4+ should be an energetically more attractiveN source for actinorhizal plants than N2. The results agree with commonly accepted views on energeticsof N uptake and assimilation in higher plants and support theconcept of a basically similar physiological behaviour betweennon-legumes and legumes. Key words: Actinorhizal symbioses, ammonium, H+/OH efflux, nitrate, N2 fixation, NRA  相似文献   

10.
White lupin (Lupinus albus L.) develops proteoid (cluster) rootsin response to phosphorus deficiency. Proteoid roots are composedof tight clusters of rootlets that initiate from the pericycleopposite protoxylem poles and emerge from every protoxylem polewithin the proteoid root axis. Auxins are required for lateralroot development, but little is known of their role in proteoidroot formation. Proteoid root numbers were dramatically increasedin P-sufficient (+P) plants by application of the syntheticauxin, naphthalene acetic acid (NAA), to leaves, and were reducedin P-deficient (-P) plants by the presence of auxin transportinhibitors [2,3,5-triiodobenzoic acid (TIBA) and naphthylphthalamicacid (NPA)]. While ethylene concentrations in the root zonewere 1.5-fold higher in -P plants, there was no effect on proteoidroot numbers of the ethylene inhibitors aminoethoxyvinvylglycine(AVG) and silver thiosulphate. Phosphonate, which interfereswith plant perception of internal P concentration, dramaticallyincreased the number of proteoid root segments in +P plants.Activities of phosphoenolpyruvate carboxylase (PEPC), malatedehydrogenase (MDH) and exuded acid phosphatase in proteoidroot segments were not different from +P controls when NAA wasapplied to +P lupin plants, but increased to levels comparableto -P plants in the phosphonate treatment. Addition of TIBAor NPA to -P plants reduced PEPC and MDH activity of -P proteoidroots to levels found in +P or -P normal root tissues, but didnot affect acid phosphatase in root exudates. These resultssuggest that auxin transport from the shoot plays a role inthe formation of proteoid roots during P deficiency. Auxin-stimulatedproteoid root formation is necessary, but not sufficient, tosignal the up-regulation of PEPC and MDH in proteoid root segments.In contrast, phosphonate applied to P-sufficient white lupinelicits the full suite of coordinated responses to P deficiencyCopyright2000 Annals of Botany Company Lupinus albus L., white lupin, proteoid roots, auxin, ethylene, phosphonate, phosphorus deficiency  相似文献   

11.
The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na2SO4 salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na+ salts. Increased growth, however, appearedto be due to a K+-sparing effect of Na+ rather than to salinityper se. The distribution of K+ and Na+ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K+ and Na+ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na+ from the young leaf lamina. Measurementsof net fluxes of K+ and Na+ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio SK, Na* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K+ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K+ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K+ retranslocation, Bladder hairs, Growth stimulation  相似文献   

12.
Nodules from six Casuarina symbioses (Casuarina cunninghamiana,C. equisetifolia, and C. glauca inoculated with each of twodifferent Frankia sources) were evaluated for (i) concentrationsof haemoglobin (measured as CO-reactive haem) at 124, 144 and165 d after inoculation, (ii) acetylene reduction and (iii)occurrence of lignin-like compounds in cell walls of the nodulesat 165 d after inoculation. Haemoglobin concentration and theoccurrence of lignin-like compounds were related to acetylenereduction at final harvest. Concentration of haemoglobin innodules ranged from 1.56 to 22.27 nmol haem (g FW)-1. Therewere marked plant species-Frankia interactions. At final harvestplant growth was greatest in C. cunninghamiana inoculated withFrankia inoculum designated SI, while haemoglobin concentrationwas intermediate in this symbiosis. Acetylene reduction activitywas detected in all the symbioses except C. equisteifolia inoculatedwith SI, and was highest in C. glauca inoculated with SI. Haemoglobinconcentration per fresh weight nodule and acetylene reductionper plant were positively correlated (r= 0.84, n= 12). The highestlevels of cumulative nitrogen fixation were always associatedwith the highest haemoglobin concentrations and the highestrates of acetylene reduction activity. Nitrogen content waslowest in C. equisetifolia inoculated with SI (23.89±3.07mg) and highest in C. cunninghamiana inoculated with the sameinoculum (217.2±22.5 mg). Irrespective of symbiotic performance,lignin-like compounds were found in the cell walls of the nodulesin all symbioses in this study. The cells containing these compoundswere mainly localized in the infected areas of the nodule. Theroles of haemoglobin and occurrence of lignin-like compoundsin relation to nitrogen fixation in Casuarina symbioses arediscussed.  相似文献   

13.
Various plant and environmental factors influence the hydraulicproperties for roots, which were examined using negative hydrostaticpressures applied to the proximal ends of individual excisedroots of a common succulent perennial from the Sonoran Desert,Agave deserti Engelm. The root hydraulic conductivity, Lp, increasedsubstantially with temperature, the approximately 4-fold increasefrom 0.5°C to 40°C representing a Q10 of 1.45. Suchvariations in Lp with temperature must be taken into accountwhen modelling water uptake, as soil temperatures in the rootzone of such a shallow-rooted species vary substantially bothdaily and seasonally. At 20°C, Lp was 2.3 x 10–7 ms{macron}1MPa{macron}1for 3-week-old roots, decreasing to abouthalf this value at 10 weeks and then becoming approximatelyhalved again at 6 months. For a given root age, Lp for rainroots that are induced by watering as lateral branches on theestablished roots (which arise from the stem base) was aboutthe same as Lp for established roots. Hence, the conventionalbelief that rain roots have a higher Lp than do establishedroots is more a reflection of root age, as the rain roots tendto be shed following drought and thus on average are much youngerthan are established roots. Unlike previous measurements onroot respiration, lowering the gas-phase oxygen concentrationfrom 21% to 0% or raising the carbon dioxide concentration from0.1% to 2% had no detectable effect on Lp for rain roots andestablished roots. Lp for rain roots and established roots wasdecreased by an average of 11% and 35% by lowering the soilwater potential from wet conditions (soil=0 kPa) to {macron}40kPa and {macron}80 kPa, respectively. Such decreases in Lp mayreflect reduced water contact between soil particles and theroot surface and should be taken into account when predictingwater uptake by A. deserti. Key words: Gas phase, rain roots, root age, soil, temperature, water potential  相似文献   

14.
Determination of a Critical Nitrogen Dilution Curve for Winter Wheat Crops   总被引:31,自引:0,他引:31  
A set of N-fertilization field experiments was used to determinethe 'critical nitrogen concentration', i.e, the minimal concentrationof total N in shoots that produced the maximum aerial dry matter,at a given time and field situation. A unique 'critical nitrogendilution curve' was obtained by plotting these concentrationsNct (% DM) vs. accumulated shoot biomass DM (t ha-1). It couldbe described by the equation: Nct = 5·35DM-0·442 when shoot biomass was between 1·55 and 12 t ha-1. Anexcellent fit was obtained between model and data (r2 = 0·98,15 d.f.). A very close relationship was found using reducedN instead of total N, because the nitrate concentrations inshoots corresponding to critical points were small. The criticalcurve was rather close to those reported by Greenwood et al.(1990) for C3 plants. However, this equation did not apply whenshoot biomass was less than 1·55 t ha-1. In this case,the critical N concentration was independent of shoot biomass:the constant critical value Nct = 4·4% is suggested forreduced-N. The model was validated in all the experimental situations,in spite of large differences in growth rate, cultivar, soiland climatic conditions; shoot biomass varying from 0·2to 14 t ha-1. Plant N concentration was found to vary by a factor of fourat a given shoot biomass level. In the heavily fertilized treatments,shoot N concentration could be 60% higher than the criticalconcentration. Most (on average 80%) of the extra N accumulatedwas in the form of reduced N. The proportion of nitrate to totalN in shoot mainly depended on the crop stage of development.It was independent of the nitrogen nutrition level.Copyright1994, 1999 Academic Press Winter wheat, Triticum aestivum, arable crops, plant N concentration, aerial biomass, critical nitrogen, dilution curve, fertilization, reduced N, nitrate  相似文献   

15.
Arahou  M.  Diem  H.G. 《Plant and Soil》1997,196(1):71-79
The effect of iron deficiency, phosphorus, NaHCO3, chelator supply and nitrogen source on the formation of cluster (proteoid) roots was investigated in Casuarina glauca growing in water culture. The addition of iron-binding chelators (e.g. EDDHA, DTPA, EDTA) or increase in nutrient solution pH with NaHCO3 resulted in the formation of cluster roots when plants were grown in solution lacking iron. Phosphorus supply even at a concentration of 500 µM did not inhibit cluster root formation if EDDHA was added to the iron-deficient medium. Cluster root formation was influenced significantly by nitrogen source and occurred only in nitrate-fed plants.C. glauca seemed to be very sensitive to iron deficiency as shown by plant chlorosis when grown on alkaline soil. The symptoms of chlorosis decreased as the chlorophyll content in shoots and the number of cluster roots increased, suggesting that the alleviation of iron deficiency in plant tissues was correlated with cluster root formation. It appears that iron deficiency is more important than phosphorus deficiency in inducing the formation of cluster roots in C. glauca.  相似文献   

16.
Chickpea cultivar ILC 482 was inoculated with salt-tolerantRhizobium strain Ch191 in solution culture with different saltconcentrations added either immediately with inoculation or5 d later. The inhibitory effect of salinity on nodulation ofchickpea occurred at 40 dS m–1 (34.2 mol m–3 NaCl)and nodulation was completely inhibited at 7 dS m–1 (61.6mol m–3 NaCl); the plants died at 8 dS m–1 (71.8mol m–3 NaCl). Chickpea cultivar ILC 482 inoculated with Rhizobium strain Ch191spcstrwas grown in two pot experiments and irrigated with saline water.Salinity (NaCl equivalent to 1–4 dS m–1) significantlydecreased shoot and root dry weight, total nodule number perplant, nodule weight and average nodule weight. The resultsindicate that Rhizobium strain Ch191 forms an infective andeffective symbiosis with chickpea under saline and non-salineconditions; this legume was more salt-sensitive compared tothe rhizobia, the roots were more sensitive than the shoots,and N2 fixation was more sensitive to salinity than plant growth. Key words: Cicer arietinum, nodulation, N2 fixation, Rhizobium, salinity  相似文献   

17.
The relationship between plant water status and distributionof 14C-labelled assimilates in cacao (Theobroma cacao L.) wasevaluated after 14CO2 pulse labelling leaves of seedlings subjectedto varying levels of water deficiency. The proportion of 14Cexported by source leaves was strongly affected by seedlingwater status. An increasing proportion of labelled assimilatesremained in source leaves at both 24-h and 72-h harvests aswater stress intensity increased. Water stress reduced the distributionof exported label to leaves and to the expanding flush in particularbut increased the proportion of label in stems and roots. Theresults suggest that current photoassimilates may be temporarilystored in source leaves and stems of cacao seedlings duringperiods of plant water deficit. The stress-induced changes inpartitioning of labelled carbon were in concordance with changesin shoot to root biomass ratios, which was likely due to greaterreduction in growth of above-ground organs to that of roots. Theobroma cacao L, assimilate partitioning, cacao, 14C-photoassimilate, water stress, water potential  相似文献   

18.
Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N2, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m–2NO2 in the flowing solutions for 14 d, and the relativeuptake of N by N2, fixation and NO3 uptake was compared.Net uptake of K+ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N2-fixationper plant (in the absence of NO2 ) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N2 fixation: NO2 uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO3 uptake over N2 fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant–1),the decline in N2 fixation as root temperature increased above11 C was compensated for by the increase in NO 3 uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m–3at 3 C to 290 mol m–3 at 25 C. Trifolium repens L, white clover, root temperature, N2 fixation, potassium uptake, nitrate uptake, flowing solution culture  相似文献   

19.
The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m–3N, varying the N03/NH4+ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N03/NH4+ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH4+ by NO3 tended to enhance the plant's growth ratewhen compared to NH4+ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO3increasing from 0 to 100%. In the presence of NH4+, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH4+. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO3in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO3 [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO3 <0.20), D. flexuosashowed a strong preference for NO3 at low external nitrate(the relative uptake ratio for N03 reaching a value of2.0 at 10% NO3). The ecological significance of thisprominent high preference for NO3 at low NO3/NH4+ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N03/NH4+ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate  相似文献   

20.
Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (Js) or in permeabilityof the roots to water (Lp). This paper investigates the effectof ABA on Lp and Js separately. It is shown that Lp increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in Js. Inhibitionof Js was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na+, Mg2+, Ca2+, and phosphate as wellas the major ion in the exudate, K+. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.  相似文献   

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