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1.
A pulse-labelling technique is described for applying specificradio substrates to detached shoots via the transpiration stream.Unlabelled bleeding sap is used as a carrier for the substratesand as a chaser during a further period of assimilation. Thesubstrates studied are 14C-U-aspartic acid, 14C-U-asparagine,14C-U-gluta-mic acid, 14C-U-glutamine, 14C-homoserine, 14C-2-glycine,and metabolically labelled samples of bleeding sap. All substratescontribute carbon to chloroform-soluble and water-soluble substances,and to the protein and other insoluble constituents of the shoot.Each substrate makes a characteristic contribution of carbonto the soluble reserves and protein of the shoot. Members ofthe aspartate and glutamate families of amino-acids dominateamong the products labelled during assimilation of the fiveradiosubstrates which are regular constituents of the sap. Resultsare contrasted with feeding 14C-2-glycine, an amino-acid normallyabsent from the bleeding sap. 相似文献
2.
Cramer M.D.; Schierholt A.; Wang Y.Z.; Lips S.H. 《Journal of experimental botany》1995,46(10):1569-1577
In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO3 reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO3 from the root solution was strongly inhibitedby salinization. Consequently, NO3 concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO3, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO3 reduction in salinized plants. Lowerstem K+ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K+shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH14CO3 followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H14CO3 incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H14CO3 incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more 14C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO3 into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity 相似文献
3.
Under conditions of apoplastic unloading from the sieve element-companioncell (se-cc) complexes in fully-elongated stems of Phaseolusvulgaris plants, gjbberellic acid (GA3 stimulated in vitro uptakeof [14C]sucrose by the stem tissues. The GA3, response dependedupon the incubate containing calcium ions and being bufferedat pH 6. The GA3 action could be accounted for by a reductionin the Michaelis-Menten constant of the uptake process. Promotedtransport by GA3 in the decapitated stems resulted in all thetissues accumulating higher levels of [14C]photosynthates. Comparisonof this response with that for in vitro uptake of [14C]sucroseindicated that GA3 stimulation of the sucrose uptake processcontributed significantly to the accumulation of photosynthatesby the pith alone. The bulk of enhanced photosynthate accumulationby the remaining stem tissues can be accounted for by a GA,-inducedelevation of the apoplast sucrose concentration. In terms ofonset and change in rate, the time-course kinetics of GA3 stimulationof [14C]photosynthate transport and of in vitro [14CJsucroseuptake were found to be similar. It is proposed that GA3 promotionof photosynthate accumulation by the pith tissues is a minorcontributing factor to GA3 regulation of phloem translocation Phaseolus vulgaris L., french bean, stem, assimilate transport, gibberellic acid, rink accumulation 相似文献
4.
Respiratory gas exchange and incorporation of 14C-leucine intoprotein were studied in proximal root segments from 25-day-oldmaize plants grown for the last ten days in 50 mM Na2SO4. 14C-leucineincorporation, and oxygen uptake in the presence of glucose,were as large in Na2SO4-grown tissues tested under saline conditionsas in tissues exposed to non-saline solutions throughout Thisadaptation was attributed to an increased metabolic capacityof Na2SO4-treated tissues, because these tissues, when returnedto non-saline solutions, evolved oxygen and incorporated 14C-leucinefaster than tissues exposed continuously to non-saline solutions. These changes are interpreted as a compensationfor the inhibitory effects found when non-adapted tissues wereexposed to 50 mM Na2SO4. Moreover, we have related them to ultrastructuralchanges observed previously in xylem parenchyma cells of thesetissues, and to the possible involvement of these xylem parenchymacells in the re-absorption of sodium from the ascending xylemfluid Zea mays L., maize, salt-stress, respiration, protein synthesis 相似文献
5.
Comparisons of respiratory metabolism among the phloem, cambiumand xylem of mature carrot root were carried out and the followingresults were obtained. 1) The activity of O2 uptake on a dry weight basis in the cambiumwas higher than that in the xylem. The phloem showed the lowestactivity. 2) RQ was close to unity in all the three tissues.3) The inhibition rate of O2 uptake by malonate was considerablyhigh in the three tissues. The highest inhibition was observedin the cambium. 4) In the phloem and xylem the malonic inhibitionrate of 14CO2 release from G-U-14C was not so high as comparedwith that of O2 uptake. In the cambium, however, those of O2uptake and 14CO2 output were comparable. 5) Malonate treatmentdid not cause any significant change in RQ. 6) The C6/C1 ratiowas highest in the cambium and lowest in the phloem. From these it is concluded that the participation of the TCAcycle in respiration is great in all the three tissues of carrotroot, and among the three the cambium has the highest activityof the cycle. As for the PP pathway, the xylem and phloem havea higher activity than the cambium. (Received April 11, 1967; ) 相似文献
6.
SHERWIN HEATHER W.; PAMMENTER N. W.; FEBRUARY ED; VANDER WILLIGEN CLARE; FARRANT JILL M. 《Annals of botany》1998,81(4):567-575
Myrothamnus flabellifoliusWelw. is a desiccation-tolerant (resurrection)plant with a woody stem. Xylem vessels are narrow (14 µmmean diameter) and perforation plates are reticulate. This leadsto specific and leaf specific hydraulic conductivities thatare amongst the lowest recorded for angiosperms (ks0.87 kg m-1MPa-1s-1;kl3.28x10-5kg m-1MPa-1s-1, stem diameter 3 mm). Hydraulic conductivitiesdecrease with increasing pressure gradient. Transpiration ratesin well watered plants were moderate to low, generating xylemwater potentials of -1 to -2 MPa. Acoustic emissions indicatedextensive cavitation events that were initiated at xylem waterpotentials of -2 to -3 MPa. The desiccation-tolerant natureof the tissue permits this species to survive this interruptionof the water supply. On rewatering the roots pressures thatwere developed were low (2.4 kPa). However capillary forceswere demonstrated to be adequate to account for the refillingof xylem vessels and re-establishment of hydraulic continuityeven when water was under a tension of -8 kPa. During dehydrationand rehydration cycles stems showed considerable shrinking andswelling. Unusual knob-like structures of unknown chemical compositionwere observed on the outer surface of xylem vessels. These maybe related to the ability of the stem to withstand the mechanicalstresses associated with this shrinkage and swelling.Copyright1998 Annals of Botany Company cavitation, desiccation, hydraulic conductivity, refilling, resurrection plant, root pressure, xylem anatomy,Myrothamnus flabellifolius 相似文献
7.
Long Distance Transport of Abscisic Acid in NaCI-Treated Intact Plants of Lupinus albus 总被引:1,自引:0,他引:1
Plants of Lupinus albus were grown for 51 d under control (1.1mol m3 NaCl) and saline (40 mol m3 NaCl) conditions.Plants were harvested and changes of carbon, nitrogen and abscisicacid (ABA) contents of individual organs were determined 41d and 51 d after germination. In the period between the twoharvests xylem and phloem saps were collected and respirationand photosynthesis of individual organs were measured. Usingflows of carbon, C/ABA ratios and increments of ABA flows ofABA in phloem and xylem and rates of biosynthesis and degradationof ABA were calculated. Both under control and saline conditionsnet biosynthesis occurred in the root, the basal strata of leavesand in the inflorescence. Metabolic degradation of ABA tookplace in the stem internodes and apical leaf strata. Salt stress increased xylem transport of ABA up to 10-fold andphloem transport to the root up to 5-fold relative to that ofthe controls. A considerable amount of ABA in the xylem saporiginated from biosynthesis in the roots, i.e. 55% in salt-treatedand smaller than 28% in control plants. The remaining part ofABA in the xylem sap originated from the shoot: it was translocatedin the phloem from fully differentiated leaves towards the rootand from there it was recirculated back to the aerial partsof the plant. The data suggest that ABA may serve as a hormonalstress signal from the root system. Key words: Lupinus albus, salt stress, abscisic acid, long distance transport 相似文献
8.
Ammonium Nutrition in Ricinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps 总被引:2,自引:0,他引:2
Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinuscommunis: its effect on plantgrowth and the chemical compositionof the whole plant, xylem and phloem saps.J. exp. Bot.37: 15991610. The growth and chemical composition of Ricinus communis cultivatedhydroponically on 12 mol m 3 NO3-N were comparedwith plants raised on a range of NH4+-N concentrations. At NH4+-Nconcentrations between 0·5 and 4·0 mol m3,fresh- and dry-weight yields of 62-d-old plants were not significantlydifferent from those of the NO3-N controls. Growth wasreduced at 0·2 mol m3 NH4+-N and was associatedwith increased root. shoot and C: organic N ratios, suggestingthat the plants were N-limited. At 8·0 mol m3NH4+-N, growth was greatly restricted and the plants exhibitedsymptoms of severe NH4+ toxicity. Plants growingon NH4+-N showed marked acidification of the rooting medium,this effect being greatest on media supporting the highest growthrates. Shoot carboxylate content per unit dry weight was lower at mostNH4+-N concentrations than in the NO3-N controls, althoughit increased at the lowest NH4+-N levels. Root carboxylate contentwas comparable on the two N sources, but also increased substantiallyat the lowest NH4+-N levels. N source had little effect on inorganic-cationcontent at the whole-plant level, while NO3 and carboxylatewere replaced by Cl as the dominant anion in the NH4+-N plants.This was reflected in the ionic composition of the xylem andleaf-cell saps, the latter containing about 100 mol m3Cl in plants on 8·0 mol m3 NH4+. Xylem-saporganic-N concentration increased more than threefold with NH4+-N(with glutamine being the dominant compound irrespective ofN source) while in leaf-cell sap it increased more than 12-foldon NH4+-N media (with arginine becoming the dominant species).In the phloem, N source had little or no effect on inorganic-cation,sucrose or organic-N concentrations or sap pH, but sap fromNH4+-N plants contained high levels of Cl and serine. Collectively, the results suggested that the toxic effects ofhigh NH4+ concentrations were not the result of medium acidification,reduced inorganic-cation or carboxylate levels, or restrictedcarbohydrate availability, as is commonly supposed. Rather,NH4+ toxicity in R. communis is probably the result of changesin protein N turnover and impairment of the photorespiratoryN cycle. Key words: Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, Phloem, amino acids, carboxylate 相似文献
9.
Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O2destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N2-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled 15 was added to the base of plant pots. After 12 h7% of root N was from applied 15 and this increased to 75% at 7 d; substantial enrichment ofN from 15 also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from15, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith 15N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm3 following addition of 15 and at 7 d 80% of the N in the citrullinehad been derived from 15N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N2-fixation. 相似文献
10.
Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.J. exp. Bot. 39: 10151023. Well-nodulated, water-cultured soybean plants were allowed toassimilate 13CO2 at a constant specific activity for 10 h andthe 13C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of 13CO2 assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, 13CO2 assimilation, xylem 相似文献
11.
The Role of Phloem Transport in the Translocation of Sucrose Along the Stem of Carnation Cut Flowers
The pathway and distribution of 14C-sugars in flower parts havebeen examined to find out in which tissue sugars are translocatedin the stem of the cut carnation; 14C-sucrose or 14C-glucosewas supplied at the base of the cut stem from a feeding solutionand the localization and chemical nature of the carbon-14 recoveredfrom flower parts were investigated. By reducing the rate oftranspiration it was found that the uptake of feeding solutionwas also reduced, but the distribution of absorbed 14C-sucrosein the flower parts was different from that which would be expectedif sucrose moved exclusively in the transpiration stream. Autoradiographsdemonstrated that 14C absorbed from the feeding solution as14C-sucrose appeared in both xylem and phloem but predominantlyin the latter; girdling failed to stop the translocation ofthe absorbed 14C-sucrose. Results of experiments with 14C-sucroseand 14C-glucose showed that sucrose was the mobile sugar andthat glucose was converted to sucrose before it was translocated.It was concluded that the translocation of sucrose absorbedfrom the feeding solution takes place both in xylem and phloemand is regulated by a mechanism involving the loading and translocationof sucrose in the phloem. 相似文献
12.
When 14CO2 was applied to the leafy shoot of a stem segment,the xylem of which was perfused with distilled water, a proportionof the activity was detected in the xylem exudate, mainly inthe form of sucrose. Experiments in which 36Rb, 22Na, or 32P-phosphatewere applied to the cambial surface of a raised portion of barkrevealed that these solutes could also pass into the xylem exudate.With the ions, the amount which moved into the xylem vesselswas dependent upon the time of year at which the experimentswere performed. 相似文献
13.
Fluid extracted from grapevine shoots by suction contained upto 0.2 per cent (w/v) of sugar. The total sugar concentrationand the relative concentrations of sucrose, glucose, and fructosechanged along the length of a shoot. Whereas sucrose is absentfrom or present only in traces in fluid bleeding from cut stumps,it was found to be a major constituent in fluid extracted fromsections of wood by suction. 14C-glucose administered to thewood of an excised shoot moved up in the transpiration stream,moved laterally, into the bark, and was partly converted tosucrose and fructose. When 14CO2 was supplied to a leaf of anintact vine, radioactive sugars were found in the fluid afterwardsextracted from the wood by suction. When a ring of bark wasremoved from the stem at a point above the 14CO2-treated leafmost of the 14C in the bark, wood, and extracted fluid belowthe ring was in the form of sugar but 14C in tissues above thering was mainly in organic acids and amino acids. It is suggestedthat a barrier within the wood prevents diffusion of sugarsinto the transpiration stream, and that the fluid extractedfrom the wood by suction cannot be regarded as ascending sapsince it contains considerable amounts of non-moving material. 相似文献
14.
The xylem and phloem transport of mineral elements from stemnodal roots to the stem and stolon of growing potato (Solanumtuberosum L. cv. Russet Burbank) plants was investigated.Adventitious roots, originating from below-ground nodes of thestem of potato seedlings, were exposed to solutions of SrCI2or MnSO4. Relative elemental concentrations were measured inthe conductive tissues using energy dispersive X-ray analysis.After a 5 h daylight uptake period, Sr (a Ca-transport analogue)levels were elevated in the stem xylem tissue, but Sr did notincrease in the stem phloem, nor was it present in either ofthe conductive tissues of stolons located 12 nodes abovethe treated roots. In contrast, elevated levels of Cl, S, andMn were found in stolon xylem and phloem tissue during the sameperiod. The absence of Sr in the stolon after 5 h suggests thatno xylem flow into the stolon occurred during the uptake periodand, furthermore, phloem flow is responsible for the transportof the Cl, S, and Mn into the stolon. Elevated levels of thesemobile nutrients in the xylem of the stolon were attributedto xylem-to-phloem transfer in the stem or leaves, transportto the stolon in the phloem, and phloem-to-xylem transfer inthe stolon. During a 19 h uptake period, some Sr was observedin the phloem tissue of the stem, demonstrating slow exchangeof Sr with sieve elements or proximal phloem parenchyma andcompanion cells. Key words: Calcium, manganese, X-ray analysis 相似文献
15.
The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO2, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution 相似文献
16.
Broccoli (Brassica oleracea var. italica cv Premium Crop) plantswere germinated in soil, transferred to vermiculite three weekslater and grown in the glasshouse, then either supplied continuouslywith boron levels ranging from 0.0 (deficient) to 12.5 (toxic)mg l1 of nutrient solution or transferred from 2.5 to0.0 mg B l1 at the initiation of inflorescence development.At commercial maturity the concentrations of various inorganicand organic solutes in phloem exudates and xylem saps, as wellas plant characteristics and elemental composition of the variousplant parts, were determined. Under deficient B levels leaf midrib and stem corkiness wereevident, together with signs of stem pith breakdown, symptomswhich resemble the initiation of the hollow stem disorder. Thexylem sap B concentration declined by about 50 % when B wasnot supplied or was removed after a period of adequate supply;the phloem concentration was unaffected. Also, the decreasingB concentration gradients from mature transpiring tissues toyoung developing sinks disappeared. Therefore, it is concludedthat when B is deficient, it is retranslocated from source leavesin the phloem stream supplying the developing leaves and inflorescence.The data also suggested that at toxic levels B undergoes extensivelateral transfer, probably from xylem to xylem, thereby enhancingthe B concentration of developing sinks. The B regime influenced dry-matter partitioning, retranslocationof some elements, and the synthesis and distribution of aminoacids and sugars, reflecting the general nature of B involvementin plant processes. Brassica oleracea var., italica, broccoli, phloem, mobility, retranslocation, boron nutrition, transport fluids, concentration gradients 相似文献
17.
Twenty-four hours after leaf 3 of a plant of Lolium multiflorumLam, was supplied with a droplet of 14C-urea and the plant enclosedin a polyethylene bag with an untreated plant, there were significantamounts of radiocarbon recovered from the untreated plant. Theleaf treated with 14C-urea was the major source of 14C leakagebut significant losses were also recorded from other parts ofthe plant. Reducing the humidity within the bag decreased theamount of 14CO2 which escaped. Losses of radiocarbon from CO2-treated plants were very low compared with those from urea-treatedplants but the pattern of assimilate distribution within thetwo types of plants was very similar. The possible causes ofthese effects are considered and the usefulness of 14C-ureaas a source of 14CO2 discussed. 相似文献
18.
The effect of 0.5 and 5.0 mM sulphate concentrations and 15,25, and 35°C root temperatures on the redistribution of35SO4 in sunflower plants was studied at two rates of transpiration.It is concluded from the results that 35SO4 was lost rapidlyfrom the free space of the roots to the external solution andthat this losa was independent of the transpiration rates androot temperatures. The fact there was no marked decrease inthe 35SO4 content of the shoots during the washingprocess indicates that the free space is likely to be confinedto the cortical region of the roots, and there seems to be adiffusion barrier between the cortex and xylem preventing arapid movement of 35SO4 from the cortex to the xylem. 相似文献
19.
Specific conductivity (ks, m2s-1MPa-1) describes the permeability of xylem and is determined by all aspects of xylem anatomy that create resistance to the flow of water. Here we test the hypothesis that ks is a function of radial and vertical position within the stem, rather than solely a function of cambial age (ring number from the pith), by measuring ks on samples excised from 35-year-old Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco] trees at six heights and two or three radial positions. Sapwood ks decreased from the cambium to the heartwood boundary, and the difference between outer and inner sapwood increased with height in the tree. Beneath the live crown, inner sapwood had 80-90% the ks of outer sapwood, but only 55% just 10 m higher in the stem (about 10 nodes down from the tree top). Outer sapwood ks peaked near the base of the crown and declined toward both the base and top of the stem. These patterns can be explained by two superimposed effects: the effect of cambial age on the dimensions of tracheids as they are produced, and the effect of xylem aging, which may include accumulation of emboli and aspiration of bordered pits. Tracheid lumen diameter and earlywood and latewood density and width, all factors known to vary with cambial age, were measured on different trees of the same age and from the same stand. Lumen diameter increased with cambial age, whereas the proportion of latewood and growth ring density increased after an initial decrease in the first 5 years. Our results suggest that the effect of cambial age on xylem anatomy is not sufficient to explain variation in ks. Instead, physical position (both vertical and radial) in the stem and cambial age must be considered as determinants of conductivity. 相似文献
20.
Physiological responses to root pruning were investigated bycomparing 14CO2 fixation rates, the partitioning of 14C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced 14CO2 fixation by 2845% but leafspecific activity (14C assimilation g-1 leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of 14Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of 14C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of 14C betweensoluble and insoluble carbohydrate fractions when plants werefed 14CO2 several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants. 相似文献