Autolysis Promotes the Extension Capacity of Zea mays Coleoptile Cell Walls in Response to Acid pH Solutions

The relationship between autolytic degradation of ß(1–3),(1–4)-D-glucanand acid pH-induced extension of isolated Zea mays cell wallshas been investigated using a constant-load extension technique.Acidic buffer (4.5) was able to induce an additional extension(E_a) on cell walls already extended at pH 6.8 buffer under a20 g-mass load, indicating that the additional extension (E_a)was the parameter that better represented the effect of thedifferent treatments on the mechanical properties of maize coleoptilecell walls. The additional extension in response to acidic pHwas higher when cell walls had been previously autolysed for24 h at pH 5.5. Furthermore, the acid-pH effect was dependenton the presence during the constant load extension of some thermo-labilefactors, suggesting the participation of expansins. Acid pHincreased E_a of native cell walls through an increase in theplastic extension (E_p) in agreement with a one step mechanismleading directly to irreversible (plastic) wall extension assuggested by Cosgrove (1977). The autolytic degradation of ß(1–3),(1–4)-D-glucan was also able to modify the mechanicalproperties of maize coleoptile cell walls increasing its elasticextension (E_e) in response to pH 4.5 buffer but that modificationonly leads to an increase in wall extension when expansins areactive, suggesting a cooperation between ß-glucanturnover and expansin action. (Received August 5, 1998; Accepted March 16, 1999)     

Comparison of Growth Responses of Barnyard Grass (Echinochloa oryzoides) and Rice (Oryza sativa) to Submergence, Ethylene, Carbon Dioxide and Oxygen Shortage

Unlike germination of wheat (Triticum aestivum), millet (Eleucinecoracana), and sorghum (Sorghum caudatum), that of Echinochloaoryzoides (barnyard grass) and Oryza satwa (rice) was not inhibitedby poorly aerated solutions with 11 k Pa oxygen (equilibriumpartial pressure) or less In the dark, seedling shoots of riceincluded a coleoptile, and in Echinochloa, a mesocotyl alsoGrowth in fresh and dry weight of shoots was strongly depressedby poorly aerated solutions in both rice and Echinochloa butthe effects on extension differed in the two species in rice,coleoptile extension was promoted by solutions partly depletedof oxygen, and also by the absence of oxygen The stimulationin partly de-oxygenated solutions resulted from the combinedpromoting effects of small oxygen partial pressures, carbondioxide, ethylene and buoyant tension in contrast, these treatmentsneither promoted nor inhibited elongation by the Echinochloacoleoptile while severely inhibiting extension of the mesocotyl Overall, poorly aerated solutions lengthened the shoot of riceand shortened it in Echinochloa when compared with those submergedin well-aerated solutions These opposite effects were broughtabout by the same gaseous changes, i e oxygen shortage, elevatedethylene and carbon dioxide The effect on Echinochloa was almostentirely restricted to the mesocotyl, coleoptile extension beingremarkably insensitive to large increases in ethylene and carbondioxide, or to extreme oxygen shortage Seedlings of the twospecies thus have contrasting strategies for survival Stress, submergence, ethylene, oxygen shortage, carbon dioxide, adaptation, anaerobiosis, rice (Oryza sativa L), barnyard grass [Echinochloa oryzoides (Ard) Fritseh]     

Effect of Peeling on IAA-induced Growth in Avena Coleoptiles

The act of peeling removes the epidermis exclusively from Avenacoleoptiles. Peeling inhibits IAA-induced growth, by inhibitingthe growth of segments incubated in the presence of IAA, andpromoting that of those incubated in water. The magnitude ofthe inhibition of IAA-induced growth is proportional to theamount of epidermis removed. It is shown that neither lateralswelling, wounding, anaerobiosis, nor exposure to supraoptimalconcentrations of IAA cause the inhibition. It is concludedthat in Avena coleoptiles the epidermis regulates the rate ofexpansion of the underlying parenchyma cells and is the principaltarget of IAA-action. Avena sativa L., oat, coleoptile, indol-3-ylacetic acid, auxin, extension growth     

Cessation of cell elongation in rye coleoptiles is accompanied by a loss of cell-wall plasticity

The mechanism by which endogenous cessation of coleoptile elongationafter emergence of the primary leaf is brought about was investigatedin rye seedlings (Secale cereale L.) that were either grownin darkness or irradiated with continuous white light. In 3-d-oldetiolated (growing) coleoptiles a turgor pressure of 0.59 MPawas measured. In 6-d-old coleoptiles, which had ceased to elongate,cell turgor was 0.51 MPa and thus only 13% lower than in therapidly growing organ. Hence, the driving force for growth (turgor)is largely maintained. Cell-wall plasticity (E_pl) and elasticity(E_Ql were determined with a constant load extensiometer bothin vivo (turgid coleoptile segments) and in vitro (frozen-thawedsamples). Cessation of coleoptile elongation was correlatedwith a 95% reduction in E_pl9 whereas E_Ql was only slightly affected.Extension kinetics were measured with living and frozen-thawedsegments cut from growing and non-growing coleoptiles. The correspondingstress-strain (load-extension) curves indicate that the cellwall of the growing coleoptile behaves like an elastic-plasticmaterial whereas that of the non-growing organ shows the behaviourof an elastic solid. These data demonstate that E_pl representsa true plastic (irreversible) deformation of the cell wall.It is concluded that cessation of coleoptile growth after emergenceof the primary leaf is attributable to a loss of cell-wall plasticity.Hence, a mechanical stiffening of the cell wall and not a lossof turgor pressure may be responsible for the deceleration ofcell elongation in the rye coleoptile. Key words: Extension growth, rye coleoptile, cell-wall extensibility, turgor pressure     

The Effects of Oxygen, Carbon Dioxide and Ethylene on Ethylene Biosynthesis in Relation to Shoot Extension in Seedlings of Rice (Oryza sativa) and Barnyard Grass (Echinochloa oryzoides)

Exposing dark-grown seedlings for 3 d to oxygen deficiency (0or 5 kPa) or to additions of carbon dioxide (10 kPa) or ethylene(0·1 Pa) slowed shoot extension in Echinochloa oryzoides,while in rice it was promoted by these treatments, except that5 kPa oxygen was without effect. In E. oryzoides this was dueto reduced growth of the mesocotyl, and in rice to enhancedgrowth of the coleoptile. These responses to carbon dioxideand oxygen deficiency were not consequences of increased ethyleneproduction, since this remained unchanged by carbon dioxideand depressed by oxygen shortage in both species. Furthermore,exogenous ethylene and the ethylene action inhibitor 2,5-norbornadieneeach failed to influence extension in anoxic seedlings, indicatingno regulatory role for ethylene in the absence of oxygen. However,concentrations of the ethylene precursor 1 -aminocyclopropane-1-carboxylic acid (ACC) were increased by carbon dioxide and0 kPa or 5 kPa oxygen, although after 72 h without oxygen totalACC production (i.e. changes in ethylene + ACC + MACC) was suppressedin both species. There was little effect on bound ACC [putativemalonyl-ACC (MACC)] formation. Transferring anaerobic (0 kPa)seedlings to oxygenated conditions (21 kPa) resulted in abnormallyfast rates of ethylene formation, possibly due to the accumulationof ACC under anoxia. This post-anoxic ethylene may have contributedto the faster extension by rice coleoptiles and slower extensionby mesocotyls of E. oryzoides compared with those of seedlingsmaintained continuously in air. Echinochloa oryzoides [Ard.] Fritsch, barnyard grass, Oryza sativa L, rice, oxygen shortage, carbon dioxide, ethylene biosynthesis, shoot extension, 1-aminocyclopropane-1-carboxylic acid (ACC), malonyl-ACC, GC-MS     

Growth-regulator Interactions in the Growth of the Shoot System of Avena sativa Seedlings: I. THE GROWTH OF THE FIRST INTERNODE

1. Segments, 3.5 mm. long, cut from the first internode of Avenasativa seedlings grown in complete darkness respond to bothauxins and gibberellic acid by accelerated extension. 2. The optimum concentration of indole-3-acetic acid (IAA) is10 p.p.m. and of gibberellic acid (GA) is 0.1 p.p.m. 3. The degree of stimulation relative to the growth of controlsegments is affected by the inclusion in the segement of thenode between the internode and coleoptile. Thus the gibberellineffect is greatly increased while the IAA effect is decreased.The optimal concentrations are not affected by inclusion ofthe node. 4. These results can best be explained in terms of the supplyby the node tissue of an endogenous auxin which is necessaryfor the expression of GA action. 5. Numerous factorial experiments demonstrated that there isno detectable interaction between applied IAA and GA in thepromotion of first-internode extension. This implies that thepostulated endogenous auxin which synergized GAA action in (4)is either an active form of IAA produced only in the node tissueor is a completely different auxin. 6. No synergism of growth-promotive action can be detected betweenGA and the two synthetic auxins I-naphthylacetic acid and 2,4-dichlorophenoxyaceticacid. 7. p-chlorophenoxy-iso-butyric acid (PCIB) anc 2,4,6-trichlorophenoxyaceticacid (2,4,6-T) act as weak auxins and thus antagonize competitivelythe promotive action of GA. 8. The anti-auxin -(I-naphythyl-methyl-sulphide)propionic acid(NMSP) antagonizes competitively the promotive action of bothIAA and GA. 9. The facts under (5)–(8) suggest that auxins and GAare acting at the same growth-promotion centres and may competefor them. 10. Growth inhibitions are induced by high concentrations ofPCIB, 2,4,6-T and NMSP. The inhibitions produced by PCIB and2,4,6-T are both synergized by supra-optimal concentrationsof IAA while that of NMSP is synergized by supra-optimal concentrationsof both IAA and GA. This similarity of the effects of IAA andGA suggests that their inhibition actions also are of a closelysimilar nature.     

Cell elongation and metabolic turnover of the cell wall as affected by auxin and cell wall degrading enzymes

A cell wall fraction (pectic substances) of oat coleoptile segmentsfed with ¹⁴C-glucose contained more radioactivity under theeffect of auxin than did the control. When labeled segmentswere grown for 6 hr in auxin or glucanase solution the labelin the hemicellulose fraction decreased as growth increased.ß-1,3-Glucanase prepared from the culture of a fungus,Sclerotinia libertiana, induces elongation of segments of thepea stem and the oat coleoptile. Traces of cellulase and pectinmethylesterase contaminating the enzyme preparation are notresponsible for the stimulatory effect. Cellulase seemed tobe rather inhibitory and pectin methylesterase showed only aslight effect on coleoptile elongation. A possible relationshipbetween the metabolic turnover of hemicellulosic polysaccharideand cell wall extension is suggested. (Received February 5, 1968; )     

Anoxia tolerance in rice seedlings: exogenous glucose improves growth of an anoxia-'intolerant', but not of a 'tolerant' genotype

This study demonstrated that, in rice seedlings, genotypic differencein tolerance to anoxia only occurred when anoxia was imposedat imbibition, but not at 3 d after imbibition. When seeds wereimbibed and grown in anoxia, IR22 (anoxia-‘intolerant’)grew much slower and had lower soluble sugar concentrationsin coleoptiles and seeds than Amaroo (anoxia-‘tolerant’),while Calrose was intermediate. After 3 d in anoxia, the sugarconcentrations in embryos and endosperms of anoxic seedlingswere nearly 4-fold lower in IR22 than in Amaroo. Sugar deficitin the embryo of IR22 is presumably due to the limitation ofsugar mobilization rather than the capacity of transport asshown by similar sugar accumulation ratios of 1.8 between embryoand endosperm in IR22 and Amaroo at 3 d in anoxia. With 20 molm^–3 exogenous glucose, coleoptile extension and freshweight increments in anoxic seedlings of IR22 were much closerto those in the two other genotypes, nevertheless protein concentrationremained lowest on a fresh weight basis in the coleoptiles ofIR22; indicating that protein synthesis has a lower priorityfor energy apportionment during anoxia than processes crucialto coleoptile extension. In contrast to these responses to anoxiaimposed at imbibition, IR22 had nearly the same high toleranceto anoxia as Calrose and Amaroo, when anoxia was imposed onseedlings subsequent to 48 h aeration followed by 16 h hypoxicpretreatment. In fact, coleoptiles of anoxic IR22 had highersugar concentrations and grew faster than Calrose, and exogenousglucose had no effect on the coleoptile extension of IR22. Excisedcoleoptile tips of IR22 and Amaroo with exogenous glucose hadsimilar rates of ethanol production and were equally tolerantto anoxia. In conclusion, much of the anoxia ‘intolerance’of IR22 when germinated in anoxia could be attributed to limitedsubstrate availability to the embryo and coleoptile, presumablydue to slow starch hydrolysis in the endosperm. Key words: Anoxia, coleoptile, embryo, endosperm, ethanol production, germination, growth, Oryza sativa L., solute net uptake or loss, sugar availability.     

Auxin- and hydrogen ion-induced cell wall loosening and cell extension in Avena coleoptile segments

Hydrogen ions and auxin induce rapid cell extension of Avenacoleoptile segments. Nojirimycin (5-amino-5-deoxy-D-glucopyranose),a potent glucanase inhibitor, inhibits auxin-induced growthbut does not affect hydrogen ion-induced extension. This inhibitorhas little effect on respiration of coleoptile segments butstrongly inhibits the in vitro activity of ß-glucosidase.Hydrogen ions and auxin decreased the minimum stress-relaxationtime of the cell wall, indicating that both enhanced cell extensionthrough cell wall loosening. The hemicellulosic glucose contentof the cell wall which was decreased by auxin after about a2-hr lag, was not affected by hydrogen ions. These results suggestthat cell wall loosening induced by hydrogen ions may not bethe same as that caused by auxin, although both phenomena arerepresented by the decrease in the minimum stress-relaxationtime. (Received November 1, 1976; )     

Growth Substance Interactions during Uptake by Coleoptile Segments of Avena sativa and Hypocoty1 Segments of Phaseolus radiatus

Further studies have been made on the interactions of plant-growthregulators during uptake by Avena sativa coleoptile and Phaseolusradiatus hypocotyl segments. 2, 4-Dichlorophenoxyacetic acid(2, 4-D) had no effect on the uptake of either indol-3yl-aceticacid (IAA) or -naphthylacetic acid (NAA) by Avena. On the otherhand, a-(i-naphthylmethylthio)-propionic acid (NMSP) stronglyinhibited IAA uptake non-competitively but was much less effectiveon NAA uptake by Avena. The ‘metabolic’ uptake ofIAA by hypocotyl segments of Phaseolus radiatus was very stronglyinhibited by 2, 3, 5-tri-iodobenzoic acid (TIBA).     

Effect of {alpha}-Tomatine on the Response of Wheat Coleoptile Segments to Exogenous Indole-3-acetic Acid

A concentration of 10^–5 M tomatine had no effect on leakagefrom, or elongation of, wheat coleoptile segments, but consistentlyreduced IAA-enhanced extension growth by c. 50 per cent. Therewas no evidence of chemical interaction between the alkaloidand the auxin in solution, and IAA action was not affected bypre-treatment for up to 3 h with 10^–5 M tomatine. Studieswith [2-¹⁴C]IAA revealed that 10^–5 M tomatine did notinhibit uptake of auxin into segments. The effect of pre-treatingsegments for up to 3 h with IAA could be virtually nullifiedby 10^–5 M tomatine, as could also IAA-induced changesin properties of coleoptile cell walls. Results are discussedin relation to the ability of tomatine to disrupt membrane functionand to current hypotheses implicating membranes in the primaryaction of auxin.     

Involvement of Cell Wall-Bound Diferulic Acid in Light-Induced Decrease in Growth Rate and Cell Wall Extensibility of Oryza Coleoptiles

Irradiation of white fluorescent light (5 W m^–2) inhibitedthe growth of Oryza coleoptiles. Light irradiation increasedstress-relaxation parameters of coleoptile cell walls, minimumstressrelaxationtime and relaxation rate, and decreased cellwall extensibility (strain/load). Under light conditions, thecontents of ferulic and diferulic acids ester-linked to thehemicellulosic arabinose residue in cell walls increased andcorrelated with the modification of the cell wall mechanicalproperties. These results suggest that light irradiation enhancesthe formation of diferulic acid bridges in hemicelluloses, makingcell walls mechanically rigid and thus inhibits cell elongationin rice coleoptiles. Also, irrespective of coleoptile age orthe presence of light, the ratio of diferulic acid to ferulicacid was almost constant, suggesting that the rate limitingstep in the formation of diferulic acid bridges in Oryza cellwalls is in the step of feruloylation. (Received September 24, 1991; Accepted December 3, 1991)     

Studies in the Physiology of Parasitism: XXXIV. Further Experiments on the Killing of Plant Cells by Fungal and Bacterial Extracts

When cucumber and turnip tissues were subjected to enzyme preparationsof Botrytis cinerea and Bacterium aroideae in the presence ofplasmolysing concentrations of various crystalloids, killingof cells was retarded out of all proportion to the slight retardingeffect on maceration. When the tissue cells were recovered fromplasmolysis they regained their sensitiveness to the toxic action.Toxicity of mercuric chloride and oxalic acid was also muchreduced when cells were plasmolysed. Fractional precipitation with various proportions of acetoneand over a wide range of pH gave preparations which varied in.activity from one treatment to another, but their maceratingand toxic actions varied together. After partial maceration, plant tissues were much more sensitiveto the toxic action of mercuric chloride or oxalic acid. Thisis interpreted as meaning that, before maceration is complete,the toxic principle has been able to reach the protoplasticsurface and to exert a deleterious effect upon it. The evidence presented gives further support to the view thatthe enzyme system of the two pathogenic organisms which maceratesthe host cell walls also brings about death of the protoplasts.     

The Geoelectric Effect in Plant Shoots:: IV. INTER-RELATIONSHIP BETWEEN GROWTH, AUXIN CONCENTRATION AND ELECTRICAL POTENTIALS IN ZEA COLEOPTILES

Incubation of Zea coleoptiles in 0.5 M mannitol totally inhibitsgrowth and geotropic curvature, but does not affect the developmentof the geoelectric effect. This pre-treatment also inhibitsthe curvature induced by the asymmetrical application of IAAto the apical end of decapitated vertical coleoptiles, but itdoes not prevent the IAA from giving rise to an electropotentialdifference between the two sides of the coleoptile. Neitherthe normal geoelectric effect, nor the auxin-induced potentialdifference in vertical coleoptiles, can therefore arise as theresult of the different rates of cell extension in the two halvesof the organ. They must be the result of the change of IAA concentrationaffecting some other aspect of the cell's physiology or metabolism. The abolition of the electrical responses in coleoptiles whichhave been plasmolysed in 1.0 M mannitol strongly suggests thatboth longitudinal and lateral transport of IAA are severelydepressed by this degree of plasmolysis. Asymmetrical application of 10^-5 M mersalyl and several othersubstances to the apical end of a decapitated vertical coleoptilegave rise to a marked electropotential difference between thetwo sides of the coleoptile, the side beneath the donor beingpositively charged with respect to the other side. Mersalyldoes not promote the growth of Zea coleoptiles. These resultsprovide additional evidence that the electropotentials do notarise from differential growth, and suggest that such substances,especially the diuretics used in clinical medicine, may provideuseful tools in the further study of the induction of surfaceelectropotentials in plant tissues at the cellular level.     

The Relationship between the Growth of the Primary Leaf and of the Coleoptile in Seedlings of Avena and Triticum

Partial inhibition of extension growth of the primary leaf occurswhen whole Triticum seedlings are immersed in aerated solutionsof IAA but is replaced by growth promotion when sucrose is addedto the external solution. In seedlings in which the coleoptilehas been excised, IAA increases the growth of the leaf bothwith and without additional sucrose. Inhibition of the leaf by moderate concentrations of IAA nolonger occurs when the seedling is detached from the endosperm.Sucrose added to the external solution raised the percentageelongation of the coleoptile almost to the level of that attainedin intact seedlings without additional carbohydrate. It alsoenabled the leaf to show a positive growth response with IAA. The results indicate that in intact seedlings treated with IAAthe growth of the primary leaf is markedly diminished owingto diversion of carbohydrate to the coleoptile if the growthof the latter is promoted as a result of the treatment. Whenthe competition of the coleoptile for carbohydrate is diminishedor eliminated, acceleration of the growth of the primary leafby IAA becomes apparent. In addition to the endogenous rhythm, with a period close to24 hours, induced in the growth-rate of the coleoptile whenseedlings of Avena are transferred from red light to darkness,a similar rhythm, with a slightly longer period, is inducedin the growth-rate of the primary leaf. This rhythm persistsin elongating leaves so long as they remain within the coleoptile.It can be recorded for at least 100 hours in deseeded seedlings. When intact seedlings of Avena are immersed for one hour inrelatively high concentrations of IAA and then transferred todistilled water for 18 hours, the elongation of the coleoptileis greater and the inhibition of the leaf is less than whenthey are transferred to humid air. Sections of the leaf of Triticum showed a slight increase inelongation in concentrations of IAA up to 5 mg./l., but no evidencewas obtained that sections of leaf and coleoptile exert any.influenceon each other's elongation when floated together on solutionsof IAA.     

Gibberellic Acid Sensitivity Determines the Length of the Extension Zone in Wheat Leaves

To test the hypothesis that gibberellic acid (GA) sensitivityaffects the length of the extension zone (LEZ) of leaf No. 1of wheat seedlings, we performed a gene dosage experiment usingRht dwarfing genes that condition GA insensitivity. We utilizednearly isogenic lines, at Rht-dosage levels of 0, 2 and 4 alleles.Anatomical markers (distances between successive stomates) wereused to infer the distribution of growth along the axis of theleaf. Interstomatal distance (ISD) and LEZ were inverse linearfunctions of Rht-dosage. The number of stomates matured perhour was independent of Rht-dosage. The relationship betweenISD and distance along the axis within the extension zone (EZ)was indistinguishable from linear. Rht-dosage did not affectthe slope of the regression of ISD against distance along theEZ. A-REST (AR; ancymidol, a potent GA synthesis inhibitor)reduced LEZ. Wild type was more sensitive to AR than doubledwarf. AR affected growth of leaf No. 1 more than length ofthe coleoptile, regardless of Rht-dosage. AR-dosage affectedcell division, whereas Rht-dosage did not. Extension zone, elongation, gibberellic acid, Rht, wheat, Triticum aesiivum L.     

Biogenesis of auxin in the coleptile of a semi-brachytic barley, uzu

The coleoptile of a semi-brachytic barley, uzu(Hordeum vulgareL. cv. Akashinriki), elongated ca 1/2 as much as the coleoptileof the contrasting normal form which is isogenic excepting theuz gene. This retarded growth of the uzu coleoptile, as comparedto the normal coleoptile, is not due to changes in the rateof basipetal transport of auxin, neither to the destructionof auxin during transport, nor to sensitivity to auxin. Furthermore,less of the extractable and bound auxins were found in the uzu(uzuz)coleoptile than in the normal(UZUZ)coleoptile, suggesting thatthe retarded growth of uzu coleoptile may be due to less auxinproduction. Apical tips of the normal coleoptile grown under sterile conditionsresponded to both tryptophan and tryptamine, but uzu coleoptiletips responded only to tryptamine. Thus, growth retardationof the uzu coleoptile may be due to lower activity of the enzymewhich converts tryptophan to tryptamine in the uzu coleoptile. (Received August 20, 1973; )     

Further Studies on the Acid Metabolism of Aspergillus niger: THE FORMATION AND UTILIZATION OF OXALIC ACID

1. Some recent works on the formation of oxalic acid by variousfungi are critically considered.
2. The present work deals withthe role of oxalic acid in the metabolismof Aspergillus niger.
3. When glucose solutions were supplied to preformed mats ofthefungus oxalic acid accumulated, attaining an equilibriumlevelwhich was not exceeded despite the presence of a considerableconcentration of glucose.
4. When the glucose supplies were depletedthe oxalic acid concentrationfell steeply to a low level.
5. Theconcentration of oxalic acid was dependent on the glucoseconcentration.In three separate series of experiments it wasshown that theoxalic acid concentration diminished with increasingglucoseconcentration.
6. Similar results were obtained when the cultureswere rearedfrom spores on culture solutions with the normalamounts ofnutrient salts but different glucose concentrations.
7. In all cases the CO₂ output increased with the glucose concentration.
8. When cultures were supplied with glucose+oxalic acid, theconcentrationof the latter fell steeply to the equilibriumlevel attainedon glucose only. In a culture receiving glucose+oxalicacid,with the oxalic acid concentration somewhat below thenormalequilibrium concentration, the formation of oxalic acidfromthe glucose ceased as soon as the equilibrium level hadbeenattained.
9. When 1 per cent. oxalic acid only was suppliedto the fungusthe concentration gradually diminished to a lowlevel. When3 per cent. oxalic acid was supplied the rate ofacid utilizationsoon fell to low value.
10. In several experimentsit was shown that the rate of CO₂ outputwas higher from culturessupplied with glucose+excess oxalicacid than from culturessupplied with glucose only.
11. The rate of oxalic acid carbonloss was always below that ofthe CO₂ carbon output both incultures supplied with oxalicacid only and in cultures receivingglucose+oxalic acid.
12. The cultures were incapable of utilizingneutral sodium oxalateand the presence of this substance hadno effecft on the ofCO₂ output.
13. The results indicate thatthe utilization of oxalic acid isassociated with the liberationof at least an equivalent amountof CO₂.
14. It is suggested thatthe utilization of oxalic acid is promotedby the presence ofglucose, thus accounting for the lower oxalicacid concentrationsand higher rates of CO₂ output of cultureswith higher glucoseconcentrations.

     

The Geoelectric Effect in Plant Shoots: III. DEPENDENCE UPON AUXIN CONCENTRATION GRADIENTS AND AEROBIC METABOLISM

The development of the geoelectric effect has been followedin Zea coleoptiles with a flowing-solution electrode system,and its dependence upon auxin concentration gradients and aerobicmetabolism assessed. A symmetrical source of IAA can effectively replace the coleoptiletip in allowing the geo-electric potential to occur. The diffusatefrom coleoptile tips, when applied asymmetrically to the apexof a vertical decapitated coleoptile, generates a potentialdifference across the coleoptile indistinguishable from thatinduced by the asymmetrical application of IAA. Asymmetricalapplication of IAA to vertical Avena and Zea coleoptiles andHelianthus hypocotyls induces closely similar responses. Neither the geoelectric effect nor a geotropic response developswhen intact Zea coleoptiles are placed horizontally after beingdeprived of oxygen, but they both occur when an aerobic atmosphereis restored. The lateral potential difference induced by theasymmetrical application of IAA to the apex of a vertical coleoptiledoes not occur under anoxic conditions. With a static-drop electrode system and a decapitated Zea coleoptile,a potential difference develops immediately after reorientationof the coleoptile into the horizontal position, and attainsa maximum value after about 10 min. This potential differencecan be further increased by the asymmetrical application ofIAA to the lower half of the apical cut surface of the coleoptile. Our data support the view that both the geoelectric potentialand the geotropic response are due to the IAA concentrationgradient which arises from the lateral transport of this substancefrom the upper to the lower half of the horizontal shoot. Theyalso bear out our previous conclusions that the ‘geoelectricpotential’ observed with static-drop electrodes and anintact shoot, is the resultant of two processes. The first isa physical phenomenon arising in the electrodes, or betweenthe electrodes and the plant tissue, and the second arises inthe living tissues of the shoot as the result of gravity-inducedchanges in auxin distribution.     

Metabolism of [14C]Indole-3-Acetic Acid by Coleoptiles of Zea mays L.

Reverse-phase high-performance liquid chromatography was usedto analyse [¹⁴C]-labelled metabolites of indole-3-acetic acid(IAA) in coleoptile segments of Zeo mays seedlings. After incubationfor 2 h in 10^–2 mol m^–3 [2-¹⁴C]IAA, methanolic extractsof coleoptiles contained between six and ten radioactive compounds,one of which co-chromatographed with IAA. The metabolic productsin coleoptile extracts appeared to be similar to those in rootextracts, with an oxindole-3-acetic-acid-like component as theprincipal metabolite, but the rate of metabolism was slowerin coleoptile than in root segments. Decarboxylation did notappear to play a major role in the metabolism of exogenous IAAduring the short incubation periods. Moreover, external IAAconcentration had little effect on the pattern of metabolism.Coleoptile segments were also supplied with [¹⁴C]IAA from agardonor blocks placed at the apical ends, and agar receiver blockswere placed at the basal ends. After incubation for 4 h, theidentity of the single radioactive compound in the receiverblocks was shown to be IAA by both reverse-phase high-performanceliquid chromatography and gas chromatography-mass spectrometrytechniques. Key words: Zea mays, Coleoptile, High-performance liquid chromatography, Indole-3-acetic acid