The nature of the dual effect of auxin on root formation in Azukia cuttings

In disbudded Azukia stem cuttings, auxin exerted a dual effecton root formation. The first phase of auxin action is identifiedwith the acceleration of cell division, especially longitudinaldivision. In cuttings treated with auxin during the first 24hr, longitudinally divided cells were observed in all 12 rootprimordia, while in water-treated cuttings, such cells wereobserved only in 8 root primordia. The second phase is the promotionof the reaction in which root primordia unable to develop furtherwithout auxin supply develop into roots. Irrespective of thetreatment during the first 24 hr, the auxin-treatment duringthe second 24 hr increased the number of roots protruding fromthe cuttings. Portulal applied during the first 24 hr increased the numberof root primordia which contained longitudinally divided cells.Gibberellin applied during the first 24 hr inhibited both transverseand longitudinal divisions in root primordia. ¹ Supported in part by Grant No. 139011 from the Ministry ofEducation, Japan. ² Present address: Junior College of Toyo University, Hakusan,Bunkyo-ku, Tokyo 112, Japan. (Received June 13, 1978; )     

Auxin Transport in Secondary Tissues1

Auxin transport was investigated in excised stem segments ofNicotiana tabacum L. by the agar block technique using [1-¹⁴C]indol-3yl-acetic acid (IAA). The ability of the stems to transportauxin basipetally increased as secondary development proceeded;by contrast the ability of the pith to transport auxin declinedwith age. By separation of the stem tissues it was shown thatthe great majority of auxin transport took place in cells associatedwith the internal phloem and in cells close to the cambium;in both cases similar velocities of transport were found (c.5.0 mm h^–1 at 22°C). The effects of osmotic gradientson auxin transport through the internal phloem were investigated.IAA was found by chromatography to account for practically allthe radioactivity in receiver blocks and other extracts of stemsegments. The significance of these results is discussed.     

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; )     

ENTRY AND EXIT OF INDOLEACETIC ACID IN CORN COLEOPTILES

The entry and exit phases of radioactive indoleacetic acid transportwere investigated in corn coleoptile sections. Compounds capableof inhibiting auxin transport, particularly p-chloromercuribenzoicacid and N-1-naphthylphthalamic acid, were found to only slightlyblock auxin entry but severely suppress auxin exit. An oxygendeficiency had little effect on auxin entry but was found tostrongly inhibit auxin transport and auxin exit. While indoleaceticacid uptake was proportional to concentration, the exit phasebecame apparently saturated at concentrations above 10^–5M. Both entry and exit were found to have temperature coefficientsof about 2 or more. The low sensitivity of auxin entry to inhibitors,or to oxygen deficiency, and the linearity of entry over a wideconcentration range suggest a diffusion component in entry.The strong sensitivity of exit to inhibitors and to oxygen deficiencyconfirms the involvement of active processes in exit, as expectedof a secretion process. ¹Present address: Research Division, Ontario Water ResourcesCommission, Toronto, Canada. Indiana, U.S.A     

Comparative studies on auxin and fusicoccin actions on plant growth

Mode of action of FC was compared with that of auxin in differentexperimental systems and the following results were obtained.

1. FC, as well as auxin, primarily induced elongation of the epidermisof pea epicotyl segments, but it also promoted elongation ofthe inner tissue, as judged by its action in split stem tests,elongation of hollow-cylinder segments and elongation of unpeeledand peeled segments.
2. FC decreased the minimum stress relaxationtime (T₀) and increasedthe extensibility (mm/gr) of the epidermalcell wall of peaepicotyl segments, as did auxin.
3. FC failedto induce expansion growth of Jerusalem artichoketuber sliceswhen given alone or in combination with kinetinor gibberellicacid.
4. FC at concentrations lower than 10^–6 M, when givenwithauxin at concentrations lower than 0.03 mg/liter, promotedelongationof Avena coleoptile segments in an additive manner,to achievethe maximum elongation at higher concentrations.
5. An antiauxin, 2,4,6-trichlorophenoxyacetic acid, inhibitedtheelongation of Avena coleoptile segments due to auxin butnotthat due to FC.
6. Nojirimycin, an inhibitor of ß-glycosidases,inhibitedelongation of pea internode segments due not onlyto auxin butalso to FC.
7. At concentrations more than 10^–5MFC promoted root elongationof intact lettuce seedlings, whichwas inhibited by exogenousauxin.

From these results it is concluded that FC and auxin have acommon mechanism, which may involve hydrogen ion extrusion,leading to cell wall loosening and thus cell elongation. Thisgrowth is limited to the extent that the cells are capable ofelongating in response to hydrogen ions. Otherwise there isa definite difference in the mode of actions between FC andauxin, including the nature of cellular receptors for thesetwo compounds. (Received August 29, 1974; )     

Meristematic Activity during Adventitious Root Primordium Development: Influences of Endogenous Auxin and Applied Gibberellic Acid

Intact brittle willows (Salix fragilis L.) were treated so that developing adventitious root primordia in the stems would be subjected to elevated gibberellic acid or reduced endogenous auxin levels. Observations were made of primordia that were initiated during the experiments and of primordia that were established before the experiments began. The results indicated that as primordia became older and contained more cells, auxin basipetally transported in the stem seemed to be of less importance in determining cell number per primordium. Thus, established primordia depended upon this auxin to a lesser extent than primordia which were being initiated. These observations were explained on the basis of differential contributions during primordium development of cell division in the cambium of the stem and in the primordia themselves. As opposed to the effects of reduced auxin levels, applied gibberellic acid reduced the cell number per primordium most in established primordia. Initiating primordia were least affected by gibberellic acid treatment. Gibberellic acid treatment seemed mainly to reduce intraprimordium cell division, on which continued development of established primordia most depends. Seemingly, at least in brittle willow, applied gibberellic acid blocks the action of auxin in primordium development subsequent to the initiation phase.     

A Fine Structural Analysis of Auxin-induced Elongation of Cucumber Hypocotyls, and the Effects of Calcium Antagonists and Ionophores

The fine structure of epidermal cells, particularly in relationto dictyosomes, has been examined in different regions of dark-growncucumber hypocotyls and in response to auxin treatment, usingboth dot overlay and image analysis techniques. The most noticeablechange in cell structure along the hypocotyls is the increasein vacuolar volume. The volume fraction occupied by dictyosomesand secretory vesicles also increased, whereas that for mitochondriaremained relatively constant. During auxin treatment, the volumefraction for dictyosomes showed an increase after 30 min followedby a fall, whereas that occupied by secretory vesicles fellsteadily over 90 min. The number of cisternae per dictyosomeshowed some increase after 2 h of auxin treatment, althoughthe increase in dictyosomal material with cell expansion waslargely accounted for by an increase in the number of dictyosomes. Auxin-stimulated elongation growth of the hypocotyls was inhibitedby a range of calcium antagonists, chelators and ionophores.The most marked inhibitions were observed with calcium chloride,the chelator chlortetracycline and the ionophores verapamil,nigericin and monensin. Linear transducer experiments showedthat these compounds generally caused an immediate reductionin the rate of growth. Fine structural observations carriedout on epidermal cells showed the most obvious effects withmonensin and nigericin which caused dictyosomes and secretoryvesicles to swell. EGTA and LaCl₃ caused secretory vesiclesto accumulate around dictyosomes, while the ionophore A23187had little effect. The results suggest that the concentration of Ca²⁺ in the cytoplasmmay be critical for cell elongation. Compounds which chelateCa²⁺ appear to be more effective inhibitors of growth in theinitial acid-induced phase, whereas those which affect ionicgradients are more disruptive in the second phase.Copyright1993, 1999 Academic Press Calcium, Cucumis sativus hypocotyle, dictyosomes, elongation growth, indoleacetic acid, stereology     

Calcium Antagonist TMB-8 Inhibits Cell Wall Formation and Growth in Pea

The effects on auxin-stimulated growth and cell wall formationof 8-(N, N-diethylamino)-octyl-3, 4, 5-trimethoxybenzoate.HCI(TMB-8), an intracellular Ca²⁺ antagonist, were investigatedin abraded stem segments from aetiolated seedlings of Pisumsativum L. cv. Alaska. Incubation of segments at pH 6.0 with200 mmol m^–3 TMB-8 resulted in a 50% inhibition of auxin-stimulatedgrowth. Added Ca²⁺ did not restore normal auxin-stimulated growth,presumably because of its well-known stiffening effect on thecell wall. In segments incubated at a pH (7–2) which preventedelongation, auxin promoted the incorporation of [³H]glucoseinto the cell wall relative to total uptake of label. TMB-8abolished about 60% of the total incorporation of label intocell walls in the presence of auxin, but was not effective inthe absence of auxin. Exogenous CaCl₂ reversed the inhibitoryeffect of TMB-8 on relative cell wall incorporation in a parabolicmanner, with a 50% reversal at about 100 mmol m^–3 andcomplete reversal at 1.0 mol m^–3 Ca²⁺. Other ions tested(Mg²⁺, Mn²⁺, Cu²⁺, Zn²⁺) were without substantial effect atconcentrations of 0.5 mol m^–3. Both apparent uptake ofCa²⁺ and consequent reversal of TMB-8 inhibition of cell wallincorporation were blocked by the Ca²⁺ channel blockers verapamiland La³⁺. The data provide further evidence that auxin-stimulatedgrowth is dependent upon continued cell wall incorporation,and suggest that a Ca²⁺ messenger system may be involved inthe promotory actions of auxin on cell wall synthesis and long-termgrowth. Key words: Auxin, calcium, cell wall synthesis     

Potassium Transport in Non-Growing Corn Root Tissue as Affected by IAA and GA3

Experiments were done to determine if the spontaneous recoveryof non-growing segments of corn root (Zea mays L.) from excisioninjury is dependent on auxin. Washing the segments with 5 runindoleacetic acid (IAA) for 2 to 4 hours gave a small but significantincrease in K⁺ (⁸⁶Rb) influx, used here as a parameter reflectingrecovery of electrogenie H⁺-efflux pumping. This promotive effectwas obtained only after an hour of washing, and was sustainedby 100 nm gibberellic acid (GA₃). Any early responses to auxinwere obscured by an adverse reaction of the root cells to externalIAA which resulted in a transitory inhibition of H⁺ pumpingand K⁺ influx. Pretreatment of excised root tips with 10 µM IAA in thegrinding medium protected a plasmalemma-enriched fraction ofthe microsomes during isolation, giving increased uncoupler-sensitiveATPase activity. Non-growing root tissue thus shows three responses to auxin:an adverse reaction at the outer surface of the plasmalemmawhich blocks H⁺ pumping; a protective or restorative effecton the H⁺-ATPase; an increased capacity for K⁺ influx duringthe developmental phase of washing, which is augmented by thepresence of GA₃. (Received March 31, 1986; Accepted September 8, 1986)     

The Stimulatory Effect of Indole-3-Acetic Acid on the Uptake of Amino-Acids by Tissue 1Helianthus annuus

Indole-3-acetic acid was observed to bring about a prompt andmarked increase in the amount of ¹⁴C accumulated by segmentsof sunflower hypocotyl from solutions of labelled glutamic acid,glycine, and lysine. The curve relating magnitude of effectto indole-3-acetic acid concentration followed the comparablecurves for water uptake and extension growth. The accumulation of ¹⁴C was related to the external concentrationof glutamic acid by a curve which departed only slightly fromlinearity. The percentage increase in ¹⁴C accumulation broughtabout by auxin did not decline to any appreciable extent withincreasing external concentration of glutamic acid. Under nitrogen the amount of ¹⁴C taken up from solutions oflabelled glutamic acid in 1·75 hour was cut down by approximatelyone-third, and the auxin effect was abolished. The Q₁₀ for ¹⁴Caccumulation between 16° C. and 26° C. was 1·2in the absence of indole-3-acetic acid, and was 1·3 inits presence. When net water uptake was eliminated by the addition of mannitolto the external solution, ¹⁴C accumulation in auxin-free mediawas not depressed. The percentage increase in ¹⁴C accumulationbrought about by auxin, however, was markedly reduced. The fate of the ¹⁴C accumulated was investigated by means ofchromatography on resin columns and on filter paper. About 30–40percent, of the ¹⁴C was in the form of glutamic acid after approximatelya hours' treatment. No marked difference in the level of glutamicacid was observed between auxin-treated and control segments.The effect of auxin was more evident on the amounts of otherradioactive derivatives, as yet unidentified. It was observed that, not only was the amount of CO₂ evolvedin respiration higher in the presence of indole-3-acetic acid,but that this CO₂ was richer in ¹⁴C, i.e. in auxin-treated tissueglutamic acid formed a larger proportion of the substrate respired. The possible implications of these observations are discussed.It is pointed out that indole-3-acetic acid may have achievedits effect by stimulating a transfer process, by lessening adiffusion resistance, or by promoting a process or processeswhich, by removing free amino-acids within the cell, maintainan inward diffusion gradient.     

PRODUCTION OF YEAST VARIANTS BY AUXIN AND EFFECTS OF PLANT GROWTH REGULATORS ON THESE VARIANTS

Using diploid strains of Saccharomyces cerevisiae and S. ellipsoideus,the following facts were found:

1. Indole-3-acetic acid, 2,4-dichlorophenoxyacetic acid and -naphthaleneaceticacid produced stable variants differing in the cell form andin the response to the actions of auxin to elongate cells, toinduce respiration- deficient mutation and to promote sporulation.
2. The auxins also produced stable variants differing in theabilityto form spores.
3. Acetic acid had no above-menthionedactions of auxin.
4. Spore-formation and cell elongation of someof auxin-inducedvariants were controlled by auxin.

Biological significance of the auxin-induced variation is discussedand the usefulness of some of these variants as experimentalmaterial for auxin physiology in general is pointed out. (Received November 1, 1966; )     

Auxin Concentrations in Nodes and Internodes ofImpatiens sultani

Greater concentrations of auxin at nodes than in internodes,resulting from some nodal barrier to basipetal transport, havelong been postulated as the cause of early differentiation ofinitially isolated xylem and cambium at the nodes. However,this study, using [¹⁴C] indole-3-acetic acid (IAA) applied apicallyand gas chromatography-mass spectrometry, found that in stemsofImpatiens sultanithe IAA concentrations (per unit f. wt) atnodes were similar to those in adjacent internodes, though alittle greater at nodes if expressed per unit length of stemand a little less per unit d. wt. By contrast, in decapitatedshoots and in stem explants of dicotyledons, loss of the apicalsource of basipetally flowing auxin can result in auxin drainagewith some auxin retention in the uppermost remaining nodes.When [¹⁴C]IAA was applied apically to shoots for 4 h and stemexplants were excised, the explants had no nodal accumulationinitially whereas comparable explants incubated for 20 h revealedsignificant nodal accumulation. If decapitation leads both tonodal auxin accumulation and to adventitious abscission justabove the node, this fits the hypothesis that abscission sitesare positioned where auxin concentration decreases locally inthe apical direction. Difficulties in quantifying nodal auxindynamics are discussed, and some crude estimates of metabolicrates and locations of the auxin are presented.Copyright 1999Annals of Botany Company Abscission, auxin,Impatiens sultani, indole-3-acetic acid, node.     

Effect of cycloheximide and cordycepin on auxin-induced elongation and {beta}-glucan degradation of noncellulosic polysaccharides of Avena coleoptile cell wall

The effect of cycloheximide (10^–5 M) and cordycepin (10^–4M) used as protein and RNA synthesis inhibitors, respectively,on auxin action in noncellulosic ß-glucan degradationof Avena coleoptile cell wall was investigated. Both depressedauxin-induced ßglucan degradation of the cell wallas well as auxin-induced elongation and cell wall loosening,suggesting that the process of ß-glucan degradationof the cell wall is closely associated with cell wall looseningand that auxin enhances the activity of an enzyme for ß-glucandegradation through de novo synthesis of RNA and protein butnot through activation of the enzyme in situ. Kinetic studywith the inhibitors showed that RNA metabolism involved in ß-glucandegradation was stimulated by auxin treatment of only 15 minwhile a longer lag phase (about 1 hr) existed for the synthesisof the enzyme. (Received December 16, 1978; )     

Location of transported auxin in etiolated maize shoots using 5-azidoindole-3-acetic Acid

A study was undertaken using the photoaffinity labeling agent, tritiated 5-azidoindole-3-acetic acid ([³H],5-N₃IAA), to identify cells in the etiolated maize (Zea mays L.) shoot which transport auxin. Transport of [³H],5-N₃IAA was shown to be polar, inhibited by 2,3,5-triiodobenzoic acid (TIBA) and essentially freely mobile. There was no detectable radiodecomposition of [³H],5-N₃IAA within tissue kept in darkness for 4 hours. Shoot tissue which had taken up [³H],5-N₃IAA was irradiated with ultraviolet light to covalently fix the photoaffinity labeling agent within cells that contained it at the time of photolysis. Subsequent microautoradiography showed that all cells contained radioactivity; however, the amount of radioactivity varied among different cell types. Epidermal cells contained the most radioactivity per area, approximately twofold more than other cells. Parenchyma cells in the mature stelar region contained the next largest amount and cortical cells, sieve tube cells, tracheary cells, and all cells in the leaf base contained the least amount of the radioactive label. Two observations suggest that the auxin within the epidermal cells is transported in a polar manner: (a) the amount of auxin in the epidermal cells is greatly reduced in the presence of TIBA, and (b) auxin accumulates on the apical side of a wound in the epidermis and is absent on the basal side. While these results indicate that auxin in the epidermis is polarly transported, this tissue cannot be the only pathway since the epidermis is only a small fraction of the shoot volume. The greater than twofold difference between the concentration of auxin in the epidermal and subtending cells demonstrates that physiological differences in the concentration of auxin can occur between adjacent cells.     

Inhibition of Cell Division and DNA Synthesis by Gibberellin in Isolated Zinnia Mesophyll Cells

A culture system of isolated mesophyll cells of Zinnia eleganswas used to examine the action of gibberellic acid (GA) on celldivision. Isolated Zinnia mesophyll cells cultured in a mediumcontaining auxin and cytokinin reinitiated cell division ina partly synchronized manner. When mesophyll cells isolatedfrom 21-day-old seedlings were used, GA added to the culturemedium at concentrations of 1 x 10^–6 M or higher suppressedthe initial rise in the number of divided cells. Tracer experimentswith [³H]-dThd revealed that GA treatment inhibited the incorporationof [³H]-dThd into DNA in the nucleus without inhibiting theuptake of [³H]-dThd into the cells, indicating that GA inhibitedDNA synthesis. GA applied at 48 h inhibited the incorporationof [³H]-dThd into DNA during the following 24 h, but GA appliedat 72 h did not inhibit the incorporation during the subsequent24 h. This suggests that GA affects the process of reinitiationof DNA synthesis, but does not affect DNA synthesis once cellshave become proliferative. (Received January 14, 1986; Accepted March 31, 1986)     

Storage Organ Development in Radish (Raphanus sativus L.). 2. Effects of Growth Promoters on Cambial Activity in Cultured Roots, Decapitated Seedlings and Intact Plants

The radish varieties Cherry Belle and Long White Icicle wereused to investigate the role of the shoot and the effects ofsynthetic growth promoters in controlling cambial activity inthe seedling axis. Development was compared in excised roots, roots with hypocotylsattached and intact seedlings cultured aseptically on a nutrientmedium. No cambial divisions were seen in isolated radicleswhich had been cultured for ten days following excision butretention of hypocotyl tissue or the entire shoot resulted incambial activity and the production of secondary vascular tissues.Enriching the culture medium by raising the sucrose conantrationto 8% and including 10^–5 M indol-3yl acetic acid (IAA)5 x 10^–6 M 6-benzylaminopurine (BA) and 5 x 10^–4Minositol enhanced root thickening, increasing stele and xylemdiameters in roots cultured both with and without attached shoottissues. The effects of shoot tissues and enrichment of themedium were additive. The effects of auxin, cytokinin and gibberellin (gibberellicacid, GA²) were also studied on daxpitated seedlings. BA wasmuch more effective in inducing cell divisions in the hypocotylthan either IAA or GA supplied separately but a mixture of IAA+GAalso produced clearly defined arcs of cambial tissue. Littlesecondary tissue had been produced after seven days' treatment,and stelar enlargement was due to the development of a cambialzone and cell expansion in the primary tissues. Only minor differencesin response were observed between the two varieties. No stimulation of storage organ development occurred when auxin,cytokinin or inositol was inwrporated into the inorganic culturesolution in which plants of Cherry Belle were grown. Rnphanus sarivus, radish, storage organ, cambial activity, growth promoters, indol-3-ylacetic acid, 6-benzylaminopurine, gibberellic acid     

Early Effects of Auxin on Growth and Nucleic-Acid Metabolism in Sterile Pea Stems

A method of obtaining sterile pea stem sections has been developedand the effect of auxin on RNA metabolism has been investigatedusing aseptic techniques. Auxin rapidly stimulated ³²P-incorporationinto ethanol precipitated nucleic acid preparations. Subsequentexperiments showed that the bulk of the ³²P in these preparationswas present in a non-nucleic acid component which could be separatedfrom the nucleic acids by combined CM cellulose-Sephadex chromatography.This method of separation was used to show that auxin treatmentdoes not appear to stimulate RNA synthesis before stimulatingextension growth.     

Auxin and Ethylene Control of Growth in Seedlings of Zea mays L. and Avena sativa L

The effects of applied ethylene on the growth of coleoptilesand mesocotyls of etiolated monocot seedlings (oat and maize)have been compared with those on the epicotyl of a dicot seedling(the etiolated pea). Significant inhibition of elongation by ethylene (10 µll^–1for 24 h) was found in intact seedlings of all three species,but lateral expansion growth was observed only in the pea internodeand oat mesocotyl tissue. The sensitivity of the growth of seedlingparts to ethylene is in the decreasing order pea internode,oat coleoptile and oat mesocotyl, with maize exhibiting theleast growth response. Although excised segments of mesocotyland coleoptile or pea internode all exhibit enhanced elongationgrowth in IAA solutions (10^–6–2 ? 10^–5 moll^–1), no consistent effects were found in ethylene. Ethyleneproduction in segments was significantly enhanced by applicationof auxin (IAA, 10^–5 mol l^–6 or less) in all tissuesexcept those of the eat mesocotyl. Segments of maize show a slow rate of metabolism of applied[2-¹⁴C]IAA (30 per cent converted to other metabolites within9 h) and a high capacity for polar auxin transport. Ethylene(10 µl l^–1 for 24 h) has little effect on eitherof these processes. The oat has a smaller capacity for polartransport than maize and the rate ef metabolism of auxin isas fast as in the pea (90 per cent metabolized in 6 h). Althoughethylene pretreatment does not change the rate of auxin metabolismin oat, there is a marked reduction in auxin transport. It is proposed that the insensitivity of maize seedlings toethylene is related to the supply and persistence of auxin whichcould protect the seedling against the effects of applied orendogenously produced ethylene. Although the mesocotyl of oatis sensitive to applied ethylene it may be in part protectedagainst ethylene in vivo by the absence of an auxin-enhancedethylene production system. The results are discussed in relationto a model for the auxin and ethylene control of cell growthin the pea.     

Effects of Growth Regulators on the Induction of Anthocyanin Synthesis in Carrot Suspension Cultures

The effects of plant growth regulators were investigated onanthocyanin synthesis induced by removing auxin from carrotsuspension cultures. Of the auxins tested, 2,4-D showed thestrongest inhibiting effect on anthocyanin synthesis and hadthe strongest promoting effect on undifferentiated growth. When2,4-D was added to anthocyanin synthesizing cells, in whichcell division had ceased, anthocyanin synthesis was repressedimmediately, accumulated anthocyanin disappeared and cell divisionresumed. All cytokinins examined promoted anthocyanin synthesisin the absence of auxin. Both gibberellic acid (GA₃) and abscisicacid inhibited anthocyanin synthesis in media lacking 2,4-D,though GA³ showed no effect on cell division. These effectsof growth regulators on anthocyanin synthesis are similar tothose reported for their effects on embryogenesis [Fujimuraand Komamine (1975) Plant Sci. Lett. 5: 359, (1979) Z. Pflanzenphysiol.95: 13, (1980) Z. PJlanzenphysiol. 99: 1]. The relationshipbetween the induction of anthocyanin synthesis, metabolic differentiation,and embryogenesis are discussed. ¹ Present address: Department of Biology, College of Arts andSciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo153, Japan. ² Present address: Biological Institute, Faculty of Science,Tohoku University, Sendai, Miyagi 980, Japan. (Received November 28, 1985; Accepted July 23, 1986)     

Photolytic Decarboxylation of Carboxyl-14C-labelled Indol-3yl-acetic Acid in Leaves of the Apple Tree

The nature and rate of degradation of carboxyl-¹⁴C-labelledindol-3y1-acetic acid (IAA-[l-¹⁴C]) were studied in apple leaves.The labelled auxin was applied to the cut surface of the growingshoot after the apical part had been removed. The respiratoryCO₂ absorbed by chromatographic paper as Na₂CO₃ then freed byphosphoric acid was quantitatively measured by an internal gascounter. It was found that the concentration of ¹⁴CO₂ evolvedby leaves was 77 times higher in daylight than in darkness.The ratio of ¹⁴CO₂/CO₂ obtained from respiration from the uppersurface of leaf blades was two and seven times higher than thatfrom the lower surface after 15 and 30 h of daylight, respectively.No such differences were noticed in darkness. Similarly, thetotal radioactivity of leaf tissues tripled in daylight, presumablybecause of photosynthetic incorporation of radioactive CO₂ evolvedduring decomposition of LAA. These facts demonstrate the photolyticcharacter of auxin decarboxylation in apple leaves. Prolongeddarkness seemed to provoke a large metabolite withdrawal fromleaves and, to some extent, to protect auxin against decarboxylation.