The glycollate pathway during the photoassimilation of acetate by Chlorella

Summary When Chlorella pyrenoidosa photoassimilates ³H–¹⁴C-acetate glycollic acid rapidly becomes labelled with both tritium and ¹⁴C. The ³H/¹⁴C ratio was 10 in glycollate, (compared with 4 in the acetate added) and the only other intermediates showing similar ³H/¹⁴C ratios to glycollate were glycerate and serine. This suggests a glycollate pathway for the formation of serine was operating in Chlorella pyrenoidosa during the photoassimilation of acetate. When Chlorella pyrenoidosa assimilated ³H–¹⁴C-acetate in the dark glycollate was not labelled with either ¹⁴C or tritium. Although glycerate and serine both became labelled with ¹⁴C and tritium in the dark they did not show the high ³H/¹⁴C ratios recorded in the light. When cells were aerated with unlabelled 5% CO₂ during the photoassimilation of ³H–¹⁴C-acetate, the ³H/¹⁴C ratios of glycollate, glycerate and serine were slightly decreased. Similarly, under anaerobic conditions in the light the ³H/¹⁴C ratio was decreased compared with aerobic conditions.     

The Role of Glycollic Acid in the Photoassimilation of Acetate by Chlorella pyrenoidosa

The kinetics of ³H-acetate assimilation by Chlorella pyrenoidosain the light were examined. The primary products of assimilationwere glycollate and succinate. After 10 sec glycollate contained45 per cent and succinate 25 per cent of the tritium incorporatedby the cells. The percentage of the total tritium in glycollateand succinate fell with time while that in citrate increased.Initially the specific activities (µc of ³H per µmoleof acid) of succinate and glycollate were greater than citrate.When ³H-¹⁴C-2-acetate was added to the cells, total dpm for³H and ¹⁴C in glycollate rapidly reached a steady state andgave a ³H/¹⁴C ratio of 10, compared with a ³H/¹⁴C ratio of 4in the acetate. This ³H/¹⁴C ratio in glycollate is found because³H is derived from ³H-¹⁴C-2-acetate and because the ¹⁴C is dilutedwith cold carbon from elsewhere. The addition of ¹⁴CO₂ at thesame time as ³H-¹⁴C acetate decreased the ³H/¹⁴C ratio in glycollatebut incorporation of ¹⁴C from ¹⁴CO₂ into glycollate was slowerthan incorporation from ¹⁴C-2-acetate. Although ¹⁴C from acetaterapidly appeared in glycollate, ¹⁴C-labelled glyoxylate wasnot detected. The ³H/¹⁴C ratio observed in glycollate rulesout formation of glycollate from acetate via glycoaldehyde.The available evidence did not support glycollate formationvia the Calvin cycle. ¹⁴C from ¹⁴C-Z-acetate appeared in glycollatebefore it did in phosphoglyceric acid. Total dpm for ³H, ¹⁴C,and ³H/^l4C ratio in Calvin cycle intermediates were not in equilibriumwith glycollic acid.     

Glycollate formation during the photoassimilation of acetate by Chlorella

Summary When ³H-¹⁴C-acetate was supplied to Chlorella pyrenoidosa in the light, glycollic acid became rapidly labelled with tritium and ¹⁴C. The ratio of glycollate was 10, whilst the ratio was 4 in the acetate added. Both ³H and ¹⁴C from acetate were present in glycollate before they were present in Calvin cycle intermediates, so that glycollate was not formed as a C₂-fragment from the Calvin cycle.     

The Effect of Hydroxymethanesulphonate on Photosynthesis in Chlorella pyrenoidosa

Photosynthesis under conditions known to favour glycollate excretionby algae did not result in glycollate excretion in a strainof Chlorella pyrenoidosa unless an inhibitor of glycollate oxidase,-hydroxypyridin-2yl-methane sulphonate (-HPMS), was present.This inhibitor increased the total amount of glycollate presentin the supernatant from the cells during photosynthetic carbondioxide fixation and gave accumulation of ¹⁴C in glycollateduring ¹⁴CO₂ fixation under conditions favouring glycollatesynthesis. At pH 8.3 -HPMS did not stimulate photosynthetic¹⁴CO₂ fixation in C. pyrenoidosa as occurs with some algae.Photoassimilation of acetate was inhibited by -HPMS, and thiswas shown to result from acetyl-CoA synthetase inhibition by-HPMS.     

The Photometabolism of Acetate by Chlorella pyrenoidosa

Chlorella pyrenoidosa can utilize sodium acetate as a carbonsource for growth in the light. Growth proceeds under aerobicconditions both in the presence and in the absence of carbondioxide, but under anaerobic conditions only in its presence.The assimilation of acetate does not result from oxidation tocarbon dioxide followed by photosynthetic fixation because theproducts of ¹⁴C-acetate assimilation are different from theproducts of ¹⁴CO₂ fixation in the presence of unlabelled acetate. In aerobic conditions 10-⁶ M DCMU induces a pattern of acetateassimilation in the light similar to that in the dark. Thus,in the presence of DCMU in the light, less acetate carbon isincorporated into cells, particularly into lipids, polysaccharide,and protein, and more is released as carbon dioxide than inits absence. The effect of 4 x 10^-3 M MFA on acetate assimilationin the presence of 10^-6 M DCMU is the same in light and dark.Acetate assimilation is unaffected by desaspidine and sodiumbisulphite. The mean generation time of C. pyrenoidosa growing on acetatein the light under aerobic conditions is 20 hours. When 10^-5M DCMU is added the mean generation time is 60 hours, the sameas that for Chlorella growing on acetate in the dark. The activityof the enzymes of the glyoxylate cycle, isocitrate lyase (E.C.4.1.3.1.)and malate synthetase (E.C.4.1.3.2.) is repressed in the light,but activity of both enzymes increases markedly when DCMU isadded.     

Some Physiological Responses of Chlorella pyrenoidosa to 2,4-Dichlorophenoxyacetic Acid

An 18-h treatment of synchronously-grown Chlorella pyrenoidosawith 2,4-D did not significantly alter the size, dry weight,degree of synchrony, or pigment content of the cells, nor weredetectable quantities of ethylene produced. When Chlorella pyrenoidosawas treated with 5?10^–4 M 2,4-D, there was a statisticallysignificant stimulation of both net oxygen uptake and productionwhile 5?10 M 2,4-D inhibited both processes. When Chlorellapyrenoidosa was treated with 5?10^–4 M and 5?10^–3M 2,4-D, significantly greater amounts of glycollate were presentin the culture medium, even though an assay for glycollate dehydrogenaseshowed that the activity of this enzyme from 2,4-D-treated Chlorellapyrenoidosa was three times greater than in control cells. Looselybound 2,4-D was partitioned from a nonaqueously isolated chloroplastfraction, while other cell fractions failed to show detectablequantities of 2,4-D. It is postulated that in Chlorella pyrenoidosathe chloroplast is a target for 2,4-D action and that interferencein photorespiratory processes may underlie the observed responses.     

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna     

Effects of Certain Inhibitors on Photorespiration by Wheat Leaf Segments

The effect on the carbon metabolism of wheat leaf segments ofcertain inhibitors of photorespiration was studied. Sodium 2-hydroxy-3-butynoatesupplied for 40 min resulted in accumulation of ¹⁴C in glycolicacid with only a 7% inhibition of photosynthesis; when suppliedfor 90 min, photosynthesis was inhibited by 47%. When ¹⁴CO₂was replaced by 1000 vpm ¹²CO₂, radioactivity in glycine decreasedbut increased more rapidly in sucrose with less release of ¹⁴CO₂.Isonicotinyl hydrazide (INH) inhibited photosynthesis from ¹⁴CO₂by 50% and glycine replaced sucrose as the main product. When,after 15 min, ¹⁴CO₂ was replaced by 150 vpm ¹²CO₂, in the presenceof INH less ¹⁴CO₂ was released, ¹⁴CO in glycine decreased moreslowly, and less [¹⁴CO]sucrose accumulated. Glycidate (potassium2,3-epoxypropionate) at 2 mM had no effect on photosyntheticrate and little effect on carbon metabolism; 20 mM glycidateinhibited photosynthesis by 64% and resulted in less radioactivityin glycine, more in phosphate esters, and less ¹⁴CO₂ released.When photosynthesis was measured in 1000 vpm CO₂ the inhibitorsgave smaller effects on metabolism than during photosynthesisfrom 150 vpm ¹⁴CO₂ but 20 mM glycidate still resulted in a 42%inhibition of photosynthesis. When U- [¹⁴CO]glycerate was appliedto leaf segments in air with 320 vpm ¹⁴CO₂ the total uptakeof glycerate was not changed by the inhibitors. INH and glycidateboth decreased the amount of glycerate metabolised. More ¹⁴COaccumulated in glycine in the presence of INH and in phosphateesters and serine in the presence of glycidate. Hydroxybutynoateincreased the production of glycolate from glycerate but didnot affect the total amount of glycerate metabolised. Although all three inhibitors affected photorespiratory metabolismnone stimulated photosynthesis. The results are consistent withthe main release of CO₂ in photorespiration arising from theconversion of glycine to serine.     

Effects of Glycollate on the Growth of a Planktonic Chlorella

The duration of the lag phase in cultures of a planktonic strainof Chlorella pyrenoidosa growing from small inocula under conditionsof light limitation was shortened by the addition of 1 mg 1^–1of glycollate to the medium. Of eight related compounds tested,none, with the possible exception of lactate, had a similareffect. Addition of 1 mg 1^–1 of glycollate also considerablyincreased the relative growth-rate of the alga at low (500 lux)but not at higher light intensities. This effect was not, however,specific to glycollate. Concentrations of glycollate (20 mg1^–1) higher than those normally produced in cultures byexcretion had an inhibitory effect on growth.     

Acetate metabolism in the process of "acetate-bleaching" of Chlorella protothecoides

Green cells of Chlorella protothecoides when incubated in amedium containing acetate but no nitrogen source, have beenshown to be bleached as strongly as in glucose-induced bleaching.Using U-¹⁴C-acetate as tracer, the acetate metabolism of algalcells during the process of acetate-induced bleaching was investigated.Changes in algal cell activities for respiration and assimilationof added ¹⁴C-acetate were followed during bleaching processesin "acetate-adapted" and "non-adapted" green cells. As in glucose-inducedbleaching of algal cells, algal cell activity for incorporating¹⁴C into lipids showed the most characteristic change, suggestingthat lipogenesis is causally related to the occurrence of bleachingin algal cells. (Received March 5, 1969; )     

Utilization Modes of Inorganic Carbon for Photosynthesis in Various Species of Chlorella

Rates of CO₂ and HCC₃^– fixation in cells of various Chlorellaspecies in suspension were compared from the amounts of ¹⁴Cfixed during the 5 s after the injection of a solution containingonly ¹⁴CO₂ or H¹⁴CO₃^–. Results indicated that irrespectiveof the CO₂ concentration during growth, Chlorella vulgaris 11h and C. miniata mainly utilized CO₂, whereas C. vulgaris C-3,C. sp. K. and C. ellipsoidea took up HCO₃^– in additionto CO₂. Cells of C. pyrenoidosa that had been grown with 1.5%CO₂ (high-CO₂ cells) mainly utilized CO₂, whereas those grownwith air (low-CO₂ cells) utilized HCO₃^– in addition toCO₂. Cells that utilized HCO₃^– had carbonic anhydrase(CA) on their surfaces. The effects of Diamox and CA on the rates of CO₂ and HCO₃^–fixation are in accord with the inference that HCO₃^– wasutilized after conversion to CO₂ via the CA located on the cellsurface. CA was found in both the soluble and insoluble fractions;the CA on the cell surface was insoluble. Independent of the modes of utilization, the apparent K_m (NaHCO₃)for photosynthesis was much lower in low-CO₂ cells than in high-CO₂ones. The fact that the CA in the soluble fraction in C. vulgarisC-3 was closely correlated with the K_m(NaHCO₃) indicates thatsoluble CA lowers the K_m. ¹ Dedicated to the late Professor Joji Ashida, one of the foundersand first president of the Japanese Society of Plant Physiologists. ⁴ On leave from Research and Production Laboratory of Algology,Bulgarian Academy of Sciences, Sofia. (Received September 14, 1982; Accepted March 1, 1983)     

Effects of Carbon Dioxide on Metabolism by the Glycollate Pathway in Leaves

When solutions of [¹⁴C]glycollate, glycine, serine, glycerate,or glucose were supplied to segments of wheat leaves throughtheir cut bases in the light, most of the ¹⁴C was incorporatedinto sucrose in air but in CO₂-free air less sucrose was made.The synthesis of sucrose was decreased because metabolism ofserine was partly blocked. Sucrose synthesis from glucose andglycerate in CO₂-free air was decreased but to a smaller extent;relatively more CO₂ was evolved and serine accumulated. Theeffects of DCMU and light of different wavelengths on metabolismby leaves of L-[U-¹⁴C]serine confirmed that simultaneous photosyntheticassimilation of carbon was necessary for the conversion of serineto sucrose. Of various products of photosynthesis fed exogenouslyto the leaves -keto acids were the most effective in promotingphotosynthesis of sucrose and release of ¹⁴CO₂ from ¹⁴C-labelledserine. This suggests that in CO₂-free air the metabolism ofserine may be limited by a shortage of -keto acid acceptorsfor the amino group. In CO₂-free air added glucose stimulatedproduction of CO₂ and sucrose from D-[U-¹⁴C]- glycerate andno competitive effects were evident even though glucose is convertedrapidly to sucrose under these conditions. In addition to asupply of keto acid, photosynthesis may also provide substratesthat can be degraded and provide energy in the cytoplasm forthe conversion of glycerate to sugar and phosphates and sucrose.     

Short-term Products of 14C-acetate Assimilation by Chlorella pyrenoidosa in the Light

The kinetics of ¹⁴C-2-acetate assimilation by Chlorella pyrenoidosain the light were examined. Under aerobic conditions the primaryproduct of acetate assimilation was succinic acid which, afterten seconds, contained over 60 per cent of the ¹⁴C incorporatedby the cells. The percentage of the total ¹⁴C in succinate fellwith time, while that in citrate and glutamate increased. After1800 sec over 60 per cent of ¹⁴C was present in two compounds,glutamic acid and an unknown compound (X). Glucose-6-phosphate,fructose-6-phosphate, phosphoglyceric acid and phosphoenolpyruvicacid became labelled after 60 sec but together never containedmore than one per cent of the total ¹⁴C incorporated. Underanaerobic conditions succinate was still the primary productof acetate assimilation, and the absence of carbon dioxide resultedin a decrease in ¹⁴C incorporation into compound X. The patternof acetate assimilation in acetate grown and acetate adaptedChlorella was very similar to that in photo-autotrophicallygrown Chlorella. In the presence of 10^–6M DCMU, succinicacid was the primary product of acetate assimilation, but therewas an early Incorporation of ¹⁴C into glutamate, aspartate,and malate. 4 x10^–3M MFA did not effect the early incorporationof ¹⁴C into succinic acid, but resulted in accumulation of ¹⁴Cin citrate and a decreased amount in glutamate and in compound X.     

The Kinetics of Extracellular Glycollate Production by Chlorella pyrenoidosa

Extracellular glycollate is liberated by Chlorella pyrenoidosaduring growth in medium bubbled with air or 3 per cent carbondioxide in air. With air the rate of release of glycollate percell decreases, with 3 per cent carbon dioxide it increases,with increase in cell number. Glycollate is released duringshort-term experiments when C. pyrenoidasa, grown under lowlight and high carbon dioxide, is transferred suddenly to highlight and low carbon dioxide. No other combination of thesefactors produces a comparable release of glycollate. The quantityof glycollate released in short-term experiments increases exponentiallywith the relative growth-rate of the culture from which thecells are derived. A crucial condition for maximum glycollaterelease is that growth of the culture prior to the experimentshould not be limited by carbon-dixoide concentration. The effectof pH is related to its effect on growth-rate; i.e. C. pyrenoidosahas a lower relative growth-rate at pH 8.3 and produces correspondinglyless glycollate than faster growing cultures at pH 6.4. Duringshort-term experiments under high light and low carbon dioxidethe rate of glycollate release drops after 50–100 minutessuggesting exhaustion of the glycollate precursor.     

Ammonia Induces Starch Degradation in Chlorella Cells

When ammonia was added to cells of Chlorella which had fixed¹⁴CO₂ photo synthetically, ¹⁴C which had been incorporated intostarch was greatly decreased. A similar effect was observedwhen potassium nitrate and sodium nitrite were added. The ammonia-induceddecrease in ¹⁴C-starch was observed in all species of Chlorellatested. With cells of C. vulgaris 11h, most of the radioactivityin starch was recovered in sucrose, indicating that ammoniainduces the conversion of starch into sucrose. The percent of¹⁴C recovered in sucrose differed from species to species andpractically no recovery in sucrose was observed in C. pyrenoidosa.In most species tested, the enhancing effects of blue lightand ammonia on O₂ uptake as well as the ammonia effect on starchdegradation were greater in cells which had been starved inphosphate medium in the dark than in non-starved cells. In contrast,the enhancing effect of ammonia on dark CO₂ fixation was muchgreater in non-starved cells. C. pyrenoidosa was unique in thatblue light did not show any effect on its O₂ uptake. (Received August 15, 1984; Accepted November 16, 1984)     

Contrasting Sensitivities of Chlorella and Higher Plant Phosphofructokinases to Dilution

A partially purified preparation of phosphofructokinase fromChlorella pyrenoidosa became unstable upon dilution. At pH 7.7and 2°C, enzyme activity remained relatively constant whenthe protein concentration was 12 mg/ml, but at 3 mg/ml almostall activity was lost after 95 min. Greater stability was affordedby lower pH, higher temperature, or the presence of fructose-l,6-P₂,ATP or P₁. Phosphofructokinases from the leaves of spinach,oat and lucerne did not show the instability characteristicsof the Chlorella enzyme. ¹Present address: Department of Biochemistry and Nutrition,University of New England, Armidale, N.S.W. 2351, Australia. (Received December 17, 1980; Accepted November 25, 1980)     

Growth of Ryegrass (Lolium perenne L.) Exposed to SO2: II. CHANGES IN THE DISTRIBUTION OF PHOTO ASSIMILATED 14C

Perennial ryegrass (Lolium perenne L.) cv. S23 was exposed to0, 50, and400 µg m– ³ SO₂ for a 29 d period, harvested,and then exposed under the same regime for a further 22 d periodof regrowth. Leaves from plants representing each exposure concentrationwere photosynthetically fed ¹⁴CO₂ for 5 min at the end of eachperiod. A significant increase in photoassimilation of ¹⁴CO₂and retention of ^I4C, concomitant with significant decreasesin [¹⁴C]glycine and [¹⁴C]serine with increasing SO₂ concentration,implied that there was an inhibition of the photorespiratorypathway. At the second harvest, leaves from plants exposed to400 µg m– ³ SO₂ also exhibited significant increasesin [¹⁴C]sucrose and [¹⁴C]fructose.     

The Oxidation of 14C-labelled Glucose by Chlorella vulgaris: 2. The Fate of Radioactive Carbon

Most of the ¹⁴C added as glucose to carbohydrate-starved cellsof Chlorella Vulgaris can be recovered as alcohol-soluble compoundsor as polysaccharide. Only 5–I6 per cent., depending onthe position of ¹⁴C in the glucose supplied, is released ascarbon dioxide. Similar results were obtained with Chlorellapyrenoidosa and Ankistrodesmus. The labelled alcohol-solublecompounds in Chlorella vulgaris include amino-acids, particularlyglutamic acid, aspartic acid, and alanine, and, when glucose-I-¹⁴Cis metabolized, the amount of ¹⁴C recovered in these amino-acidsis about the same as that recovered as carbon dioxide. Degradationof the glucose incorporated into polysaccharide shown that theC₁ and C₆ atoms of glucose rapidly interchange when in the cells.The bearing of these results on attempts to estimate the relativeimportance of different pathways of glucose breakdown is discussed.     

Enzymatic Inactivation of Extracellular Carbonic Anhydrase and its Effect on K1/2 (CO2) for Photosynthesis in Chlorella ellipsoidea C-27

The ratio of the extracellular to the intracellular activityof carbonic anhydrase (CA) in cells of Chlorella ellipsoideaC-27, adapted to low levels of CO₂ for 24 h (low-CO₂ cells),was about one to one. Treatment of intact cells with PronaseP inactivated about one-half of the extracellular CA activitywithout affecting photosynthetic activity. The CA activity incell homogenates and in cell-wall ghosts liberated during celldivision was completely inactivated by the same treatment. Pretreatmentwith Glycosidase mix, Chitosanase and Macerozyme enhanced theinactivation of the CA activity in intact cells. These resultssuggest that extracellular CA is evenly distributed throughoutthe whole cell-wall region. The apparent K_1/2 for dissolved inorganic carbon (DIC) in low-CO₂cells doubled when extracellular CA was inactivated by treatmentwith Pronase P, but the K_1/2 obtained was still one-half ofthat in high-CO₂ cells. Photosynthetic ¹⁴CO₂-fixation in low-CO₂cells was enhanced by acetazolamide, whereas H¹⁴CO₃^–-fixationwas suppressed. The results suggest that CO₂ is a dominant substrateutilized by cells and that HCO₃^– is utilized after conversionto CO₂. The present results show that both intracellular andextracellular CA contribute to the increase in affinity forDIC during photosynthesis in low-CO₂ cells of Chlorella ellipsoideaC-27. (Received May 7, 1990; Accepted July 18, 1990)     

Distribution of Radioactivity from Exogenously Supplied [1-14C]Indol-3-yl-acetic Acid and [3, 4-3H (N)] Gibberellin A, in Geotropically-stimulated Picea abies (L.) Karst. Roots

The distribution of exogenously-supplied radioactive labelledindol-3-yl-acetic acid (IAA) and gibberellin A₁ (GA₁) in geotropicallystimulated roots of Norway spruce (Picea abies (L.) Karst.)has been demonstrated. Seedlings were positioned with theirroot tips in 2.1 x 10^–6 M [¹⁴C]IAA or 1.3 x 10^–8m ³H-GA₁ for 4 and 20 h, respectively. After geotropic stimulationfor 90 min in the horizontal position the root tips were cutlongitudinally in 50 µm thick sections, using a freeze-microtome.The radioactivity in the ¹⁴C-IAA treated roots occurred in higherconcentration in the lower than in the upper halves (ratio 1.25:1). A similar trend was observed in the [³H]GA₁-treated rootswhere the ratio lower: upper halves was 2.04: 1. The ratio ofradioactivity in right and left halves of vertical roots wasapproximately the same in roots supplied with [¹⁴C]IAA and [³H]GA₁(1.09: 1). The supplied radioactive compounds were analysed chromatographicallyafter extraction in methanol of 6 mm apical root segments. Onlya small fraction (7–8 per cent) of the supplied [¹⁴C]IAAwas revealed unchanged in the segments. The major part of thechromatographed, labelled compound has not been identified,but on basis of its R_F value it is suggested that it may beindol-3-acetyl-aspartic acid (IAAasp). The chromatographic analysis of the [³H]GA,-treated segmentsshowed that only small fractions of this gibberellin has beenconverted to other compounds. These results have been discussed and correlated with knowledgeof plant growth regulators and their participation in root geotropism. Picea abies, spruce, geotropism, gibberellin A₁, indol-3-yl-acetic acid, growth regulators, redistribution in roots