Auxin Balance in the Avena Coleoptile

Coleoptiles of Avena possessed the capacity to degrade infiltrated indole-3-acetic acid (IAA). This activity decreased along the length of the coleoptile from apex to base on the bases of fresh weight, dry weight and protein; the apical 1 cm segment degraded more IAA than segments from other parts of the coleoptile. The naturally occurring inhibitor of the IAA oxidase activity increased in concentration up to 20 mm from the coleoptile apex; beyond, it decreased gradually towards the base. The spatial distribution of this inhibitor does not explain the gradient in IAA oxidase activity. Growth in length of the coleoptile and the IAA inactivating capacity of the apical 1 cm segment, increased 5- and 4,4-fold, respectively, between the ages of 70 and 130 h; but auxin secretion into agar platelets by the apical 2 mm of the coleoptile registered only a 2.7-fold increase. Deseeding and derooting the seedlings reduced the subsequent growth, diffusible auxin content and the IAA oxidase activity of the coleoptiles; derooting proved to be more deleterious than deseeding. A parallel reduction was evident in auxin content and IAA degrading activity following these treatments. Application of the cytokinin 6-benzylaminopurine (BAP) to coleoptiles of derooted seedlings failed to influence their capacity to degrade IAA. Nor was the activity of the aldehyde oxidase, which converts indole-3-acetaldehyde (IAAld) to IAA, affected by such treatment.     

The Growth-time Relationships of the Auxin- induced Growth in Avena Coleoptile Sections

1. 1. The growth rate of Avena coleoptile sections in the presenceof indoleacetic acid (IAA) is constant with time over a widerange of time intervals and IAA concentrations.
2. 2. Constancyof growth rate is dependent upon the maintenanceof constantconditions in which the concentration of IAA availableto thesection remains the chief factor limiting growth rate.
3. 3.Control of the pH of the medium in which the sections aregrownis essential to the maintenance of constant growth rate,particularlyin the presence of high concentrations of IAA.
4. 4. The lagperiod in establishment of steady growth rate bysections inthe presence of IAA is less than 10 minutes andis not detectableby present methods of measurement.

     

Inhibitory Effects of Dichlorophenoxyacetones on Auxin-induced Growth of Avena Coleoptile Sections

Six dichlorophenoxyacetones were synthesized and examined as potential metabolic antagonists utilizing Avena coleoptile sections and the straight growth assay procedure. Supplements of indoleacetic acid promoted growth of the sections which were inhibited by the analogs; the most inhibitory derivatives were 2,3-; 2,4-; 2,5-; and 3,4-dichlorophenoxyacetone which produced half-maximal growth responses (relative to the unaug-mented control growth) at concentrations of 106, 86, 80, and 62 μg/ml, respectively. A Lineweaver-Burk plot of the data for the inhibition by 2,4-dichlorophenoxyacetone and its reversal by indoleacetic acid appeared to represent an uncompetitive-like inhibition.     

The Intercellular Spaces of the Avena Coleoptile

In the transverse sections of fresh Avena coleoptile certain intercellular spaces are transparent, others are dark. The transparent spaces represent the result of water-logging of the originally water-lined air passages. The dark spaces are lined with a plastic lipid-containing membrane which can be impregnated with melted paraffin. In the living tissue this membrane can be cut transversely and the cut sections presumably seal off the gas inside thus causing the dark interfacial refraction. Because of the high permeability of lipids to carbon dioxide and the virtual impermeability to oxygen and nitrogen, there is a reason to believe that the lipid-lined spaces are filled with gas rich in carbon dioxide, and the lipid membrane may function as a regulator of the diffusion pressure of this gas.     

Growth Regulation of Dark-grown Dwarf Barley Coleoptile by the Endogenous IAA Content

Growth curves of dark-grown coleoptiles of 11 isogenic coleoptilardwarf strains of barley (Hordeum vulagare L. cv. Akashinriki:uzu, 5, 77, 97, 105, 125, 131, 133, 136, 145 and 148) were simulatedwith a logistic equation and the endogenous IAA contents ofthe barley strains were determined. Growth analysis of the dwarfbarley coleoptiles revealed that the final coleoptile lengthwas correlated with the growth rate on the 2nd day after germination(r=0.897), when the growth rate was about maximum. The endogenousIAA Content of the barley strains, measured fluorometrically,indicated that on the 2nd day, the dwarf strains contained lessendogenous IAA than the normal Strain. The IAA content on the2nd day was correlated to the growth rate on the 2nd day (r=0.907,except for Strain 145) and the final coleoptile length (r=0.933,except for strains 77 and 145). The correlation, however, wasnot significant on the 3rd day. These results suggested thatthe dwarfism of the dark-grown coleoptiles of the barley Strainsexamined is primarily controlled by the endogenous IAA content. ¹ Present address: Department of Biology, Faculty of Science,Osaka City University, Osaka 558, Japan. (Received February 1, 1982; Accepted April 13, 1982)     

Stress-relaxation Properties of the Avena Coleoptile Cell Wall

Changes in the cell wall properties of Avena coleoptile segments were studied under various conditions by stress-relaxation analysis. Rheological models consisting of four or an infinite number of Maxwell viscoelastic components were used. The stress-relaxation parameters of these models, t₁, t_o, T, Gi and stress/strain ratio, were determined. The following results were obtained. 1. The 1/T₁ increased and stress/strain ratio decreased with the age of the coleoptiles. Decapitation caused a decrease in l/t₁. 2. Auxin increased I/T₁ but decreased t_o and stress/strain ratio within 5 minutes after application. 3. Treatment with a fungal β-l,3-glucanase increased 1/T₁ both in living and methanol-killed, pronase-treated coleoptiles. Cellulase did not cause the changes observed in the parameters of the isolated cell wall of the coleoptile segments. This held true for all treatments (with and without auxin, killed and pronase-treated). The results obtained suggest that auxin primarily causes a partial degradation of the non-cellulosic physaccharide components of the cell wall.     

Investigations of the Geotropic Curvature of the Avena Coleoptile

The geotropic reaction in Avena coleoptiles is studied as a function of the stimulation time. The direction of the stimulation with respect to the vascular bundles must be defined when studying geotropic responses. It is found that the threshold time to evoke geotropic response is less than half a minute, i.e., at least ten times lower than the presentation time usually reported in the literature. An extrapolation procedure can be used to give a so-called extrapolated presentation time t_b, which is intimately related to the logarithmic part of the geotropic response curve and has a physical meaning in the reciprocity rule. The problem of the duration of the true threshold time for stimulation with 1 g is discussed. An experiment indicates that it is not necessary for mass particles (“statoliths”) to settle on the lateral cell wall in order to start the geotropic reaction chain. The slope of the logarithmic part of the geotropic response curve is independent of the transverse force applied to the coleoptiles. Support is given to the view that the slope is determined by the number of sedimenting mass particles.     

An Endogenous Rhythm in the Rate of Carbon Dioxide Output of Bryophyllum: I. SOME PRELIMINARY EXPERIMENTS

A techinique is described for recording automatically, withthe aid of an infrared gas analyzer, the rate CO₂ output orabsorption by plant material under controlled conditions. An examination of the rate of CO₂ output by excised leaves of16 species of succulent plants in darkness and in a CO_-2-freeatmosphere revealed clearly defined rhythms in only Bryophyllumfedtschenkoi, B. daigremontianum and B. calycinum (pinnatum). Further investigation of the rhythm in leaves of B. fedtschenkoirevealed that: (1) daylength has no effect upon the period ofthe rhythm in subsequent darkness, the phase being set at thetime the lights are extinguished; (2) normal air suppressesthe rhythm; (3) removel of the epidermis and cutting the mesophyllinto pieces 1 cm² does not effect either the phase or periodof the rhythm; (4) continuous illumination at an intensity of3,000 lux inhibits the rhythm which restarts when the lightsare extinguished. The phase of the rhythm can be set at anytime of day according to the time at which the lights are extinguished.The time which elapses between the onset of darkness and thefirst peak decreases as the length of the light treatment isincreased. The endogenuous nature of the rhythm is fully established. Theresults are compared with of other researches.     

An Endogenous Rhythm of Spore Discharge in Sordaria fimicola

The sporulation process in Sordaria fimicola has been studiedunder controlled conditions, with particular reference to discharge.Discharge occurred with a circadian periodicity for the greaterpart of the sporulation life of the culture, with peaks at about18.00 hours in either light or darkness. The consistency ofthis phenomenon was confirmed by an analysis of previous work.Fluctuations in light intensity, temperature, relative humidity,carbon dioxide concentration, and vibration were eliminatedas far as possible, as causes of this periodicity. Minor fluctuationsin discharge rate with a period of less than 24 h were probablyrandom. The general nature of the periodicity was shown to be consistentwith some aspects of endogenous rhythms in other organisms.However, spore discharge in S. fimicola is very sensitive tolight, and it is readily entrained to discharge out of phasewith the natural period. The rate of development of two stages of spore formation (delimitationand maturation) were determined, and these were related to therate and periodicity of discharge. From the data it was impossibleto establish at which stage of sporulation the periodicity arose.     

Inhibition of the Coleoptile Growth in Avena sativa by Endosperm Removal--Changes in Auxin, Osmotica and Cell Wall

Removal of the endosperm from 84-h-old etiolated oat seedlingsstrongly retarded the subsequent growth of coleoptiles. Thecontribution of the endosperm to coleoptile growth was studied.Endosperm removal was found to: (1) decrease the endogenouslevel of indole-3-acetic acid (IAA) in the coleoptile tip. IAAapplied to the coleoptile tip stimulated coleoptile growth inseedlings with and without the endosperm. The sensitivity ofthe coleoptile to a suboptimal concentration of IAA was higherin seedlings without the endosperm than in intact ones. At theoptimal concentration of IAA, however, the final length of thecoleoptile was larger in intact seedlings than in those withoutthe endosperm. (2) decrease the concentration of the solublesugars and amino acids in the cell sap. (3) retard the increasein the amount of polysaccharides in the cell wall of the coleoptile,particularly noncellulosic ones. (4) make the cell wall mechanicallyrigid according to stress-relaxation analysis of the cell wall.(5) induce an increase in the osmotic potential of the coleoptilecell sap. From these results, it was concluded that the endosperm suppliesthe coleoptile with IAA, sugars and amino acids, thus promotingcoleoptile growth. (Received September 24, 1987; Accepted February 3, 1988)     

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.