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.     

The Geoelectric Effect in Plant Shoots: II. SENSITIVITY OF CONCENTRATION CHAIN ELECTRODES TO REORIENTATION

A concentration chain, static-drop electrode system has beenused by several investigators to measure the geoelectric effectin plant shoots. This paper describes investigations of theinherent sensitivity of this electrode system to reorientationwith respect to gravity. When the gelatine plug of the electrodeis made up with distilled water, and the contact solution is0.1 mM KCl, a potential difference develops immediately afterelectrodes in direct contact are rotated through 90° intothe vertical plane. A similar response is found when the contactsolution is 5 mM CaCl₂. Increasing the concentration of thecontact solution, or incorporating KCl, K₂SO₄, or ZnSO₄ intothe gelatine plug, drastically reduces the potential differencedeveloped after reorientation. The potential difference acrosselectrodes in direct contact decreases as the electrodes age.The potential difference measured with these electrodes acrossa decapitated, horizontally placed, hypocotyl of Helianthusarmuus also decreases as the electrodes age. The polarity ofthe charge is reversed as compared with that found when theelectrodes are in direct contact. The kinetic characteristicsof the geoelectric potential difference developed across a non-decapitated,horizontal coleoptile of Zea mays change as the electrodes age.With fresh electrodes the potential develops immediately afterreorientation and continues to increase with time. With 4-day-oldelectrodes, however, no potential difference develops until9 min after the moment of reorientation, but then it increaseswith time. The characteristics of the geoelectric potentialdifference developed with the aged concentration-chain, static-dropelectrodes are similar to those found with several other typesof electrodes which do not themselves have an inherent sensitivityto reorientation with respect to gravity. The results supportour earlier suggestion that the potential difference which apparentlydevelops with the static-drop electrode system, immediatelyafter a shoot is turned through 90° in reality developsin the electrode system itself and not in the plant tissue.The geoelectric effect which arises in the living plant shootbegins to develop approximately 10 min after reorientation.     

The Geoelectric Effect in Plant Shoots: V. A DISCUSSION

In earlier papers Woodcock and Wilkins, and Hertz and Grahmreported measurements of the geoelectric effect (GEE) in Zeamays coleoptiles. While Woodcock and Wilkins attached contactelectrodes to the coleoptile directly, Grahm and Hertz employeda vibrating-electrode technique that avoided touching the plant. The results obtained by these two methods agree on all pointsexcept the magnitude of the GEE, where a difference of morethan 100 per cent is reported. In this paper this differenceis investigated and explained by certain properties characteristicof the measuring techniques used as well as by differences inexperimental procedure.     

The Geoelectric Effect in Plant Shoots: I. THE CHARACTERISTICS OF THE EFFECT

A comparative study has been made of the geoelectric effectin Helianthus hypocotyls and Zea coleoptiles using two electrodesystems. With the static-drop electrode system a potential differencedeveloped between the upper and lower surfaces of the shootsimmediately after they were turned into the horizontal position.The lower surface of the shoot became positively charged withrespect to the upper surface. In non-decapitated shoots thispotential difference continued to increase for at least 20 min,whereas in decapitated shoots no further increase occurred afterabout 10 min from the moment of reorientation. In contrast,with a flowing-solution electrode system no potential differencedeveloped in non-decapitated shoots until about 12 min afterthey were placed in the horizontal position. Thereafter thelower surface became increasingly positively charged with respectto the upper surface for at least a further 12 min. In decapitatedshoots there was no tendency whatsoever for the lower surfaceof the horizontal shoot to become positively charged with respectto the upper surface, even after 25 min in the horizontal position.The static-drop electrode system has an inherent sensitivityto reorientation in a gravitational field; a potential differencedevelops immediately after reorientation, and increases to amaximum value within the first 10 min of reorientation, regardlessof whether or not the electrodes are in contact with plant tissue.The continued increase in the potential difference measuredacross the shoots with the static-drop electrode system, andthe entire development of the potential difference measuredwith the flowing-solution electrode system, are both dependentupon the shoot apex being intact. These facts have enabled usto show that the electro-potential difference measured acrosshorizontally placed non-decapitated tissues with the static-dropelectrode system is the resultant of two distinct processes:(a) an immediate, purely physical electrical effect generatedin the electrodes themselves, and (b) a delayed geoelectriceffect which arises solely in the living tissues of the shootand which is dependent upon the apex of the shoot being intact.     

The Role of the Coleoptile Apex in Controlling Organ Elongation: II. EFFECTS OF AUXIN SUBSTITUTION AND AUXIN TRANSPORT INHIBITORS ON DECAPITATED COLEOPTILES

The most widely quoted evidence that auxin controls coleoptileelongation is that auxin applied to the cut surface of a coleoptilecan obviate the effects of decapitation. A quantitative studyof this old observation shows that exogenously applied auxinonly overcomes the effects of decapitation if supplied at unphysiologicallyhigh concentrations. Furthermore, when a ring of an auxin transportinhibitor is applied just beneath the apex of an intact coleoptile,so that auxin supplied from the apex is abolished, the growthrate of the coleoptile is not reduced significantly. These experimentssuggest that coleoptile elongation is not dependent on auxintransported from the apex. Key words: Coleoptile, auxin, transport inhibitors     

PHYTOCHROME ACTION IN ORYZA SATIVA L. III. THE SEPARATION OF PHOTOPERCEPTIVE SITE AND GROWING ZONE IN COLEOPTILES, AND AUXIN TRANSPORT AS EFFECTOR SYSTEM

• 1 In 4-day-old etiolated rice seedlings, 3 mm of the coleoptile tip did mainly perceive the photostimulus to cause the phytochrome-dependent inhibition of coleoptile elongation. At this age, cell elongation occurred most in the middle portion of coleoptiles in the dark, and was reversibly controlled by a brief exposure of the tip to red and far-red light. Thus, the photoperceptive site was evidently separated from the growing zone in intact rice coleoptiles.
• 2 The red-light-induced inhibition of coleoptile elongation was nullified by the removal of tip followed by the exogenous application of IAA. The sensitivity of thus treated coleoptiles to IAA was gradually lost during intervening darkness between the irradiation and the decapitation, and a 50% loss was obtained at ca. 6th hour at 26°C.
• 3 Polar auxin transport from coleoptile tips was remarkably prevented at the period between, at least, 2nd and 4th hour after red irradiation, and it recovered to the level of dark control by the 6th hour. Far-red light given immediately after red irradiation reversed the yield of diffusible auxin up to that of far-red control.

     

Dormancy in Seeds of Charlock: IV. INTERRELATIONSHIPS OF GROWTH, OXYGEN SUPPLY AND CONCENTRATION OF INHIBITOR

Respiration measurements were made over a period of 24 h at25 °C on seeds and excised embryos maintained in Warburgflasks with partial pressures of oxygen ranging from 0 to 1atm. In the initial phase (0 to 4 h), the rate of oxygen uptake(Q_O2 of excised embryos increased linearly with external oxygenconcentration (C_O from 0 to 0.1 atm O₂ from 0.1 to 0.2 atm O₂the relation was curvilinear, and from 0.2 to 1.0 atm O₂ uptakewas independent of concentration. In later stages the relationbetween Q_O2 and C_o changed, and from 20 to 24 h the rate ofoxygen uptake increased with concentration to 1.0 atm O₂. Thechanges with time were associated with increase in rate of respiration,increase in cell size and cell number, and the oxidation offats. The decline in concentration of oxygen from the surfaceto the centre of embryos was calculated to be relatively smallat each external oxygen concentration. Althugh the rate of diffusionfailed to keep pace with consumption, the main parameters whichdetermined the internal oxygen status of the embryos were thesurface concentrations and the permeability of the seed coat.The resistance of the seed coat to diffusion of oxygen was foundto be very high, the coefficient of diffusion being about 10^–7mm² s^–1. The concentration of oxygen and in air were estimatedto be approximately 0.04 and 0.02 atm O₂, respectively. Sincea smaller concentration of oxygen (0.012 atm O₂) in the tissueswas found to be sufficient for growth, the dormancy of the seedswas not due to lack of oxygen. Dormancy appeared to be due tothe activity of growth-inhibiting substances, the concentrationof which increased with decrease in oxygen supply; below 0.1atm O₂ their rate of production increased with decrease in theoxygen concentration of the tissues. They accumulated withinthe testas of dormant seeds and prevented cell elongation. Extractsof the inhibitory substances were partially purifield by partitioningthe aqueous fraction with ether and separating chromatographically.The active principle(s) was not abscisic acid ((+)—AbscisinII, ‘Dormin’) or the mustard oil, allylisothiocyanate.     

Gravitropism in Higher Plant Shoots : IV. Further Studies on Participation of Ethylene

Ethylene at 1.0 and 10.0 cubic centimeters per cubic meter decreased the rate of gravitropic bending in stems of cocklebur (Xanthium strumarium L.) and tomato (Lycopersicon esculentum Mill), but 0.1 cubic centimeter per cubic meter ethylene had little effect. Treating cocklebur plants with 1.0 millimolar aminoethoxyvinylglycine (AVG) (ethylene synthesis inhibitor) delayed stem bending compared with controls, but adding 0.1 cubic centimeter per cubic meter ethylene in the surrounding atmosphere (or applying 0.1% ethephon solution) partially restored the rate of bending of AVG-treated plants. Ethylene increases in bending stems, and AVG inhibits this. Virtually all newly synthesized ethylene appeared in bottom halves of horizontal stems, where ethylene concentrations were as much as 100 times those in upright stems or in top halves of horizontal stems. This was especially true when horizontal stems were physically restrained from bending. Ethylene might promote cell elongation in bottom tissues of a horizontal stem or indicate other factors there (e.g. a large amount of `functioning' auxin). Or top and bottom tissues may become differentially sensitive to ethylene. Auxin applied to one side of a vertical stem caused extreme bending away from that side; gibberellic acid, kinetin, and abscisic acid were without effect. Acidic ethephon solutions applied to one side of young seedlings of cocklebur, tomato, sunflower (Helianthus annuus L.), and soybean (Glycine max [L.] Merr.) caused bending away from that side, but neutral ethephon solutions did not cause bending. Buffered or unbuffered acid (HCl) caused similar bending. Neutral ethephon solutions produced typical ethylene symptoms (i.e. epinasty, inhibition of stem elongation). HCl or acidic ethephon applied to the top of horizontal stems caused downward bending, but these substances applied to the bottom of such stems inhibited growth and upward bending—an unexpected result.     

Whole Plant Studies Using Radioactive 13-Nitrogen: III. THE RELATIONSHIP BETWEEN NITRATE AND AMMONIUM ION UPTAKE AND WATER FLUX FOR ZEA MAYS

Nitrogen-13 labelled nitrate and ammonium ions were used tomeasure uptake and assimilation rates in non-destructive experimentsin which the water flux through hydroponically grown maize plantswas doubled by changing the ambient humidity. Although no immediateeffects on ion uptake were observed, longer term observationsshowed different patterns of coupling for the uptake parametersfor nitrate and for ammonium. The patterns were compared withpatterns predicted for different combinations of processes foruptake and for discharge to the xylem. Those which were compatiblewith the experimental patterns indicated that for nitrate theuptake into the root showed coupling to water flux whereas dischargeto the xylem did not. For ammonium there appeared to be couplingto the water flux of both uptake into the root and of dischargeto the xylem though it was less evident in the former when theexternal ammonium ion concentration was low. Key words: Ion-water coupling, Ammonium uptake, Nitrate uptake     

Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: II. RELATIONSHIP BETWEEN GROWTH RATE, CARBOHYDRATE CONCENTRATION AND 14C-PARTITIONING WITHIN TUBERS

Potato plants (Solanum tuberosum L.) were grown in water culturein a controlled environment. The growth rates of individualtubers were closely reflected by their ¹⁴C-content 20 h after¹⁴CO₂ had been applied to the aerial parts of the shoot for4 h. The ¹⁴C-content of the tuber (sink strength) was significantlycorrelated to the ¹⁴C-concentration of the tuber tissue (¹⁴Cg^–1 fr. wt.=sink activity). The sink activity, which differedbetween individual tubers by up to a factor of 10, was alsoclosely related to the conversion rates of ¹⁴C into the starchand the remainder as well as to the ¹⁴C-content in the ethanolsoluble fraction. This indicates the simultaneous use of photosynthatefor growth and storage in the growing tubers. No preferenceof photosynthate utilization for either of these processes couldbe detected in relation to the sink activity of the tubers.Tubers with high sink activity imported ¹⁴C-labelled photosynthateat higher rates although their tissue contained higher concentrationsof reducing sugars and sucrose than the tissue of tubers withlow sink activity. Despite the close relationship between sinkactivity and the rate of starch synthesis (¹⁴C-conversion intostarch), no significant correlation was found between sink activityand the actual starch concentration of the tissue. The applicationof zeatin riboside directly onto individual tubers increasedtheir growth rates in comparison to non-treated tubers of thesame plant. The results indicate the importance of both growthand storage processes for the regulation of sink activity inyoung potato tubers. Key words: Potato tuber, ¹⁴C-photosynthate partitioning, zeatin riboside application     

Direct Measurements of Turgor Pressure Potentials: IV. NATURALLY OCCURRING PRESSURES IN GUARD CELLS AND THEIR RELATION TO SOLUTE AND MATRIC POTENTIALS IN THE EPIDERMIS

Measurements were made of turgor pressures in epidermal, subsidiary,and guard cells at different degrees of stomatal opening. Theresults are correlated with estimates of solute potentials ofthe different cell saps and with a postulated matric potentialof cell walls. The results contribute to an understanding ofthe role of the subsidiary cells in the stomatal mechanism especiallyin the Graminaceous type and the role of ‘tissue tension’as a pressure involved in turgor phenomena in epidermal tissue.The results make it possible to explain the abnormally largestomatal openings found after floating illuminated leaf tissueon water or keeping it in humid CO₂-free air.     

Studies in Plant Growth Hormones: IV. CHROMATOGRAPHY OF HORMONES AND HORMONE PRECURSORS IN CABBAGE

1. Acid and neutral ethereal fractions and the non-ether-solubleaqueous fraction of an extract of cabbage leaves were chromatographed,and the chromatc-grams assayed using oat coleoptile sections. 2. In the aqueous fraction an acidic precursor of 3-indolylacetonitrile(IAN) was found. When chromatographed in isopropanol/ammonia,the precursor travelled at the position of 3-indolylacetic acid(IAA), but in n-butanol/ammonia it was much closer to the starting-line.IAN is liberated from the precursor under conditions of alkalinehydrolysis including ammoniacal chromatography, and is alsoliberated by heat. Precursor and IAN zones promoted coleoptilegrowth, but the former when sprayed with ferric chloride/perchloricacid or nitrous/nitric acid gave a yellow colour showing thatthere was no free IAN on this part of the chromatogram. Hypothesesto account for this activity are discussed. 3. A neutral inhibitor was present in the aqueous fraction.It is volatile, ether-soluble, and is thought to have been liberatedfrom a water-soluble substance. 4. The neutral fraction, chromatographed in isopropanol/ammonia,contained IAN and a growth promoter at Rf o–o·1:thelatter stimulates cress root growth above that in water. Thispromoter could be formed from a precursor in the aqueous fractionby heat treatment followed by shaking with sodium bicarbonatesolution. It is suggested that this neutral hormone is the accelerator-of Bennet-Clark and Kefford. The data of these workers are analysedto show that this interpretation is consistent with their results. 5. The acid fraction contained IAN but no IAA. The former isthought not to have been liberated from the precursor in thisfraction but to have entered into it from the neutral fractionduring separation with sodium bicarbonate solution. AlthoughIAA may have been absent from the plant material, it is possiblethat any present was destroyed during the process of extraction. 6. Evidence is presented that there are other growth promoterspresent at low concentration in the extract in addition to thosealready mentioned.     

Whole Plant Studies using Radioactive 13-Nitrogen: IV. A COMPARTMENTAL MODEL FOR THE UPTAKE AND TRANSPORT OF AMMONIUM IONS BY ZEA MAYS

Presland, M. R. and McNaughton, G. S. 1986. Whole plant studiesusing radioactive 13-nitrogen IV. A compartmental model forthe uptake and transport of ammonium ions by Zea mays.—Jexp. Bot. 37: 1619–1632 Ammonium ion uptake by roots of hydroponically grown maize seedlingsand the transport of ammonium-sourced nitrogen to the shootwere measured using the short-lived isotope 13-nitrogen. Theyare shown to be described by a five compartment model—ammoniumin the root bathing solution, ammonium entering the root, nitrogenbound in the root, nitrogen outside the load region but stillwithin the root, and nitrogen in the shoot. Once taken intothe root symplasm, ammonium-sourced nitrogen was not exchangedwith the external solution. Activity-time profiles for nitrogentransported both basally and apically from the load region ofthe root implied the existence of large well-mixed pools inthe transport path, though the capacity of the pools inferredfrom modelling the profiles was greater than found by dissectingroots following loading with labelled ammonium. Key words: Ammonium uptake, Zea mays, compartmental model, 13-nitrogen     

Action Potentials in Lupinus angustifolius L. Shoots: V. SPREAD OF EXCITATION IN THE STEM, LEAVES, AND ROOT

The spread of excitation in the stem, leaves, and root of Lupinuswas investigated. The stem was electrically stimulated (d.c.pulses). Potential changes in the plant were measured with extracellularcontact electrodes applied at various points to the plant surface.The action potential (AP) was found to propagate solely in thestem. It is extinguished in the region of the stem apex andthe root neck. It does not propagate in leaves. There arise,however, in the leaves, root, and stem apex characteristic potentialchanges at the moment when the AP reaches these regions, butthey do not have the characteristics of AP. Attempts at evokingexcitation by electrical stimulation of leaves, root, or theitem apex were unsuccessful. It appears therefore that the stemplays a special role in the spread of excitation in the plant. The regularities described here differ from those obtained bystimulation of plants with damage stimuli such as touching theshoot or a leaflet with a warmed glass rod (Paszewski and Zawadzki,1976).     

The Role of Boron in Plant Growth: I. THE EFFECT ON GENERAL GROWTH, SEED PRODUCTION AND CYTOLOGICAL BEHAVIOUR

The relation of boron to vegetative growth and seed productionof Trifolium pratense L. and Vicia faba L. has been studiedusing sand and water culture techniques. It was found that theaddition of a very small quantity of boron was sufficient torestore deficient plants to normal growth and it would appearthat boron fertilizers will only be beneficial where deficiencysymptoms have been shown. The rapid cessation of cell division under conditions of borondeficiency was demonstrated and was responsible for a decreasein both vegetative and seed yields through reduced branchingand flower differentiation. No special association of boronwith the functioning of pollen or with chromosome behaviourwas found. The cessation of cell division is responsible forthe abnormalities associated with boron deficiency but the cellularreactions involved remain unknown. Previous suggestions of the role of boron in plant metabolismare discussed in the light of the conclusion that boron is essentialfor the maintenance of meristems.