The Electroshock-Induced Inhibition of Phloem Translocation

The transient inhibition of phloem translocation which is producedby an electric shock was studied by applying controlled-currentstimuli to short lengths of bean stem. Translocation was monitoredby observing the accumulation of carbon-11 label into the plantapex. The principal findings are: (i) For constant-current electricshocks whose (current) x (time) product was held constant at10 mC, those of higher current (and shorter duration) gave longerlasting inhibitions, (ii) Breaking a 5 s pulse into a trainof 100 ms pulses slightly shortened the duration of the inhibition,even though the same total charge was passed; however, the separationof the pulses within the train did not seem to matter. But,when the inhibition produced by a pair of 50 ms pulses was studied,the duration of inhibition increased with pulse spacing. (iii)Single pulses as short as 10 ms could produce a detectable inhibition.The duration of inhibition grew rapidly with pulse length forpulse lengths between 10 ms and 100 ms. (iv) When a pair of2.5 s pulses was applied, a longer inhibition was obtained ifthe two pulses were of opposite polarity. To explain these phenomena,a qualitative conceptual model is provided in terms of ion channelsin the phloem. In the experiments reported here and in all previous pulse-labellingexperiments from this laboratory, it has been noted that rapidlyincreasing stochastic variability in the count data made itextremely difficult to obtain useful results for a period ofmore than 2 h after loading, even if the number of counts perunit time remained fairly high. A quantitative theory for thislimitation is worked out and shown to agree with experimentaldata. Therefore, since biological variability from plant toplant makes it advisable to compare matched stimuli on the sameplant in order to detect trends reliably, the number of challengeswhich can usefully be applied following a single pulse-labellingwith a short-lived isotope is quite limited. Key words: Phloem translocation, transient inhibition, electric shock     

The Inhibition of Phloem Translocation by Ammonia

Bean plants (Phaseolus vulgaris L. cv. Fardenlosa Shiny) werelabelled with carbon-11 via their first trifoliate leaves when3-weeks-old and the transient inhibitions of translocation causedby the application of ammonium chloride solutions (10 mol m^–3)to a peeled region of stem were studied. At pH 6·5 theammonium was without effect. At pH 11·0 even a briefapplication inhibited translocation for many minutes, whilelonger applications inhibited translocation for considerablylonger. Solutions of 10 mol m^–3 sodium chloride were withouteffect at either pH. At pH 6·5 ammonium chloride solution contains predominantlyammonium ions (NH₄⁺) and at pH 11·0 predominantly dissolvedammonia gas (NH₃). Hence we conclude that phloem transport withinbean stems is inhibited by dissolved ammonia gas but not ammoniumions. Key words: Phloem translocation, transient inhibition, ammonia, ammonium ion     

The Effect of 2,4-Dinitrophenol on the Growth of Bordetella pertussis in Chick Tracheal Organ Culture

Among various metabolic inhibitors tested, only 2, 4-dinitrophenol inhibited the growth of Bordetella pertussis in chick tracheal organ culture at concentrations nontoxic both for bacterial organisms and for ciliary motility of the tracheal fragments. Although this effect of 2, 4-dinitrophenol was reversible in its early stage, longer treatment with this inhibitor resulted in an irreversible inhibition of bacterial growth due to secondary damage of the tracheal fragments. From these observations, it was postulated that the energy required for bacterial growth might be derived from cellular metabolism sensitive to inhibition with 2, 4-dinitrophenol.     

A Rate of Cooling Response in Phloem Translocation

A transient inhibition of phloem transport caused by coolingis found to depend on the rate of cooling. This inhibition hasbeen observed for a temperature drop of only 2.5 C?. Possiblemechanisms are discussed. Key words: Phloem, Translocation, Cooling     

The Effect of Petiolar Temperature upon the Translocation Rate of 137Cs in the Phloem of Nymphoides peltata

A fixed length of the petiole of Nymphoides peltata was subjectedto a variety of temperature treatments. A Q₁₀ is calculatedfor the velocity of flow, from the mass transport rate of ¹³⁷Csin the phloem at temperatures between 0 and 50?C. Associatedwith lowered temperatures there was a temporary interruptionof transport overriding the longer term temperature dependence.The characteristics of the interruption were dependent uponthe size of the temperature drop and the magnitude of the ‘new’temperature.     

The Phloem Translocation of Enantiomeric Pairs of Weak Acids in Ricinus communis

A possible role for specific carrier mechanisms in the phloemtranslocation of xenobiotics in Ricinus communis L. var. Gibsonii(Nichols) has been investigated by comparing the phloem transportof enantiomers of three acidic compounds. No differences intranslocation were found between the R– and S-enantiomersof phenylalanine, 2-methoxy-2-phenylpropanoic acid or 5-methyl-5-phenyloxazolidine-2,4-dione.These similarities between transport of enantiomers indicatethat stereospecific carriers do not occur, even for the endogenousamino acid phenylalanine of which only one enantiomer occursnaturally in phloem sap. Whilst transport of the enantiomerswas similar, there were differences between the mobility ofthe three compounds, and these could largely be explained interms of their physicochemical properties, without the needto invoke the existence of specific carrier mechanisms. Key words: Phloem transport, enantiomers, Ricinus communis, weak acids     

Effect of 2,4-Dinitrophenol on Auxin-induced Ethylene Production and Auxin Conjugation by Mung Bean Tissue

Auxin-induced ethylene production by mung bean (Phaseolus mungo L.) hypocotyl segments was markedly inhibited by 2,4-dinitrophenol regardless of whether or not kinetin was present. Uptake of indoleacetic acid-2-¹⁴C was also inhibited in the presence of 2,4-dinitrophenol. Segments treated only with indoleacetic acid rapidly converted indoleacetic acid into indole-3-acetylaspartic acid with time whereas kinetin suppressed indoleacetic acid conjugation. Formation of indole-3-acetylaspartic acid was significantly reduced when 2,4-dinitrophenol was present. The suppression of indoleacetic acid conjugation by kinetin and 2,4-dinitrophenol appeared to be additive, and the free indoleacetic acid level in segments treated with 2,4-dinitrophenol in the presence of indoleacetic acid or indoleacetic acid plus kinetin was remarkably higher than in corresponding segments which received no 2,4-dinitrophenol.     

Phloem Pressure Differences and C-Assimilate Translocation in Ecballium elaterium

The role of phloem turgor pressure in ¹⁴C-assimilate translocation in Ecballium elaterium A. Rich was studied. The direction of translocation was manipulated by two methods: darkening, or defoliation, of the upper or lower halves of the shoots. After 24 hours of labeled assimilate movement, sieve tube turgor levels were measured with the phloem needle technique. Distribution of label, determined by autoradiography and counting, revealed a direct correlation between the direction of assimilate transport and the pressure difference. Phloem turgor levels always decreased in the stem of darkened shoots; this resulted in greater pressure differences in the stem between the source leaf receiving ¹⁴CO₂ and treated regions.     

The Interrelation between the Effects of 2,4-Dinitrophenol and of Substrate Concentration on the Rate of Exogenous Respiration

1. The specific activity of the CO₂ evolved by sunflower hypocotylsegments supplied with labelled glucose or glutamic acid isrelated to the external concentration by a rectangular hyperbolaover a concentration range of approx. 2–60 mM. The overallrespiration rate over this range is approximately 15 per cent.above that of a buffer control, and rises only slightly withincreasing external concentration. 2. The curve for total exogenous substrate respired is alsoclose to a rectangular hyperbola over this concentration range. 3. 2,4-dinitrophenol (DNP) reduces the slope of the hyperbolafor specific activity, but not its maximum value. Increasingthe glucose concentration while maintaining the DNP concentrationconstant reduces the degree of inhibition. 4. The effect of DNP on absolute amount of exogenous substraterespired depends on the external concentration of the substrate.At lower concentrations, absolute exogenous respiration is inhibited:at higher concentrations, it is stimulated. 5. The results have been analysed with the object of determiningwhether the respiratory sites lie in the ‘sugar free space’of the cell. Kinetic analysis indicates (a) that exogenous substratesdo not diffuse to these sites through an appreciable diffusionbarrier; (b) that there is no evidence for an active transportstep; and (c) that the data accord well with the view that exogenoussubstrates diffuse freely to the respiratory sites where theycompete on equal terms with endogenous substrates.     

The Influence of Low Temperature on the Rate of Translocation in the Phloem of Salix viminalis L.

The rate of phloem translocation in willow stems was found tobe little reduced on cooling considerable lengths of the transportpathway. Even when 65 cm lengths of stem were cooled from about23 to 0 ?C the reduction in rate was judged to be less than20 per cent. It is argued that the velocity of translocationis well maintained in cooled stems and that there is no goodcase for supposing that the concentration of the translocatehad altered. The rate of radial transfer of materials from thephloem to the xylem did show a decrease in cooled zones. Theimplications of the results on current theories of phloem transportare discussed.     

The Role of Phloem Transport in the Translocation of Sucrose Along the Stem of Carnation Cut Flowers

The pathway and distribution of ¹⁴C-sugars in flower parts havebeen examined to find out in which tissue sugars are translocatedin the stem of the cut carnation; ¹⁴C-sucrose or ¹⁴C-glucosewas supplied at the base of the cut stem from a feeding solutionand the localization and chemical nature of the carbon-14 recoveredfrom flower parts were investigated. By reducing the rate oftranspiration it was found that the uptake of feeding solutionwas also reduced, but the distribution of absorbed ¹⁴C-sucrosein the flower parts was different from that which would be expectedif sucrose moved exclusively in the transpiration stream. Autoradiographsdemonstrated that ¹⁴C absorbed from the feeding solution as¹⁴C-sucrose appeared in both xylem and phloem but predominantlyin the latter; girdling failed to stop the translocation ofthe absorbed ¹⁴C-sucrose. Results of experiments with ¹⁴C-sucroseand ¹⁴C-glucose showed that sucrose was the mobile sugar andthat glucose was converted to sucrose before it was translocated.It was concluded that the translocation of sucrose absorbedfrom the feeding solution takes place both in xylem and phloemand is regulated by a mechanism involving the loading and translocationof sucrose in the phloem.     

Respiration and Phloem Translocation in the Roots of Chickpea (Cicer arietinum)

Root growth in chickpea (Cicer arietinum) has been studied fromthe early vegetative phase to the reproductive stage in orderto elucidate its growth and maintenance respiration and to quantifythe translocation of assimilates from shoot to root. A carbonbalance has been drawn for this purpose using the growth andrespiration data. The increase in the sieve tube cross-sectionalarea was also followed simultaneously. Plants growing in a nutrient culture medium were studied todetermine the relative growth rate (RGR) 5–60 d aftergermination. RGR declined from 113 to 41 mg d^–1 g^–1during the measurement period. Simultaneous with the RGR analysis,respiration rate was also measured using an oxygen electrode.The respiration rate declined as the plants aged and a drasticreduction was recorded following anthesis. The relationshipbetween RGR and respiration rate was used to extrapolate themaintenance respiration (m) and growth respiration (1/Y_EG).The respiration quotient (r.q.) of the roots was 1.2 and theQ₁₀ in the range 20–25 °C was 2·2. A carbon balance for the roots was constructed by subtractingthe carbon lost during respiration from that gained during growth.The roots were found to respire no less than 80% of the carbontranslocated. The increase in the cross-sectional area composed of sieve tubeswas measured near the root-shoot junction as the plants grew.Chickpea has storied sieve plates which simplifies these measurements.Their cross-sectional area increased during growth mainly becauseof an increase in sieve tube number. The diameter of individualsieve tubes remained constant. Specific mass transfer (SMT) values for seive tubes into theroots have been computed during various stages of growth. SMTvalues were relatively constant before anthesis (approx. 6·5g h^–1 cm^–2), but decreased following anthesis. Wedid not evaluate possible retranslocation from roots: any suchretranslocation would have the effect of increasing our SMTvalues. Chickpea, Cicer arietinum, legume, root, respiration, phloem, translocation, carbon balance, specific mass transfer, sieve-tube dimensions     

Respiratory Response of Acer pseudoplatanus Cells to Pyruvate and 2,4-Dinitrophenol

The endogenous respiration rate of unstarved cultured cells of Acer pseudo-platanus L. is markedly stimulated by 2,4-dinitrophenol. Pyruvate is also stimulatory but to a lesser degree than dinitrophenol. Exogenously supplied sugars cause no short-term stimulation. Pyruvate does not enhance the elevated rate of O₂ uptake in the presence of dinitrophenol but does cause additional CO₂ evolution. The endogenous concentration of pyruvate is elevated in the presence of dinitrophenol. These observations suggest that the rate of O₂ uptake by the unstarved intact cells is limited by the rate of glycolysis and that rate of glycolysis is regulated by the intracellular concentration of adenine nucleotides or inorganic phosphate. Dinitrophenol stimulation of endogenous respiration is due in part to an indirect acceleration of glycolysis but also to a more direct facilitation of oxidation in the presence of excess mitochondrial substrate.     

A Quantitative Investigation of a Peristaltic Model for Phloem Translocation

An analysis is made of a peristaltic model of phloem translocationIt is postulated that the periodic action of contractile orbending organelles drive a longitudinal flow of solution withintubules which connect sieve pores in successive sieve plates Plausible values are assumed for the velocity of propagationof the contraction wave, the frequency, the amplitude, and theviscosity of the solution and its concentration. Using relationswhich describe peristaltic flow, predictions are made for thevalues of parameters such as the velocity of solution, the drivingpressure, and rate of energy dissipation. These predicted valuesare seen to be reasonable when compared with the known propertiesof other biological contractile systems. Thus the model is quantitativelyacceptable.     

Phloem Translocation and Heat-induced Callose Formation in Field-grown Gossypium hirsutum L

Phloem translocation rates in field-grown cotton (Gossypium hirsutum L.) dropped from morning to afternoon and continued to decline toward evening, except that recovery occurred following the hottest afternoon when the maximum temperature was 44 C. Water deficits increased from morning to evening, and severity of deficits generally were proportional to daytime heating. Water stress contributed toward reducing translocation but was not always the governing factor. Callose breakdown appeared to be slower than heat-induced synthesis, and in the evening callose still reflected the influence of high afternoon temperatures. Translocation was considerably reduced when about 50% or more of the hypocotyl sieve plates had large amounts of callose. While heat-induced callose may have reduced translocation because of sieve plate pore constriction, temperatures of 39 to 44 C appeared to inhibit an additional component of translocation as well, possibly in the leaf blade.     

Calculation of Translocation Coefficients from Phloem Anatomy for use in Crop Models

Translocation coefficients, computed for unit ground area, arerequired in crop models as part of the simulation of the partitioningof assimilates. An equation for translocation is derived byconsidering pressure-driven flow and the physical dimensionsof the pathway in the phloem. Numerical estimates of the translocationcoefficients for grasses and trees are calculated using anatomicaldata. The values found are compatible with published rates oftranslocation in the phloem.Copyright 1995, 1999 Academic Press Model, Münch, phloem, sieve tube, translocation     

Supplemental Substrate Enhancement of 2,4-Dinitrophenol Mineralization by a Bacterial Consortium

A Janthinobacterium sp. and an actinomycete, both capable of mineralizing 2,4-dinitrophenol (DNP), were used to construct a consortium to mineralize DNP in nonaxenic bench-scale sequencing batch reactors (SBRs). Average K_m values for DNP mineralization by pure cultures of the Janthinobacterium sp. and the actinomycete were 0.01 and 0.13 μg/ml, respectively, and the average maximum specific growth rate (μ_max) values for them were 0.06 and 0.23/h, respectively. In the presence of NH₄Cl, nitrite accumulation in pure culture experiments and in the SBRs was stoichiometric to initial DNP concentration and the addition of nitrogen enhanced DNP mineralization in the SBRs. Mineralization of 10 μg of DNP per ml was further enhanced in SBRs by the addition of glucose at concentrations of 100 and 500 μg/ml but not at 10 μg/ml. Possible mechanisms for this enhanced DNP mineralization in SBRs were suggested by kinetic analyses and biomass measurements. Average μ_max values for DNP mineralization in the presence of 0, 10, 100, and 500 μg of glucose per ml were 0.33, 0.13, 0.42, and 0.59/h, respectively. In addition, there was greater standing biomass in reactors amended with glucose. At steady-state operation, all SBRs contained heterogeneous microbial communities but only one organism, an actinomycete, that was capable of mineralizing DNP. This research demonstrates the usefulness of supplemental substrates for enhancing the degradation of toxic chemicals in bioreactors that contain heterogeneous microbial communities.     

2,4-Dinitrophenol as a specific inhibitor of the breakdown of the actomyosin-phosphate-ADP complex.

2,4-Dinitrophenol (DNP) was found to cause a "clearing response" of myosin B in a medium in which "superprecipitation" of myosin B would otherwise take place. The effect of actin concentration on Mg-ATPase [EC 3.6.1.3] of HMM was studied in the presence and absence of DNP. The results indicate that DNP causes an increase rather than a decrease in the affinity of HMM for actin, and that it causes a decrease only in the actin-activated portion of the Mg-ATPase activity. Using a light-scattering technique, it was shown that neither the ATP-induced dissociation of acto-HMM nor subsequent reassociation is significantly affected by the presence of DNP. As for the formation of the myosin-phosphate-ADP complex in the myosin-ATPase reaction, it was shown that formation of the reactive complex is not affected by DNP. It can thus be concluded that DNP inhibits the decomposition of the actomyosin-phosphate-ADP complex, which is thought to be coupled with superprecipitation.