Light Actions in the Germination of Cocklebur Seeds: III. EFFECTS OF PRE-TREATMENT TEMPERATURE ON GERMINATION RESPONSES TO FAR-RED LIGHT AND ON DARK GERMINATION IN THE RED LIGHT-REQUIRING UPPER SEEDS

Esashi, Y., Oota, H., Saitoh, H. and Kodama, H. 1985. Lightactions in the germination of cocklebur seeds. III. Effectsof pre-treatment temperature on germination responses to far-redlight and on dark germination in the red light-requiring upperseeds.—J. exp. Bot. 36: 1465-1477. Red light (R) responsiveness in R-requiring upper cocklebur(Xanthium pennsylvanicum Wallr.) seeds changed in differentpatterns during a soaking period at different temperatures.At temperatures above 23°C, the responsiveness increasedand then decreased. At lower temperatures (3–18°C),however, it continued to increase throughout an experimentalperiod. The lower temperatures caused germination in the subsequentdark at 33°C, regained the R responsiveness and acquiredthe dark germinability when subsequently exposed to 8°C,to an extent proportional to the duration of the chilling. Far-red (FR) was inhibitory to germination in an earlier soakingperiod at lower temperatures, but its effect gradually decresed,and finally turned promotive. The negative FR response was repeatedlycontrolled by the following R irradiation. However, the positiveFR response was enhanced by an immediate R irradiation, andFR/R reversibility occurred after the second FR. In contrastto the R responsiveness and dark germinability, the positivegermination response to FR was not induced by soaking at 3°C,in which the growth of the axial tissue as a photoreceptivesite did not occur at all. Similarly, it was not manifestedwhen the seeds soaked at 33°C were subsequently subjectedto 8°C. Key words: Cocklebur seeds, dark germination, far-red light, low temperature, red light, seed germination, Xanthium pennsylvanicum     

Light Actions in the Germination of Cocklebur Seeds: I. DIFFERENCES IN THE LIGHT RESPONSES OF THE UPPER AND LOWER SEEDS

Germination responses to light were studied in the upper andlower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Thelower seed was dark-germinating and negatively photoblastic;the upper one had a red-light (R) requirement and was positivelyphotoblastic. Germination of the lower seeds was inhibited bya prolonged single irradiation with R, blue (B) or far-red (FR)light applied during imbibition. The maximal inhibitory effectof a single irradiation occurred 9 h and 13 h after the startof soaking at 33 °C and 23 °C, respectively. However,the inhibitory effect of R differed from that of B and FR, byonly delaying germination. A single exposure to B or FR lightcould be replaced by intermittent B or FR irradiation, and theireffects were repeatedly reversible by the following R irradiation.If the upper seeds were not exposed to R during imbibition,they failed to germinate even at 33 °C which was optimalfor germination, and the promotive effect of R increased withdelay of its application time. The photoperceptive locus incocklebur seeds was the axial tissue for all B, R and FR. Lightreceived by the cotyledonary tissue had little effect. Germinationdimorphism in response to light is discussed with respect tothe phytochrome content and the ageing of axial tissues. Key words: Blue light, Dimorphism, Far red light, Germination, Red light, Xanthium seed     

Light Actions in the Germination of Cocklebur Seeds: V. EFFECTS OF ETHYLENE, CARBON DIOXIDE AND OXYGEN ON GERMINATION IN RELATION TO LIGHT

Esashi, Y., Hase, S. and Kojima, K. 1987. Light actions in thegermination of cocklebur seeds. V. Effects of ethylene, carbondioxide and oxygen on germination in relation to light.–J.exp. Bot. 38: 702–710. Effects of ethylene, CO² and O² on the germination of after-ripenedupper cocklebur (Xanthium pennsylvanicum Wallr.) seeds wereexamined in relation to pre-irradiation by red (R) or far-red(FR) light In order to remove the pre-existing Pfr, seeds weresoaked in the dark for various periods prior to light irradiationand gas treatments. Regardless of light, 0.3 Pa C₂H₄ promotedgermination at 23 ?C, but it strongly inhibited germinationwhen applied at 33 ?C, the optimal temperature for the germinationof this seed. However, delayed application of C₂H₄ during 33?C incubation stimulated germination independently of lightin a similar manner to that seen at 23 ?C. It is, therefore,suggested that the germination-regulating action of C2H4 iscompletely independent of phytochrome. In contrast, the germination-promoting effect of 3–0 kPaCO₂ was pronounced only when the seeds were previously irradiatedby R, regardless of temperature, suggesting that CO₂ actionto promote germination depends upon Pfr. A synergism betweenCO₂ and C₂H₄ at 23 ?C was observed only in the germination ofseeds pre-irradiated by R, while at 33 ?C an antagonism occurredindependently of light. The stimulation of C₂H₄ production byCO₂ was most striking in the cotyledonary tissue pre-irradiatedby R. However, the R-dependent enhancement of CO₂-stimulatedC₂H₄ production was negated by the subsequent FR and it wasnot found in the presence of 1-aminocyclopropane-1-carboxylicacid (ACC). Moreover, the R dependency of the germination-promotingCO₂ effect disappeared in the presence of C₂H₄. The R-dependentC₂H₄ production enhanced by CO₂ may thus be involved, at leastpartially, in some step of conversion from methionine to ACC. The germination-promoting effect of C₂H₄, but not CO₂, was enhancedby O₂ enrichment regardless of light. However, the germination-promotingeffect of pure O₂ itself appeared to depend upon pre-irradiationwith R Key words: Carbon dioxide, cocklebur seed, ethylene, far-red light, germination, oxygen, red light, Xanthium pennsyloanicum     

Light Actions in the Germination of Cocklebur Seeds II. Possible Origin of the Diversity of Light Responses in Seed Germination

In negatively photoblastic, lower seeds of cocklebur (Xanthiumpennsylvanicum Wallr.), the respective germination-inhibitingeffects of red (R) and far-red (FR) lights were found in theproximal and near-tip zones of the axial tissues. In contrast,the germination-stimulating effect of R in positively photoblastic,upper cocklebur seeds was manifested in the near-tip zone ofthe axes, the R effect being reversed when FR was given to thezone. The R-sensitive zone in the upper seeds, however, shiftedtowards the more proximal zone as the period of pre-soakingat low temperatures increased. This shift was accompanied bythe ability to germinate in the dark in the upper seeds. In the lower seeds, R inhibited axial growth in the near-tipzone, whereas FR inhibited it in the proximal zone. In contrast,axial growth in the near-tip zone of the upper seeds was promotedby R. In both seeds, light had little effect on the growth ofthe radicle tip. Pre-soaking at low temperatures induced dark-germinationby hastening the axial growth of the upper seeds, thus allowingthe upper seed to resemble the lower one. We therefore proposea hypothesis that explains the diversification of photoresponsesin seed germination. (Received August 7, 1984; Accepted December 24, 1984)     

Physiological Studies on Germination of Phaseolus mungo Seeds: II. GLUCOSE AND ORGANIC-ACID METABOLISMS IN THE EARLY PHASES OF GERMINATION

The metabolism of glucose and organic acids was studied in theearly phases of germination of Phaseolus mungo seeds. Duringthe first 5–6 h of imbibition, ethanol fermentation activelyoccurred and in the following stage the TCA cycle seemed tobe markedly activated. CO₂ fixation seemed to be low, at leastin the early stage of germination. Aspartic acid was convertedactively into malic acid. This seemed to be a cause of the activesynthesis of malic acid observed.     

Physiological Studies on Germination of Phaseolus mungo Seeds: I. DEVELOPMENT OF RESPIRATION AND CHANGES IN THE CONTENTS OF CONSTITUENTS IN THE EARLY STAGES OF GERMINATION

Changes in respiration rate and in the contents of various constituentsduring the early period of germination of Phaseolus mungo seedswere studied. The course of the respiration developed in threephases. A sharp rise was observed in the first phase (Phasea), followed by the second phase (Phase b) of fairly constantrespiration rate. The respiration rate increases again in thethird phase (Phase c). The O² uptake in Phases a and b was notinhibited by iodoacetate and fluoroacetate, while that in Phasec was inhibited. The contents of aspartic and glutamic acidsand amides were very high. A decrease of aspartic acid contentwas notable during the first few hours of germination. Citricand malic acids were the major organic acid constituents. Citricacid content in the seeds decreased, while that of malic acidremained unchanged. The leaching of malic acid into the soakingmedium was remarkable during the first 6 h of imbibition     

Studies on the Physiology of Rice: XV. CHANGES IN THE NITROGEN METABOLISM OF SEEDS DURING GERMINATION AND THEIR RELATION TO THE APPLICATION OF GROWTH REGULATORS

Seeds of rice variety ‘Chinsurah Boro’ were treatedwith two concentrations of indoleacetic acid (IAA) and maleichydrazide (MH), 0.1 mg. per litre and 0.01 mg. per litre, duringthe first 7 days of germination, and the resulting changes innitrogen metabolism of both endosperm and embryo were determined.The seedling growth at different times during the treatmentwas recorded. Uptake of water by the embryo increased up to 72 hours, butthereafter the percentage water content fell, owing to large-scaletranslocation of dry matter to it. The endosperm showed increasingwater content for 5 days. There are two marked stages of nitrogen metabolism of the seedduring germination. During the first 72 hours hydrolysis ofprotein in the endosperm and translocation of soluble nitrogento the embryo are the predominant features. Thereafter the embryoactively synthesizes protein from the products of translocation. IAA and MH affected the seedling growth similarly. Initiallythere was a small retardation of leaf growth, but at later periodsrecovery took place. Root growth was more adversely affectedthan shoot growth, the effect also persisting longer. Thesechanges in growth were reflected in the nitrogen metabolismof the seedlings. For the first few days IAA and MH retardedthe supply of soluble nitrogen from the endosperm to the embryo,consequently there was less soluble nitrogen in the embryo thoughprotein synthesis there was affected only slightly. The differencein soluble nitrogen between the treated and untreated embryospersisted throughout the seven days. An attempt is made to explainthis on the basis that primarily IAA and MH retard the enzymaticactivity responsible for the hydrolysis of protein in the endosperm.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: IV. PROTEIN SYNTHESIS AND ENZYME ACTIVITY CHANGES WITHIN THE COTYLEDONS OF RICINUS COMMUNIS L. SEEDS

Drying of immature seeds of Ricinus communis L. cv. Hale (castorbean) during the desiccation-tolerant phase of development causesthem to germinate upon subsequent rehydration. This desiccation-inducedswitch from development to germination is also mirrored by achange in the pattern of soluble and insoluble protein synthesiswithin the cotyledons of the castor bean. Following rehydrationof seeds prematurely dried at 40 d after pollination (DAP),cotyledonary proteins characteristic of development (e.g. storageproteins) are no longer synthesized; hydrolytic processes resultingin their degradation commence (after 12 h) in a manner similarto that observed following imbibition of the mature seed. Apattern of protein synthesis recognizable as germination/growth-associatedoccurs; premature drying has elicited a redirection in metabolismfrom a developmental to a germinative mode. Desiccation is alsorequired for the induction (within cotyledons of 35 DAP seeds)of enzymes involved in protein reserve breakdown (leucyl ß-naphthylamidase;LeuNAase) and lipid utilization (isocitrate lyase; ICL), anevent intimately associated with the post-germinative (growth)phase of seedling development. Thus, at a desiccation-tolerantstage of development, premature drying results in the suppressionof the developmental metabolic programme and a permanent switching-onof the germination/growth metabolic programme. Key words: Desiccation, metabolism, seed development, seed germination, castor bean, cotyledons     

Dormancy and Impotency of Cocklebur Seeds: IV. Effects of Gibberellic Acid, Benzyladenine, Thiourea, and Potassium Nitrate on the Growth of Embryonic Axis and Cotyledon Segments

Germination of nondormant but impotent small cocklebur seeds (Xanthium pennsylvanicum Wallr.) was promoted profoundly with thiourea or benzyladenine, and slightly with gibberellic acid. Gibberellic acid was ineffective in causing the germination of dormant cocklebur seeds, although thiourea and benzyladenine were effective. Experiments with excised seed pieces showed that the promotive effects of thiourea, benzyladenine, and gibberellic acid on cocklebur seed germination were associated with the enhancement of growth of seed parts; thiourea stimulated predominantly the axial growth, whereas benzyladenine stimulated predominantly the cotyledonary growth.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): III. INHIBITION OF GERMINATION BY SHORT EXPOSURE TO HIGH TEMPERATURE

Seeds of pearl millet were germinated on wet filter paper attemperatures up to 50 ?C. In one experiment, the temperaturewas held at 50 ?C during imbibition and was then lowered to32 ?C or 25 ?C. Germination rate and the maximum fraction ofseeds germinating (G_m) both decreased as the time of exposureto 50 ?C increased. In contrast, exposure to 50 ?C after imbibitionfor 8 h slowed germination but did not significantly reduceG_m. When the ‘high’ temperature imposed after imbibitionwas reduced from 50 ?C to 45 ?C, there was a small reductionin the rate of germination but not in G_m. The responses haveimplications for the optimum time of sowing in the tropics whenmaximum daytime soil temperature at the depth of sowing is inthe range of 45–50 ?C. Key words: Pennisetum typhoides, Temperature, Germination     

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.     

Studies on the Chemical Induction of a Light Requirement for Germination in Seeds of Lettuce, Lactuca sativa L., cv. 'Great Lakes'

Detailed time courses of germination were recorded for threeseed batches of lettuce, cv. ‘Great Lakes’. Thegermination of this variety is usually considered to be insensitiveto light, but considerable variability was noted between seedbatches with respect to the effect of light. Previous reportson the chemical induction of a light requirement for germinationin lettuce seeds were not substantiated for this variety. Naringeninwas found to be relatively ineffective, and although coumarin,ABA, and IAA all delayed germination with increasing concentration,this delay was not diminished by continuous light. The inhibitionsof germination by sucrose, mannitol, Carbowax, sodium chloride,potassium chloride, and potassium nitrate were also similarin light and darkness; these inhibitions appeared to relateto the osmotic pressures of the solutions. It is postulatedthat previous reports of chemical induction of a light requirementmay be the result of presenting germination figures at one pointin time. Analysis of germination by detailed time courses showedthat while prolonged light irradiation slightly acceleratedgermination, perhaps by raising seed temperature, it in no wayovercame chemical inhibition.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: III. INSOLUBLE PROTEIN SYNTHETIC PATTERN CHANGES WITHIN THE ENDOSPERM OF Ricinus communis L. SEEDS

Kermode, A. R., Gifford, D. J. and Bewley, J. D. 1985. The roleof maturation drying in the transition from seed developmentto germination. III. Insoluble protein synthetic pattern changeswithin the endosperm of Ricinus communis L. seeds.—J.exp. Bot. 36: 1928–1936. Immature seeds of Ricinus communisL. cv. Hale (castor bean) removed from the capsule at 30 or40 days after pollination (DAP) can be induced to germinateby being subjected to drying. This desiccation–inducedswitch from development to germination is mirrored by a change,upon subsequent rehydration, in the pattern of insoluble proteinsynthesis within the endosperm storage tissue. During normaldevelopment from 25–40 DAP there is rapid synthesis ofthe insoluble (11S) crystalloid storage protein. At later stagesof development (45 and 50 DAP), crystalloid protein synthesisdeclines markedly and synthesis of new insoluble proteins commences.Following premature drying at 30 or 40 DAP, the pattern of insolubleprotein synthesis upon rehydration is virtually identical tothat following imbibition of the mature seed. Proteins synthesizedduring normal late development (at 45 and 50 DAP) are producedup to 48 h after imbibition; a subsequent change in the patternof insoluble protein synthesis occurs between 48 and 72 h. Thus,in contrast to the rapid switch in the pattern of soluble proteinsynthesis induced by drying, insoluble protein syntheses withinthe endosperm are redirected towards those uniquely associatedwith a germination/growth programme only after a considerabledelay following mature seed imbibition, or following rehydrationof the prematurely dried seed. Nevertheless, these results supportour contention that drying plays a role in the suppression ofthe developmental metabolic programme and in the permanent inductionof a germination/growth programme. Key words: Desiccation, crystalloid storage proteins, castor bean, seed development, seed germination     

Factors controlling Flowering in the Chrysanthemum: IV. THE SITE OF VERNALIZATION AND TRANSLOCATION OF THE STIMULUS

By confining low-temperature treatment to the growing tip ofthe Chrysanthemum plant, the apex has been shown to be the seatof perception of vernalization, a result which is in full accordwith earlier experiments on other plants requiring vernalization.Experiments on the translocation of the stimulus through maturetissues involving stock/scion and ‘approach’ graftswith or without defoliation gave negative results. By decapitatingin turn the main axis and the resulting laterals produced, ithas been shown that the stimulus is passed on from existingapices to laterals of up to the seventh order at least, lateralapices which were formed well after the time of chilling. Theseresults are discussed in relation to prior work on the existenceof a specific vernalization substance as well as a floweringhormone and their translocation.     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.): VI. INFLUENCE OF PRIMING ON GERMINATION RESPONSES TO TEMPERATURE AND WATER POTENTIAL DURING SEED DEVELOPMENT

Seed priming (imbibition in water or osmotic solutions followedby redrying) generally accelerates germination rates upon subsequentre-imbibition, but the response to priming treatments can varyboth within and among seed lots. Seed maturity could influenceresponsiveness to priming, perhaps explaining variable primingeffects among developmentally heterogeneous seed lots. In thecurrent study, muskmelon (Cucumis melo L.) seeds at two stagesof development, maturing (40 d after anthesis (DAA)) and fullymature (60 DAA), were primed in 0?3 M KNO₃ for 48 h at 30 ?C,dried, and imbibed in polyethylene glycol 8000 solutions of0 to –1?2 MPa at 15, 20, 25, and 30 ?C. Germination sensitivitiesto temperature and water potential () were quantified as indicatorsof the influence of seed maturity and priming on seed vigour.Germination percentages of 40 and 60 DAA control seeds weresimilar in water at 30 ?C, but the mean germination rate (inverseof time to germination) of 40 DAA seeds was 50% less than thatof 60 DAA seeds. Germination percentages and rates of both 40and 60 DAA seeds decreased at temperatures below 25 ?C. Reductionsin also delayed and inhibited germination, with the 40 DAAseeds being more sensitive to low than the 60 DAA seeds. Primingsignificantly improved the performance of 40 DAA seeds at lowtemperatures and reduced , but had less effect on 60 DAA seeds.Priming lowered both the minimum temperature (T_b) and the minimum (_b) at which germination occurred. Overall, priming of 40 DAAseeds improved their germination performance under stress conditionsto equal or exceed that of control 60 DAA seeds, while 60 DAAseeds exhibited only modest improvements due to priming. Asthe osmotic environment inside mature fruits approximates thatof a priming solution, muskmelon seeds may be ‘primed’in situ during the late stage of development after maximum dryweight accumulation. Key words: Cucumis melo L., seed priming, germination, vigour, development, temperature     

The Role of Maturation Drying in the Transition from Seed Development to Germination: I. ACQUISITION OF DESICCATION-TOLERANCE AND GERMINABILITY DURING DEVELOPMENT OF Ricinus communis L. SEEDS

Kermode, A. R. and Bewley, J. D. 1985. The role of maturationdrying in the transition from seed development to germination.1. Acquisition of desiccation–tolerance and germinabilityduring development of Ricinus communis L. seeds.—J. exp.Bot. 36: 1906–1915. Seeds of Ricinus communis L. cv. Hale(castor bean) undergo a transition from desiccation–intoleranceto desiccation–tolerance approximately midway throughtheir development. Tolerance of slow desiccation is gained overonly a few days of development (between 20 and 25 d) and isachieved well before the completion of major developmental events,such as reserve deposition and the onset of normal maturationdrying. A tolerance of very rapid water loss brought about bydrying over silica gel is not acquired by this seed until nearmaturity. Coincident with the acquisition of tolerance to slowdesiccation the seeds gain the capacity to germinate upon subsequentrehydration. Germinability and capacity for normal post–germinativegrowth during the tolerant phase are not fully expressed unlessthe seed is dried at an optimal rate, which is dependent uponthe developmental stage of the seed. Drying presumably actsto terminate developmental processes and to initiate those metabolicprocesses necessary to prepare the seed for germination andgrowth. Key words: Desiccation-tolerance, germinability, seed development, castor bean     

The Role of Maturation Drying in the Transition from Seed Development to Germination: II. POST-GERMINATIVE ENZYME PRODUCTION AND SOLUBLE PROTEIN SYNTHETIC PATTERN CHANGES WITHIN THE ENDOSPERM OF Ricinus communis L. SEEDS

Kermode, A. R. and Bewley, J. D. 1985. The role of maturationdrying in the transition from seed development to germination.II. Post–germinative enzyme production and soluble proteinsynthetic pattern changes within the endosperm of Ricinus communisL. seeds.—J. exp. Bot. 36: 1916–1927. Immature seedsof Ricinus communis L. cv. Hale (castor bean) removed from thecapsule at 30 or 40 d after pollination (DAP) do not germinateunless first subjected to a desiccation treatment. This changefrom development to germination elicited by premature desiccationis also mirrored by a change, upon subsequent rehydration, inthe pattern of soluble protein synthesis within the endospermstorage tissue. Following rehydration of prematurely dried 30or 40 DAP seeds, soluble proteins characteristic of developmentcease to be synthesized after 5 h of imbibition, and those uniquelyassociated with germination and growth are then produced. Apattern of soluble storage protein breakdown comparable to thatfound in endosperms from mature seeds following imbibition isalso observed. In contrast, hydration of 40 DAP seeds immediatelyfollowing detachment from the mother plant results in a continuationof the developmental pattern of protein synthesis. Prematuredesiccation at 40 DAP elicits the production within the endospermof enzymes involved in protein reserve breakdown (leucyl ß–naphthylamidase;LeuNAase) and lipid utilization (isocitrate lyase; ICL) to levelscomparable to those observed in mature–hydrated endosperms.It is proposed that drying plays a role in redirecting metabolismfrom a developmental to a germinative mode; it also appearsto be a prerequisite for the induction of hydrolytic enzymesessential to the post–germinative (growth) phase of seedlingdevelopment. Key words: Desiccation-tolerance, germinability, seed development, castor bean     

STUDIES OF THE TRIAD : IV. Structure of the Junction in Frog Slow Fibers

The structure of the triadic junction in frog slow fibers has been studied and compared with that of twitch fibers. The junctional gap is wider (by approximately 13%) in slow fibers. The junctional feet have the same size and disposition as in twitch fibers, although the size and shape of the junctional areas are different. It is concluded that the role of triads in slow fibers is the same as in twitch fibers     

Studies in the Principles of Phytotoxicity: IV. THE EFFECTS OF THE DEGREE OF NITRATION ON THE TOXICITY OF PHENOL AND OTHER SUBSTITUTED BENZENES

1. The relative toxicity of phenol, o- and p-nitro, 2:4-dinitro-,and 2:4:6- trinitrophenol was measured by up to five techniques.Determinations were made of the concentrations required to reduceby half (i) the growth rate of Trichoderma viride at pH 3, (ii)the frond multiplication rate of Lemna minor at pH 5·1–5·4,(iii) the respiration rate of yeast at pH, (iv) the respirationrate of infiltrated leaf disks of Brassica alba at pH 3, and(v) the number of B. alba seedlings surviving after sprayingwith buffered solutions of the compounds. Whenever possibletoxicity was measured at a pH level below pK so as to eliminatethe effect of pH on toxicity, but in some of the measurementswith 2:4-dinitro- phenol (pK = 4·0) and with picric acid(pK = 0·8) this was not feasible. It was then necessaryto measure toxicity at a pH at which the compound was much dissociated;from the measured values of toxicity of dinitrophenol the toxicityat a lower pH was estimated from the generalized pH-activityrelationship of Simon and Beevers (5952). 2. The relative toxicity of the nitrophenols is very similarwhichever test procedure is employed. The first two nitrationsare accompanied by large increases in toxicity, but with thethird successive nitration there is a decrease. 3. The toxicity of the corresponding mono, di, and trinitroderivatives of anisole, phenetole, toluene, and nitrobenzenewas determined using T. viride. In each series the first andthird stages of nitration brought about large increases in toxicitybut the intermediate one had little effect. 4. All the p-nitro compounds were more toxic than their o- isomers. 5. On the basis of the concepts put forward by Ferguson (1939)and Albert (1951) it is concluded that the toxic action of thenitroanisoles and nitrotoluenes is ‘structurally non-specific’,i.e. the effect is not dependent on the presence or absenceof a particular chemical group in the molecule. With both thenitrobenzene and the phenol series the results suggest thatthe exact chemical structure is an important factor.