Dormancy in Dioscorea: Rapid Germination of Detached Embryos from Dormant Seeds of D. tokoro

The intact dormant seeds of Dioscorea tokoro germinate slowlyif at all between 11-23°C; for full and rapid germinationthey require prior chilling treatment [Okagami and Kawai (1982)Bot. Mag. Tokyo 95: 155]. The germination abilities of zygoticembryos detached from dormant seeds of this species were studiedunder various nutritional and temperature regimes. For germinationof embryos, the minimum nutritional components in Murashigeand Skoog's (1962) medium that were required were sucrose andNO^–₃ or SO^2–₄. As the source of carbohydrate forgermination of detached embryos, sucrose, mannose and maltosewere effective; glucose and fructose were less effective; andrhamnose was entirely unable to support germination. Embryos detached from dormant seeds, incubated with the sucroseplus KNO₃, germinated more rapidly with increasing temperatureup to 35°C. However, application of sucrose and KNO₃ didnot induce germination of intact seeds above 26°C. Therefore,it is very possible that the endosperm exerts an inhibitoryfunction on germination at such high temperatures. When seeds were incubated after a cut was made over a smallpart of the edge of the endosperm in which the radicle of theembryo is encased, germination occurred rapidly but the increasein germination percentage was slight. This result suggests thatthe endosperm suppots part of the germination inhibition bymeans of a mechanical barrier or its impermeability to wateror gases. Physiological features of the endosperm alone or interactionsbetween the embryo and endosperm may contribute significantlyto the characteristics of dormancy of intact seeds of this species. (Received May 30, 1988; Accepted January 11, 1989)     

Structural Associations Between Mitochondria and Chloroplasts in the Bundle Sheath Cells of Portulaca oleracea

Mitochondria and chloroplasts are structurally associated inthe bundle sheath cells of Portulaca oleracea L., an NAD malicenzyme type C₄ plant. These associations occur in some restrictedregions where the mitochondrial cristae extend inwards. Exposureof plants to 1.0 µ11^–1 SO₂ for 3 h induced shrinkageof mitochondria of the bundle sheath cells, which further visualizedthe structural association between the organelles. C₄ plant, chloroplast, mitochondrion, Portulaca oleracea, sulphur dioxide     

Priming and accelerated ageing affect L-isoaspartyl methyltransferase activity in tomato (Lycopersicon esculentum Mill.) seed

Damage and degradation of cellular proteins is observed duringage-induced seed deterioration. L-Isoaspartyl protein methyltransferase(EC 2.1.1.77 [EC] ) is an enzyme hypothesized to play a role in limitingand repairing age-induced damage to proteins. Tomato (Lycopersiconesculentum Mill. ‘New Yorker’) seeds were assayedfor changes in L-isoaspartyl methyl-transferase activity duringaccelerated ageing and after osmotic priming. Accelerated ageingof seeds for 1–4 d at 45C and 100% relative humidityreduced germination from 94% to 71%, increased the mean timeof germination (MTG) from 2.4 to 5.8 d, and was accompaniedby a correlative decrease in L-isoaspartyl methyltransferaseactivity (r²=0.90). Aged and untreated seeds were primed for7 d at 20C in darkness using aerated solutions of 3% KNO₃ orpolyethylene glycol 8000 (PEG) with equivalent osmotic potential(–1.25 MPa). Priming with KNO₃ decreased the MTG, butdid not improve germination percentage for untreated seeds.Priming did not affect L-isoaspartyl methyltransferase activityin untreated seeds, but restored activity in aged seeds primedin KNO₃ to levels near that of untreated seeds. Priming withPEG did not effectively improve the MTG or increase L-isoaspartylmethyltransferase activity. During germination, L-isoaspartylmethyltransferase activity remained constant for 48 h post-imbibitionand then declined, suggesting that the enzyme was developmentallyregulated and inactivated or degraded as radicle emergence occurred. Key words: L-Isoaspartyl methyltransferase, protein repair, seed priming, accelerated ageing, Lycopersicon esculentum     

Quantal Response of Seed Germination in Seven Genera of Cruciferae to White Light of Varying Photon Flux Density and Photoperiod

The response of the germination of seeds of Barbarea vema (Mill.)Aschers, Brassica chinensis L., Brassica juncea (L.) Czern.& Coss., Brassica oleracea L. var. gongylodes L., Camelinasaliva (L.) Crantz, Eruca saliva Mill., Lepidium sativum L.,Nasturtium officinale R. Br., and Rorippa palustris (L.) Besserto white fluorescent light of different photon flux densitiesapplied for different daily durations in a diurnal alternatingtemperature regime of 20 °C/30 °C (16 h/8 h) was quantifiedby linear relations between probit percentage germination andthe logarithm of photon dose, the product of photon flux densityand duration. The low energy reaction, in which increasing dosepromotes germination, was detected in all the seed populationsbut in Barbarea vema and Brassica Juncea the lowest photon doseapplied (10^–5–2 and 10^{–5 7} mol m^–2 d^–1,respectively) was sufficient to saturate the response. Comparisons,where possible, between photoperiods demonstrated reciprocity,i.e. germination was proportional to photon dose irrespectiveof photoperiod, for the low energy reaction in Brassica oleracea(1 min d^–1 to 1 h d^–1), Camelina saliva (1 min d^–1to 8 h d^–1), Eruca saliva (1 min d^–1 to 24 h d^–1),Lepidium sativum (I min d^–1 to 8 h d^–1) and Rorippapalustris (1 min d^–1 to 8 h d^–1), but not in Brassicachinensis and Nasturtium officinale. The high irradiance reaction,in which increasing dose inhibits germination, was detectedin Barbarea vema, Brassica chinensis, Brassica juncea, Brassicaoleracea, and Camelina saliva. The minimum dose at which inhibitionwas detected was lO^–0–3 mol m^–2 d^–1.These results are discussed in the context of devising optimallight regimes for laboratory tests intended to maximize germination The response of germination to photon dose was also quantifiedwith 3 x 10^–4 M GA₂, co-applied (Brassica chinensis, Camelinasaliva, and Lepidium sativum) and with 2 x 10^–2 M potassiumnitrate co-applied (Brassica chinensis). In the latter casepotassium nitrate had no effect in the dark and inhibited germinationin the light, but GA₂, promoted germination substantially inall three species. Variation amongst seeds in the minimum photondose required to stimulate germination was not affected by co-applicationof GA₂, in Brassica chinensis and Camelina saliva, whereas seedsof Lepidium salivum showed a narrower distribution of sensitivitiesto the low energy reaction in the presence of GA₂ Barbarea vema (Mill.) Aschers, Brassica chinensis L., Brassica juncea (L.) Czern. & Coss., Brassica oleracea L. var. gongylodes L., Camelina saliva (L.) Crantz, Eruca saliva Mill., Lepidium satiaum L., Nasturtium officinale R. Br., Rorippa palustris (L.) Besser, Cruciferae, light, gibberellic acid, seed germination, seed dormancy     

Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

Imsande, J. 1986. Inhibition of nodule development in soybeanby nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max(L.) Merr.] is inhibited by continuous growth in the presenceof 4· mol m^–3 KNO₃ The presence of 4·0 molm^–3 ‘starter nitrate’ for 3-6 d during noduledevelopment, however, subsequently stimulates nodule dry weightaccumulation and nitrogenase activity. These stimulations occureven though 4· mol m^–3 nitrate temporarily delaysnodule development, i.e. the late steps of nodule developmentare reversibly inhibited by a short-term exposure to 4·0mol m^–3 nitrate. On the other hand, treatment with 4·0mol m^–3 nitrate in excess of 14 d significantly reducesnodule dry weight Thus, extended growth in the presence of 4·0mol m^–3 KNO₃ seems to block both early and late stepsof nodule development. Nodulation of hydroponically grown soybeansis also inhibited by continuous growth in the presence of 2·0mol m^–3 (NH₄)₂SO₄ This inhibition is not caused by acidityof the growth medium. On the other hand, nodule development6 d after inoculation with Rhizoblum japonicum is not delayedby a 7-d exposure to 2·0 mol m^–3 (NH₄)₂SO₄ butis partially inhibited by a prolonged exposure to (NH₄)₂SO₄Because repression of nodulation by 4·0 mol m^–3KNO₃ is more severe than that by 2·0 mol m^–3 (NH₄)₂SO₄and because ammonium taken up by the soybean plant is not activelyoxidized to nitrate, it is suggested that there are at leasttwo mechanisms by which nitrate utilization represses noduleformation in soybean. Key words: Glycine max, nitrogen, nitrogen fixation, nodulation     

Preliminary characterization of 1-aminocyclopropane-1-carboxylate oxidase properties from embryonic axes of chick-pea (Cicer arietinum L.) seeds

For a deeper understanding of the germination of chick–pea(Cicer arietinum) seeds, which is dependent upon ethylene synthesis,a crude extract containing authentic ACC oxidase (ACCO) activitywas isolated in soluble form from the embryonic axes of seedsgerminated for 24 h. Under our optimal assay conditions (200mM HEPES at pH 7.0, 4µM FeS0₄, 6 mM Na–ascorbate,1 mM ACC, 20% 0₂, 3% CO₂ , and 10%glycerol) this enzyme was5–fold more active than under the conditions we used initiallyin the present work. The enzyme has the following K_m: 28 µMfor ACC (approximately 4–fold less than in vivo), 1.2%for O₂ (in the presence of an optimal CO₂ concentration of 3%),and 1% for CO₂ in the presence of O₂ (20%). The enzyme is inhibitedby phenanthroline (PNT) (specific chelating agent of ferrousion), and competitively inhibited (K₁, =0.5 mM) by 2–aminoisobutyricacid (AIB), and the enzymatic activity was not detectable inthe absence of CO₂. Under optimal assay conditions, the enzymehas two optimum temperatures (28 C and 35 C) and is inhibitedby divalent metal cations (Zn²⁺> CO²⁺>Ni²⁺>Cu²⁺>Mn²⁺>Mg²⁺) and by salicylic acid, propylgallate, carbonyl cyanidem–chlorophenyl hydrazone (CCCP), dinitrophenol (DNP),and Na–benzoate. The in vitro ACCO activity which we recoveredin soluble form is equivalent to approximately 80–85%of the apparent activity evaluated in vivo. Key words: ACC oxidase, Cicer arietinum, ethylene, germination, seeds     

Effect of Ethylene and Culture Environment on Rice Callus Proliferation

Modifications to the gaseous envelope by callus during culturein Petri dishes were shown to reduce growth and promote necrosisof several rice (Oryza sativa L.) cultivars. Incubatingcallusunder a continuous flow of gas mixtures of known compositionsuggested that the inhibition of growth was caused by the accumulationof ethylene, the depletion of oxygen and, to a lesser extent,the accumulation of carbon dioxide. In order to evaluate theimportance of ethylene accumulation aminoethoxyvinylglycine(AVG), 1-aminocyclopropane-l-carboxylic acid (ACC and silvernitrate (AgNO₃), were added to the nutrient medium and ethylenemeasurements performed during callus culture. Ethylene restrictedcallus growth particularly under high (35 °C) as comparedto moderate (25 °C) temperatures and under illuminated ascompared to darkened incubation. Under illuminated incubationat 25 °C AVG (5 mmol m^–3) and AgNO°(50 mmol m^–3)significantly improvedcallus growth (100 and 60% respectively)while ACC (200 mmol m^–3) significantly decreased growth(40%). AVG and AgNO₃ were less effective under dark incubationat 25 °C where ethylene production was lower. Furthermore,callus growth was significantly better in large as comparedto small culture vessels since the ethylene concentration wasdiluted and more oxygen was available for respiration. Bettercontrol of ethylene and increased oxygen availability couldbe a way ofproducing healthy callus for the formation of embryogenictissues of otherwise recalcitrant cultivars of rice (e.g. IndicaIR42) and may be a way of improving manipulation of other cerealspecies. Key words: 1-Aminocyclopropane-1-carboxylic acid, aminoethoxyvinylglycine, callus, ethylene, Oryza sativa, silver nitrate     

Ethylene Evolution from Bracts and Leaves of Poinsettia, Euphorbia pulcherrima Willd

Woodrow, L. and Grodzinski, B. 1987. Ethylene evolution trombracts and leaves ol Poinsettia, Euphorbia pulcherrima Willd.—J.exp. Bot. 38: 2024–2032. Ethylene release from fully expanded, red and white bracts andleaves of poinsettia, Euphorbia pulcherrima Willd., was compared.On a laminar (area) basis leaves contained about 50 times morechlorophyll and demonstrated 10 times the photosynthetic rateof the bracts. Both tissues contained starch, however, solublecarbohydrate in the bracts consisted primarily of reducing hexoseswhile the leaves contained mainly sucrose for translocation.The total free alpha-amino nitrogen content of the bract tissuewas twice that of the leaf tissue. The leaves contained moreACC (1-aminocyclopropane-1-carboxylic acid) and produced proportionallymore endogenous C²H⁴ than either the red or white bracts. ACC-stimulated₂H₄ release was also greatest from the green tissue indicatingthat the EFE (ethylene forming enzyme) was most active in theleaves. The specific activity of the ¹⁴C₂H₄/¹²C₂H₄ releasedfrom [2,3-¹⁴C]ACC confirmed ACC as the primary precursor ofC₂H₄ in this tissue. Ethylene release from the non-photosynthetic,bract tissue was not markedly affected by alterations in CO₂or light conditions. In green leaf tissue endogeneous ethylenerelease increased from 1·5 to 6·0 pmol C₂H₄ cm^–2h^–1 while ACC-stimulated ethylene release increased from10 to 35 pmol C₂H₄ cm^2– h^1– as the CO₂ partial pressureincreased from 100 to 1 200 µbar. Key words: Poinsettia, ethylene, bracts     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

The Effects of Priming and Ageing on Seed Vigour in Tomato

A comparison was made of the effects of seed priming or ageingtreatments on the performance of tomato (Lycopersicon esculentumMill. cv. UC204C) seeds according to a number of indices ofseed vigour. A single lot of tomato seeds was primed in 120mol m^–3 K2HPO4 + 150 mol m^–3 KNO₃ for 5 d at 20?C, or aged at 13% moisture content (dry weight basis) and 50?C for 6 d. Germination percentage (>98%) was unaffectedby priming and reduced to 85% by ageing. X-ray photographs andlongitudinal sections revealed the formation of free space surroundingthe embryo in dry primed seeds, which was not evident in controlor aged seeds. Priming increased the rate of germination atall temperatures above the base temperature (T_b), while ageingdecreased it. T_b was unaffected by priming and only slightlyincreased by ageing. The variation in individual times to germinationwas approximately doubled in both primed and aged seed comparedto the control, based upon the slopes of probit germinationpercentage versus log thermal time curves. Root growth aftergermination tests and seedling growth in both greenhouse andfield tests were not influenced by either priming or ageing.The conductivity test was found to be unreliable as a vigourtest for tomato seeds. The results identify several indiceswhich can be used to quantify seed vigour in tomato. They alsoillustrate that seed priming can enhance seed performance accordingto some criteria, while having no effect or decreasing qualityaccording to other criteria. Seed vigour can apparently be separatedinto various components which can be independently influencedby seed enhancement treatments. Key words: Tomato, seed germination rate, seed priming, seed vigour     

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     

Contrasted Effects of CO2 on the Regulation of Dormancy and Germination in Xanthium pennsylvanicum and Setaria faberi Seeds

The effects of CO₂ on dormancy and germination were examinedusing seeds of cocklebur (Xanthium pennsylvanicum Wallr.) andgiant foxtail (Setaria faberi Herrm.). The rate of germinationof the giant foxtail seeds as well as cocklebur was promotedby exogenously applied CO₂ at a concentration of 30 mmol mol^-1regardless of the sowing conditions. However, seeds which failedto germinate in the presence of CO₂, entered a secondary phaseof dormancy under unfavourable germination conditions. If CO₂was applied to seeds under conditions such as water stress imposedwith a 200 mol m^-3 mannitol solution, a hypoxic atmosphere of100 mmol mol^-1 O₂ or a treatment of 0·1 mol m^-3 ABA,development of secondary dormancy was accelerated. These contrastedeffects of CO₂ were observed in ecological studies. Under naturalfield conditions germination of buried giant foxtail seeds respondedpositively to CO₂ during a period of release from primary dormancyfrom Feb. to May, but CO₂ accelerated secondary dormancy commencingin early Jun. In other words, in the presence of CO₂, both theenvironmental conditions and the germination states of the seedsclearly showed secondary dormancy-inducing effects. Thus, itseems that CO₂ has contrasted effects on regulation of dormancyand germination of seeds depending on the germination conditions.Copyright1995, 1999 Academic Press Xanthium pennsylvanicum, cocklebur, Setaria faberi, giant foxtail, CO₂, water stress, hypoxia, ABA, germination, secondary dormancy     

Germination of Leymus arenarius and its Significance for Land Reclamation in Iceland

Leymus arenarius is used to stabilize the extensive areas oferoded volcanic sand in Iceland, both inland and on the coast.It has been reported previously to produce seeds of generallylow viability. We investigated the potential for seed dormancyand the responses of germination to temperature, light and salinity,as part of a re-assessment aimed at improving reclamation procedures. Contrary to previous reports, high rates of germination couldbe obtained under certain conditions. All caryopses were soakedin water (24 h) and stratified for 2 weeks (5 °C) beforethe subsequent germination tests. Constant temperatures or exposureto light resulted in very poor germination. Close to 100% germinationcould be obtained within 2 weeks in continuous darkness, underalternating temperatures with an amplitude of 10-20 °C ona 12 h cycle; high day temperatures appear to be important.Diurnal fluctuations in temperature of this order occurred underaverage weather conditions in the black, volcanic sands in Icelandduring the growing season (May-Sep.). The dark requirement isinterpreted as a selective response to the adverse conditionsfor establishment at the surface of the sand; the alternatingtemperature requirement may be a response to ensure dormancyunder deep burial with accreting sand, although it could havea role in gap-sensing under established canopies. Scarification,surface sterilization and treatment with n -butanol or KNO₃were all generally ineffective in promoting germination. Nearlyall caryopses that did not germinate, in all treatments, remainedviable. Caryopses of coastal populations of Leymus arenarius showedsignificantly higher total germination and more rapid germinationin 100 mmol l^-1 and 300 mmol l^-1 NaCl solutions than inlandpopulations. The inhibition of germination by salinity was anosmotically enforced dormancy effect rather than a lethal, toxicone; caryopses that had not germinated in saline solution generallywere able to germinate subsequently, when transferred to non-salineconditions.Copyright 1994, 1999 Academic Press Germination, Leymus arenarius (lymegrass), Triticeae, survival analysis, fluctuating temperature, sand stabilization, salinity tolerance, Iceland     

Nitrate-Ammonium Ratio Required for pH Homeostasis in Hydroponically Grown Soybean

Imsande, J. 1986. Nitrate-ammonium ratio required for pH homeostasisin hydroponically grown soybean.—J. exp. Bot. 37: 341–347. Plant acid-base homeostasis is achieved when the mmoles of hydroxylions produced in the plant equal the mmoles of protons. Reductionof nitrate to ammonia is the major source of hydroxyl ions whereasammonium uptake-assimilation and the metabolism of neutral sugarsto organic acids are the primary sources of protons. Soybean[Glycine max (L.) Merr plants were grown hydroponically on mediumsupplemented with 3·0 mol m^–3 nitrogen providedas various combinations of KNO₃ and NH₄NO₃ Plant growth consumedessentially all available nitrogen in each case; however, onlyin flasks supplemented with approximately 1·8 minolesof KNO₃ plus 0·6 mmole of NH₄NO₃ was the pH of the mediumunchanged. Thus, for every mmole of nitrogen assimilated, approximately0·6 mmole of dissociable protons must have been producedby the conversion of neutral sugars to carboxylic acids. Also,it was shown that a plant obtaining all of its nitrogen fromnitrate must neutralize or excrete approximately 0·5mmole of hydroxyl ion d^–1. Conversely, the plant derivingall of its nitrogen from dinitrogen must excrete or neutralizeat least 0·8 mmole of hydrogen ion d^–1 whereasthe plant deriving all of its nitrogen from aminonium must excreteor neutralize approximately 2·1 mmoles of hydrogen iond^–1. Nevertheless, plants grown on medium supplementedwith 2·4 mol m^–1 nitrate plus 0·6 mol m^–3ammonium did not achieve a higher growth rate than plants grownon 3·0 mol m^–3 nitrate. Key words: Glycine max, nitrogen fixation, nitrate utilization     

Effects of Seed Treatments on the Emergence of Oil Seed Rape Seedlings from Compacted Soil

Imbibition of seeds of oil seed rape (Brassica napus cv Jetneuf)in 10^–3 M aminoethoxyvinylglycine (AVG) or 10^–2silver thiosulphate (Ag⁺) had no effect on germination nor onthe emergence of seedlings from uncompacted or lightly compressedsoil, but significantly reduced emergence from moderately compressedsoil of 68.4 or 143.3 N cm^–2 impedance. Exertion of force by emerging control seedlings against a staticcantilever bar fitted with strain gauges reached a maximum (F_max)of 6 g over 10 h. Higher axial forces were achieved when theseedlings were emerging from compressed soil, without any changein the time required to reach F_max, so that the build-up offorce was considerably (1.8 fold) faster than in uncompressedsoil. This adaptive response to soil impedance was modified by theseed pretreatments employed. Seedlings from AVG or Ag⁺pretreatedseeds produced lower axial forces than controls, and neitherF_max nor the rate at which force developed showed any responseto soil compression. After pretreatment in 10^–3 ethephon or 10^–4 naphthaleneacetic acid (NAA) the seedlings achieved similar F_max to controlseedlings, but responded more rapidly to soil compression sothat the rate of build up of emergence force was 2.3 fold (NAA)or 2.8 fold (ethephon) faster in compressed than in uncompressedsoil. The results suggest that the exertion of force by a seedlingagainst a barrier involves a dynamic response to impedance onthe part of the seedling. This response can be either enhancedor suppressed by substances which affect ethylene productionor ethylene action. Such compounds may have promise for modifyingseedling emergence from impeding soils. Brassica napus, oil seed rape, seedling emergence, soil compaction, ethylene, Ethrel, silver, aminoethoxyvinylglycine, naphthalene acetic acid     

Influence of Environmental Stress on the Germination of Anabaena vaginicola Akinetes

Germination of akinetes of Anabeana vaginicola v. fertilissimaPrasad in response to environmental stress was studied. Additionof nitrate to the medium induced early and maximum germination(96 per cent), whereas less than half of the akinetes germinatedwhen either nitrate or phosphate was omitted from the medium.The pH range over which germination occurred was 7.0–9.0.The desiccated akinetes after rehydration germinated after acertain lag period, depending upon the dehydration state. Thetemperature optimum for germination and vegetative growth wasthe same (25 °C) and germination did not occur at 5 °Cor above 35 °C. The limit of heat shock tolerated was 55°C for 4 min. In addition to white light, only the red partof the visible spectrum induced germination. Ultraviolet radiationreduced germination rate presumably by inducing thymine dimersin DNA. The photoreactivating system (s) in akinetes is certainlynon-photosynthetic. LD₅₀ photon flux densities were 300 Jm^–2for akinetes and 240 Jm^–2 for vegetative cells. Anabaena vaginicola, blue-green alga, akinete, germination, environmental stress     

The effect of temperature and CO2 on seed quality development in wheat (Triticum aestivum L.)

Winter wheat (Triticum aestivum L.) cv. Hereward was grown inthe field in two double-walled polyethylene-covered tunnelswithin each of which a temperature gradient was superimposedon diurnal and seasonal fluctuations in temperature. The meantemperature between anthesis and harvest maturity varied from14.3 to 18.4C among plots within these tunnels. The CO₂ concentrationwas controlled at different values in each tunnel; seasonalmean concentrations were 380 and 684 µmol CO₂ mol^–1air. Crops were also grown outside the tunnels at ambient temperaturesand CO₂. Samples of seeds were harvested sequentially from eachplot between anthesis and harvest maturity. Seed germinationand seed survival during subsequent air-dry storage were determinedfor each sample. The onset of both ability to germinate anddesiccation tolerance (ability to germinate after rapid desiccationto 10–15% moisture content and subsequent rehydration)coincided in all environments. Full germination capacity (>97%, determined at 10C) was reached 4–18 d before theend of the seed-filling phase (mass maturity) in most cases.There was little or no decline in germination capacity duringsubsequent seed development and maturation. Differences in seedquality were evident, however, throughout seed development andmaturation when seed survival curves during subsequent storagewere compared. Potential longevity in air-dry storage (assessedby the value K₁ of the seed viability equation) improved consistentlyboth before and after mass maturity. There was a significantpositive relation between the rate of increase in potentiallongevity (dK₁Idt) and temperature (the minimum temperaturefor seed quality development was 4.8 C), but neither CO₂ concentrationnor production within the polyethylene tunnels affected thisrelation. Key words: Wheat, Triticum aestivum L., seed development, seed longevity, carbon dioxide, temperature     

Nitrogen utilization by plant species from acid heathland soils: II. Growth and shoot/root partitioning of NO3- assimilation at constant low pH and varying NO3-/NH4+ ratio

The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m^–3N, varying the N0₃^–/NH₄⁺ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N0₃^–/NH₄⁺ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH₄⁺ by NO₃^– tended to enhance the plant's growth ratewhen compared to NH₄⁺ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO₃^–increasing from 0 to 100%. In the presence of NH₄⁺, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH₄⁺. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO₃^–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO₃^– [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO₃^– <0.20), D. flexuosashowed a strong preference for NO₃^– at low external nitrate(the relative uptake ratio for N0₃^– reaching a value of2.0 at 10% NO₃^–). The ecological significance of thisprominent high preference for NO₃^– at low NO₃^–/NH₄⁺ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N0₃^–/NH₄⁺ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate     

Proton Translocation Associated with Denitrification in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

H⁺ translocation driven by NO₃^–, NO₂^– and N₂O reductionswith endogenous substrates in cells of Rhodopseudomonas sphaeroidesforma sp. denitrificans was investigated by the oxidant pulsemethod. Upon injection of nitrogenous oxides to anaerobic cellsin darkness, an alkaline transient in the external medium wasobserved, followed by acidification. The alkaline transientwas enhanced by carbonyl cyanide m-chlorophenylhydrazone. When a viologen dye was used as an electron donor in the presenceof 1 mM Af-ethylmaleimide and 0.1 mM 2-n-heptyl-4-hydroxyquinoline-N-oxideto preclude respiration-linked H⁺ extrusion, addition of KNO₃,KNO₂ and N₂O caused only a rapid alkalinization. The H⁺ consumptionstoichiometries, H⁺/2e^– ratios for NO₃^– reductionto NO₂^–, NO₂^– reduction to 1/2 N₂O and N₂O reductionto N₂ were –1.90, –3.18 and –2.04, respectively.These values agreed well with the fact that all reductions ofnitrogenous oxides in denitrification occur on the periplasmicside of the cytoplasmic membrane. When corrected for H⁺ consumption in the periplasm, the H⁺ extrusionstoichiometries, H⁺/2e^– ratios with endogenous substratesin the presence of K⁺/valinomycin for NO₃^– reduction toNO₂^–, NO₂^– reduction to 1/2 N₂O and N₂O reductionto N₂ were 4.05, 4.95 and 6.01, respectively. (Received August 4, 1982; Accepted January 13, 1983)     

The Effect of Ethylene and Temperature on ATP Accumulation from Various Substrates in Peanut (Arachis hypogaea L. ) Seeds

Peanut (Arachis hypogaea L. ) seed powder accumulated ATP fromAMP and phosphoenolpyruvate (PEP) at a rate of approx. 100 pmolmin^–1mg^– powder at 35° C. When peanut seed powderwas incubated with various substrates, which may result in PEPor AMP (ADP) synthesis, then ATP accumulated. The best substratecombinations examined so far were AMP + succinate, NADH₂, andAMP + malate + NAD, with activities of 33, 12 and 12 pmol min^–mg^–1powder,respectively; AMP + malate showed very low activity. Some combinationsexhibited linear activities with time, while others had an exponential-typeprofile. The temperature dependence of the ATP accumulationdemonstrated by the Ahrrenius plot had a double phase with atransition point at 25° C. The Ea values between 15°C and 25° C were 25 000–50 000 cal/mol, while above25° C the Ea values fluctuated between 6000 and 8000 cal/mol(depending on the substrate). The AMP + PEP combination exhibiteda single-phase profile between 15° C and 40° C, withan Ea value of 22 000 cal/mol. In the presence of some substrates,ethephon (ethylene) had a stimulatory effect and caused an increasein the Ea values at the high temperature phase. A comparisonof seed powder from dormant seeds with that from non-dormantseeds revealed that some substrate combinations accumulate ATPfaster in non-dormant seeds and others do so in dormant seeds. Key words: Arachis hypogaea, ATP, Ethylene, Dormancy, Peanut, Seed