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     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: I. GERMINATION AT SUBOPTIMAL TEMPERATURE

The bases of differences in germination rates (GR_g = inverseof time to germination [t_g] of percentage g) among three cold/salt-toleranttomato (Lycopersicon esculentum Mill.) accessions (PI 341988,PI 120256, and PI 174263) and one cold/salt-sensitive tomatocultivar (T5) were investigated. The effects of seed priming(6 d imbibition in aerated –1.2 MPa polyethylene glycolsolution at 20 ?C followed by redrying) and of removing theendosperm/testa cap covering the radicle on the temperaturesensitivity of GR_g, and the interaction of these treatmentswith genotypes, were also examined. GR_g decreased linearly withdecreasing temperature for all genotypes and seed treatments.The minimum or base temperatures for germination (T_b) variedby 1 ?C among the tomato lines, so genotypic differences inGR_g were due to differing thermal time requirements for germination.The mean thermal time requirement for germination of T5 seeds was 22% and 19% greater than that of PI 341988 andPI 120256 seeds, respectively, but only 9% greater than thatof PI 174263 seeds. Seed priming did not lower Tb of any genotype,but significantly reduced by 24, 49, 41, and 49% in T5, PI 341988, PI 120256, and PI 174263, respectively,indicating that priming increased the rate at which the seedsprogressed towards germination when T>T_b, but did not lowerthe minimum temperature at which germination could occur. Primingincreased the GR_g of T5 seeds to equal or exceed those of control(non-primed) seeds of the cold/salt-tolerant genotypes at anyT>T_b, but the PI lines exhibited an even greater responseto priming. Times to germination within each seed lot were normallydistributed on a logarithmic scale. Priming increased the variancein tg within a seed lot when compared to control seeds. However,the variation in thermal time for germination between the 10thand 90th percentiles of the seed population (_T(10–90))was relatively unaffected by priming due to the reduction in in primed seeds. Removing the endosperm cap and testa opposite the radicle tip decreased almost 6-fold and and reduced T_b by 5 ?C in T5 and PI 341988,implicating processes in the endosperm/testa as the limitingfactors in germination at suboptimal temperatures. Key words: Lycopersicon esculentum Mill., tomato, genetic variation, seed priming, thermal time, germination rate     

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 Effects of Priming and 'Natural' Differences in Quality amongst Onion Seed Lots on the Response of the Rate of Germination to Temperature and the Identification of the Characteristics under Genotypic Control

Ellis, R. H. and Butcher, P. D. 1988. The effects of primingand ‘natural’ differences in quality amongst onionseed lots on the response of the rate of germination to temperatureand the identification of the characteristics under genotypiccontrol —J. exp. Bot. 39: 935–950. A screening procedure was applied to define the response ofthe rate of seed germination to sub-and supra-optimal temperaturesfor different lots or sub-lots of two onion (Allium cepa L.)cultivars.Three sub-lots of the cultivar White Lisbon were derived froma control lot by osmotic priming (–1.4 MPa, 20 °C.7 d) alone, by priming and drying and by priming, drying andsubsequently storing the seeds for 7 weeks at 2–5 °C.The major effect of priming was to reduce the thermal time forgermination at both sub- and supra-optimal temperatures. Primingalone also altered the distribution of thermal times at sub-optimaltemperatures. A new equation is presented to describe this variation.In contrast, priming had no consistent effect on base temperature(T_b and little effect on the distribution of ceiling temperatures[T_e(G)]. For the control lot of White Lisbon T_b was 4°C,whilst the best common estimate of T_b for all four sub-lotswas 3.5°C. The mean estimate of T_c(50) for the control,primed and primed and dried sub-lots was 35.5°C.Comparisonof three lots of the cultivar Senshyu Semi Globe Yellow of widely-differingviability showed substantial differences in the thermal timefor germination at sub-optimal temperatures, but no significantdifferences in T_b (P>0.10), the common estimate being 4°C.There was a significant negative correlation between probitpercentage viability and the logarithm of the thermal time for50% germination at sub-optimal temperatures amongst the threelots (P<0.05). The work suggests that base temperature forgermination is a genotypic characteristic which is unaffectedby differences in seed quality. It also shows that the effectof priming, quantified as a reduction in thermal time requirementsfor germination, varies amongst the seeds within a lot. Key words: -Onion, seed germination rate, temperature, priming     

The Effects of Priming and Ageing on Resistance to Deterioration of Tomato Seeds

The influence of seed priming and ageing treatments on viabilityand rate of germination of tomato (Lycopersicon esculentum Mill.)seeds was examined under both long-term and controlled-deteriorationstorage conditions. Seeds of a single lot of tomato were eitherprimed or aged to increase or decrease the rate of germination(Argerich and Bradford, 1989). They were then stored at 6% moisturecontent (dry weight basis) at either 4 ?C or 30 ?C for 1 year.Both viability and germination rate were unaffected by eitherstorage temperature in control seeds, or by 4 ?C storage inprimed or aged seeds. At 30 ?C, however, viability and germinationrate of primed and aged seeds was markedly reduced after 6 monthsof storage. The temperature dependence of the germination rateand the spread of germination times within the population wasalso adversely affected by high temperature storage, particularlyfor primed seeds. Under controlled deterioration conditions(13.5% moisture content and 50 ?C), the rate of loss of viabilitywas greater for primed seed than for control or aged seeds.The relationship between seed viability and the mean germinationrate, however, was not influenced by the seed treatments. Thesedata are analysed in relation to current models of seed deteriorationduring storage and seed repair during priming. The results indicatethat enhancement of seed germination rates by priming treatmentssimultaneously lowers the resistance of seeds to deterioration.Primed tomato seeds must, therefore, be considered to be vigorousseeds with a reduced storage life. Key words: Tomato, controlled deterioration, seed germination rate, seed viability     

Ageing and Osmotic Priming in Wheat Seeds: Effects Upon Certain Components of Seed Quality

Deteriorated wheat seed lots were osmotically primed and surface-dried,dried-back, then aged for two more weeks Osmotic priming didnot affect the final germination in any of the aged seed lots,but mean germination time was decreased The response of unagedseed lot to treatments was studied in the following the timecourses of germination, as well as L-[4, 5-²H]leucine and [6-³H]thymidineincorporation (as a measure of protein and DNA synthesis, respectively)into dissected embryos during the early hours of germinationIt was found that the variation in mean germination time andthe rate of synthetic reactions were related The greatest improvementin the components of seed lot quality was achieved after primingand surface drying The beneficial effects of priming were apparenteven after dehydration followed by two further weeks of ageingCumulative correlations between mean germination time and variousbiochemical tests in variously treated aged seed lots were highlysignificant The physiological processes involved in deteriorationand recovery and the potential application of biochemical testsin detecting vigour changes are discussed Triticum durum, seed, ageing, osmotic priming, mean germination time, protein DNA synthesis     

Amounts of Nuclear DNA and Internal Morphology of Gibberellin- and Abscisic Acid-deficient Tomato (Lycopersicon esculentum Mill.) Seeds During Maturation, Imbibition and Germination

Using X-ray photography and flow cytometry, the internal morphologyand DNA replication activity of wild type (wt), GA- (gib-1 )and ABA-deficient (sit^w ) tomato (Lycopersicon esculentum Mill.cv. Moneymaker) mutant seeds were studied. During seed formation,from 30 to 45 d after pollination (DAP) the endosperm becomessolid and the seed starts to gain desiccation tolerance. Atthis time significant changes occur in the amounts of DNA inradicle tip cells. At 30 DAP, radicle tip cells of the threegenotypes manifest about 60% of 2C, 30% of 4C and 10% of 8Camounts of DNA. Upon maturation (45 DAP onwards), most cellsin the seeds of the three genotypes arrest in the G₁phase ofthe cell-cycle with 2C amounts of DNA. However, a relativelyhigh proportion of cells with 4C amounts of DNA was detectedin the radicle tip cells ofsit^w compared with wild type andgib-1. At the well-matured stage (60 DAP), there were about 2% ofseeds with free space in wild type andgib-1 , and about 13%insit^w . At the over-matured stage (75 DAP), even more seedswith free space were found insit^w , whereas no increase in theproportion of the seeds with free space was detected in theother two genotypes. In -1.0 MPa PEG-6000 with or without 10µM GA₄₊₇, no germination occurred in well-matured wildtype andgib-1 seeds, whether or not they were dried after harvest.However,sit^w seeds were able to germinate both in over-maturefruit and in -1.0 MPa PEG-6000. Priming of dried seeds in -1.0MPa PEG induced a large amount of free space in almost all seedsof the three genotypes, and nuclear DNA synthesis in the radicletip cells of wild type andsit^w seeds. However, PEG priming offresh (non-dried) seeds had no effect on the amount of freespace and 2C/4C DNA ratios in wild type orgib-1 seeds, but didinduce free space in about 20–25% ofsit^w seeds and provoked4C signals insit^w seeds. Removal of the endosperm and testaopposite the radicle tip of seeds resulted in root protrusion,the induction of free space and an increase of 4C DNA signalsin the three genotypes. It is concluded that ABA is crucialfor the efficient arrest of tomato embryo radicle tip cellsin G₁phase upon maturation, whereas GAs play an important rolein re-initiating 4C DNA levels upon germination. Dormancy; flow cytometry; free space; Lycopersicon esculentum ; maturation; priming; seed; tomato     

The Effects of Priming and Accelerated Ageing upon the Nucleic Acid Content of Leek Seeds and their Embryos

Using seed priming and accelerated ageing techniques, a singlelot of leek (Allium porrum) seeds was manipulated to producefour lots of seeds with different germination performance. Changesin content of the major nucleic acid species in whole seedsand embryos of two of these lots (primed and unprimed), weredetermined over the early stages of germination. The major effectof priming was an increased level of RNA species in the seedsand embryos, and this difference was maintained during germination.Comparison of nucleic acid levels in the dry seeds of thesetwo lots and two others (aged and aged then primed) indicatedthat there was no correlation with germination performance.Similar comparisons of the nucleic acid levels in the embryosof seeds imbibed for 1 d showed only a limited correlation betweenrRNA levels and germination performance. Analysis of these datasuggests that accelerated ageing has an adverse effect uponendosperm cells, which results in the degradation of their nucleicacids during priming. Furthermore, the viability of these agedseeds also falls during priming. The data also indicate thatratios of rRNA to DNA correlate with germination performanceof the four lots of seeds studied. It is proposed that sucha relationship is indicative of the efficiency of a primingtreatment, and may be useful in comparisons of naturally varyingseed lots. Key words: Leek, seed, germination, priming, nucleic acids     

The Role of Anaerobic Respiration in Germinating Muskmelon Seeds: I. IN RELATION TO SEED LOT QUALITY

Five muskmelon (Cucumis melo L.) cultivar seedlots from a commercialsource and freshly produced seeds of two cultivars, when artificiallyaged, were found to differ in their viability and vigour asdetermined by germination tests. Furthermore, the various commercialseedlots without ageing also exhibit a range of deteriorationlevels. Low vigour seeds had higher respiratory quotient valuesthan the high vigour seeds as a result of a higher level ofCO₂ production. This high level of CO₂ evolution in low vigourseeds may have been due to anaerobic respiration. Levels of acetaldehyde and ethanol produced by imbibing seedswere negatively associated with seed viability and vigour. After6 h of imbibition low vigour seeds produced significantly moreethanol and acetaldehyde than high vigour seeds. After 24 hof imbibition, ethanol continued to accumulate in the commercialseedlots up to 10-fold the amount produced after 6 h of imbibition,whereas, acetaldehyde levels increased less. However, in thefreshly produced, artificially aged seeds (except the most extremeageing), ethanol levels were reduced and no acetaldehyde productioncould be detected, indicating re-utilization of ethanol. It is suggested that ethanol production in the first hours ofimbibition can be used as a reliable index to predict germinationin muskmelon seedlots. Key words: Germination, Cucumis melo L., Seeds, Anaerobic respiration     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.): VII. INFLUENCE OF AFTER-RIPENING AND AGEING ON GERMINATION RESPONSES TO TEMPERATURE AND WATER POTENTIAL

The effects of storage conditions on the germination of developingmuskmelon (Cucumis melo L.) seeds were tested to determine whetherafter-ripening is required to obtain maximum seed vigour. Seedswere harvested at 5 d intervals from 35 (immature) to 60 (fullymature) days after anthesis (DAA), washed, dried, and storedat water contents of 3·3 to 19% (dry weight basis) at6, 20, or 30°C for up to one year. Germination was testedin water and in polyethylene glycol 8000 solutions ( –0·2to –1·2 MPa osmotic potential) at 15, 20, 25 or30°C. Germination percentages and rates (inverse of meantimes to radicle emergence) were compared to those of newlyharvested, washed and dried seeds. For 40 and 60 DAA seeds,one year of storage at 20°C and water contents <6·5%significantly increased germination percentages and rates at20°C, but had little effect on germination at 25 and 30°C.Storage reduced the estimated base temperature (T_b) and meanbase water potential (_b) for germination of both 40 and 60 DAAseeds by approximately 5°C and 0·3 MPa, respectively.Immature 35 DAA seeds showed the greatest benefit from storageat 3 to 5% water content and 30°C, as germination percentagesand rates increased at all water potentials (). Storage underthese same conditions had little effect on the germination ofmature seeds in water, but increased germination percentagesand rates at reduced 's. Accelerated ageing for one month at30°C and water contents from 15 to 19° increased germinationrates and percentages of mature seeds at reduced 's, but longerdurations resulted in sharp declines in both parameters. Immatureseeds lost viability within one month under accelerated ageingconditions. An after-ripening period is required at all stagesof muskmelon seed development to expand the temperature andwater potential ranges allowing germination and to achieve maximumgerminability and vigour. Post-harvest dormancy is deepest atthe point of maximum seed dry weight accumulation and declinesthereafter, both in situ within the ripening fruit and duringdry storage. Key words: Muskmelon, Cucumis melo L., seed, development, dormancy, germination, vigour, after-ripening     

Activation of the Cell Cycle in Tomato (Lycopersicon esculentum Mill.) Seeds during Osmoconditioning as Related to Temperature and Oxygen

Using flow cytometric analyses of the nuclear DNA content, westudied the effects of various conditions of osmopriming onthe activation of the cell cycle in embryo root tips of tomato(Lycopersicon esculentum‘Elko’) seeds. In dry untreatedseeds, 90.7% of the nuclei revealed 2C signals. Priming of seedsin polyethylene glycol-8000 (PEG) improved the germination rateof seeds transferred onto water at 15 °C. This was associatedwith an increase in 4C signals when priming was carried outat -1.0 and -1.5 MPa. Priming at -2.0 MPa enhanced subsequentgermination but had no effect on DNA replication. For temperaturesduring priming up to 25 °C, a positive linear correlationexisted between the efficiency of the treatment, evaluated bythe reciprocal of time to obtain 50% germination at 15 °C,and the frequency of 4C nuclei or the 4C/2C values. Such a correlationdid not exist when priming was performed at higher temperatures.At least 5% oxygen in the atmosphere was required during primingfor the induction of DNA replication and for the enhancementof subsequent germination. In the presence of 5x10^-4M and 10^-3MNaN₃during priming, most of the cells were maintained with 2CDNA levels and the treatment had no stimulatory effect on germination.The results show a positive linear relationship between thefrequency of 4C DNA nuclei or the 4C/2C ratio and the improvingeffect of priming. However, in suboptimal conditions of priming(-2.0 MPa or temperatures higher than 25 °C), the improvementof seed germination was not associated with the onset of DNAreplication.Copyright 1999 Annals of Botany Company Cell cycle, germination, osmopriming, oxygen, temperature, Lycopersicon esculentum, tomato.     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: II. INTRASPECIFIC VARIATION IN CHICKPEA (Cicer arietinum L.) AT CONSTANT TEMPERATURES

Positive linear relationships were shown between constant temperaturesand the rates of progress of germination to different percentiles,G, for single populations of each of five genotypes of chickpea(Cicer anetinum L.). The base temperature, T_b, at which therate of germination is zero, was 0·0°C for all germinationpercentiles of all genotypes. The optimum temperature, T_o(G),at which rate of germination is most rapid, varied between thefive genotypes and also between percentiles within at leastone population. Over the sub-optimal temperature range, i.e.from T_b to T_o(G), the distribution of thermal times within eachpopulation was normal. Consequently a single equation was appliedto describe the influence of sub-optimal temperatures on rateof germination of all seeds within each population of each genotype.The precision with which optimum temperature, T_b(G), could bedefined varied between populations. In each of three genotypesthere was a negative linear relationship between temperatureabove T_b(G) and rate of germination. For all seeds within anyof these three populations thermal time at supra-optimal temperatureswas constant. Variation in the time taken to germinate at supra-optimaltemperatures was a consequence of normal variation in the ceilingtemperature, T_o(G)—the temperature at or above which rateof progress to germination percentile G is zero. A new approachto defining the response of seed germination rate to temperatureis proposed for use in germplasm screening programmes. In two populations final percentage germination was influencedby temperature. The optimum constant temperature for maximumfinal germination was between 10°C and 15°C in thesepopulations; approximately 15°C cooler than the optimumtemperature for rate of germination. It is suggested that laboratorytests of chickpea germination should be carried out at temperaturesbetween 10°C and 15°C. Key words: Chickpea, seed germination rate, temperature     

Ethylene, Nitrate and Nitrite Interactions in the Promotion of Dark Germination of Common Purslane Seeds

Ethylene (10 µ1^–1) caused about one-third of highlydark-dormant seeds of common purslane (Portulaca oleracea L.)to germinate in the dark. Attempts were made to increase germinationin the dark with nitrate and ethylene combinations. When applieddirectly to the seeds, KNO₃ did not stimulate germination andKNO₃ plus ethylene did not increase germination above that ofethylene alone. Pre-incubation of seeds in KNO₃ for 4 to 7 dbefore the ethylene applications significantly increased germination.The effects of the KNO₃ pre-incubation were additive at eachof four ethylene concentrations (0.1–100 µ11^–1).Potassium nitrate was effective only when ethylene followedthe KNO₃ pre-incubation period. Potassium nitrite stimulatedabout 25 per cent of the seeds to germinate without a pre-incubationperiod and without ethylene. Also, ethylene plus KNO₂ enhancedgermination above that achieved by either stimulus alone. Silvernitrate did not block the ethylene promotion of germination,but reversed the typical ethylene inhibition of seedling growthfollowing germination. The results support the views that nitrateexerted its effect via conversion to nitrite within the seedand that the rate of nitrate conversion may be a limiting factorin the dark germination of common purslane seeds. Ethylene mayfacilitate nitrite activity by increasing seed sensitivity tothe stimulus. Common purslane, Portulaca oleracea L., ethylene, nitrate, nitrite, germination, dormancy     

Evidence for Repair Processes in the Invigoration of Seeds by Hydration

Seeds with low vigour as a result of ageing showed a large improvementin seed quality following a 2 h soak in water, reflected inan increased rate of germination, higher emergence in soil,and the retention of high germination after 24 h rapid ageingin a controlled deterioration vigour test. Similar, but smallerimprovements were seen after a priming treatment using polyethyleneglycol. Seeds dried to their initial moisture content aftertreatment retained these beneficial effects. The treatmentshad little effect on high vigour seed. These findings suggestthat seed invigoration by hydration treatments results fromthe repair of previously sustained deterioration. Brussels sprouts, Brassica oleracea var. gemmifera, priming, repair, seed moisture content, ageing     

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     

Effects of Chilling, Light and Nitrogen-containing Compounds on Germination, Rate of Germination and Seed Imbibition of Clematis vitalba L.

Effects of chilling (5 °C) period, light and applied nitrogen(N) on germination (%), rate of germination (d to 50% of totalgermination; T_50%) and seed imbibition were examined inClematisvitalba L. In the absence of chilling, light and N, germinationwas minimal (3%). When applied alone, both chilling and N increasedgermination. Chilling for 12 weeks increased germination to64%, and 2.5 mM NO^-₃or NH⁺₄increased germination to 10–12%.Light did not increase germination when applied alone, but didwhen applied in combination with chilling and/or N. Half theseed germinated when light was combined with 2.5 mM NO^-₃or NH⁺₄.The influence of chilling, light and/or N on germination wasgreater when combined, than when either factor was applied alone.Both oxidized (NO^-₃) and reduced (NH⁺₄) forms of N increasedgermination, but non-N-containing compounds did not, suggestingthe response was due to N and not ionic or osmotic effects. Without additional N, T_50%decreased from 16–20 d at zerochilling, to around 5 d at 8 and 12 weeks chilling. AlthoughT_50%was not influenced by an increase in NO^-₃or NH⁺₄from 0.5to 5.0 mM , it did increase with additional applied N thereafter.However, the magnitude of the N effect was small compared tothat of chilling. Like germination, seed imbibition increasedwith a longer chilling period, but in contrast imbibition decreasedslightly with increased applied NO^-₃or NH⁺₄. It is argued thatincreased imbibition is not directly related to an increasein total germination, but that it may be related to the rateof germination. Possible mechanisms involved in the reductionin dormancy ofC. vitalba seed are discussed. Clematis vitalba L.; germination; dormancy; imbibition; rate of germination; chilling; light; nitrate; ammonium; nitrogen; phytochrome     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation     

Flow Cytometric Determination of Nuclear Replication Stages in Tomato Seeds during Priming and Germination

Flow cytometric determination of DNA levels in embryos of fullymatured dry tomato (Lycopersicon esculenium) seeds revealedlarge amounts of 2C DNA signals, indicating that most cellshad arrested in the cell cycle at the presynthetic G₁ phaseof nuclear division. After imbibition in water, an augmentationof the 4C signal in the embryonic root tip region was found.This increase could be ascribed to cells entering the syntheticphase of nuclear division leading towards the doubling of chromosomalmaterial. In the root tip cells, 4C:2C ratios increased I dafter imbibition in water though radicle emergence started 2d later. Apparently, DNA synthesis preceded germination. Onlya small increase in the number of cells with 4C DNA levels wasfound in the rest of the embryonic tissues. In whole dry seeds,DNA histograms revealed both a 2C signal and a considerable6C peak, the latter originating from the endoreduplicated endosperm. A priming period of 14 d in PEG-6000 considerably enhanced therate and uniformity of germination. In the ungerminated seeds,the 4C DNA signal of root tip cells started to increase after3 d incubation in PEG. The ratio of 4C:2C steadily increasedduring the 14 d priming period, though did not reach the levelobtained after hydration in water. Upon priming, the 4C:2C ratiowas constant after redrying the seeds towards the original moisturecontent, indicating that the chromosomal material in the rootcells had stably ceased cell cycle activity at the G² phase.The present results indicate that the beneficial effects ofpriming on seedling performance are associated with the actionof replicative DNA synthetic processes prior to germination. Lycopersicon esculeniumMill, tomato, DNA content, flow cytometry, priming, seed, nuclear replication stage, C levels     

Catalase is a key enzyme in seed recovery from ageing during priming

Ageing induces seed deterioration expressed as the loss of seed vigour and/or viability. Priming treatment, which consists in soaking of seeds in a solution of low water potential, has been shown to reinvigorate aged seeds. We investigate the importance of catalase in oxidation protection during accelerated ageing and repair during subsequent priming treatment of sunflower (Helianthus annuus L.) seeds. Seeds equilibrated to 0.29 g H₂O g⁻¹ dry matter (DM) were aged at 35 °C for different durations and then primed by incubation for 7 days at 15 °C in a solution of polyethylene glycol 8000 at −2 MPa. Accelerated ageing affected seed germination and priming treatment reversed partially the ageing effect. The inhibition of catalase by the addition of aminotriazol during priming treatment reduced seed repair indicating that catalase plays a key role in protection and repair systems during ageing. Ageing was associated with H₂O₂ accumulation as showed by biochemical quantification and CeCl₃ staining. Catalase was reduced at the level of gene expression, protein content and affinity. Interestingly, priming induced catalase synthesis by activating expression and translation of the enzyme. Immunocytolocalization of catalase showed that the enzyme co-localized with H₂O₂ in the cytosol. These results clearly indicate that priming induce the synthesis of catalase which is involved in seed recovery during priming.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): I. CONSTANT TEMPERATURE

The germination of pearl millet (Pennisetum typhoides S. &H.) seeds was investigated at constant temperatures between12 ?C and 47 ?C on a thermal gradient plate. The rate of germination increased linearly with temperaturefrom a base T_b to a sharply defined optimum T_o beyond whichthe rate decreased linearly with temperature, reaching zeroat T_m. The linearity of the response both above and below T_oallowed time and temperature to be combined in a thermal timeat which a specified fraction of the seeds germinated. Withinthe population T_b and T_m were constant.