Freeze Desiccation: a Second Mechanism for the Survival of Hydrated Lettuce (Lactuca sativa L.) Seed at Sub-zero Temperatures

Differential Thermal Analysis of hydrated lettuce cv. GreatLakes achenes using a rapid cooling rate (20 °C h^–1)produced two exotherms per achene. Both exotherms representedthe freezing of supercooled water. The high temperature exothermoccurred at –93 °C and was produced by freezing ofwater inside the pericarp but exterior to the endosperm. Thetemperature at which it occurred could be altered by the additionof nucleating agents. The low temperature exotherm produced by freezing of the embryooccurred at –162 °C and marked the death of the seed.Its temperature was not changed by the addition of nucleatingagents but its occurrence required the structural integrityof the endosperm. At low cooling rates (1 and 2 °C h^–1)low temperature exotherms were not recorded and samples removedat –25 °C had high viability. Slow cooling causeda redistribution of water within the seed whereby ice formingoutside the endosperm caused desiccation of the embryo and preventedits freezing. A mechanism is proposed, in terms of established supercoolingand nucleation theory, to explain the observed results and thevalue of freeze tolerance to the species in its natural habitatis discussed. Cooling rate, differential thermal analysis, freezing avoidance, Lactuca sativa L., lettuce, seed, supercooling, water migration     

The Potential for Newly-Germinated Cabbage Seed Survival and Storage at Sub-Zero Temperatures

The moisture content of newly germinated cabbage seed (radicles1 05 mm long) was reduced to 14% of f.wt without loss of viability.As the moisture content was reduced below 45%, the temperatureat which the germinated seeds froze, and therefore died, decreasedprogressively to a minimum of –34 C at 19% moisture content.No freezing exotherms were recorded in seeds with moisture contentsbelow 19%. Seeds with a moisture content between 14 and 16%maintained viability for at least 1 week when cooled at 26C.min^–1to –20 C and held at this temperature, indicating thepotential for prolonged storage of these low-moisture-contentgerminated (LMCG) seeds. Brassica oleracea, cabbage, germinated seed, seed storage, fluid drilling, freezing exotherm, thermal analysis     

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     

The Effect of Temperature on Reproduction in FivePrimulaSpecies

Primula vulgarisHuds.,P. verisL.,P. frondosaJanka, and threepopulations ofP. farinosaL. were legitimately and illegitimatelypollinated, and the self-fertileP. scoticaselfed and cross-pollinatedand then subjected to uniform temperature conditions of 6, 15or 26 °C for 4 d before gynoecia were examined for pollengermination and pollen tube growth, or plants progressed toseed set at 15 °C, after which seeds were weighed, germinated,and seedlings grown on. The temperature responses of pollengermination and pollen tube growth were not always congruent,and varied between species, populations, and often between morphs(pin and thrum) in the distylous species. Nevertheless, optimaltemperature responses tended to be lower for vernal species(P. vulgarisandP. veris) and for subarcticP. scoticathan forlater flowering montane species. However, no relationship wasfound between pollen temperature response, and fertility. Thegreatest seed set occurred after legitimate pollination at 15°C in most cases; a flowering temperature of 26 °C tendedto impede seed set, except forP. scoticaand the low altitudepopulation ofP. farinosa. InP. veris, P. frondosaand the highaltitude population ofP. farinosa,some illegitimate pollen germinationand pollen tube growth occurred at 26 °C, but this did notlead to increased within-morph seed set in these self-incompatiblespecies at this relatively high temperature. Temperature atflowering frequently affected average seed weight, and inP.verisand two populations ofP. farinosathis attribute may havebeen influenced by seed number, the average seed weight of few-seededcapsules tending to be greater than for many-seeded capsules.A high seed weight might mitigate the disadvantageous effectsof low fecundity resulting from interactions with floweringtemperature. However, inP. vulgarisandP. scoticainteractionsbetween flowering temperature and seed weight may have other,undetermined, causes. The seed of four species germinated leastwell in standard conditions when set following a flowering temperatureof 6 °C, which tends to support the hypothesis that temperatureat flowering can affect seed physiology; in contrast the seedof the two upland populations ofP. farinosagerminated leastwell after flowering at 26 °C. We conclude that much morework is needed on interactions between temperature and reproductiveefficiency, but that preliminary indications suggest that aglobal increase in temperature at flowering might adverselyaffect the quantity and quality of seed set in some species.Copyright1998 Annals of Botany Company Distyly, pollen tube,Primula farinosa, Primula frondosa, Primula scotica, Primula veris, Primula vulgaris,reproduction, seed set, temperature.     

Freezing pattern in apple seeds as affected by the temperature of fruit storage

Storage of apple fruits ( Pyrus malus L. cv. Golden Delicious) at different temperatures (0, 12 and 35°C) markedly altered the pattern of water freezing in the seeds. Higher temperatures of storage brought about a shift from the sequential to the discontinuous type of freezing in seeds, the low temperature exotherm (LTE) being much more pronounced than the high temperature one (HTE). The occurrence of LTE was highly correlated with embryo death. Fruit storage at higher temperatures also caused a decrease in the threshold super cooling temperature of seeds and of naked embryos, removed from the seed coat and endosperm. The decrease was less pronounced in naked embryos than in intact seeds. The results presented show that the frost resistance of apple seeds relies mainly on the supercooling ability of the embryos and is increased by the presence of seed coat and endosperm. The broad peak of the LTE indicates that, in contrast with other seeds and many super cooling organs, massive ice nucleation in apple seeds occurs within the embryo tissues and that extra organ freezing seems to be of less importance. Therefore, the increased super cooling ability of apple seeds, isolated from fruits stored at higher temperatures, seems to rely on those seed properties that protect embryo cells against heterogeneous ice nucleation.     

Storage Behaviour of Salix alba and Salix matsudana Seeds

Effects of dehydration, storage temperature and humidificationon germination of Salix alba andS. matsudana seeds were studied.Newly released seeds showed 100% germination before and afterdehydration to 11–12% moisture content. Germination ofthe high vigour lot (100% initial normal germination) was notaffected by dehydration to 6.7% moisture content but germinationdecreased with further dehydration to 4.3%. The lower vigourlot (75% initial normal germination) was more susceptible todehydration and germination decreased following dehydrationto 6.7% moisture content. Dry seeds of both species survivedimmersion in liquid nitrogen without loss of viability. Thegermination of seeds stored with 9% moisture content decreasedto 35–40% in 5 months at -20°C or in 2 months at 5°C.However, at 25°C seeds entirely lost viability within 2weeks. Seeds showed improved performance when stored at -70°C> - 20°C > 5°C > 25°C and tolerated dehydrationto a moisture content in equilibrium with 15% relative humidity.Results suggest that they are orthodox in storage behaviouralthough they are short-lived. Humidification treatment of lowvigour seed lots resulted in a remarkable increase in germinationpercentage. Copyright 2000 Annals of Botany Company Salix alba, Salix matsudana, willow, seed storage behaviour, dehydration, humidification, cryopreservation     

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     

Survival and Vigour of Lettuce (Lactuca sativa L.) and Sunflower (Helianthus annuus L.) Seeds Stored at Low and Very-low Moisture Contents

Seeds of lettuce (Lactuca sativa L.) and sunflower (Helianthusannuus L.) were stored hermetically at 35 °C with 11 differentmoisture contents between 1·3 and 6·9%, and between1·3 and 7·1% of fresh mass, respectively. Germinationand vigour (mean germination time, root length, seedling dryweight) were determined after storage for 0, 8, or 16 weeks(sunflower) or 0, 8, 16, or 48 weeks (lettuce) in these environmentsfollowed by various humidification treatments (to avoid imbibitioninjury). The range of seed storage moisture contents over whichdeterioration was minimized depended upon the criterion of deteriorationused, and varied somewhat between species. Comparison of theseranges for seeds stored for the longest durations showed thatfor some criteria seed performance was poorer (P < 0·05)at both the lowest and highest moisture contents investigatedthan at certain of the intermediate storage moisture contents(e.g, most rapid germination occurred in sunflower followingstorage at 2·2-4·7% moisture content), whereasfor other criteria all the drier storage moisture contents weresuperior to the more moist (e,g. greatest seedling growth occurredin sunflower following storage at 1·3-5·1% moisturecontent). But none of these results suggested that lettuce andsunflower seeds stored hermetically at 2·5-3·0%or 2·2-2·5% moisture content, respectively, wereless vigorous than at any other moisture content tested. Inboth species, these storage moisture contents are in equilibriumwith about 8-10% relative humidity (r.h.) at 20 °C, whichis similar to and indeed marginally less than the 10-13% r.h.recommended following earlier studies on the longevity of seedsin hermetic storage at much warmer temperatures. Thus, theseresults show no evidence that the optimum seed moisture contentfor storage increases with decrease in temperature, at leastover the range 35-65 °C, as has been suggested elsewhere.We conclude that the international recommendation for the long-termseed storage for genetic conservation at 5 ± 1% moisturecontent should not be revised upwardly, and that in situationswhere refrigeration cannot be provided storage at even lowermoisture contents is worthy of further investigation for thoseseeds in which desiccation at 20 °C to equilibrium at 10%r.h. results in moisture contents well below 5%.Copyright 1995,1999 Academic Press Helianthus annuus L., sunflower, Lactuca sativa L., lettuce, desiccation, seed storage, seed vigour     

Effects of Temperature and dl-Strigol on Seed Conditioning and Germination of Witchweed (Striga asiatica)

Seed conditioning and germination in witchweed (Striga asiatica(L.) Kuntze) were temperature-dependent. With higher conditioningtemperatures, shorter conditioning time was required for germinationwith terminal dl-strigol (strigol) treatment at 30 °C. Maximumgermination (80–100%) was obtained by conditioning inwater at 20, 25, 30 and 35 °C for 14, 7, 5 and 3 d, respectively,and terminally treating with 10^–6 M strigol at 30 °C.Seeds conditioned in 10^–8 M strigol instead of water germinatedmuch less with the same terminal strigol treatment. Generally,conditioning was slower when seeds were conditioned in strigolrather than water. The reduction in germination rate by pretreatmentin strigol or pretreatment at low temperatures could be overcomeby increasing the terminal strigol concentration in the germinationtest. Conditioned seeds did not germinate at 10 and 15 °Cwith a terminal 10^–6 M strigol treatment but yielded closeto maximum germination at 25, 30 and 35 °C with the sameterminal strigol treatment. To obtain maximum germination, boththe minimum conditioning temperature and the minimum germinationtemperature for conditioned seeds were 20 °C. Factors suchas conditioning time, and strigol concentration and temperatureduring conditioning and/or germination determine whether seedsremain in the conditioning phase or shift to a germination phase. dl-Strigol, germination stimulation, parasitic plants, seed conditioning, seed germination, Striga asiatica, temperature, weed control     

Changes in Lipids of Bean Seeds (Phaseolus vulgaris) and Corn Caryopses (Zea mays) Aged in Contrasting Environments

Seeds of French bean (Phaseolus vulgaris L. cv. The Prince)and caryopses of sweet corn (maize; Zea mays L.F1 hybrid Firstof All) were stored in environments of 79% relative humidityand 25 °C, 80% r.h. and 40 °C or 100% r.h. and 45 °C,giving ageing (loss of gcrminability and vigour) over periodsof months, weeks or days, respectively. The relationship betweenchanges in lipids and changes in germinability or vigour wasunaffected in general by the speed of ageing. In the corn caryopsesthere was no evidence of hydrolysis of phospholipids or peroxidationof fatty acids during ageing. In the bean seeds phosphatidicacid increased during the ageing period and phosphatyl cholinedeclined. The percentage of fatty acids as hnolenic acid initiallyfell during bean ageing, but in the slower ageing conditionsit rose again as germination reached zero. In bean, the presenceof phosphatidic acid could be a sensitive indictator of lossof vigour, but relative proportions of the different fatty acidswould be a misleading indicator of quality. Rapid artificialageing may be an adequate model, in some species, of ageingat moderate speeds and of ageing under some ambient conditions. French bean (Phaseolus vulgaris L. cv. The Prince), sweet corn (maize, Zea mays L., Fl hybrid First of All), seed ageing, phospholipids, fatty acids     

Nuclear Replication Activity During Seed Development, Dormancy Breakage and Germination in Three Tree Species: Norway Maple (Acer platanoidesL.), Sycamore (Acer pseudoplatanusL.) and Cherry (Prunus aviumL.)

Flow cytometric analyses of nuclear DNA levels were carriedout during development, stratification and germination of dormantseeds from three tree species with contrasting characteristics.Norway maple (Acer platanoides) and sycamore (Acer pseudoplatanus)have orthodox (desiccation-tolerant) and recalcitrant (desiccation-sensitive)storage behaviours, respectively, and require only a periodof cold to break dormancy, whereas, orthodox cherry (Prunusavium) seeds require an initial warm period before cold stratificationto fully stimulate germination. Whole embryos and radicle tipsof both Norway maple and sycamore were found to have stablehigh levels of 4C DNA during the latter stages of developmentand both contained nuclei arrested at the 2C and 4C levels atmaturity. Mature cherry embryos had nuclei predominantly arrestedat the 2C level. This suggests that the acquisition of desiccationtolerance is not correlated with the arrest of the cell cycleat any particular nuclear DNA level. Neither DNA replicationin radicle cells nor germination occurred when seeds were maintainedmoist at a constant 20 °C. However, in the late stages ofcold treatment during stratification, nuclear DNA levels inradicle cells changed in advance of radicle emergence in theorthodox Norway maple and cherry, whereas in the recalcitrantsycamore, change was not recorded until after radicle emergence.These results show that DNA replication has potential use asan indicator of the progress of tree seeds through stratificationtreatments used to break some types of dormancy. The ways inwhich this indicator could be exploited for seed quality andperformance testing are discussed.Copyright 1998 Annals of BotanyCompany Norway maple,Acer platanoidesL., sycamore,Acer pseudoplatanusL., cherry,Prunus aviumL., DNA replication, flow cytometry, seed dormancy, stratification     

The Effect of Temperature Change on Leakage from Pea Seeds

Intact seeds of P. sativum and P. elatius leaked electrolytes,sugars and proteins for as long as 48 h after the beginningof imbibition. Initial leakage was higher at 25 °C thanat 5 °C, but its extent was much greater at the lower temperature.Transfer of seed from 5 °C to 25 °C after 5 h imbibitionresulted in leakage for 48 h at the initial rate at 5 °C.The transfer from 25 °C to 5 °C gave leakage at a rateequivalent to the initial rate at 25 °C. The results arediscussed in relation to behaviour of membranes and to the sensingof the initial temperature. Key words: Pisum sativum, P. elatius, Leakage, Electrolytes, Sugars, Proteins, Temperature     

Changes in Trifolium arvense Seed Quality Following Alternating Temperature Treatment using Liquid Nitrogen

The effects of alternating temperatures, using liquid nitrogen(–196°C) as the low temperature phase, on Trifolium arvense L.seed quality was investigated. Treatment reduced hardseedednesswith the seeds exhibiting a linear increase in the probit ofgermination with the logarithm of the number of alternatingtemperature cycles. Germination levels equal to ‘chipped’controls were achieved after four cycles or more, dependingon the initial depth of hardseededness, and the response wasunaffected by either the rate of cooling or the rate of warming. Imbibition time was reduced with increasing alternating temperaturecycles. Resealing experiments identified the main area of wateruptake as the lens, and SEM showed that deepening of alreadypresent cracks may be important in removing hardseededness.Pre-treatment of seeds with alternating temperatures predisposesthe seeds to subsequent damage by sulphuric acid treatment,but not by impaction. Stress effects were not restricted to the seed coat surfaceas between 10 and 20 % of seeds treated for 3–15 cyclesdeveloped into abnormal seedlings with detached cotyledons.Seed vigour is also reduced as treatment leads to contractionof the temperature response curve for germination. The resultshave implications for the cryopreservation of papilionoid legumeseeds as a means of genetic conservation. Trifolium arvense, Hare's foot, hardseededness, quantal response, cryopreservation, liquid nitrogen     

Supercooling ability of Rhododendron flower buds in relation to cooling rate and cold hardiness

The freezing process and supercooling ability in flower budsof 11 native Rhododendron species were examined with referenceto the cooling rate and cold hardiness by differential thermalanalysis. The freezing patterns of the excised whole buds variedwith the season: in autumn, buds froze as whole units, whilein winter, freezing was initiated in the scales and propagatedto each floret. The supercooling ability of florets was enhancedduring winter. The freezing patterns in winter buds were stronglyinfluenced by the cooling rate (1 to 30°C/hr). Althoughthe first exotherm in scales occurred at –5 to –10°Gand was rate-independent, the occurrence of several floret exothermsshifted considerably to lower subzero temperatures at slowerrates. The most reliable cooling rate for testing maximum supercoolingability was l°C/hr. The exotherm in florets of hardier speciesoccurred at –20 to –25°C and at –7 to–20°C for less hardy ones, and were well correlatedwith their killing temperatures. Water relations within budtissues in response to freezing are briefly discussed. (Received June 26, 1980; )     

Supercooling ability of Rhododendron flower buds in relation to cooling rate and cold hardiness

The freezing process and supercooling ability in flower budsof 11 native Rhododendron species were examined with referenceto the cooling rate and cold hardiness by differential thermalanalysis. The freezing patterns of the excised whole buds variedwith the season: in autumn, buds froze as whole units, whilein winter, freezing was initiated in the scales and propagatedto each floret. The supercooling ability of florets was enhancedduring winter. The freezing patterns in winter buds were stronglyinfluenced by the cooling rate (1 to 30°C/hr). Althoughthe first exotherm in scales occurred at –5 to –10°Gand was rate-independent, the occurrence of several floret exothermsshifted considerably to lower subzero temperatures at slowerrates. The most reliable cooling rate for testing maximum supercoolingability was l°C/hr. The exotherm in florets of hardier speciesoccurred at –20 to –25°C and at –7 to–20°C for less hardy ones, and were well correlatedwith their killing temperatures. Water relations within budtissues in response to freezing are briefly discussed. (Received June 26, 1980; )     

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     

Constant, Fluctuating and Effective Temperature and Seed Longevity: a Tomato (Lycopersicon esculentum Mill.) Exemplar

Tomato seeds with a moisture content of 16.4% were stored hermeticallyat one of five constant temperatures (10, 20, 30, 40, 50 °C)or in one of nine alternating temperature (24 h/24 h) regimes(10/30, 10/40, 10/50, 20/30, 20/40, 20/50, 30/40, 30/50, 40/50°C) for up to 224 d. In each regime, seed survival conformedto cumulative negative normal distributions and all 14 survivalcurves could be constrained to a common origin. Estimates ofthe constants C_Hand C_Qof the viability equation determined atconstant temperatures were 0.0346 (s.e. 0.0058) and 0.000401(s.e. 0.000096), respectively. The effective temperature forseed survival of each alternating temperature regime was alwaysmuch higher than the mean. Tomato seeds were also stored hermeticallyat 15.9% moisture content at 40 °C for 0, 7, 14, 21 or 28d before transfer to 50 °C. This investigation showed thatthe standard deviation of the subsequent survival curves at50 °C was unaffected by the duration of previous storageat 40 °C. The results of both investigations were consistentwith the hypothesis that loss in probit viability is solelya function of the current storage environment, with no effectof change in temperature per se. The application of the viabilityequation to seed survival in fluctuating environments was validatedagainst independent observations for rice in uncontrolled storageconditions. Copyright 2001 Annals of Botany Company Temperature, seed storage, longevity, moisture content, viability equation, tomato, rice     

Freezing Tolerance in Hydrated Lactuca sativa (L) Seed: A Model to Explain Observed Variation Between Seed Lots

Low temperature tolerance was investigated in the imbibed seedof 15 seed lots compnsmg seven cultivars of Lactuca sativa L.During rapid cooling (20 °C h^–1) some seeds of allseed lots survived to –16 °C but none to –20°C. The majority of seed lots retained over 50 per centviability above –14 °C due to isolation of the embryofrom external ice by the endosperm, and subsequent embryo super-cooling.Certain seed lots, including all three seed lots of cv. TomThumb, showed high mortality at temperatures above –10°C. Correlation of mortality with the formation of externalice suggested that the endosperm is not an effective nucleationbarrier in these seed lots. Survival to –20 °C was increased at slower coolingrates (6 to 1 °C h^–1) due to freeze desiccation ofthe embryo, but seed lots varied considerably in their toleranceof specific cooling rates. A model to explain this variationwas developed incorporatmg (1) seed lot super-cooling limittemperature, (2) the rate at which freeze dehydration of thesupercooled embryo took place, (3) the moisture content at whichnucleation (at –20 °C) was no longer certain and (4)the.initial equilibrium moisture content of the fully imbibedseed. Factors (1), (2) and (3) were found to be relatively constant,but low (or artificially reduced) seed moisture content wasclosely correlated with high survival at natural cooling rates.Seed size fractions of similar moisture content from a singlecultivar showed that more small seeds survive cooling at 3 °Ch^–1 to –20 °C than larger seed. Seed with pierced endosperms or ineffective nucleation barrierswere capable of surviving to at least –10 °C if cooledslowly (1 °C h^–1) but were killed by rapid (20 °Ch^–1) cooling. Lactuca sativa L, lettuce, seed germination freezing tolerance, super-cooling     

Effects of Grain Development and Thermal History on Grain Maturation and Seed Vigour of Pennisetum americanum

Studies were made of the viability and vigour of seeds of pearlmillet (Pennisetum americanum) harvested at different stagesof grain development and from different controlled-temperatureenvironments. Seed viability and vigour of the next generationwere dependent on the extent of grain development at harvest.Where grain had developed for only one-third of the potentialgrain-filling period before harvest, seed viability and vigourwere greatly reduced. Harvest at or after the middle of grain-fillingdid not reduce seed viability or vigour. The temperature atwhich the grains had developed did not affect seed viability,but grains that had developed at 21/16 °C (day/night) producedseedlings of greater height and dry weight than those from grainswhich had developed at higher temperatures.     

Effects of Temperature of Mother-plant Environment on Yield and Germination of Seeds of Lettuce (Lactuca sativa)

Experiments on the production of two separate crops of lettuceseeds, each in three different temperature environments, andsubsequent tests on the seed are described. Low production temperatures(20 °C day, 10 °C night) gave a low yield of large seeds,and high temperatures (30 °C, 20 °C) gave a higher yieldof much smaller seed; the highest yield came from medium temperatures(25 °C, 15 °C), which gave medium-sized seed. After-ripening,manifested as an increase in percentage germination at hightemperatures with increase in seed age, occurred in seed fromall three production environments of the first crop, thoughthere were differences in degree, and in that from the two higherproduction temperatures, but not the lowest, of the second crop.Measurements of the forces required to penetrate the layerssurrounding the embryo showed an inverse relationship with temperatureof the production environment for pericarps but not for endosperms,and a gradual reduction during storage for pericarps but notendosperms. Measurements of germination potential showed thatembryos from seeds produced in cool conditions were less ableto cope with high temperatures than those from warner conditions.These results are discussed in relation to the control of germinationin lettuce. Lettuce, Lactuca sativa (L.), seed production, germination, seed coverings, germination potential