Germination of Talinum triangulare L. Seeds as Affected by Various Chemical and Physical Treatments

Three-month ‘old’ and ‘fresh’ seedsof Talinum triangulare were subjected to various treatmentsto induce early and rapid germination. Scarification and activated carbon were the most effective treatmentsin improving total germination in fresh seeds, while the 3 and5 per cent thiourea treatments were most effective in improvingtotal germination in old seeds. Activated carbon, scarificationand 5 per cent thiourea treatments enhanced early germinationin both old and fresh seeds. Cumulative percentage germinationwas very high in fresh seeds after scarification or after treatmentwith activated carbon and 5 per cent thiourea, and lowest inseeds treated with 3 per cent thiourea and hot water. In oldseeds, highest cumulative percentage germination was obtainedwith 3 and 5 per cent thiourea treatments and scarification.Generally, higher germination was obtained with fresh seedsthan with old seeds. Partial seed-coat removal and treatment with 5 per cent thiouriaresulted in a higher rate of and cumulative percentage germinationcompared with seeds with the coat partially removed but nottreated with thiourea. Constantly high temperature (34 °C) increased both rateand total germination compared with seeds planted at room temperature(20–23 °C). Treatments that did not induce germinationwere 1 per cent thiourea, H₂SO₄, cold water soaking at roomtemperature, 6 per cent hydrogen peroxide and soil planting.These treatments effected less than 3 per cent germination. Talinum triangulare L, seed scarification, activated charcoal, thiourea, germination     

Germination of Tagetes minuta L. I. Temperature Effects

Initial studies have indicated that Tagetes minuta achenes haveboth a temperature and a light requirement for germination.Temperatures tested were 10, 20, 25, 30 and 35 °C. Germinationwas optimal at 25 °C under white light conditions. Underthese conditions 100 per cent of achenes germinated within 7days of imbibition. There was no germination at 10 or 35 °Ceither in the light or in the dark. Achenes imbibed and incubatedat 35 °C for 4 days showed no visible signs of germinationbut on transfer to 25 °C, 100 per cent of these achenesgerminated within 24 h. Furthermore, achenes given this hightemperature (35 °C) treatment could be dried at 25 °C,re-imbibed at 25 °C and again 100 per cent of achenes germinatedwithin 24 h of re-imbibition. This rapid germination responsefollowing removal from the high temperature regime could alsobe induced by transfer to temperatures of 20 °C or 20 °C(16 h) alternating with 10 °C (8 h). Tagetes minuta L., weed seeds, germination, temperature, light     

Seed Germination of a Halophytic Grass Aeluropus lagopoides

Aeluropus lagopoides(Linn.) Trin. Ex Thw. (Poaceae) is a perennialgrass distributed from coastal Sindh and Balochistan to salineflats of Punjab, Pakistan. Seeds collected from an inland populationofA. lagopoides located on the University of Karachi campuswere germinated under various levels of salinity (0, 100, 200,300, 400 and 500 m M NaCl) and temperature regimes (10/20, 15/25,20/30 and 25/35 °C) in a 12 h dark/12 h light photoperiod.Highest germination was obtained under non-saline conditions,and an increase in NaCl concentration progressively inhibitedgermination. Inhibition of germination was greater at coolertemperatures (10/20 °C) when no seed germinated above aconcentration of 300 m M NaCl. The germination response at moderatetemperatures (20/30 °C) was optimal, with 30% of seeds germinatingin 500 m M NaCl. The rate of germination decreased as salinityincreased. Germination rate was highest at 20/30 °C andlowest at 10/20 °C. Seeds were transferred from salt solutionsto distilled water after 20 d and those from high salinitiesrecovered quickly at warmer temperatures with an optimal responseat 20/30 °C. Copyright 2001 Annals of Botany Company Aeluropus lagopoides, germination, halophyte, Karachi, salinity, temperature     

Control of the Germination Responses of Amaranthus retroflexus L. Seeds by their Parental Photothermal Environment

Germination responses of the seeds of Amaranthus retroflexusL. were affected by the photoperiod, temperature, and levelof solar radiation experienced by their parent plants. Seedsfrom parents grown continuously in short days (SD, 8 h) lostpost-harvest dormancy more rapidly and had a higher dark germination,as well as a greater responsiveness (at 30?C) to pretreatmentsat low temperature (5 or 10?C) and to short illuminations, thanseeds from parents grown continuously in long days (LD, 16 h).Dark germination and responsiveness of the seeds to promotivetreatments were both higher when their parents were transferredat flowering from LD to SD than when grown continuously in LD.These responses were lower when their parents were similarlytransferred from SD to LD than when grown continuously in SD.The promotive effects of parental post-flowering SD on darkgermination (at 30?C) were enhanced by reduction of parentaltemperature (from 27/22?C to 22/17?C), but the responsivenessof the seeds to low temperature pretreatment was reduced. Inflorescencesdeveloping in LD produced seed with higher germinability whenfloweringwas not induced (LD throughout) than when it was induced (eitherby SD till flowering, or by three SD cycles when 4–5 leavesappeared). Reduced levels of solar radiation had opposite effectsin the different parental photoperiods: dark germination andthe responsiveness to low temperature pretreatments were reducedin LD, but were increased in SD. Differences in the germination responses resulting from differencesin the parental environment could not be correlated with differencesin seed coat thickness or seed dry weight.     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation     

Effects of microhabitat,light and temperature on seed germination of a critically endangered Himalayan medicinal herb,Swertia chirayita: Conservation implications

Abstract

Swertia chirayita, a critically endangered medicinal herb, is being over-harvested in the wild. Understanding seed germination is a pre-requisite to ensure species conservation. The germination of seeds collected from six microhabitats was studied at 20°C, 25°C, and 30°C, both under a 14/10 h light/dark photoperiod and in continuous darkness. Two-way ANOVA indicated that microhabitat and temperature significantly affect seed germination, germination rate, germination recovery (GR), and GR rate. Overall, the seeds collected from under canopy showed a significantly (p < 0.05) higher germination than those from open habitats, at 20°C, 25°C, and 30°C (14/10 h light/dark photoperiod). Germination was negligible in continuous darkness but after transfer to a 14/10 h light/dark photoperiod, the seeds from under canopy significantly recovered at 20°C and at 25°C (p < 0.05), and showed the highest germination percentage compared to seeds collected from tree base, stump base, shrubberies, and grassy slope. Similarly, at 30°C, seeds from under canopy recorded the highest GR percentage. In general, seed germination, mean germination rate, seed GR, and GR rate were significantly greater (p < 0.05) at 25°C. Among the microhabitats tested, variation in GR rate was significant (p < 0.05). Seeds were confirmed to be positively photoblastic.     

Photocontrol of seed germination in the hemiparasite Buchnera hispida (Scrophulariaceae)

Abstract Buchnera hispida, a facultative root parasite of grasses and graminaceous crops, has a light requirement for germination. Studies were carried out on the effects of varying photoperiods with or without preceding dark incubation, on seed germination. Buchnera seeds showed long-day behaviour, since they germinated at all photoperiods including continuous light, and longer photoperiods were more effective in triggering seed germination than shorter photoperiods. Also, effects of red and far-red light indicated that the phytochrome system is operative in the light-induced germination of Buchnera. Although dark incubation in water before illumination was not absolutely necessary for germination, it caused the seeds to respond more rapidly to light. The longer the time of the dark incubation the more responsive the seeds were to photoperiod except when 15 min light was given. The effectiveness of a preceding dark incubation in making Buchnera seeds sensitive to rapid light action was completely inhibited at 4°C. This is in agreement with the hypothesis that a reaction partner of the far-red absorbing form of phytochrome is produced during dark incubation of Buchnera seeds. Such an intermediate has also been reported in some positively photoblastic seeds of non-parasitic flowering plants.     

Effects of Photoperiod, Nitrogen Nutrition and Temperature on Inflorescence Initiation and Development in Onion (Allium cepa L.)

The effects of photoperiod, nitrogen nutrition and temperatureon inflorescence initiation and development in onion cv. Rijnsburgerand cv. Senshyu Semi-globe Yellow were studied in controlledenvironments. Rates of inflorescence initiation were estimatedusing the data for leaf numbers formed prior to flower formationand the rates of leaf initiation. At 9 °C inflorescenceinitiation was accelerated by long photoperiods particularlyfor cv. Rijnsburger where the average time for initiation was86 days in 8 h and 38 days in 20 h photoperiods. Initiationwas as rapid at 12 °C as at 9 °C but was slower at 6°C. A reduction in the nitrate concentration in the nutrientsolution from 0.012 to 0.0018 M greatly accelerated inflorescenceinitiation particularly in photoperiods and temperatures notconducive to rapid initiation. Cv. Senshyu initiated more slowlythan cv. Rijnsburger and was less sensitive to photoperiod andnitrogen level. The development rate of inflorescences afterinitiation was accelerated by long photoperiods and increasesin temperature from 6 to 12 °C but was retarded by the lowernitrogen level. Allium cepa L., onion, flower initiation, inflorescence development, photoperiod, nitrogen nutrition, temperature, vernalization     

Effects of Temperature and Photoperiod on Winged Beans [Psophocarpus tetragonolobus (L.) D.C.]

The effects of temperature and photoperiod on winged beans werestudied using 15 University of New Guinea (UPS) selections andfive Sri Lanka (SL) selections. They were grown at 25/20 or30/25 °C day/ night temperature at 11 or 14 h photoperiodwith 12 h thermoperiod. Differences in stomatal density wereobserved among selections and between photoperiods. Higher densitiesoccurred at 14 h photoperiod than at 11 h photoperiod. Whenstomatal density was high due to a photoperiod or temperatureeffect, there was a corresponding increase in leaf area andd. wt of plants. Total chlorophyll content at 25/20 °C was higher at 11 hphotoperiod than at 14 h photoperiod in all selections whilethe total chlorophyll content at 30/25 °C varied with thephotoperiod and selection. Leaf area of SL selections was greater than that of UPS selections.Also greater leaf area was observed at 14 h photoperiod thanat 11 h photoperiod, irrespective of the growing temperature. Temperature was as important as photoperiod in controlling floweringof winged beans. All the UPS selections and two SL selectionsflowered at 11 h photoperiod at 25/20 °C but failed to flowerat the same photoperiod at 30/25 °C indicating an interactionbetween temperature and photoperiod. It is likely that wingedbeans have a narrow photoperiodic range, particularly the SLselections. Psophocarpus tetragonolobus (L.) D.C., winged bean, stomatal density, leaf area, flowering, temperature, photoperiod     

Light Actions in the Germination of Cocklebur Seeds: IV. DISAPPEARANCE OF RED LIGHT-REQUIREMENT FOR THE GERMINATION OF UPPER SEEDS SUBJECT TO ANOXIA, CHILLING, CYANIDE OR AZIDEPRETREATMENT

Esashi, Y., Fuwa, Nn Kojima, K. and Hase, S. 1986. Light actionsin the germination of cocklebur seeds. IV. Disappearance ofred light-requirement for the germination of upper seeds subjectto anoxia, chilling, cyanide or azide pretreatmenL—J.exp. Bot. 37: 1652–1662. The effects on the germination of positively photoblastic uppercocklebur (X anthium pennsylvanicum Wallr.) seeds by pretreatingwith anoxia, chilling, cyanide or azide, which stimulates theirdark germination, were examined in relation to light actions.Prior to experiments, seeds were pre-soaked at 23 °C inthe dark for 1 or 2 weeks to remove the pre-existing Pfr. Whenthe prctreatment conditions were suboptimal for germinationinduction, the stimulating effects of the pretreatments on germinationduring a subsequent dark period at 23 °C were manifest onlywhen seeds were irradiated with red light before or after thepretreatment Red light promotion was reversed by blue or far-redlight treatment. However, both prc-chilling for 6 d at 8 °Cand prctreatment with 1· 5 mol m ^{– 3} NaN³ for 2d could induce full germination without red light exposure.On the other hand, both pre-exposure to anoxia for 8 d and pretreatmentwith 30 mol m^–3 KCN could induce the dark germinationonly when germination occurred at 33 °C which is known toaugment the ratio of an alternative respiration flux to a cytochromeone. Moreover, the dark germination in response to these inductionswere strongly inhibited by the inhibitors of alternative respiration,propyl gallate and benzohydroxamic acid, applied during a subsequentdark period. It was thus suggested that Pfr has some relationto the operation of two respiration systems of cocklebur seeds,but it is not indispensable to germination of this positivelyphotoblastic seed. Key words: Anoxia, azide, blue light, chilling cyanide, dark germination, far-red light, red light, seed germination, X anthium pennsylvanicum     

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

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

A Comparative Study of Germination Responses to High Irradiance Light

Seeds of 19 native British herbaceous species (14 grasses andfive forbs) were exposed to white light at three photon fluencerates: high (19–2 mol m^–2 d^–1), medium (9·6mol m^–1 d^–1) and low (2·3 mol m^–2 d^–2) These photon doses have been found by previous workers to inhibitgermination in several species. High and low photon doses wereapplied only as continuous light, but the medium dose was appliedas both continuous light and as a 12 h light/12 h dark photoperiod.All four treatments, plus a dark control, were carried out at15 °C; the high and low doses were also applied with a dailyalternation of 10/20 °C. The majority of species (15) fell into one of two groups. Inseven species germination was relatively high and consistentacross all treatments, including darkness; in the other eightspp germination was inhibited only in darkness. Mostly thesedata confirmed published results for the same species In contrast in Agrostis capillaris and A. stolonifera germinationwas high only at alternating temperatures, irrespective of photondose, but was also slightly promoted by a constant temperaturecombined with light/dark alternations. Only in Bromus sterilisand B. ereclus was germination inhibited by light, in B. erectusat all photon doses and in B. sterilis only at the highest photondose These results suggest that inhibition of germination by highirradiance light is not widespread among native British species Aira caryophyllea, Arrenatherum elatius, Festuca ovina, F. rubra, Hordeum murinum, Milhim effusum, Silene dioica, Achillea millefolium, Brachypodium syhaticum, Digitalis purpurea, Holcus lanatus, Leucanlhemum vulgare, Phleum pratense, Poa trivialis, Taraxacum officinale, Agrostis capillaris, A. stolonifera, Bromus erectus, B. sterilis, seed, dormancy, germination, light, High irradiance reaction, alternating temperatures, photoperiod     

Dormancy in seeds of Phellodendron wilsonii is Mediated in Part by Abscisic Acid

A regime of temperatures alternating between 35°C (8 h)and 10°C (16 h) proved to be a specific method for breakingdormancy of seeds of Phellodendron wilsonii. The relationshipbetween the germination capacity after about 13 days of incubationunder this regime and the logarithm of the amount of ABA inthe seed before incubation yielded a high correlation coefficient.The final germination capacity under this regime was poorlycorrelated with the amount of ABA in seeds before incubation.These observations suggest that at least part of the germinationbehavior is controlled by the amount of abscisic acid in theseed. Stratification decreased the amount of ABA but resultedin only 8–25% germination relative to other dry storedseeds failed to germinate under suboptimal conditions, namely,incubation at 22°C with 12 h light. This phenomenon indicatesthat some unknown mechanism is the major factor that controlsthe dormant state, and the effect of this mechanism are overcomeonly by the specific alternating-temperature regime. Fluctuationsin temperature failed, however, to reduce the ABA content ofseeds during the incubation period. We conclude that some unknownfactor contributes to the dormancy of seeds of P. wilsonii althoughthe level of ABA definitely plays a minor role in the maintenanceof seed dormancy. (Received January 25, 1993; Accepted November 25, 1993)     

Tomato Seed Germination. Osmotic Pretreatment and Far Red Inhibition

At 25 °C germination of tomato (Lycopersicon lycopersicum)seeds is inhibited by continuous and intermittent far red illumination.It is also inhibited by a single 30 min far red irradiationgiven 8 h from the start of imbibition. The incubation of seedsin a mannitol solution inhibitory for germination has no effecton the final germination percentage after seeds are subsequentlytransferred to water. A 30 min far red irradiation at the timeof transfer results in partial inhibition of germination. Thisinhibition can be released by the continuation of osmotic incubationfor several days before the transfer to water. At the end ofa 7 d dark period of osmotic incubation, inhibition of subsequentgermination in water can be realized only by continuous farred illumination. Seeds osmotically pretreated for 7 d and afterwardsdried-back show a mean time to 50% germination significantlylower than that of untreated seeds. Moreover, besides singleand intermittent, even continuous far red light has no inhibitoryeffect on the germination of these seeds. It is concluded that,in addition to the already known germination advantages, osmoticpresowing treatment also induces the ability of seeds to germinateunder unfavourable light conditi.     

Influence of Temperature and Light Conditions on Germination,Growth and Conidiation of Oidium neolycopersici

The effect of temperature and light conditions (spectral quality, intensity and photoperiod) on germination, development and conidiation of tomato powdery mildew (Oidium neolycopersici) on the highly susceptible tomato cv. Amateur were studied. Conidia germinated across the whole range of tested temperatures (10–35°C); however, at the end‐point temperatures, germination was strongly limited. At temperatures slightly lower than optimum (20–25°C), mycelial development and time of appearance of the first conidiophores was delayed. Conidiation occurred within the range of 15–25°C, however was most intense between 20–25°C. Pathogen development was also markedly influenced by the light conditions. Conidiation and mycelium development was greatest at light intensities of approximately 60 μmol/m² per second. At lower intensities, pathogen development was delayed, and in the dark, conidiation was completely inhibited. A dark period of 24 h after inoculation had no stimulatory effect on later mycelium development. However, 12 h of light after inoculation, followed by continuous dark, resulted in delayed mycelium development and total restriction of pathogen conidiation (evaluated 8 days postinoculation). When a longer dark period (4 days) was followed by normal photoperiod (12 h/12 h light/dark), mycelium development accelerated and the pathogen sporulated normally. When only inoculated leaf was covered with aluminium foil while whole plant was placed in photoperiod 12 h/12 h, the intensive mycelium development and slight subsequent sporulation on covered leaf was recorded.     

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     

Photoinduced Seed Germination of Oenothera biennis L: I. General Characteristics

General characteristics of light-induced germination of Oenothera biennis L. seeds were investigated at 24°C. During dark imbibition, seeds reached maximal respiration in 7 hours and maximal water content and photosensitivity in 24 hours. After dark imbibition of 24 hours, seeds required a long exposure (>36 hours) to red or white light for maximal germination. Two photoperiods (12 and 2 hours) separated by a period of darkness of 10 to 16 hours gave near maximal germination. For the two photoperiod regime, the first light potentiates a reversible phytochrome response by the second light. A 35°C treatment for 2 to 3 hours in the dark immediately prior or subsequent to 8 hours of light caused a higher percentage of germination. A 2 hour treatment at 35°C also potentiates a reversible phytochrome response. Halved seeds germinated at 100% in light or darkness indicating that the light requirement of the seeds is lost in the halving procedure. After-ripened seeds required less light and germinated more rapidly and at higher percentages than seeds tested shortly after maturation.     

Seed Germination and Seedling Development in Cyclamen persicum

Cyclamen persicum Mill, seeds germinate in a narrow range oftemperature and germination is strongly inhibited by continuousirradiation with white light. The thermal optimum is approx.15 °C in both darkness and light. Seed germination is alsovery sensitive to oxygen deprivation and this sensitivity ismore pronounced at 20 °C than at the optimum 15 °C.Very immature seeds cannot germinate at any temperature, butgerminability increases during seed maturation Seedling development is unusual since seed reserves are usedimmediately for tuber formation. Tuberization is optimal at15–20 °C in light and in darkness. Supra-optimal temperatures(25–30 °C) or hypoxia inhibit tuber formation andlead to very elongated tubers These results allow the producers to improve the productionof homogeneous populations of cyclamen seedlings Wheat seeds, Triticum aestwum L., acetylcholinesterase, electrophoresis, germination, assay     

Flowering in Pisum: the Effect of Genotype, Plant Age, Photoperiod, and Number of Inductive Cycles

The genotypes If e Sn hr, Lf e Sn hr, and If e Sn Hr requirefewer inductive cycles as they age. It is suggested that thisresults from a decrease in the activity of the Sn gene in theleaves as they age, resulting in a higher ratio of promoterto inhibitor. Gene Lf does not affect the rate of this agingbut it does increase the number of inductive cycles requiredfor flower induction over the first 5 weeks of growth. The geneHr has no effect until week 4 but thereafter causes a reductionin the effect of age on the Sn gene. The genotype If e Sn Hrcan be induced by a single inductive cycle (32 h of light) fora relatively long period. The length of dark period required for the expression of theSn gene is shown to be less than 4 h providing a relativelylong photoperiod precedes the dark period. It appears that noper manent induction of tissue by photoperiods favourable toflowering occurs in peas. The critical photoperiod for plantsof genotype if e Sn Hr is shown to be between 12 and 14 h atl7·5 °C and the usefulness of the term ‘criticalphotoperiod’ is discussed with respect to quantitativelong-day plants.     

SUCCESSIVE PROCESSES INVOLVED IN THE GERMINATION RESPONSE OF NASTURTIUM SEEDS

1. The seeds ofNasturtium palustreDC. do not germinate, eitherin the light or darkness, at various constant temperatures,but require for their full germination a certain period of alow temperature (5°) applied immediately after light irradiation.These results indicate the existance of at least two processes,a light-dependent process and a low temperature-requiring process,in the initiation of germination ofNasturtiumseeds. Experimentalevidence indicated further that the light exposure causes twodifferent processes in the seed germination. 2. When a dark period at 23° was inserted between the lightirradiation and the low temperature treatment the germinationwas suppressed. The inhibitory effect of the inserted dark periodat 23° was eliminated by a short irradiation during thedarkness (light-break). 3. Prolonged exposure ofNasturtium seeds to any concentrationof gibberellin brought about no germination when exposure wasgiven in complete darkness. The germination was promoted onlywhen light irradiation was applied to the seeds. A short applicationof gibberellin at a fairly high concentration was, however,remarkably effective for the germination even in the darkness,and the germination was inhibited as the gibberellin applicationwas lengthened. It was considered that gibberellin could substitutefor the combined effect of light irradiation and low temperaturetreatment to induce the germination of Nasturtium seeds, andthat gibberellin was inhibitive toward the reactions followingthe above treatments which induced the germination (Received October 31, 1996; )