Studies with Liatris spicata Willd. 1. Effect of Temperature on Sprouting, Flowering and Gibberellin Content

Dormancy was induced during storage of Liatris spicata cormsgrown in Israeli summer conditions, but plants left in soilcontinued vegetative growth. Corms of winter-grown plants sproutedfreely. Treatment with GA₃ restored both sprouting and floweringin summer-grown corms, but in winter corms GA₂ was effectiveonly after corms were stored at low temperature. All the plantsflowered after 4 weeks at 2 °C and GA³ treatment. The content of gibberellins in the main bud of freshly excavatedcorms decreased during the first 18 d of storage but increasedto the initial level after 4 months of cold storage. The numberof flowering stems increased to 2.5 per corm when corms werecold-stored up to 75 d, but decreased with a longer storage. Liatris spicata, dormancy, flowering, gibberellin, sprouting     

Studies in the Physiology of the Onion Plant: IV. THE INFLUENCE OF DAY-LENGTH AND TEMPERATURE ON THE FLOWERING OF THE ONION PLANT

At temperatures above about 17° C. inflorescence initiationin growing onion plants, as in stored sets, is suppressed whetherthe plants are kept in long or short days. Independently ofcurrent day-length and of previous day-length treatment, ifthe plants are sufficiently large initiation begins very shortlyafter the temperature falls below c. 15° C. Emerged infiorescencesappear some ten or so weeks later. Small plants are unable toinitiate inflores cences under any of the conditions tested,and actual size (perhaps leaf area) rather than leaf or nodenumber seems to be the important factor. Inflorescence emergenceis suppressed at high temperatures in short days or long days;in long days bulb formation also suppresses emergence at lowertemperatures. In long days at temperatures sufficiently lowfor bulbing to be delayed, however, emergence is accelerated.Plants which have produced bulbs in long days in the summershow a delay of inflorescence emergence in the following winter.     

Dormancy and Flowering in Anemone coronaria L. as Affected by Photoperiod and Temperature

The effects of day-length and temperature on flowering and dormancyinduction were studied in Anemone coronaria L., with plantsraised either from corms or achenes. An Israeli hybrid sourcewas used (de Caen cv. Hollandia x Israeli wild type). Dormancy onset is characterized by the cessation of foliageleaf production, the appearance of leaf scales protecting theperennating bud, and leaf senescence. Dormancy was induced byhigh temperature and long days but increasing temperatures (from17/12 °C to 32/12 °C) induced earlier dormancy thanprolonging the photoperiod (range 8–16 h). A significant(P = 0.01) interaction was found between these factors, withsmaller photoperiodic effects the higher the temperature. At22/17 °C the critical day-length for dormancy inductionwas between 11 and 12 h. The transition from the vegetative to the reproductive stageappears to be an autonomous process that occurs with developmentin plants raised from either corms or achenes and does not requireenvironmental induction. Photo- and thermoperiodic effects onflowering were indirect, being mediated through their influenceon dormancy induction. Anemone coronaria L., dormancy, flowering, photoperiod, thermoperiod     

Photoperiod and Temperature Affect Sprouting of Onion bulbs (Allium cepa L.)

The effects of photoperiod and temperature on sprouting wereinvestigated using three cultivars of onion which exhibit differentresponses to daylength during bulb growth. Two photoperiods(8 h and 14 h) and two constant temperatures (15 C and 20 C)were used. Two cultivars showed enhanced sprouting and leafemergence in short days, and one sprouted poorly in all conditions.In long days, some plants showed growth of new bulbs withinthe old one, without sprouting. Photomorphogenesis, photoperiodism, Allium cepa L, onion, dormancy, bulb sprouting     

Flowering of Contrasting White Clover Varieties in Relation to Temperature in Controlled Environments

Effects of temperature on floral initiation of ten white clovervarieties growing in controlled environments are described.Plants grown under long days (16 h) were subjected to constanttemperatures of 26, 18 and 10 ^°C. Relationships betweenmorphological and physiological traits and flowering were examined. Most plants flowered at the two higher temperatures but only10 per cent of plants flowered at 10 ^°C. Larger leaved typestended to produce more reproductive buds per stolon at the highertemperatures than did smaller leaved varieties. Of the floral characters studied, floret number was least affectedby temperature. Ovule number and peduncle length were greatestat 18 ^°C. Variation in, and absolute level of nectar secretionwas greatest at the highest temperature. Trifolium repens, white clover, flowering, temperature     

Ribulose 1, 5-bisphosphate Carboxylase/ Oxygenase Activities in Leaves of Greenhouse Roses

An enzyme assay was developed to measure the initial and Mg²⁺–CO₂activated forms of Ribulose 1,5-bisphosphate Carboxylase/Oxygenase(Rubisco) in rose leaves. The assay was verified by co-extractionof the leaflets with partially purified spinach Rubisco andthrough correlation with net photosynthetic rates of individualleaflets (r²=0.7324). Changes in activities were measured asa function of depth of leaves in the canopy for two cultivarsof greenhouse hybrid tea roses. Initial Rubisco activity declinedwith increasing canopy depth for both cultivars. The activatedform of the enzyme, however, remained constant with canopy depthfor cv. Red Success; but increased with canopy depth, then declinedafter mid-canopy in the cv. Royalty. Rubisco activities werealso measured in the cv. Red Success grown in CO₂ enriched environments(100 mm³ dm^–3) at three humidity levels. The activitieswere not significantly affected by humidity treatment. However,there was a trend for plants grown at lower humidity to havehigher activated activities. Key words: Humidity, Rubisco, Rosa ? hybrida, Royalty, Red Success     

The preconditioning effect of day-length and light intensity on adult plant resistance to powdery mildew in oats

Oat genotypes with partial resistance to mildew exhibited a higher level of adult plant resistance (a.p.r.) when grown in 16 h compared with 8 h day-length. This increased resistance was expressed in the upper leaves, and to some extent in the lower, earlier-developed leaves. The expression of increased resistance coincided with floral initiation in Maldwyn, but preceded it in CC4761. The development of a.p.r. and floral initiation were thus not inherently linked, but conditions which accelerated the plants' ontogeny also caused earlier expression of resistance. For the most efficient testing of breeding material, plants should be grown at long day-length and high light intensity. When plants are grown at 16 h day-length and constant temperature of 15 ^oC, the leaf segments should be inoculated at the 6 to 8 leaf stage because normally susceptible genotypes showed marked resistance when inoculated later.     

The Effects of Temperature and Light Integral on the Phases of Photoperiod Sensitivity inPetuniaxhybrida

Flowering in petunias is hastened by long days, but little isknown about when the plants are most sensitive to photoperiod,or how light integral or temperature affect such phases of sensitivity.The effects of these factors on time to flowering was investigatedusing reciprocal transfer experiments between long (16 h d^-1)and short days (8 h d^-1). The effect of light integral on thephases of photoperiod sensitivity was examined using two sowingdates and a shading treatment (53% transmission). The effectsof temperature were investigated by conducting reciprocal transferexperiments in glasshouse compartments at five temperature regimes(means of 13.7, 19.2, 22.3, 25.0 and 28.7 °C). The lengthof the photoperiod-insensitive juvenile phase of development,when flowering cannot be induced by any environmental stimulus,was sensitive to light integral; low light integrals prolongedthis phase, from 23 d at 2.6 MJ m^-2d^-1to 36 d at 1.6 MJ m^-2d^-1(totalsolar radiation). The length of this development phase was shortest(12.5 d) at 21 °C; it was longer under cooler (21 d at 13.5°C) and warmer temperatures (17.6 d at 28.3 °C). Afterthis phase, time to flowering was influenced greatly by photoperiod,with long days hastening flowering by between 28 and 137 d,compared with short days. Plants also showed some sensitivityto both temperature and light integral during this phase, butthe duration of the final phase of flower development, duringwhich plants were photoperiod-insensitive, was dependent primarilyon the temperature at which the plants were grown; at 14.5 °C,33.9 d were required to complete this phase compared with 11.4d at 25.5 °C. The experimental approach gave valuable informationon the phases of sensitivity to photothermal environment duringthe flowering process, and could provide the basis of a morephysiologically-based quantitative model of flowering than hashitherto been attempted. The information is also useful in thescheduling of lighting and temperature treatments to give optimalflowering times of high quality plants.Copyright 1999 Annalsof Botany Company Petunia,Petuniaxhybrida, juvenility, flowering, photoperiod, temperature, light integral, reciprocal transfer.     

Properties of Phosphoenolpyruvate Carboxylase and Carbohydrate Accumulation in the C3-CAM Intermediate Sedum telephium L. Grown Under Different Light and Watering Regimes

A comparison of the activity and properties of the enzyme phosphoenolpyruvatecarboxylase (PEPC) was made for plants of Sedum telephium L.grown under low (70 µmol m^–2 s^–1) or high(500µmol m^–2 s^–1) PPFD and subjected to varyingdegrees of water stress. Under well-watered conditions onlyplants grown under high PPFD accumulated titratable acidityovernight and the extractable activity of PEPC was almost 2-foldhigher in these plants than in plants grown under low PPFD.Increasing drought stress resulted in a substantial increasein the activity of PEPC extracted both during the light anddark periods and a decrease in the sensitivity to inhibitionby malic acid. The magnitude of these changes was determinedby the severity and duration of drought and by light intensity.A comparison of the kinetic properties of PEPC from severelydroughted plants revealed that plants droughted under high PPFDhad a lower K_m for PEP than plants under low PPFD. Additionof 2·0 mol m^–3 malate resulted in an increase inthe K_m for PEP, with plants draughted under low PPFD havinga significantly higher K_m in the presence of malic acid comparedto those under high _PPFD. Response to the activator glc-6-P,which lowered the K_m for PEP, also varied between plants grownunder the two light regimes. Under well-watered conditions PEPCextracted from plants under high PPFD was more sensitive toactivation by glc-6-P than those under low PPFD. After the severedrought treatment, however, the K_m for PEP in the presence ofglc-6-P was similar for enzyme extracted from plants grown underboth light regimes. Soluble sugars and starch were depletedovernight and were both possible sources of substrate for PEPC.With increasing drought, however, the depletion of starch relativeto soluble sugars increased under both light regimes. The propertiesof PEPC and the characteristics of carbohydrate accumulation/depletionare discussed in relation to the regulation of CAM in S. telephiumgrown under different light and watering regimes. Key words: PEP carboxylase, CAM, carbohydrates, Sedum telephium     

Light Quality and Vernalization Interact in Controlling Late Flowering in Arabidopsis Ecotypes and Mutants

The late flowering ecotypes of Arabidopsis thaliana L. (Heyn.)Eifel, Pitztal and Innsbruck responded to 10 d vernalization(cold treatment) by flowering earlier with less with less thanhalf the number of leaves of non-induced plants. The vernalizationresponse was cumulative: increased numbers of days of vernalizationinduced earlier flowering up to an apparent saturation in responseafter 30 to 40 d. The ratio of red:far-red (R:FR) light alsoaffected non-vernalized time-to-flower. When grown under fluorescentplus incandescent lamps (R:FR = 1·0), time-to-flowerwas approximately half that required by plants grown under fluorescentlamps (R:FR = 5·8) at the same photon flux density andphotoperiod. Leaf production rate was unaffected by either vernalizationor light quality changes and time-to-flower and leaf numberwere highly correlated (r² = 0·973). The late flowering mutants of Landsberg erecta were grown underlighting which displayed a gradient of R:FR. Some mutants likeco, flowered at the same time in all R:FR treatment, while otherlike fca took nearly twice as long to flower, with double thenumber of leaves at R:FR ratio of 5·8 compared with theR:FR = 1 treatment. The ranking of the response from least tomost responsive was co, fe, gi, WT, fd, fwa, ft, fha, fpa, fy,fve and fca. Vernalization of these Landsberg mutants always resulted inearlier flowering, although only fca, fve, fy and fpa were significantlymore sensitive to thermoinduction than the wild type parent.There was a high correlation (r² = 0·89 between the responseto thermoinduction and to R:FR ratio. Vernalization of fca for24 d largely eliminated the R:FR time-to-flower response. Vernalizationand photoinduction similarly affect late flowering and can substitutefor each another.Copyright 1993, 1999 Academic Press Light quality, vernalization, flowering, Arabidopsis thaliana, phytochrome, thermoinduction, photoperiod, photoinduction, growth conditions, photon flux density, daylength, spectral quality, far-red light     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Responses of Respiration to Increases in Carbon Dioxide Concentration and Temperature in Three Soybean Cultivars

The purpose of this experiment was to determine how respirationof soybeans may respond to potential increases in atmosphericcarbon dioxide concentration and growth temperature. Three cultivarsof soybeans (Glycine max L. Merr.), from maturity groups 00,IV, and VIII, were grown at 370, 555 and 740cm³m^-3carbon dioxideconcentrations at 20/15, 25/20, and 31/26°C day/night temperatures.Rates of carbon dioxide efflux in the dark were measured forwhole plants several times during exponential growth. Thesemeasurements were made at the night temperature and the carbondioxide concentration at which the plants were grown. For thelowest and highest temperature treatments, the short term responseof respiration rate to measurement at the three growth carbondioxide concentrations was also determined. Elemental analysisof the tissue was used to estimate the growth conversion efficiency.This was combined with the observed relative growth rates toestimate growth respiration. Maintenance respiration was estimatedas the difference between growth respiration and total respiration.Respiration rates were generally sensitive to short term changesin the measurement carbon dioxide concentration for plants grownat the lowest, but not the highest carbon dioxide concentration.At all temperatures, growth at elevated carbon dioxide concentrationsdecreased total respiration measured at the growth concentration,with no significant differences among cultivars. Total respirationincreased very little with increasing growth temperature, despitean increase in relative growth rate. Growth respiration wasnot affected by carbon dioxide treatment at any temperature,but increased with temperature because of the increase in relativegrowth rate. Values calculated for maintenance respiration decreasedwith increasing carbon dioxide concentration and also decreasedwith increasing temperature. Calculated values of maintenancerespiration were sometimes zero or negative at the warmer temperatures.This suggests that respiration rates measured in the dark maynot have reflected average 24-h rates of energy use. The resultsindicate that increasing atmospheric carbon dioxide concentrationmay reduce respiration in soybeans, and respiration may be insensitiveto climate warming. Glycine max L. (Merr.); carbon dioxide; respiration; temperature; climate change     

Long Term Effects of High CO2 Concentration on Photosynthesis of Water Hyacinth (Eichhornia crassipes (Mart.) Solms)

The photosynthetic response to CO₂ concentration, light intensityand temperature was investigated in water hyacinth plants (Eichhorniacrassipes (Mart.) Solms) grown in summer at ambient CO₂ or at10000 µmol(CO₂) mol^–1 and in winter at 6000 µmol(CO₂)mol^–1 Plants grown and measured at ambient CO₂ had highphotosynthetic rate (35 µmo1(CO₂) m^–2 s^–1),high saturating photon flux density (1500–2000) µmolm^–2 s^–1 and low sensitivity to temperature in therange 20–40 °C. Maximum photosynthetic rate (63 µmol(CO₂)m^–2 s^–1) was reached at an internal CO₂ concentrationof 800 µmol mol^–1. Plants grown at high CO₂ in summerhad photosynthetic capacities at ambient CO₂ which were 15%less than for plants grown at ambient CO₂, but maximum photosyntheticrates were similar. Photosynthesis by plants grown at high CO₂and high light intensity had typical response curves to internalCO₂ concentration with saturation at high CO₂, but for plantsgrown under high CO₂ and low light and plants grown under lowCO₂ and high light intensity photosynthetic rates decreasedsharply at internal CO₂ concentrations above 1000 µmol^–1. Key words: Photosynthesis, CO₂, enrichment, Eichhornia crassipes     

Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO₂ on the response to hightemperatures in terms of carbon assimilation (net photosynthesis,stomatal conductance, amount and activity of Rubisco, and concentrationsof total soluble sugars and starch) and of photochemistry (forexample, the efficiency of excitation energy captured by openphotosystem II reaction centres) were studied in cork oak (Quercussuber L.). Plants grown in elevated CO₂ (700 ppm) showed a down-regulationof photosynthesis and had lower amounts and activity of Rubiscothan plants grown at ambient CO₂ (350 ppm), after 14 monthsin the greenhouse. At that time plants were subjected to a heat-shocktreatment (4 h at 45C in a chamber with 80% relative humidityand 800–1000 mol m^–2 s^–1 photon flux density).Growth in a CO₂-enriched atmosphere seems to protect cork oakleaves from the short-term effects of high temperature. ElevatedCO₂ plants had positive net carbon uptake rates during the heatshock treatment whereas plants grown at ambient CO₂ showed negativerates. Moreover, recovery was faster in high CO₂-grown plantswhich, after 30 min at 25C, exhibited higher net carbon uptakerates and lower decreases in photosynthetic capacity (A_max aswell as in the efficiency of excitation energy captured by openphotosystem II reaction centres (F_vJF_m than plants grown atambient CO₂. The stomata of elevated CO₂ plants were also lessresponsive when exposed to high temperature. Key words: Elevated CO₂, temperature, acclimation, photosynthesis, Quercus suber L.     

Changes in Endogenous Growth Regulators in the Gladiolus Corm during Dormancy

Changes in endogenous growth regulators in gladiolus corms during dormancy were studied using paper and column chromatography followed by a bioassay with the test for straight growth of Avena coleoptile. Corms were grown in the field or in a glass room of a phytotron at 20°C in the light. Another lot was grown in a dark room at 20°C in the dark. Half of the daughter corms in each lot were cold-treated for about one month and the other half were stored at room temperature after harvest. The earliest sprouting was seen in dark grown corms with cold treatment, and the latest sprouting in light grown corms without cold. This pattern was similar in each cultivar over a period of three years. Corms from both lots contained considerable amounts of inhibiting substance just after harvest. However, dark grown corms treated with cold showed a rapid decrease in inhibitor activity and an increase in promoter activity. On the other hand, in light grown corms without cold treatment there was inhibitor activity found consistently even after two months. —There appear to be two inhibiting zones in the chromato-grams. One of these contains two inhibitory substances, one of which was assumed to be abscisic acid.     

Size of the Chrysanthemum Shoot Apex in Relation to Inflorescence Initiation and Development

The size of the apical dome of Chrysanthemum morifolium Ramat.at the transition to inflorescence initiation in continuouslight (long days) was not systematically influenced by eitherthe temperature or the irradiance under which the plants weregrown. It was generally 0.26 mm in diameter and c. 3.6 x 10^–3mm³ in volume when the first bract was initiated. The dimensionsof the apical dome of plants in short days were only slightlysmaller at this stage. Similarly, each step in the further developmentof the chrysanthemum inflorescence was associated with a narrowrange of apex sizes, indicating that inflorescence initiationand development are closely related to apex size. Chrysanthemum morifolium Ramat, shoot apex, inflorescence initiation     

The Effects of Temperature, Photoperiod and Light Integral on the Time to Flowering of Pansy cv. Universal Violet (ViolaxwittrockianaGams.)

The effects of temperature, photoperiod and light integral onthe time to first flowering of pansy (ViolaxwittrockianaGams)were investigated. Plants were grown at six temperatures (meansbetween 14.8 and 26.1 °C), combined with four photoperiods(8, 11, 14 and 17 h). The rate of progress to flowering increasedlinearly with temperature (up to an optimum of 21.7 °C)and with increase in photoperiod (r²=0.91, 19 d.f.), the latterindicating that pansies are quantitative long day plants (LDPs).In a second experiment, plants were sown on five dates betweenJuly and December 1992 and grown in glasshouse compartmentsunder natural day lengths at six temperatures (means between9.4 and 26.3 °C). The optimum temperature for time to floweringdecreased linearly (from 21.3 °C) with declining light integralfrom 3.4 MJ m^-2d^-1(total solar radiation). Data from both experimentswere used to construct a photo-thermal model of flowering inpansy. This assumed that the rate of progress to flowering increasedas an additive linear function of light integral, temperatureand photoperiod. Independent data from plants sown on threedates, and grown at five temperatures (means between 9.8 and23.6 °C) were used to validate this model which gave a goodfit to the data (r²=0.88, 15 d.f.). Possible confounding ofthe effects of photoperiod and light integral are discussed. Pansy;Violaxwittrockiana; flowering; photo-thermal model; temperature; photoperiod; light integral     

The Interactive Effects of Carbon Dioxide Enrichment and Daylength on Growth and Development inPetunia hybrida

Plants were grown at either 350 or 1000 µl l^-1CO₂and inone of three photoperiod treatments: continuous short days (SD),continuous long days (LD), or short switched to long days atday 41 (SD–LD). All plants received 9 h of light at 450µmol m^-2s^-1and LD plants received an additional 4 h oflight at 8 µmol m^-2s^-1. Growth of SD plants respondedmore positively to elevated CO₂than did LD plants, due largelyto differences in the effect of CO₂on unit leaf rate. High CO₂increasedheight and decreased branching under SD conditions, but hadno effect under LD conditions. Elevated CO₂also increased thenumber of buds and open flowers, the effect for flower numberbeing greater in short than in long days. The specific leafarea of plants grown at 1000 µl l^-1CO₂was reduced regardlessof daylength. High CO₂also decreased leaf and increased reproductiveallocation, the magnitude of these effects being greater underSD conditions. Bud formation and flower opening was advancedunder high CO₂conditions in SD plants but bud formation wasdelayed and there was no effect on flower opening under LD conditions.The effects of CO₂on plants switched from SD to LD conditionswere largely intermediate between the two continuous treatments,but for some parameters, more closely resembled one or the other.The results illustrate that daylength is an important factorcontrolling response of plants to elevated CO₂. Petunia hybridaHort. ex Vilm; carbon dioxide; photoperiod; functional growth analysis; daylength; global change; development; phenology     

Control of Flower Number in the First Inflorescence of Tomato (Lycopersicon esculentum Mill.): The Role of Gibberellins

The number of flowers in the first inflorescence of tomato plantswas increased by low temperatures and reduced by the applicationof GA³. The effect of GA³, was greater in a low temperatureregime (12 °C minimum) than at normal temperatures (16 °Cminimum). Increases in flower number could be produced by theremoval of young developing leaves but the treatment was nolonger effective if plants wen grown at low temperatures orwere treated with GA³. Young developing leaves were shown to be sources of diffusiblegibberellin-like substances. Leaves from plants grown in a normaltemperature regime yielded greater amounts of gibberellin-likesubstances than leaves from plants grown in the low temperatureregime. It is suggested that high levels of endogenous gibberellinsact to reduce the number of flowers formed in the first inflorescence,and that leaf removal and low temperatures influence flowernumbers by lowering levels of diffusible gibberellins in theplants. Lycopersicon esculentum, tomato, flower number, gibberellins, temperature     

Durations of the Photoperiod-sensitive and Photoperiod-insensitive Phases of Development to Flowering in Four Cultivars of Soyabean [Glycine max (L.) Merrill]

Four cultivars of soyabean [Glycine max (L.) Merill] of diverseorigin were grown in pots in a plastic-house maintained at day/nighttemperatures of 30/20°C. Plants were transferred at varioustimes after sowing from short (11·5 h d^-1) to long (13·5h d^-1) days and vice versa. The times from sowing to first floweringfor control plants grown continuously in short days varied from38 to 53 d, whereas the flowering of plants grown continuouslyin long days was delayed by about 20 d in each cultivar. Theduration of the initial photoperiod-insensitive phase (oftencalled the juvenile phase) varied three-fold between cultivars,i.e. from 11 to 33 d. As expected, the duration of the photoperiod-sensitivephase was greater in long days, but there was comparativelylittle genetic variation in photoperiod-sensitivity as definedin terms of days delay in time to flowering per hour increasein photoperiod (9-11 d h^-1). Similarly, there was little variationin the photoperiod-insensitive post-inductive phase; it rangedfrom 15 to 20 d. In consequence, the duration of the initialphotoperiod-insensitive phase was a strong determinant of timeto first flowering in these cultivars. The importance of thisso-called juvenile trait is discussed in terms of preventingthe premature flowering of USA-adapted cultivars when grownin short tropical daylengths and thus improving the adaptationof the crop to the lower latitudes.Copyright 1993, 1999 AcademicPress Glycine max (L.) Merill, soyabean, photoperiodism, juvenility, flowering