The role of temperature in the seed germination of two species of the Solanum nigrum complex

The germination behaviour of S. nigrum L. and S. physalifoliumRusby var. nitidibacatum (Bitter) Edmonds is compared, basedon temperature requirements during imbibition. Three seed lotsof S. nigrum had base temperatures (T_b) between 7.5C and 10C,showing a lower T_b when the period of freezing days, duringwhich each population was collected, was reduced. S. physalifoliumhas a higher value for T_b at constant temperatures (21C) thatcan be interpreted as a dormancy constraint. This constraintis released by alternating temperatures at amplitudes exceeding5C and with the high temperature above 21C by apparently reducingT_b to 12.5C. This implies that for S. physaiifolium temperature has a dualeffect on germination. It is the driving force for changes indormancy, but germination also depends on the temperature. Therole of temperature for S. nigrum is simpler: each populationcollected showed differences in the thermal time required forgermination that could be related to the temperature regimenof the original environment. Key words: Dormancy, S. nigrum, S. physalifolium, thermal time     

Leaf Extension in Zea mays: I. LEAF EXTENSION AND WATER POTENTIAL IN RELATION TO ROOT-ZONE AND AIR TEMPERATURES

The temperature of the roots and shoots of Zea mays plants werevaried independently of each other and the rates of leaf extensionand leaf water potentials were measured. Restrictions of leafextension occurred when root temperatures were lowered from35 to 0 °C, but leaf water potentials were lowered onlyat root temperatures below 5 °C. Similar changes in ratesof leaf extension were measured at air temperatures from 30to 5 °. Between 30 and 35 °C air temperature, in anunsaturated atmosphere, restrictions of leaf extension wereassociated with low leaf water potentials. It was concluded that, at root temperatures 5 to 35 °C,and shoot temperatures 5 to 30 °C, water stress was notthe main factor restricting the extension of Zea mays leaves.     

The Effect of Temperature on Photosynthesis of Ryegrass and White Clover Leaves

The rate of photosynthesis of leaves of perennial ryegrass (Loliumperenne L.) and white clover (Trifollum pratense L.) grown atdifferent temperatures was measured at a range of temperatures.There was a small effect of the temperature at which a leafhad grown on its photosynthetic rate, but a large effect ofmeasurement temperature, especially in bright light, where photosyntheticrates at 15°C were about twice those at 5°C. It appearsthat temperature could affect sward photosynthesis in the field.Ryegrass and clover had similar photosynthetic rates which respondedsimilarly to temperature. Lolium perenne L., ryegrass, Trifolium pratense L., white clover, photosynthesis, temperature, irradiance     

A Relationship between Membrane Permeability and Ethylene Production at High Temperature in Leaf Tissue of Phaseolus vulgaris L.

Leaf discs cut from primary leaves of Phaseolus vulgaris L cvMasterpiece were incubated at temperatures higher than the growthtemperature of 25 °C Both basal and wound ethylene productionincreased up to temperatures of 35–37 5 °C, thereafterdeclining rapidly There was no detectable ethylene productionat temperatures above 42 5 °C Exposure of leaf discs tohigh temperature for 60 mm resulted in a large production ofwound ethylene when they were returned to 25 °C The magnitudeof ethylene production was related to the initial incubationtemperature as was the length of the lag period before maximumproduction was achieved The results are discussed in relationto the requirement for continued membrane integrity for ethyleneproduction ethylene, temperature, membrane permeability, Phaseolus vulgaris L, dwarf bean     

Alternating Temperatures and Rate of Seed Germination in Lentil

The effect of alternating temperatures on the times taken byseeds of lentil (Lens culinaris Medikus) to germinate was investigatedusing a two-way temperature-gradient plate. Between 5 and 25°C,warmer temperatures increased the rate of germination. Variationamong the individual seeds in the times required for germinationat different constant temperatures within this range were describedwell by a log-normal distribution of thermal times, accumulatedabove a base temperature of 1·5°C. Even with amplitudesas great as 20°C, no effect of alternation per se on thethermal time required for germination was detected—whetherthe cool temperature was applied for 8 or 16 h d^-1. Similarly,in alternating temperature regimes where the minimum temperatureof the diurnal cycle was between 0°C and the base temperature,the thermal times required for germination (where no thermaltime accrued during the periods when temperature was below T_b)were in close agreement with those values provided by the modeldetermined at warmer constant temperatures. However, where theminimum temperature applied was < 0°C the germinationof all but the earliest germinators was delayed beyond modelpredictions, and more so where the sub-zero minimum temperaturewas applied for 16 rather than 8 h d^-1. The results, therefore,contradict the view that alternation in temperature per se reducesthe thermal time required for seed germination. Rather, rateof germination responds instantaneously to current temperature,but prolonged exposure to sub-zero temperatures can result indamage sufficient to delay germination when seeds are returnedto regimes warmer than the base temperature.Copyright 1994,1999 Academic Press Lens culinaris Medikus, lentil, seed germination, alternating temperatures, thermal time, temperature-gradient plate     

An Analysis of the Effect of Alternating Temperatures on Germination of Lycopus europaeus L.

The germination of Lycopus europaeus seeds depends absolutelyon exposure to light and fluctuating temperatures. Studies oftemperature responses were made to establish the minimum fluctuationrequired for a response, the interaction of temperature andexposure time in different parts of the alternating temperaturecycle, and the effects of successive transfers between cyclingtemperature conditions. There was a complex interaction betweenthese three. The minimum fluctuation never fell below 6.5 °Cbut varied up to c. 15 °C according to other test conditions.High temperatures favoured rapid responses, and exposure totemperatures above 20 °C in one or other phase of the temperaturecycle was essential for a full response. No response occurredeither at any temperature under constant conditions, or if onlyone temperature change was given. Under some conditions a singlecycle of alternating temperatures, including two changes oftemperature, promoted high germination rates.     

Quantal Response of Fruit and Seed Germination Rate in Quercus robur L. and Castanea sativa Mill, to Constant Temperatures and Photon Dose

A linear relationship between constant temperatures in the sub-optimaltemperature range and germination rate is shown in both Quercusrobur L and Castanea sativa Mill germinated under nominal darkconditions. The mean base temperature was interpolated for Qrobur as 0 8 ? or 2-4 ?, depending on seed lot provenance, andfor C. sativa as 1 -4? The optimum temperature for germinationin Q. robur was about 20? compared with around 28 ? in C. sativaOver the sub-optimal temperature range the distribution of thermaltimes was log-normal for each population studied their spreadvarying both between Q robur seed lots and between species However,in C. sativa germinated close to the mean base temperature,the distribution in thermal times was reduced Thermal timesto germination were decreased in Q. robur and C sativa by approximately0 3 and 0-5 log-units, respectively, when the pericarp was removed,i.e in the seeds, but the sensitivity of the response remainedrelatively unaltered In both species the germination rate was the same when nominaldark or safe green light conditions were employed during thegermination test. However, at 21 ? Q robur exhibited the highirradiance reaction (HIR) at photon doses above 30mmol m^–2d^–1. HIR first affected the germination rate by an inhibitionof radicle extension The sensitivity of the response to thermaltime was reduced as photon dose increased. This photo-inhibitionwas exacerbated at supra-optimal temperatures. In contrast,C. sativa germination rate at 26 ? was little influenced bylight at a photon dose of 752 mmol m^–2 d^–1 Key words: Seed germination rate, temperature, thermal time, light, photon dose     

The synergistic effect of carbon concentration and high temperature on lipid peroxidation in Peridinium gatunense

Lipid peroxidation in Peridinium samples taken from two differentdepths in Lake Kinneret fluctuated throughout the spring withan overall increasing trend. Samples from 0.5 and 5 m showeda similar peroxidation pattern, which was maximal after thefall off in algal biomass. The rapid decline in Peridinium biomasscoincided with ambient lake temperatures of 21–23C. Fattyacid composition profiles were similar at both depths, althoughafter the peak of the bloom, a significant increase in polyunsaturatedfatty acids and oleic acid was only found at 0.5 m, togetherwith a decrease in the percentage of polyunsaturated fatty acids.These effects were related to ambient light stress rather thana result of lipid peroxidation. Lake samples taken at differentperiods of the bloom and incubated at various temperatures showeddifferential peroxidation. Higher temperatures caused increasedlipid peroxidation, but this appeared to be dependent on thesampling period. Samples withdrawn from the lake at the beginningof the bloom showed little peroxidation after a 5 day incubationat 14C, room temperature (25C) or ambient lake temperature(16C) compared to mid-bloom samples in which there was a significantincrease in peroxidation when they were incubated at room temperature(25C) or ambient lake temperature (22C). Incubation at 14Cinhibited peroxidation; however, samples from mid-bloom againshowed enhanced peroxidation compared with those from the beginningof the bloom. These in situ results suggested a relationshipbetween temperature, another environmental variable during thebloom and lipid peroxidation in Peridinium. As total dissolvedinorganic carbon (DIC) concentrations fall significantly duringthe progress of the bloom and represent an important sourceof environmental stress, laboratory experiments were establishedto investigate the synergistic effect of temperature and carbonnutrition on lipid peroxidation in Peridinium cultures. Increasedtemperature alone caused a slight increase in lipid peroxidation,but this was greatly augmented by carbon limitation. Althoughcarbon limitation induced increased catalase activity, at highertemperatures activity declined after 48 h, allowing for thesubstantial increase in lipid peroxidation.     

The Influence of Temperature on Root Hair Infection of Trifolium parviflorum and T. glomeratum by Root Nodule Bacteria: I. THE EFFECTS OF CONSTANT ROOT TEMPERATURE ON INFECTION AND RELATED ASPECTS OF PLANT DEVELOPMENT

The onset and rate of infection in root hairs of T. parviflorumand T. glomeratum inoculated with Rhizobium trifolli strain5 varied much with root temperature. At moderate root temperature(18, 24, and 30 °C) infections were initiated earlier andin larger numbers than at low (6 and 12 °C) or moderatelyhigh (36 °C) temperatures. Both species showed a broad temperatureoptimum between 18 and 30 °C. The site of thread initiation(apically or laterally in a hair) was independent of temperature,as was also the proportion of successful threads penetratingthe root cortex, which increased with seedling age. Threadsgrew more slowly at low temperatures. The size of hair nucleinear infection threads remained unaffected by temperature, butnuclei associated with laterally originating threads were largerthan those associated with apical threads. Infection was non-randomly distributed along the main root atall temperatures. More zones of infection were found at moderatetemperatures than at temperature extremes (6–12 or 36°C). Probit plots of numbers of infections for individualplants were steplike, the linear sloping parts correspondingto normal distributions of infection within zones. Between 18 and 30 °C numbers of infections increased exponentiallyin two phases, the first more rapid phase ending at about thetime nodules appeared. A model devised for the infection processand fitted to the data suggested the existence of two kindsof infections: primary ones occuring randomly at a slow rateand probably not affected by temperature and secondary infectionsthat appeared to increase with rising temperatures in the range12 to 30 °C. Nodule numbers were relatively more sensitive to high and lowtemperatures than infection. The numbers of infections and nodulesand the root lengths of T. parviflorum were twice those of T.glomeratum except at the temperature extremes. Numbers of infectionswere otherwise unrelated to root length or cotyledon or leafareas. The development of lateral organs (primordia, lateralroots, and nodules) was reduced at temperatures below 18 °Cand above 30 °C.     

Differential Inhibition of Shootvs.Root Growth at Low Temperature and its Relationship with Carbohydrate Accumulation in Different Wheat Cultivars

Shoot and root growth rate, carbohydrate accumulation (includingfructan), reducing sugar content and dry matter percentage weremeasured in six wheat cultivars, ranging from winter to springtypes, grown at either 5 or 25 °C. At 5 °C (comparedwith 25 °C), the relative growth rate (RGR) of shoots wassimilarly reduced in all cultivars, but the RGR of shoots wasmore affected in winter wheats. This difference resulted insmaller root:shoot ratios than in spring wheats, which alsodeveloped more first-order lateral roots. A direct relationshipbetween carbohydrate accumulation at low temperatures and reductionin root growth was established. These results suggest that differentialshootvs.root growth inhibition at low temperature may play akey role in carbohydrate accumulation at chilling temperatures.This differential response might lead to improvements in survivalat temperatures below 0 °C, regrowth during spring, andwater and nutrient absorption at low temperatures.Copyright1997 Annals of Botany Company Wheat; Triticum aestivum; low temperatures; root growth; root: shoot ratio; sugar accumulation     

Influences of Water Status, Temperature, and Root Age on Daily Patterns of Root Respiration for Two Cactus Species

Daily patterns of root respiration measured as CO₂, efflux werestudied at various soil water potentials, temperatures, androot ages for individual, attached roots of the barrel cactusFerocactus acanthodes and the platyopuntia Opuntia ficus-indica.The daily patterns of root respiration for both establishedroots and rain roots followed the daily patterns of root temperature.Root respiration increased when root temperature was raisedfrom 5 °C to 50 °C for F. acanthodes and from 5 °Cto 55 °C for O. ficus-indica; at 60 °C root respirationdecreased 50° from the maximum for F. acanthodes and decreased25° for O. ficus-indica. Root respiration per unit d. wtdecreased with root age for both species, especially for rainroots. Root respiration rates for rain roots were reduced tozero at a soil water potential (     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: III. A COMPARISON OF FIVE FABA BEAN GENOTYPES AT CONSTANT TEMPERATURES USING A NEW SCREENING METHOD

Ellis, R. H., Simon, G. and Covell, S. 1987. The influence oftemperature on seed germination rate in grain legumes. III.A comparison of five faba bean genotypes at constant temperaturesusing a new screening method.—J. exp. Bot. 38: 1033–1043. A screening procedure which requires information on the progressof germination at only four temperatures was able to definethe response of the rate of seed germination to sub- and supra-optimaltemperatures for whole seed populations of each of five fababean (Vicia faba L.) genotypes. In one population of the cultivarSutton the models for sub- and supra-optimal temperatures derivedfrom the screen satisfactorily explained observations from anearlier separate investigation at a wider range of temperatures.Two discrete groups of genotypes were identified. Within eachgroup the base temperature T_b did not differ significantly:for the landraces Lebanese Local Large and Syrian Local Largethe value was estimated to be –7·5°C and forthe landrace Lebanese Local Small and the cultivars Sutton andAquadulce it was –4·0°C. The optimum temperaturefor the 50th percentile [T_o(50), at which temperature the rateof germination is maximal] also varied between these two groupsof genotypes, being 20·5–21·5°C forthe first group and 24·5–26·0°C forthe second. In several temperature regimes some of the viableseeds within a seed population failed to germinate. Nevertheless,even at temperatures where a substantial proportion of the seedsfailed to germinate the models defined by the screening methodpredicted the germination times of those seeds which did germinate. Key words: Faba bean, seed gemination rate, temperature     

Effect of Chilling Temperatures on the Protoplasmic Streaming of Plants from Different Climates

A microscope mount was designed so that specimen temperaturescould be monitored and controlled without impairing phase contrastoptics and used to measure rates of protoplasmic streaming between0 and 25 ?C in trichome cells of Lycopersicon esculentum, Lycopersiconhirsutum, Citrullus vulgaris, Tradescantia albiflora, Digitalispurpurea, and Veronica persica. Between 10 and 20 ?C the rates of streaming varied from 2–6µm s^–1 depending on the temperature, and differencesbetween the species were small. The temperature coefficientof streaming rates was found to increase as the temperaturewas lowered so that the plot of log rate against temperaturehad a steeper slope at the lower temperatures. The largest temperature cofficients were for the warmth-requiringL. esculentum (tomato) and C. vulgaris (water melon), and thesmallest for the temperate-zone plants V. persica (speedwell)and D. purpurea (foxglove). The changes in rate always occurredover a range of temperature; no ‘critical temperature’wasobserved below which streaming abruptly stopped and above whichit was active, although the amount of streaming as well as therate decreased as the lowest temperatures were approached. The temperatures experienced by the specimens during the experimentdid not affect the recovery of normal streaming rates betweenabout 10 and 20 ?C. In a population of a wild tomato, Lycopersicon hirsutum Humb.and Bonpl., collected from different altitudes in Peru and Ecuador,i.e. from locations of different environmental temperature,the rate of protoplasmic streaming at 5 ?C was greatest in thevarieties collected from the highest altitudes. The resultssuggest that streaming rates correlate with genetic adaptationto low temperature in the species examined.     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. in Flowing Solution Culture: I. GROWTH

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. in flowing solution culture. I. Growth.—J.exp. Bot. 38: 42–52 Oilseed rape (Brassica napus L. cv. Bien venu) was grown for49 d in flowing nutrient solution at pH 6?0 with root temperaturedecrementally reduced from 20?C to 5?C; and then exposed todifferent root temperatures (3, 5, 7, 9, 11, 13,17 or 25?C)held constant for 14 d. The air temperature was 20/15?C day/nightand nitrogen was supplied automatically to maintain 10 mmolm^–3 NH₄NO₃ in solution. Total dry matter production wasexponential with time and similar at all root temperatures givinga specific growth rate of 0?0784 g g^–1 d^–1. Partitioningof dry matter was influenced by root temperature; shoot: rootratios increased during treatment at 17?C and 25?C but decreasedafter 5 d at 3?C and 5?C. The ratio of shoot specific growthrate: root specific growth rate increased with the ratio ofwater soluble carbohydrates (shoot: root). Concentrations ofwater soluble carbohydrates in shoot and root were inverselyrelated to root temperature; at 3, 5 and 7?C they increasedin stem + petioles throughout treatment, coinciding with a decreasein the weight of tissue water per unit dry matter. These resultssuggest that the accumulation of soluble carbohydrates at lowtemperature is the result of metabolic imbalance and of osmoticadjustment to water stress. Key words: Brassica napus, oilseed rape, root temperature, specific growth rate     

Seed Viability Constants for Lettuce

Seeds of two lettuce cultivars (Lactuca sativa L., cv. Meikoninginand cv. Grand Rapids) were hermetically stored with constantmoisture contents ranging between 3.6 and 17.9 per cent (freshweight basis) at constant temperatures ranging between 5 and75 °C. The decline with time in percentage germination andpercentage normal seedlings was determined for each storagetreatment. The data were fitted to an equation which containsthe constants: K₁, the probit of the initial percentage germinationor normal seedlings; K_E, a species constant; C_W, the constantof a logarithmic moisture term; C_H, the constant of a lineartemperature term and C_Q, the constant of a quadratic temperatureterm. Regression analysis of data from storage periods up to5.5 years at temperatures of 5–75 °C and seed moisturecontents of 3.6–13.6 per cent yielded the following values:K_E= 8.218, C_W=4.797±0.163, C_H=0.0489±0.0050 andC_Q=0.000365±0.000056. Although this equation consistentlyprovided a better fit, simplified equations, assuming eithera log-linear relationship between seed longevity and temperature,or a log-linear relationship between seed longevity and bothmoisture content and temperature, accounted for more than 94per cent of the variation at the restricted temperature rangeof 5–40 °C. Longevity of the same seed lots at sub-zero temperatures (–5,–10 and –20 °C) was studied in separate tests.Freezing damage, resulting in abnormal seedlings in the germinationtest, occurred at –20 °C when the moisture contentof the seeds exceeded 12 per cent. No decline in percentagenormal seedlings was observed after a storage period of 18 monthsor longer at –20 °C, provided the seed moisture contentdid not exceed 9.5 per cent. For seeds stored at –5 and–10 °C with 9.6–12.5 per cent moisture content,the observed rate of decline of percentage normal seedlingswas adequately predicted by the viability equation, using theabove values for the constants. This suggests that for low moisturecontents the viability equation can be applied to estimate longevityat sub-zero temperatures. Lettuce, Lactuca sativa (L.), seed longevity, seed storage, viability constants, storage conditions     

Use of the Weibull Function to Calculate Cardinal Temperatures in Faba Bean

The onset of germination of faba bean seeds at constant temperaturewas progressively delayed as that temperature diverged froman optimum of 25.5 ?C. At temperatures below 10 ?C, or above28 ?C, the maximum germination percentage fell to below 90%.There was no germination at 39 ?C. Positive and negative linearrelationships were established between the constant temperaturesand the rates of progress of germination to different percentiles,at sub-optimal and supra-optimal temperatures, respectively.Like germination rates, base temperature (T_b) declined from3.71 to –0.83 ?C as the percentile value increased from10% to 80%. Caution was urged in extrapolating beyond the experimentaldata set. Differences in the ceiling temperature (T_c) with percentilecould not be discerned. Cumulative germination progress curves at each temperature weremodelled by the Weibull, logistic, and cumulative normal distributionfunctions. Cardinal temperatures (T_b and T_c) calculated fromthese data reasonably approximated the actual data. The Weibullfunction demonstrated a good approximation at all percentilelevels, while the logistic and cumulative normal distributionfunctions, as a result of their inherent symmetry, deviatedat the extreme percentiles. It was concluded that the Weibullfunction not only accurately modelled cumulative germinationbut could also be used in the calculation of cardinal temperatures. Key words: Seed germination rate, cardinal temperatures, faba bean, Weibull function, probit and logic scales     

Transplantation Immunity in Annelids. III. Effects of Temperature on Xenograft Rejection in Earthworms

Earthworms are capable of destroying body wall xenografts. Xenograftstransplanted Eisenia foetidaLumbricus terrestris at varioustemperatures showed that graft survival time is inversely proportionalto temperature in both cases. Colder temperatures slow graftrejection, but higher temperatures, up to optimal limits, accelerategraft destruction. At 21 C, Eisenia grafts on Lumbricus hostsare rejected faster than at any other experimental temperature;the variability in individual times is lessened. Beginning at23 C sub-optimal conditions are approached. Thus, graft survivaltimes are more prolonged. Lumbricus grafts on Eisenia hostsare rejected most quickly at 23 C; at 27 C all Eisenia are dead.A review of the earthworm's general thermal dependence is presentedand a rationalization for prolonged second-set graft survival(negative memory) at low temperatures is given. Temperatureeffects probably account for the lack of positive memory at15 C from previous studies.     

The Temperature Response of Gas Exchange in Sorghum Leaves and the Effect of Heterosis

The aim of this research was to describe the temperature responseof gas exchange in grain sorghum (Sorghum bicolor L. Moench)leaves as temperature was increased in a fashion similar toits daily increase in a hot environment. The hypothesis thatsorghum hybrids manifested heterosis for carbon dioxide exchangerate on a wider temperature range as compared with their parentallines was also evaluated. Gas exchange of detached turgid leaveswas measured in four sorghum hybrids and their parents as leaftemperature rose steadily from 32 °C to 43 °C in 4 h.CER was maximal at about 37–40 °C depending on thegenotype. In one genotype (Tx378), CER was maximal at 42 °C.In three out of four hybrids significant heterosis was foundfor CER at the lower, the higher or at the full range of temperaturestested depending on the hybrid. Consequently, all three heterotichybrids displayed a greater temperature range for high CER ascompared with their parents. Heterosis in CER was largely explainedby heterosis in stomatal conductance, though the effects ofnon-stomatal components could not be elucidated here. Heterosisin transpiration was revealed in the same three hybrids, especiallyat moderate temperatures. Since heterosis in CER was relativelygreater than heterosis in transpiration, significant heterosisin transpiration ratio at moderate temperatures was seen intwo of the four hybrids. An effect of heat hardening on CERwas observed in parental line Tx430 and its two hybrids as seenin the reduction of CER at 36–37 °C and its subsequentrecovery as temperatures rose slowly to 43 °C. The effectof heat hardening on photosynthesis was also observed in someof the genotypes as a hysteresis in the association betweenstomatal conductance and CER as temperatures increased. Thishysteresis indicated that, for the same stomatal conductance,CER was greater after than before leaves were exposed to hightemperature. Key words: Photosynthesis, hybrid vigour, heat stress     

Distinct Influence of Root and Shoot Temperature on Nitrogen Fixation by White Clover

White clover (Trifolium repens L.) was grown in controlled environmentsto determine the distinct effects of root and shoot temperatureon the accumulation of total and fixed (¹⁵ N dilution) nitrogenat two levels of nitrate (10 and 75 mM). Nitrogen fixation(BNF) showed a positive response to higher shoot temperature(23 vs. 13 C day temperature), irrespective of whether or notroot temperature was increased in parallel. Low root temperature(5 C) caused a marked reduction in the accumulation of totalnitrogen at both nitrate levels, and led to a lower proportionof N derived from BNF. The temperature response of BNF was attributedfor the major part to an adaptation to the demand for fixedN. It is therefore concluded that BNF is not primarily responsiblefor the reduced clover growth at low temperatures. White clover, Trifolium repens L., temperature, nitrogen fixation, nitrate, root, shoot     

Characterization of the Germination Responses to Temperature of Lettuce (Lactuca sativa L.) Achenes

Germination tests were done on 23 cultivars of lettuce (Lactucasativa L.) comprising a variety of different morphological formsselected for cultivation at various seasons. Significant differences at the upper limit of temperature tolerancewere found between different cultivars: maximum temperaturesfor 50 per cent germination ranged from 23 to 32 °C, andsusceptibility to the induction of secondary dormancy by hightemperatures varied widely from one cultivar to another. Nocorrelations were established between germination responsesand heading type, achene colour or growing season The germination responses of all cultivars at temperatures fromjust above 0 to 20 °C were closely similar and results froma large number of experiments were used to produce a standardgermination reference curve foti culvars of L. sativa. Departfuresrom this curve were found to arise predominantly from variationsin seed quality or test conditions rather than from the genotypeof the cultivar. Detailed comparisons of germination at all possible combinationsof alternate and constant day/night temperatures are presentedfor two cultivars. One with relatively high tolerance to high-temperatureinducedinhibition, the other with low tolerance. The results are discussed in relation to the original distributionof the species as a wild plant and its history of cultivationby man. Lactuca sativa L., Lettuce, achene germination, temperature response