Low Temperature Affects Pattern of Leaf Growth and Structure of Cell Walls in Winter Oilseed Rape (Brassica napus L., var. oleifera L.)

Three-week acclimation of winter oilseed rape (Brassica napusL. var. oleifera L.) plants in the cold (2 °C) resultedin a modified pattern of leaf cell enlargement, indicated bythe increased thickness of young leaf blades and modified dimensionsof mesophyll cells, as compared with non-acclimated tissuesgrown at 20/15 °C (day/night). The thickness of leaf cellwalls also increased markedly during cold acclimation but itdecreased in response to a transient freezing event (5 °Cfor 18 h followed by 6 or 24 h at 2 °C, in the dark). Cellwalls of the upper (adaxial) epidermis were most affected. Theirultrastructure was modified by cold and freezing treatmentsin different ways, as revealed by electron microscopy. Possiblereasons for the cold- and freezing-induced modifications inthe leaf and cell wall morphology and their role in plant acclimationto low temperature conditions are discussed. Copyright 1999Annals of Botany Company Acclimation, Brassica napus var. oleifera, cell wall ultrastructure, cold, freezing, leaf structure, winter oilseed rape.     

Effect of Temperature on Growth of Crotalaria juncea L. and Crotalaria sericea Retz.

Growth performances of Crotalaria juncea L. and C. sericea Retz.have been compared at two controlled temperatures, 16–20°C, and 28–32 °C, with respect to increase ind. wt and leaf area, relative growth rate, leaf area ratio,specific leaf area, leaf weight ratio, net assimilation rate,the ratio of mean relative growth rate to mean relative rateof leaf area increase () and shoot/root ratios. Both the speciesgrew better at the higher temperature; however the relativegrowth rate was more affected by temperature in C. sericea thanin C. juncea. Further, it was observed to be more dependenton net assimilation rate than on the leaf area ratio. Crotalaria juncea L., Crotalaria sericea Retz., relative growth rate, leaf area ratio, specific leaf area, leaf weight ratio, leaf area increase, net assimilation rate, shoot/root ratio     

The Growth and Photosynthesis of Seedling Plants of White Clover at Low Temperature

The growth of white clover (Trifolium repens L.) in conditionstypical of April in Southern England (8 °C day/4 °Cnight, 12 h photoperiod of 90 J m^–2 s^–1 visibleradiation) was extremely slow, whether the plants were dependentfor nitrogen on fixation by their root nodules or were suppliedwith abundant nitrate; although growth was slower in the nodulatedplants. The reasons for slow growth were a large root: shootratio and a small leaf area, particularly in the nodulated plants,and a low photosynthetic rate in all plants. The probable effectsof these characteristics on the growth of white clover withgrasses in mixed pastures are discussed. Trifolium repens L, white clover, low temperature, leaf area, photosynthetic rate, nitrogen supply, growth     

Temperature and Seed Storage Longevity

Seed survival data for eight diverse species, namely the cerealbarley (Hordeum vulgare L.), the grain legumes chickpea (Cicerarietinum L.), cowpea [Vigna unguiculata (L.) Walp.] and soyabean [Glycine max (L.) Merr.], the timber trees elm (Ulmus carpinifoliaGleditsch.), mahogany (Swietenia humilis Zucc.), and terb (Terminaliabrassii Exell.), and the leaf vegetable lettuce (Lactuca sativaL.) were compared over a wide range of storage environments(temperatures from –13 °C to 90 °C, seed moisturecontents from 1.8 to 25% f. wt) using a viability equation developedpreviously. In accordance with that equation, the effect oftemperature on seed longevity was dependent upon the temperaturerange. The temperature coefficients of the viability equationdid not differ significantly (P > 0.05) among the eight speciesdespite their contrasting taxonomy. Thus the quantitative relationbetween seed longevity and temperature does not vary among diversespecies. The same conclusion was obtained for the coefficientsof a proposed alternative model of the relation between seedlongevity and temperature. The implications of the two temperaturemodels in the viability equation for extrapolations to low andvery low temperatures are discussed. Seed storage, seed longevity, seed moisture, temperature, viability equation, genetic resources conservation, Cicer arietinum L., Glycine max (L.) Merr., Hordeum vulgare L., Lactuca sativa L., Swietenia humilis Zucc., Terminalia brassii Exell., Ulmus carpinifolia Gleditsch., Vigna unguiculata (L.) Walp     

Effects of the Reversal of Day and Night Temperatures on Tillering and on the Elongation of Stems and Leaf Blades of Wheat

Different cultivars of wheat (Triticum aestivum L.) were grownin cabinets, under a 12 h photoperiod, at constant temperatures,and high day/low night and low day/high night temperatures.Plants were also transferred at different ages, between 18/10°C and 10/18 °C regimes. Application of the growth regulatorsCCC and TIBA was tested at 18/10 °C and GA₃ and IAA at 10/18°C. The reversal of day and night temperatures did not affect spikedifferentiation or the numbers of leaves and elongating internodes.However, tillering and tiller development were markedly promotedby the low day/high night temperature regimes whereas the elongationof leaf blades and stem internodes were suppressed under theseregimes. These effects were attributed to the effects of thetemperature regimes on the endogenous hormone balance of theplants. Considering the results of the transfer and growth regulatortreatments it was concluded that there were no obligatory associationsamong the number of tillers appearing, their subsequent development,leaf blade length, and stem elongation. It is suggested thatthe study of the physiological mechanisms controlling thesecharacters may benefit from experimentation under reciprocallydiffering day night temperature regimes.     

The Effect of Temperature on Growth and Development of Echinochloa Millets

Accumulation of dry weight and leaf plus stem area were measuredin Echinochloa utilis and E. frumentacea grown at temperatureregimes from 15/10°C to 33/28°C (day/night). Tilleringand height were recorded in addition to leaf number which wassubsequently used as a developmental index. In both species shoot dry weight increased with temperatureup to 33/28°C; the increase in relative growth rate (RGR)was negligible above 27/22°C. Below 27/22°C the RGRof E. frumentacea decreased sharply and at 15/10°C it madeno effective growth. At low temperatures the RGR of E. frumentaceawas lower than that of E. utilis due to slow leaf area expansion,and in particular smaller individual leaves. E. frumentaceatillered more than E. utilis. Plant development was retardedat low temperatures but was not as responsive to temperatureas dry weight and leaf area. The different responses to temperatureof the two species were described in equations suitable forinclusion in predictive growth models. Echinochloa spp., millet, growth, development, temperature, relative growth rate     

One-Leaf Cuttings as a Model to Study Second Growth in the Potato (Solanum tuberosum) Plant

Second growth is an important physiological disorder of thepotato (Solanum tuberosum L.) plant. A model system to studysecond growth was developed using one-leaf cuttings. Photoperiod,temperature, decapitation and leaf removal treatments were carriedout on the plants from which the cuttings were taken and onthe cuttings themselves. Tuberized, one-leaf cuttings takenfrom moderately-induced plants and exposed to 35 °C afterleaf removal showed 95% second growth within 10 d after treatmentinitiation. Conditions that promoted second growth also reducedstarch and dry-matter content, even in tubers that did not developsecond growth. Cuttings, second growth, potato, Solanum tuberosum L, cv, Bintje, Solanum tuberosum L. cv., Désirée, Solanum tuberosum L. cv., Russet Burbank, tuberization, starch content, dry-matter, heat, photoperiod, decapitation, leaf removal     

The Development of Primary Root Nodules on Vicia faba L. Grown at Two Temperatures

The development of primary root nodules on the field bean (Viciafaba L.) grown at 10 and 18 °C was examined. The sequenceof anatomical changes observed was the same in both temperatureregimes. Nodules developed much more slowly at 10 °C andthe nodules were much larger when corresponding anatomical changesoccurred. Primary root nodules eventually ceased growth in bothtemperature regimes but ultimate nodule volume was nearly twiceas great as 10 °C as at 18 °C. The larger size did notcompensate for the lower specific nitrogenase activity of cold-grownnodules: nitrogen fixation (acetylene reduction) rates on awhole plant basis were much lower at 10 °C than at 18 °C.There was no difference in the total number of primary rootnodules in the two temperature regimes but their distributionwas biased towards the upper part of the root and the epicotylat 18 °C. Vicia faba L., field bean, nodulation, nitrogen fixation, temperature     

Water Use Characteristics of Dactylis glomerata L., Lolium perenne L. and L. multiflorum Lam. Plants

Cocksfoot (Dactylis glomerata L.). perennial ryegrass (Loliumperenne L.) and Italian ryegrass (L. multiflorum Lam.) plantswere grown on deep (75–95 cm) columns of soil in glasshousesand growth rooms with and without irrigation. The species inwhich growth declined least rapidly after water had been withheldwere those which transpired most slowly. During early establishmentin the glasshouse cocksfoot transpired least because of slowroot growth. In the growth room, when root systems were deeperand denser, perennial ryegrass transpired least because of lowleaf water conductance. Results are discussed in relation to(a) drought resistance in the three species; (b) breeding forincreased drought resistance through modifying root distributionand leaf water conductance; and (c) the use of isolated soilcolumns in water relations studies. Dactylis glomerata L., Lolium perenne L., Lolium multiflorum Lam., cocksfoot, perennial ryegrass, Italian ryegrass, transpiration, roots, leaf water conductance     

Leaf and Tiller Production of Prairie Grass (Bromus willdenowii Kunth) and Two Ryegrass (Lolium) Species

A detailed morphological study of three prairie grass cultivars(Bromus willdenowii Kunth) was conducted under ‘vegetative’and ‘reproductive’ growth conditions (short andlong photoperiods) and at different temperatures. Perennialryegrass (Lolium perenne L.) and Westerwolds ryegrass (Loliummuhiflorum Lam.) were compared during vegetative growth. Prairie grass had higher leaf appearance rates (leaves per tillerper day) and lower site filling (tillers per tiller per leafappearance interval) than the ryegrass species. Tillering rates(tillers per tiller per day) were also lower, except under vegetativeconditions at 4C. Low tiller number in prairie grass was notdue to lack of tiller sites but a result of poor filling ofthese sites. Lower site filling occurred because of increaseddelays in appearance of the youngest axillary tiller and lackof axillary tillers emerging from basal tiller buds. In prairiegrass, no tillers came from coleoptile buds while only occasionallydid prophyll buds develop tillers. Low tiller number in prairiegrass was compensated for by greater tiller weight. Prairiegrass had more live leaves per tiller, greater area per leafand a high leaf area per plant. Considerable variation between cultivars was found in prairiegrass. The cultivar ‘Bellegarde’ had high leaf appearance,large leaves and rapid reproductive development, but had lowlevels of site filling, tillering rate, final tiller numberand herbage quality during reproductive growth. ‘Primabel’tended to have the opposite levels for these parameters, while‘Grasslands Matua’ was intermediate and possiblyprovided the best balance of all plant parameters. prairie grass, Bromus willdenowii Kunth, perennial ryegrass, Lolium perenne L., Westerwolds ryegrass, Lolium multiflorum Lam., temperature, photoperiod, leaf appearance, leaf area, tillering, site filling, tillering sites, yield     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

Effects of Photoperiod and Temperature on Shoot, Root and Rhizome Growth in ThreeLotus uliginosusSchkuhr Populations

The growth of three populations of greater lotus (Lotus uliginosusSchkuhrsyn.L. pedunculatusCav.) was compared at photoperiods of 10,12 and 14 h at a maximum day/minimum night temperature of 21/16°C and at maximum day/minimum night temperatures of 27/22,21/16, 18/13 and 15/10 °C at a photoperiod of 12 h. Shortdays (10 h) favoured root and rhizome development compared tolong days (14 h). A temperature regime of 15/10 °C restrictedrhizome development compared to the 18/13 and 21/16 °C regimes.Shoot growth was restricted at the highest temperature regime(27/22 °C). The cultivar Sharnae had fewer, but heavier,rhizomes than Grasslands Maku; this may indicate adaptationto the dry summers at its site of origin (Algarve, Portugal).The response of rhizome growth to temperature and photoperiodexplains part of the performance of greater lotus in the fieldat a wide range of latitudes. Grazing management to encouragethe persistence ofL. uliginosusin pasture in temperate environmentsmay include the exclusion of grazing livestock in autumn. Inthe sub-tropics, monitoring of rhizome production in the fieldwould be required before deciding the appropriate time intervalbetween grazing.Copyright 1998 Annals of Botany Company Lotus uliginosus(Schkuhr); greater lotus; temperature; daylength; shoots; roots; rhizomes.     

The Effects of Position and Temperature on the Expansion of Leaves of Vicia faba L.

The growth in area of the first eight leaves of broad bean plantswas investigated in growth room experiments. Plants were grownat either 20 or 14 °C or transferred from 20 to 14 °C.Rates of leaf appearance and unfolding increased with temperature.The duration of growth of a leaf increased with leaf numberfor the first five leaves and then remained constant The meangrowth rate declined or remained constant with increasing leafnumber Durations of growth were shorter and growth rates largerat 20 °C than at 14 °C Plants responded immediatelyto the change in temperature Final areas of leaves which expandedafter transfer from 20 to 14 °C were larger than those grownat 20 °C Vicla faba L., broad bean, leaf expansion, temperature responses     

The Effect of Air Humidity on Photosynthesis of Ryegrass and White Clover at Three Temperatures

Increasing leaf-air vapour pressure deficit (VPD) decreasedthe stomatal conductance and the photosynthetic rate of leavesof ryegrass (Lolium perenne L.) and white clover (Trifolhimrepens L.) at light saturation and at lower irradiance. In ryegrassboth conductance and photosynthesis, and in clover photosynthesis,decreased less with increasing VPD in low irradiance than theydid at an irradiance which saturated photosynthesis. In ryegrass,relative to their values at 10 mb, photosynthesis and conductancedecreased less with increasing VPD at 25 °C than at 20 or16·5 °C. In white clover, relative conductance (butnot photosynthesis) was less reduced at 25 than at 16·5°C Measurements of VPD of air in the leaf canopy of a field-growncrop are combined with the observed responses of photosynthesisto VPD and temperature in a model. This shows that high VPDis likely to depress photosynthesis significantly and that,during a typical day, the rate of light saturated photosynthesismay remain fairly steady, because the depression of photosynthesisdue to rising VPD is offset by the stimulation due to risingtemperature Perennial ryegrass, Lolium perenne L., White clover, Trifolhim repens L., photosynthesis, leaf conductance, water vapour pressure deficit, temperature     

Tuberization in Potato at High Temperatures: Gibberellin Content and Transport from Buds

Warm temperatures (35°C day/30°C night) which inhibittuberization in potato (Solanum tuberosum L., cv. Sebago) increasedgibberellin activity in crude extracts from buds, but not frommature leaves, as determined by the lettuce hypocotyl bioassay.Changes in the growth of tubers and stolons indicate the occurrenceof basipetal movement of GA₃ applied to the terminal bud ora mature leaf. ¹⁴C labelling from GA₃ or mevalonic acid injectedjust below the terminal bud was recovered in the lower shoot,stolons and tubers, but the amount transported was greater atcool temperatures (20/15°C). It is concluded that high temperaturespromote the synthesis of gibberellin in the buds rather thantransport to the stolons. Solanum tuberosum L., potato, tuberization, gibberellin     

A Comparison of the Growth of the C4 Grass Spartina anglica with the C3 Grass Lolium perenne at Different Temperatures

Dunn, R., Thomas, S. M., Keys, A. J. and Long, S. P. 1987. Acomparison of the growth of the C₄ grass Spartina anglica withthe C₃ grass Lolium perenne at different temperatures.—J.exp. Bot. 38: 433–441. S. anglica is one of the few C₄ species which occurs naturallyin cool temperate zones. It is known to attain photosyntheticrates which equal or exceed those of C₃ grasses over the temperaturerange typical of the spring and summer in cool temperate climates.This study examines whether S. anglica can also attain comparablegrowth rates at these temperatures. Seedlings of S. anglicaand L. perenne cv. S23 were grown in controlled environmentsat 10,15,20 and 25 °C. Quantitative growth analysis wasconducted by taking frequent harvests to determine the progressionsof leaf area and plant weight of individual plants with time.Quadratic regressions were found to describe these progressionswell. Instantaneous derived growth parameters were calculatedfrom the fitted regressions. Both absolute and relative growthrates of S. anglica were significantly lower than for L. perenne,this being largely attributable to a lower ratio of leaf areaproduction per unit of plant dry weight. Although the amountof dry matter invested into leaves was similar, the leaf areaper unit of leaf dry weight was lower in S. anglica. In comparisonto L. perenne, the rate of dry matter accumulation per unitof leaf area (ULR) was higher in S. anglica at 25 °C andinitially equal at 10 °C. Prolonged exposure to 10 °Csteadily reduced ULR in S. anglica which approached zero at80 d. Although growth in S. anglica is reduced more by low temperaturethan it is in L. perenne, by comparison to other C₄ speciesthe assimilatory capacity of S. anglica is more tolerant oflow temperature exposure. Key words: C4 photosynthesis, temperature, quantitative growth analysis     

Temperature Response of Vernalization in Wheat: A Developmental Analysis

The vernalization response of wheat ( Triticum aestivum L.)was reinterpreted from a developmental perspective, using currentconcepts of the developmental regulation of wheat morphologyand phenology. At temperatures above 0 °C, the effects ofthe process of vernalization per se in wheat are confoundedby the effects of concurrent vegetative development. These effectsare manifested by differences in the number of leaves initiatedby the shoot apex prior to floral initiation, which in turnaffects the subsequent rate of development to ear emergenceand anthesis. Leaf primordia development during vernalizationand total leaf number at flowering were used to develop criteriato define both the progress and the point of saturation of thevernalization response. These criteria were then used to reinterpretthe results of Chujo ( Proceedings of the Crop Science Societyof Japan 35 : 177–186, 1966), and derive the temperatureresponse of vernalization per se for plants grown under saturatinglong day conditions. The rate of vernalization increased linearlywith temperature between 1 and 11 °C, such that the timetaken to saturate the vernalization response decreased from70 d at 1 °C to 40 d at 11 °C. The rate declined againat temperatures above 11 °C, and 18 °C was apparentlyineffective for vernalization. Total leaf number at saturation,however, increased consistently with temperature, as a resultof the balance between the concurrent processes of leaf primordiuminitiation and vernalization. Total leaf number at saturationincreased from 6 at 1 °C to 13.3 at 15 °C, which inturn influenced the time taken to reach ear emergence. The advantagesof using this developmental interpretation of vernalizationas the basis for a mechanistic model of the vernalization responsein wheat are discussed. Triticum aestivum L.; wheat; vernalization; rate; temperature; primordia; leaf number; flowering     

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     

Carbon Dioxide Exchange of Spring-wheat in Relation to Age and Photon-flux Density at Different Growth Temperatures

CO₂-exchange rates (CER) of the sixth and the flag leaves oftwo spring-wheat varieties, Kolibri and Famos, were comparedusing an open-circuit infrared gas analysing system. Measurementswere repeated every two weeks starting when leaf blades werefully expanded. Single plants were grown in a controlled environmenthaving a photopuiod of 15 h and a day/night temperature of 24/19°C(H), 18/13 °C (M), and 12/7 °C (L) respectively untilapprox. 2 weeks after anthesis and at 18/13 °C until maturity.The photosynthetic photon-flux density (PPFD) at the top ofthe plants was 500 µE m^–2 sec^–1. During themeasurements PPFD was gradually reduced from 2000 to 0 µEm^–2 sec^–1 whereas the temperature was maintainedat the respctive growth-temperatures during the light period.The CER of the sixth leaf declined fairly similarly for bothvarieties, except for Kolibri where a faster decline was observedduring the first two weeks after full leaf expansion. The CERof the flag leaf declined more slowly than that of the sixthleaf. With the flag leaf of Famos, the decline was nearly linear,whereas with Kolibri it was very slow during the first few weeksbut rapid as the leaves further senesced. This pattern becamemore pronounced as the growth temperature decreased. The declinein relation to leaf age was much smaller at low PPFD than athigh PPFD during the same period. At full leaf expansion Kolibrireached higher maximum CER than Famos except at H. As the PPFDwas reduced the difference became smaller and at very low PPFDsuch as 50 µE m^–2 sec^–1 was reversed for thesixth leaf. Under optimum growth conditions maximum values ofCER were greater than 50mg CO₂ dm^–2h^–1 and PPFDfor light saturation was close to 2000 µE m^–2 sec^–1.A comparison between the actual CER and a fitted curve widelyused, PN=(a+b/l)^–1–DR, showed that the goodnessof fit strongly depends on cultivar, treatment and leaf ageas well as on the number and the level of PPFD from which datafor calculations are taken. Triticum aestivum, L., wheat, photosynthesis, photon-flux density, light response, carbon, dioxide exchange     

Quantal Response of Seed Germination in Seven Genera of Cruciferae to White Light of Varying Photon Flux Density and Photoperiod

The response of the germination of seeds of Barbarea vema (Mill.)Aschers, Brassica chinensis L., Brassica juncea (L.) Czern.& Coss., Brassica oleracea L. var. gongylodes L., Camelinasaliva (L.) Crantz, Eruca saliva Mill., Lepidium sativum L.,Nasturtium officinale R. Br., and Rorippa palustris (L.) Besserto white fluorescent light of different photon flux densitiesapplied for different daily durations in a diurnal alternatingtemperature regime of 20 °C/30 °C (16 h/8 h) was quantifiedby linear relations between probit percentage germination andthe logarithm of photon dose, the product of photon flux densityand duration. The low energy reaction, in which increasing dosepromotes germination, was detected in all the seed populationsbut in Barbarea vema and Brassica Juncea the lowest photon doseapplied (10^–5–2 and 10^{–5 7} mol m^–2 d^–1,respectively) was sufficient to saturate the response. Comparisons,where possible, between photoperiods demonstrated reciprocity,i.e. germination was proportional to photon dose irrespectiveof photoperiod, for the low energy reaction in Brassica oleracea(1 min d^–1 to 1 h d^–1), Camelina saliva (1 min d^–1to 8 h d^–1), Eruca saliva (1 min d^–1 to 24 h d^–1),Lepidium sativum (I min d^–1 to 8 h d^–1) and Rorippapalustris (1 min d^–1 to 8 h d^–1), but not in Brassicachinensis and Nasturtium officinale. The high irradiance reaction,in which increasing dose inhibits germination, was detectedin Barbarea vema, Brassica chinensis, Brassica juncea, Brassicaoleracea, and Camelina saliva. The minimum dose at which inhibitionwas detected was lO^–0–3 mol m^–2 d^–1.These results are discussed in the context of devising optimallight regimes for laboratory tests intended to maximize germination The response of germination to photon dose was also quantifiedwith 3 x 10^–4 M GA₂, co-applied (Brassica chinensis, Camelinasaliva, and Lepidium sativum) and with 2 x 10^–2 M potassiumnitrate co-applied (Brassica chinensis). In the latter casepotassium nitrate had no effect in the dark and inhibited germinationin the light, but GA₂, promoted germination substantially inall three species. Variation amongst seeds in the minimum photondose required to stimulate germination was not affected by co-applicationof GA₂, in Brassica chinensis and Camelina saliva, whereas seedsof Lepidium salivum showed a narrower distribution of sensitivitiesto the low energy reaction in the presence of GA₂ Barbarea vema (Mill.) Aschers, Brassica chinensis L., Brassica juncea (L.) Czern. & Coss., Brassica oleracea L. var. gongylodes L., Camelina saliva (L.) Crantz, Eruca saliva Mill., Lepidium satiaum L., Nasturtium officinale R. Br., Rorippa palustris (L.) Besser, Cruciferae, light, gibberellic acid, seed germination, seed dormancy