Environmental Control of Flowering in some Northern Carex Species

The environmental control of flowering in some arctic-alpineCarexspecies has been studied in controlled environments.Carex nigra,C. brunnescens, C. atrata, C. norwegica andC. serotina all hada dual induction requirement for flowering. In all exceptC.nigra either low temperature (12 °C or lower) or short days(SD) over a wider range of temperatures were needed for primaryfloral induction and inflorescence formation. InC. nigra primaryfloral induction took place in SD only (9–21 °C),8–10 weeks of exposure being required for a full response.In all these species long days (LD) were required for, or stronglypromoted, culm elongation and inflorescence development (secondaryinduction). Quantitative ecotype differences in both primaryand secondary induction were demonstrated. Unlike the otherspecies,C. bicolor proved to be a regular LD plant which requiredLD only for inflorescence initiation and development. In allspecies leaf growth was strongly promoted by LD, especiallyin the higher temperature range (15–21 °C). In SDand temperatures below 15 °C the leaves became senescentand the plants entered a semi-dormant condition which was immediatelyreversed by LD. The results are discussed in relation to growthform and life history of shoots. Carex ; dual induction; ecotypic diversity; flowering; growth; photoperiod; sedges; temperature     

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     

Leaf Extension in Zea mays: II. LEAF EXTENSION IN RESPONSE TO INDEPENDENT VARIATION OF THE TEMPERATURE OF THE APICAL MERISTEM, OF THE AIR AROUND THE LEAVES, AND OF THE ROOT-ZONE

The temperatures of the roots, the apical meristem, and theshoots of Zea mays plants were varied independently of eachother and the rates of leaf extension were measured. When thetemperature of the apical meristem and region of cell expansionat the base of the leaf was kept at 25 °C, changes of leafextension in response to changes of root and shoot temperatureswere less pronounced. When the temperature of the meristematicregion was changed by increments of 5 or 10 °C from 0 to40 °C, and the root and shoot temperatures were kept at25 °C, rapid changes in leaf extension occurred. It was concluded that the rates of leaf extension were controlledat root-zone temperatures of 5 to 35 °C by heating or coolingof the meristematic region. Changes in rates of leaf extensionin response to changes in air temperature were attributed todirect effects on the temperature of the meristematic regionand on the physiology of the leaf.     

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     

ENVIRONMENTAL CONTROL OF INITIATION OF THE INFLORESCENCE,REPRODUCTIVE STRUCTURES,AND PROLIFERATIONS IN POA BULBOSA

Youngner , Victor B. (U. California, Los Angeles.) Environmental control of initiation of the inflorescence, reproductive structures, and proliferations in Poa bulbosa. Amer. Jour. Bot. 47(9): 753–757. Illus. 1960.—Environmental factors controlling inflorescence initiation and development of inflorescence proliferations were studied using controlled environments. Inflorescence initiation was promoted by long days (16 hr.) following vernalization of 1 wk. or more'at 10°C. Normal florets developed under high temperatures (21°–27°C.) and long days following initiation. Bulbils developed in place of normal florets at short days (8 hr.) and low temperatures (ca.20°C, or below). Long days and low temperatures or short days and high temperatures produced panicles containing both bulbils and normal florets. Two flower-promoting stimuli are proposed; one promoting inflorescence initiation and the second promoting floret differentiation. Both stimuli appear to be quantitative in their mode of action.     

Day length affects the dynamics of leaf expansion and cellular development in Arabidopsis thaliana partially through floral transition timing

Background and Aims: Plant aerial development is well known to be affected by daylength in terms of the timing and developmental stage of floraltransition. Arabidopsis thaliana is a ‘long day’plant in which the time to flower is delayed by short days andleaf number is increased. The aim of the work presented herewas to determine the effects of different day lengths on individualleaf area expansion. The effect of flower emergence per se onthe regulation of leaf expansion was also tested in this study. Methods: Care was taken to ensure that day length was the only sourceof micro-meteorological variation. The dynamics of individualleaf expansion were analysed in Ler and Col-0 plants grown underfive day lengths in five independent experiments. Responsesat cellular level were analysed in Ler plants grown under variousday lengths and treatments to alter the onset of flowering. Key Results: When the same leaf position was compared, the final leaf areaand both the relative and absolute rates of leaf expansion weredecreased by short days, whereas the duration of leaf expansionwas increased. Epidermal cell number and cell area were alsoaltered by day-length treatments and some of these responsescould be mimicked by manipulating the date of flowering. Conclusions: Both the dynamics and cellular bases of leaf development arealtered by differences in day length even when visible phenotypesare absent. To some extent, cell area and its response to daylength are controlled by whole plant control mechanisms associatedwith the onset of flowering.     

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     

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.     

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     

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     

Grass Leaf Elongation Rate as a Function of Developmental Stage and Temperature: Morphological Analysis and Modelling

Elongation of successive leaves was measured following defoliationof tall fescue plants in controlled environments. Measurementswere made under constant temperatures of 24 °C and 14 °C,and after temperature changes from 24 to 14 °C andvice versa.A morphological analysis of the growing leaf was made from thetime it was 1 mm long until it was fully elongated. The timeelapsed from initiation until the leaf was 1 mm long was estimated.Young leaves less than 1.5 mm long elongated slowly at a constantleaf elongation rate (LER). By extrapolating this LER back toleaf initiation from the apex it was calculated that elongationlasted 42.5 d at 24 °C and 51 d at 14 °C. Lengths ofthe division zone (DZ) and the extension-only zone (E-OZ) increasedto a maximum and then decreased during leaf development. Temperaturechange had an immediate effect on LER but the response varieddepending on the direction of the temperature change. To describethese different features, an empirical model of DZ and E-OZwas designed. Its five parameters were optimized at constanttemperature. The model was then used to simulate the LER ofplants subjected to temperature changes. Instant and lastingeffects of the initial temperature on mean LER in plants transferredfrom 14 to 24 °C andvice versawere well simulated. It wasconcluded that the major reason for differences was due to thegrowth stage (DZ and E-OZ lengths) at which the changes occurredat both temperatures.Copyright 1999 Annals of Botany Company Festuca arundinaceaSchreb., tall fescue, growth zone, division zone.     

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.     

Photothermal Time for Flowering in Faba Bean (Vicia faba) and the Analysis of Potential Vernalization Responses

One cultivar and one land-race of faba bean were subjected to18 potentially vernalizing pre-treatments (constant temperaturesof 1, 5 or 9 °C factorially combined with photoperiods of8 or 16 h d^–1 for 10, 30 or 60 d), and then transferredinto four different growing regimes (‘day’/‘night’temperatures of 18/5 °C or 24/13 °C factorially combinedwith photoperiods of 11 or 16 h d–1). Control plants weregrown entirely in the latter four regimes. The times from sowingto appearance of first open flowers were recorded for all plants.Control plants of the land-race Zeidab Local flowered soonerin long days and in the warmer regime. Pre-treatment reducedthe subsequent time to flower in the four growing-on regimesbut most of the variation in the total time to first flowerfor the pre-treated plants was accounted for by differencesin the combined photothermal time accumulated in the two successiveenvironments - which was predicted by a simple photothermalmodel. Thus, there was neither a specific low-temperature nora short-day vernalization response in this accession. Similarly,no true low-temperature or short-day vernalization responsewas detected in the cv. Maris Bead. However, this UK cultivarflowered later than predicted in the 24/13 °C regime, indicatingthat the 24 °C ‘day’ temperature was supraoptimal.Delays to flowering at 24/13 °C were, however, less evidentwhen plants were grown in long days or following prolonged (30–60d) pre-treatments at cool temperatures. Viciafaba faba, bean, flowering, photoperiodism, vernalization, photothermal time, screening germplasm.     

Kinetics of Leaf Growth in Lolium temulentum at Optimal and Chilling Temperatures

Lolium temulentum plants were grown at 20 °C, under an 8-hdaylength, in a controlled-environment chamber, and the kineticsof leaf expansion were observed by measuring the movement ofan optical grid attached to the fourth leaf. The leaf emerged23–24 d after sowing and was fully expanded 9–10d later. Extension rate was maximal between the second and fifthdays after emergence and declined markedly thereafter. Duringthe rapid growth phase the rate of elongation exhibited a distinctdiurnal rhythm, fluctuating between 1.9 to 2.3 mm h^–1in the light period, and 1.3 to 1.7 mm h^–1 in the dark.A circadian oscillation with a period of about 27 h was observedin leaves elongating in continuous darkness. When plants weretransferred to 5 °C soon after emergence of the fourth leafthere was an immediate reduction in rate of growth to about22 per cent of the rate at 20 °C: the Q₁₀ for the mean elongationrate in the range 20–5 °C was 3.7. When plants weretransferred from 20 to 2 °C at fourth leaf emergence, meanextension rate declined to less than 5 per cent, correspondingto a Q₁₀ in the range 5–2 °C of more than 300. Furthermore,growth at 2 °C was confined almost entirely to the darkphase of the photoperiod cycle. The responsive tissue was shownto be a small area of expanding leafless than 1.5 cm above theshoot apex and the possible mechanisms underlying low temperatureeffects in this region are discussed. Lolium temulentum L., leaf growth, auxanometer, low temperature, diurnal rhythm     

GLOCHIDIA OF THE FRESHWATER MUSSEL LAMPSILIS OVERWINTERING ON FISH HOSTS

Largemouth Bass were infected with glochidia of the freshwatermussel Lampsilis cardium. Three fishes each were held at 4.5,10, and 15.5°C; five fish were held at 21°C. By 64 days,metamorphosed juveniles were found in the 15.5 and 21°Ctrials but not in the 5.5 and 10°C trials, indicating thatthe lower threshold temperature for metamorphosis was between10 and 15.5°C for the duration. In a second experiment,Largemouth Bass were infected with glochidia of L. cardium andheld at 10°C. A sample of fishes was removed monthly andbrought to 21°C. Numbers of glochidia that metamorphosedafter being warmed were compared to the number that metamorphosedwithout warming. The percentage that metamorphosed after warmingdecreased linearly with time. At one month, 100% of the glochidiametamorphosed after warming. This decreased to 80% by two months,to 30% by four months and 3% by six months. Although this post-warmingpercentage decreased with time, the total percentage of metamorphosedjuveniles (at all temperatures) was not correlated with time.Controls kept at 21°C required three weeks to reach peakmetamorphosis, but test subjects subjected to 10°C requiredless than nine days to metamorphose once warmed. Many overwinteringglochidia therefore complete a portion of their developmenton the host at winter temperatures, but stop short of excystment.Some glochidia metamorphosed without being warmed, but thisphenomenon is not understood. This study confirms that glochidiamay overwinter on hosts, with some glochidia persisting formore than six months before metamorphosing when warmer conditionsreturn. (Received 29 September 1998; accepted 18 January 1999)     

Environmental Control of Flowering in Vicia faba L.

There is a heteroblastic change in leaflet number in many stocksof Vicia faba, the rate of change being affected by the temperatureand photoperiod under which the plants are grown. In all exceptthe earliest flowering stocks of broad beans, and particularlyat high temperatures, flower initiation shows a quantita-tivelong-day response. For full development of the initiated inflorescenceslong days are required. Flower initiation may be accelerated in all except the earliestflowering stocks of V.faba by brief exposures to low temperatures,particularly when the plants are grown in short days at hightemperatures. The response to low temperatures is more rapidat I0° C. than at 4° C. but eventually approaches saturationat both temperatures. More prolonged exposure to low temperaturesdelays flower initia-tion. The response to low temperaturesincreases with increasing plant age but can occur during embryodevelopment on the mother plant. At temperatures above 14° C, and particularly above 23°C, a reaction inhibitory to flower initiation occurs. This reactionis probably restricted to the diurnal dark periods but is operativeat all stages of the life cycle, including embryo development.Its inhibitory effects may be overcome by subsequent cold treatment,and when the low temperature processes have reached saturationsubsequent high temperatures are no longer inhibitory. Although nucleosides could accelerate flower initiation, purineand pyrimidene analogues did not, with one exception, reducethe response to low temperature treatment.     

Environmental Control of Flowering in Barley (Hordeum vulgare L.). II. Rate of Developement as a Function of Temperature and Photoperiod and its Modification by Low-temperature Vernalization

Plants of six contrasting genotypes of barley were raised fromvernalized (imbibed at 1 °C for 30 d) or non-vernalizedseeds and grown in 12 different controlled environments comprisingfactorial combinations of three photoperiods (10, 13 and 16h d^–1), two day temperatures (18 and 28 °C) and twonight temperatures (5 and 13 °C). Except at longer daysfor Athenais or Arabi Abiad, the 28 °C day temperature wasgenerally supra-optimal and delayed awn emergence. At lowertemperatures and in photoperiods shorter than the critical value,P_C, which delay awn emergence, the time from sowing to awn emergencefor five of the genotypes conformed to the equation 1/f=a +bT{macron}+cPwhere f is the time to awn emergence (d), T{macron} is meandiurnal temperature (°C), P is photoperiod (h d^–1)and a, b and c are genotype-specific constants. In Arabi Abiad,however, significant responses to temperature were not detected.The low temperature pre-treatment of the seeds reduced the subsequenttime to awn emergence in Athenais and the autumn-sown genotypesAger, Arabi Abiad and Gerbel B, especially in longer days, buteither had no effect or tended to delay awn emergence in thespring-sown types Emir and Mona. In the spring-sown types P_Cwas outside the range investigated (i.e. > 16 h d^–1),but in Ager it was approx. 13 h d^–1 and in Gerbel B justover 13 h d^–1. For plants of Arabi Abiad grown from vernalizedseeds P_c was almost 15 h, but     

THE EFFECTS OF STEADY AND CYCLING TEMPERATURES ON THE ACTIVITY OF THE SLUG DEROCERAS RETICULATUM

At constant temperatures, crawling activity in the slug Derocerasreticulatum has an optimum at 13°C in late winter, and 17°Cin early summer. The optimum for feeding remains at 14°C.Activity has anendogenous basis, but daytime activity increaseswith temperature and long daylengths. Cycling temperatures producemore locomotor activity and less feeding in 24 h than constanttemperatures, even when the temperature is higher at night.However, cooling stimulates and warming inhibits activity duringthe period of change and when cooling occurs at dusk, over halfthe 24 h activity occurs in the next three hours. Faster ratesof cooling produce greater responses, but can result in loweroverall activity because the slugs remain longer at low temperatures. (Received 25 May 1984;     

Growth and Development in the Young Tomato: I. THE EFFECT OF TEMPERATURE AND LIGHT INTENSITY ON GROWTH OF THE SHOOT APEX AND LEAF PRIMORDIA

Tomato seedlings were grown at constant temperatures of 25°and 15° C. in a 12-hour day at light intensities of 1,600,800, and 400 f.c. The rate of increase in size of the shootapex and the rates of formation and growth of leaf primordiaduring the vegetative phase were followed by dissecting samplesfrom the time of cotyledon emergence onwards. The rate of enlargement of the shoot apex increased with lightintensity, but apical enlargement was delayed at the highertemperature, the delay being longer the lower the light intensity.The rates of leaf formation and leaf growth increased with bothtemperature and light intensity. Temperature had a larger effecton leaf growth than on leaf formation. More leaves were formedbefore flowering at 25° C. than at 15° C., the increasein leaf number being greater the lower the light intensity. It is suggested that the delay in the enlargement of the apexat high temperature can be explained in terms of competitionfor assimilate, the competitive potential of the expanding leafprimordia exceeding that of the apex at higher temperatures.     

Effects of Soil Temperature on the Growth and Hormone Content of Dactylis glomerata L. (Cocksfoot) in Controlled Environments

A comparison was made of effects of ‘low’ and ‘high’soil temperature (LST and HST, about 9 and 21·5 °Crespectively) on shoot growth of Norwegian and Portuguese populationsof Dactylis glomerata. In experiments lasting 8 short days (SD,8 h photoperiods) LST decreased leaf extension more markedlyin the Portuguese population. No differential effect of LSTon leaf growth was recorded in experiments lasting 20 or 21SD or in experiments of 8 d duration in long days (LD, 16 hphotoperiods). Since the meristem and region of cell extensionis close to the soil surface LST could directly influence bothroot and shoot growth. The application of gibberellic acid enhanced leaf extension,particularly in plants grown at HST. 6-Benzylaminopurine tendedto decrease leaf length. Extractable gibberellin levels wererelatively low after 8 SD in shoots of both races grown at LST.Cytokinin levels increased at LST, more so in shoots of Portugueseplants which sustained the greater reduction in leaf extension. The data suggest that LST may reduce the production of endogenousgibberellins important for leaf growth in Dactylis glomerata.Cytokinins are probably necessary for growth processes but theirlevel may reflect, rather than direct, the rate of leaf growth.