The Respiration of Mature Field Bean (Vicia faba L.) Leaves During Prolonged Darkness

Dark respiration in attached and detached mature leaves of thefield bean (Vicia faba L.) was studied whilst leaves experiencedup to 60 h of darkness. The results showed: (1) the initialrespiration rate to vary according to the irradiance duringthe previous photoperiod; (2) the dark respiration rate (perunit area) of attached leaves to be essentially constant duringa normal 12 h night although there was a rapid loss in leafd. wt during this time; (3) after 12 h, the respiration rateof attached leaves decayed to an asymptotic value at about 36h; (4) the respiration rate of leaves detached at the end ofthe photoperiod and maintained in the dark on deionised water,decayed only after 36 h of darkness; (5) there was no differencebetween the respiration rate of attached and detached leavesduring the normal 12 h night. It is concluded that the dark respiration of attached fieldbean leaves is intially related to the synthesis and translocationof sucrose in addition to maintenance. After about 36 h, whenthe rate of CO₂ efflux is more or less steady, the CO₂ effluxreflects the intensity of maintenance processes only. The maintenancerespiration rate (determined after 60 h in the dark) rangedfrom 062 to 151 mg CO₂ (g d. wt)^–1 h^–1 but wasrelatively unaffected by several applied treatments. Vicia faba L., field bean, respiration, maintenance, nitrate, non-structural carbohydrate, export     

Studies on Dark Fixation of Carbon Dioxide in Kalanchoe: II. Effect of Interaction of Photoperiodic Induction with Water Stress and Growth Retardants

In Kalanchoë blossfeldiana von Poellnitz cv. Tom Thumband cv. Feuer Blute interaction of CO₂ fixation with photoperiodicinduction and water stress was examined. It was found that TomThumb raised in dilute culture solution and kept in photoinductivecycles (8 h light and 16 h dark) flowered but failed to showa net dark CO₂ fixation. A net dark fixation was observed whenthe concentration of the culture solution was increased or plantswere sprayed with 300 or 750 mg 1^–1 CCC or 300 or 2000mg 1^–1 B-9. In a non-inductive photoperiod no net darkfixation was observed with these treatments although there wasa tendency for dark fixation to increase. Feuer Blute did not flower in inductive photoperiods when keptin half strength solution. It is suggested that in Tom Thumbboth photoperiodic induction and water stress are required forinitiation of net CO₂ dark fixation. In Feuer Blute where CAMis occurring normally photo-induction is sufficient to induceflowering. In half strength solution CO2 dark fixation is disturbedand floral induction also does not occur.     

Studies on the Photoreceptors for the Promotion and Inhibition of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy

Three-day-old etiolated seedlings of Pharbitis nil were exposedto red light for 10 min and sprayed with N⁶-benzyladenine beforetransfer to a 48-h inductive dark period, after which they weregrown under continuous white light. A second red irradiationpromoted flowering when given at the 5 and 24th hour of theinductive dark period but inhibited flowering at the 10 and15th hour. Far-red light inhibited flowering when given at anytime during the first 24 h of the dark period. Red/far-red reversibilitywas clearly observed at the 0, 5, 10 and 24th hour, but notat the 15th hour when both red and far-red lights completelyinhibited flowering. The action spectrum for the inhibition of flowering at the 15thhour of the inductive dark period had a sharply defined peakat 660 nm and closely resembled the absorption spectrum of theP_R form of phytochrome. The photoreceptors involved in thesephotoreactions are discussed. (Received June 10, 1983; Accepted July 6, 1983)     

Respiratory Efflux of Carbon Dioxide from Mature and Meristematic Tissue of Uniculm Barley During Eighty Hours of Continuous Darkness

Young plants of uniculm barley were grown singly in pots ina growth room at 23/21 °C, and an irradiance of 655 µEm^–2s^–1 during each 12 h photoperiod. At the fifth leaf stage,they were subjected to 80 h of continuous darkness during whichthe rates of CO₂ efflux of vegetative shoot meristems, and maturefully expanded leaves, were separately monitored. Respiratoryefflux from the meristematic tissue was initially high, 12–15mg CO₂ g^–1 h^–1, equivalent to a daily loss in weightof 20–25 per cent. It remained high, or even rose slightly,during what would have been the normal dark period, but thenfell sharply. Even so, it was still three times that of themature tissue at the end of the experimental period. The rateof CO₂ efflux of the mature tissue began low, and fell evenfurther during the first 12 h of darkness. It then levelledoff at a rate of 2·0–2·5 mg CO2 g^–1h^–1, equivalent to a daily loss in weight of about 3 percent. It is suggested that the rate of ‘mature tissue’respiration, established after 12–24 h of darkness, mightbe a useful selection criterion to employ in attempts to increasethe total dry matter yield of the grass crop by breeding. Hordeum vulgare L., barley, respiration, synthetic respiration, maintenance respiration, meristem, mature tissue respiration, simulated sward     

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

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

Variation in the Durations of the Photoperiod-sensitive and Photoperiod-insensitive Phases of Development to Flowering Among Eight Maturity Isolines of Soyabean [Glycine max (L.) Merrill]

In soyabean [Glycine max (L.) Merrill] the period between sowingand flowering is comprised of three successive developmentalphases—pre-inductive, inductive and post-inductive—inwhich the rate of development is affected, respectively, bytemperature only, by photoperiod and temperature, and then againby temperature only. A reciprocal-transfer experiment (carriedout at a mean temperature of 25°C) in which cohorts of plantswere transferred successively between short and long photoperiodsand vice-versa showed that eight combinations of three pairsof maturity alleles (E₁/e₁, E₂ /e₂, E₃ /e₃) had their greatesteffect on the duration of the inductive phase in long days.This phase was increased with the increasing photoperiod sensitivityinduced by the different gene combinations, and ranged fromabout 27 to 54 d according to genotype. In a short day regime(11·5 h d^-1), less than the critical photoperiod, theduration of the inductive phase was brief—requiring about11 photoperiodic cycles in the less photoperiod-sensitive genotypesand only about seven cycles in the more sensitive ones. Thematurity genes also affected the duration of the two photoperiod-insensitivephases; these durations were positively correlated with thephotoperiod-sensitivity potential of the gene combinations.The largest effect was on the pre-inductive phase which variedfrom 3 to 11 d, while the post-inductive phase varied from about13 to 18 d. As a consequence of these non-photoperiodic effectsof the maturity genes, even in the most inductive regimes (daylengthsless than the critical photoperiod) the time taken to flowerby the less photoperiod-sensitive combinations of maturity geneswas somewhat less than in the more sensitive combinations—rangingfrom about 28 to 34 d. The genetic and practical implicationsof these findings are discussed.Copyright 1994, 1999 AcademicPress Glycine max (L.) Merrill, soyabean, maturity genes, isolines, flowering, photoperiod     

Spectral Dependence of Night-break Effect on Photoperiodic Floral Induction in Lemna paucicostata 441

A single dark period of longer than 8 hr induced flowering inLemna paucicostata 441 cultured in E medium. Monochromatic lightsof 450, 550, 650 and 750 nm with a half-power bandwidth of 9nm given for 10 min at the 8th hour of a 14-hr dark period inhibitedflowering. The fluence rates required for 50% inhibition were10, 0.5, 0.1 and 3 µmol m^–2. sec^–1, respectively.When applied between the 4th and the 10th hour of the dark period,lights of 450, 550 and 650 nm were inhibitory showing a maximumeffect at the 8th hour. But 750-nm light completely inhibitedflowering when applied at any time during the first 8 hr ofthe dark period. The inhibitory effect of 750-nm light givenat the beginning of the dark period was totally reversed bya subsequent exposure to 650-nm light, and the fluence-responsecurves for the effect of 750-nm light given at the 0, 4th and8th hour were essentially the same. This suggests that the presenceof P_FR is crucial for the floral initiation throughout the first8 hr of the inductive dark period. The role of phytochrome inthe photoperiodic flower induction of L. paucicostata is discussed. (Received January 4, 1982; Accepted March 19, 1982)     

Characterization of the Photoperiodic Response of Post-flowering Development in Maturity Isolines of Soyabean [Glycine max(L.) Merrill] 'Clark'

Plants of all eight isolines of three maturity genes (all combinationsof two alleles at the three lociE₁/e₁,E₂/e₂,E₃/e₃) of soyabean[Glycine max(L.) Merrill] were grown in four different photoperiods(12, 13, 14 or 15 h d^-1) at 30/24 °C from first flower openingto harvest maturity. Photoperiod, isoline, and their interaction,affected significantly (P<0.01) the duration between firstand last flowering, and reproductive duration. The interactionsbetween genotype and photoperiod were sufficiently strong thatconsiderable differences in these durations were detected amongisolines in the least-inductive environment (15 h d^-1) whereasdifferences were negligible in the most-inductive regime (12h d^-1). There was a negative linear relation between photoperiodand both rate of progress from the appearance of the first tothe last flower, and rate of progress from first flowering toharvest maturity; sensitivity to photoperiod varied (P<0.05)six- and five-fold, respectively, among the extreme isolines(e₁e₂e₃andE₁E₂E₃). The three dominant allelesE₁,E₂andE₃, singly,had comparatively little effect on post-flowering traits, butconsiderable epistasis (particularly betweenE₁andE₂) was detectedfor sensitivity to photoperiod in respect of rates of progressfrom the appearance of the first to the last flower, and fromfirst flower to harvest maturity. Thus the large variationsdetected for these traits are the consequence of genexgene (xgene)xenvironmentinteractions.Copyright 1998 Annals of Botany Company. Glycine max(L.) Merrill, soyabean, maturity genes, flowering, photoperiod.     

Carbon Partitioning and Export in Mature Leaves of Pepper (Capsicum annuum)

The partitioning of recently fixed carbon by mature pepper leaveshas been examined over a 10 h photoperiod using a constant specificradioactivity ¹⁴CO₂ labelling technique. Changes in the ratesof carbon partitioning into export, starch, sucrose and hexoseswere examined following changes in irradiance during the photoperiod.Leaves grown under 80 W m^–2 PAR were exposed to this irradiancefor the first 4 h of the photoperiod then the iiradiance wasdecreased. Leaves accumulated sufficient reserves in the first4 h to maintain export at the initial rate (approximately 20µg carbon cm^–2 leaf h^–1) over the following6 h of the photoperiod when the net photosynthesis rate (Pn)was decreased to 10% of the initial rate by the decreased irradiance.Export was initially maintained by the depletion of sucroseand hexose and then by carbon from the degradation of starchin the light. If leaves were exposed to low irradiance at the beginning ofthe photoperiod, then the export rate was linearly related tothe Pn during that period. When Pn exceeded that required tomaintain an export rate of approximately 20 µg carboncm^–2 h^–1, then more carbon was partitioned intostarch. At low initial irradiance, a greater proportion of photosynthatewas partitioned into export rather than starch and at high initialirradiancc the reverse occurred. There was a linear relationship between starch accumulationrate and Pn for all leaves but the relationship between Pn andexport rate was only significant for leaves with low levelsof reserve carbon. The results show that mature pepper leaves subjected to differentirradiances maintain constant export rates through alterationsof carbon partitioning. Export at low Pn is maintained at theexpense of sugar and starch reserves, with partitioning in highirradiance being predominantly to starch. Key words: Carbon partitioning, Starch, Export, Pepper (Capsicum annuum L.)     

Respiration losses in planktonic green algae cultivated in raceway ponds

Rates of respiratory oxygen uptake were measured in darkenedsamples of green microalgae (Coelastrum sphaericum, Scenedesmusfalcatus) growing in raceway ponds under field conditions andindoors. The rates measured over 6–26h varied between1 and 1.5 and 8.25 µl O₂ mg^–1 h^–1 and dependedon incubation, temperature, time in the dark, and on the temperatureand irradiance at which the algae were cultivated before. Q₁₀values ranged from 1.43 to 1.68. Arrhenius relationships describedthe influence of temperature upon respiration rates below theoptimum temperature, even after many hours of incubation inthe dark. Respiration rates were lower after growth at optimaltemperature than after cultivation at other temperatures. Thelight history also influenced the rates, being high when thealgae were darkened after exposure to high irradiance and lowerafter weaker pre-illumination. For the algae under study theoverall loss during 12h of darkness was estimated to be 2–10%of the biomass prior to darkening. Higher losses are expectedfor natural conditions and stronger irradiances.     

The Diurnal Pattern of Nitrate Uptake and Reduction by Spinach (Spinacia oleracea L.)

Spinach plants were grown in bowls of aerated nutrient solutionin a controlled environment chamber for 24 h, and harvestedevery 3·5-5 h to record their growth, nitrate and wateruptake, and plant nitrate concentration. Twelve such experimentsare described, either with a 14/10 h dark/light regime, or continuouslight or darkness. The irradiance was either 110, 320, or 510µmol m^-2 s^-1 (PPFD). All these regimes began at the endof the light period of a 14/10 h dark/light regime (510 µmolm^-2 s^-1) lasting approximately 2 weeks. Nitrate uptake rate per g of dry weight of plant continued almostunabated at about 17 µmol h^-1 through the initial 14-hdark period, and then fell away sharply if the light was notrestored, but increased slightly when it was. With continuouslight at 510 µmol m^-2 s^-1, uptake rate rose steadily forthe first 24 h of light, and then fell sharply for about 6 h.Shoot nitrate concentration increased about three-fold in thedark phase, and declined in the light at a rate which was positivelyrelated to the irradiance. Root nitrate concentration was severaltimes higher than that of the shoot: its diurnal change wassmaller (relative to the mean) than that of the shoot. Nitratereduction occurred to a small extent in the dark, and increasedrapidly as soon as the lights came on, to remain at a roughlyconstant rate (related to the irradiance) throughout the lightphase. Dry matter increase in the light was related to irradiance,but with little increase above 320 µmol m^-2 s^-1. Respiratoryweight loss in the dark was not detectable. Rate of fresh weightincrease was approximately constant throughout light and darkperiods. The results compare quite well with the predictions of a simplesimulation model, based on the pump/leak principle.Copyright1994, 1999 Academic Press Spinacia oleracea, nitrate, uptake, reduction, influx, efflux, diurnal, regulation, model, simulation     

Pathway of dark inorganic carbon fixation in two species of diatoms: influence of light regime and regulator factors on diel variations

Planktonic algae submitted to vertical mixing with a short periodicitycommute many times a day from low to high irradiance levels.To study the influence of this light periodicity, two diatoms,Skeletonema coslatum and Nitzschia turgiduloides, were cultivatedunder alternating conditions of 2 h light/2 h dark (2 h/2 h),simulating vertical mixing in the natural environment. Two otherlight regimes were used: continuous light (CL) and alternatecycles of 12 h light/12 h dark (12 h/12 h). Products synthesizedin the dark by S.costmum during 60 s incubation for 2 h/2 hculture or during 5 min for 12 h/12 h culture were determined.They were essentially sugars, malate, aspartate and glyceratefor 2 h/2 h cells and 12 h/12 h cells taken at the beginningof the light period. In contrast, 12 h/12 h cells taken duringthe darkness or in the middle of the light period and set inthe dark synthesized only amino acids. Our results corroborateprevious reports on dark CO₂ fixation via phosphoenolpyruvatecarboxykinase (PEPCKase, enzyme allowing the fixation of CO₂on PEP and the synthesis of amino acids) with involvement ofa substrate synthesized during the light period, but demonstratethat incorporation also occurs by the C-3 pathway (pathway responsiblefor the major CO₂ fixation in the light) in the very early stagesof the dark period. Another important result highlighted bythis study is the appreciable rate of dark fixation: on average6.7, 8.3 and 12.7% of photosynthesis at saturating photon fluxdensity for N.turgiduloides cultivated under 2 h/2 h, CL and12 h/12 h regime respectively and nearly 12% for S.costatumin the 2 h/2 h light regime. Variation of dark fixation wasinvestigated as a function of hour in the two species. Skeletonemacostatum cells submitted to the 2 h/2 h cycle show a constantrate of light-independent assimilation throughout the day. Bycontrast, both N.turgiduloides grown under the 12 h/12 h or2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycleundergo fluctuations in the rate of dark fixation over the light/darkcycle. The mean dark fixation rate is controlled by the lengthof the photoperiod or the frequency of light fluctuations, dependingon species. We argue that this phenomenon must be taken intoconsideration in primary production calculations. Dependingon whether they are synthesized at the beginning or at the endof the light period, products are somewhat different and therate of fixation varies. This leads us to suggest that the pathwayof dark fixation may be regulated by at least two factors: amountof available substrate and enzyme (RuBPCase and PEPCKase) activityand/or amount.     

Variation in the Durations of the Photoperiod-sensitive and Photoperiod-insensitive Phases of Post-first Flowering Development in Maturity Isolines of Soyabean [Glycine max(L.) Merrill] 'Clark'

Plants of eight isolines of soyabean [Glycine max(L.) Merrill],comprising all combinations of two alleles at the three lociE₁/e₁,E₂/e₂andE₃/e₃inthe cultivar ‘Clark’ background, were transferredafter different periods following first flowering from longdays (LD, 14 h d^-1) to short days (SD, 12 h d^-1) andvice versaina reciprocal-transfer experiment in a plastic house maintainedat 30/24 °C (day/night). Photoperiod (0.10>P>0.05),transfer time (P<0.001),>isoline (P<0.001), and theirinteractions (P<0.001) all affected flowering duration, i.e.the period from first flowering until the appearance of thelast flower. The flowering duration comprised two distinct phases:a photoperiod-sensitive phase beginning at first flowering,and a subsequent photoperiod-insensitive phase. The durationof the photoperiod-sensitive phase varied much more among theisolines in LD than in SD. Only the dominant alleleE₁increasedthe sensitivity of the photoperiod-sensitive phase of floweringduration to photoperiod singly, but positive epistatic effectswere detected betweenE₁andE₂,E₁andE₃, and especially among allthree dominant alleles. The increases in flowering durationresulting from the combined effects of gene and environment(i.e. photoperiod) were associated with considerable increasesin biomass and seed yield at harvest maturity.Copyright 1998Annals of Botany Company. Glycine max(L.) Merrill, soyabean, maturity genes, flowering, photoperiod, reciprocal transfer, yield.     

Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C.E. 1987. Diurnal production and utilization of photosynthatein nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state ¹⁴C-labelling technique was used to examine thediurnal carbon fixation, storage and export characteristicsof white clover leaves. Approximately 70% of fixed carbon wasexported to other organs during the photoperiod. The remainingcarbon was stored mainly as starch (80% at the end of the photoperiod)with smaller amounts of sucrose, hexoses and charged compounds.Carbon export from the leaf at night was provided by remobilizationof starch. During the photoperiod it was estimated that c.60% of carbonexported from the leaf was directed towards the nodulated root;45% to nodules and 15% to roots. The 40% directed towards theshoot was supplemented by a further 11% of carbon (in the formof amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulatedcarbohydrate which was available for use during darkness, inconjunction with a diminished supply of exported carbon fromleaves. Nodules exhibited a striking pattern of carbohydratestorage and depletion. The levels of sucrose and starch in thenodules at the end of the photoperiod were sufficient to maintainN₂ fixation for 8–9 h of the 12 h dark period. We proposethat continued import from leaves provided the additional sucrosenecessary to support undiminished nodule function throughoutthe entire dark period. Key words: White clover, photosynthate, starch, carbohydrate, nodules, N₂ fixation     

Light dependency of the mating process in Closterium acerosum

Sexual cell division and activation of gametangial cells forconjugation in Closterium acerosum were induced by light. L₂₀₀cells conjugated at maximum level under the following conditions;(i) a light intensity higher than 1,000 lux in a 16-hr lightand 8-hr dark regime and (ii) an illumination time longer than12 hr at 3,000 lux. L₂₀₀ cells also conjugated under continuousillumination at 3,000 lux. The action spectrum for the activation of gametangial cellshad peaks around 450, 611 and 665 nm. 3-(4'-Chlorophenyl)-l,l-dimethylurea (CMU) inhibited the accumulationof carbohydrates and sexual cell division at 10^–5 M andthe activation of gametangial cells for conjugation at 10^–4M. (Received August 15, 1977; )     

Factors Influencing Induction of Resistance to Dark Abscission by Malformin

Factors influencing induction of resistance to dark abscissionby malformin on cuttings of Vigna radiata during treatment inlight were examined. When light duration (13.5 W m^–2)increased from 0 to 48 h, the effect of malformin on subsequentdark abscission changed from stimulation only (0 to 4 h), stimulationfollowed by inhibition (8 to 12 h), to inhibition only (24 to48 h). Maximum abscission resistance occurred after 48 h whenirradiance was 6.6 W m^–2. Kinetin treatment in light reducedsubsequent dark abscission by controls but did not reduce abscissionon malformintreated cuttings. Hadacidin had no effect on inductionof abscission resistance by malformin. IAA, hydroxyproline,CaCl₂, sucrose, and NH₄NO₃ were inactive. ABA and ethephon completelyblocked induction of abscission resistance by malformin. Inhibitionof abscission induced by kinetin was also blocked by ABA. Becauseboth puromycin and malformin inhibited dark abscission followingtreatment in light, malformin may induce abscission resistanceby inhibiting protein synthesis or promoting formation of othersubstances which inhibit protein synthesis. Leaf blade removalfrom the distal end of the petioles abolished malformin-inducedabscission resistance. It is suggested that in light malformininduces formation of abscission-inhibiting compounds in leaveswhich are responsible for development of abscission resistance. (Received May 17, 1983; Accepted November 8, 1983)     

Photoperiodic sensitivity of the cotyledon and first true leaf of Xanthium

Three days after germination of Xanthium seeds, the seedlingswith only the first pair of opposite leaves attained full photoperiodicsensitivity and flowered in response to a 16-hr dark periodtreatment. The cotyledons of three- to six-day-old seedlingswere completely insensitive to inductive dark treatment. Six-day-oldseedlings could not be induced to flower with a 9-hr dark period.But with a dark period of 10 hr or longer, floral inductionoccurred and the optimum dark period was 15 hr. Developmentof the flowering apices started within 2 days after the inductivedark treatment and was completed by 9 to 11 days. ¹This work was supported by the United States Atomic EnergyCommission Contract AT (11-1)-1338. ²Present address: Plant Physiology Section, Crops Research Institute,P.O. Box 3785, Kumasi, Ghana, West Africa. (Received March 28, 1974; )     

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     

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     

Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Predicting responses of plant and global carbon balance to theincreasing concentration of carbon dioxide in the atmosphererequires an understanding of the response of plant respirationto carbon dioxide concentration ([CO₂]). Direct effects of thecarbon dioxide concentration at which rates of respiration ofplant tissue are measured are quite variable and their effectsremain controversial. One possible source of variation in responsivenessis the energy status of the tissue, which could influence thecontrol coefficients of enzymes, such as cytochrome-c oxidase,whose activity is sensitive to [CO₂]. In this study we comparedresponses of respiration rate to [CO₂] over the range of 60to 1000 µmol mol^-1in fully expanded leaves of four C₃andfour C₄herbaceous species. Responses were measured near themiddle of the normal 10 h dark period, and also after another24 h of darkness. On average, rates of respiration were reducedabout 70% by the prolonged dark period, and leaf dry mass perunit area decreased about 30%. In all species studied, the relativedecrease in respiration rate with increasing [CO₂] was largerafter prolonged darkness. In the C₃species, rates measured at1000 µmol mol^-1CO₂averaged 0.89 of those measured at 60µmol mol^-1in the middle of the normal dark period, and0.70-times when measured after prolonged darkness. In the C₄species,rates measured at 1000 µmol mol^-1CO₂averaged 0.79 of thoseat 60 µmol mol^-1CO₂in the middle of the normal dark period,and 0.51-times when measured after prolonged darkness. In threeof the C₃species and one of the C₄species, the decrease in theabsolute respiration rate between 60 and 1000 µmol mol^-1CO₂wasessentially the same in the middle of the normal night periodand after prolonged darkness. In the other species, the decreasein the absolute rate of respiration with increase in [CO₂] wassubstantially less after prolonged darkness than in the middleof the normal night period. These results indicated that increasingthe [CO₂] at the time of measurement decreased respiration inall species examined, and that this effect was relatively largerin tissues in which the respiration rate was substrate-limited.The larger relative effect of [CO₂] on respiration in tissuesafter prolonged darkness is evidence against a controlling roleof cytochrome-c oxidase in the direct effects of [CO₂] on respiration.Copyright 2001 Annals of Botany Company Carbon dioxide, respiration, Abutilon theophrasti(L.), Amaranthus retroflexus(L.),Amaranthus hypochondriacus (L.), Datura stramonium(L.), Helianthus annuus(L.), Solanum melongena(L.), Sorghum bicolor(L. Moench), Zea mays