Effects of Green Light on the Chloroplasts of Spinach Leaf Discs

The development and photosynthetic capacity of chloroplastsformed in green light in cultured spinach leaf discs has beenstudied. At intensities of 4 to 6 mW cm^–2 green lightstimulates chloroplast replication to about the same extentas white, blue, and red light. However, practically no chloroplastreplication occurs in discs grown in low intensity green orwhite light but considerable chloroplast growth takes place.Ultrastructural studies have shown that these chloroplasts,which can be two to five times the area of control plastids(high intensity white light), have an essentially normal thylakoidsystem. Double isotope labelling experiments have establishedthat the synthesis of chloroplast ribosomal-RNA is similar incontrols and in discs grown in low intensity green or whitelight. On a per unit chlorophyll basis the CO₂ fixation rateof spinach discs grown in low intensity green (or white) lightsaturates with increasing light intensity or increasing CO₂concentration at values well below control discs. In this respecttheir photosynthetic characteristics bear a similarity to thoseof shade plants.     

Environmental Anoxia is Unnecessary for Inhibiting Chloroplast Photomorphogenesis in Rice Coleoptiles (Oryza sativa L.)

High population densities of germinating rice seedlings in initiallyair-saturated sealed aquatic environments exhibited d^–seedling growth consisting solely of coleoptile emergence inlight and dark environments. Residual oxygen tensions of 17–23%of the initially air-saturated water containing the d^–seedlings were evident after 15 d in both the light and dark.Coleoptiles of all d^– seedlings were stark white in appearance,lacked protochlorophyllide, and contained proplastids and amyloplasts,there being no evidence of normal etioplast development in thelight or dark and no chloroplast development in the light. Thus,complete environmental anoxia was observed to be unnecessaryfor inhibiting normal chloroplast photomorphogenesis in coleoptilesof light-germinated rice seedlings. Increasing the oxygen tensionsof the 15-d-old aquatic environments of light- and dark-germinatedd^– seedlings placed in the light resulted in normal chloroplastphotomorphogenesis in coleoptiles, shoots, and roots. Key words: Oryza sativa, environmental anoxia, chloroplast photomorphogenesis, rice coleoptiles     

Light-induced changes in membrane potential in Spirogyra

Spirogyra cells exhibited changes in membrane potential whenthey were exposed to light. Cells made chloroplast-free didnot show any light-induced potential change (LPC) upon illuminationwith white light and also monochromatic red (680 nm) and farred (720 nm) light. LPC was observed when the cell containedonly a small fragment of chloroplast, whether the cell had anucleus or not. The magnitude of LPC depended on the amountof chloroplast in the cell. DCMU at 10^–5 M, CCCP at 10^–5 M and DNP at 10^–4M at pH 5.5 suppressed LPC, while CCCP at 1–5 ? 10^–6M, NH₄Cl at 5 ? 10^–2 M and DNP at 10^–4 M at pH 7.0stimulated LPC. PMS at 10^–4 M stimulated LPC and couldinduce LPC which was completely inhibited by DCMU. These factssuggest that LPC is related to noncyclic and cyclic electronflows. The influences of light and dark conditions and various metabolicinhibitors (DCMU, DNP, CCCP, NH₄Cl) on ATP level have been investigated.No significant difference in the ATP level was observed betweencells in the light and dark. DNP at 10^–4 M (pH 5.5) andCCCP at 5 ? 10^–6 M decreased the ATP level significantly,while DCMU and NH₄Cl only slightly. Good correlation was notfound between the total ATP level and LPC in Spirogyra. LPC occurred even when the external medium contained only asingle salt such as KCl, NaCl or CaSO₄. LPC was also recorded in chloroplasts in situ and in vitro.The mode of LPC of chloroplasts was quite different from thatof the cell. On illumination, the chloroplast potential changedvery rapidly and transiently in the positive direction thenrecovered spontaneously to almost the original potential level. Possible causes of LPC are discussed in relation to the electrogenicion pump. ¹ Present address: Department of Botany, Faculty of Science,University of Tokyo, Hongo, Bunkyo, Tokyo 113, Japan. (Received November 9, 1977; )     

Negative Phototropism in Vaucheria terrestris Regulated by Calcium I. Dependence on Background Blue Light and External Calcium Concentration

In the presence of 1–8 mM Ca²⁺ unilateral 10–15min irradiation with blue light can elicit negative phototropicbending in the tip-growing coenocytic fresh water alga, Vaucheriaterrestris (Xanthophyceae), when it is simultaneously irradiatedwith strong blue or green background light. By changing wavelength,fluence rate, and duration of background light and holding thoseof unilateral light (456 nm, 1.7 Wm^–2) negative phototropicresponse was analyzed: the wavelength of the background lighthad to be shorter than 540 nm; red light (660 nm) was ineffectiveeven at very high fluence rates (100W.m^–2). The negativebending was strongly and specifically dependent on the externalCa²⁺ concentration. Other divalent cations, Mg²⁺ and Ba²⁺, wereeither toxic or quite ineffective; Sr²⁺ could partly supportthe growth, but mediated neither positive nor negative phototropicbending. The rate of tip-growth was not significantly alteredbetween 10⁶M and 10^-6 mM Ca²⁺. Pre-irrradiation with the backgroundlight slightly increased the negative curvature; whereas thatwithout subsequent simultaneous irradiation does not cause negativebending, but rather increases the positive curvature. Three-mindelayed start of background light did not cause negative bendingany more. The present results strongly suggest that blue light elicitsan influx of Ca²⁺ at the apex of Vaucheria and that the increasedcytoplasmic Ca²⁺ regulates the sensitivity and direction ofphototropic response. (Received June 23, 1988; Accepted September 7, 1988)     

Inhibition of Phototaxis and Motility by UV-B Irradiation in Dictyostelium discoideum Slugs

The effect of monochromatic irradiation in and near the UV-Bregion (280–320 nm) on motility (speed of movement) andwhite light-induced phototactic orientation was studied in slugsof the cellular slime mold Dictyostelium discoideum (NC-4).Motility decreased to about 50% in UV-B fluences (about 100Jm^–2 at 280 nm) well below those required to inhibit slugand sorocarp development or to impair viability. At wavelengthsof 270–300 nm, the phototactic orientation was almosttotally eliminated by an exposure to U V (about 0.1 Wm^–2)for as short a time as half an hour when administered at thebeginning of the 24 h exposure to white light. (Received July 22, 1983; Accepted September 27, 1983)     

Exposure of Dunaliella salina to Low Temperature Mimics the High Light-Induced Accumulation of Carotenoids and the Carotenoid Binding Protein (Cbr)

We report that growth of Dunaliella salina at either 13°C/150µmol m^–2s^–1 or 30°C/2,500 µmol m^–2s^–1 results in the accumulation of comparable levels ofcarotenoids and the zeaxanthin-binding protein, Cbr. We concludethat carotenoid and Cbr abundance in this green alga respondto changes in PSII ‘excitation pressure’ ratherthan to high light per se. (Received September 19, 1996; Accepted November 20, 1996)     

Action Spectrum for Light-induced Chloroplast Accumulation in a Marine Coenocytic Green Alga, Bryopsis plumosa

A marine coenocytic green alga, Bryopsis plumosa exhibited multistriatetype protoplasmic streaming of a velocity less than 100 µm.min^–1.When the alga was illuminated locally, chloroplasts and othercell organelles accumulated in the illuminated zone. The actionspectrum for this reaction showed that blue light between 380and 500 nm was most effective. The velocity of chloroplast movement decreased when the cellwas totally illuminated with blue light, but no comparable changewas observed under red light illumination. Therefore, chloroplastaccumulation probably was caused by the reduced streaming ratein the illuminated zone. Electron microscopy showed cytoplasmic microtubules arrangedparallel to the cell axis in the vicinity of the chloroplasts.Chloroplast movement was inhibited heavily by treatment withantimicrotubule agents, but was little affected by cytochalasinB at a concentration of 10 µg/ml. (Received May 30, 1981; Accepted August 24, 1981)     

The Regulation of Root Growth in Cress Seedlings by Light and Gravity

Horizontal growth of seedling roots of Lepidium sativum L. cv.Curled is sensitive to white light with a photon flux densityas low as 7.5 µEinsteins m^–2 S^–1. Inhibitionof growth is positively correlated with light intensity. Rootsgrowing in the vertical plane are much less sensitive to light.The light inhibition of horizontal root growth can be overcomeby the application of an axial force. The effects of light andaxial force are reversible. Although geotropic responsivenessis not dependent on exposure to light, there is evidence thatlight induces a heightened geosensitivity. The results are discussedin relation to the counter-current theory of the regulationof root growth.     

Light-driven movements of the trifoliate leaves of bean (Phaseolus vulgaris L.). Activity of blue light and red light

The puhrinule of the terminal leaflet in the trifoliate leafof bean (Phaseolus vulgaris L.) responds to its continuous exposureto directional overhead light by increasing the elevation ofits attached lamina. Blue light drives this response, but theeffectiveness of unfiltered white light equalled, or exceededthe effectiveness of blue light at equivalent irradiances (200–800µmol m^–2 s^–1). Adding red light to blue lightenhanced the initial rate of response, and increased its steady-state.These effects of red light increased with irradiance. Adding200–800 µmol m^–2 s^–1 red light to 50µmol m^–2 s^–1 blue light was more effectivein enhancing the initial rate of response than adding blue lightat equivalent irradiances, whereas added blue light was moreeffective in increasing the steady-state. In continuous bluelight the initial (maximal) angular velocity of laminar reorientation,as well as the eventual steady-state of the response increasedlinearly with log PFD (up to 800 µmol m^–2s^–2).Laminar reorientation also took place in continuous red lightby itself, and the angular velocity of the response was initiallyhigh, then became considerably slower. The initial phase wasapparently independent of irradiance up to PFD 100 µmolm^–2 s^–1 but increased progressively with log PFDat higher irradiances. During the second phase, the rate increasedlinearly with irradiance, becoming saturated at PFD 200 µmolm^–2 s^–1. Key words: Phaseolus, phototropism, pulvinule, spectral dependence, trifoliate leaf movements     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Effect of Growth Regulators and Light on Pollen Germination and Pollen Tube Growth in Pinus roxburghii Sarg

Pine (Pinus roxburghii) pollen grown in suspension cultureswas used to study the effects of growth regulators and lightconditions on germination and pollen tube growth. Indol-3-ylacetic acid, gibberellic acid, ethylene, abscisic acid and cyclicAMP (cAMP) at low concentrations (1–10 mg 1^–1) promotedgermination and tube growth. Addition of 1 and 10 mg 1^–1cAMP to any of the growth regulators had a promotory effect.Pollen tube growth decreased in white light as compared to thedark, and was increased in red light. Far-red light counteractedthe effect of red light. The effect of growth regulators incausing the enhanced tube growth appears to be manifested throughsubstances such as cAMP, and phytochrome seems to be involved. Pinus roxburghii, pine, pollen germination, pollen tube growth, growth regulators, cyclic AMP, phytochrome     

The involvement of photosynthesis in inducing bud formation on excised leaf segments of Heloniopsis orientalis (Liliaceae)

The role of photosynthesis in inducing adventitious bud formationon leaf segments of Heloniopsis orientalis was investigated.The effect of white light reached a maximum at about l25 J?m^–2?sec^–1.White, red, blue and far-red light were effective in inducingbud formation, but green light was not. In darkness, bud formationwas induced if sugar was added to the nutrient medium. The photosyntheticinhibitors DCMU and AT blocked the effect of light. Bud formationwas inhibited in CO₂-free air. The requirement of sucrose forbud formation in darkness could be replaced by citrate. It wasconcluded from these results that light appears to induce budson leaf segments through some processes dependent upon photosynthesis. (Received January 11, 1978; )     

DNA Biosynthesis in Chloroplasts: I. CHARACTERIZATION OF AND DEVELOPMENTAL VARIATION IN BOTH LIGHT- AND ATP-DRIVEN THYMIDINE INCORPORATION INTO DNA IN ISOLATED INTACT CHLOROPLASTS

Intact chloroplasts from young pea leaves were able to incorporate[³H]thymidine into DNA at relatively high rates (50 pmol mg^–1chlorophyll h^–1 or more), using light as the sole energysource. The intact plastids were also able to synthesize DNAin darkness, but only if ATP and MgCl₂ (MgATP) were both present.The rates of MgATP-driven assimilation in darkness were equalto or greater than light-driven activity. Neither light nordithiothreitol pretreatments enhanced thymidine incorporationin darkness, suggesting that enzymes of chloroplast DNA (ctDNA)biosynthesis are not regulated via a thioredoxin-type system.Although exogenous nucleosides (other than [³H]thymidine) werenot an absolute requirement, dramatically elevated rates ofincorporation (over 300 pmol mg^–1 chlorophyll h^–1)were seen when adenosine, cytidine, guanosine and thymidinewere supplied in combination (500 mmol m^–3 each). RadiolabelledDNA synthesized by the isolated chloroplasts was prepared usinga new heat extraction method. After digestion by restrictionendonucleases, ctDNA synthesized in organello was found to givetypical autoradiography patterns for chloroplast DNA. ExonucleaseIII studies suggested that 5% to 15% of the newly synthesizedDNA might be in a closed circular form. MgATP-driven synthesisin darkness was highly age-dependent. Chloroplasts from young(6 to 8-d-old) plants, or alternatively the youngest leavesof more mature plants, were 4–10 times more active thanthose from older tissues. Although these data do not establishconclusively that replication-type synthesis was occurring inthe isolated chloroplasts, they are consistent with this suggestion. Key words: Chloroplast DNA replication, isolated chloroplasts, chloroplast DNA synthesis     

Photosynthetic electron transport in Dunaliella tertiolecta (Chlorophyceae) measured by fast repetition rate fluorometry: relation to carbon assimilation

A comparison of photosynthesis-irradiance response curves (P–Eresponse curves) obtained through fast repetition rate (FRR)fluorometry and radiocarbon (¹⁴C) tracer method was made inthe chlorophyte, Dunaliella tertiolecta, grown under differentirradiance conditions. In FRR-based P–E response curveexperiments, actinic light provided by white light-emittingdiodes (LEDs) was increased gradually from 0 to 1500 µmolquanta m^–2 s^–1 and the rate of photosyntheticelectron transport was determined at each light level. Short-termexperiments (20 min) of ¹⁴C-based P–E response curvewere carried out with an improved photosynthetron, which containswhite LEDs as the light source. Irrespective of growth irradiance,the ratios of FRR to ¹⁴C-based initial slopes were almost uniform.The ratios of FRR- to ¹⁴C-based maximum rates were 25–36%higher than those of FRR- to ¹⁴C-based initial slopes. The relationshipbetween electron transport and carbon assimilation was non-linearwith increasing discrepancy towards high actinic light. Thisnon-linear relationship between FRR- and ¹⁴C-based estimatesis primarily due to the effect of physiological processes stimulatedat high levels of light, such as cyclic electron flow and theMehler reaction. The results of this study indicate that theFRR fluorometry can be used as a good indicator of photosyntheticrates from low to middle light levels, but becomes increasinglyquestionable as the maximum photosynthetic rate is approached.The degree to which this relationship is further affected bynutrient-status warrants investigation.     

The Effects of Light Intensity and External Potassium Level on Root/Shoot Ratio and Rates of Potassium Uptake in Perennial Ryegrass (Lolium perenne L.)

Loliun perenne L. (cv.S. 23) was grown on vermiculite in winterin a heated greenhouse for 8 weeks under factorial combinationsof two potassium regimes (nominally 6 parts/10⁶ and 156 parts/10⁶in Hewitt's solution) and three densities of artificially supplementedvisible radiation flux (36.1, 7.3, and 2.2 W m^–2). Growthand potassium uptake were studied through the calculation ofvarious growth functions from fitted curves. There was little effect of potassium treatment but the experimentalmaterial responded markedly to light. Leaf-area ratio in thethree treatments showed extreme plasticity in increasing from2–3 x 10^–2 through 6 x 10^–2 to 8–9 x10^–2 m² g^–1 as light intensity decreased. Correspondingdecreases in unit leaf rate, however, caused over-all reductionsin relative growth rate. Specific absorption rates for potassium (A_K, dry-weight basis)were strongly reduced at the lower light intensities but alsodisplayed complex ontogenetic drifts. Values of the allometricconstant, k (the ratio of root and shoot relative growth rates),decreased from c. 0.7 at 36.1 W m^–2 through c. 0.3 at7.3 W m^–2 to a value not significantly different fromzero (P < 0.05) at 2.2 W m^–2. In material grown under the two higher light intensities a constantinverse relationship was found between the mass ratio of rootand shoot and the corresponding activity ratio. The resultsconform to this model: Mass ratio = –0.001+45.0 (1/activityratio) where activity ratio is expressed as specific absorptionrate for potassium (in µg g root^–1 h^–1)/unitshoot rate (rate of increase of whole-plant dry weight per unitshoot dry weight, in mg g shoot^–1 h^–1). The implicationsof this relationship are discussed.     

Spectral response of corn (Zea mays) in root geotropism

The primary roots of corn (Zea mays, Wisconsin hybrid 64A ?22R) show positive geotropism following exposure to light. Thisconfirms the works of other investigators. The curvature responsebegins at about 1 hr following irradiation and reached a plateauat 5 hr. A study of wavelengths 350–760 nm, using energiesof 2.24 ? 10¹⁴ photon cm^–2 and exposure times 60 sec,shows that the most effective light is at 660 nm with lessereffectiveness at 460 and 560 nm. The responses at 660 and 460nm are reversible by a far-red (730 nm) exposure, indicativeof the possible participation of phytochrome. Analyses of freshtips of corn roots with a dual-wavelength difference photometershow the phytochrome content in the root to be about 0.16 (OD) per gram fresh wieght. The requirement of light for thegeotropic growth response of corn roots might be an adaptivephenomenon. The occurrence of photomorphogenic activity in thegreen light should be of concern to those who use green as the"safe" light in "dark" experiments. ¹Work supported by U. S. NASA and U. S. ERDA. (Received September 26, 1977; )     

Photoresponses of the copepod Mesocyclops edax

The predatory copepod Mesocyclops edax is an important componentof many zooplankton communities where it typically makes extensivedid vertical migrations. To describe the effect of light onadults we measured their photoresponses in the laboratory. Theresponse spectrum is characterized by a wide plateau of greatestsensitivity from about 480 – 580 nm. These animals areadapted to perceive light during the day since their regionof maximum sensitivity overlaps the spectral region of highestquantal intensity underwater (575 – 700 nm). The thresholdintensity for positive phototaxis by dark adapted animals wasabout 5 x 10^–1 Wm^–2 at 540 nm, and they were positivelyphototactic up to an intensity of 5 x 10^–1 Wm^–2.Above this intensity phototaxis is no longer observed. Light-adaptedanimals were less sensitive than dark-adapted, but their generalpattern of response to light intensity did not differ. Thereis no rhythm in phototaxis. Their photoresponses may providea mechanism for controlling vertical migration so as to minimizeexposure to planktivorous fish. ¹Contribution No. 1375-AEL from UM-CEES, Appalachian EnvironmentalLaboratory.     

An Analysis of the Growth of Young Tomato Plants in Water Culture at Different Light Integrals and CO2 Concentrations: I. Physiological Aspects

Young tomato plants were grown from germination in water cultureat light-flux densities from 6 to 110 W m^-2 (400–700 nm),daylengths from 8 to 24 h and CO₂ concentrations from 0.4 to2.2 g CO² m^-3 in controlled environment cabinets. The growth rates and net assimilation rates of 14–17-day-oldplants at the highest light integrals were appreciably greaterthan most values previously recorded for tomato, and diminishedwith time. Plants in the lowest light conditions had leaf arearatios five times larger than those in the highest light, attributablemainly to a difference in leaf dry weight/area. Such flexibilityin leaf area ratio has not previously been associated with ‘sun’plants such as the tomato. Relatively normal growth was obtained in continuous light, incontrast to most other reports. This may have been due to theuse of conditions which would minimise water stress. The efficiency of the conversion of incident light energy tochemical energy by the whole plant ranged from 15 per cent inseedlings in low continuous light to about 6 per cent, tendingto be higher in young plants in long days under CO₂ enrichment.The higher values are probably overestimates because of theexclusion of reflected light from the energy receipt values.     

Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

The effects of blue light (B) pretreatments on internode extensiongrowth and their possible interaction with phytochrome mediatedresponses were examined in Sinapis alba seedlings grown for11 d under 280 µmol m^–2 s^–1 of continuousblue-deficient light from low pressure sodium lamps (SOX). SupplementaryB (16 µmol m^–2 s^–1) caused no detectable inhibitionof the first internode growth rate under continuous SOX, butgrowth rate was inhibited after transfer to darkness. This effect,and the growth promotion caused by far-red bend-of-day' lightpulses were additive. The addition of B at 16 µmol m^–2s^–1 during 11 d, or only during the first 9 or 10 d orthe latest 0.75, 1 or 2 d of the SOX pretreatment caused approximatelythe same extent of inhibition after the transition to darkness.A single hour of supplementary B before darkness caused morethan 50% of the maximum inhibition. However, 24 h of lower fluencerates of B (4 or 7 µmol m^–2 s^–1) were ineffective.Covering the internode during the supplementary B period didnot prevent the response to B after the transition to darkness.Far-red light given simultaneously with B (instead of the SOXbackground) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly inthe leaves inhibits extension growth in subsequent darkness,provided that high phytochrome photo-equilibria are presentduring the irradiation with B. Once triggered, this effect doesnot interact significantly with the ‘end-of-day’phytochrome effect. Key words: Blue light, extension growth, phytochrome     

Ethylene Evolution from Bracts and Leaves of Poinsettia, Euphorbia pulcherrima Willd

Woodrow, L. and Grodzinski, B. 1987. Ethylene evolution trombracts and leaves ol Poinsettia, Euphorbia pulcherrima Willd.—J.exp. Bot. 38: 2024–2032. Ethylene release from fully expanded, red and white bracts andleaves of poinsettia, Euphorbia pulcherrima Willd., was compared.On a laminar (area) basis leaves contained about 50 times morechlorophyll and demonstrated 10 times the photosynthetic rateof the bracts. Both tissues contained starch, however, solublecarbohydrate in the bracts consisted primarily of reducing hexoseswhile the leaves contained mainly sucrose for translocation.The total free alpha-amino nitrogen content of the bract tissuewas twice that of the leaf tissue. The leaves contained moreACC (1-aminocyclopropane-1-carboxylic acid) and produced proportionallymore endogenous C²H⁴ than either the red or white bracts. ACC-stimulated₂H₄ release was also greatest from the green tissue indicatingthat the EFE (ethylene forming enzyme) was most active in theleaves. The specific activity of the ¹⁴C₂H₄/¹²C₂H₄ releasedfrom [2,3-¹⁴C]ACC confirmed ACC as the primary precursor ofC₂H₄ in this tissue. Ethylene release from the non-photosynthetic,bract tissue was not markedly affected by alterations in CO₂or light conditions. In green leaf tissue endogeneous ethylenerelease increased from 1·5 to 6·0 pmol C₂H₄ cm^–2h^–1 while ACC-stimulated ethylene release increased from10 to 35 pmol C₂H₄ cm^2– h^1– as the CO₂ partial pressureincreased from 100 to 1 200 µbar. Key words: Poinsettia, ethylene, bracts