Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna     

The influence of irradiance on growth, photosynthesis and respiration of Gyrodinium cf. aureolum

The bloom-forming marine dinoflagellate Gyrodinium cf. aureolumwas grown in batch cultures over a range of irradiances (35–380µmolm^–2 s^–1 and growth, photosynthesis and respirationrates determined. Saturation of growth occurred at irradiancesof 100µmol m^–2 s^–1 Below this light level,decreases in growth rates and cell size, and a relative increasein carbon specific respiration rates, were observed. On theother hand, photosynthesis-irradiance relationships determinedfrom dissolved oxygen incubations showed that on a cellularand carbon basis, cultures grown at low irradiances had higherrates of light-limited and light-saturated photosynthesis, mainlyas a result of large increases in cell chlorophyll content.This adaptation strategy enables low-light-grown organisms toexploit available high irradiance through a relatively highphotosynthetic capacity. In cells grown at higher light levels(>100µmol m^–2 s^–1), excess photosynthatemay be diverted to storage rather than used for growth.     

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.     

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     

A Cytochrome P450 Mediated Naringenin 3'-Hydroxylase from Sweet Orange Cell Cultures

A microsomal flavonoid 3'-hydroxylase (F3'H) catalyzing themetabolism of naringenin to eriodictyol in Citrus sinensis (L.)Osbeck cv. ‘Hamlin’ cell suspension cultures wasshown to be a cytochrome P450 enzyme. This reaction requiredO₂ and NADPH and was inhibited by CO, with partial reversalof CO-inhibition by light at 450 nm. Cytochrome P450 contentranged from 10–20 pmol (mg microsomal protein)^–1.The F3'H reaction was shown to be linear in regard to proteinconcentration between 2.5 and 25 µg of microsomal protein.The optimum pH for the reaction was 7.4–7.6 and the temperatureoptimum was between 30 and 37°C. The apparent K_m and V_maxfor naringenin were 24 µM±3.2 and 81.4±7.9pmol eriodictyol min^–1 (mg protein)^–1, respectively.The microsomal F3'H was also capable of forming dihydroquercetinfrom dihydrokaempferol (40 pmol min^–1 (mg protein)^–1)and of quercetin from kaempferol (3.25 pmol min^–1 (mgprotein^–1). Cytochrome c and ketoconazole were the bestinhibitors of WH activity followed by piperonyl butoxide anda-naphthoflavone. Light was shown to be an inducer of the F3'Halmost doubling the specific activity and increasing the microsomalcytochrome P450 content by 30% over that of dark grown cells.F3'H activity was also confirmed in microsomal preparationsof young (new flush) leaves from ‘Hamlin’ treesand flavedo of ‘Hamlin’ oranges, ‘Marsh’grapefruit, and ‘Lisbon’ lemon. No activity wasobserved in older, hardened leaves and albedo of all the fruittested. Initiation of embryogenesis in the ‘Hamlin’cell suspension cultures by switching from a sucrose mediumto a glycerol-based medium resulted in the down-regulation ofF3'H. ¹Mention of a trademark, warranty, proprietary product, or vendordoes not constitute a guarantee by the U.S. Department of Agricultureand does not imply its approval to the exclusion of other productsor vendors that may also be suitable.     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

Acclimation of single plants of Lolium temulentum to changing[CO₂] was studied on plants grown in controlled environmentsat 20°C with an 8 h photoperiod. In the first experimentplants were grown at 135 µ;mol m^–2 s^–1 photosyntheticphoton flux density (PPFD) at 415µl l^–1 or 550µll^–1 [CO₂] with some plants transferred from the lowerto the higher [CO₂] at emergence of leaf 4. In the second experimentplants were grown at 135 and 500 µmol m^–2 s^–1PPFD at 345 and 575 µl l^–1 [CO₂]. High [CO₂] during growth had little effect on stomatal density,total soluble proteins, chlorophyll a content, amount of Rubiscoor cytochrome f. However, increasing [CO₂] during measurementincreased photosynthetic rates, particularly in high light.Plants grown in the higher [CO₂] had greater leaf extension,leaf and plant growth rates in low but not in high light. Theresults are discussed in relation to the limitation of growthby sink capacity and the modifications in the plant which allowthe storage of extra assimilates at high [CO₂]. Key words: Lolium, carbon dioxide, photosynthesis, growth, stomatal density     

The Uptake and Distribution of Copper in Some Forage Grasses as Affected by Nitrate-nitrogen Supply in Flowing Solution Culture

The effect of changes in nitrate-nitrogen supply on the absorptionand distribution of copper was examined in grasses grown inflowing solution culture with a maintained concentration ofcopper. Absorption by roots (µg Cu g^–1 dry root)decreased markedly when nitrogen had been depleted or was maintainedat 0.1 mg l^–1 N, but there was an immediate increase whennitrogen was maintained at 1.0 or 10.0 mg l^–1. There werealso large increases in the concentration of copper in the shootsof plants grown with 1.0 and 10.0 mg 1^–1 N. The rootsof plants grown with 0.1 or 1.0 mg 1^–1 N retained similarproportions of uptake, but a lower proportion was retained whenthe plants were grown with 10.0 mg 1^–1. Although a lowerproportion of the copper was associated with cell walls in theplants grown at 10.0 mg 1^–1 N this was the result of alower content of cell walls rather than an effect on copperitself. In a longer-term experiment in conventional solutionculture with a range of nitrogen concentration, the concentrationof copper in shoots was largely determined by shoot growth. Dactylis glomerata, Festuca arundinacea, Lolium perenne, cell walls, copper absorption, copper distribution, flowing solution culture, nitrate-nitrogen     

Ecological studies on the phytoplankton of Boknafjorden, western Norway. II. Environmental control of photosynthesis

Both predicted (incubator) and measured (in situ) ¹⁴C-assimilationrates were studied from February to November 1981 at three stationsin Boknafjorden, a deep silled fjord of western Norway. Sampleswere taken from different light depths within the euphotic zone.A high degree of conformity was found between the two approaches.Daily values of carbon assimilation integrated over the euphoticzone varied between 0.05 and 1.4 g C m^–2. Yearly primaryproduction varied between stations from 82 to 112 g C m^–2(120–148 g C m^–2 when based on average light conditions).The light-saturation curve parameters ^B and P^B_max ranged from0.0056 to 0.0537 mg C mg Chla^–1 h^–1 µE^–1m² and from 0.7 to 8.5 mg C mg Chla^–1 h^–2 (in situassimilation numbers ranged from 0.9 to 9.3 mg C mg Chla^–1h^–1) respectively, which compare well with those publishedfrom the northwestern side of the Atlantic. The overall importanceof light in controlling photosynthesis throughout the year wasrevealed by the light utilization index , estimated to be 0.43mg C mg Chla^–1 E^–1 m². The maximum quantum yieldwas encountered on August 17, with 0.089 mol CE^–1. Chla/Cratios above and below 0.010 were found to be typical for shade-and light-adapted cells respectively. Assimilation numbers andgrowth rates were linearly related only when considering light-adaptedcells. Consistent with the findings of this study, the applicabilityof I_K, ^B and P^B_max as indicators of light-shade adaptation propertiesshould be questioned. Maximum growth rates were encounteredduring an autumn bloom of the dinoflagellate Gyrodinium aureolum(1.0 doublings day^–1), while 0.7–0.8 doublings day^–1were found for a winter bloom (water temperature of 2°C)of the diatom Skeletonema costatum. No unambiguous temperatureeffect on assimilation number and growth of phytoplankton couldbe recognized in Boknafjorden. A tendency towards increasedassimilation numbers coinciding with increased water columnstability was revealed. The highest P^B_max values were oftenencountered at almost undetectable nutrient concentrations.At least during summer this could be attributed to recyclingof nutrients by macro- and/or microzooplankton, responsiblefor a greater part of the primary production now being grazeddown. This study supports the convention that the depth of theeuphotic zone may extend considerably below the 1% light depth.     

Somatic Embryogenesis from Clonal Leaf Tissues of Cassava

Leaf lobes were isolated from palmate leaves of clonal cassava(Manihot esculenta Crantz) material growing in vitro or in glasshouseconditions and subjected to a two-stage culture procedure involvingincubation on Murashige and Skoog (MS2) basal medium supplementedwith 2–12 mg l^–1 2,4-D for 20 d (Stage I) beforetransfer to MS2 basal medium supplemented with 0.01 mg l^–12,4-D and 0.1 mg l^–1 6-benzylamino purine (BAP) (StageII medium). Embryogenetic tissues, foliose structures and somatic embryosdeveloped from leaf lobes at all Stage I 2,4-D concentrations,except on those explants isolated from shoot-tip cultures incubatedon MS2 basal medium supplemented with 0.1 mg l^–1 NAA and1.0 mg l^–1 BAP. Leaf lobes isolated directly from glasshouse plants showed optimalembryogenetic competence when subjected to a Stage I cultureperiod of 17 d, although foliose structure initiation was optimalwith shorter Stage I durations. Leaf lobes of 2–4 mm lengthand those isolated from phyllotaxic leaf numbers 4 and 5 showedthe greatest embryogenetic competence. Manihot esculenta, cassava, somatic embryogenesis, tissue culture, morphogenetic competence     

Somatic Embryogenesis in Sugarcane (Saccharum officinarum L.): Growth and Plant Regeneration from Embryogenic Cell Suspension Cultures

Embryogenic cell suspension cultures were established from calliderived from young leaves of sugarcane (Saccharum officinarumL.) by placing them in liquid medium containing 5 per cent coconutwater (CW), 2–3 mg 1^–1 2, 4-D and 500 mg 1^–1casein hydrolysate (CH). The cultures were maintained by transferring2.5–5.0 ml of the suspension to 35 ml of fresh mediumevery 4–5 days. Organized structures resembling the earlystages of embryogeny were formed when 2, 4-D in the medium waslowered (0.1–1.0 mg 1^–1) but these did not developbeyond the globular or early scutellar stages. High levels ofsucrose (6–10 per cent) promoted the formation of proembryoids.Plating of the suspension on MS agar medium supplemented with0.25–2.0 mg 1^–1 2, 4-D, 5 per cent CW, 500 mg 1^–1CH, with or without activated charcoal, resulted in the formationof embryogenic calli. A large number of embryoids were formedin media containing lower 2, 4-D concentrations. Transfer ofembryoids to half-strength MS medium with 6 per cent sucroseestablished plantlets which were successfully transferred tosoil. Saccharum officinarumL, sugarcane, suspension culture, embryogenesis, regeneration     

Action Potentials Evoked by Light in Traps of Dionaea muscipula Ellis

At low light intensities (less than 50 µmol m^–2s^–1) illumination evokes transient depolarization of membranepotential in mesophyll cells of the leaf-trap of Dionaea muscipulaEllis. Darkening causes hyperpolarization approximately symmetricto the response to illumination. The amplitude as well as therate of potential changes depend on light intensity. After exceedinga definite threshold (usually between 50 and 80 µmol m^–2s^–1)the depolarization plays the role of a generator potential andan all-or-none action potential (AP) is released. Switchinglight off in a depolarization phase of an AP does not changeits shape and the amplitude. When the light intensity is increasedto 80–150 µmol m^–2 s^–1 a single lightstimulus triggers two successive APs. The time interval betweenthe two APs decreases with increasing stimulus strength andreaches the minimum between 300 and 400 µmol m^–2s^–1. At higher light intensities the interval increasesagain, and finally only a single AP is triggered. It was shownthat the effect was evoked by light but not by temperature changeaccompanying illumination. An inhibitor of the photosyn-theticelectron transport chain, DCMU, blocked the generator potentialsmediating between light absorption and APs. Residual responsesto light stimuli in plants treated with DCMU had reverse polarityand strongly reduced the amplitudes. (Received September 16, 1997; Accepted January 16, 1998)     

The phycocyanin to chlorophyll {alpha} ratio and other cell components as indicators of nutrient limitation in two planktonic cyanobacteria subjected to low-light exposures

The cell composition of the planktonic cyanobacteria, Oscillatoriaagardhii (Gomont) and Oscillatona redekei (van Goor), was comparedfor cultures grown under nitrogen (N) and phosphorus (P) limitation,and light climates which were energy (E) limited (photoperiods3:21 and 6:18 light:dark (LD) and irradiances 12–153 µmolm^–2s^–1). Increases in carbohydrate/protein ratio(CHO/Prot) and declines in chlorophyll a (Cha) and phycocyanin(PC) resulted from N and P limitation. N-, P- and E-limitedcultures could be distinguished on the basis of P content andthe ratio of PC/Cha. The P content range of 0.1–0.55%of ash-free dry weight (AFDW) for P-limited cultures was lowerthan that for N- and E-limited cultures (0.56–2.2 %AFDW).Cultures limited by N were distinguishable from E-limited cellsby lower PC/Cha ratios, ranging from 0 to 4.08, compared to3.9–6.9 for E-limited cells. Under the 3:21 LD cycle,the minimum PC/Cha ratio of E-limited cells was 4.5. Increasesin the CHO/Prot ratios were proportional to the difference betweenthe nutrient-limited growth rate and the non-nutrient-limitedgrowth rate. A comparison of the composition of the two speciesshowed greater accumulation of carbohydrate by O.agardhii undernutrient-limiting conditions, but that O. redekei had higherlevels of protein, chlorophyll a and phycocyanin and, in theabsence of P limitation, higher P contents than O.agardhii.     

The Effect of Inorganic Nutrients on the Hormonal Requirements of Normal, Habituated, and Crown-gall Tissue 4

The addition of Braun and Wood's inorganic supplements (845mg l^–1 KCl, 1800 mgl^–1 NaNO₃, 300 mg l^–1 NaH₂PO₄.2H₂O,790 mg l^–1 (NH₄)₂SO₄) to White's medium caused markedincreases in the growth of normal tissues of Helianthus annuus,Nicotiana rustica, Daucus carota, and Vinca rosea and crown-galltumour tissues of H. annuus. However, no evidence was obtainedwhich suggested that the presence of these extra salts markedlyinfluenced the essential requirements of normal callus for auxinsand kinetin. In contrast their presence significantly influencedthe hormonal requirements of certain habituated cultures ofH. annuus and V. rosea. These habituated cultures had specificauxin requirements on White's medium while either an auxin orkinetin was sufficient on high-salts medium. These results arediscussed in relation to previous reports which suggested thatthe biosyntheses of auxins and other growth factors in normaland crown-gall cultures are specifically activated by certaininorganic ions.     

Effects of Growth Regulators and Glutamine on In Vitro Development of Zygotic Embryos of Taro (Colocasia esculenta var. antiquorum)

Zygotic embryos of taro, Colocasia esculenta var. antiquorumcultured on Linsmaier-Skoog (LS) medium without the additionof hormones develop into mature plants only in the presenceof endosperm tissue. Growth is usually evident within the firstweek of culture when embryos swell and become green. Embryosexcised from endosperm and cultured on LS containing 0-01 mg1^–1 naphthaleneacetic acid (NAA), and 0–01 mg 1^–16-dimethylaminopurine (6-DMAP) grow at a rate comparable withcontrols for the first week of culture. During the second week,growth rates are higher than controls primarily because embryosform elongated hypocotyl regions which often produce swollentissues and/or callus. In the presence of 200 mg 1^–1 glutamineand a range of concentrations of 6-dimethylaminopurine, benzyladenine,or NAA, elongation of the hypocotyl axis is inhibited, and acompact callus may develop. Embryos grown on LS containing 200mg 1^–1 glutamine and 2.0 mg 1^–1 2, 4, 5-trichlorophenoxyaceticacid form friable callus which was used to generate short-livedsuspension cultures. Growth Regulators, Glutamine, tygotic embryos, Colocasia esculenta, endosperm     

The productive and optical status of the oligotrophic waters of the Southern Aegean Sea (Cretan Sea), Eastern Mediterranean

Primary production, pigment concentrations and spectral measurementsof downwelling irradiance were made at four stations in fourseasons (spring, summer, autumn, winter) during 1994 in thewaters of the South Aegean Sea (Cretan Sea), Eastern Mediterranean.Rates of production were determined using in Situ incubationtechniques and included measurements at the surface microlayer.Depth-integrated values averaged over season were 5.66 mg Cm^–2 h^–1 for primary production and the correspondingchlorophyll (Ch1) a and phaeophytin (Phaeo) a values had meansof 4.87 and 1.21 mg m^–3 respectively. The assimilationratio remained very low (mean over season: 1.19 mg C mg^–2Chl a h^–1 as did the Phaeo a/Chl a ratio (mean over season:0.24). The annual production for the area was estimated to yield24.79 g C m^–2 year^–1. Primary production and Chla estimates showed statistically significant seasonal, spatialand depth variations. The spectral values of the attenuationcoefficient Kd (     

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     

Hormonal Control of Xylogenesis in Pith Parenchyma Explants of Lactuca

The time course of xylem differentiation was determined in explantsof lettuce pith parenchyma (Lactuca sativa L. cv. Romana) culturedon Murashige and Skoog (1962) medium using different concentrationsof auxin (IAA) and one cytokinin (zeatin or kinetin). Increasinglevels of auxin from I mg 1^–1 to 15 mg 1^–1 in thepresence of a constant level of a cytokinin (zeatin or kinetin)yielded up to 10 mg 1^–1 IAA, an increase in the numberof tracheary element formations. Cytokinin concentrations aboveand below o.1 mg 1^–1 interacting with an optimal xylogenicamount of auxin inhibited xylogenesis. The IAA (10 mg 1^–1)-zeatin(0.1 mg ^1–1) treatment produced the greatest number oftracheids, while kinetin compared to zeatin did not producesuch an effect. The different effectiveness of zeatin and kinetinin inducing tracheary element formations was not due to a differentcapacity of the two cytokinins to stimulate cell division butit seems likely that zeatin, because of interaction with IAA,is more active than kinetin in the determination of the dividingcells in a specific type of cytodifferentiation. The IAA (10mg 1^–1)-zeatin (0.1 mg 1^–1) treatment produced about6.9 per cent tracheids with respect to cell division while IAA(10 mg 1^–1)-kinetin (0.1 mg 1^–1) produced 4.2 percent. These results are discussed with reference to the problemsof hormonal control of xylem differentiation.     

Differentiation of Shoot Buds in vitro in Tissue 1Sisymbrium irio L.

Shoot bud formation was induced in the stem callus of Sisymbriumirio L., a Cruciferous plant. The callus was established onMurashige and Skoog medium with IAA (1?0 mg l^–1) and kinetin(0?5 mg l^–1). The effect of three purines (kinetin, 6-benzylaminopurine,and 6-methylaminopurine) incorporated singly along with IAAin MS medium was investigated. It was found that kinetin orMAP (3–5 mg l^–1) along with IAA (0?5 mg l^–1)were the most effective in inducing shoot bud formation. Adeninesulphate (10 mg l^–1) with kinetin (1?0 mg l^–1) alsoinduced bud differentiation. The morphogenetic potential of the callus to differentiate shootbuds was seemingly lost in 2 year old callus cultures. However,on successively subculturing on a regeneration medium shootbuds differentiated and the number of buds formed improved onfurther subculture. Two types of meristematic outgrowths were recognized: (i) arisingfrom superficial cells and (ii) arising from deep-seated cellsin the vicinity of tracheidal elements. However, both typesformed meristematic nodules on the surface of which shoot budsdifferentiated. Some embryoids were also recognized arisingsuperficially.     

Some Physiological Responses of Chlorella pyrenoidosa to 2,4-Dichlorophenoxyacetic Acid

An 18-h treatment of synchronously-grown Chlorella pyrenoidosawith 2,4-D did not significantly alter the size, dry weight,degree of synchrony, or pigment content of the cells, nor weredetectable quantities of ethylene produced. When Chlorella pyrenoidosawas treated with 5?10^–4 M 2,4-D, there was a statisticallysignificant stimulation of both net oxygen uptake and productionwhile 5?10 M 2,4-D inhibited both processes. When Chlorellapyrenoidosa was treated with 5?10^–4 M and 5?10^–3M 2,4-D, significantly greater amounts of glycollate were presentin the culture medium, even though an assay for glycollate dehydrogenaseshowed that the activity of this enzyme from 2,4-D-treated Chlorellapyrenoidosa was three times greater than in control cells. Looselybound 2,4-D was partitioned from a nonaqueously isolated chloroplastfraction, while other cell fractions failed to show detectablequantities of 2,4-D. It is postulated that in Chlorella pyrenoidosathe chloroplast is a target for 2,4-D action and that interferencein photorespiratory processes may underlie the observed responses.     

Size-fractionated primary production studies in the vicinity of the Subtropical Front and an adjacent warm-core eddy south of Africa in austral winter

Results are presented of size-fractionated primary productionstudies conducted in the vicinity of the Subtropical Front (STF),an adjacent warm-core eddy, and in Sub-antarctic waters duringthe third South African Antarctic Marine Ecosystem Study (SAAMESIII) in austral winter (June/July) 1993. Throughout the investigation,total chlorophyll (Chl a) biomass and production were dominatedby small nano- and picophytoplankton. No distinct patterns intotal Chl a were evident. At stations (n = 7) occupied in thevicinity of the STF, total integrated biomass values rangedfrom 31 to 53 mg Chl a m^–2. In the vicinity of the eddy,integrated biomass at the eddy edge (n = 3) ranged from 24 to54 mg Chl a m^–2 and from 32 to 43 mg Chl a m^–2 inthe eddy (n = 2). At the station occupied in the Sub-antarcticwaters, total integrated biomass was 43 mg Chl a m^–2.Total daily integrated production was highest at stations occupiedin the vicinity of the STF and at the eddy edge. Here, totalintegrated production ranged from 150 to 423 mg C m^–2day^–1 and from 244 to 326mg C m^–2 day^–1, respectively.In the eddy centre, total integrated production varied between134 and 156 mg C m^–2 day^–1. At the station occupiedin the Sub-antarctic waters, the lowest integrated production(141 mg C m^–2 day^–1) during the entire survey wasrecorded. Availability of macronutrients did not appear to limittotal production. However, the low silicate concentrations duringthe survey may account for the predominance of small nano- andpicophytoplankton. Differences in production rates between theeddy edge and eddy core were related to water column stability.In contrast, at stations occupied in the vicinity of the STF,the control of phytoplankton production appears to be relatedto several processes, including water column stability and,possibly, iron availability.