Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Growth of Ceratium hirundinella in a subtropical Australian reservoir: the role of vertical migration

A study into the photophysiology, growth and migration of Ceratiumhirundinella in Chaffey Reservoir in subtropical northern NewSouth Wales, Australia, revealed that a proportion of cellsformed subsurface accumulations at depths that optimized lightintensity (212–552 µmol photons m^–2 s^–1)for photosynthesis and cell growth. At high incident irradiance,Ceratium migrated downwards from the near-surface waters, avoidinghigh-light-induced, slow-recovering non-photochemical quenchingof photosystem II. Overnight deepening of the surface mixedlayer by convective cooling produced homogeneous distributionsof Ceratium with a significant proportion of the populationbelow the depth where light saturation of photosynthesis occurred.Ceratium migrated towards the surface from suboptimal lightintensities, at a velocity of 1.6–2.7 x 10^–4 m s^–1.Subsurface accumulations occurred under a variety of turbulenceintensities; however, accumulation was significantly reducedwhen the turbulent velocity scale in the mixed layer was >5x 10^–3 m s^–1, beyond which turbulent diffusion dominatedadvection by swimming. The formation of subsurface accumulationswith increased computed water column integral photosynthesisby 35% compared to a uniform cell distribution.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

Population structure and swarm formation of the cyclopoid copepod Dioithona oculata near mangrove cays

The cyclopoid copepod Dioithona oculata forms swarms in water>30 on deep among prop roots of red mangroves (Rhizophoramangle) which fringe protected areas of two lagoonal cays, TwinCays, Belize. During 7 of 8 months surveyed by in situ observation,swarms were present but differed in size from small cylindricalswarms (5–10 cm diameter) to bands extending up to 1200m Swarms were never observed at night Swarms formed at dawnwhen light intensities reached an average value of 13.82 (logioquanta cm^–Abstract. s^–1) and dispersed at dusk atsimilar intensities Swarms observed in June formed earlier anddispersed later in the day than swarms observed in January,their swarming behavior followed seasonal changes in light intensityMean dioithonan density in swarms (10 ml^–1) was much higherthan the mean density (0 15 ml^–1) of non-swarming dioithonansaround mangrove prop roots. In open water 3–5 m away fromthe mangroves, mean dioithonan density was 7 9 x 10^–5ml¹ during the day, and 2 68 x10^–3 ml^–1 at nightSwarms were composed predominantly of adults and copepodid stagesIV and V, although younger copepodid stages could be presentNauplii were never present. The ‘average copepodid stage’for all 95 swarms sampled was 5 3, where 6 0 represents a swarmwith only adults In open water 3–5 m away from the mangroves,the youngest copepodids (stage one) dominated the dioithonanpopulation during the day. At night when swarms dispersed toopen waters, average copepodid stage was higher (3 5) comparedwith the day value (1.2) in open waters. Although densitiesin swarms were higher in June than January, average copepodidstage in June was higher (5 6) than that in January (4.9). Ahigher percentage of adults were females during June than January.Therefore higher densities did not result from increases ofsmaller stages in swarms, but perhaps changes in behavior orpopulation structure.     

Bacterivory in seawater samples estimated by a dual radioactive-labelling technique

A dual radioactive-labelled bacteria technique using Vibrio(DRLV), developed for laboratory studies on bacterivory, hasbeen refined for use at the concentrations of prey and predatorstypcially found at sea. Experiments with estuarine water collectedin spring and in autumn showed that bacterivorous nanoflagellates(HNF) (concentration 1.38±0.35x10³ HNF ml^–1) ingested2.7±0.96 DRLV flagellate1^–1 h^–1 at concentrationsof 0.8–2.2x10⁶ DRLV ml^–1 in the presence of 2.04±0.68x10⁶natural bacteria ml^–1. The method was also applied tosamples collected in October in the Celtic Sea, when on average1 ml of water from the surface layer contained 1.41±0.16x10⁶natural bacteria, 14.6x10³ cyanobacteria, 530±170 HNF,7.3±3.0x103 phototrophic nanoflagellates (1.5–4µm), 49.0±26.5 phototrophic dinoflagellates, 36.3±12.6heterotrophic dinoflagellates and 21.3±9.5 Leucocryptosmarina. Under these conditions the grazing rate in most samplesdid not exceed the coefficient of variation of the method (2%),although we estimate the grazing rate was -1.6 DRLV HNF^–1h^–1 and on one occasion a rate of 2.45 was recorded. Thegross growth efficiency for protein of -30% displayed by naturalHNF means that they could release about     

Studies on the Movement of Water Through Apple Trees

Resistances to the flow of water through young potted appletrees were estimated by measuring the transpiration rate oftrees with and without root systems. Root system resistanceswere obtained by difference. Whole-plant resistances were ofthe order 10 x 10¹³ Pa s m^–3 and there was some evidencethat root resistances (R_r) varied with transpiration rate; theratio R_r:R_x (where R_x is resistance to water flow in the stemsystem) altered from 2:1 at relatively high transpiration ratesto 1:1 at lower rates. The trunk of a 9-year-old orchard tree (trunk diameter {smalltilde}7 cm, height {small tilde}2.5 m) was cut under water andestimates of the flow resistances in this tree were obtained.These were much lower than the resistances to flow in the pottedtrees. Capacitance (defined as the change in stored water content perunit change in plant water potential) values were calculatedfor the small trees and the large tree from measurements ofweight and water potential changes after the trees were removedfrom water. They were very similar on a weight basis (approx.2.0 x 10^–8 kg kg^–1 Pa^–1). Leaf capacitancevalues ({small tilde}1 x 10^–8 kg Pa^–1 m^–2)were also obtained. Stomatal conductances decreased with water potential and increasedwith short-wave radiation, but the relationships were not definitive.Estimates of boundary layer conductance in a greenhouse (verylow wind speeds) were of the same order ({small tilde}5 mm s^–1)as values obtained previously.     

Zooplankton fecal pellet fluxes and vertical transport of particulate organic material in the pelagic environment

Fecal pellet fluxes were determined using a series of multireplicatetraps set at 35, 65, 150, 500, 750 and 1500 m in the northeastPacific. Fecal pellets appear to be important contributors to total carbonfluxes. In near-surface waters (35–150 m), pellet fluxesranged from 2–3 x 10⁵ pellets m^–2 day^–1. Minimumpellet fluxes were observed at 500m({small tilde}0.4x 10⁵ pelletsm^–2day^–1). In contrast, the 1500 m pellet flux valueincreased to approximately 0.8 x 10⁵ pellets m^–2 day^–1relative to the 500 m depth, and is probably the result of insitu repackaging. Analyses of pellet content suggest multiplesources of "large" particle input throughout the water column.The implications of this phenomenon are discussed in terms ofthe detrital rain and ladder of migration theories.     

Temporal variability of diversity and biomass of tintinnids (Ciliophora) in a southwestern Atlantic temperate estuary

Seasonal variations in diversity and biomass of tintinnids (Ciliophora:Tintinnida) were investigated at two fixed stations in the innerpart of the Bahía Blanca Estuary (38°42' S, 61°50'W) during an annual cycle. The variations were analysed in relationto surface temperature, salinity, transparency, solar radiationand chlorophyll a (Chl a)concentration. Biomass was calculatedin terms of biovolume and carbon units. Diversity was estimatedas the number of species and the Shannon Index (H', ln based).Density of tintinnids ranged from 100 to 7800 individuals L^–1H' ranged from 0 to 1.81. The biomass varied from 0.3 to 127.78x 10⁶ µm³ L^–1 (0.02–39.4 µg C L^–1).Density was significantly related to temperature, solar radiationand Secchi distance (P < 0.01); diversity was significantlyrelated to temperature (P < 0.01) and solar radiation (P< 0.05). Biomass was significantly related only to temperature(P < 0.01) in one of the stations. According to principalcomponents analysis (PCA) tintinnids exhibited a segregationof three groups: winter, spring–summer and autumn forthe most internal station and winter, spring and summer–autumnfor the most external station. H' values were lower than thoseobserved in other coastal systems found at about the same latitudein the northern hemisphere.     

The Osmotic Pressure of Concentrated Solutions of Polyethylene Glycol 6000, and its Variation with Temperature

The vapour pressures of aqueous solutions of polyethylene glycol6000 have been measured (by equilibration with sucrose solutions)up to the saturation point at 25 °C (1.45 g g^–1 water).The reduced-osmotic-pressure (/c), when plotted versus concentration(c), rapidly and linearly increased up to a concentration ofabout 0.8 g g^–1 (crossing the similar plot for sucrose).Above this concentration, the reduced-osmotic-pressure rosemore slowly, but still more rapidly than sucrose. The maximumosmotic pressure achieved at saturation was nearly 18 MPa. Usingthe virial equation: /c= RT/M + RTA₂c, the calculated secondvirial coefficient (A₂) for the linear part is 4.5 x 10^–3mol g^–1, a value slightly greater than most literaturevalues at 25 °C. Data are cited showing that A₂ varies linearlyfrom 5–6 x 10^x3 at 0 °C, to zero at 80–90 °C     

Spatial and temporal distribution of mesozooplankton in the Gulf of Aqaba and the northern Red Sea in February/March 1999

The distribution pattern, taxonomic composition and communitystructure of mesozooplankton was studied along a transect with10 positions between the Gulf of Aqaba and the northern RedSea. Five positions were resampled two or three times duringa cruise of RV ‘Meteor’ in February/March 1999.In spite of clear differences in the density stratificationbetween the Gulf of Aqaba and the northern Red Sea, the mesozooplanktoncomposition was very similar: Copepods were by far the mostabundant taxon, contributing 76–95% to the total community.The remainder was composed largely of ostracods, chaetognaths,appendicularians and molluscs. The mesozooplankton of the deeplymixed stations was homogeneously distributed, at all other stationsthe bulk of the mesozooplankton (>70%) was concentrated inthe mixed surface layer with peaks of calanoids, cyclopoidsand appendicularians in the vicinity of the chlorophyll a (Chla) maximum layer. Ostracods and poecilostomatoids dominatedthe layers below. Standing stocks within the total water column(550–1200 m) varied between 93 and 431 x 10³ individualsm^–2 for copepods and 5–76 x 10³ individuals m^–2for other mesozooplankton with highest numbers in the northernGulf of Aqaba, where vertical mixing was deep (400–500m) and Chl a and mesozooplankton distributions homogeneous throughoutthe water column. Towards the south, the mixed depth decreasedfrom 300 m in the central Gulf of Aqaba to 50 m in the Red Sea.Cluster analysis separated three distinct groups of stations,compounding the observed differences between the northern Gulfof Aqaba (Position I) and the other positions. The analysisalso revealed temporal differences between the February andMarch sections of the cruise, indicating the winter–springtransition. The stations sampled in March are characterisedby a higher total abundance and by a higher percentage of appendiculariansand ostracods than the stations sampled in February     

Effects of Colchicine, Vinblastine, Cytochalasin B and Phalloidin on the Seismonastic Movement of Mimosa pudica Leaf and on Motor Cell Ultrastructure

Fleurat-Lessard, P., Roblin, G., Bonmort, J. and Besse, C. 1988.Effects of colchicine, vinblastine, cytochalasin B and phalloidinon the seismonastic movement of Mimosa pudica leaf and on motorcell ultrastructure.—J. exp. Bot. 39: 209–221. Colchicine at 1 x 10^–3 mol dm^–3 does not affectthe seismonastic movement of Mimosa pudica leaves but disruptsmicrotubules in motor cells. Vinblastine at 5 x 10^–3 moldm^–3 does not affect this movement and partly disruptsmicrotubules. Vinblastine at 1 x 10^–4 mol dm^–3 alwaysdisrupts microtubules, even after a 12 h reversibility whenthe movement is restored. These drugs, applied at the same respectiveconcentrations, do not alter cytoplasmic and vacuolar fibrils.Cytochalasin B and phalloidin alter the seismonastic movementof Mimosa leaves when applied at concentrations of 1.25 x 10^–3and 2.4 x 10^–4 mol dm^–3 respectively. These drugs,used at the same respective concentrations, also affect themotor cell structure and, in particular, modify the arrangementand the structure of the fibrils but they do not destroy themicrotubules. These data suggest that microtubules are not directly involvedin the seismonastic reaction whereas fibrils, formed by thin(3.0 nm wide) filaments, may be implicated in this reaction. Key words: Colchicine, cytochalasin B, phalloidin, Mimosa pudica, motor cells, vinblastine     

L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake

The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H⁺ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The K_m values of thethree affinity sites were 1.4 x 10^–3 M, 1.3 x 10^–4M, 4.3 x 10^–5 M; the maximum velocity values were 3.3x 10^–8, 4.5 x 10^–9, 1.8 x 10_–9 mol/10 min/4x 10⁶ cells. (Received April 18, 1981; Accepted August 25, 1981)     

The Effect of Morphactin (Methyl 2-Chloro-9-hydroxyfluorene-9-carboxylate) on Basipetal Transport of Indol-3-ylacetic Acid in Hypocotyl Sections of Phaseolus vulgaris L.

The effect of morphactin (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate)on basipetal transport of auxin (Indol-3-ylacetic acid-2-¹⁴C)was studied in bean (Phaseolus vulgaris) hypocotyl with thedonor-receiver block method. Morphactin (5 x 10^–6m) reduced IAA (5 x 10^–6m) transportintensity by an average of 83 per cent and auxin transport capacityby 90 per cent, but transport velocity was not affected. Morphactin did not inhibit uptake of IAA into hypocotyl tissue,but it did prevent transfer of IAA from the tissue into receiverblocks. Chromatographic analysis of the tissue after 4 h IAA-2-¹⁴Ctransport showed that 54 per cent of the total activity wasin the form of IAA in the control and 42 per cent in the morphactintreated tissue. No difference was found in the rate of decarboxylationof IAA-1-¹⁴C between control and morphactin treated tissue sections.Nor could any difference between control and morphactin be shownin the radioactivity associated with a TCA ppt fraction. Ina study of the transportable auxin pool, morphactin decreasedthe size of the pool and increased the half-life of decay ofauxin transport from 1•22 h to 3•85 h. In a kineticanalysis of the reversal of morphactin (5 x 10^–6m) inhibitionby increasing concentration of IAA-2-14C (5 x 10^–6m to2 x 10^–5m), it was shown that IAA transport resemblesMichaelis-Menten enzyme reaction kinetics, and that inhibitionby morphactin fitted a ‘mixed type’ model. IAA hada dissociation constant of 8•5 x 10^–6m and morphactinthat of 4•3 x 10^–7m with a K_m for the transport processof 8•5 x 10^–6m.     

Cyst formation: an important mechanism for the termination of Scrippsiella trochoidea (Dinophyceae) bloom

A sediment trap study was conducted at Daya Bay, South ChinaSea, to investigate the relationships between encystment andpopulation dynamics of Scrippsiella trochoidea from December1999 to January 2001. A dense bloom of S. trochoidea occurredduring the study period from August to September 2000, withthe maximum cell number of 3.18 x 10⁴ cells mL^–1.Two morphotypes of cysts, one with a thick calcareous wall (calcifiedcyst) and another without the obvious calcareous cover (non-calcifiedcyst), were observed during this investigation. The morphologicaland excystment characteristics of these two cyst types werestudied as well. Mass encystments of S. trochoidea, with themaximum of 3.05 x 10⁵ cysts m^–2 d^–1for calcified cyst, and 1.54 x 10⁷ cysts m^–2 d^–1for non-calcified cyst, coincided with the maximum abundanceof the vegetative cells. Encystment caused the transfer of atotal of 2.24–4.49 x 10⁸ cells m^–2 vegetativecells from the water column to the sea bottom during the bloomand resulted in a considerable loss of the bloom population.High assemblages of cysts of S. trochoidea were detected inthe surface sediments as well. This rich ‘seed bed’in the surface sediments caused by the high efficiency of encystmentafter blooms acting as a benthic reservoir for future vegetativepopulation, together with the short dormant period (15–26days) and high germination rate (50–90%), may explainthe repeated occurrence of S. trochoidea blooms in Daya Bay.     

Pyrosoma atlanticum (Tunicata, Thaliacea): grazing impact on phytoplankton standing stock and role in organic carbon flux

Pyrosomas are the large group of pelagic tunicates whose trophicrole in pelagic communities has not yet been sufficiently studied.We ran across a local area of high concentration of the mostwidespread and commonest species of pyrosomas, Pyrosoma atlanticum,450 miles off the Congo river mouth. The following was estimated:gut pigment content, defecation rate, organic carbon and pigmentcontent of fecal pellets, and sinking rate. Based on these dataand the measured number of pyrosomas colonies the grazing impacton phytoplankton and the fecal pellet flux were calculated.During the night swarms of 50–65 mm P.atlanticum removed53% of phytoplankton standing stock in the 0–10 m layer;sparsely distributed pyrosomas consumed only 4%. The grazingimpact in the 0–50 m layer was only 12.5 and <1% respectively.The fecal pellet flux resulting from nocturnal feeding of P.atlanticumwhile swarming made up 1.4–1.6 x 10⁶ pellets m^–210 h^–1 or 305–1035 mg C m^–2 10 h^–1 and1.4 x 10⁵ pellets m^–2 10 h^–1 or 87.4 mg C m^–210 h^–1 while non-swarming. Incubation experiments showedthe rapid degradation of fecal pellets at 23°C: the lossof pigment and carbon content was {small tilde}60–70%after 45 h. We believe that given the sinking rate of 70 m day^–1the main part of fecal material does not leave the upper watercolumn and is retained in the trophic web of the epipelagiclayer.     

Abundance and productivity of heterotrophic nanoplankton in Georgia coastal waters

The population abundances and rates of biomass production ofheterotrophic nanoplankton (HNAN) in Georgia coastal waterswere evaluated by epifluorescence microscopy. HNAN populations(mostly non-pigmented microflagellates <10 µm in diameter)ranged from 0.3 x 10³ cells ml^–1 in shelf waters 15 kmoffshore to 6.3 x 10³ cells ml^–1 in waters 0.25 km fromthe coast. There was a strong correlation (r = 0.83) betweenHNAN and free bacterioplankton population abundances, but noapparent relation (r = 0.38) between HNAN and phototrophic nanopLankton(PNAN) abundances. HNAN biomass production in estuarine andnearshore shelf waters, as estimated from increases in HNANpopulations during laboratory incubations of natural water samples,ranged from 0.10 to 0.79 mg C m^–3 h^–3, with populationgeneration times of 9.7 to 26.5 h. There was a significant linearrelation (r = 0.95) between HNAN biomass and HNAN productivity.We calculated that HNAN may graze at least 30% to 50% of dailybacterioplankton production in Georgia coastal waters.     

Fluxes of diatoms in the Dona Paula Bay, west coast of India

Sediment traps were deployed at a station in the Dona PaulaBay to collect sedimenting particles at weekly intervals fromNovember to May during 1995–1997. Sedimented particleswere analysed for total diatom flux, chlorophyll a (Chl a) andparticulate organic carbon (POC). The highest diatom flux wasrecorded in April–May for both the years. Fluxes of diatomsvaried from0.6 x 10⁴ cells m^–2 day^–1 (November 1995)to 121.47 x 10⁴ cells m^–2 day^–1 (December 1996).In all, 19 diatom genera were identified in the sedimented material.Navicula, Nitzschia, Pleurosigma, Licmophora, Coscinodiscus,Rhizosolenia and Surirella were the most abundant genera inthe sedimented material throughout the sampling period. Meanflux of POC and diatom carbon was 251 and 0.39 mg C m^–2day^–1, respectively. The diatom carbon accounted for 0.15%of the POC flux. Mass flux of diatoms showed significant negativecorrelation with the concentration of nitrate and phosphate.This suggests that the nutrient concentration played an importantrole in influencing the sedimentation of diatoms at this coastalstation.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. II. The Effects of Abscisic Acid and Cytokinins

Guttation was used as a non-destructive way to study the flowof water and mineral ions from the roots and compared with parallelmeasurements of root exudation. Guttation of the leaves of barley seedlings depends on age andon the culture solution. Best rates of guttation were obtainedwith the primary leaves of 6- to 7-day-old seedlings grown onfull mineral nutrient solution. The growing leaf tissue becomessaturated with K⁺ below 1.5 mM K⁺ in the medium, whereas K⁺concentration in the guttated fluid still increases furtheras K⁺ concentration in the medium is raised. At 3 mM K⁺ averagevalues of guttation were 1.4–2.4 mm³ h^–1 per plantwith a K⁺ concentration of 10–20 mM; for exuding plantsthe flow was 4.2–7.6 mm³ h^–1 per plant and K⁺ concentration35–55 mM. Abscisic acid (ABA) at 10^–6 to 10^–4 M 0–2h after addition to the root medium increased volume flow ofguttation and exudation and the amount of K⁺ exported. Threeh after addition of ABA both volume and amount of K⁺ were reduced.There was an ABA-dependent increase in water permeability (Lp)of exuding roots shortly after ABA addition. Later Lp was decreasedby 35 per cent and salt export by 60 per cent suggesting aneffect of ABA on salt transport to the xylem apart from itseffect on Lp. Benzyladenine (5 x 10^–8 to 10^–5 M)and kinetin (5 x 10^–6 M) progressively reduced volumeflow and K⁺ export in guttation and exudation and reduced Lp. Guttation showed a qualitatively similar response to phytohormonesas found here and elsewhere using exuding roots. Hordeum vulgare L., barley, guttation, abscisic acid, cytokinins, benzyl adenine, kinetin     

Ageing of Cucumber and Onion Seeds: Phospholipase D, Lipoxygenase Activity and Changes in Phospholipid Content

Cucumber (Cucumis sativus L. cv. Mesa) and onion (Allium cepaL. cv. Rijnsburger Heldis) seeds were rapidly aged at 40 °Cand 74% relative humidity. Onion seeds were also slowly agedat 40 °C with 15% relative humidity for 11 months and onemore month at 28% relative humidity. Significant loss of totaland individual phospholipids was an early event during bothstorage treatments. With slow ageing of onion, loss of phosphatidylcholineoccurred several months before loss of viability and vigourwas detected. Phosphatidic acid, the lipid product of phospholipaseD action, increased during rapid ageing of both cucumber andonion. Phosphatidic acid was present in onion seeds before theageing treatments and its content remained unchanged in theslowly aged seeds. There was 1600 (cucumber) and 2000 (onion)times more phospholipase D activity (6 x 10⁵ and 2·9x 10⁵ nmol g^–1 d^–1 in cucumber and onion, respectively)in crude extracts from non-aged seeds than was required to accountfor the fastest fall in phospholipids (72, 372 and 144 nmolg^–1 d^–1 for cotyledons and radicles of cucumberand onion, respectively, over the first 9 d [cucumber] or 1d [onion] of ageing) and fastest increase in phosphatidic acid(7, 162 and 37 nmol g^–1 d^–1). How accurate a guidethe in vitro activity of phospholipase D was to the in vivoactivity was unclear. However, the considerable excess activityseen with the formation of phosphatidic acid supports the proposalthat hydrolysis of phospholipids by phospholipase D is a firststep in deterioration during ageing. Substantial lipoxygenaseactivity was also detected (58 x 10³ and 54 x 10³ nmol g^–1d^–1 respectively, for non-aged cucumber and onion seeds).However, the increase in conjugated dienes (an early productof peroxidation) in ageing cucumber seeds was comparativelysmall (90 nmol g^–1 d^–1 over 21 d ageing), and increasein malondialdehyde could not be detected, indicating that peroxidationmay not have been a major factor in cucumber. The increase inconjugated dienes during rapid ageing of onion seeds was larger(1·5 x 10³ nmol g^–1 d^–1 over days 0–2of rapid ageing), much greater than the decrease in phospholipidacyl groups (260 nmol g^–1 d^–1 over days 0–2of rapid ageing) indicating the occurrence of peroxidation offatty acids released from reserve as well as from membrane lipids.This higher level of conjugated dienes during onion ageing wasthe main difference between cucumber and onion, indicating thatthe level of peroxidation could be an important difference betweenfast and slow ageing seeds. However, peroxidation is not theonly possible deleterious process since hydrolysis of the normalmembrane phospholipids to phosphatidic acid increased the contentof non-bilayer-forming lipids and this too could be a directmembrane-destabilizing consequence of phospholipase D actionduring ageing. Key words: Cucumis sativus L. (cucumber), Allium cepa L. (onion), seed ageing, phospholipase D, lipoxygenase, phospholipids