Picoplankton dynamics in Davies Reef lagoon, the Great Barrier Reef, Australia

The diel variations in abundance and frequency of dividing cells(FDC) of coccoid cyanobacteria in a coral reef lagoon were investigatedin June, September and December 1989, and April 1990. Cyanobacteriaand picoplanktonic eukaryotes (<3 µm) were sampledmonthly from January to December 1990. The average abundancesof cyanobacteria and eukaryotes ranged between 1.17–10.0610⁴cells ml^–1 and 0.16–2.4110⁴ cells ml^–1, respectively,with abundances of both being higher in summer (November-April)than in winter (May-October). The ratio of cyanobacteria toeukaryotes fluctuated from 1.93 to 8.67, independent of theseasonal variation in their abundances. The instantaneous growthrate of cyanobacteria, which was estimated from the daytimeabundance increment, ranged between 0.430 and 3.144 day^–1The estimated daily specific growth rate of cyanobacteria bythe FDC method ranged between 0.231 and 0.966 day^–1. InApril, despite the high specific growth rate and low flushingconditions. cyanobacterial abundance showed a cyclic diel pattern,suggesting a strong grazing impact on their population.     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Relocalization of the V-ATPase B2 subunit to the apical membrane of epididymal clear cells of mice deficient in the B1 subunit

An acidic luminal pH in the epididymis contributes to maintaining sperm quiescent during their maturation and storage. The vacuolar H⁺ATPase (V-ATPase), located in narrow and clear cells, is a major contributor to luminal acidification. Mutations in one of the V-ATPase subunits, ATP6v1B1 (B1), cause distal renal tubular acidosis in humans but surprisingly, B1^–/– mice do not develop metabolic acidosis and are fertile. While B1 is located in the apical membrane of narrow and clear cells, the B2 subunit localizes to subapical vesicles in wild-type mouse, rat and human epididymis. However, a marked increase (84%) in the mean pixel intensity of B2 staining was observed in the apical pole of clear cells by conventional immunofluorescence, and relocalization into their apical membrane was detected by confocal microscopy in B1^–/– mice compared with B1^+/+. Immunogold electron microscopy showed abundant B2 in the apical microvilli of clear cells in B1^–/– mice. B2 mRNA expression, determined by real time RT-PCR using laser-microdissected epithelial cells, was identical in both groups. Semiquantitative Western blots from whole epididymis and cauda epididymidis showed no variation of B2 expression. Finally, the luminal pH of the cauda epididymidis was the same in B1^–/– mice as in B1^+/+ (pH 6.7). These data indicate that whereas overall expression of B2 is not affected in B1^–/– mice, significant redistribution of B2-containing complexes occurs from intracellular compartments into the apical membrane of clear cells in B1^–/– mice. This relocation compensates for the absence of functional B1 and maintains the luminal pH in an acidic range that is compatible with fertility. male reproductive tract; male fertility; luminal acidification; proton pump; vacuolar H⁺ATPase     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

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.     

Synthesis of an S-(2-Aminoethyl)-L-Cysteine Conjugate in S-(2-Aminoethyl)-L-Cysteine- Resistant Adenine-Auxotrophic Cells of Datura innoxia Mill

A line of S-(2-aminoethyl)-L-cysteine-resistant adenine-auxotrophiccells (Ad^–AEC^r strain) was isolated from adenine-auxotrophiccells (Ad^– strain) of Datura innoxia Mill by a stepwiseselection method. Ad^–AEC^r and Bl cells, which were clonedfrom the original Ad^–AEC^r cells, were able to grow activelyon medium that contained 10 mM S-(2-aminoethyl)-L-cysteine (AEC),whereas the growth of Ad^– cells ceased completely in thepresence of 0.5 mM AEC. The resistant phenotype has been maintainedfor at least 10 months in culture on medium without AEC. Levels of free lysine in Ad^–AEC^r and Bl cells were similarto that in Ad^– cells. By contrast, the level of free AECin Ad^–AEC cells was 10-fold lower than in Ad^– cellsand no free AEC was detectable in Bl cells. However, acid hydrolysisof extracts from Ad^–AEC^r and Bl cells resulted in a remarkableincrease in levels of detectable AEC. This result indicatesthat conjugated AEC is synthesized and accumulated in the AEC-resistantcells. The level of the AEC conjugate in Bl cells increasedwith increases in the concentration of AEC in the culture medium,while intracellular levels of AEC were so low as not to be detectablein the case of cells grown on medium supplemented with AEC atless than 1 mM. The AEC conjugate was also detected in Ad^–cells, but at lower levels than in the AEC-resistant cells.In addition, AEC was found to be incorporated into soluble proteinsin Ad^– cells. These results suggest that the resistance of AEC-resistant cellsof Datura innoxia is accomplished via acceleration of the synthesisof the AEC conjugate which prevents any increase in intracellularlevels of free AEC. ¹Present address: Institute for Biology and Chemistry, TsumuraCo.Ltd., Inashiki, Ibaraki, 300-03 Japan. ²Present address: North Kanto Shop, Sakata Seed Co. Ltd.,Saitama,347 Japan.     

The effect of detrital addition on the development of nanoflagellates and bacteria in Lake Kinneret

The effect of adding dissolved substrates derived from algalcells on the patterns of nutrient cycling and growth of bacteria,heterotrophic nanoflagellates (HNAN) and photoautotrophs wasdetermined in samples of near-surface waters from Lake Kinneret.Supplementation of substrates always resulted in an increasedpeak of HNAN numbers and had little effect on bacterial numbers.HNAN-mediated nutrient remineralization of nitrogen and phosphoruswas also stimulated. In light-incubated samples the remineralizednutrients were taken up by photoautotrophic cells. Maximum growthrates observed for HNAN ranged from 0.03 to 0.11 h^–1,clearance rates for bacteria 1.1–7.3 nl HNAN^–1 h^–1and remineralization rates 6.4–8.4 µg N mg dry wt^–1h^–1 and 0.37–0.99 µg P mg dry wt^–1 h^–1.     

Studies on the Growth in Culture of Plant Cells: X. FURTHER STUDIES ON THE CONDITIONING OF CULTURE MEDIA BY SUSPENSIONS OF ACER PSEUDOPLATANUS L. CELLS

The standard synthetic culture medium (Stuart and Street, 1969)has been modified by adjustment of its initial pH to 6.4 andby the addition of gibberellic acid (0.25 mg/l) and of a mixtureof 15 L-amino acids formulated from an analysis of the conditionedmedium. The minimum effective density for the growth of sycamorecell suspensions in the standard medium is 9–15 x 10³cells ml^–1, for the modified synthetic medium it is 2.0x 10³ cells ml^–1, and for conditioned medium 1.0–1.25x 10³ cells ml^–1. Using either conditioned medium (Stuart and Street, 1969) orthe modified synthetic medium it is demonstrated that the growthof cultures initiated at low density is enhanced by a volatilefactor released from actively growing cell suspensions. In presenceof conditioned medium and this volatile factor cultures canbe established from stationary-phase cells at a density of 6x 10² cells ml^–1. The volatile factor can be absorbedin 40 per cent w/v KOH but attempts to replace the factor byair containing carbon dioxide at concentrations up to 5 percent have so far been unsuccessful.     

Carbon and nitrogen content of Noctiluca scintillans in the Seto Inland Sea, Japan

The carbon and nitrogen content of Noctiluca scintillans cellsfrom the Seto Inland Sea, Japan was investigated in order toestimate its biomass in natural samples. The carbon contentof N.scintillans ranged from 123 to 627 ng C cell^–1 witha mean value of 353 ng C cell^–1, or 1.12 to 2.67 fg Cµm^–3 with a mean value of 1.98 fg C µm^–3.The nitrogen content ranged from 36.0 to 232 ng N cell^–1with a mean value of 131 ng N cell^–1, or 0.499 to 0.910fg N µm^–3 with a mean value of 0.694 fg N µm^–3.Total cell carbon and nitrogen increased but the carbon andnitrogen per cell volume decreased with increasing cell volume.The C/N ratio of the cells ranged from 2.3 to 4.4, which wasrelatively low compared with the Redfield ratio. The carbonand nitrogen content was extremely low (91.2 ng C cell^–1,41.8 ng N cell^–1) for starved cells, whereas it was extremelyhigh (528 ng C cell^–1, 205 ng N cell^–1) for cellswhich had ingested the large diatom, Coscinodiscus wailesii.Our results suggest that the carbon and nitrogen content ofN.scintillans varies depending on its physiological conditionand the type of food that it has recently consumed.     

Occurrence of viable photoautotrophic picoplankton in the aphotic zone of Lake Biwa, Japan

The distribution and abundance of photoautotrophic picoplankton(PPP. Synechococcus group) in the aphotic bottom sediments ofLake Biwa were investigated by direct counting and viable counting(most probable number, MPN) methods. In the surface layer ofbottom sediments (0–1 cm). where large PPP blooms occurredin the past 5 years, >10⁵ cells cm^–3 of PPP were foundto be viable throughout the year. Furthermore, the density ofPPP deposited on the sediment surface (0–0.1 cm) was oneorder of magnitude higher (MPN = 1.3 x 10⁶ cells cm^–3.direct count = 9.9 x 10⁶ cells cm^–3) than that of bulkedsurface sediments (0–1 cm). Even in the deeper layer (13–14cm) of bottom mud, viable PPP were still found (10¹ cells cm^–1.In winter, viable PPP in the aphotic bottom sediments were 10⁴–10⁵times greater per Unit volume than those in the euphotic lakewater. Since the aphotic bottom sediments have high levels ofPPP, as well as high growth potential (high ratio of viablecount/total direct count), they are likely to seed PPP bloomsin the North Basin of Lake Biwa.     

Bacterial biomass and production in a shallow bay

Bacterial biomass (BB, acridine orange) and bacterial secondaryproduction (BSP, [³H]thymidine incubations) were measured forthe first time in Concepción Bay (37°35'S, 73°01'W).BB ranged from 1.8x10⁶ to 22.5x10⁶ cells ml^–1 (the lattervalue observed near to an industrial effluent), BSP from 0.27x10⁶to 2.5x10⁶ cell ml^–1 day^–1 and heterotrophic turnoverrates between 0.15 and 1.0 doublings day^–1.     

Boron Nutrition of Cultured Tobacco BY-2 Cells: I. Requirement for and Intracellular Localization of Boron and Selection of Cells that Tolerate Low Levels of Boron

Cultured cells of tobacco (Nicotiana tabacum L. cv. Bright Yellow2) grown under the standard culture conditions (1 mg boron liter^–1medium as boric acid) contained boron at a concentration of2.26 mg boron kg^–1 oven-dried cells and the protoplastcontained 1.26% of the boron in the cells. The cells requiredboron for growth and the half-maximum growth rate was obtainedwith 0.056 mg of boron liter^–1 medium. Subculturing thecells in media with lower concentrations of boron allowed selectionof cells that can grow even in the presence of 1 µg boronliter^–1 medium. Cell walls of the selected cells seemedto be thicker than those of the control cells and Golgi bodieswere accompanied by more secretory vesicles than those in thecontrol cells. (Received May 25, 1992; Accepted September 10, 1992)     

Vertical and temporal heterogeneity of planktonic ciliated protozoa in a humic lake

Seasonal population dynamics and the vertical distribution ofciliates were studied in relation to the particular food resourcesoccurring in a humic and moderately acidic lake (Lake Vassivière).The abundance (1.4 x 10³–20.4 x 10³ cells l^–1 mean= 4.8 x 10³ cells l^–1) and biomass (0.5–34.6 µgC l^–1, mean = 6.0 µg C l^–1) of ciliated protozoawere low and close to values reported for oligotrophic environments.The species composition of the population varied greatly withdepth. Whereas large-sized species of oligotrichs, some of whichwere mixotrophic, dominated at the surface, haptorids were bestrepresented in deep waters. The spatial distribution of thevarious groups of ciliates was largely determined by light andthe vertical distribution of microbial food resources (detritus,bacteria, algae) within the water column of this brown-coloredlake.     

Internal Factors Regulating Nitrate and Chloride Influx in Plant Cells

The primary factor determining the observed decrease in activeC1^– influx during salt accumulation in carrot and barleyroot cells has been shown to be the concentration of C1^–+ NO₃^– in the vacuole. The relationship between C1^– influx and the vacuolar concentrationsof various substances was examined after the tissues had accumulatedions from various salt solutions. After accumulating K⁺ malate,C1^– influx was not reduced, but after accumulating C1^–or NO₃^– salts, C1^– influx was reduced by up to 90per cent. Considering all treatments, C1^– influx was notcorrelated with the vacuolar concentration of K⁺, Na⁺, (K⁺+Na⁺),reducing sugars, malate, C1^–, or NO₃^–, nor withthe cellular osmotic pressure. The correlation coefficient betweenCl^– influx and log (C1^– + NO₃^– concentrationin the vacuole) was highly significant, and accounted for allthe variation in C1^– influx in this experiment. Net NO₃^– influx is similarly reduced by a high C1^–concentration in the vacuole. External Cl^– and NO₃^–have quantitatively different, apparently competitive, effectson C1^– influx. These differ from the apparently negative-feedbackeffects of C1^– and NO₃^– in the vacuole, which arequantitatively similar. Decreasing the internal hydrostatic pressure by raising theexternal osmotic pressure increased active K⁺ influx in Valoniaventricosa, but had no effect on C1^– or K⁺ influx in carrotor maize root cells. Cl^– influx is not related to thereducing sugar concentration during ageing drifts in excisedcarrot root tissue. Acetazolamide did not inhibit C1^– influx to carrot tissue. The implications of this type of negative feedback regulation,and the relationship between C1^– and NO₃^– transportare discussed.     

Role of NBC1 in apical and basolateral HCO3- permeabilities and transendothelial HCO3- fluxes in bovine corneal endothelium

Corneal transparency and hydration control are dependent on HCO₃^– transport properties of the corneal endothelium. Recent work (13) suggested the presence of an apical 1Na⁺-3HCO₃^– cotransporter (NBC1) in addition to a basolateral 1Na⁺-2HCO₃^– cotransporter. We examined whether the NBC1 cotransporter contributes significantly to basolateral or apical HCO₃^– permeability and whether the cotransporter participates in transendothelial net HCO₃^– flux in cultured bovine corneal endothelium. NBC1 protein expression was reduced using small interfering RNA (siRNA). Immunoblot analysis showed that 5–15 nM siRNA decreased NBC1 expression by 80–95%, 4 days posttransfection. Apical and basolateral HCO₃^– permeabilities were determined by measuring the rate of pH_i change when HCO₃^– was removed from the bath under constant pH or constant CO₂ conditions. Using either protocol, we found that cultures treated with NBC1 siRNA had sixfold lower basolateral HCO₃^– permeability than untreated or siCONTROL siRNA-treated cells. Apical HCO₃^– permeability was unaffected by NBC1 siRNA treatment. Net non-steady-state HCO₃^– flux was 0.707 ± 0.009 mM·min^–1·cm² in the basolateral-to-apical direction and increased to 1.74 ± 0.15 when cells were stimulated with 2 µM forskolin. Treatment with 5 nM siRNA decreased basolateral-to-apical flux by 67%, whereas apical-to-basolateral flux was unaffected, significantly decreasing net HCO₃^– flux to 0.236 ± 0.002. NBC1 siRNA treatment or 100 µM ouabain also eliminated steady-state HCO₃^– flux, as measured by apical compartment alkalinization. Collectively, reduced basolateral HCO₃^– permeability, basolateral-to-apical fluxes, and net HCO₃^– flux as a result of reduced expression of NBC1 indicate that NBC1 plays a key role in transendothelial HCO₃^– flux and is functional only at the basolateral membrane. corneal endothelium; sodium bicarbonate cotransporter; small interfering RNA; bicarbonate transport     

Nitrate transport in intact wheat roots:II. Long-term effects of NO3- concentration in the nutrient solution on NO3- unidirectional fluxes and distribution within the tissues5

We have examined the long-term effects of NO₃^– concentrationson NO₃^– (¹⁵NO₃^–) fluxes and cellular pool sizesin roots of intact 30-d-old wheat (Triticum aestivum cv. Courtot)grown hydroponically. Compartmental analysis was performed understeady-state conditions at five different levels of NO₃^–concentration (from 0.1 up to 5 mol m^–3 taking into accountmetabolism and secretion into the xylem (Devienne et al., 1994).Nitrate and reduced nitrogen levels in the tissues were largelyindependent of external NO₃^– concentration although below1.5 mol m^–3 NO₃^–; concentration limited plant growth.In the chamber, marked diurnal variations in net uptake occurredand, in the light, higher NO₃^– concentrations yieldedhigher NO₃^– uptake rates. After transfer of the plantsto the laboratory, the increase in net uptake linked to elevationof NO₃^–; concentrations was even larger (from 0.1 to 8.8µmolh^–1 g^–1 FW) as a result of a marked increase (x10–11) in the unidirectional influx at the plasmalemmawhile NO₃^– efflux was less enhanced (x 4–5). Underthese conditions, influx into the vacuole was also higher (x2–4) while efflux from the vacuole was little affected(x 1–3). NO₃^– concentrations within the cell compartmentswere estimated under the clas sical assumptions. The vacuolarconcentration was a little modified by NO₃^– availabilitywhereas that in the cytosol increased from about 10 mol m^–3to about 20 mol m^–3 indicating that (1) the absolute valuefor the cytosol was high and (2) it displayed only a small increasedespite very large changes in NO₃^– fluxes. NO₃^–distribution within the cells did not seem to involve an activeaccumulation of NO₃^– in the vacuole. Key words: Wheat, ion transport, nitrate, ¹⁵N, compartmentation     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

Autotrophic and heterotrophic nanoplankton in the diet of the estuarine copepods Eurytemora affinis and Acartia bifilosa

The ingestion of autotrophic and heterotrophic nanoplanktonby two estuarine copepods, Eurytenora affinis and Acarith bifilosa,was measured in various environmental conditions using the incubationmethod and epifluorescence microscopy. Egg production of thespecies was also deter mined in order to estimate their carbonrequirements. Assuming a gross efficiency of egg productionof 0.3, nanoplanktonic carbon ingested always met the carbonrequirements suggesting that, most of the time, other carbonsources could be unnecessaly. Nanoplankton ingestion by A.bifilosa(from 128 to 1693 cells ind.^–1 h^–1) was dominatedby autotrophic forms (60–97%) and was seriously affectedby high (>100 mg l^–1 suspended particulate matter (SPM)concentrations. Nanoplankton ingestion by E.affinis (from 300to 1049 cells ind^–1 h^–1) was relatively stable incomparison, but this latter species seemed to switch its grazingpressure from autotrophic to heterotrophic forms when SPM concentrationsincreased. Thus, two copepod species, living in the same estuary,presented two different feeding behaviours, probably to maximizeenergy input per unit of energy expenditure. Such differencescould contribute to the spatial and seasonal segregation ofthese species which is usually observed.      

Laboratory study of the spawning rate of the calanoid copepod Centropages typicus: effect of fluctuating food concentration

The spawning rate of laboratory-reared Centropages typicus fedHymenomonas elongala increases with food concentration, up toa value of {small tilde}2800 µg C (16 500 cells) ml^–1.An alternation of a low food (1000 cells ml^–1) and highfood concentration (16 500 cells ml^–1) is not favourableto egg release when its periodicity is 1 or 2 days, whereasit may be of advantage if it is longer (3–6 days). Inthe latter case, Centropages typicus will benefit best fromthe rich food diet if this coincides with (or just follows)the last moult.     

Differential Responses of Buckwheat Leaf Cells to Growth Substances Stimulating Cell Division

Isolated buckwheat cotyledons form calli, roots or buds whencultured in an appropriate medium. A medium containing high2,4-D (5 mg 1^–1) and low KN (01 mg I^–1), which inducescallus formation, was found to stimulate cell division in thelayer between palisade and spongy parenchyma tissue after 72h. Low 2,4-D and low KN (01 mg I^–1 each), which stimulatesroot formation in buckwheat cotyledons, induces divisions primarilyin spongy parenchyma cells. In a high benzylaminopurine (10^–5M) and a low IAA (10^–6 M) medium, which favours bud induction,cell divisions were localized to the palisade layer. The differentialresponsiveness of leaf cells to various hormone treatments isdiscussed.