Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Photoadaptation in Antarctic phytopfankton: variations in growth rate, chemical composition and P versus I curves

The response of phytoplankton to variations in the light regimewas studied during the VULCAN and ACDA cruises in the Antarctic.Unenriched batch cultures of 12–19 days' duration reachedchl concentrations of 10–50 µg^–1 and exhibitedexponential growth rates, with the maximal rate being 0.41 doubl,day^–1. Ice edge algae exhibited maximum growth rates atphoton flux densities (PFD) of 30–100 µE m^–2S^–1and the growth rate was reduced by about 30% at 500–1000µE m^–2S^–1 The chl/C ratio ranged between 0.004and 0.018, with the lowest ratios at PFDs above 500 µEm^–2S^–1 chl/C ratios were also below maximum at PFDsbelow 40–50 µE m^–2S^–1 The C:N:P ratioswere close to the Redfield ratios; the Si/C ratio averaged 0.16(atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050in different culture senes. When thawed after having been frozenfor 10 days, shade-adapted cultures were in a much better conditionthan sun-adapted ones. P versus I data showed that the maximumassimilation number varied from 0.75 to 4.4 µg C (µgchl)^–1h^–1. It varied inversely with the chl/C ratio;therefore the maximum carbon turnover rate varied little betweensamples (0.024/0.035 h^–1). Low biomass communities exhibitedrelatively high values for (the initial slope of P versus Icurves), low values for 1_sat (160–330 µE m^–2S^–1),and they were susceptible to photoinhibition. In contrast, communitiesdominated by Odontella weissflogii exhibited low values for, a high value for I_sat (560 µE m^–2S^–1 andthey tolerated high PFDs. The photo-adaptational status of thephytoplankton in natural water samples is discussed relativeto the profile of water column stability and mixing processes.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Gibberellins and the Growth of Excised Tomato Roots: Comparison of gib-1 Mutant and Wild Type and Responses to Applied GA3 and 2S, 3S Paclobutrazol

Clones of excised roots of wild type tomato (Lycopersicon esculentum,Mill., cv. Moneymaker) and a near-isogenic GA-deficient mutant(gib-1/gib-1) were cultured in modified White's medium containing1.5% w/v sucrose. The linear elongation rate of the main axisof the gib-1 mutant was 40% less than that of the wild type.In addition, the main axis of the gib-1 mutant was thicker thanthat of the wild type but main axis volume growth was the samein both genotypes, indicating that the gib-1 allele was affectingthe orientation of root expansion. There was no evidence tosuggest that the gib-1 allele affected either the pattern ofemergence or the density of lateral roots. Elongation rate andthickness of gib-1 mutant roots were restored to those of thewild type by the addition of low concentrations (0.1–1.0µM) of gibberellic acid (GA3). These concentrations ofGA3 caused a slight reduction in extension growth of wild typeroots, indicating that endogenous GAs were not limiting elongationof normal roots in culture. The GA biosynthesis inhibitor, 2S,3S paclobutrazol, at 0.1 µM, significantly reduced elongationof wild type roots and this inhibition was counteracted by 0.1µM GA₃. It is concluded that the difference in growthbetween the gib-1 mutant and the wild type represented GA-dependentgrowth. Low concentrations of 2S, 3S paclobutrazol caused onlya small (5%) reduction in growth of the gib-1 mutant and thisgrowth inhibition was not reversed by GA₃. This observation,and the fact that gib-1 mutant roots grow in the absence ofadded GA₃, suggested that part of root growth was GA-independent.However, the possibilities that the gib-1 mutant is ‘leaky’and that paclobutrazol does not inhibit GA biosynthesis completelycannot be excluded. Key words: gib-1 mutant, gibberellic acid, Lycopersicon esculentum, 2S, 3S paclobutrazol, root growth     

Effect of irradiance and vapour pressure deficit On stomatal response to CO2 enrichment of four tree species

The stomatal response of seedlings grown in 360 or 720 µmolmol^–1 to irradiance and leaf-to-air vapour pressure deficit(VPD) at both 360 and 720 µmol mol^–1 to CO₂ wasmeasured to determine how environmental factors interact withCO₂ enrichment to affect stomatal conductance. Seedlings offour species with different conductances and life histories,Cercis canadensis (L.), Quercus rubra (L.), Populus deltoides(Bartr. ex Marsh.) P. nigra (L.), and Pinus taeda (L.), weremeasured in hopes of identifying general responses. Conductanceof seedlings grown at 360 and 720 µmol mol^–1 CO₂were similar and responded in the same manner to measurementCO₂ concentration, irradiance and VPD. Conductance was lowerfor all species when measured at 720 than when measured at 360µmol mol^–1 CO₂ at both VPDs ({small tilde}1.5 and{small tilde}2.5 kPa) and all measured irradiances greater thanzero (100, 300, 600,>1600 µmol m^–2 S^–2)The average decrease in conductance due to measurement in elevatedCO₂ concentration was 32% for Cercis, 29% for Quercus, 26% forPopulus, and 11% for Pinus. For alt species, the absolute decreasein conductance due to measurement in CO₂ enrichment decreasedas irradiance decreased or VPD increased. The proportional decreasedue to measurement in CO₂ enrichment decreased in three of eightcases: from 0.46 to 0.10 in Populus and from 0.18 to 0.07 inPinus as irradiance decreased from>1600 to 100 µmolm^–2 s^–1 and from 0.35 to 0.24 in Cercis as VPD increasedfrom 1.3 to 2.6 kPa. Key words: Stomatal conductance, CO₂ enrichment, irradiance, vapour pressure deficit     

Biochemical Limitations to Carbon Assimilation in C3 Plants--A Retrospective Analysis of the A/Ci Curves from 109 Species

Species-specific differences in the assimilation of atmosphericCO₂ depends upon differences in the capacities for the biochemicalreactions that regulate the gas-exchange process. Quantifyingthese differences for more than a few species, however, hasproven difficult. Therefore, to understand better how speciesdiffer in their capacity for CO₂ assimilation, a widely usedmodel, capable of partitioning limitations to the activity ofribulose-1,5-bisphosphate carboxylase-oxygenase, to the rateof ribulose 1,5-bisphosphate regeneration via electron transport,and to the rate of triose phosphate utilization was used toanalyse 164 previously published A/C_i, curves for 109 C₃ plantspecies. Based on this analysis, the maximum rate of carboxylation,Vc_max, ranged from 6µmol m^–2 s^–1 for the coniferousspecies Picea abies to 194µmol m^–2 s^–1 forthe agricultural species Beta vulgaris, and averaged 64µmolm^–2 s^–1 across all species. The maximum rate ofelectron transport, J_max, ranged from 17µmol m^–2s^–1 again for Picea abies to 372µmol m^–2 s^–1for the desert annual Malvastrum rotundifolium, and averaged134µmol m^–2 s^–1 across all species. A strongpositive correlation between Vc_max and J_max indicated that theassimilation of CO₂ was regulated in a co-ordinated manner bythese two component processes. Of the A/C_i curves analysed,23 showed either an insensitivity or reversed-sensitivity toincreasing CO₂ concentration, indicating that CO₂ assimilationwas limited by the utilization of triose phosphates. The rateof triose phosphate utilization ranged from 4·9 µmolm^–2 s^–1 for the tropical perennial Tabebuia roseato 20·1 µmol m^–2 s^–1 for the weedyannual Xanthium strumarium, and averaged 10·1 µmolm^–2 s^–1 across all species. Despite what at first glance would appear to be a wide rangeof estimates for the biochemical capacities that regulate CO₂assimilation, separating these species-specific results intothose of broad plant categories revealed that Vc_max and J_maxwere in general higher for herbaceous annuals than they werefor woody perennials. For annuals, Vc_max and J_max averaged 75and 154 µmol m^–2 s^–1, while for perennialsthese same two parameters averaged only 44 and 97 µmolm^–2 s^–1, respectively. Although these differencesbetween groups may be coincidental, such an observation pointsto differences between annuals and perennials in either theavailability or allocation of resources to the gas-exchangeprocess. Key words: A/C_i curve, CO₂ assimilation, internal CO₂ partial pressure, photosynthesis     

Ethylene Biosynthesis and Xylogenesis in Lactuca Pith Explants Cultured In Vitro in the Presence of Auxin and Cytokinin: The Effect of Ethylene Precursors and Inhibitors

The possible involvement of ethylene in the induction of xylemdifferentiation was studied in lettuce (Lactuca saliva L. cv.Romaine) pith parenchyma explants. The addition of the ethyleneprecursors L-methionine (0.25 µM), S-adenosylmethionine(25 µM) and 1-aminocyclopropane-l-carboxylic acid (0.01µM), or the ethylene-releasing agent 2-chloroethylphosphonicacid (1.0 µM), to a standard IAA-kinetin-containing mediumenhanced xylogenesis compared to control explants cultured inthe absence of these compounds. In the presence of the ethyleneinhibitors aminoethoxyvinylglycine, Co(NO₃)₂ and AgNO₃, xylogenesiswas inhibited. Inhibition of xylogenesis by aminoethoxyvinylglycine(75 µM), Co(NO₃)₂ (50 µM) and AgNO₃ (6.0 µM)was reversed by exogenous 1-aminocyclopropane-l-carboxylic acid(0.01 µM), 2-chloroethylphosphonic acid (5.0 µM)and L-methionine (0.25 µM), respectively. Ethylene productionby explants cultured on media containing L-methionine or 1-aminocyclopropane-l-carboxylicacid was greater than the biosynthesis of ethylene by explantscultured in the absence of these compounds. The incorporationof 2-chloroethylphosphonic acid into the culture medium resultedin higher rates of ethylene production compared to explantscultured on the IAA-kinetin medium. The presence of either aminoethoxyvinylglycineor Co(NO₃)₂ inhibited ethylene production by explants culturedon the IAA-kinetin medium. The data support the hypothesis thatethylene plays a positive role in the initiation of xylem differentiation. Key words: Xylogenesis, Differentiation, Ethylene, IAA, Kinetin, Lactuca sativa     

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 µM Zn²⁺, and 50 µM Cu²⁺. Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC₅₀ for ATP stimulation of internalization was 440 µM in saline containing 2 mM Ca²⁺ and 2 mM Mg²⁺, and 33 µM in low-Mg²⁺, nominally Ca²⁺-free saline. Overall, the results are most consistent with activation of a P2X₇ receptor by ATP^4–. However, low permeability to N-methyl-D-glucamine⁺ and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X₇ receptor activation. ATP stimulation of internalization occurs in Na⁺-free, Ca²⁺-free, or low-Mg²⁺ saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X₇ receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx. purinergic receptor; internalization; patch clamp     

Diurnal mixed layers and the long-term dominance of Microcystis aeruginosa

The population dynamics of the cyanobacterium Microcystis aeruginosain a hypertrophic, subtropical lake (Hartbeespoort Dam, SouthAfrica) were followed over 4.5 years. We examined the hypothesisthat M.aeruginosa dominated (>80% by volume) the phytoplanktonpopulation up to 10 months of each year because it maintaineditself within shallow diurnal mixed layers and was thus ensuredaccess to light, the major limiting resource. Wind speeds overHartbeespoort Dam were strong enough to mix the entire epilimnionthrough Langmuir circulations only 12% of the time. At othertimes solar heating led to the formation of diurnal mixed layers(z₁) that were shallower (<2 m) than the euphotic zone (z_cu;mean = 3.5 m, range: 0.45–8.4 m) while the seasonal mixedlayer (z_m) was always deeper than z_cu (range: 7–18 m).By means of its buoyancy mechanism M.aeruginosa maintained thebulk of its population within z₁, while non-buoyant speciessank into dark layers. Adaptation to strong light intensitywas implicated from low cellular chlorophyll a content (0.132µg/106 cells) and high I_k (up to 1230 µE m^–2s^–1). Ensured access to light, the post-maximum summerpopulations persisted throughout autumn and winter, despitesuboptimal temperatures, by sustaining low losses. Increasedsedimentation losses caused a sharp decline of the populationat the end of winter each year, and a short (2–3 months)successional episode followed, but by late spring M.aeruginosawas again dominant. The data from Hartbeespoort Dam point outthe importance of distinguishing between z_m and z₁, and showthe profound effect that the daily pattern of z₁ as opposedto the seasonal pattern of z_m, can have on phytoplankton populationspecies composition.     

Effects of some SH-inhibitors and EDTA on flowering in Lemna perpusilla 6746

Lemna perpusilla 6746, a short-day duckweed, flowered undercontinuous illumination on M-sucrose medium containing CuSO₄,AgNO₃ and HgCl₂, which are SH-inhibitors. The optimum concentrationsof CuSO₄, AgNO₃ and HgCl₂ were 5, 1 and 20 µM, respectively.Other metal ions tested were ineffective, but at least two otherSHinhibitors, potassium ferricyanide and iodoacetamide, alsoinduced long-day flowering at the concentrations of 0.1-1 µM. Adding 50 µM EDTA to the medium prevented the effect ofcupric ion, but not that of other SH-inhibitors. EDTA at 200µM induced some long-day flowering when added to a mediumwith no SH-inhibitors. It also permitted some flowering whenadded together with cupric ion, and accelerated flowering inthe presence of the other SHinhibitors listed above. EDTA andSH-inhibitor effects appeared to be additive. (Received May 25, 1973; )     

Experimental evaluation of herbivory in the ctenophore Mnemiopsis leidyi relevant to ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA

Herbivory of Mnemiopsis leidyi and its interactions with phytoplanktonand non-gelatinous zooplankton were examined in small-scalemicrocosm experiments. Clearance rates for M. leidyi incubatedwith phytoplankton were generally negative, but ranged up to4.5 1 ctenophore^–1 day^–1 when the large (80 µmø) diatom Ditylum brightwelli was offered as a food source.These highest ingestion rates would provide Mnemiopsis withonly 21 % of its daily carbon requirements for respiration.Mean shrinkage of M. leidyi was 8.2–51% when incubatedwith phytoplankton. Although M. leidyi neither fed activelyon phytoplankton, nor satisfied its nutritional needs on sucha diet, the chain-forming diatom Skeletonema costatum becameentangled in mucus strands and balls produced by M. leidyi inthe absence of zooplankton. Attachment onto mucus occurred atphytoplankton concentrations commonly observed in NarragansettBay and may be important in the formation of "marine snow" duringsummer M. leidyi pulses; phytoplankton sinking rate and the"package size" available to herbivores would also be affected.The experiments support our previous hypothesis based on fieldobservations in Narragansett Bay that M. leidyi indirectly regulatesphytoplankton abundance there during the summer bloom as a consequenceof predation on zooplankton. The extent to which M. leidyi influencedphytoplankton dynamics in the microcosms was dependent on therelative abundance and physiological state of the three trophiclevels. A food web diagram for M. leidyi is presented.     

Phytoplankton and zooplankton development in a lowland, temperate river

The longitudinal and seasonal patterns of plankton developmentwere examined over 2 years in a lowland, temperate river: theRideau River (Ontario, Canada). Following an initial decreasein phytoplankton and zooplankton biomass as water flowed fromthe headwaters into the Rideau River proper, there was an increasein chlorophyll a (chl a) and zooplankton biomass with downstreamtravel. At approximately river km 60, both phytoplankton andzooplankton reached their maximum biomass of 27 µg l^–1(chl a) and 470 µg l^–1 (dry mass), respectively.Downstream of river km 60, the biomass of both planktonic communitiesdeclined significantly despite increasing nutrient concentrationsand favorable light conditions. These downstream declines maybe due to the feeding activity of the exotic zebra mussel (Dreissenapolymorpha) which was at high density in downstream reaches(>1000 individuals m^–2). There was no evidence forlongitudinal phasing of phytoplankton and zooplankton, as increasesand decreases in chl a and zooplankton biomass appeared to coincide.Overall, chl a was best predicted by total phosphorus (R²=0.43),whereas zooplankton biomass was best predicted by chl a (R²=0.20).There was no evidence for significant grazing effects of zooplanktonon phytoplankton biomass.     

Characterization of Sulfate Transport in the Green Alga Chlorella ellipsoidea

A rapid induction of sulfate transport was observed in the greenalga Chlorella ellipsoidea during sulfur-limited growth. Bothaffinity and V_max increased about five-fold within 6 h of transferringcells from Bold's basal medium with 350 µM MgSO₄ to sulfur-deficientBold's medium. High affinity sulfate transport was induced within15 min and reached maximum rate within 3 h of transferring cellsto sulfur-deficient condition, indicating that a new, high-affinity-sulfatetransport system is induced by sulfur starvation in C. ellipsoidea.Eadie-Hofstee plots of initial rates of sulfate uptake indicatedthat the K of sulfur-starved cells was about 17 µM. Bothsulfur-starved and unstarved cells grown in air had a V_max of1.5 times higher than that of high-CO₂ grown cells. Sulfatetransport was completely inhibited by 30 µM CCCP or 800µMKCN both in the light and the dark but transport in the lightwas not inhibited by 20 µM DCMU. Treatment with 50 µMor 500 µM vanadate caused 50% inhibition of uptake. Therate of sulfate uptake in the dark was twice that in the lightand was stimulated by low pH. These results suggest that thesulfate transport system in C. ellipsoidea is operated by protonsymport across the plasmamembrane which is partially mediatedby P-type ATPase and that these systems depend exclusively onenergy derived from oxidative phosphorylation in the mitochondria. (Received June 28, 1995; Accepted August 8, 1995)     

A Comparison between the Uptake of Potassium by Plants from Solutions of Constant Potassium Concentration and during Depletion

A comparison was made between two methods of measuring the relationshipbetween the external [K⁺] and the flux of K⁺ into whole plantsof Lolium perenne and Raphanus sativus. The values of flux obtainedfrom solutions of 1.2 µM K⁺ held constant around the rootswere three and six times greater for Lolium and Raphanus respectivelythan the values obtained at the same concentration in a depletionexperiment in which the solutions, initially 100 µM K⁺,were depleted to below 1.2 µM K⁺ by plant uptake. In thedepletion experiment with Lolium, the flux was higher into plantsgrown at low [K⁺] than into plants grown at 100 µM eventhough [K⁺] within the plant was about the same for all groupsof plants. It is suggested that Lolium grown at low [K⁺] hasan efficient mechanism for K⁺ uptake which continues to operatefor some time after the plants have been transferred to a higherconcentration. With both species, K_m was 15–20 µMin the depletion experiment and below 1 µM when concentrationswere held constant.     

Vitamins, phytoplankton and bacteria: symbiosis or scavenging?

The conclusion that over 25% of global primary production dependson direct algal/bacterial symbiosis involving vitamin B₁₂ [Croftet al., (2005) Algae acquire vitamin B₁₂ through a symbioticrelationship with bacteria. Nature, 438, 90–93] is patentlyfalse, for it is based on a misconception of the probable levelof the vitamin B₁₂ requirement in marine pelagic algae. A reviewof the various published attempts at measuring this requirementsuggests that it is likely to be so low that oceanic and coastalconcentrations of the vitamin would usually be sufficient tosustain the populations that occur without the assistance ofdirect algal/bacterial symbiosis. The levels measured are discussedin relation to method (batch or continuous culture) and protocolsused. Requirement values considered by the author to be acceptablerange from 0.1 to 0.3 pM for the vitamin growth saturation constant(K_S) and from 30 to 100 µL algal biomass pmol^–1vitamin for the yield.     

The Meristem and Quiescent Centre in Cultured Root Apices of the gib-l Mutant of Tomato (Lycopersicon esculentum Mill.)

Cultured root apices of tomato bearing the gib-I mutation, whichreduces the levels of endogenous gibberellins, grew slower andwere thicker than wild-type contols. This was the result ofshorter and broader cells in the menstem of the mutant. Cellsof both cortex and stele were affected, but this did not causeany alteration to the volume fraction occupied by these twotissues in the root meristem. Root caps were longer in the mutantand there were also more layers of rhizodermis. All these effectscould be reproduced in wild-type roots by addition of 0.1µM2S, 3S paclobutrazol (an inhibitor of gibberellin biosynthesis)to the culture medium and could be normalized in mutant rootsby 0.1 µM GA₃. Cell doubling times in the proximal regionof the meristem were similar in mutant and wild-type roots,but were faster in both the quiescent centre (QC) and the capmeristem of the mutant. This latter feature of the mutant rootsis likely to be the cause of their longer caps, while the fasterrate of division in the QC accounts for the additional tiersof cells that were found to build up in the cortical portionof this zone These additional tiers failed to form in mutantroots grown in GA₃, but they could be induced in wild-type rootsby 2S, 3S paclobutrazol. These results suggest that endogenousgibberellins may be partly responsible for the slow rate ofcell growth and proliferation in the QC. Gibberellins, gib-I mutation, Lycopersicon esculentum, meristem, roots, 2S, 3S paclobutrazol, quiescent centre, tomato     

Effect of Sodium on Phosphate Uptake in Unicellular and Filamentous Cyanobacteria

The effect of Na⁺ on phosphate uptake was studied in four strainsof cyanobacteria: Synechococcus PCC 7942, Gloeothece PCC 6501,Phormidium sp. and Chlorogloeopsis PCC 6912. Phosphate uptakewas stimulated by Na⁺ in all cases. Li⁺ and K⁺ acted as partialanalogues for Na⁺. Half-saturation [K_1/2(Na⁺)] of phosphateuptake was reached with Na⁺ concentrations ranging from 317µM in Chlorogloeopsis to 659 µM in Phormidium. Theconcentration of phosphate required to reach half-saturationof phosphate uptake [K_1/2(P_i)]was not changed by the presenceof Na⁺. (Received April 11, 1994; Accepted July 5, 1994)     

In situ Effects of Elevated Atmospheric CO2on Leaf Freezing Resistance and Carbohydrates in a Native Temperate Grassland

The objectives of this study were to quantify changes in leaffreezing resistance and carbohydrate concentrations caused bylong-term (6 years) exposure to elevated CO₂(ambient: 360 µll^-1, elevated: 600 µl l^-1) in five dominant plant speciesgrowing in situ in a native temperate grassland. Across allfive species tested from three functional groups, the mean temperatureat which all leaves were damaged (T₁₀₀) significantly (P = 0.016)increased from -9.6 to -8.5 °C under elevated CO₂ , anda similar marginally significant (P = 0.079) reduction was observedfor the mean temperature that caused 50% leaf damage (T₅₀),from -6.7 to -6.0 °C. The mean temperature at which initialleaf damage was observed (T₀) was not significantly influencedby elevated CO₂ . Although concentrations of soluble sugars(+25%,P = 0.042), starch (+53%, P < 0.001), and total non-structuralcarbohydrates (TNC, +40%, P < 0.001) were significantly higherunder elevated CO₂ , leaf freezing resistance actually decreasedunder elevated CO₂ . Concentrations of soluble sugars were positivelycorrelated with freezing resistance when viewed across all fivecommunity dominants, but within any individual species, no suchrelationships were found. We also found no evidence for ouroriginal hypothesis that increased concentrations of solublesugars increase freezing resistance. Thus, future atmosphericCO₂levels may instead increase the risk of late spring freezingdamage. Furthermore, the strong differences in freezing resistanceobserved among the species, along with decreased freezing resistance,may increase the risk of losing species that have inherentlyweak freezing resistances from the plant community. Copyright2001 Annals of Botany Company CO₂enrichment, frost hardiness, sugar, starch, total non-structural carbohydrates (TNC)     

Food and suspended sediment influences on the naupliar and copepodid durations of freshwater copepods: comparative studies on Tropodiaptomus and Metadiaptomus

Post-embryonic durations of Tropodiaptomus spectabilis (Kiefer)and Metadiaptomus colonialis(van Douwe) were determined at 20°Cin laboratory factorial experiments involving four algal foodenrichment levels (0, 100, 500 and 2500 µg l^–1 Cof Selenastrum added to 20 µm filtered water from respectivesource-lakes) and three suspended sediment levels (filtered,natural, and 2- to 3-fold sediment-enriched lake water). Foodeffects (30, 75, 225 and 600 (µg ^–1 C of Scenedesmus)were tested alone at 20°C for Metadiaptomus meridianus (vanDouwe). Total naupliar (D_n) and total copepodid (D_c) developmenttimes [summed to give total post-embryonic duration (D_t)] andmetasome lengths at maturity were measured In all taxa, foodsupply maximally affected D_c values 2- to 3-fold, whereas itsmaximal influence on D_n values was relatively slight (generally25%). The measured effect of food supply on D_t, was as strongas the predicted influence of temperature over an appropriateannual range. Food supply influenced size at maturity, and probablythereby fecundity, thus exerting additional demographic influences.Sediment effects were inconsistent, and quantitatively weakerthan food effects Total development of T.spectabilis was 20%raster, and that of M.colonialis 15% slower in sediment-enrichedthan in natural sediment level treatments; contrasting baselinesediment levels (2–3 times higher for the latter species)and different enrichment procedures confound interpretation.Unexpectedly, and inexplicably, development almost invariablyfailed in sediment-free water, implying an apparent dependencyon inorganic particles in these taxa This contrasts with thegenerally adverse influences of high sediment concentrationsupon zooplankton. Minimal male and female D_t values at 20°Cwere comparable and significantly longer in M.colonialis (15.5and 17 7 days) and M meridianus (16.5 and 21.5 days) than inT.spectabilis (11.7 and 12 2 days). These differences in durationare ecologically incongruous in relation to expected r –K life history strategies of genera characteristic of temporaryor semipermanent waters and permanent waters respectively.     

A Comparison of the Growth, Photosynthesis, Stomatal Conductance and Water Use Efficiency of Moricandia and Brassica Species

When grown in pots and well-watered, the relative growth ratesof the above ground parts of two species of Moricandia (M. arvensis,an intermediate C₃–C₄ species, and M. moricandioides,a C₃ species) were inferior to those of two cultivated Brassicaspecies (B. campestris and B. napus). The Moricandia specieshad thicker leaves (greater d.wt per unit leaf area) with morechlorophyll than the Brassica species and had slightly greaterrates of photosynthesis per unit leaf area at an irradiance(400–700 nm) of 2000 µmol quanta m^–2 s ^–1.Leaves of M. arvensis, known to have a CO₂ compensation pointbetween that of C₃ and C₄ species, had a lower ratio of theintercellular to atmospheric partial pressure of CO₂ (C₁/C_a)and a greater instantaneous water use efficiency (WUE) thanthose of M. moricandioides and the Brassica species. Carbon isotope discrimination (