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.     

Flower-inducing Activity of Vitamin K in Lemna paucicostata

Vitamins K₁ K₃ and K₅ induced flowering in Lemna paucicostata151, a short-day plant, cultured in 1/10 strength M medium (1/10M medium) under continuous light, and their activity was greatlyintensified by simultaneous application of benzyladenine. Themost active of these was vitamin K₅ L. paucicostata 6746 ismore sensitive to vitamin K₅ than strain 151, but the effectof vitamin K₅ on strain 6746 was not intensified by benzyladenine.The flower-inducing activity of vitamin K₅ was intensified bythe addition of benzoic acid in both strains and by the additionof copper or ferricyanide in Strain 6746, when these chemicalswere added at such low concentrations that they would scarcelyinduce flowering. In strain 6746, vitamin K₅ added to 1/10 M had little effecton flowering under a subcritical photoperiod, while it clearlyinduced flowering under continuous light. In this strain, vitaminK₅ added to full strength M medium, in which this plant wasmore sensitive to short photoperiods than in 1/10 M medium,did not induce flowering even under continuous light, and wasrather inhibitory under short photoperiods. (Received August 14, 1984; Accepted October 16, 1984)     

A carrier-mediated mechanism for pyridoxine uptake by human intestinal epithelial Caco-2 cells: regulation by a PKA-mediated pathway

Vitamin B₆ is essential for cellular functions and growth due to its involvement in important metabolic reactions. Humans and other mammals cannot synthesize vitamin B₆ and thus must obtain this micronutrient from exogenous sources via intestinal absorption. The intestine, therefore, plays a central role in maintaining and regulating normal vitamin B₆ homeostasis. Due to the water-soluble nature of vitamin B₆ and the demonstration that transport of other water-soluble vitamins in intestinal epithelial cells involves specialized carrier-mediated mechanisms, we hypothesized that transport of vitamin B₆ in these cells is also carrier mediated in nature. To test this hypothesis, we examined pyridoxine transport in a model system for human enterocytes, the human-derived intestinal epithelial Caco-2 cells. The results showed pyridoxine uptake to be 1) linear with time for up to 10 min of incubation and to occur with minimal metabolic alteration in the transported substrate, 2) temperature and energy dependent but Na⁺ independent, 3) pH dependent with higher uptake at acidic compared with alkaline pHs, 4) saturable as a function of concentration (at buffer pH 5.5 but not 7.4) with an apparent Michaelis-Menten constant (K_m) of 11.99 ± 1.41 µM and a maximal velocity (V_max) of 67.63 ± 3.87 pmol · mg protein^-1 · 3 min^-1, 5) inhibited by pyridoxine structural analogs (at buffer pH 5.5 but not 7.4) but not by unrelated compounds, and 6) inhibited in a competitive manner by amiloride with an apparent inhibitor constant (K_i) of 0.39 mM. We also examined the possible regulation of pyridoxine uptake by specific intracellular regulatory pathways. The results showed that whereas modulators of PKC, Ca⁺²/calmodulin (CaM), and nitric oxide (NO)-mediated pathways had no effect on pyridoxine uptake, modulators of PKA-mediated pathway were found to cause significant reduction in pyridoxine uptake. This reduction was mediated via a significant inhibition in the V_max, but not the apparent K_m, of the pyridoxine uptake process. These results demonstrate, for the first time, the involvement of a specialized carrier-mediated mechanism for pyridoxine uptake by intestinal epithelial cells. This system is pH dependent and amiloride sensitive and appears to be under the regulation of an intracellular PKA-mediated pathway. vitamin B₆; intestinal transport; transport regulation; Caco-2 cell     

DISTRIBUTION OF QUINONES IN SOME PHOTOSYNTHETIC BACTERIA AND ALGAE

The distribution and the concentration of quinones in 5 speciesof photosynthetic bacteria and 4 species of algae were investigated.The Athiorhodaceae, except for Rhodospirillum rubrum, containedubiquinone as the major quinone component. Rhodospirillum rubrumcontained two kinds of quinones, i.e. ubiquinone and rhodoquinone.Chromatium, strain D contained two kinds of quinones, i. e.ubiquinone and vitamin K₂. The green sulfur bacterium, Chloropseudomonasethylicum, contained vitamin K₂ and a Chlorobium quinone-likesubstance. The marine algae tested, Porphyra yezoensis, Porphyra sp. andHizikia fusiforme, contained vitamin K₁, plastoquinone A, -tocopherylquinoneand ubiquinone. In Chondrus ocellatus, the presence of a ubichromenol-likesubstance was detected instead of ubiquinone. Other types ofplastoquinone and tocopherylquinone were not found in thesealgae. In the thalli of Porphyra sp., vitamin K₁, plastoquinoneA and -tocopherylquinone were found to be localized in chloroplasts. ¹Present address: Department of Biophysics and Physical Biochemistry,Johnson Research Foundation, University of Pennsylvania, Philadelphia,Pennsylvania, U.S.A.     

Photosynthetic Characteristics of C3-C4 Intermediate Flaveria Species II. Kinetic Properties of Phosphoenolpyruvate Carboxylase from C3, C4 and C3-C4 Intermediate Species

The kinetic properties of phosphoenolpyruvate (PEP) carboxylasehave been studied among several Flaveria species: the C₃ speciesF. cronquistii, the C₃–C₄ species F. pubescens and F.linearis, and the C₄ species F. trinervia. At either pH 7 or8, the maximum activities (in µmol.mg Chl^–1.h^–1)for F. pubescens and linearis (187–513) were intermediateto those of the C₃ species (12–19) and the C₄ species(2,182–2,627). The response curves of velocity versusPEP concentration were hyperbolic for the C₃ and C₃–C₄species at either pH 7 or 8 while they were sigmoidal for theC₄ species at pH 7 and hyperbolic at pH 8. The K_m values forPEP determined from reciprocal plots were lowest in the C₃ species,and of intermediate value in the C₃–C₄ species comparedto the K' values of the C₄ species determined from Hill plotsat either pH 7 or 8. Glucose-6-phosphate (G6P) decreased theK_m values for PEP at both pH 7 and 8 in the C₃ and C₃–C₄species. In the C₄ species, G6P decreased the K' values at pH8 but increased the K' values at pH 7. In all cases, G6P hadits effect by influencing the activity at limiting PEP concentrationswith little or no effect on the maximum activity. At pH 8 andlimiting concentrations of PEP the degree of stimulation ofthe activity by G6P was greatest in the C₄ species, intermediatein F. linearis, a C₃–C₄ species, and lowest in the C₃species. In several respects, the PEP carboxylases of the C₃–C₄Flaveria species have properties intermediate to those of theC₃ and C₄ species. (Received April 30, 1983; Accepted August 22, 1983)     

Effects of Elevated Carbon Dioxide on Plant Biomass Production and Competition in a Simulated Neutral Grassland Community

Using open-top chambers, four prominent species (Lolium perenne,Cynosurus cristatus, Holcus lanatusandAgrostis capillaris) ofIrish neutral grasslands were grown at ambient and elevated(700 µmol mol^-1) atmospheric CO₂for a period of 8 months.The effects of interspecific competition on plant responsesto CO₂enrichment were investigated by growing the species ina four-species mixture. The results indicate that the speciesdiffer in their ability to respond to elevated CO₂. CO₂-enrichmenthad the largest effect on the biomass production ofH. lanatus,but substantial stimulations in biomass production were alsofound for the other three species. The CO₂-stimulation of biomassproduction forH. lanatuswas accompanied by increased tillering.In addition, reductions in specific leaf area were found forall species. Exposure to elevated CO₂increased the communitybiomass of the four-species mixture. This increase can be mainlyattributed to a significant increase in the biomass ofH. lanatusatelevated CO₂. No statistically-significant changes in speciescomposition of community biomass were found. However,H. lanatusdidincrease its share of community biomass at each of the harvests,with the other three species, mainlyL. perenne, suffering lossesin their shares at elevated CO₂. The results show that: (1)the species varied in their response to elevated CO₂; and (2)species composition in natural plant communities is likely tochange at elevated CO₂, but these changes may occur rather slowly.Much longer periods of exposure to elevated atmospheric CO₂maybe required to permit detection of significant changes in speciescomposition.Copyright 1998 Annals of Botany Company Carbon dioxide (CO₂) enrichment, competition, Lolium perenne,Cynosurus cristatus, Holcus lanatus, Agrostis capillaris, biomass, specific leaf area, tillering.     

Cold-Tolerant Crop Species Have Greater Temperature Homeostasis of Leaf Respiration and Photosynthesis Than Cold-Sensitive Species

Some plant species show constant rates of respiration and photosynthesismeasured at their respective growth temperatures (temperaturehomeostasis), whereas others do not. However, it is unclearwhat species show such temperature homeostasis and what factorsaffect the temperature homeostasis. To analyze the inherentability of plants to acclimate respiration and photosynthesisto different growth temperatures, we examined 11 herbace-ouscrops with different cold tolerance. Leaf respiration (R_area)and photosynthetic rate (P_area) under high light at 360 µll^–1 CO₂ concentrations were measured in plants grown at15 and 30°C. Cold-tolerant species showed a greater extentof temperature homeostasis of both R_area and P_area than cold-sensitivespecies. The underlying mechanisms which caused differencesin the extent of temperature homeostasis were examined. Theextent of temperature homeostasis of P_area was not determinedby differences in leaf mass and nitrogen content per leaf area,but by differences in photosynthetic nitrogen use efficiency(PNUE). Moreover, differences in PNUE were due to differencesin the maximum catalytic rate of Rubisco, Rubisco contents andamounts of nitrogen invested in Rubisco. These findings indicatedthat the temperature homeostasis of photosynthesis was regulatedby various parameters. On the other hand, the extent of temperaturehomeostasis of R_area was unrelated to the maximum activity ofthe respiratory enzyme (NAD-malic enzyme). The R_area/P_area ratiowas maintained irrespective of the growth temperatures in allthe species, suggesting that the extent of temperature homeostasisof R_area interacted with the photosynthetic rate and/or thehomeostasis of photosynthesis.     

Effects of Elevated CO2 and Simulated Seasonal Changes in Temperature on the Species Composition and Growth Rates of Pasture Turves

Large turves from a ryegrass/white clover based pasture wereexposed to 350 or 700 µl l^-1 CO₂ for a period of 217 din controlled environment rooms. The temperature was increasedduring the experiment from 10/4 °C day/night to 16/10 °Cand finally to 22/16 °C. The turves were cut to a heightof 2 cm at intervals and growth rates calculated from the regrowth. Growth rates over the duration of the experiment were 8% higherat elevated CO₂; the difference between CO₂ treatments beingstatistically significant only at the highest temperature. Speciescomposition of the turves at 350 µl l^-1 CO₂ showed seasonalchanges similar to those measured in the field. The effect ofCO₂ was to exaggerate the normal decline of ryegrass at warmertemperatures and increase the proportion of white clover. About30% of the total growth rate was from other species (notablyBromus hordeaceus L. and Poa trivialis L.) and this fractionwas similar between CO₂ levels. Root mass was measured at theend of the experiment and was 50% higher at elevated CO₂. The modest above-ground response to CO₂ was a result of CO₂stimulation occurring only at the higher temperature. Becauseof the CO₂ x temperature interaction, the effect of CO₂ in temperateregions will be seasonal. When this is matched with seasonalgrowth patterns of herbage species, a complex response of pasturecommunities to CO₂ is possible. In our case, white clover wasgrowing most strongly during the period of greatest CO₂ stimulationand consequently its growth was enhanced more than that of ryegrass;however, the cooler season growth of ryegrass gives it a temporalniche which is little affected by CO₂ and this may be importantfor ryegrass stability if it is an inherently poor responderto CO₂. The results indicate that for temperate species theeffects of competition at elevated CO₂ cannot be easily determinedfrom experiments conducted at a single temperature.Copyright1994, 1999 Academic Press CO₂ enrichment, seasonal growth, species composition, turves, Trifolium repens L., Lolium perenne L., climate change     

Photosynthetic efficiency of Panicum hians and Panicum milioides in relation to C3 and C4 plants

Panicum hians and Panicum milioides were found to have characteristicsintermediate to those of C₃ and C₄ species with respect to CO₂compensation point, percentage inhibition of photosynthesisby O₂ at various O₂/CO₂ solubility ratios, and water use efficiency.C₄ species have a higher carboxylation efficiency than eitherthe intermediate or C₃ species. During photosynthesis, evenunder 2.5% O₂, C₄ species have a higher affinity for intercellularCO₂ (Km 1.6 µM) apparently due to the initial carboxylationthrough PEP carboxylase. Under low O₂ the intermediate and C₃species had a similar affinity for intercellular CO₂ duringphotosynthesis (Km 5–7 µM) consistent with carboxylationof atmospheric CO₂ through RuDP carboxylase. There were considerablevariation in photosynthesis/unit leaf area at saturating CO₂levels in the species examined which in part is due to differencesin RuDP carboxylase /unit leaf area. The highest rates of photosynthesis/unitleaf area under CO₂-saturating conditions were with the C₃ specieswhich had a correspondingly high level of RuDP carboxylase/unitleaf area. Possibilities for the greater efficiency of P. hiansand P. milioides in comparison to C₃ species in utilizing lowlevels of CO₂ in the presence of atmospheric O₂ are discussed. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and theUniversity of Wisconsin Research Committee with funds from theWisconsin Alumni Research Foundation. (Received June 25, 1977; )     

Mechanisms of Thermoregulation in Flying Bees

SYNOPSIS. Thermoregulation of elevated thorax temperatures isnecessary for bees to achieve the high rates of power productionrequired for flight, and is a key factor allowing them to occupywidely varying thermal environments. However, the mechanismsby which bees thermoregulate during flight are poorly understood.Thermoregulation is accomplished by balancing heat gain andheat loss via the following routes: convection, evaporation,and metabolic heat production. There appears to be a diversityof thermoregulatory mechanisms employed during flight amongbee species. Some species, particularly Bombus spp., activelyincrease the distribution of thoracic heat to the abdomen duringflight as air temperature (T_a) rises, and apparently thermoregulateby varying convective heat loss. However, thermal variationin convection has not been directly measured for any free-flyingbee. Above 33°C, flying Apis mellifera greatly increaseevaporative heat loss with T_a, and many other species "tongue-lash"during flight at high T_as or when artificially heated. Thus,evaporation seems to be important for preventing overheatingduring flight at very high T_as. Flying A. mellifera and Centrispallida strongly decrease metabolic rate as T_a increases, suggestingthat they are varying metabolic heat production for thermoregulationand not aerodynamic requirements. Variation in metabolic heatproduction appears to be mediated by changes in wingbeat kinematics,since wingbeat frequency decreases with T_a for A. melliferaand Centris spp. It is unknown if the decrease in flight metabolicrate at higher T_as occurs secondarily as a consequence of greaterefficiency or if it is truly an active response.     

Water relations and hydraulic architecture of woody hemiepiphytes

Several parameters related to the water relations of eight woodyhemiepiphytes with different photosynthetic pathways were studiedin situ and in the laboratory on Barro Colorado Island, Panama.As a group, woody hemiepiphytes tended to have less conductivestems per unit leaf area (lower k_L) and invested less wood cross-sectionper unit leaf area compared to free-standing trees, while theirspecific conductivity (K_s) was significantly higher. Among hemiepiphytes,there were significant differences between C₃ and CAM (CrassulaceanAcid Metabolism) species in respect to leaf characteristics,transpiration rates, diel patterns of water flow through aerialroots, and in hydraulic architecture parameters. Average transpirationrates of the two Clusia species (C₃-CAM) were lower by aboutan order of magnitude compared to the C₃ species. In all C₃species, sap flow through aerial roots (F) closely followedtranspiration (E), whereas E and F were decoupled in time inthe C₃-CAM species Clusia uvitana: considerable long-distancewater flow occurred at night. The hydraulic efficiency of theother C₃-CAM species, C. minor was the lowest of the five speciesinvestigated. Key words: Hemiepiphytes, water relations, hydraulic architecture, aerial roots, CAM     

Synthesis of Receptor Sites for Vitamin B12 in Euglena gracilis

In Euglena gracilis, vitamin B₁₂ uptake follows a biphasic patternconsisting of an initial rapid phase followed by a slower secondaryphase. Chase experiments showed that vitamin B₁₂ was tightlybound to its receptor-sites during either rapid or slow phaseof uptake. The slow phase was markedly inhibited when Euglenacells were preincubated with cycloheximide for 30 min at a concentrationof 100µg/ml. When the preincubation time was longer than30 min, a gradual inhibition of the rapid phase occurred andreached 84% after 4 h. This inhibitory effect of cycloheximideis reversible. On the other hand, tunicamycin at 1 µ/mlirreversibly inhibited the B₁₂ uptake suggesting that the receptor-sitefor B₁₂ is glycoprotein in nature. These results suggest thatthe rapid phase is also dependent on protein synthesis and representsB₁₂ binding on preexisting free receptors whereas the secondaryslow phase represents B_l2 binding on newly synthesized receptors.Both phases of uptake seem to be controled by the same receptor.The half-life of the free receptor is estimated to be 66 minwhereas the B₁₂ receptor complex is very stable. (Received October 3, 1988; Accepted February 15, 1989)     

Stomatal Responses to Sulphur Dioxide and Vapour Pressure Deficit

Stomatal conductances (g_s) of plants of Vicia faba, Raphanussativus, Phaseolus vulgaris, Heilanthus annuus, and Nicotianatabacum were measured in chambers containing either clean airor air containing between 18 and 1000 parts 10^–9 SO₂ atwater vapour pressure deficits (vpd) ranging from 1·0to 1·8 kPa. When vpd was low (<1·3 kPa at 22 °C) and stomatawere open, exposure to SO₂ induced rapid and irreversible increasesin g_s in V. faba. This response persisted throughout the exposure(3 d). The increase in g_s, 20–30% compared with cleanair, was independent of SO₂ concentration up to 350 parts 10^–9Stomatal conductances of polluted plants at night were greaterthan controls. When stomata were closed before exposure to SO₂,there was no effect on g_s. When vpd was varied, g_s of unpolluted plants of P. vulgarisshowed no response, but that of R. sativus increased slightlywith increasing vpd. In both species exposure to SO₂ causedan increase in g_s at all vpd values. g_s of unpolluted plantsof V. faba, H. annuus, and N. tabacum decreased with increasingvpd. At low vpd values exposure to SO₂ in these species causedan increase in g_s, but, above a certain value of vpd, dependingon species, g_s decreased with exposure to SO₂. It is postulated that SO₂, once in the substomatal cavity, entersthe stomatal complex via adjacent epidermal cells and at lowconcentrations leads to a reduction in turgor in these cellsand consequently to stomatal opening. In vpd-sensitive species,increased transpiration from guard cells or epidermal cellsadjacent to the stomata induced by SO₂ may lead to stomatalclosure at large vpd levels. Stomatal sensitivity to vpd insuch cases may be enhanced because adjacent epidermal cell turgoris lowered by SO₂. At high SO₂ concentrations direct disruptionof guard cell structure may lead to a loss of turgor and stomatalclosure.     

Expression, regulation, and function of P2X4 purinergic receptor in human cervical epithelial cells

Micromolarconcentrations of ATP stimulate biphasic change in transepithelialconductance across CaSki cultures on filters, an acute transientincrease (phase I response; triggered by P2Y₂ receptor and mediated by calcium mobilization-dependent cell volume decrease) followed by a slower decrease in permeability (phase II response). Phase II response is mediated byaugmented calcium influx and protein kinase C-dependent increase intight junctional resistance. The objective of the study was todetermine the role of P2X₄ receptor as a mediator ofphase II response. Human cervical epithelial cells expressP2X₄ receptor mRNA (1.4-, 2.2-, and 4.4-kb isoforms byNorthern blot analysis) and P2X₄ protein. Depletion ofvitamin A reversibly downregulated P2X₄ receptor mRNA andprotein and ATP-induced calcium influx. Depletion of vitamin Aabrogated phase II response, and the effect could bepartially reversed only with retinoic acid receptor (RAR)-selectiveretinoids but not retinoid X receptor (RXR) agonists. Depletion ofvitamin A also abrogated protein kinase C increase in tight junctionalresistance, and the effect could not be reversed with retinoids.Depletion of vitamin A also abrogated phase I increase inpermeability and reversibly downregulated P2Y₂ receptormRNA and ATP-induced calcium mobilization. However, in contrast tophase II response, both RAR and RXR agonists could fullyreverse those effects. These results suggest that phase IIresponse is mediated by a P2X₄ receptor mechanism.

     

Effects of light intensity and quality on the relative N and P requirement (the optimum N:P ratio) of marine planktonic algae

The relative requirement of N and P (the optimum N:P ratio)by Dunaliella tertiolecta, Phaeodactylum tricornutum, Prymnesiumparvum and Thalassiosira pseudonana was studied under variouslight intensities and spectra. The ratio was determined as theratio of the minimum cell N and P concentrations (q₀N and q₀pwhen either nutrient was limiting. The ratio varied widely amongspecies; under light-saturation for growth (116 µEin ^m–2s^–1 it ranged from 11.8 in ^D. tertiolecta to 36.6 in P.tricornutum. The ratio appeared to be higher at a sub-saturatingintensity (24 µEin m^–2 s^–1 in all except P.tricornutum, mainly because of higher q_oN with little changein q_oP. In T. pseudonana Q_oP also increased, resulting in aninsignificant change in the ratio. The ratio varied little withinthe range of saturation intensity. Light quality affected q_oNand q_oP as well as the ratio, and the pattern of change variedfrom species to species. The optimum ratio of individual specieswas linearly correlated to their q_oN except in P. tricornutum.q_oN for all species showed a linear correlation with cell proteinconcentrations irrespective of light conditions. The changeof optimum N:P ratios in the three species thus appears to berelated to changes in cell protein contents. The ratio of carbohydratesto protein remained constant regardless of light intensity orquality and was higher in P-limited cultures. We conclude thatchanges in light regime can strongly influence algal nutrientrequirements and species interrelationships by altering theoptimum cellular N:P ratio.     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: I. A COMPARISON OF CHICKPEA, LENTIL, SOYABEAN AND COWPEA AT CONSTANT TEMPERATURES

For a single seed population of each of four species of grainlegume positive linear relationships were shown between temperatureand rate of germination for different fractions (G) of eachpopulation, from a base temperature, T_b(G), at which germinationrate is zero, to an optimum temperature, T_o(G) at which germinationrate is maximal. At constant temperatures warmer than T_o(G)there were negative relations (probably linear) between temperatureand rate of germination to the maximum temperature for germination,T_m(G), Within each population T_b(G) did not differ, but it didvary between species, viz.0.0?C, 0.25?C, 4.and 8.5?C for chickpea(Cicer arietinum L.), lentil (Lens culinaris Medic.), soyabean(Glycine max [ Merr.) and cowpea (Vigna unguiculata [L.] Walp.),respectively. In contrast, T_o(G) varied both within each populationand also between the four species: 80% of seeds in each populationhad T_o(G) values within the range 31.8?C to 33.8 ?C, 24.0?Cto 24.4?C, 34.0?C to 34.5?C and 33.2?C to >40?C, respectively.Values of T_m(G) were much more vanable: the 80% population rangewas 48 .0?C to 60.8?C for chickpea, 31.8?C to 34.4?C for lentiland 46.8?C to 55.2?C for soyabean; reliable estimates couldnot be made for cowpea, but the results suggest higher and morevariable values of T_m(G) than in the other three species. Atsub-optimal temperatures the distribution of thermal time forthe different fractions of each population was normal, exceptfor lentil where it was log-normal. A single equation is proposedto describe the influence of sub-optimal temperatures on ratesof germination for whole seed populations. At supra-optimaltemperatures, variation in thermal time for the different fractionsof each population was only slight. The implications of thesefindings for the adaptation of grain legume crops to differentenvironments, and for the screening of germplasm, are discussed. Key words: Seed germination rate, temperature, grain legumes     

NUTRITIONAL STUDIES OF THE EDIBLE SEAWEED PORPHYRA TENERA I. THE INFLUENCE OF DIFFERENT B12 ANALOGUES, PLANT HORMONES, PURINES AND PYRIMIDINES ON THE GROWTH OF CONCHOCELIS

The red alga Porphyra tenera has been obtained in axenic cultureby the "dip and drag" technique in an agarized medium containingantibiotics. In the axenic culture, the Conchocelis of P. teneraneeds vitamin B₁₂ for growth. The addition of other vitaminsdoes not increase growth further. The pattern of specificityis similar to that of Escherichia coli 113-3. Factor B, pseudovitaminB₁₂ and Factor Z₂ support as much growth as vitamin B₁₂, while2-methylmercaptoadenine and 5-methyl benzimidazole cobalamineincrease the growth more than B₁₂. All the analogues containingbenzimidazole can replace B₁₂. Kinetin, adenine, indoleaceticacid and especially gibberellic acid increase growth. Threepurines, (xanthine, hypoxanthine and guanine) and three pyrimidines(uracil, methylcytosine and thymine) also promote growth inthe presence of vitamine B₁₂. (Received January 18, 1965; )     

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 (     

Reactive oxygen species are important mediators of taurine release from skeletal muscle cells

The present study illustrates elements ofthe signal cascades involved in the activation of taurine effluxpathways in myotubes derived from skeletal muscle cells. Exposingprimary skeletal muscle cells, loaded with ¹⁴C-taurine, to1) hypotonic media, 2) the phospholipaseA₂ (PLA₂) activator melittin, 3)anoxia, or 4) lysophosphatidyl choline (LPC) causes anincrease in ¹⁴C-taurine release and a concomitantproduction of reactive oxygen species (ROS). The antioxidants butulatedhydroxy toluene and vitamin E inhibit the taurine efflux after cellswelling, anoxia, and addition of LPC. The muscle cells possess twoseparate taurine efflux pathways, i.e., a swelling- andmelittin-induced pathway that requires 5-lipoxygenase activity foractivation and a LPC-induced pathway. The two pathways aredistinguished by their opposing sensitivity toward the anion channelblocker DIDS and cholesterol. These data provide evidence forPLA₂ products and ROS as key mediators of the signalcascade leading to taurine efflux in muscle.

     

Quantity and Kinetic Properties of Ribulose 1,5-Bisphosphate Carboxylase in C3, C4, and C3-C4 Intermediate Species of Flaveria (Asteraceae)

The kinetic properties of ribulose 1,5-bisphosphate carboxylase(RuBPC) appear to have been modified during evolution of photosynthesisto adjust to changes in substrate availability. C₄ plants areconsidered to have a higher concentration of CO₂ available toRuBPC than C₃plants. In this study, the K_m(CO₂ and catalyticcapacity (k_cat) of RuBPC and the ratio of RuBPC protein to totalsoluble protein from several Flaveria species, including C₃,C₃-C₄ intermediate, and C₄ species, were determined. The C₃and intermediate species had similar K_m(CO₂) values while theC₄ species on average had higher K_m(CO₂) values. The mean ratioof K_cat/K_m for species of each group was similar, supportingthe hypothesis that changes in K_m and K_cat, are linked. Theallocation of total soluble protein to RuBPC was lowest in theC₄ Flaveria species, intermediate in the C₃-C₄ species, andhighest in the C₃ species. The results suggest that during evolutionof C₄ photosynthesis adjustments may occur in the quantity ofRuBPC prior to changes in its kinetic properties. (Received January 4, 1989; Accepted April 11, 1989)