A comparison of in situ and simulated in situ methods for estimating oceanic primary production

Primary production data measured by in situ (IS) and ‘simulated’in situ (SIS) incubations were compared. To minimize differencesbetween the two types of incubations, SIS experiments were conductedin temperature-controlled incubators in which the spectral distributionand irradiance were adjusted to approximate IS conditions. ISavailable irradiance (I_Is) was computed from vertical attenuationof integrated surface irradiance. Vertical attenuation was estimatedusing a spectral irradiance model, validated by measured profilesof the vertical attenuation coefficient. IS incubations werecarried out using two methods. The first involved deploymentof bottles on a drifting array for whole-day (dawn to dusk)incubations. The second method employed an autonomous submersibleincubation device that performed short term (<1 h) incubationsat multiple depths. Differences between whole-day IS and SISincubation estimates were attributed partially to differencesbetween I_IS and SIS-available irradiance (I_SIS). Photosynthesis-irradiance(P-I) properties of IS and SIS populations from the whole-dayincubations were not significantly different. P-I propertiesof the short-term IS and SIS populations were significantlydifferent, although estimates of P^B (mg C mg Chl^–1 h^–1)from contemporaneous IS and SIS incubations did not differ by>40%. Integrated water-column primary production (IPP) estimatedusing P-I models derived from SIS data were within 15% of ISestimates of IPP.     

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

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

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

Irradiance and daylength effects on growth and chemical composition of Gyrodinium aureolum Hulburt in culture

For Gyrodinium aureolum significant irradiance and daylengtheffects were found on the division rate and on the growth-relevantChla-normalized photosynthetic rate (^gP^B). Optimum conditionsof irradiance and daylength were found at 230 µmol m^–2s^–1 and 14 h for the division rate, and at >260 µmolm^–2 s^–1 and <6 h for ^gP^B.^gP^B showed no photoinhibition,while the division rate decreased markedly at irradiances abovesaturation. This difference and the difference in optimum irradiancebetween the division rate and ^gP^B are explained by a decreasein cellular Chla/carbon ratio with increasing irradiance. Thecellular content of carbon and nitrogen decreased significantlywith increasing irradiance. Total phosphorus was independentof irradiance and daylength. Below the saturation irradiancefor ^gP^B the daily Chla-normalized carbon yield may be describedas an exponential function of the daily irradiance (irradiancex daylength).     

Effects of vertical-horizontal coupling on the horizontal distribution of chlorophyll a

The distributions of chlorophyll a and salinity were sampledon successive days along horizontal transects in central LongIsland Sound. Changes in the variance spectra over a periodof three days following a wind event suggest that there wasan input of variance at wave numbers of the order 6 ? 10^–3radians per meter. There was an associated increase in coherencysquared between chlorophyll and salinity. During this periodvertical structure in both chlorophyll and salinity was reestablished.It appears that horizontal structure was generated by the interactionof this vertical structure with a vertical shear in horizontalcurrents at semidiurnal and lower frequencies and possibly byshort period internal waves with frequencies near the V?is?l?frequency.     

Characteristics of the Wave of Depolarization Induced by Wounding in Bidens pilosa L.

Heating locally the hypocotyl of Bidens pilosa L. elicits awave of depolarization. The mechanism of the wave has been investigatedby means of microelectrophysiological techniques. The amplitudeof the transmembrane potential variation induced by an extracellularion concentration change (K⁺, Na⁺, Ca²⁺, Cl^–) was thesame in the resting conditions as during the slow wave. At pH4.0, the amplitude of the slow wave was reduced by 56% comparedwith the control performed at pH 7.0. In the presence of theuncoupler CCCP, the slow wave was not observed. The Ca²⁺ -chelatorEGTA and the Ca2²⁺ -channel blocker La³⁺ reduced, respectively,the amplitude of the slow wave by 78% and 68%. These resultsindicate the involvement of Ca²⁺ in triggering the slow wave.A transient modification of the electrogenic H⁺ pump activity(inactivation-activation) and of the transmembrane H⁺ flux inthe slow wave are discussed. Key words: Slow wave (of depolarization), wounding, electrogenic pump, calcium, Bidens pilosa L     

Pacemaker potentials generated by interstitial cells of Cajal in the murine intestine

Pacemaker potentials were recorded in situ from myenteric interstitial cells of Cajal (ICC-MY) in the murine small intestine. The nature of the two components of pacemaker potentials (upstroke and plateau) were investigated and compared with slow waves recorded from circular muscle cells. Pacemaker potentials and slow waves were not blocked by nifedipine (3 µM). In the presence of nifedipine, mibefradil, a voltage-dependent Ca²⁺ channel blocker, reduced the amplitude, frequency, and rate of rise of upstroke depolarization (dV/dt_max) of pacemaker potentials and slow waves in a dose-dependent manner (1–30 µM). Mibefradil (30 µM) changed the pattern of pacemaker potentials from rapidly rising, high-frequency events to slowly depolarizing, low-frequency events with considerable membrane noise (unitary potentials) between pacemaker potentials. Caffeine (3 mM) abolished pacemaker potentials in the presence of mibefradil. Pinacidil (10 µM), an ATP-sensitive K⁺ channel opener, hyperpolarized ICC-MY and increased the amplitude and dV/dt_max without affecting frequency. Pinacidil hyperpolarized smooth muscle cells and attenuated the amplitude and dV/dt_max of slow waves without affecting frequency. The effects of pinacidil were blocked by glibenclamide (10 µM). These data suggest that slow waves are electrotonic potentials driven by pacemaker potentials. The upstroke component of pacemaker potentials is due to activation of dihydropyridine-resistant Ca²⁺ channels, and this depolarization entrains pacemaker activity to create the plateau potential. The plateau potential may be due to summation of unitary potentials generated by individual or small groups of pacemaker units in ICC-MY. Entrainment of unitary potentials appears to depend on Ca²⁺ entry during upstroke depolarization. pacemaker activity; slow waves; gastrointestinal motility; calcium channel     

Ammonium release by nitrogen sufficient diatoms in response to rapid increases in irradiance

It has been hypothesized that nitrogen-replete diatoms, butnot flagellates, may release NO₂^–, NH₄⁺ or dissolved organicnitrogen (DON) following rapid increases in irradiance (andconsequently an increase in cellular electron energy), as mightbe expected to occur in a vertically well mixed estuarine system.Just as the increase in irradiance leads to an increase in cellularenergy, so too would a decrease in temperature, due to the temperaturedependency of biosynthetic enzymes. This hypothesis was testedby comparing the response of nitrogen-replete diatoms (Skeletomenacostatum, Thalassiosira weissflogii and Chaetoceros sp.) andflagellates (Dunaliella tertiolecta, Pavlova lutheri and Prorocentrumminimum) to rapid increases in irradiance and decreases in temperature.Short-term (<3 h) changes in extracellular NO₂^– andNH₄⁺ concentrations were measured in cultures following theseexperimental shifts, as well as in cultures retained at thegrowth irradiance. Net rates of NO₂^– and NH₄⁺ releasewere calculated from the time course of extracellular nitrogenconcentrations. As a fraction of NO₃^– uptake, NO₂^–release rates under the increased irradiance increased marginallyrelative to NO₂^– release rates under the growth irradiance.Release rates of NH₄⁺ under the increased irradiance increasednearly fivefold over release rates at the growth irradiance,and accounted for 84% of the NO₃^– uptake rate. In directcontrast to the diatom species, the flagellate species releasedNO₂^– under the higher experimental irradiance at ratesone half those of the release rates under the growth irradiance,and continued to take up NH₄⁺ under both irradiance conditions.Within the experimental boundaries, these findings have importantphysiological and ecological implications. The magnitude ofthe observed nitrogen release represents a significant physiologicalsink for electrons and, in fact, calculations suggest that upto 62% of the total electrons harvested could be consumed. Froman ecological perspective, these findings add to the body ofliterature which suggests that a significant fraction of thenitrogen that is taken up is ultimately released in dissolvedform. More importantly, these data suggest that DON is not theonly compound that phytoplankton may release in the aquaticenvironment.     

Potential contributions of vertically migrating Rhizosolenia to nutrient cycling and new production in the open ocean

We present a numerical model of nutrient uptake and photosynthesisduring migrations of the marine diatom Rhizosolenia that wasdeveloped to estimate fluxes of carbon and nitrogen due to thesemigrations in the open ocean. The predicted specific rate ofincrease of Rhizosolenia was 0.11–0.15 day¹, whereas thetotal time for one migration cycle ranged between 3 and 5 days.Using published estimates of Rhizosolenia abundance, we estimatethat new primary production due to Rhizosolenia migrations rangesbetween 0.018 and 0.033 mmol N m^–2 day^–1. Thesevalues represent up to 17% of new production due to turbulentdiffusive fluxes of nitrate into the euphotic zone and are ofthe same order of magnitude as new production due to nitrogenfixation in tropical oceans. Large-scale contributions of Rhizosoleniato oceanic new production are limited by their relatively lowstanding crop. Variations in the formulation of losses withdepth greatly affected gross and net fluxes of carbon and nitrogen.Better characterization of losses of Rhizosolenia and improvedestimates of its abundance will help determine more accuratelythe contributions of Rhizosolenia to global biogeochemical cycles. ¹Present address: Department of Agricultural and EnvironmentalScience, The Queen's University of Belfast New forge Lane, BelfastBT95PX, UK     

Infection of Coscinodiscus spp. by the parasitoid nanoflagellate Pirsonia diadema: II. Selective infection behaviour for host species and individual host cells

The parasitoid nanoflagellate (PNF) Pirsonia diadema is hostspecific for the marine centric diatom Coscinodiscus spp. Experimentsshowed that flagellates significantly prefer C. wailesii overC.granii as host species (interspecific selectivity). This preferencewas independent of light conditions (dark, irradiance of 10and 70 µmol m^–2 s^–1) and temperature (10 and15C). Among unicellular host diatoms, the infection behaviourwas selective for individual cells: already infected C.graniicells were more attractive for further flagellate attachmentthan non-infected cells (intraspecific selectivity). Individualcells (     

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     

Primary production in lakes Huron and Michigan: in vitro and in situ comparisons

Oxygen- and carbon-14-based primary production estimates from9–16 h in vitro incubations were compared in lakes Huronand Michigan. For surface mixing layer compansons, gross O₂/¹⁴Cphotosynthetic quotients (gross PQ) averaged 2.2, and net O₂/¹⁴Cphotosynthetic quotients (net PQ) averaged 1.4. The mean grossPQ is consistent with a theoretical P0 based on the CO₂ andNO₃ assimilation ratio. However, within the deep chlorophylllayer, gross PQ and net PQ averaged 4.9 and 2.8 respectively.These higher values were likely due to excess NO₃ reductionat the expense of CO₂ uptake. Thus, during short experimentsunder low light conditions, oxygen evolution and CO₂ uptakemay not be tightly coupled. In vitro and in situ O₂ productionestimates were compared in four diurnal (dawn to dusk) experimentsin Lake Huron. In situ production estimates were determinedby measuring water-mass oxygen changes and oxygen transfer acrossthe air-water interface. In situ production estimates were approximatelytwice in vitro production estimates for both surface mixinglayer and deep chlorophyll layer comparisons. The differencebetween estimates was attributable to containment effects manifestin 13–16 h bottle incubations. Short-term (1–2 h)in vitro production was also compared to diurnal in vitro production.Rates of short-term production were {small tilde}1.6 times higherthan rates of diurnal production, suggesting that short-termin vitro production experiments may provide reasonable estimatesof in situ primary production.     

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

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

Control of Ca2+ wave propagation in mouse pancreatic acinar cells

We haveinvestigated control mechanisms involved in the propagation ofagonist-induced Ca²⁺ waves inisolated mouse pancreatic acinar cells. Using a confocal laser-scanningmicroscope, we were able to show that maximal stimulation of cells withacetylcholine (ACh, 500 nM) or bombesin (1 nM) caused an initialCa²⁺ release of comparable amountswith both agonists at the luminal cell pole. SubsequentCa²⁺ spreading to the basolateralmembrane was faster with ACh (17.3 ± 5.4 µm/s) than with bombesin(8.0 ± 2.2 µm/s). The speed of bombesin-inducedCa²⁺ waves could be increased upto the speed of ACh-induced Ca²⁺waves by inhibition of protein kinase C (PKC). Activation of PKCsignificantly decreased the speed of ACh-inducedCa²⁺ waves but had only littleeffect on bombesin-evoked Ca²⁺waves. Within 3 s after stimulation, production of inositol1,4,5-trisphosphate [Ins(1,4,5)P₃]was higher in the presence of ACh compared with bombesin, whereasbombesin induced higher levels of diacylglycerol (DAG) than ACh. Thesedata suggest that the slower propagation speed of bombesin-inducedCa²⁺ waves is due to higheractivation of PKC in the presence of bombesin compared with ACh. Thehigher increase in bombesin- compared with ACh-induced DAG productionis probably due to activation of phospholipase D (PLD). Inhibition ofthe PLD-dependent DAG production by preincubation with 0.3% butanolled to an acceleration of the bombesin-induced Ca²⁺ wave. In further experiments,we could show that ruthenium red (100 µM), an inhibitor ofCa²⁺-inducedCa²⁺ release in skeletal muscle,also decreased the speed of ACh-induced Ca²⁺ waves. The effect ofruthenium red was not additive to the effect of PKC activation. Fromthe data, we conclude that, following Ins(1,4,5)P₃-inducedCa²⁺ release in the luminal cellpole, secondary Ca²⁺ release fromstores, which are located in series between the luminal and the basalplasma membrane, modifies Ca²⁺spreading toward the basolateral cell side byCa²⁺-inducedCa²⁺ release. Activation of PKCleads to a reduction in Ca²⁺release from these stores and therefore could explain the slower propagation of Ca²⁺ waves in thepresence of bombesin compared with ACh.

     

Growth Responses of Two Solanum Species to Contrasting Temperatures and Irradiance Levels: Relations to Photosynthesis, Dark Respiration and Chlorophyll Fluorescence

Potato production in the tropical lowlands during the rainyseason is constrained by high temperature and low irradiance.This study examined the effect of these two variables on drymatter production and allocation, using plant growth, leaf anatomy,gas exchange and chlorophyll fluorescence measurements. Plantsof two clones, Solanum goniocalyx cv. Garhuash Huayro (GH) andDTO-33, a heat tolerant clone of S. tuberosum x S. phureja,were grown in growth chambers at 33/25 °C or 20/10 °Cday/night temperature. At each temperature, plants were grownin either 12 h high irradiance (430–450 µmol m^–2s^–1 PAR) or 12 h low irradiance (250–280 µmolm^–2 s^–1) both with a 6–h photoperiod extensionof 6 µmol m^–2 s^–1. Plants were harvested after10 d (initial harvest) and after 20 d (final harvest). By theend of the study DTO-33 had produced more dry matter and hadtuberized, whereas GH had a greater leaf area ratio (LAR) andspecific leaf area (SLA). The highest relative growth rate (RGR)was at low temperature and low irradiance, possibly due to acombination of thin leaves with a large surface area. At thehigh temperature, low irradiance had the opposite effect, producingthe lowest net assimilation rate (NAR) and lowest RGR. Bothtuber number and weight were markedly reduced by high temperature.Low irradiance, in combination with high temperature, producedvirtually no tubers. Stomatal density, which was greater onGH than in DTO-33, was increased at high temperature. When measuredat 30 °C both clones, especially DTO-33, showed heat-adaptationin terms of ability to maintain a high rate of net photosynthesisat 30 °C. Plants grown at high irr-adiance and low temperaturehad the lowest net photosynthetic rate at 30 °C. Concurrentmeasurements of chlorophyll fluorescence indicated that onlythe initial (O) fluorescence parameter was affected. The dataconfirm the field observation that reduction in potato growthat high temperature can be aggravated by lower irradiance. Thisreduction is associated with a reduced leaf area and NAR. Growth analysis, heat adaptation, light     

Size of the Chrysanthemum Shoot Apex in Relation to Inflorescence Initiation and Development

The size of the apical dome of Chrysanthemum morifolium Ramat.at the transition to inflorescence initiation in continuouslight (long days) was not systematically influenced by eitherthe temperature or the irradiance under which the plants weregrown. It was generally 0.26 mm in diameter and c. 3.6 x 10^–3mm³ in volume when the first bract was initiated. The dimensionsof the apical dome of plants in short days were only slightlysmaller at this stage. Similarly, each step in the further developmentof the chrysanthemum inflorescence was associated with a narrowrange of apex sizes, indicating that inflorescence initiationand development are closely related to apex size. Chrysanthemum morifolium Ramat, shoot apex, inflorescence initiation     

Light Distribution in Discontinuous Canopies: Calculation of Leaf Areas and Canopy Volumes Above Defined 'Irradiance Contours' for use in Productivity Modelling

Plant canopies can be considered as assemblages of leaves, stemsand fruits growing in zones of differing irradiance demarcatedby contours of mean irradiance as measured on a horizontal surface. The following general equations have been derived to calculatethe leaf area (L_I) and the canopy volume (CV_I) in zones externalto any chosen contour of mean irradiance: (1) L_I = ((1nl)/(–K)(I–T_f) or leaf area index (LAI) if this is less (2) CV_I = L_I/(leaf area density m² m^–2), where I is the specified value of irradiance (horizontal surface)expressed as a decimal fraction of that above the canopy, Kis the appropriate extinction coefficient and T_f is the proportionof the total of available radiation which, if the canopy isdiscontinuous, would reach the ground by passing through gapsbetween the discrete canopy units. Where the canopy is continuousT_f is zero so expression (1) simplifies to L₁ = 1n I/–K(or LAI if this is less). For a range of model hedgerow orchards of varying dimensions,spacings and LAIs, it has been shown that the use of these equationsgives very similar results to those obtained by detailed calculationof light penetration. They therefore seem to be of potentialuse in calculating both potential dry-matter production by discontinuouscanopies of any type and, in the case of orchard fruit crops,the potential effect of changes in tree size, leaf area density,spacing etc. on the canopy volume in which irradiation is adequatefor fruit bud initiation and fruit colour development. light distribution, discontinuous canopy, irradiance contours, leaf area index, orchards     

Field Measurements of the Gas Exchange of Woody Plant Species in Simulated Sunflecks

The photosynthetic responses of eight tree and shrub speciesto simulated sunflecks was measured in the field. The net carbonexchange (NCE) of Corylus avellana and Ulmus glabra increasedwith irradiance up to the maximum irradiance of 230 µmolm^–2 s^–1. The NCE of Fraxinux excelsior, Hedera helixand the sun and shade forms of Rhododendron ponticum saturatedat about 120 µmol m^–2 s^–1 whereas the NCEof Ilex aquifolium, Daphne laureola and Fagus sylvatica hadeffectively saturated at 27 µmol m^–2 s^–1. In all cases the quantum efficiency of NCE could be predictedfrom measurements of chlorophyll fluorescence and the maximumvalue for NCE from measurements of stomatal conductance. Therelationships were combined into a model for predicting NCE/irradiancecharacteristics. Corylus avellana L., Daphne laureola L., Fagus sylvatica L., Fraxinus excelsior L., Hedera helix L., Ilex aquifolium L., Prunus laurocerasus L., Rhododendron ponticum L., Ulmus glabra Huds., gas exchange, stomatal resistance, water use efficiency, chlorophyll fluorescence, quantum efficiency     

Theoretical insights into the mechanism of spiral Ca2+ wave initiation in Xenopus oocytes

Spiral waves of intracellularCa²⁺ have often been observed inXenopus oocytes. Such waves can beaccounted for by most realistic models forCa²⁺ oscillations taking diffusionof cytosolic Ca²⁺ into account,but their initiation requires rather demanding and unphysiologicalinitial conditions. Here, it is shown by means of numerical simulationsthat these spiral Ca²⁺ wavesnaturally arise if the cytoplasm is assumed to be heterogeneous both atthe level of the synthesis and metabolism ofD-myo-inositol 1,4,5-trisphosphate[Ins(1,4,5)P₃]and at the level of the distribution of theIns(1,4,5)P₃receptors. In such conditions, a spiral can be initiated in thesimulations after an increase inIns(1,4,5)P₃ concentration, with the direction of rotation being determined by theposition of the region of high receptor density with respect to thelocus ofIns(1,4,5)P₃production.

     

The Effects of Temperature and Light Intensity on Embryo Numbers in Wheat Doubled Haploid Production through WheatxMaize Crosses

Triticum aestivumxZea mayscrosses are now widely used in theproduction of wheat doubled haploids to produce homozygous lines.Seasonal effects are known to influence the number of haploidembryos produced through wheatxmaize crosses, but the effectsof temperature and light have not been quantified. This studyinvestigated the effect of temperature and light intensity onhaploid embryo production. New Zealand wheat cultivars weregrown in a glasshouse until booting when they were transferredto growth cabinets at three temperatures (day/night; 17/12,22/17 or 27/22 °C at an irradiance of 250 µmol m^-2s^-1PAR).In another experiment, wheat lines were transferred to a growthcabinet at one of three light intensities (300, 500 or 1000µmol m^-2s^-1PAR at 22/17 °C day/night, with a photoperiodof 16 h). The temperature and light intensity at which pollinationswere made and subsequent fertilisation and embryo developmentoccurred, significantly (P<0.01) influenced the frequencyof haploid embryo production. The optimal temperature for embryorecovery was 22/17 °C. The greatest number of embryos wasproduced at a light intensity of 1000 µmol m^-2s^-1. Thesefindings will result in improvements in the overall efficiencyof the wheatxmaize system for wheat doubled haploid production.Copyright1998 Annals of Botany Company Intergeneric crossing, temperature, light intensity,Triticum aestivum,wheat,Zea mays,maize.