Dynamics of a stable, large volume, laboratory ecosystem containing Daphnia and phytoplankton

Daphnia magna and phytoplankton were maintained in stable, large-volume(3400 l), indoor, planktonic model ecosystems for up to 20 weeks.Ecosystem stability was demonstrated by damped oscillationsin population abundance, and by a return to steady-state conditionsif the system was perturbed. The steady-state biomass of daphniaand algae responded to variations in nutrient loading ratesand daphnia harvesting rates in accordance with predictionsfrom a mathematical model. Increased nutrient addition resultedin an increase in biomass of daphnia, but not phytoplankton,if both were present. Increased harvesting caused an increasein chlorophyll and decreased daphnia biomass. Phosphorus additionswere balanced by sedimentation losses which were proportionalto daphnia biomass. Increased harvesting rates resulted in decreasedphosphorus loss rates and increased efficiency of phosphorusconversion into harvested daphnia. Damped oscillations had aperiodicity of 3.5 weeks and could be successfully modelledassuming a half-saturation constant for phosphorus control ofalgal growth of 1 µg l^–1. The ecosystem became unstableif nutrient loading rates exceeded a defined limit which wasa function of harvesting rates. Steady-state chlorophyll anddaphnia biomass levels were similar to levels approached inLake Michigan in summer in 1983 and 1984, when the zooplanktonbiomass was dominated by Daphnia pulicaria.     

A comparison between methods used to control nutrient supply

Experimental methods to supply nutrients to culture solutionsin order quantitatively to control plant nutrition are compared.In experiments with tomato and birch plants, for which the dataare available in databases (Ingestad et al., 1994a, b), thenutrients were supplied at constant relative addition rates(R_A over sufficiently long periods of time to achieve acclimatedplants and reliable measurements of plant responses. The plantswere maintained under steady-state conditions, i.e. the internalnutrient concentrations (c₁) remained constant, as a resultof a numerical equality between the relative uptake rate (R_U)and the relative growth rate (R_G). These results are comparedto experiments with pea plants (Macduff et al., 1993). In oneseries (a), R_A was applied, but without strict control of internalsteady-state, and in the other series (b), the external concentration(c_e) was maintained constant. With limiting nitrogen, in bothseries, there was a substantial deviation from equality betweenR_U and R_G. In (a), c_I changed during the experimental periodand the purpose of the R_A approach was lost. In (b), a constantc_e had little effect on nitrogen uptake and plant growth. Atthe three highest concentrations, steady-states were obtainedat non-limiting uptake rates. At the lowest concentration, theuptake rate of nitrogen was about the same, but there was adecrease of R_a, which apparently was not caused by reduced uptake.Clear-cut relationships can not therefore be established betweentreatment variables and plant responses and the conclusionsreached by Macduff et al. (1993) have little support in theirexperimental results. This indicates an urgent need to updateboth theories and experimental methods together: in particular,it is important to identify the system under investigation andto distinguish between control of the medium and control ofthe plant. Key words: Experimental control, external nutrient concentration, non-limiting and limiting nutrient supply, relative addition rate, relative uptake rate, relative growth rate, steady-state     

Developmental Analysis of Steady-State Levels of Cu/Zn and Mn Superoxide Dismutase mRNAs in Maize Tissues

The superoxide dismutase (Sod) steady-state mRNA levels in maizeseedlings and developing kernels were examined by RNA blot analysis,using Cu/Zn and Mn cDNA probes encoding the cytosolic and mitochondrialSOD isozymes, respectively. The cytosolic (Cu/Zn) Sod steady-statemRNA levels remained relatively constant for the various tissuesexamined. In contrast, the mRNA levels of the mitochondrial(Mn) SOD-3 isozyme increased in postgerminative scutella. Thesteady-state mRNA of the Atp2 gene, F₁ ATPase ß subunit,was compared to the Sod3 (Mn) mRNA levels. Results of this comparativestudy suggest that the steady-state levels of mRNAs transcribedby nuclear genes encoding essential mitochondrial proteins areindependently regulated throughout development. (Received April 2, 1990; Accepted September 10, 1990)     

Biosynthesis of Anthraquinone by Cells of Galium mollugo L. Grown in a Chemostat with Limiting Sucrose or Phosphate

Growth and anthraquinone biosynthesis by Galium cells were examinedin steady-state substrate-limited conditions using a chemostatcontinuous culture technique. Steady-state growth was obtainedin both sucrose- and phosphate-limiting conditions for periodsup to 60 d. In sucrose-limiting conditions three growth rateswere investigated with doubling times (t_d) of 25 h, 35 h and40 h, and phosphate-limited growth was obtained at t_d= 35 h.The kinetics of the growth response to a change in limitingsubstrate concentration in sucrose-limiting conditions was examinedand found to follow closely that predicted by the applicationof Monod's (1950) model obtained for micro-organisms. The anthraquinone content of cells grown in phosphate and sucroselimitation was uniformly similar and at a relatively low level(0.68 mg g^–1 dry wt.). When the substrate limitation wasrelieved by the addition of the limiting substrate, either phosphate,or sucrose, anthraquinone synthesis was markedly stimulated.The addition of the anthraquinone precursor, orthosuccinyl benzoicacid (OSB) greatly enhanced anthraquinone synthesis in phosphate-limitingconditions but not in sucrose-limited cells. The results show that growth limitation by phosphate and bysucrose causes a suppression of the rate of synthesis of thesecondary metabolite anthraquinone in Galium cells and suggeststhat the metabolic point of suppression is different in eachcase. Key words: Anthraquinone biosynthesis, Galium, Continuous culture, recursor feeding     

A Dynamic Model for Plant Growth: Validation Study Under Changing Temperatures

A dynamic simulation model to describe vegetative growth ofplants, for which some functions and parameter values have beenestimated previously by optimization search techniques and numericalexperimentation based on data from constant temperature experiments,is validated under conditions of changing temperatures. To testthe predictive capacity of the model, dry matter accumulationin the leaves, stems, and roots of tobacco plants (Nicotianatabacum L.) was measured at 2- or 3-day intervals during a 5-weekperiod when temperatures in controlled-environment rooms wereprogrammed for changes at weekly and daily intervals and inascending or descending sequences within a range of 14 to 34°C. Simulations of dry matter accumulation and distributionwere carried out using the programmed changes for experimentaltemperatures and compared with the measured values. The agreementbetween measured and predicted values was close and indicatesthat the temperature-dependent functional forms derived fromconstant-temperature experiments are adequate for modellingplant growth responses to conditions of changing temperatureswith switching intervals as short as 1 day. Nicotiana tabacum, tobacco, translocation, partitioning, root growth, shoot growth     

A Simple and Inexpensive Design of Chemostat Enabling Steady-state Growth of Acer pseudoplatanus L. Cells under Phosphate-limiting Conditions

Operational and constructional details are given of a relativelysimple and inexpensive chemostat designed for the continuousculture of plant cells in suspension. This apparatus permitscontrol of the growth rate of sycamore, Acer pseudoplatanusL. cells in steady-state conditions. By alteration of the rateof input of medium different steady-state growth rates wereobtained over a wide range (mean doubling times from 182 h to36 h). In order to establish a growth-limiting nutrient thetime course of nutrient uptake in batch culture was measured.In batch culture the maximum growth obtained was proportionalto the initial concentration of phosphate when this was belowa concentration of 17 µg P per ml (as phosphate). It isalso shown in chemostat culture that the steady-state cell densityis proportional to the phosphate concentration in the mediumwhen this is below 17 µg P per ml (as phosphate). Phosphatewas therefore established to be the growth rate-limiting nutrientin chemostat culture at a concentration of 8•5 µgP per ml (as phosphate).     

Resource competition between two rotifer species (Brachionus rubens and B.calyciflorus): an experimental test of a mechanistic model

A mechanistic model of competition on a single resource wastested experimentally with two freshwater rotifers, Brachionusrubens and B.calyciflorus, both grown on the alga Chlorellapyrenoidosa. Using open culture systems for each species wemeasured: (i) the resource-saturated exponential growth rate,µ_max, and (ii) the relationship between specific growthrate, µ, at steady-state and the residual algal concentrationover a range of system turnover rates, or dilution rates, D.The µ_max of B.calyciflorus was {small tilde}60% higherthan B. rubens. These results were then used to construct agraphical model for predicting the victor in interspecific competitionbetween the two rotifers. Since the two resource-dependent growthrates crossed, one species, B.calyciflorus, was predicted tobe the victor at a high D while B. rubens was predicted as thevictor at low D. Finally, the outcome of competition was determinedfor two turnover rates. As predicted by the graphical competitionmodels, B.calyciflorus was the dominant species at rapid D (0.029h^–1) and B.rubens was dominant at slow D (0.0044 h^–1).These studies support recent conclusions that mechanistic competitionmodels may be applied to predict dominant species from a prioriinformation on growth potential and resource levels, which isnot possible with traditional Lotka-Volterra models.     

Quantitative analysis of the combined effects of temperature, evaporative demand and light on leaf elongation rate in well-watered field and laboratory-grown maize plants

The respective effects of meristem temperature, vapour pressuredeficit (VPD) and photosynthetic photon flux density (PPFD)on leaf elongation rate (LER) of maize, in the absence of waterdeficit in the soil have been quantified. This analysis wascarried out in a series of field experiments in northern andsouthern France over several seasons and years, and in growthchamber experiments. LER was measured with 10 min steps, togetherwith meristem temperature, VPD and PPFD at leaf level in threetypes of experiments: in growth chamber experiments with stepsin PPFD or VPD at constant meristem temperature, in growth chamberexperiments with several combinations of constant, but contrasting,PPFDs, VPDs and meristem temperatures, and in the field withfluctuating conditions, (i) When evaporative demand was low(night or day with low air VPD), LER was only linked to meristemtemperature, regardless of other climatic conditions, (ii) Lighthad no effect per se on LER in the range from 0 to 1500 molm^–2 s^–1 for time-scales longer than 2 h, providedthat its indirect effects on meristem temperature and on evaporativedemand were corrected (in the growth chamber) or taken intoaccount (in the field), and provided that cumulated PPFD overa weekly time-scale was compatible with field conditions, (iii)Evaporative demand sensed by growing leaves, as estimated bymeristem-to-air vapour pressure difference, markedly affectedLER in the range from 1–4 kPa, at all time-scales understudy, with a unique relationship in the growth chamber (constantconditions) and in the field (fluctuating conditions). Thiseffect was only observed when PPFD was high enough for stomatato open. The negative effect of evaporative demand on LER wasprobably not due to long distance root-to-shoot signalling,since soil was wet, calculated root water potential remainedclose to 0 MPa and concentration of ABA in the xylem sap wasvery low. Therefore, it is proposed to model maize LER witha two-step process, involving the calculation of the maximumLER at a given meristem temperature and then the calculationof the reduction in LER due to evaporative demand. Joint analysisof the whole set of data by using the two equations yieldeda r² of 0.75. This two-step process would be more accurate thanthe provision of LER from temperature only in cases where airVPD frequently exceeds 2 kPa. Key words: Leaf growth, light, evaporative demand, temperature, thermal time, water deficit, ABA, Zea mays L.     

Comparative Growth Behaviour and Leaf Nutrient Status of Native Trees Planted on Mine Spoil With and Without Nutrient Amendment

The effect of nutrient amendment on growth of nine indigenoustree species planted on coal mine spoil was studied. Greatergrowth in fertilized plots was accompanied by greater foliarN and P concentrations in all species. The response to fertilizationvaried among species and was greater in non-leguminous thanin leguminous species. Furthermore, leguminous species exhibitedhigher growth rates compared to non-leguminous species. Thelog-transformed height-diameter relationships were significantfor all tree species and treatments. Acacia catechu, Dalbergiasissoo, Gmelina arborea and Azadirachta indica fitted the elasticsimilarity model of tree growth; whereas Pongamia pinnata andPhyllanthus emblica followed the constant stress model. Tectonagrandis was the only species which fitted the geometric similaritymodel. In Albizia lebbeck and Terminalia bellirica, the b -values(the gradients of the log-transformed height to diameter relationships)were considerably lower (<0.5), and these two species didnot fit any model of tree growth. In several cases, the b -valueswere considerably influenced by nutrient amendment. The log-transformedcrown mass and trunk mass relationships were significant forall treatments and species. The slope of the crown mass:trunkmass relationship was near unity in A. indica, D. sissoo, G.arborea, P. emblica, P. pinnata, T. grandis, and T. bellirica.However, in A. catechu and A. lebbeck, this slope was well belowunity suggesting a greater allocation to non-photosynthetictissue. Fertilizer amendment resulted in a heavier crown relativeto trunk in A. indica, T. grandis and T. bellirica. Copyright2001 Annals of Botany Company Diameter increment, fertilizer application, foliar N, foliar P, height increment, tree growth, volume increment     

A Model for Plant and Crop Growth, Allowing for Competition for Light by the use of Potential and Restricted Projected Crown Zone Areas

Relations for competition for light are developed and used ina plant growth model applicable to the isolated plant, to plantsin even-aged monoculture and to plants in mixed-aged monoculture.In an isolated plant, it is assumed that a leaf area, proportionalto the plant mass, is contained within a crown whose projectedzone area is proportional to plant mass to the 2/3 power. Self-shadingprogressively reduces the specific growth rate. If light werethe sole limiting resource and were constant, one can derivea growth equation, dw/dt = rw[1 - exp (-KW^1/3)]KW^1/3, which,integrated, gives w^1/3 = K^-1 ln {1 + [exp (KW^1/3₀)-1] exp (rt/3)}.It approximates, initially, to a particular case of the Richards(1959) empirical growth equation. In even-aged evenly-spaced monocrops competing only for light,it is assumed that the zone areas merge at canopy closure, andgrowth then follows the expolinear equation of Goudriaan andMonteith (1990), giving a continuous function based on groundcover. For mixed-aged monocrops, we assume a phase of canopyclosure that affects the younger plants earlier than the olderones. Under varying environmental conditions in the field, plant growthmay be affected by other factors in addition, e.g. temperature.In the growth conductance model of Aikman and Scaife (1993),the shading expressions are applied to the light-dependence. Data from two sowings of cabbage and carrot in even-aged andmixed-aged monocrops were used to test the model. The parametervalues derived from the even-aged monocultures predict the growthrates in the mixed-aged monocultures better than models whichassume uniform canopies.Copyright 1994, 1999 Academic Press Growth, model, monocrop, even-aged, mixed-aged, PAR, density, competition, light, shading, zone area, ground cover, temperature, carbon dioxide, expolinear, carrot, Daucus carota L., cabbage, Brassica oleracea L     

Regulation of Photosystem Stoichiometry in the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714 in Response to Light-Intensity

Light-induced changes in stoichiometry among three thylakoidcomponents, PS I, PS II and Cyt b₆-f complexes, were studiedwith the cyanophyte Synechocystis PCC 6714. Special attentionwas paid to two aspects of the stoichiometric change; first,a comparison of the patterns of regulation in response to differencesin light-intensity with those induced by differences in light-quality,and second, the relationship between regulation of the stoichiometryand the steady state of the electron transport system. Resultsfor the former indicated that (1) the abundance of PS I on aper cell basis was reduced under white light at the intensityas high as that for light-saturation of photosynthesis, butPS I per cell was increased under low light-intensity, (2) PSII and Cyt b₆-f complexes remained fairly constant, and (3)changes in the abundance of PS I depended strictly on proteinsynthesis. The pattern was identical with that of chromaticregulation. For the second problem, the redox steady-statesof Cyt f and P700 under white light of various intensities weredetermined by flash-spectroscopy. Results indicated that (1)Cyt f and P700 in cells grown under low light-intensity [highratio of PS I to PS II (PS I/PS II)] were markedly oxidizedwhen the cells were exposed to high light-intensity, while theyremained in the reduced state under low light-intensity. (2)After a decrease in the abundance of PS I, most of P700 remainedin the reduced state even under high light-intensity, whilethe level of reduced Cyt f remained low. (3) Both Cyt f andP700 in cells of low PS I/PS II were fully reduced under lowlight-intensity, and Cyt f reduction following the flash wasrapid, which indicates that the turnover of PS I limits theoverall rate of electron flow. After an increase in the abundanceof PS I, the electron transport recovered from the biased state.(4) The redox steady-state of the Cyt b₆-f complex correlatedwell with the regulation of PS I/PS II while the state of thePQ pool did not. Based on these results, a working model ofthe regulation of assembly of the PS I complex, in which theredox steady-state of the Cyt b₆-f complex is closely relatedto the primary signal, is proposed. (Received August 2, 1990; Accepted December 10, 1990)     

Cell-cycle effects on the kinetics of silicic acid uptake and resource competition among diatoms

Current models of nutrient utilization by planktonic algae generallyassume that cells transport nutrients continuously. This assumptionis violated in the case of Si utilization by diatoms. Silicicacid transport is confined to specific portions of the cellcycle such that only a fraction of the cells in a populationare likely to be transporting silicic acid at any given time.A theoretical framework for describing the growth and nutrient-uptakekinetics of populations of cells exhibiting cell-cycle-dependentnutrient uptake is presented. Thalassiosira weissflogii wasused as a model organism. Kinetic parameters determined at steadystate, assuming cells take up Si continuously, underestimatedboth the V_m and K_s of individual cells by nearly an order ofmagnitude. Likewise, kinetic constants determined from short-termexperiments assuming continuing uptake were shown to be dependenton the initial physiological state of the population and todramatically underestimate the kinetic parameters of individualcells. The response of steady-state populations to large pulsesof dissolved Si was also examined. The ratio of the averagemaximum specific uptake rate of the culture to the average maximumobservable specific growth rate (V' _mavg/     

Kinetic study of hybridoma cell growth in continuous culture. I. A model for non-producing cells

The kinetic behavior of a nonproducing hybridoma clone AFP-27-NP was investigated in continuous culture under glucose-limited conditions. A total of more than 21, 000 h of cultures were operated at dilution rates ranging from 0.01 to 0.06 h(-1). The viable cell concentrations, dead cell concentrations, and cell volumes all varied with the dilution rate. A steady-state model was developed based on the biomass concentration and the glucose concentration. The specific growth rate as a function of glucose concentration is described by a model similar to the Monod model with a threshold glucose concentration and a minimum specific growth rate incorporated; the model is meaningful only at glucose concentrations and specific growth rates above these levels. A death rate is included in the model which is described by an inverted Monod-type function of glucose concentration. The yield coefficient based on glucose is constant in the lower range of specific growth rates and changes to a new constant value in the upper region of specific growth rates. No maintenance term for glucose consumption was needed; in the plot of specific glucose consumption rate vs. specific growth rate, the line intercepted the specific growth rate axis at a value close to the minimum growth rate. The values for the model parameters were determined from regression analysis of the steady-state data. The model predictions and experimental results fit very well.     

Analysis of Dry Matter Partitioning in Dactylis glomerata during Vegetative Growth Using a Carbon Budget Model

The dry matter partitioning in vegetative plants of Dactylisglomerata was studied from experiments performed in controlledenvironments. Plants were grown hydroponically in growth chambers,at two constant temperatures (17 and 25 °C). In both experimentsthe root fraction decreased regularly with time, an effect thatwas more accentuated in the higher temperature regime. In orderto explain the change in dry matter partitioning, the experimentalshoot and root growth were analysed using a carbon budget modelwhich includes shoot and root maintenance requirements. Themodel predicts a relationship between the root specific growthrate and the product of shoot specific growth rate and shootto root dry weight ratio. In the range of experimental accuracy,this relationship was found to be linear at both temperatures,which should indicate that the partitioning coefficients andthe root maintenance coefficient remained constant during vegetativegrowth. The effect of temperature on the value of these coefficientscan be specified from a linear regression analysis. Between17 and 25 °C, the root maintenance coefficient increasedby about a factor of two, whereas the partitioning coefficientsdid not vary significantly. On the basis of these results, itwas shown that the decrease in root fraction during vegetativegrowth should be mainly attributed to the decrease in net specificactivity of shoots.Copyright 1994, 1999 Academic Press Dactylis glomerata L., vegetative growth, model, partitioning, root:shoot ratio, shoot specific activity, maintenance requirements     

Leaf Growth in Cotton (Gossypium hirsutum L.) 1. Growth in Area of Main-stem and Sympodial Leaves

A model is presented for growth of individual and successivemain-stem leaves of cotton, based on a series of indoor experimentsand data sets from the literature. Three variable parametersare used to describe individual leaf growth: relative growthrate of meristematic tissue (R¹), relative rate of approachof final area (R₂) and a ‘position parameter’ (t_0.5)which governs the transition from meristematic to extensiongrowth. Final area of a leaf does not occur in the model asa deterministic quantity but it is a result of the processesduring growth. The model generates successive mainstem leavesand sympodial leaves as an integrated system. Assimilate shortagesoccurring in the plant operate on R₁ leading to the characteristicchange of final leaf area along the mainstem. Gossypium hirsutumL., cotton, leaf growth, relative growth rate, meristematic tissue, extension growth, mathematical model     

A Second-order Dynamic Model for the Frost Hardiness of Trees

The development of frost hardiness in forest trees is describedby a dynamic model in which the input variables are the prevailingenvironmental conditions and the developmental stage of trees.The assumption of the model is that for each temperature andphotoperiod there is a discrete stationary level of frost hardiness,which is attained if these environmental factors remain constant.The dependence of the stationary level on temperature and photoperiodis assumed to be piece-wise linear and additive. The rate ofacclimation, i.e. frost hardening or dehardening, is describedas a second-order dynamic process with two time constants, thesecond of which changes depending on the stage of the annualdevelopment of the trees. The frost hardiness model was calibratedand tested using experimental data from Douglas fir [Pseudotsugamenziesii var. glauca (Beissn.) Franco] seedlings. The resultssuggest that the second-order model describes the changes infrost hardiness better than the first-order model with onlyone time constant.Copyright 1995, 1999 Academic Press Acclimation, developmental stage, Douglas fir, dynamic model, frost hardiness, photoperiod, Pseudotsuga menziesii, temperature     

Quantitative Aspects of a Theory of Translocation

An analysis is made of two aspects of the behaviour of a proposedmechanism of translocation based on the mixing effect withina sieve-tube of streaming transcellular strands of protoplasmpassing through the sieve-pores and through a vacuole-like reservoirof sucrose solution in each sieve-element. First, the behaviour of the model is considered when it is transportinga solute (sucrose, for example) in the steady-state conditionof constant consumption and supply. It is found that the concentrationsof solute in the regions of the model are readily determinedat all points of the sieve-tube, and that the gradients of concentrationare all equal, the concentrations differing by constant amountsat all points. Transport is analogous to diffusive transferin that the steepness of the concentration gradient is proportionalto the rate of transfer and simple expressions are derived forthe net transport of solute and for the apparent diffusion coefficientin terms of the properties of the model. Transfer is found tobe rapid over a short path and slower over a longer path; thereis a maximum distance beyond which a given rate of transfercannot take place. Second, an analysis is made of the unsteady behaviour of themodel when changing concentrations of solute are introducedinto the system. The concept of strand diffusion time is introduced.This is the time taken for the concentration in the strand surroundedby an infinite reservoir to fall to I/e of its initial value.The results of this analysis are used to determine how radioactivesolute will become distributed when it is introduced into thesteady-state model. If the time over which the translocationof radioactive solute has proceeded is small compared with thestrand diffusion time, a wave-like profile of radioactivitywith distance is produced, and if large, the profile is of adiffusive kind; intermediate conditions give a profile displayingboth characteristics which is termed the transition type. Thesethree types of profile of radioactivity are exemplified by experimentsin different plants (Salix, Tropaeolum, and Helianthus), andthe experimental profiles are used to estimate (1) the apparentdiffusion coefficient of translocated sucrose in the sense ofMason and Maskell, and (2) the permeability constant of thestrand-reservoir boundary. The close correspondence betweenthese two sets of figures derived from the radioactive profileswith the experimental values obtained directly is regarded asstrong confirmation of the theory.     

Growth and light absorption of some planktonic cyanobacteria, diatoms and Chlorophyceae under simulated natural light fluctuations

Specific growth rates of Limnozhrix redekei, Planktothrix agardhii(cyanobacteria), Synedraacus, Stephanodiscus minutulus (diatoms),Scenedesmus acuminatus and Scenedesmus armatus (Chlorophyceae)were compared under different time structures of illumination,but the same daily light exposure, at 20C. Fluctuating irradiancesimulating a uniform rapid transport of the algal cells acrossthe aquatic light field on a cloudless day with Z^eu/Z^mix=1 wascompared with constant irradiance throughout the same photoperiodof 12 h length as well as a photoperiod of 6 h length. Fluctuatinglight (30 min for a cycle) resulted in a decrease in specificgrowth rates as compared with constant irradiance at the samephotoperiod length. This decrease amounts to 15–20% fordiatoms, 20–25% for Chlorophyceae and 35–40% forcyanobacteria, respectively. The decrease is somewhat lowerif the fluctuations simulating mixing are slower (60 min fora cycle). The specific growth rate is also decreased by a shorterphotoperiod, but this effect is more species specific. Regardingthe in vivo absorption spectra, fluctuating light or a shorterphotoperiod has little or no effect on the Chlorophyceae anddiatoms studied, whereas cyanobacteria show an increase in lightabsorption by chlorophyll a and phycobilins.     

An architectural analysis of the elongation of field-grown sunflower root systems. Elements for modelling the effects of temperature and intercepted radiation

The effects of photosynthetic photon flux density (PPFD) andsoil temperature on root system elongation rate have been analysedby using an architectural framework. Root elongation rate wasanalysed by considering three terms, (i) the branch appearancerate, (ii) the individual elongation rates of the taproot andbranches and (iii) the proportion of branches which stop elongating.Large ranges ofPPFD and soil temperature were obtained in aseries of field and growth chamber experiments. In the field,the growth of root systems experiencing day-to-day natural fluctuationof PPFD and temperature was followed, and some of the plantsunder study were shaded. In the growth chamber, plants experiencedcontrasting and constant PPFDs and root temperatures. The directeffect of apex temperature on individual root elongation ratewas surprisingly low in the range 13–25C, except forthe first days after germination. Root elongation rate was essentiallyrelated to intercepted PPFD and to distance to the source, bothin the field and in the growth chamber. Branch appearance ratesubstantially varied among days and environmental conditions.It was essentially linked to taproot elongation rate, as theprofile of branch density along the taproot was quite stable.The length of the taproot segment carrying newly appeared brancheson a given day was equal to taproot elongation on this day,plus a 'buffering term' which transiently increased if taprootelongation rate slowed down. The proportion of branches whichstopped elongating a short distance from the taproot rangedfrom 50–80% and was, therefore, a major architecturalvariable, although it is not taken into account in current architecturalmodels. A set of equations accounting for the variabilitiesin elongation rate, branch appearance rate and proportion ofbranches which stop elongating, as a function of interceptedPPFD and apex temperature is proposed. These equations applyfor both field and growth chamber experiments. Key words: Sunflower, root system, model, temperature, radiation