Consequences of Wheat Breeding on Nitrogen and Phosphorus Yield, Grain Nitrogen and Phosphorus Concentration and Associated Traits

Several studies have analysed the effects of wheat breedingon dry matter accumulation and partitioning, but little hasbeen done to understand the effects on nutrient economies. Theobjective of this study was to identify the changes producedby wheat breeding in the economy of nitrogen and phosphorusunder field conditions. Two experiments were carried out withseven genotypes (including a commercial hybrid) representingdifferent eras of plant breeding. Wheat breeding has increased grain nitrogen and phosphorus yieldbut total absorbed nutrients have not shown any trend duringthis century. The main attribute closely related to the increasein grain nitrogen and phosphorus yields was their harvest indices.The higher nutrient partitioning in the newer cultivars wasassociated with lower grain nitrogen and phosphorus concentrationsin their grains. Therefore, there was a negative effect of geneticimprovement in grain nitrogen and phosphorus concentrations.The main cause for the decreased concentration of these nutrientsin the grains of the modern cultivars appeared to be a dilutionby an even more increased dry matter partitioning. It is suggestedthat future breeding should be aimed to select for higher nitrogenuptake as a way to increase the level of this nutrient in grain.Copyright1995, 1999 Academic Press Triticum aestivum L., wheat breeding, genetic improvement, nitrogen, phosphorus, wheat, grain nitrogen concentration, grain phosphorus concentration     

Spatial and temporal patterns of phytoplankton composition in subtropical coastal lagoon, the Indian River Lagoon, Florida, USA

A 2 year study of the phytoplankton community was carried outin the Indian River Lagoon, USA. In terms of biovolume, thephytoplankton community was generally dominated by dinoflagellates,diatoms or cyanobacteria. Mean phytoplankton standing cropswere highest in the most flow-restricted regions of the lagoon,which had the lowest mean salinity values and comparativelyhigh total nitrogen:total phosphorus ratios. In this region,blooms of dinoflagellates were common in the first year of thestudy, which was characterized by an El Niño event thatyielded exceptionally high rainfall levels and freshwater outflow.Picoplanktonic cyanobacteria blooms became more prominent inthe second year of the study, which was characterized by belowaverage rainfall conditions. In unrestricted flow regions ofthe lagoon, located near inlets to the Atlantic Ocean, diatomswere most often the dominant taxa. Regions of intermediate waterturnover rates and high external loading of phosphorus had aprevalence of diatom blooms. However, the average phytoplanktonstanding crops in the latter regions did not reach the levelsexperienced in the flow-restricted parts of the lagoon. In termsof individual phytoplankton taxa, the most common bloom-formingdiatoms in the Indian River Lagoon system included: Skeletonemacostatum, Dactyliosolen fragilissimus, Skeletonema menzelii,Cerataulina pelagica, Odontella regia, Chaetoceros lorenzianus,Rhizosolenia setigera and Thalassionema nitzschioides. The majorbloom-forming dinoflagellate species included: Pheopolykrikoshartmannii, Akashiwo sanguinea, Prorocentrum micans, the potentiallytoxic species Pyrodinium bahamense var. bahamense and Prorocentrumminimum. Several picoplanktonic cyanobacteria were also prominentmembers of the phytoplankton community, including Synechococcuselongates. The spatial and temporal patterns observed in someof these dominant species were attributable to patterns in keyenvironmental variables, including salinity, temperature andnutrient concentrations.     

Osmotic Stress in Coleoptiles and Primary Leaves of Wheat: I. EFFECTS ON SIZE, WEIGHT, TOTAL PROTEIN, DNA, AND PHOSPHORUS

Seedlings of wheat (Triticum durum, cv. Balcarceño-INTA)were water-stressed in darkness with 20% polyethylene glycol(PEG) 6000 or 0.3 M mannitol added to the root medium. At differenttimes and up to a total of 36 h of treatment the coleoptileand primary leaves were cut and analysed. The height and freshweight of shoots were lower in treated plants than in controlplants. Dry weight was not significantly different between controland water-stressed plants. Total protein concentration decreasedsignificantly (P < 0.01) after 36 h of PEG 6000 treatment.Total DNA concentration decreased in controls but not significantly(P < 0.025) in treated seedlings. This result was interpretedas indicating that cell elongation prevailed over cell divisionin controls and that cell enlargement was affected in stressedplants. Total phosphorus concentration fell in control and treatedseedlings. However, phosphorus specific radioactivity increasedby 116% in control plants, 93% in mannitol-treated plants, and22% in PEG 6000-treated seedlings. These data suggest that anearly metabolic effect of water stress may be on phosphorusturnover in shoots.     

Nutrient Relations of Groundsel (Senecio vulgaris) Infected by Rust (Puccinia lagenophorae) at a Range of Nutrient Concentrations I. Concentrations, Contents and Distribution of N, P and K

Groundsel (Senecio vulgaris L.), healthy or infected with therust fungus Puccinia lagenophorae was grown in sand and fedwith a complete nutrient medium diluted to give a range of concentrations.Analysis of bulked, dried tissues of the plant showed that undernutrient-rich conditions rust infection resulted in increasedconcentrations of total (Kjeldahl) nitrogen and potassium buthad little effect on phosphorus concentration. Thus, despitereduced dry weight growth, total plant nitrogen contents wereno less in rusted than control plants. Although total contentsof phosphorus and potassium were reduced by rust, effects wereprobably related to loss of these nutrients in fungal spores. Interactions between rust infection and nutrient supply weresignificant but differed between nutrients: rust caused increasednitrogen concentrations only under nutrient-rich conditionsbut increased phosphorus concentrations only when nutrient supplywas limited. Increased concentrations were not confined to infectedtissues. Mechanisms underlying rust-nutrient interactions appearto be complex and to depend inter alia on the partitioning andrecycling of the particular nutrient within the plant. Rust-inducedincreases in potassium concentration occurred under both highand low nutrient conditions but were confined to infected tissues.Potassium accumulation in nutrient deficient conditions wasprobably due to increased transpirational flux into infectedtissues, but under nutrient-rich conditions reduced potassiumexport appeared to assume greater significance. The possible implications of the changed nutrient relationsfor the wider interactions of rust-infected plants in naturalvegetation are discussed. Senecio vulgaris, Puccinia lagenophorae, rust infection, nutrient deficiency, nutrient content, nutrient concentration, nutrient distribution     

Effect of Nitrogen, Phosphorus and Potassium Additions on Plant Biomass and Soil Nutrient Content of a Swale Barrier Strand Community in Louisiana

Fertilization of a swale plant community with various levelsand combinations of nitrogen, phosphorus and potassium resultedin increased plant growth. Nitrogen addition produced the greatestincrease in biomass. At some high nitrogen levels, phosphoruslimited plant production. Potassium fertilization did not influenceplant growth. Phosphorus fertilization decreased the levelsof soil calcium, magnesium and manganese. A shore-perpendiculargradient in apparent saltwater inundation influenced soil pHand the concentrations of iron, phosphorus, and magnesium. Fertilizer effects, swale Scirpus americanus, Spartina patens, nitrogen limitation     

Nutrient levels and the development of diatom and blue-green algal blooms in a shallow Australian estuary

In the Peel-Harvey estuary system, Western Australia, some 90%of riverflow and nutrient loading occurs in three winter months.Diatom blooms follow riverflow, but are replaced by blooms ofthe blue-green Nodularia spumigena Mert., especially in HarveyEstuary. By analysis of time series data from 1977–1983,it is shown that the magnitude of the Nodularia bloom in summeris related to the minimum salinity of the estuary (and hencetotal river flow), maximum phosphate concentration and totalriverine phosphorus loading, in the previous winter. The relationshipshave a predictive capacity. It is argued that diatom bloomstrap phosphorus, which is sedimented largely as faecal pellets;the phosphorus is recycled and supports Nodularia growth underwarmer conditions, and the amount available determines Nodulariabiomass. Nodularia blooms collapse when summer salinities reach30     

The Uptake and Utilization of Phosphorus and Nitrogen by Diploid, Tetraploid and Hexaploid Wheats (Triticum spp.)

Twenty genotypes of Triticum and Aegilops wheats including diploid,tetraploid and hexaploid types, were grown under contrastingphosphorus (P) regimes (control and low P) at 15 °C by dayand 10 °C at night. Dry-matter production and phosphorusand nitrogen uptake and distribution were measured on matureplants. Phosphorus efficiency (PE) was considered in terms of yieldper unit of P in the main shoot and concentration of phosphorusin grain (per cent P). In the low-P set, PE, which ranged from110 to 715 mg grain mg^–1 P, increased as the yield perculm and dry-matter partitioning (harvest index) increased,with hexaploid > tetraploid > diploid. In both the controland low-P plants percentage P in grain decreased in the orderdiploids > tetraploids > hexaploid wheats. Grain phosphoruswas highly negatively correlated with the log of grain yield(r = –0.74; –0.88) and the log of harvest index(r = –0.80 and –0.88) for control and low-P plants,respectively. This suggests that future gains in plant harvestindex will cause smaller reductions in grain phosphorus concentrations.But, within either a high or low phosphorus supply, wheats witha given grain harvest index have significantly different grainphosphorus concentrations, and conscious selection for thischaracter is feasible. Low-P plants had similar grain nitrogen concentrations but lowernitrogen harvest indexes than control plants. Aegilops spp., Triticum spp., wheat, yield components, harvest index, polyploidy, evolution, phosphorus efficiency     

The Effect of Wind and a Reduced Supply of Phosphorus and Nitrogen on the Growth and Water Relations of Festuca arundinacea Schreb

Festuca arundinaceae was grown at high and low wind-speed attwo levels of either soil phosphorus or soil nitrogen. At increasedwind-speed, mean relative growth rate and leaf extension ratewere reduced when plants were grown with high nutrient concentrationsand further reduced when phosphorus or nitrogen stress was imposedon the plants. Transpiration was increased at high wind-speedexcept under conditions of phosphorus stress, where the ratewas actually decreased. Relationships between water stress,wind and nutrient status are discussed, especially in relationto the possible role of phosphorus stress in causing sclerophylly. Festuca arundinacea Schreb., relative growth rate, water stress, wind, nitrogen, phosphorus, sclerophylly     

Cytokinins and the Growth Responses of Seedlings of Betula pendula Roth. and Acer pseudoplatanus L. to Nitrogen and Phosphorus Deficiency

Deficiencies in the supply of nitrogen and phosphorus broughtabout the cessation of shoot extension and the formation ofterminal resting buds in seedlings of Acer pseudoplatanus maintainedunder long days, whereas seedlings of Betula pendula maintainedunder the same conditions continued extension growth of themain axis over a long period, although they showed symptomsof nitrogen deficiency, including a reduced chlorophyll content.The endogenous cytokinin levels of the base/nucleoside fractionin leaves of B. pendula were significantly lower in the nitrogen-deficientplants than in the controls only 3 d after transfer to deficientconditions. Similar, though less marked, changes were observedin response to phosphorus deficiency. Changes in the levelsof cytokinin activity in the nucleotide fraction in responseto nitrogen deficiency were much less marked. No significantchanges in cytokinin levels of the base/nucleoside or nucleotidefractions could be detected in leaf material from seedlingsof A. pseudoplatanus, even after a nitrogen or phosphorus deficiencytreatment of 4 weeks. Application of exogenous benzyladenineto seedlings of B. pendula subjected to nitrogen deficiencyresulted in a rapid regreening and to outgrowth of inhibitedlateral buds after only 2 d.     

The Effects of Shaking on the Growth and Water Relations of Festuca arundinacea Schreb

The extension growth of Festuca arundinacea was reduced by shaking,and the leaves formed during shaking were thinner and narrowerthan the controls. The response to shaking was modified by thenutritional status of the plant — shaken plants took upmore phosphorus from a phosphorus deficient soil than the controlsand this advantage seemed to offset the influence of shaking. Shaken plants displayed a higher stomatal conductance and alower water potential than the controls, though it was consideredthat the effect of shaking could not be attributed entirelyto differences in water potential. Festuca arundinacea Schreb., water potential, stomatal conductance, phosphorus, shaking     

Plant allometry, leaf nitrogen and phosphorus stoichiometry, and interspecific trends in annual growth rates

• Background Life forms as diverse as unicellular algae,zooplankton, vascular plants, and mammals appear to obey quarter-powerscaling rules. Among the most famous of these rules is Kleiber's(i.e. basal metabolic rates scale as the three-quarters powerof body mass), which has a botanical analogue (i.e. annual plantgrowth rates scale as the three-quarters power of total bodymass). Numerous theories have tried to explain why these rulesexist, but each has been heavily criticized either on conceptualor empirical grounds. • N,P-Stoichiometry Recent models predicting growth rateson the basis of how total cell, tissue, or organism nitrogenand phosphorus are allocated, respectively, to protein and rRNAcontents may provide the answer, particularly in light of theobservation that annual plant growth rates scale linearly withrespect to standing leaf mass and that total leaf mass scalesisometrically with respect to nitrogen but as the three-quarterspower of leaf phosphorus. For example, when these relationshipsare juxtaposed with other allometric trends, a simple N,P-stoichiometricmodel successfully predicts the relative growth rates of 131diverse C₃ and C₄ species. • Conclusions The melding of allometric and N,P-stoichiometrictheoretical insights provides a robust modelling approach thatconceptually links the subcellular ‘machinery’ ofprotein/ribosomal metabolism to observed growth rates of uni-and multicellular organisms. Because the operation of this ‘machinery’is basic to the biology of all life forms, its allometry mayprovide a mechanistic explanation for the apparent ubiquityof quarter-power scaling rules.     

Changes in the Distribution of Magnesium, Potassium, Calcium and Phosphorus during Growth of Cucurbita Seedlings

Ockenden, I. and Lott, J. N. A. 1988. Changes in the distributionof magnesium, potassium, calcium and phosphorus during growthof Cucurbita seedlings—J. exp. Bot. 39: 973–980. Etiolated seedlings of Cucurbita maxima cv. Warted Hubbard andCucurbita andreana, grown in distilled water, showed considerablegrowth. However, only a small percentage of the minerals storedin the large cotyledons was mobilized to the root-shoot axisof the etiolated seedlings. The proportion of minerals removedfrom the cotyledons to the root-shoot axis was about two-timeshigher in C. andreana than in C. maxima. When the seedlingswere grown in distilled water in the presence of light, thepercentage of cotyledon mineral content that was mobilized waslower than the amount mobilized during growth in the dark. Cotyledonsof light-grown seedlings that were given adequate nutrients,took up minerals from the medium. These fully-expanded and photosyntheticcotyledons had slightly more P and Mg than did the cotyledonsof the dry embryos but had increased their K content 5-foldand their Ca content 50-fold. Key words: Cucurbita, minerals, seeds, seedings, NAA     

Advantages from diel vertical migration can explain the dominance of Gonyostomum semen (Raphidophyceae) in a small, steeply-stratified humic lake

In late summer, a large flagellated alga, Gonyostomum semen(Raphidophyceae), constituted most of the phytoplankton biomassin a small steeply-stratified humic lake. Its diel verticalmigration (DVM) was very distinct and extended at night intothe anoxic hypolimnion. After midsummer, the depletion of hypolimneticoxygen led to a gradual release of soluble reactive phosphorus(SRP) from the sediment, but one month later, irrespective ofcontinuing stratification, the concentrations again returnedto undetectable levels down to the bottom. As this coincidedwith the rapid increase in G.semen population, the latter wasprobably responsible for the depletion of SRP. The flux of SRPfrom the sediment to the epilimnion was virtually interrupted,making non-migrating phytoplankton dependent only on regeneratedand inflowing inorganic phosphorus. Besides nutrient availability,DVM also benefited G.semen in the reduction of metabolic andgrazing losses. In this lake, the remarkable multiple advantagesof DVM probably explain the dominance of the large G.semen inthe late summer phytoplankton biomass over much smaller algae.     

Crown Gall Tumorigenesis in the Forage Legume Medicago sativa L.

Seedlings of the forage legume Medicago sativa formed crowngall tumours at low frequency following inoculation with octopineand nopaline strains of Agrobacterium tumefaciens. There occurreda plant variety-bacterial strain specificity with respect totumorigenesis, some Medicago varieties failing to develop tumoursfollowing inoculation. In comparison, all bacterial strainstested incited tumours on stem explants of Nicotiana tabacumcv. Xanthi. DNA-DNA hybridization revealed a simple T-DNA patternin an uncloned and cloned Medicago tumour incited by the octopinestrain ACH5, protoplast cloning indicating the T-DNA copy numberto be about 5 copies per genome. Comparison of the T-DNA ofoctopine positive and octopine negative B6 tumours showed acorrelation between absence of part of the T-DNA and loss ofopine synthesis. Although non-transformed tissues of Medicagoreadily regenerate plants, this ability was suppressed in transformedtissues. ¹Present address: Centro Miglioramento Genetica Piante Foraggere,Facolt? di Agraria, Borgo XX Giugno, 06100 Perugia, Italy (Received September 14, 1983; Accepted February 7, 1984)     

Nitrogen, Phosphorus, and Potassium Nutrition of the Edaphic Ecotypes in Cynodon dactylon (L.) Pers.

A differential response to nitrogen, phosphorus, and potassiumwas observed for the three edaphic ecotypes in Cynodon dactylon(L.) Pers. naturally occurring at Malla, Shiwalik, and Chandigarh.The soils of these sites differ markedly with respect to availablenutrients. The better growth of the Malla population comparedto the Chandigarh population in low potassium and low nitrogenmedia could be related to the low availability of these nutrientsin the calcareous Malla soil. The Shiwalik population tendedto show an intermediate behaviour. Whilst the Malla populationwas susceptible to ‘phosphate toxicity’, the growthyield of the Shiwalik and the Chandigarh populations improvedwith increase in phosphorus in the medium. The three populationsalso differed from each other with respect to the uptake ofvarious nutrients. These results could be correlated with thenatural habitat conditions in which the three ecotypes occur.It was concluded that besides calcium which was shown to bepartly responsible for the restriction of the three ecotypesto their respective habitats, the availability of nitrogen,potassium, and also phosphorus may play an important role.     

Minerals, Carbohydrates and Amino Acids of Pollens from some Woody and Herbaceous Plants

The mineral, carbohydrate, protein, amino acid, lipid and energycontents of the seven main pollens gathered by honey-bees insouth-east Scotland were determined. Neither dry matter (mean73 per cent) nor ash (mean 3 per cent) were very variable. Thecalcium content of all pollens was relatively similar as werethe magnesium, potassium and sodium contents but manganese whichwas relatively high in ericaceae and phosphorus which was lowin Trifolium were more variable. Acer pollen contained mostprotein and Fagus least, and of the amino acids only serine,cystine and histidine were significantly different in the differentpollens. Over 30 per cent of all pollens was a mixture of fructoseand glucose, ericaceae containing most and Acer least. Hemicellulose(mean 7 per cent) was high in Ranunculus and low in Acer, cruciferaeand Fagus. ‘Cellulose’ (mean 0•5 per cent)was high in cruciferae and low in ericacea. Ether extract (mean5 per cent) was high in cruciferae and Fagus. The mean grossenergy of pollen was 23 kJ g^–1. Fresh pollen is comparable with lean meat as a source of energyand protein, and is more like dormant plant seeds than herbagein its water, ash and protein content but stores energy as sugarrather than starch or lipid.     

Crown Gall on Castor Bean Leaves: II. FORMATION OF SECONDARY TUMOURS

Secondary tumours were formed on the cotyledonary leaf petiole,the hypocotyl, and first true leaf of castor bean seedlingsafter inoculating the blades of the cotyledonary leaves withAgrobacterium tumefaciens. Depending on the strain of bacteriaemployed, 0 to 80 per cent of the plants developed secondarytumours. The ability of different strains to initiate secondarytumours was not obviously correlated with their relative effectivenessin initiating primary tumours. Though all produced primary tumours,five out of ten auxotrophic strains failed to yield secondarytumours, whereas only one out of 14 prototrophic strains failedto do so. Both the number of plants developing secondary tumoursand the frequency with which these tumours occurred on differentparts of the plant were positively correlated with the concentrationof the primary inoculum. Tumours also developed on the cotyledonaryleaf petiole and on the hypocotyl after vacuum infiltrationof A. tumefaciens into the blade of cotyledonary leaves. Inmost instances (9 out of 11 plants) no tumours were formed onthe blade of the infiltrated leaf. Thus, tumour formation equivalentto secondary tumours can occur in the absence of a primary tumouror an overt primary wound. Excision of inoculated leaves showedthat the stimulus for secondary tumour formation moves fromthe blade to the hypocotyl within 24 h. Attempts to demonstratethe presence of a sub-cellular tumour-initiating agent in homogenatesof inoculated leaves were unsuccessful. A. tumefaciens, however,was found in the petiole of the cotyledonary leaf and in thehypocotyl within 24 h of inoculation. The migrating agent responsiblefor secondary tumour formation in castor beans is concludedto be intact bacteria.     

Mycorrhizal Response of Two Tomato Genotypes Relates to their Ability to Acquire and Utilize Phosphorus

The purpose of this study was to determine how a plant's responseto colonization by mycorrhizal fungi relates to its abilityto acquire and utilize phosphorus for growth and reproduction.Two tomato genotypes previously found to be either responsive(‘LA1709’) or unresponsive (‘large cherry’)to mycorrhizal colonization during early vegetative growth wereexamined in detail. Plants were grown at four levels of addedphosphate or with mycorrhizal inoculum. Vegetative and reproductivegrowth, phosphorus uptake and root length density were measuredduring the course of plant development. Mycorrhizal symbiosissignificantly increased above-ground dry mass, root length,phosphorus content and yield under low phosphorus conditionsin ‘LA1709’, while it had less effect on these characteristicsin ‘large cherry’. When uninfected, however, ‘LA1709’grew and reproduced poorly unless high amounts of phosphoruswere added to the soil, while ‘large cherry’ grewwell under very low phosphorus conditions. This was because‘large cherry’ had significantly higher root lengthdensities than ‘LA1709’, enabling plants from thisgenotype to explore more soil volume and acquire greater amountsof phosphorus when grown without mycorrhizal fungi in low phosphorussoil. ‘Large cherry’ also had higher phosphorususe efficiency and allocated a greater proportion of phosphorusto reproduction when uninfected than ‘LA1709’. Itappears traits that affect a plant's ability to acquire andutilize phosphorus efficiently for growth and reproduction canalso affect its response to mycorrhizal colonization in tomato.Copyright1998 Annals of Botany Company. Tomato,Lycopersicon esculentum,mycorrhiza,Glomus etunicatum,phosphorus, reproduction, lifespan.     

Phytoplankton and zooplankton development in a lowland, temperate river

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

Elemental composition (C:N:P) and growth rates of bacteria and Rhodomonas grazed by Daphnia

The elemental composition and growth rate of Rhodomonas andheterotrophic bacteria were studied in batch cultures in thepresence and absence of Daphnia and at two different levelsof phosphorus limitation. The elemental content of single cellswas measured with X-ray microanalysis. Simultaneously, dilutionexperiments were performed in order to estimate grazing losses,growth rates and dominant nutrient sources for bacteria andRhodomonas. The phosphorus:carbon (P:C) ratios of the bacteriawere generally higher in the experiment with the stronger Plimitation of the system. High P:C ratios were taken as an indicationthat bacteria were carbon limited. The presence of Daphnia resultedin a further increase in bacterial P:C ratios and increasedspecific growth rates. Thus, grazing increased the availabilityboth of inorganic nutrients and organic substrates, stimulatingthe growth of the bacteria. P:C ratios of Rhodomonas decreasedwith increasing P limitation of the system. Only at strong Plimitation did the presence of Daphnia result in increased P:Cratios of Rhodomonas compared with the control without daphnids.This study shows that the elemental content and growth rateof heterotrophic bacteria and Rhodomonas are influenced by grazingand nutrient regeneration by daphnids. The response is dynamicand depends on the level of nutrient limitation of the system. Present address: Department of Microbiology, University of BergenJahnebakken 5, NO-5020 Bergen, Norway