Phytoplankton blooms: a 'loophole' in microzooplankton grazing impact?

Phytoplankton size and relations between phytoplankton and microzooplankton(ciliates and heterotrophic dinoflagellates) biomass are analysedin 12 globally distributed areas. In view of the results, ahypothesis is posed where blooming species are those able toescape control by microzooplankton through a combination ofpredation avoidance mechanisms (e.g. larger size, colonies,spines, and toxic compounds) at the beginning of the bloom.Factors that help to enhance subsequent bloom development includepositive feedback from the poor nutritional status of the phototrophicprey which adversely affects predation, inter-microzooplanktongrazing and top–down grazing by mesozooplankton on microzooplankton.Blooming conditions are interpreted as physical or chemicalperturbations disrupting the predator–prey controls thatnormally operate at the level of the microbial loop, opening‘loopholes’ into which some phytoplankton speciespopulations can explode.     

Effects of Flooding and Drainage and their Alternation on the Growth and Uptake of Nutrients by Rice (Oryza sativa L., indica, var. IR-8)

Continuous flooding of the soil (‘flooded’ treatment)gave best growth of IR-8 variety of rice whereas soil drainedfor 4 weeks and then flooded for 8 (‘drained and flooded’treatment) resulted in poorest growth and chlorotic plants.Plants grown in the continuously drained soil (‘drained’treatment) and those in the soil flooded for 4 weeks and thendrained for 8 (‘flooded and drained’ treatment)showed intermediate growth. There were no differences in therelative water content of plants growing in the various treatments.Analyses of plant tissues showed that a consideration of therelative concentration of Fe, Mn, and P in the shoots is mostclosely related to the performance of rice under various culturalconditions. An increase in the concentration of Fe in the planttissues following flooding was correlated with the best growth(‘flooded’ treatment) unless it was accompaniedby high level of Mn (as in the ‘drained and flooded’treatment) which may have proved toxic, e.g. by interferencewith Fe metabolism as was evidenced by chlorosis. Measurementsof oxidation-reduction potentials, oxygen diffusion rates, andthe concentration of exchangeable and soluble Fe and Mn in thesoils have shown that the ‘drained and flooded’treatment caused the most extreme reducing conditions. Floodingaccompanied by the development of extreme reducing conditionsled to a greater accumulation of Mn in the shoots (‘drainedand flooded’ treatment) whereas flooding accompanied bythe maintenance of oxidizing conditions (‘flooded’treatment) resulted in a lower uptake of Mn. Growth of riceplants for 4 weeks in the drained soil did not fit them forthe reduced conditions which developed during subsequent flooding(‘drained and flooded’ treatment).     

Microzooplankton grazing of phytoplankton in a tropical upwelling region

We measured grazing by herbivorous zooplankton (<200 μm fraction) in coastal and slope regions of the South Brazil Bight. Using the dilution technique, we performed nine experiments during the austral summer, when nutrient-rich South Atlantic Central Water is present on the shelf, and five during winter. These experiments provide the first estimates of microzooplankton grazing in the western South Atlantic Ocean. Model II regression showed a strong relationship between phytoplankton intrinsic growth rates and grazing, with a slope of 0.64 (±0.28; 95% confidence interval) indicating that microzooplankton grazing could account for the majority of phytoplankton mortality. Both phytoplankton growth and microzooplankton grazing were higher during the summer upwelling season, compared to winter. For the two experiments that were conducted in oligotrophic slope water, grazing accounted for >80% of phytoplankton production. A comparison of incubations with and without added inorganic nutrients showed no consistent stimulation of phytoplankton growth (slope of enriched versus unenriched treatments not significantly different from 1). Estimates from microscopic counts of heterotrophic organisms >10 μm indicated that copepod nauplii comprised the largest share of the microzooplankton biomass (mean 62.4 ± 5.8% SE). Grazing estimates were not correlated with microzooplankton biomass, whether or not nauplii were included, suggesting that most of the grazing was done by nano-sized zooplankton. Electronic Supplementary Material Electronic supplementary material is available in the online version of this article at and is accessible for authorized users. Handling editor: S. Wellekens     

Effects of mesozooplankton removal and ammonium addition on planktonic trophic structure during a bloom of the Texas 'brown tide': a mesocosm study

A bloom of the alga Aureoumbra lagunensis, known as the Texas‘brown tide’, persisted in the Laguna Madre of Texasfor most of the 1990s. The dominant mesozooplankter in LagunaMadre, Acartia tonsa, does not feed on A. lagunensis, and duringblooms there are few other suitably sized phytoplankton cellsavailable to feed on. We hypothesized that these copepods increasedtheir feeding on microzooplankton, thereby reducing grazingpressure by microzooplankton on A. lagunensis and contributingto the persistence of this bloom. A mesocosm experiment wascarried out to test this hypothesis during the summer of 1999.Twelve fiberglass corral-type mesocosms were deployed in thefield for 16 days, each enclosing 1.2 m³ of Laguna Madre waterand 1.1 m² of natural benthos. Mesozooplankton were removedfrom six mesocosms with a 153 µm mesh dip net every 4days; the other six mesocosms were treated in the same way,except that the contents of the net were returned to the mesocosm.For each zooplankton treatment, half of the mesocosms were dosedwith 40 µm NH₄ at 4 day intervals, and half received noadditions. Phytoplankton populations in these mesocosms at thestart of the experiment were dominated by A. lagunensis andthe cyanobacterium Synechococcus spp. Growth rates of A. lagunensiswere higher in mesocosms without ammonium additions, providingno evidence for nitrogen limitation. Acartia tonsa populationswere reduced by 50% in the zooplankton removal mesocosms, andciliate populations were significantly higher. The increasein ciliate population had no significant impact on A. lagunensispopulation dynamics, however, providing no evidence to supportthe hypothesis that a trophic cascade reducing microzooplanktonpopulations contributed to the persistence of the brown-tidebloom. In contrast, populations of Synechococcus sp. showedevidence of both ‘top–down’ and ‘bottom–up’control; they grew faster in nutrient addition mesocosms andhad lower populations in mesocosms with increased densitiesof ciliate grazers.     

The Effect of Applications of a Synthetic Cytokinin on Shoot Dominance and Grain Yield in Spring Barley

Spring barley cv Ark Royal was grown in pots in the open, andsoil drenches containing 6-benzyl-aminopurine were repeatedlyapplied during the pre-heading phase starting either ‘early’when the main shoot apex had reached the glume primordium stage,during tillering, or ‘later’ at the anther primordiumstage, at the start of stem elongation Although grain yield was unaffected by the ‘late’cytokinin treatments, it was increased by up to 57 per centby the ‘early’ treatments. The increase was dueto greater grain yields of the smaller shoots of each plantthere was no effect on the yields of the two largest shootsor on the total number of fertile shoots The increased grain yield of these smaller shoots was attributableto increased weights of their individual kernels, especiallyin the basal and distal parts of the ears such that a greateruniformity of kernel size within the ear was achieved Thus the cytokinin treatment increased uniformity both betweenshoots and between spikelets within the ears Hordeum vulgare L, spring barley, cytokinin, 6-benzylaminopunne, shoot dominance, harvest index, grain yield     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Food web interactions in a Calanus finmarchicus dominated pelagic ecosystem--a mesocosm study

The significance of nauplii versus copepodite stage V of Calanusfinmarchicus grazing and their effects on the structure of thefood web were investigated during two sampling periods of 7–8days in March and April in experimental mesocosms held in aNorwegian fjord over a 2 month period. The mesocosms were manipulatedby the addition of two different levels of inorganic nutrients(control versus enriched). During the ‘naupliar’period in March, the phytoplankton was characterized by a diatombloom while during the ‘copepodite’ period in April,it was in a post-bloom phase characterized by small-celled species,mainly Phaeocystis pouchetii. Phytoplankton, bacterial and protozooanbiomass and production rates were measured in addition to copepodbiomass. Copepod grazing was estimated by three different methods:(i) gut fluorescence; (ii) chlorophyll clearance from the water;and (iii) growth method measured as body carbon increase. Thetwo latter methods gave similar results for nauplii, but allthree gave different results for the copepodites. Independentsomatic growth, based on changes in abundance and individualcarbon content, and grazing estimates revealed an overall growthefficiency of 0.66 ± 0.20 (mean ± S.E.) for copepodites.Empirical carbon flow models were constructed, which indicatedthat the nauplii could not control either phytoplankton or protozoangrowth in either the control or in the enriched system. Ignoringrecycling and sedimentation, the fate of the primary productionfor the nauplii-dominated community was to be grazed by a diverseand abundant protozooplankton community. In the copepodite-dominatedcommunity, the copepods grazed >100% of the daily primaryproduction, and also grazed heavily on a protozooplankton communityof low biomass and diversity and presumably on detritus. Thefate of the primary production in the two different copepodscenarios followed predicted routes for ‘low meso-zooplankton’and for ‘high meso-zooplankton’ biomass systems,as suggested by Wassmann (Wassmann, 1998).     

ASYNCHRONOUS REPRODUCTIVE ACTIVITY IN THE BROADCAST SPAWNER CELLANA CAPENSIS (GMELIN, 1791) (GASTROPODA: PATELLIDAE)

Short-term fluctuations in the reproductive condition of thelimpet Cellana capensis are described. Gravimetric and histologicalanalyses of gonadal development were carried out on samplescollected at 4 day intervals over a 3 month period from theMkambati Nature Reserve, Transkei. The results indicate a distinctlack of reproductive synchrony within the population, whichapplied not only to gametogenic development per se but alsoto spawning activity. Some of the spawning events identifiedduring the study period were more marked than others, thesemay reflect a greater degree of synchronization in activity.Results presented suggest that C. capensis is probably a ‘partial’rather than a ‘complete’ spawner. The implicationsof asynchronous spawning activity for the reproductive successof broadcast spawners are also discussed. (Received 6 December 1988; accepted 15 February 1989)     

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.     

Fast microzooplankton grazing on fast-growing,low-biomass phytoplankton: a case study in spring in Chesapeake Bay,Delaware Inland Bays and Delaware Bay

Dilution experiments were performed to examine the growth and grazing mortality rates of picophytoplankton (<2 μm), nanophytoplankton (2–20 μm), and microphytoplankton (>20 μm) at stations in the Chesapeake Bay (CB), the Delaware Inland Bays (DIB) and the Delaware Bay (DB), in early spring 2005. At station CB microphytoplankton, including chain-forming diatoms were dominant, and the microzooplankton assemblage was mainly composed of the tintinnid Tintinnopsis beroidea. At station DIB, the dominant species were microphytoplanktonic dinoflagellates, while the microzooplankton community was mainly composed of copepod nauplii and the oligotrich ciliate Strombidium sp. At station DB, nanophytoplankton were dominant components, and Strombidium and Tintinnopsis beroidea were the co-dominant microzooplankton. The growth rate and grazing mortality rate were 0.13–3.43 and 0.09–1.92 d⁻¹ for the different size fractionated phytoplankton. The microzooplankton ingested 73, 171, and 49% of standing stocks, and 95, 70, and 48% of potential primary productivity for total phytoplankton at station CB, DIB, and DB respectively. The carbon flux for total phytoplankton consumed by microzooplankton was 1224.11, 100.76, and 85.85 μg C l⁻¹ d⁻¹ at station CB, DIB, and DB, respectively. According to the grazing mortality rate, carbon consumption rate and carbon flux turn over rates, microzooplankton in study area mostly preferred to graze on picophytoplankton, which was faster growing but was lowest biomass component of the phytoplankton. The faster grazing on Fast-Growing-Low-Biomass (FGLB) phenomenon in coastal regions is explained as a resource partitioning strategy. This quite likely argues that although microzooplankton grazes strongly on phytoplankton in these regions, these microzooplankton grazers are passive. Handling editor: K. Martens     

Microzooplankton grazing in the surface water of the Southern Ocean during an austral summer

Microzooplankton grazing was investigated in surface waters of the Indo-Pacific and Atlantic sectors of the Southern Ocean by the dilution method. Phytoplankton growth varied mainly between 0.1 and 0.4 day⁻¹, and microzooplankton grazing between 0.0 and 0.3 day⁻¹. Great fluctuations in phytoplankton growth rate were observed at one station within 3 weeks and between closely spaced stations. Microzooplankton grazing rates were similar to phytoplankton growth rate despite the variation of phytoplankton growth rates, although in some cases, phytoplankton growth overwhelmed microzooplankton grazing. These observations suggest that microzooplankton are the main consumers of primary producers, and that steady state between phytoplankton growth and microzooplankton grazing is usually established in the Southern Ocean in austral summer. Received: 5 November 1996 / Accepted: 4 March 1997     

The Extraction, Solubility, and Characterization of Two Groups of Barley Storage Polypeptides

This paper reports further studies on the characteristics ofthe storage protein fraction (hordein) of barley. Hordein consistsof two groups of polypeptides (termed ‘B’ and ‘C’)coded by two separate but linked loci. Whereas the ‘C’polypeptides are readily soluble and extracted in 60% (v/v)ethanol at room temperature, the ‘B’ group is moresoluble in, and therefore more efficiently extracted by, 50%(v/v) propan-1-ol or 45% (v/v) propan-2-ol at elevated temperaturesand in the presence of 2-mercaptoethanol. However, the mostefficient conditions for hordein extraction (50% propan-1-ol+ 2% (v/v) 2-mercaptoethanol at 60 °C) also extract somecontaminating non-hordein polypeptides resulting in an apparentlyincreased lysine content of the hordein fraction. Amino acid analysis of the purified ‘B’ and ‘C’hordein groups shows that, whereas ‘C’ hordein containsmore glutamate + glutamine, proline, and phenylalanine than‘B’ hordein, it contains only traces of lysine andsulphur amino acids in contrast to ‘B’ hordein whichcontains 0·5% lysine 0·6% methionine, and 2·5%cysteine. Equilibrium sedimentation analyses carried out on the purified‘B’ and ‘C’ groups indicates that thepreparations were reasonably monodisperse with molecular weightsof approximately 32 000 and 52 000 respectively. These valuesare considerably lower than those previously determined by SDS-PAGE.     

Factors Affecting the Abscission of Reproductive Organs in Yellow Lupins (Lupinus luteus L.): III. ENDOGENOUS GROWTH SUBSTANCES IN VIRUS-INFECTED AND HEALTHY PLANTS AND THEIR EFFECT ON ABSCISSION

Extracts of small and mature-size lupin pods yielded four substancesaffecting the growth of wheat-coleoptile sections: one acidpromotor (A), two acid inhibitors(B and X), and one neutralinhibitor(Y). Inhibitor B was extremely active, however, coleoptile sectionsshowed no signs of toxic effects; they resumed growth at a rapidrate after rinsing them and adding ß-indolylaceticand (IAA) to the medium. 1 µg of IAA was required to counteractthe effect of ‘B’ extracted from 230 mg. Of tissue.On an equal fresh weight basis the inhibiting action of ‘B’in lupin pods was 500–1,500 times more potent than thatof ‘inhibitor ß’ in etiolated pea seedlings. Small pods of plants infected with pea-mosaic virus yielded3 times the amount of ‘A’ of healthy plants (equivalentto 1 µg. IAA 0.3 µg. IAA per 25 g. of tissue respectively),and approximately the amount of ‘B’. Mature podsof virus-infected plants again yielded more‘A’,but also 2? times more ‘B’ than pods of healthyplants. Healthy pods yielded more ‘A’ than virus-infectedpods, and there was no difference in ‘X’. A lupin abscission test was developed and the effects of proximaland distal application of -naphthyl acetic acid (NAA) are presented,and discussed with respect to results of other abscission tests. ‘A’ accelerated abscission when applied proximally,and delayed or prevented it when applied distally. ‘B’strongly accelerated abscission when applied in either way.A possible mechanism explaining the abscission-inducing effectof developing pods on later flowers is discussed in terms ofthe substances ‘A’ and ‘B’. The partlyprevented abscission observed on virus-infected plants was foundto agree well with the proposed mechanism.     

Phytoplankton development and turbulent mixing in Lake Kinneret (1992-1996)

We have utilized data from a recently developed three-dimensionalvelocity fluctuation meter to compute the dissipation of turbulentkinetic energies (TKE) and the intensity of turbulent mixingin horizontal and vertical planes in the pelagic, epilimnicwater of Lake Kinneret, Israel. These characteristics of wind-inducedturbulent movement have been monitored from January 1992 throughDecember 1996. The turbulence parameters were strongly correlatedto wind energy inputs, calculated daily as 5 day cumulativeinputs. There have been dramatic changes in the annual and seasonaldevelopment of phytoplankton, together with unusually high levelsof primary production in this lake since 1994. We observed differentpatterns of vertical and horizontal turbulent movement and ofTKE dissipation rates during the years when ‘unusual’phytoplankton development occurred (1994–1996) comparedto ‘normal’ years (1992, 1993). The first appearanceof the filamentous cyanobacterium Aphanizomenon in this lakein August–September 1994 coincided with a period of markedlylower rates of TKE dispersion and a shift from vertical to horizontaldominance of the turbulent eddy spins. The absence of a regularwinter-spring bloom of the dinoflagellate, Peridinium, in 1996occurred when dissipation rates of TKE were extremely high,while record high amounts of dinoflagellates (1994, 1995) appearedwhen dissipation rates were very low. Correlations were shownbetween phytoplankton parameters (chlorophyll, primary productionand the ratio of primary production to chlorophyll) and boththe dissipation rate of TKE and the intensity of water turbulentmixing in the vertical plane. We suggest that the changes inthe ‘turbulence climate’ of Lake Kinneret were animportant factor in determining shifts in phytoplankton successionand the population composition of the algal assemblage.     

The Influence of the Plant Growth-regulator, 2-Methyl-4-Chlorophenoxyacetic Acid, on the Metabolism of Carbohydrate, Nitrogen and Minerals in Solanum lycopersicum (Tomato)

The effects of the foliar application of phytocidal concentrationsof 2-methyl-4-chlorophenoxyacetic acid (MCPA) on change in totaldry weight, and in ‘available carbohydrate’ (starch,‘total’ and ‘reducing’ sugars), totalnitrogen, phosphorus, potassium, calcium, and magnesium of ‘tops’and roots of tomato plants have been followed over a periodof 14 days following spraying. There were two main treatments—‘nutrient’(nutrient supply to roots continued after spraying) and ‘water’(distilled water only supplied to roots after spraying) and‘water’ (distilled water only supplied to rootsafter spraying)—the sub-treatments consisting of ‘MCPA’versus ‘no-MCPA’ for each of the main treatments.Twelve different times of sampling were used. In analysing the present data, the quantity ‘residualdry weight’ (total dry weight less ‘available carbohydrate’),which was originally introduced by Mason and Maskell as a basisof reference for analyses of plant organs in short-period experimentsnot involving appreciable growth, has been used as an estimateof the permanent structure of plant growth. This new use ofthe ‘residual dry weight’ basis has brought outimportant features which were obscured when the data were leftin their primary form (as percentages of total dry weight oramounts per plant). Growth, as measured by increase in ‘residual dry weight’,was greatly inhibited by 2-methyl-4-chlorophenoxyacetic acidshortly after spraying, in both the presence and the absenceof nutrient. In the presence of 2-methyl-4-chlorophenoxyacetic acid, netassimilation rate (estimated as rate of increase in total dryweight per gram ‘residual dry weight’ of the ‘tops’)was greatly diminished while uptake of total nitrogen and ofP₂O₅ (estimated as increase in total nitrogen or of P₂O₅ ofthe whole plant per day per 1 g. ‘residual dry weight’of the roots) appeared to undergo a similar but much smallerdiminution. It seemed probable, however, that in the presenceof MCPA a larger proportion of the carbohydrate actually formedwas utilized for synthesis of aminoacids and protein. In the plant as a whole there was no evidence of actual depletionof ‘available carbohydrate’ as a result of MCPAtreatment, this fraction showing a steady increase in all treatmentsthroughout the experiment. The rate of increase was, however,much reduced by MCPA treatment. The ‘tops’ presentedmuch the same picture as the whole plant, but for the rootsthe situation was quite different. While the roots of the ‘no-MCPA’plants and also of the ‘MCPA-water’ plants showeda steady increase in available carbohydrate, those of the ‘MCPA-nutrient’plants rose only very slightly (from the initial value of 8mg. per plant to about 10 mg.) during the first 2 days, andthen in the next 2 days declined to a value (about 6 mg.) belowthe initial and remained at this low level for the rest of theexperiment. It is suggested that the phytocidal effect of 2-methyl-4-chlorophenoxyaceticacid in the presence of nutrient may be due to depletion ofthe ‘available carbohydrate’ supplies in the roots,which is shown to be brought about, in part, by reduced transportfrom the tops, and partly by the relatively greater utilizationof the carbohydrate present. These results offer an explanationfor the facts that plants showing vigorous growth are more easilykilled by MCPA and that perennial plants, particularly thosewith storage tissues in their roots, are more resistant. Further,they suggest the useful practical application that MCPA treatmentshould be given when the carbohydrate reserves of the rootsare at a minimum. For perennial plants, conditions might beexpected to be optimal for the application of MCPA in late spring,at a time when the first ‘flush’ of growth is slowingdown and before any appreciable new reserves of carbohydratehave been accumulated. It was also shown that 2-methyl-4-chlorophenoxyacetic acid preventedthe net synthesis of starch, but still permitted an appreciablenet formation of sucrose. 2-methyl-4-chlorophenoxyacetic acid appeared to have no effecton the uptake of potassium, calcium, or of magnesium. The lackof effect on potassium is contrasted with the previous observationby Rhodes, Templeman, and Thruston (1950) that sub-lethal concentrationsof MCPA, applied over a relatively long period to the rootsof tomato plants, specifically depressed the uptake of potassium.     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

Relative Sensitivity and Tolerance of some Gladiolus Cultivars to Sulphur Dioxide

Five Gladiolus cultivars, namely ‘Aldebaran’, ‘BrightEye’, ‘Illusion’, ‘Manisha’ and‘Manmohan’, were exposed to 1 and 2 µg l^–1sulphur dioxide to test their relative-sensitivity toleranceto the pollutant Plants were fumigated experimentally for 2h daily Foliar injury symptoms were observed first in ‘Manisha’followed by ‘Aldebaran’ and ‘Illusion’at the higher dose Photosynthetic pigments and leaf extractpH were significantly decreased, particularly in ‘Manisha’and ‘Illusion’ Overall disturbances in the plantmetabolism due to SO₂ treatment led to retarded growth of plants,as evident from decreased shoot length and phytomass valuesThe order of sensitivity of the five Gladiolus cultivars toSO₂ was as follows, with the greatest first Manisha, Illusion,Aldebaran, Bright Eye, Manmohan Cultivars, Gladiolus, sensitivity, sulphur dioxide, tolerance     

Microzooplankton grazing and the control of phytoplankton biomass in the Suwannee River estuary,USA

Microzooplankton grazing can have significant impacts on the distribution and abundance of phytoplankton, thereby influencing the frequency and duration of algae blooms. Observations of high ciliate abundances in the Suwannee River estuary, Florida, suggest a significant potential for top-down pressure on the phytoplankton community by microzooplankton. We examined the composition of the microzooplankton and determined grazing mortality losses for phytoplankton within the Suwannee River estuary from 2001 to 2002. Our results indicated grazing mortality rates of 1.4 d⁻¹, equivalent to a loss of up to 76% of phytoplankton standing crop and up to 83% of total daily primary production. The microzooplankton community was primarily composed of ciliates, dinoflagellates, and copepod nauplii. The densities of ciliates in the estuary were comparable to densities reported in highly eutrophic ecosystems (9,400–72,800 ciliates l⁻¹). Grazing pressure on small phytoplankton may be further enhanced because ciliates and small dinoflagellates have growth rates similar to those of phytoplankton, and therefore can keep up with surges in abundance. Handling editor: Judit Padisak     

Induction of 'Pathogenesis-related' Proteins in Tobacco Leaves by Physiological (Non-pathogenic) Disorders

The PR protein-inducing effect of naturally-occurring physiologicaldisorders of tobacco (‘sharilka’, due to water excess,and Mn toxicity) was established. This supports the view ofPR proteins as ‘stress’ macromolecules rather thanexclusively pathogenesis-related macromolecules. The effectsof the physiological disorders were compared with those of PVY^N-infection;all these treatments produced similar symptoms of necrotic spottingon leaves. It was demonstrated that non-pathogenic (‘physiological’)necrobiosis induced the same PR proteins as did infectious necrobiosis. Key words: PR proteins, tobacco, physiological disorders     

The effect of sedimentation and microbial nitrogen regeneration in a plankton community: a simulation investigation

A computer simulation model was developed to investigate nitrogenfluxes associated with microbial interactions in plankton communities.A short time scale was used, appropriate to the build-up anddecline of phytoplankton blooms in temperate shelf waters aftera mixing or upwelling event. The model depicts a continuum ofevents, many of which have been observed in coastal, upwellingand oceanic systems, including two phytoplankton peaks correspondingto ‘new production’ and ‘regenerated production’.It predicts that nitrogen loss through sedimentation of phytoplanktonand faeces may result in a smaller bloom with a delayed onsetand prolonged duration. Microbial regeneration of nitrogen wasfound to be important in sustaining the middle stages of a phytoplanktonbloom, whereas micro- and meso-zooplankton regeneration occurredtowards the end of the bloom.