The Suction Potential of Plant Cells and Some Related Topics

This paper is a theoretical discussion of the concept of suctionpotential or suction pressure as applied to plant cells, especiallywith reference to the possible occurrence of ‘active’water-secreting mechanisms. Following an attempt to define whatis meant by suction potential there is a discussion of the distinctionbetween ‘active’ and ‘passive’ agenciesin relation to water movement and a tabulation of such agencies. Some of the less commonly considered mechanisms are discussedin some detail, and an attempt is made to evaluate how effectivethey might be in producing increments of turgor pressure. Theconclusion is reached that if active mechanisms are operativein cell-water relations, then suction potential cannot adequatelybe denned in the ordinary way—in fact the ‘water-absorbingeffort’ of a cell cannot be completely specified in termsof pressure alone. There are appendixes on the possible role of frictional or contactelectrification in cell physiology; on the increase of permeabilityof the plasma membrane to ions and water caused by electricalforces; and on the energy requirement of active water-secretingmechanisms.     

Why do fast- and slow-growing grass species differ so little in their rate of root respiration,considering the large differences in rate of growth and ion uptake?

Herbaceous plants grown with free access to nutrients exhibit inherent differences in maximum relative growth rate (RGR) and rate of nutrient uptake. Measured rates of root respiration are higher in fast-growing species than in slow-growing ones. Fast-growing herbaceous species, however, exhibit lower rates of respiration than would be expected from their high rates of growth and nitrate uptake. We investigated why the difference in root O₂ uptake between fast- and slow-growing species is relatively small. Inhibition of respiration by the build-up of CO₂ in closed cuvettes, diurnal variation in respiration rates or an increasing ratio of respiratory CO₂ release to O₂ uptake (RQ) with increasing RGR failed to explain the relatively low root respiration rates in fast-growing grasses. Furthermore, differences in alternative pathway activity can at most only partly explain why the difference in root respiration between fast- and slow-growing grasses is relatively small. Although specific respiratory costs for maintenance of biomass are slightly higher in the fast-growing Dactylis glomerata L. than those in the slow-growing Festuca ovina L., they account for 50% of total root respiration in both species. The specific respiratory costs for ion uptake in the fast-growing grass are one-third of those in the slow-growing grass [0·41 versus 1·22 mol O₂ mol (NO₃^–)^–1]. We conclude that this is the major cause of the relatively low rates of root respiration in fast-growing grasses.     

Pico-, nano- and microplankton communities in hydrothermal marine coastal environments of the Eolian Islands (Panarea and Vulcano) in the Mediterranean Sea

This paper reports the relationship among pico-, nano- and microplanktoniccommunities observed in two different shallow marine hydrothermalenvironments. Seawater samples from five stations in the coastalarea of the ‘Porto di Levante’ (Vulcano Island)and from three stations off the Island of Panarea (Eolian Islands,Italy) were collected in May and July of 1989, respectively.Microbiological investigations were carried out in order todetermine the density of: (i) the total picoplankton (both autotrophicand heterotrophic): (ii) the total picophytoplankton (autofluorescentpicoplanktonic cells); (iii) the larger phytoplankton (>2µm): (iv) the ‘metabolically active’ cellsof total picoplankton and cyanobacteria; (v) the heterotrophicaerobic bacteria. The peak values of picoplankton and picophytoplanktoncomponents, with an order of magnitude of 10⁹ and 10⁷ cells1¹, respectively, were registered in the wannest water samples(30–75°C) collected from the Vulcano area. At Panarea.eukaryotic picophytoplankton and ‘metabolically active’coccoid cyanobacteria showed an opposite trend. A possible competitionbetween the two groups is to be considered. Cyanobacteria, diatomsand the genus Licmophora. in particular, were prevalent in thewarmest hydrothermal vents of Vulcano. Nano- and microphytoplanktoniccommunities in the offshore waters of the Island of Panareashowed more variability than in Vulcano. Moreover, in the Panareawaters the prevalence of phytoflagellates above the thermoclinewas observed, whereas diatoms were predominant below this layer.     

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.     

Soil Carbon Storage Response to Temperature: an Hypothesis

Recently, global and some regional observations of soil carbonstocks and turnover times have implied that warming may notdeplete soil carbon as much as predicted by ecosystem models.The proposed explanation is that microbial respiration of carbonin ‘old’ mineral pools is accelerated less by warmingthan ecosystem models currently assume. Data on the sensitivityof soil respiration to temperature are currently conflicting.An alternative or additional explanation is that warming increasesthe rate of physico-chemical processes which transfer organiccarbon to ‘protected’, more stable, soil carbonpools. These processes include adsorption reactions, some ofwhich are known to have positive activation energies. Theoretically,physico-chemical reactions may be expected to respond more towarming than enzyme-mediated microbial reactions. A simple analyticalmodel and a complex multi-pool soil carbon model are presented,which separate transfers between pools due to physico-chemicalreactions from those associated with microbial respiration.In the short-term, warming depletes soil carbon. But in thelong-term, carbon losses by accelerated microbial respirationare offset by increases in carbon input to the soil (net production)and any acceleration of soil physico-chemical ‘stabilization’reactions. In the models, if net production rates are increasedin response to notional warming by a factor of 1.3, and microbialrespiration (in all pools) by 1.5, then soil carbon at equilibriumremains unchanged if physico-chemical reactions are acceleratedby a factor of about 2.2 (50% more than microbial reactions).Equilibrium soil carbon increases if physico-chemical reactionsare over 50% more sensitive to warming than soil respiration.Copyright 2001 Annals of Botany Company Soil organic matter, carbon, respiration, temperature, stabilization, decomposition, model     

Active Iron in Plant Leaves

Tomato plants were grown with limited or luxury water supplyand given nitrogen either as the ammonium ion or the nitrateion. The ‘active’ iron fractions of the leaves asextracted by etherized tenth molar hydrochloric acid showedno relationship with the total iron but a very significant linearrelationship was found between the active iron fraction andthe ratio of total phosphorus to total iron. Recalculation of results presented by other investigators formaize plants grown in soil with various levels of moisture,phosphorus and iron also showed a highly significant linearrelationship between the active iron fraction and the ratioof total phosphorus to total iron. Lycopersicon esculentum L., tomato, ‘active iron’, phosphorus, Zea mays     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Pattern of Respiration of a Perennial Ryegrass Crop in the Field

‘Dark’ respiratory losses of CO₂ were measured ona one year old sward of S24 perennial ryegrass (Lolium perenneL.) at intervals during a 74 day reproductive growth period,between April and June, and a 21 day vegetative growth period,in July and August. Part of the sward was shaded for one weekbefore measure ments commenced. Measurements of ‘dark’respiration continued for 46 hand it was possible to distinguishtwo components which are designated ‘maintenance’and ‘synthetic’ ‘Maintenance’ respiration was taken to be the meanrate of CO₂ efflux after 40–46 h darkness. When calculatedon a plant d. wt basis at 15°C it ranged between 6 to 32mgCO₂ g^-1 day^-1 during reproductive growth and 10–14 mgCO₂ g^-1 day^-1 during vegetative growth. During reproductivegrowth, sward protein content ranged between 7–23 percent and when maintenance respiration was recalculated on thebasis of protein content it changed relatively little throughoutthe growth period (90–140 mg CO₂ g pro tein^-1 day^-1);the value for vegetative growth ranged between 70–100mgCO₂ g protein-day^-1. Total ‘synthetic’ CO₂ flux was determined duringreproductive growth and a rate of ‘synthetic’ CO₂flux was determined during both reproductive and vegetativegrowth. Between 15 and 35 per cent of the CO₂ fixed in the previousphotoperiod was lost in ‘synthetic’ respirationof above-ground material in reproductive swards. Previous shadingincreased the proportion of ‘synthetic’ CO₂ lossfrom above ground. The rate of ‘synthetic’ CO₂ outputduring the first hours of darkness increased with amount ofCO₂ fixed in the previous photoperiod, although it was not proportionalto it. There is some evidence that assimilate is ‘carried-over’from one photoperiod to the next.     

Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

Fiskeby V soya bean was grown from seed germination to seedmaturation with two contrasting patterns of nitrogen metabolism:either wholly dependent on dinitrogen fixation, or with an abundantsupply of nitrate nitrogen, but lacking root nodules. The carbonand nitrogen economies of the plants were assessed at frequentintervals by measurements of photosynthesis, shoot and rootrespiration, and organic and inorganic nitrogen contents. Plantsfixing atmospheric nitrogen assimilated only 25–30 percent as much nitrogen as equivalent plants given nitrate nitrogen:c. 40 per cent of the nitrogen of ‘nitrate’ plantswas assimilated after dinitrogen fixation had ceased in ‘nodulated’plants. The rates of photosynthesis and respiration of the shootsof soya bean were not markedly affected by source of nitrogen;in contrast, the roots of ‘nodulated’ plants respiredtwice as rapidly during intense dinitrogen fixation as thoseof ‘nitrate’ plants. The magnitude of this respiratoryburden was calculated to increase the daily whole-plant respiratory loss of assimilate by 10–15 per cent over thatof plants receiving abundant nitrate. It is concluded that ‘nodulated’plants grew more slowly than ‘nitrate’ plants inthese experiments for at least two reasons: firstly, the symbioticassociation fixed insufficient nitrogen for optimum growth and,secondly, the assimila tion of the nitrogen which was fixedin the root nodules was more energy-demanding in terms of assimilatethan that of plants which assimilated nitrogen by reducing nitratein their leaves.     

Modelling the Components of Plant Respiration: Some Guiding Principles

Respiration is poorly represented in whole plant or ecosystemmodels relative to photosynthesis. This paper reviews the principlesunderlying the development of a more mechanistic approach tomodelling plant respiration and the criteria by which modelbehaviour might be judged. The main conclusions are as follows:(1) Models should separate C substrate from structure so thatdirect or indirect C substrate dependence of the componentsof respiration can be represented. (2) Account should be takenof the fact that some of the energy for leaf respiration isdrawn from the light reactions of photosynthesis. (3) It ispossible to estimate respiration associated with growth, nitratereduction, symbiotic N₂fixation, N-uptake, other ion uptakeand phloem loading, because reasonable estimates are availableof average specific unit respiratory costs and the rates ofthese processes can be quantified. (4) At present, it is lesseasy to estimate respiration associated with protein turnover,maintenance of cell ion concentrations and gradients and allforms of respiration involving the alternative pathway and futilecycles. (5) The growth-maintenance paradigm is valuable but‘maintenance ' is an approximate concept and there isno rigorous division between growth and maintenance energy-requiringprocesses. (6) An alternative ‘process-residual’approach would be to estimate explicitly respiratory fluxesassociated with the six processes listed in (3) above and treatthe remainder as a residual with a phenomenological ‘residual maintenance’ coefficient. (7) Maintenance or‘residual maintenance’ respiration rates are oftenmore closely related to tissue N content than biomass, volumeor surface area. (8) Respiratory fluxes associated with differentprocesses vary independently, seasonally and during plant development,and so should be represented separately if possible. (9) Anunforced outcome of mechanistic models should be a constrained,but non-constant, ratio between whole plant gross photosynthesisand respiration. Copyright 2000 Annals of Botany Company Respiration, photosynthesis, growth, maintenance, substrate, N uptake, nitrate reduction, symbiotic N₂fixation, phloem loading, model.     

Respiratory energy requirements of roots vary with the potential growth rate of a plant species

The rates of growth, net rate of nitrate uptake and root respiration of 24 wild species were compared under conditions of optimum nutrient supply. The relative growth rate (RGR)of the roots of these species varied between 110 and 370 mg g^-1 day^-1 and the net rate of nitrate uptake between 1 and 7 mmol (g root dry weight)^-1 day^-1. The rate of root respiration was positively correlated with the RGR of the roots. Root respiration was also calculated from the measured rate of growth and nitrate uptake, using previously determined values for the costs of maintenance, growth and ion uptake of two slow-growing species. The calculated rate of respiration was slightly lower than the measured one for slow-growing species, but twice as high as measured rates for rapid-growing species. This discrepancy was not due to a relatively smaller electron flow through the alternative pathway and, consequently, a more efficient ATP production in the fast-growing species. Neither could variation in specific costs for root growth or maintenance explain these differences. Therefore, we conclude that fast-growing species have lower specific respiratory costs for ion uptake than slow-growing ones. Due partly to these lower specific costs of nutrient uptake, the fraction of respiration that rapid-growing species spend on anion uptake is lower than that of slow-growing species, in spite of the much higher rate of ion uptake of the fast-growing ones.     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 2. Grown as Young Plants from Seed

Young plants of two selection lines of Lolium perenne cv. S23with ‘fast’ and ‘slow’ rates of ‘maturetissue’ respiration were individually grown from seed,together with plants of S23, their common parent, in 9.2 cmpots in a controlled environment at 20/15 °C day/night temperatures. No significant differences were found between the genotypesin leaf extension and tiller production during this early stageof their growth. They did differ however, by an average of 26%,in the rate of dark respiration of fully expanded leaf laminae.The use of a simple model demonstrated that such a differencein respiration could alone account for the different rates ofdry matter production shown by the selection lines when grownas young crops from seed. Possible penalties of ‘slow’respiration are also considered. Lolium perenne L., ryegrass, respiration, maintenance respiration, stimulated swards, leaf growth, tiller production, carbon economy     

The growing outer epidermal wall: design and physiological role of a composite structure

Background: The cells of growing plant organs secrete an extracellular fibrouscomposite (the primary wall) that allows the turgid protoplaststo expand irreversibly via wall-yielding events, which are regulatedby processes within the cytoplasm. The role of the epidermisin the control of stem elongation is described with specialreference to the outer epidermal wall (OEW), which forms a ‘tensileskin’. Novel Facts: The OEW is much thicker and less extensible than the walls ofthe inner tissues. Moreover, in the OEW the amount of celluloseper unit wall mass is considerably greater than in the innertissues. Ultrastructural studies have shown that the expandingOEW is composed of a highly ordered internal and a diffuse outerhalf, with helicoidally organized cellulose microfibrils inthe inner (load-bearing) region of this tension-stressed organwall. The structural and mechanical backbone of the wall consistsof helicoids, i.e. layers of parallel, inextensible cellulosemicrofibrils. These ‘plywood laminates’ containcrystalline ‘cables’ orientated in all directionswith respect to the axis of elongation (isotropic material).Cessation of cell elongation is accompanied by a loss of order,i.e. the OEW is a dynamic structure. Helicoidally arranged extracellularpolymers have also been found in certain bacteria, algae, fungiand animals. In the insect cuticle crystalline cutin nanofibrilsform characteristic ‘OEW-like’ herringbone patterns. Conclusions: Theoretical considerations, in vitro studies and computer simulationssuggest that extracellular biological helicoids form by directedself-assembly of the crystalline biopolymers. This spontaneousgeneration of complex design ‘without an intelligent designer’evolved independently in the protective ‘skin’ ofplants, animals and many other organisms.     

An Agar-diffusion Technique Applied to the Study of Gibberellins in Tomato (Lycopersicon esculentumMill.) Plants

Chromatography of extracts of agar blocks on which shoot tipsof tomato had stood for 24 h revealed the presence of at leasttwo gibberellin-like substances. Gibberellin-like activity detectedby the dwarf pea bioassay was shown to be associated with activityin promoting elongation growth in tomato seedlings. The activesubstances were acidic; tests for the presence of ‘neutral’,‘basic’ and ‘bound’ gibberellins inextracts of the agar blocks gave negative results. Amounts ofgibberellin-like substances obtained from the shoot tips wereincreased by the incorporation of EDTA into the agar blocksbut were apparently unaffected by the temperature (within therange 5–25 °C) at which the diffusion was carriedout. It is suggested that the technique measured levels of diffusiblegibberellins in the tissues at the time of their excision ratherthan rates of gibberellin synthesis during the diffusion period.     

The Occurrence of Translocated Antibiotics in Expressed, Plant Sap

The details of a convenient laboratory press are presented anda procedure by which plant sap is readily obtained is described.The pressing procedure has been shown to release 53–74per cent. of the total plant water as expressed sap. Preliminary observations on the uptake and translocation ofantibiotics by higher plants obtained by examining expressedsap are described. It is shown that the assay of expressed sapprovides a measure of the griseofulvin and chloramphenicol concentrationin leaves of plants grown in solutions of these antibioticsthat does not differ significantly from the value obtained bythe assay of water extracts. Extraction of leaves from plants grown in griseofulvin solutionswith organic solvents demonstrates a much greater antibioticcontent than is indicated by the assay of expressed sap. Toexplain this difference a ‘free’ antibiotic fraction,obtainable in expressed sap or water extracts, and a ‘bound’fraction recovered from the leaf only by organic solvent extractionare postulated.     

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.     

Algal 14C and total carbon metabolisms. 1. Models to account for the physiological processes of respiration and recycling

A consistent set of equations has been written to describe thenet rate of algal ¹⁴CO₂ uptake (and where appropriate respirationand photosynthesis) which take into account separately complicationsdue to respiration of the labelled photosynthetic products andthe recycling of respiratory CO₂. Written specifically intothe equations is the concept of ‘new’ and ‘old’carbon, the coefficient q is used in the respiration model toallow for the differential respiration of organic material fromthe ‘new’ and ‘old’ carbon pools. Analyticalintegrals have been found for respiration and recycling models,and the behaviour of the models studied over periods of 12 h(i.e. up to 70% of the intrinsic generation time). The rateconstant for respiration has a greater effect on the behaviourof the recycling than the respiration model. Over short timecourses (up to 30% of the intrinsic generation time), the effectsof respiration and recycling on net ¹⁴CO₂ uptake are quite distinct,especially at high P/R ratios, and not complicated by assumptionsover the value of q. Although the value of q will have a time-dependentsecondary effect on the modelled total carbon-specific respirationrate, this was found not to give rise to major problems of interpretation.Beyond 50% of the intrinsic generation time, the separate treatmentof respiration and recycling in the models becomes less satisfactory.It was concluded that the present equations, which are not constrainedby mass balance considerations, would not be appropriate fora model that combines the two processes. The pattern of recyclingat low P/R values is identified as one of the major uncertaintiesin producing models of ¹⁴C uptake. The effect of the releaseof dissolved organic material can be anticipated in a generalway. The models have been used to define an experimental strategyto establish the separate effects of respiration and recyclingon the time course of net ¹⁴C uptake. The initial rates givethe clearest resolution of the two processes and it would appearthat with photosynthetic rates in the region of 1 day^–1,incubation periods up to 3–6 h would be suitable to determinethe importance of recycling in controlling net ¹⁴C uptake. Withthe present models, only in the absence of recycling could theeffect of respiration be studied and the value of q established.     

Evidence for a high proportion of inactive ribosomes in slow-growing yeast cells.

From the protein and RNA content of Saccharomyces cerevisiae growing in different media we calculate that ribosome efficiency is changed: incorporation of amino acids into protein decreases from 8.8 amino acids/s per ribosome in fast-growing cells (0.54 doubling/h) to 5.2 amino acids/s per ribosome in slow-growing cells (0.30 doubling/h). We could not detect significant protein turnover in either fast-or slow-growing cultures, so the lower ribosome efficiency does not seem to be an artifact caused by changes in unstable protein production at different growth rates. Nor is the lower ribosome efficiency due to slower migration of ribosomes along mRNA: the times required to complete polypeptides of known molecular weights are the same in slow-growing cells as those previously determined for fast-growing cells [Waldron, Jund & Lacroute (1974) FEBS Lett. 46, 11-16]. We therefore deduce that ribosome efficiency changes in yeast because the fraction of ribosomes engaged in protein synthesis falls (from 84% in fast-growing cells to 50% in slow-growing cells.     

Characterization of RNA-dependent RNA Polymerases in Healthy and Tobacco Mosaic Virus-infected Tomato Plants

Healthy tomato plants were shown to contain high levels of RNA-dependentRNA polymerase activity, mainly in a ‘soluble’ form,but also partly in a ‘ bound’ form. The ‘bound’enzyme was solublized by EDTA treatment. Both forms of enzymewere partially purified and characterized. The ion and pH optimaof the two forms were identical at all stages of purification.Both enzymes exhibited uridylyl transferase activity, whichmade up 35 per cent of total incorporation. Infection with tobacco mosaic virus (TMV) increased activityof ‘soluble’ enzyme by twofold, and of solubilized‘bound’ enzyme by less than twofold. Uridylyl transferaseactivity was also increased by infection. General propertiesof the enzymes were unaltered by infection with one exception:in the presence of TMV RNA as added template, the ‘soluble’enzyme from infected plants incorporated ³H-UTP into productswith the electrophoretic properties and RNase sensitivitiesexpected for replicative form and replicative intermediate ofTMV. ‘Soluble’ enzyme from healthy plants, and solublized‘ bound’ enzyme from either healthy or infectedplants did not synthesize these products. The ‘soluble’ and solubilized ‘bound’enzymes behaved differently on ion-exchange chromatography.Under the conditions used, ‘soluble’ enzyme didnot bind to the column, whereas solublized ‘bound’enzyme did. No differences in chromatographic behaviour werefound between enzymes from healthy or infected plants. Withboth ‘soluble’ and solublized ‘bound’enzymes, the uridylyl transferase activity co-chromatographedwith the polymerase activity. Tomato, Lycopersicon esculentum, RNA-dependent RNA polymerase, tobacco mosaic virus, tobacco mosaic virus replicase     

Studies in Stomatal Behaviour: VII. EFFECTS OF ANAEROBIC CONDITIONS UPON STOMATAL MOVEMENT--A TEST OF WILLIAMS'S HYPOTHESIS OF STOMATAL MECHANISM

An experiment was carried out to investigate stomatal responsesin wheat to four ‘closing treatments’, viz. highcarbon dioxide concentration, darkness, dry air and nil, eachgiven under both aerobic and anaerobic conditions. Thus theeffect of lack of oxygen on the closing (or opening) tendencywas estimated. Changes in calculated from resistance porometer readings were used as data and reasonsare given for thinking this is the best available measure forinvestigating stomatal dynamics in wheat. Williams's hypothesisdemands that lack of oxygen should cause stomatal opening orprevent closure; the present experiment shows that anaerobicconditions significantly increase the closing tendency when‘closing treatments’ are first applied. There isalso some suggestion that oxygen-lack itself tends to causeclosure in the absence of any other ‘closing treatment’.Williams's hypothesis in its original form is thus disproved(for wheat) but the present results would be consistent withan ‘active’ uptake of water by the guard cells contributingto stomatal opening. A nearly significant interaction betweencarbon dioxide and oxygen suggests that under anaerobic conditionsa ‘closing substance’ may perhaps be formed, forexample, by the union of some intermediate in glycolysis withcarbon dioxide.