Rhizomorph Behaviour in Armillaria mellea (Fr.) Quel.: With one Figure in the text: Saprophytic Colonization of Woody Substrates in Soil

Rhizomorphs put out by Armillaria mellea from small woody inoculain glass tubes of moist soil were tested for their ability tocolonize segments of willow shoots, previously killed by autoclavingand then buried for various periods in soil before inoculationwith A. mellea. The degree of saprophytic colonization of thesesubstrate segments by A. mellea was assessed by reburying themin fresh tubes of moist soil, and recording weekly growth incrementsof rhizomorphs put out from them over a period of 5 weeks. Aperiod of previous burial in soil up to 3 weeks was found notto diminish availability of the substrate segments to A. mellea,but with longer periods substrate value for A. mellea progressivelydeclined. Segments of living, green shoots of willow provideda consistently better substrate for A. mellea than did deadsegments, and these living segments maintained their substratevalue over periods (up to 7 weeks) of previous burial in soil.These results are interpreted in terms of competition betweenA. mellea and other soil fungi. In the infection of living tissues,A. mellea benefits from the exclusion of competitors by hostresistance. In the saprophytic colonization of dead tissues,A. mellea has the advantage of its capacity to decompose celluloseand lignin, which are substrates restricted to a minority ofsoil fungi.     

Stimulation of Armillaria mellea by phenolic fungicides

Phenolic fungicides, which were initially fungicidal to mycelium of Armillaria mellea on the surface of well‐colonised wood billets, eventually stimulated the growth of A. mellea. An extensive growth of rhizomorphs was produced from A. mellea inoculum, which had been exposed to phenolic chemicals for 3 months, compared to few or no rhizomorphs produced from inoculum exposed to water or a suspension of a non‐phenolic fungicide, fenpropidin. Inoculated privet plants grown either in pots or under field conditions were treated with a range of fungicides; fenpropidin, phenyl phenol, cresylic acid or water (control) was applied every 6 months over 21/2 yr. Fenpropidin caused a slightly (but significantly) lower incidence of infection than occurred in untreated plants, but the phenolic fungicides, cresylic acid and phenyl phenol, did not reduce the incidence of infection. The severity of infection (% root circumference colonised at 5 cm depth) was greater following cresylic acid treatments than the other fungicides or water‐treated controls. Use of phenolic fungicides such as cresylic acid for the control of A. mellea may therefore be counter‐productive.     

Infection Foci of Armillaria mellea in First-rotation Hardwoods

Infection foci of Armillaria mellea, often 10–20 m indiameter, were discovered in first-rotation plantings of oakand beech on former arable or heathland sites—they werecommonly associated with thinning stumps containing the fungus.Since the foci were scattered and contained different mycelialtypes of A. mellea it seemed probable that they had arisen asa result of stump infection by basidiospores. There was evidencethat many years had elapsed between stump infection and theproduction of rhizomorphs, and that subsequent growth of thesedepended upon the presence of suitable living roots. Largeroaks seemed little affected whereas many small and medium-sizedtrees had been killed. Armillaria mellea, infection foci in plantations, tree stumps, basidiospores     

Determination of Water, Solute and Turgor Potentials of Mycelium of Various Basidiomycete Fungi causing Wood Decay

Using thermocouple psychrometry, the water, solute and turgorpotentials of hyphae of Phallus impudicus and Serpula lacrimans,cords of P. impudicus, Phanerochaete velutina and S. lacrimansand rhizomorphs of Armillaria mellea were determined. A comparisonwas made between laboratory- and field-grown cords of P. impudicusand P. velutina. Comparison was made also between the waterrelations of the hyphae at the growing front, and hyphae ofthe more distal, basal regions. Similarly, the growing regionsof cords/rhizomorphs were compared with the older, non-extendingbasal regions of the same structures. In all cases gradientsof water and turgor potentials were found from basal, olderregions to younger, extending regions. These results are discussedin relation to the mechanism of translocation in fungi. Key words: Mycelium, Basidiomycotina, Turgor gradients, Translocation     

Interrelation Between Growth Rate of Individual Potato (Solanum tuberosum L.) Tubers and Their Cell Number

In potato plants fast and slow growing tubers develop on thesame plant. A hypothetical causality between tuber growth rateand tuber cell number was investigated by determining the tubercell number with the aid of an automatic counting procedure.Our data show a close correlation between tuber size and cellnumber over the whole range of tuber volumes considered (3–28cm³). If the influence of tuber size on cell number is eliminatedby means of a partial correlation analysis, the cell numberof the entire tuber is not significantly correlated with itsgrowth rate. An exclusive consideration of the smaller cells(10–30 µm) in the apical tuber region, where thecell division rate in potato tubers is highest, reveals a loosebut significant partial correlation to tuber growth rate (r= 0.383, P < 0.05). The growth rate of the slow growing tubers of any potato plantmay be enhanced by removing the fast growing tubers. In thefirst few days this enhanced growth rate is not due to a stimulationof cell division rate, but rather due to cell expansion. Potato, Solanum tuberosum L., tuber growth rate, tuber cell number     

Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: III. RELATIONSHIP BETWEEN GROWTH RATE OF INDIVIDUAL TUBERS, TUBER WEIGHT AND STOLON GROWTH PRIOR TO TUBER INITIATION

Potato plants (Solanum tuberosum L.) were grown in water culture.About 14 d after tuber initiation no significant differenceswere found between apical and basal tuber parts in ¹⁴C-uptakeand partitioning into various fractions from ¹⁴C-labelled photosynthate.Thus, the fresh weight of these tubers could be used as a parameterfor the sink size. The ¹⁴C-content per tuber (sink strength)20 h after ¹⁴CO₂-supply to the foliage was significantly correlatedwith the tuber fresh weight. No correlation was found betweenthe ¹⁴C-concentration of the tuber (sink activity; ct. min^–g^– fr. wt.) and tuber fresh weight. Consequently, tuberfresh weight (sink size) per se must have been a factor whichinfluenced sink strength. Stolon parameters characterizing theirgrowth prior to tuber initiation (e.g. stolon volume) and theircapacity for photosynthate transport (diameter, length) weremeasured at the time of tuber initiation. Significant correlationswere found between these stolon parameters and subsequent growthof individual tubers. Anatomical studies on the proportion ofvarious tissues in the cross sectional area of stolons supportthe idea of a negative relation between growth of individualtubers and transport resistance in the phloem of the stolons.It is concluded that in the initial phase of tuber growth, mainlyfactors outside of the tuber determine its growth rate. In laterstages of tuber growth, when the sink strength increases, thecompeting strength of individual tubers for photosynthate isdominated mainly by factors within the tuber itself, such astheir sink size and sink activity. Key words: Potato tuber, sink size, tuber initiation, transport resistance     

Translocation in rhizomorphs ofArmillaria mellea

The uptake and transport of exogenously applied [U-¹⁴C]glucose and³²PO₄ by the root-like structures termed rhizomorphs of the fungusArmillaria mellea were studied. Rhizomorphs were grown in petri dishes from a defined medium, across and air space, into either defined medium or water agar. Uptake of label and transport of absorbed label from bases to tips of rhizomorphs occurred in cultures growing aerobically. Label was present in all segments sampled along the lengths of such rhizomorphs with the concentration of label generally decreasing with increasing distance from the base to which isotope has been applied. The air space eliminated background due to diffusion of isotope directly through agar. Control experiments revealed that translocated compounds do not diffuse on the surface of rhizomorphs or their cells, but instead, are carried within the cells of the rhizomorphs. Both absorption and transport of label were eliminated by glutaraldehyde fixation. Rhizomorphs living under anaerobic conditions absorbed, but did not transport, label. This suggests a mechanism of transport that is dependent upon aerobic respiration. Essentially no transport of label was observed in the reverse direction from tip to base of rhizomorphs under the experimental conditions used.     

Pithiness and Growth of Radish Tubers as Affected by Irradiance and Plant Density

Pithiness in radish tubers (Raphanus sativus L.) is a syndromewhere xylem parenchyma cells die and are filled with air. Featuresassociated with rapid tuber growth such as large cells, greatdistances between strands of vascular tissue and rupture ofxylem vessels have been suggested to induce pithiness. The objectiveof this study was to analyse the extent to which pithiness inradish was related to tuber growth. Growth rates were variedby altering irradiance and plant density. In addition, the relationshipbetween pithiness and tuber growth (length, diameter, freshmass) was investigated in batches of plants grown under identicalconditions. The growth rates of shoot and tuber strongly increased withincreasing inter-plant distance and irradiance. Our resultssuggest a ‘true’ effect of assimilate supply ondry matter partitioning, as not only the ratio between shootand root dry mass, but also the slope of this relationship decreasedwith increasing irradiance and inter-plant distance. Withina treatment the ratio between tuber growth and shoot growthremained constant with plant age. Increase in length of thetuber did not appear to be an important factor in inducing pithiness.Within a batch of plants, a positive correlation between tuberdiameter and pithiness was observed only for tubers smallerthan 14 mm. An increase in both irradiance and inter-plant distanceincreased tuber size (diameter, mass) and pithiness, but theeffects on pithiness could not be ascribed only to differenttuber sizes. Pithiness strongly increased with plant age andit was concluded that pithiness occurred, to a great extent,according to genetic programming. The data showed that for treatmentswhich affect tuber growth, conclusions about their effects onpithiness depend on whether comparisons were made at the sametuber size or the same time, explaining apparent contradictionsin the literature. Raphanus sativus L.; radish; pithiness; genetic programming; cell death; light; plant density; growth rate; dry matter partitioning; shoot; tuber     

Internal Structure and Hydraulic Conductivity of Basidiomycete Translocating Organs

Eamus, D., Thompson, W., Cairney, J. W. G. and Jennings, D.H. 1985. Internal structure and hydraulic conductivity of basidiomycetetranslocating organs.–J. exp. Bot. 36: 1110–1116. The presence in rhizomorphs of Armillaria mellea and cords ofPhallus impudicus of wide diameter hyphae (5–20 {diaeresis}m),which run for considerable distances along these linear organs,has been demonstrated. The longitudinal hydraulic conductivityof these organs has been determined experimentally and similarvalues were obtained when the hydraulic conductivity was calculatedtheoretically on the basis that the vessel hyphae were the solechannel for water movement along these organs. The experimentaldata have been discussed in relation to other data for long-distancetranslocation in basidiomycete linear organs. It is concludedthat the vessel hyphae are the main channels for turgor-driventranslocation. Key words: -Basidiomycete fungi, translocation, hydraulic conductivity     

The Ecology of Armillaria mellea in Britain Biological Control

The first trial in Britain of a method developed and used inEast Africa to control Armillaria mellea is described. Contraryto experience in East Africa, colonization of hardwood rootsby A. mellea was not reduced either by ringbarking trees beforefelling, or by frill-girdling and poisoning. Five years afterfelling, roots of trees treated in this manner were found tobe fully colonized, while invasion of the roots of untreatedtrees appeared to be still in progress. However, the formerwere far less effective as food bases for A. mellea than thelatter. Further experimentation may-reveal whether the treatmentsdescribed are practical methods for reducing infection in plantationsestablished on infested hardwood sites. Stump poisoning (withno pre-felling treatment) may also prove effective.     

Biological species ofArmillaria and their mycoparasitic associations withRhodophyllus abortivus in Hokkaido

Specimens of basidiomes and/or rhizomorphs ofArmillaria mellea complex and basidiomes ofRhodophyllus abortivus, developing on the same decaying stumps or stems of forest trees, were collected in three forests in Hokkaido. Normal basidiomes ofR. abortivus were found near to, but free from, the rhizomorphs and/or basidiomes ofArmillaria, while abnormal basidiomes, as carpophoroid forms, were developed on the rhizomorphs ofArmillaria. Of three mycoparasiticArmillaria isolates found withR. abortivus, one was identified asA. gallica and two asA. jezoensis. The isolates ofR. abortivus showed excellent mycelial growth and rhizomorph formation on PDA. However, on MDA, RMDA and BMDA, they showed poor aerial mycelia growth and no rhizomorphs. In the contrapositional cultures, the growth ofA. gallica was completely inhibited byR. abortivus on PDA but only slightly inhibited on MDA and RMDA. On the other hand, mutual inhibition at a distance was observed on BMDA. The mycelial growth and rhizomorph formation inA. jezoensis were severely inhibited by the colony ofR. abortivus on PDA, but only slightly inhibited on MDA. On RMDA and BMDA, the colonies of twoArmillaria species andR. abortivus showed mutual inhibition at a distance and apparent rhizomorph formation by bothArmillaria species.     

Biological Nitrogen Fixation in Pachyrhizus ahipa (Wedd.) Parodi

The patterns of plant growth and N₂ fixation capability in Pachyrhizusahipa (Wedd) Parodi inoculated with Bradyrhizobium ‘PachyrhizusSpec 1’ strains (Lipha Tech) were investigated in a zero-Nculture system under greenhouse conditions The P ahipa plantis day-neutral with respect to reproductive development Competitionoccurred between the two storage organs (legume and tuber) andprevented high tuber yield in P ahipa The symbiotic effectivenessof the association was high, as the profuse nodulation providedthe inoculated plants with adequate amounts of N Nodules werepresent throughout the cycle of P ahipa The change in rate ofN₂ fixation (RNF) and relative growth rate (RGR) was almostparallel during ontogenesis The developmental pattern of N₂fixation activity revealed that 65% of total N₂ fixation occurredafter N began to accumulate in the reproductive (pod wall plusseed) tissue During pod filling allocation of N compounds tothe seeds exceeded N₂ fixation, the pod walls being the primarysource of redistributed N, followed by the leaves. Pachyrhizus ahipa (Wedd) Parodi, ahipa, tuber crop, dinitrogen fixation, dry matter, N partitioning, reproductive growth     

Comparative Analysis of Tuber Development in Six Sweet Potato (Ipomoea batatas (L.) Lam) Cultivars: 2. Interrelationships between tuber shape and yield

Interrelationships between tuber dimensions and yield duringdevelopment of six sweet potato cultivars with different tubershapes were examined. Maximum tuber lengths and tuber stalklengths were attained after 16 and 8 weeks of growth, respectively.Rates of increase in tuber width in the 16- to 24-week periodand final tuber widths were related to the mean tuber weightand yield of the cultivars studied, e.g. rates were highestin the high yielding cv. O₄₉ and low in cv. A16/15. Relationshipsbetween parameters of tuber development, tuber dimensions, sinkstrength and yield were evaluated and tuber width appeared tobe the most important single dimensional determinant of highyield. However, tuber shape in cv. 03/62 was different fromother cultivars and this cultivar was an exception in the interrelationshipsdescribed. Some factors known to affect tuber shape and developmentwere discussed.     

Allocation of Photosynthate to Individual Tubers of Solanum tuberosum L.: I. RELATIONSHIP BETWEEN TUBER GROWTH RATE AND ENZYME ACTIVITIES OF THE STARCH METABOLISM

Potato plants (Solanum tuberosum L.) were grown in water culturein a controlled environment. Cooling (+8°C) of individualtubers decreased their growth rates and increased the growthrates of non-cooled tubers of the same plant. The carbohydrateconcentration in non-cooled and cooled tubers did not differsignificantly, but ¹⁴C-import from labelled photosynthate waslower in cooled than in non-cooled tubers. The markedly lowerconversion rate of ethanol-soluble ¹⁴C to starch in cooled,in comparison to non-cooled tubers, was not associated withsignificant differences in the in vitro activities of starchsynthase, ADPG-pyrophosphorylase and starch phosphorylase understandard assay conditions (+30°C). However, the Q₁₀-valuesof the enzymes differed in vitro in the temperature range between30°C and 8°C, leading to a marked decrease in the activityratio of ADPG-pyrophosphorylase/starch phosphorylase in cooledtubers. In tubers differing in growth rates without manipulation, 14d after tuber initiation significant positive correlations werefound between ¹⁴C-concentration of tuber tissue and the in vitroactivities of starch synthase and ADPG-pyrophosphorylase anda significant negative correlation between ¹⁴C-concentrationand starch phosphorylase. In contrast, in tubers which wereanalysed 5 d after initiation, there were only small differencesbetween tubers in growth rate, ¹⁴C import and the activity ratioADPG-pyrophosphorylase/starch phosphorylase. From various directand indirect evidence it is concluded that the growth rate ofindividual tubers, and thus the sink strength, is at least inpart controlled by the activity of starch synthesizing enzymes. Key words: Potato tuber, cooling, starch synthesizing enzymes     

An Analysis of the Relation between Dry Matter Allocation to the Tuber and Earliness of a Potato Crop

Compared with late cultivars, early potato cultivars allocatea larger part of the available assimilates to the tubers earlyin the growing season, leading to shorter growing periods andlower yields. A dynamic simulation model, integrating effectivetemperature and source –sink relationships of the crop,was used to analyse this relation, using data from experimentsin the Netherlands carried out over 5 years. Dry matter allocationto the tuber in these field experiments was simulated well whenthe tuber was considered as a dominant sink that affects earlinessof a potato crop in two ways: early allocation of assimilatesto the tubers stops foliage growth early in the season and reducesthe longevity of individual leaves. In a sensitivity analysisthe influence of tuber initiation, leaf longevity and the maximumrelative tuber growth rate (R_tb) on assimilate allocation andcrop earliness was evaluated. It was found that the maximumrelative tuber growth rate can influence crop earliness morethan the other two factors, but when conditions for tuber growthare optimal, the leaf longevity is most important. Solanum tuberosumL.; simulation model; source –sink relationships; cultivars     

Tuberization in Potato at High Temperatures: Responses to Gibberellin and Growth inhibitors

The responses of potato plants (Solanum tuberosum L., cv. Sebago)to high temperatures (32 day/28 C night or 32/18 °C) andgibberellin are similar, in that they promote haulm growth andsuppress tuber production, whereas low temperatures (22/18 °C)abscisic acid and CCC have the opposite effect, promoting tuberproduction and reducing the growth of the haulms. The inhibitoryeffect of the high temperatures on tuber production, under aphotoperiod of 14 h, was almost completely reversed in theseexperiments by the application of CCC, and partly reversed byABA. Single-leaf cuttings from plants grown at the various temperaturesand chemical treatments responded in the same way as the wholeplant. It is suggested that both haulm growth and tuber initiationare influenced by a common hormonal control, and that temperatureexerts its influence by altering the balance between the levelsof endogenous gibberellins and inhibitors. These substancesapparently act directly on the stolon tip, rather than throughtheir general influence on haulm growth. Solanum tuberosum L., potato, tuberization, temperature response, gibberellin, abscisic acid, 2-chloroethyltrimethylammonium chloride (CCC)     

Translocation of Assimilates Within and Between Potato Stems

Three aspects of translocation in potato were examined: (i)translocation within stems (ii) translocation between individualstems of a plant (iii) translocation between tubers followinginjection of ¹⁴C sucrose into a single daughter tuber. Assimilatesexported from single leaves of evenly illuminated potato stemsremained confined to the same side of the stem as the sourceleaf in a pattern consistent with the internal arrangement ofvascular bundles in the stem, and tubers borne on stolons verticallybelow the source leaf contained higher concentrations of ¹⁴Cthan those on the opposite side. Consequently ¹⁴C import intothe tubers bore little relationship to tuber growth rates. However,alteration of source/sink relations by pruning stems to a singlesouce leaf resulted in an even distribution of ¹⁴C throughoutthe vascular bundles of the stem and ¹⁴C import into the tubersbore a stronger relationship to tuber growth rates than to thephyllotactic relationship of the tubers with the source leaf. Labelling one stem of a potato plant resulted in little or nomovement of ¹⁴C into tubers on other unlabelled stems. However,removal of the unlabelled stems at ground level induced a significantmovement of ¹⁴C from the labelled stem to the tubers on unlabelledstems, this movement occurring via the mother tuber. Shadingthe unlabelled stems had less effect than stem removal. ¹⁴C sucrose injected into single daughter tubers was translocatedto other tubers on the same stem and also to tubers on a secondstem at the opposite end of the mother tuber. The sucrose wasconverted to starch in these tubers. The results favour the view that each potato stem functionsas an independent unit with potential for assimilate redistributionwithin a stem but with little or no carbon exchange occurringbetween stems, unless under severely altered source/sink patterns. Assimilates, ¹⁴C, autoradiography, potato (Solanum tuberosum L.), tuber growth     

The Stem as a Temporary Sink before Tuberization in Helianthus tuberosus L.

The growth of individual Helianthus tuberosus (Jerusalem Artichoke)plants, and that of their component parts, was measured overone growing season of 33 weeks. The stem served as a temporarysink for assimilates before tuberization began and during thisperiod constituted up to 65 per cent of the total dry weightof the plant. Greatest tuber growth occurred when the leaveswere senescing rapidly and it was sustained by transfer of drymatter from the stem. The relationship of this growth patternto tuber and stem metabolism and its possible hormonal controlis discussed.     

Studies on the Physiology of Nodule Formation: VIII. The Influence of the Size of the Rhizosphere Population of Nodule Bacteria on Root Hair Infection in Clover

With an adequate inoculum the number of infected root hairsin three species of clover (Trifolium parviflorum, T. patensand T. glomeratum) increased exponentially with time in twophases; the increase was rapid during the first 8-10 days beforenodulation begins, but slower afterwards. T. parviflorum hadmost infections and T. glomeratum the fewest. Experiments on varying inoculum size, using an avirulent mutanstrain of Rhizobhtm trifolii as diluent, showed that root-hairinfection was differentially limited by inoculum size duringthe two phases. Infection in all three species was about doubledby doubling the density of the virulent bacteria in the rhizospherebefore nodulation begins. After nodulation bacterial densityhad to be increased much more than twice to double the numberof infections. This increase in the infecting population wasinversely related to the numbers of infections formed on thethree host species. Early infection and nodulation were promoted by high bacterialdensity in the rhizosphere.     

Production of Potato Spindle Tuber Viroid (PSTV) by Large Scale Fermentation of PSTY-Infected Potato Cell Suspension Cultures

Berlin, J., Wray, V., Forche, E., Reng, H.–G , Schler,H, Luckinger, R. and Mhlbach, H.–P. 1985. Production ofpotato spindle tuber viroid (PSTV) by large scale fermentationof PSTV–infected potato cell suspension cultures.—J.exp. Bot 36: 1985–1995. Cell suspension cultures of Solatiumdemissum, infected with the potato spindle tuber viroid (PSTV),were scaled up to volumes of up to 800 dm³ to provide sufficientand uniform plant material for subsequent studies on viroidbiosynthesis. Here we describe the technological aspects ofproducing the required amounts of biomass and viroid. The cells,which had been maintained on a medium containing expensive coconutmilk, were first adapted to rapid growth on the less expensiveB5–medium. The physiological state of the cells was monitoredby in vivo ³¹P–NMR spectroscopy Under the chosen conditionsthe scale–up from 10 dm³ inoculum from shake flasks tothe harvest of the 800 dm³ stirred fermenter lasted 38 d andprovided 112 kg biomass. Growth characteristics and viroid productionin shake flasks and large bioreactors were rather similar. Gelelectrophoretic analysis of isolated nucleic acids using silverstaining and Northern blot hybridization revealed a PSTV–contentof approximately 700 µg PSTV per kg fresh mass of culturedcells. Key words: Solanum demissum, plant cell cultures, potato spindle tuber viroid, biomass production, fermentation, in vivo ³¹P-NMR