bluuebunny/crossref_metadata_2025 huggingface dataset:
THE DEGRADATION OF CHLOROPHYLL — A BIOLOGICAL ENIGMA
Authors: Hendry GEORGE A. F. , HOUGHTON JENNIFER D. , BROWN STANLEY B.
Published: April 2006

Summary

Some 109tonnes of chlorophyll are destroyed each year on land and in the oceans. The fate of these chlorophylls is, however, largely unknown. This review describes the developmental stages at which chlorophyll breakdown occurs in aquatic and terrestrial biological systems, and the destruction arising from herbivory, disease, pollution and other physical hazards. At the cellular level, an attempt is made to separate the breakdown of chlorophyll during senescence from the many other events associated with cell destruction and death.

A consideration of the more important chemical and biophysical properties of chlorophylls and their derivatives is provided, together with data on their spectral properties. The biosynthetic and biodegradative pathways of chlorophyll metabolism are, so far as is possible, described with some predictions as to the likely fate of the missing tonnes. Two types of degradation are recognized; the first involves up to five defined enzymes concerned with the early stages, the second covers the less well defined enzymic and non‐enzymic destruction of the macrocyclic structure. These degradative reactions are compared with the reactions implicated in the breakdown of other porphyrins including haems in plants and animals.

A brief description is given of the occurrence of breakdown products of chlorophyll in past biomass, including those of geological significance and those in a more recent archaeological context.

Finally, the economic significance of chlorophyll breakdown is considered in the context of agriculture and horticulture, veterinary and medical sciences, food colouring and cosmetic industries, and the multi‐million‐dollar attraction of autumn leaf fall to tourism.

ContentsSummary256I.Introduction256II.Chlorophylls: global production and destruction259III.Chlorophylls: nomenclature and chemical characteristics260IV.Chlorophyll metabolism268V.Chlorophyll degradation during senescence274VI.Other degradative conditions278VII.Breakdown products in past biomass287VIII.Pathways of degradation289IX.Economic importance291Acknowledgements294References294
EFFECT OF NITROGEN FEEDING SOURCE ON THE SUPPLY OF NITROGEN FROM ROOT TO SHOOT AND THE SITE OF NITROGEN ASSIMILATION IN MAIZE (ZEA MAYS L. CV. R201)
Authors: MURPHY A. T. , LEWISF O. A. M.
Published: April 2006

Summary

Maize plants (Zea mays L. cv. R201) were grown to 21 d in pH‐controlled gravel culture with 2 mM inorganic N supplied as nitrate alone, ammonium alone or 1:1 nitrate + ammonium. At 21 d, the 14N feeding solutions were replaced with 15N solutions, and xylem sap collections were made 4 and 8 h after the commencement of feeding. Leaf and root material was harvested also for in vitro nitrate reductase and glutamine synthetase activity assays.

Xylem sap analyses showed that in nitrate‐only fed plants the major supply of nitrogen from root to shoot was in the nitrate form (60%) with 35 % carried as amino compounds. However, 93% of 15N was transported to the shoot as nitrate and only 6% in amino compounds, indicating the more direct routing of newly absorbed nitrogen to the shoot via the former. Leaf NRA was seven‐fold greater than that of the root, confirming the shoot as the major site of nitrogen assimilation in plants fed only nitrate. In ammonium‐only fed plants, 84% of xylem N was found in organic form (66%16N), the remainder translocating as ammonium, identifying the root as the major site of ammonium N assimilation. In ammonium + nitrate fed plants, 64% of xylem N was present as organic N (55%16N), 34% as nitrate (43 %16N), indicating shared N assimilation between shoot and root, with root assimilation predominating. In plants receiving nitrate, glutamine was the major N compound translocated, in plants receiving only ammonium, asparagine predominated. GS activity was approximately the same in root and shoot and showed no response to N source.

The significance of these results is discussed with respect to the reported increased productivity of maize fed a mixed nitrate‐ammonium N source.
MEMBRANE THERMOSTABILITY AND NITRATE REDUCTASE ACTIVITY IN RELATION TO WATER STRESS TOLERANCE OF YOUNG SUGAR‐CANE PLANTS
Authors: VENKATARAMANA S. , NAIDU K. MOHAN , SINGH SUDAMA
Published: April 2006

Summary

An assessment of tolerance to water stress based upon measurements of cellular membrane thermostability and in vivo nitrate reductase activity (NRA) was made using young leaves of sugar‐cane (Saccharum officinarum L. commercial hybrids, Co 419, Co 740 and Co 1148) subjected to stress (51 to 61 d old) and subsequent hydration.

An average reduction in leaf water potential (ΨL) from −0.97 to −1.91 MPa was associated with a decrease in NRA from 2.68 to 1.13 Ψmol g f. wt−1h−1, and an increase of the membrane injury from 30.8 to 70.9%. A highly significant positive correlation between ΨL and NRA and a negative correlation between ΨL and membrane injury were demonstrated. As ΨL declined, the concentration of malondialdehyde, a lipid peroxidation product, doubled. Following rehydration, at 63 d, increases in ΨL and NRA were higher than those of membrane thermostability and malondialdehyde concentration.
15N NUCLEAR MAGNETIC RESONANCE STUDY OF AMMONIUM ION ASSIMILATION BY LEMNA GIBBA L.
Authors: MONSELISE EDNA BEN‐IZHAK , KOST DANIEL , PORATH DAN , TAL MOSHE
Published: April 2006

Summary

15N Nuclear magnetic resonance (NMR) spectroscopy was used to follow nitrogen assimilation and amino acid production in Lemna gibba L. exposed to 0.4 mM 15NH4Cl solution for 24 h. NMR analysis of the 16N content of the treated plant tissues (aqueous extract) revealed 15N incorporation into glutamine (N‐amide plus amino‐N or amide‐N only) and glutamic acid and no detectable free ammonium ion.

Methionine sulphoximine (MSO), an inhibitor of glutamine synthetase, at 1.0 mM inhibited completely the incorporation of 15N. In the presence of 0.5 mM azaserine (AZA), a glutamate synthase inhibitor, the incorporation of 15N was detected only in the amide group of glutamine. The results confirm the involvement of the glutamine synthetase/glutamate synthase (GS/ GOG AT) pathway in the assimilation of ammonium ions.
FOLIAR ENTRY AND INCORPORATION OF ATMOSPHERIC NITROGEN DIOXIDE INTO BARLEY PLANTS OF DIFFERENT NITROGEN STATUS
Authors: ROWLAND A. J. , DREW M. C. , WELLBURN A. R.
Published: April 2006

Summary

The uptake of atmospheric NO2 through leaf surfaces and the influence on uptake of combined nitrogen by the roots were investigated in hydroponically grown barley plants exposed to 0.3 μI 1−1nitrogen dioxide (NO2). The response to NO2 was dependent on the nitrogen status of the plant at the time of exposure. Only with low nitrate supplies to the roots did exposure of leaves to atmospheric NO2 cause significant increases in the nitrogen (N) content of barley. Levels of both nitrate and reduced forms of N increased in plants in response to the pollutant and were directly due to uptake of NO2 by leaves, since the root environment was completely sealed from atmospheric sources of NO2. Moreover, these increases were not due to the stimulation of nitrate uptake by the roots from the nutrient solution, although levels of activity of both nitrate and nitrite reductases were affected by exposure to NO2. Atmosphere NO2 did, however, affect the ability of a plant to respond to a change in the amount of nitrate supplied to the roots, and the use of both 18N‐labelled NO2 and nitrate indicated that the cycling of nitrogen within the plant was also influenced by exposure to atmospheric NO2.
TILLERING AND LEAF AREA OF WINTER BARLEY EXPOSED TO SULPHUR DIOXIDE IN THE FIELD
Authors: BAKER C. K. , FULLWOOD A. E. , COLLS J. J.
Published: April 2006

Summary

Winter barley (Hordeum vulgare L., cv. ‘Igri’), grown as a crop in the field, was exposed to SO2 concentrations ranging from 40 to 200 ppb [parts per billion (109) by volume] above ambient. From spring onwards, plants at 40 and 50 ppb were consistently the heaviest, having slightly more tillers than plants in the other plots. In the range 80 to 200 ppb SO2, the plants usually had fewer tillers than the controls, but this difference was not consistent throughout the season. Trends in the numbers and weights of tillers were interpreted as circumstantial evidence that 80 to 200 ppb of SO2 may have decreased assimilate production.

Leaf area in May was markedly bigger in fumigated plants than in the controls. At 80 to 200 ppb the increases may have resulted from the plants having diverted a greater proportion of shoot dry weight into leaf tissue and from increased specific leaf area. These responses of ‘Igri’ barley in the field resembled those obtained by other workers for the same variety grown in controlled environments.
EVIDENCE THAT ARSENIC TOLERANCE IN HOLCUS LANATUS L. IS CAUSED BY AN ALTERED PHOSPHATE UPTAKE SYSTEM
Authors: MACNAIR MARK R. , CUMBES QUINTON
Published: April 2006

Summary

The uptake of arsenate by arsenic tolerant and non‐tolerant plants of Holcus lanatus L., and the interaction of this uptake with phosphate level, was studied. The tolerance of the different plants, as measured by root growth, was also investigated. Arsenate inhibited the root growth of non‐tolerant plants much more than that of tolerant ones, but at low levels of arsenate, the high phosphate treatment reduced the toxicity of arsenate in non‐tolerants but not in tolerants. Tolerant plants took up less arsenate than non‐tolerants. The effect of phosphate on arsenate uptake was very small in the tolerant plants, but in non‐tolerants phosphate inhibited arsenate uptake when the phosphate was at a higher concentration than arsenate. These results are compatible with the hypothesis that arsenate is taken up by the phosphate uptake system in non‐tolerant plants, but in tolerant plants this system has become altered so that it no longer recognizes arsenate.
REVERSAL OF CAESIUM INHIBITION OF GROWTH BY POTASSIUM IN HYPOCOTYLS OF TOMATO SEEDLINGS (LYCOPERSICON ESCULENTUM L.)
Authors: KORDAN H. A.
Published: April 2006

Summary

Stunted, misshapen, necrotic hypocotyls of tomato seedlings (Lycopersicon esculentum L.) germinated on caesium cations in continuous red or white light indicated caesium‐associated hypocotyl photo‐injury. Continuous, far red irradiation inhibited seedling germination in control and caesium treatments but was reversed by continuous white light. Hypocotyl photo‐injury also occurred in far red pretreated seedlings germinated on caesium in continuous white light. Except for suppressed elongation, hypocotyls of caesium‐treated seedlings were normal in appearance. Etiolated caesium‐treated seedlings placed in continuous white light also exhibited hypocotyl photo‐injury. The effects of caesium cations were reversed by potassium cations but not by cations of other alkali metals.
MORPHOLOGICAL PLASTICITY AND MINERAL NUTRIENT CAPTURE IN TWO HERBACEOUS SPECIES OF CONTRASTED ECOLOGY
Authors: CRICK J.C. , GRIME J.P.
Published: April 2006

Summary

Developmental plasticity and nitrogen capture were compared in Agrostis stolonifera and Scirpus sylvaticus grown in compartmentalized root growth arenas which allowed simulation of the spatial and temporal patchiness distinguishing the conditions of mineral nutrient supply associated with fertile and infertile soils.

Despite the small root system, higher rates of nitrogen capture and dry matter production were achieved by A. stolonifera at both high and low external concentrations of mineral nutrients. This species also showed a marked ability rapidly to adjust partitioning of growth between parts of the root system, thus allowing local proliferations of fine roots into sectors of high mineral nutrient concentration. Under the conditions of the experiment, this form of plasticity was only weakly developed in S. sylvaticus, which instead maintained a large but relatively unresponsive root mass under each experimental treatment.

It is concluded that the high plasticity observed in A. stolonifera allows the dynamic exploitation of fertile soil by competing roots. It is predicted that the more stable pattern of root development in S. sylvaticus will confer a selective advantage in environments where mineral nutrients are strongly limiting upon productivity and become available in temporally unpredictable pulses.
INTERRELATIONSHIPS BETWEEN ASPECTS OF NITROGEN METABOLISM AND SOLUTE ACCUMULATION AND THE DISTRIBUTION OF SUBTROPICAL WOODY PLANTS IN SOUTHEAST AFRICA
Authors: PRIOR J. , TUOHY J. , WHITING J.
Published: April 2006

Summary

Little is known of the mechanisms governing nitrogen metabolism and stress tolerance in woody plants of subtropical southeast African savannas, although many of the more widely occurring genera, particularlyAcaciaandCombretumspecies, include valuable multi‐purpose trees. Basal nitrate reductase activities in vigorous leaves from 59 species and five savanna areas were used as a measure of nitrate availability. Consistently low activities were found in many of the legumes and in members of the Combretaceae. When shoots from 35 of these same species were provided with supplementary nitrate solution, increased reductase activity was recorded in a number of taxa, including non‐nodulating legumes belonging to the Caesalpinioideae. Nodulating legumes did not behave in the same way. Either proline or quaternary ammonium compounds, examples of stress metabolites, accumulated in the shoots of a range of taxa belonging to a number of families. The highest concentrations occurred in shoots collected from the driest areas. Amounts of tannin were high in some species but showed no consistent pattern.
ECTOMYCORRHIZAL VS ENDOMYCORRHIZAL FUNGI WITHIN THE SAME ROOT SYSTEM
Authors: CHILVERS G. A. , LAPEYRIE F. F. , HORAN D. P.
Published: April 2006

Summary

Vesicular‐arbuscular (VA) mycorrhizal fungi and ectomycorrhizal fungi were observed together in the same root system, and even within the same root apices, of seedlings of Eucalyptus dumosa A. Cunn. ex Schau. On a population basis, there was a succession of two overlapping mycorrhizal epidemics‐VA mycorrhizas followed by ectomycorrhizas.

This succession was interpreted as follows. Early build‐up of VA mycorrhizas was related to initial higher inoculum potential of that fungus and to rapidity of colonization of individual roots. The ectomycorrhizal fungus was more successful later in achieving secondary infections by hyphal spread and in colonization of lateral root branches. The ectomycorrhizal fungus had no difficulty infecting pre‐existing VA mycorrhizas, but the ectomycorrhizal sheath provided a barrier to subsequent VA mycorrhizal infections. While the proportion of VA mycorrhizas to ectomycorrhizas changed between two and five months, the overall proportion of mycorrhizal roots remained constant, suggesting that there could be competition between the different fungi for limiting substrate.
THE INFLUENCE OF ROOT GROWTH AND INOCULUM DENSITY ON THE DYNAMICS OF ROOT DISEASE EPIDEMICS: THEORETICAL ANALYSIS
Authors: JEGER M. J.
Published: April 2006

Summary

Including root growth and inoculum density as variables in a simple model of disease progress of a monocyclic root pathogen often leads to a sigmoid curve. If the product of root density and inoculum density is constant, then a monomolecular curve (without an inflexion point) results. When the product is a function of time, then either an asymptotic exponential or a sigmoid curve results, depending upon the parameter values. Where the product of root density and inoculum density increases exponentially, then a Gompertz function in the incidence of surviving plants results. Where root growth is fast relative to the reduction in inoculum through infection, then an asymptotic value of disease less than 1.0 is predicted.

Detailed models of dynamics of root infection, incorporating root extension and loss of inoculum due to infection, and cases without and with lesion expansion, lead to the following conclusions: (1) increase in lesion numbers without lesion expansion, does not constrain or provide an upper limit to root growth; (2) where there is no root growth, then the proportion of root surface covered by lesions approaches an asymptote strictly less than 1.0 in the case without lesion expansion, but approaches 1.0 in the case with lesion expansion; (3) where the rate of infection is greater than the rate of root extension (without lesion expansion), then there is an upper limit to lesion surface area on roots; otherwise both root surface area and lesion numbers increase without limit; (4) where the rate of lesion expansion is greater than the rate of root extension, then the proportion of root surface area covered by lesions approaches 1.0 asymptotically. Explicit solutions giving the healthy root area as a function of time are obtained.

Analysis of the dynamics of root infection indicates that root disease control strategies should aim to reduce pathogen density, maintain a low rate of root extension relative to root infection and restrict lesion expansion.
MODELS FOR THE CONTROL OF BRANCH POSITIONS AND FLOWERING SEQUENCES OF CAPITULA IN MYCELIS MURALIS (L.) DUMONT (COMPOSITAE)
Authors: JANSSEN J. M. , LINDENMAYER A.
Published: April 2006

Summary

The development of highly branched inflorescences is considered. Two main characteristics of these structures are the flowering sequences (acropetal, basipetal or divergent) and the branch production (acrotonic, basitonic or mesotonic), which can be different on various branching orders (paracladia). A widespread pattern, found especially among Compositae, is a basipetal or divergent flowering sequence combined with basitonic or mesotonic branch production. We present quantitative models for this type of inflorescence development, based on the transport of a flower‐inducing hormone (florigen) and of a hormone responsible for apical dominance (auxin), together with some additional control factors. The computed structures obtained with these models are compared with observations on the inflorescence development of wall lettuce, Mycelis muralis.
INTERNAL AERATION AND DEVELOPMENT OF STELA ANOXIA IN SUBMERGED ROOTS
Authors: ARMSTRONG W. , BECKETT P. M.
Published: April 2006

Summary

A case is presented for progressing from mathematical models of root aeration which assume complete radial homogeneity in respiratory demand and oxygen diffusivity to models which accommodate radial differences between the various tissue cylinders. The described model simulates aeration in submerged roots and allows for axial gas‐phase diffusion in the cortex and radial liquid‐phase diffusion elsewhere. It also accommodates diffusion into the rhizosphere and axial and radial differences in the root's properties on a tissue cylinder basis.

The results, based on the characteristics of aerenchymatous graminean roots, highlight the potential for stelar anoxia and some wall‐layer anoxia in submerged, and even non‐submerged, roots. They show that, (1) stelar anoxia should raise oxygen levels elsewhere and enhance root extension; (2) with high and constant oxygen diffusivity in the stele, anoxia should first occur close to the apex and spread basally and laterally with further root extension; (3) there will be competition between the oxygen demands of the root and rhizosphere; (4) extension may be reduced by radial oxygen losses into the rhizosphere, but the subapical decline in root‐wall permeability in wetland species safeguards against this effect; (5) differences in critical oxygen pressure for extension growth (COPE) and respiration (COPR) may arise; and (6) survival may be helped by the manner in which anoxia spreads with aerobic conditions persisting in the phloem and pericycle until only traces of oxygen remain in the cortex. Sectional profiles of the roots show how oxygen gradients in the stele and wall layers will increase with increasing subapical impedance in these zones and, in the case of the wall layers, by oxygen throughflux to the rhizosphere.

Some of the results suggest that inner‐stele anoxia might account for some of the hypoxic responses found in roots, and it is predicted that declining stelar oxygen diffusivity in subapical regions will raise COPR but not COPE, the inner‐stele becoming anoxic subapically and the apical regions remaining wholly aerobic. This implies the possibility that where roots are described in the literature as hypoxic, because of ethanol production, raised alcohol dehydrogenase activity and reduced energy charge, the symptoms may be sometimes indicative only of some subapical stelar anoxia.

The results also serve to illustrate how variable the occurrence and shape of anoxic stelar zones might be. With only a slight alteration in subapical stelar oxygen diffusivity, anoxia could arise simultaneously in two positions along the root, or the first site of anoxia in the growing root might be both subapical and hypobasal. The likelihood of the endodermis in subapical parts being a possible cause of anoxia in roots is also considered.
REGULATORY ASPECTS OF NH4+ UTILIZATION IN THE ACIDOPHILIC THERMOPHILIC UNICELLULAR RED ALGA CYANIDIUM CALDARIUM
Authors: DI MARTINO RIGANO VITTORIA , VONA VINCENZA , DI MARTINO CATELLO , RIGANO CARMELO
Published: April 2006

Summary

Cyanidium caldarium is an acidophilic thermophilic red alga. At pH 1.9 with cells grown in batch culture with excess ammonium, removal of CO2 or darkness caused immediate inhibition of ammonium assimilation. By contrast, with cells grown in chemostat culture with nitrogen limitation, assimilation of ammonium was not inhibited by darkness, but it was inhibited by CO2 deprivation. In the case of these cells, inhibition was not immediate and behaved as a time‐dependent phenomenon. Ammonium assimilation was restored when CO2was supplied in either light or darkness.

In contrast, at pH 7.4, ammonium assimilation decreased with time both in the presence and absence of CO2, and it ceased completely after about 90 min incubation at this pH. The inhibition was not due to the high pH per se. When resuspended at pH 2, these inhibited cells were unable to assimilate ammonium during a 25 min period in the presence of CO2. Thereafter, they become able to assimilate ammonium but at a rate which was only 30% of that of the control. If they were pre‐incubated at pH 2 in nitrogen‐free medium for 3 h, however, they then assimilated ammonium immediately after its addition. It is suggested that a reversible inactivation at pH 1.9 and a denaturation at pH 7.4 of the system for ammonium uptake occurs, and that this uptake system is stabilized by carbon compounds against inactivation at pH 1.9 but not against denaturation at pH 7.4.
THE INFLUENCE OF LOW TEMPERATURE ON DIEL PATTERNS OF CARBOHYDRATE METABOLISM IN LEAVES OF POA ANNUA L. AND POA×JEMTLANDICA (ALMQ.) RICHT.
Authors: BORLAND ANNE M. , FARRAR J. F.
Published: April 2006

Summary

The effect of low temperature on diel changes in net gas exchange, carbohydrate content and export, and on the extractable activities of sucrose phosphate synthase and acid invertase from leaves of Poa annua (a ruderal grass) and Poa×jemtlandica (a subarctic grass) was investigated. Exposure for 5 d to a temperature of 7°C significantly reduced the rate of net photosynthesis of P. annua but had no effect that of P.×jemtlandica. In both species, soluble sugars constituted 20 % of the dry weight and showed net accumulation over 24 h. There was approximately twice the concentration of fructans in the leaves of P. x jemtlandica compared with P. annua.

During the 16 h photoperiod, the leaves of P.×jemtlandica stored twice as much photosynthate as the leaves of P. annua. However, the higher rate of export from the leaves of P.×jemtlandica in the dark compared to P. annua resulted in the allocation of similar proportions of photosynthate to the processes of storage, respiration and export in leaf blades of both species over a 24 h period.

Low temperature resulted in a decrease in the extractable activity of sucrose phosphate synthase from P. annua but not from P.×jemtlandica. The activity of acid invertase was reduced by exposure in both species.
VARIATIONS IN ACTIVITIES OF GLYCOGEN PHOSPHORYLASE AND TREHALASE DURING THE PERIODIC FRUITING OF THE EDIBLE MUSHROOM AGARICUS BISPORUS (LANGE) IMBACH.
Authors: WELLS TERENCE K. , HAMMOND JOHN B. W. , DICKERSON ANTHONY G.
Published: April 2006

Summary

Activities of trehalase and glycogen phosphorylase were determined for stage 1 (pin) sporophores of Agaricus bisporus harvested daily throughout several fruiting cycles (flushes). Both enzymes exhibited maximum activity coincident with peak flush, as defined by maximum yield of stage 2 to 4 sporophores and minimum activity during the interflush periods. Glycogen phosphorylase activity sometimes peaked on more than one occasion during a flush, whereas trehalase gave only a single peak of activity per flush. These results are discussed in relation to metabolic events associated with carbohydrate changes during the periodic fruiting cycle of this fungus.
METABOLISM OF MANNITOL IN MYCORRHIZAL AND NON‐MYCORRHIZAL FUNGI
Authors: RAMSTEDT MAURITZ , JIRJIS RAIDA , SÖDERHÄLL KENNETH
Published: April 2006

Summary

The activity of mannitol dehydrogenase, mannitol 1‐phosphatase and mannitol 1‐phosphate dehydrogenase in five mycorrhizal basidiomycetes, one mycorrhizal ascomycete and five non‐mycorrhizal basidiomycetes was investigated. No mannitol 1‐phosphate dehydrogenase could be detected in any of the basidiomycetes. Mannitol dehydrogenase, present in all the tested fungi except Armillariella mellea, was strictly NAD‐dependent for the non‐mycorrhizal basidiomycetes, whereas in the mycorrhizal species it was mainly NADP‐dependent. The mannitol 1‐phosphatase of the mycorrhizal basidiomycetes was specific towards mannitol 1‐phosphate in contrast to that of the non‐mycorrhizal species, which was a non‐specific phosphatase. In the ascomycete Sphaerosporella brunnea all the mannitol metabolizing enzymes were found, including mannitol 1‐phosphate dehydrogenase. Growth of S. brunnea on mannitol slightly decreased the activity of the mannitol dehydrogenase (NADP) and mannitol 1‐phosphate dehydrogenase, but there was a slight increase in activity of glucose 6‐phosphate dehydrogenase.
THE EFFECT OF DESICCATION ON CELL SHAPE IN THE LICHEN PARMELIA SULCATA TAYLOR
Authors: BROWN D. H. , RAPSCH S. , BECKETT A. , ASCASO C.
Published: April 2006

Summary

Comparisons have been made of the shape of algal cells in the lichen Parmelia sulcata which was subjected to controlled desiccation regimes inducing substantial water loss. The spherical appearance of algal cells obtained by conventional techniques for transmission electron microscopy (TEM) was shown by low temperature scanning electron microscopy (LTSEM) to be the consequence of rapid rehydration during fixation. Collapse of the walls of algal cells and fungal hyphae in the medulla and algal layer when desiccated were observed with LTSEM and shown to be reversible on rehydration. Desiccation‐induced contraction of cortical fungal protoplasts was detected with LTSEM. Fixation with osmium vapour only before TEM demonstrated a similar contraction of algal protoplasts.
PHENOTYPIC FLEXIBILITY OF CAREX FLACCA SCHREB.
Authors: HEATHCOTE C. A. , DAVIES M. S. , ETHERINGTON J. R.
Published: April 2006

Summary

Nine populations of Carex flacca Schreb., collected from sites which vary widely in their liability to soil flooding, were grown on an experimental soil with free drainage, transient flooding or continuous flooding for 26 weeks. Although the flooding treatments resulted in low redox potentials and high concentrations of reduced iron and manganese in the soil, the growth of C. flacca plants was little affected by flooding. All populations grew equally well on the flooded soils. All populations produced adventitious roots at the soil surface in the flooded treatments; these were absent from plants grown on the freely drained soil. Flooding produced an increase in the manganese concentration of roots and shoots but, although the iron content of roots was markedly increased by flooding, there was no increased transport to the shoots.

In view of the absence of detectable phenotypic differentiation between these populations from widely contrasting sites and their unimpaired growth on severely waterlogged soils, it is argued that C. flacca plants have a broad tolerance (phenotypic flexibility) of soil flooding. This may be important in the wide ecological amplitude of this species.
MORPHOLOGICAL AND ANATOMICAL EFFECTS OF SEVERE DROUGHT ON THE ROOTS OF LOLIUM PERENNE L.
Authors: JUPP A. P. , NEWMAN E. I.
Published: April 2006

Summary

Three experiments using soil‐grown Lolium perenne plants were performed in order to investigate the effects of a gradually increasing drought stress on the death of root cells and the growth of lateral roots. Water potentials of ‐2 to ‐10 MPa caused death of the root cortex, but death of root tips occurred only at soil water potentials below ‐10 MPa. Low soil water potentials promoted lateral root initiation and elongation, the total length of lateral roots being between three and five times that of control plants. On rewetting severely droughted plants, root growth continued by elongation of existing, previously initiated, lateral roots.
ECTOMYCORRHIZA FORMATION IN EUCALYPTUS
Authors: MALAJCZUK N. , DELL B. , BOUGHER N. L.
Published: April 2006

Summary

Cortinarius and Hysterangium species are a dominant component of the macrofungi in eucalypt forests of Western Australia, and their hyphae occupy 10 % of the soil surface area. Anatomical studies of jarrah (Eucalyptus marginata Donn ex Sm.) and karri (E. diversicolor F. Muell) roots collected in association with hyphae of Cortinarius globuliformis Boug., C. ochraceus Clel. and Hysterangium inflatum Rodway showed mycorrhizal structures with mantles one to five cells thick and Hartig nets penetrating to the hypodermis. These superficial ectomycorrhizas were similar in size to those of non‐colonized feeder roots and lacked the expanded cortex and broad mantle of pyramidal ectomycorrhizas formed by Laccaria laccata (Scop, ex Fr.) Berk. & Br. Colonized host roots responded with polyphenol accumulation in the epidermis/hypodermis and lignification of the outer cortex for both the superficial and pyramidal types.
IN VITRO FRUITING UNDER CONTROLLED CONDITIONS OF THE ECTOMYCORRHIZAL FUNGUS HEBELOMA CYLINDROSPORUM ASSOCIATED WITH PINUS PINASTER
Authors: DEBAUD J. C. , GAY G.
Published: April 2006

Summary

The ectomycorrhizal fungus Hebeloma cylindrosporum Romagnési produced sporulating fruit bodies under axenic conditions on a synthetic medium when cultivated with its usual host plant Pinus pinaster (Sol.). In the absence of the host plant, H. cylindrosporum only formed primordia or immature fruit bodies. In 11 Erlenmeyer flasks, the fungal fruiting started about one and half months after the inoculation of one‐month‐old P. pinaster plants by the mycelium of H. cylindrosporum, and a total of 144 fruit bodies appeared in 45 flasks during a 9.5‐month fruiting period. At 18 °C, the mean number of basidiomes developed per flask was about 3–5. A thermal shock of ± 6 °C for 5 d did not significantly affect the mean number of basidiomes per flask but modified the time course of their rate of formation. Spores were collected from the fruit bodies and were found to be capable of germinating.
THE INHIBITORY EFFECT OF TRICHODERMA SPECIES AND OTHER SOIL MICROFUNGI ON FORMATION OF MYCORRHIZA BY LACCARIA BICOLOR IN VITRO
Authors: SUMMERBELL RICHARD C.
Published: April 2006

Summary

Ten microfungal isolates from the rhizosphere of mycorrhiza of black spruce were tested for their ability to inhibit formation of mycorrhiza by Laccaria bicolor (Maire) Orton on axenically cultivated black spruce seedlings. Two Trichoderma species, T. viride Pers. and T. polysporum (Link ex Pers.) Rifai, were strongly antagonistic towards mycorrhizal colonization. This antagonism was seen both in trials where Trichoderma and L. bicolor were inoculated together and in a trial designed to allow establishment of the mycorrhizal fungus in the rhizosphere prior to a challenge with T. viride. Despite its antagonism in flask trials, T. viride did not parasitize the hyphae of L. bicolor in agar culture. Two other soil fungi, Tolypocladium inflatum Gams and Trichosoporon beigelii (Küch et Rab.) Vuill., also had small but significant inhibitory effects on the formation of mycorrhiza in flask trials. However, a variety of Mucoralean fungi, heavily sporulating soil Hyphomycetes and non‐sporulating root‐associated Hyphomycetes had no deleterious effect on the process. It is concluded that Trichoderma spp. are likely to be significant antagonists of mycorrhizal colonization in vivo, particularly in fumigated soils.
THE FORM, DISTRIBUTION AND SEASONAL ACCUMULATION OF CALCIUM IN KIWIFRUIT LEAVES
Authors: CLARK C. J. , SMITH G. S. , WALKER G. D.
Published: April 2006

Summary

Seasonal changes in the content of Ca and Mg in leaf blades collected from mature kiwifruit [Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson var. deliciosa] vines in a high‐producing orchard were measured by a sequential fractionation procedure (14 M acetic acid, 0.25 M HC1 and residual). Total Ca and Mg concentrations decreased within the first 4 weeks of growth, but increased linearly thereafter to be 1322 and 197 μmol g−1 dry weight, respectively, by leaf fall. Total quantities of Ca and Mg accumulated at this time were approximately 2500 and 380 μmol per blade. Calcium oxalate, dissolved in the HC1 extract, was the predominant fraction at every harvest, accounting for up to 79 % of the total Ca content shortly after leaf emergence, and decreasing to 42 % at leaf fall. The quantity of physiologically active Ca (acetic acid‐soluble) ranged from a minimum of 15 % (8 weeks after emergence) to a maximum of 41 % at the end of the season. Forms of Ca remaining in the solid residue after extraction made only a minor contribution (3 to 21 %) to the total content at any time during the season. In contrast, however, 89 to 96% of the total Mg was extracted by acetic acid at every harvest, with the remainder dissolved by HC1. Scanning electron microscopy and X‐ray microanalysis of the lamina of mature leaves indicated the presence of two crystalline, Ca‐rich products which differed in their morphology, location and distribution. Large styloids (120 to 270 μm in length) were concentrated in idioblasts adjacent to vascular tissue, while bundles of four‐sided raphides, containing wedge‐shaped apices, were located predominantly between the major transport vessels of the vascular tissue. X‐ray diffraction analysis of crystalline isolates confirmed that both morphological forms are calcium oxalate monohydrate (whewellite).
THE PROCESSES OF ANTHER DEHISCENCE AND POLLEN DISPERSAL
Authors: KEIJZER C. J.
Published: April 2006

Summary

The process of anther opening was followed in different species by means of light and electron microscopical techniques, stereo microscopical observation inside living tissues and micromanipulation. From these data and a review of the mainly very old literature on this subject, the opening process is outlined. It consists of the following stages: (1) Expansion of the epidermis and endothecium cells and deposition of U‐shaped wall thickenings inside the latter. (2) Enzymatic opening of the septum between two locules. (3) Mechanical rupture of the tapetum. (4) Mechanical opening of the stomium by the centripetal force of the highly turgescent epidermis and endothecium. (5) Outward bending of the locule walls by the centrifugal force of the dehydrating epidermis and endothecium. Steps (4) and (5) clearly reflect the mechanical functions of the two latter tissues. The roles of the other anther tissues and the filament during the earlier stages are discussed.
THE HISTORY OF ENDEMIC FLORA OF ST HELENA: A RELICTUAL SERIES
Authors: CRONK Q. C. B.
Published: April 2006

Summary

When the endemic plant species of St Helena are ordered according to their increasing taxonomic isolation, their corresponding ‘sister groups’ show a ‘relictual series’ of increasing geographical disjunction. This is best explained by progressive extinction and evolution in continental areas rather than by particularly high rates of evolutionary change on the island (i.e. the taxonomically isolated endemics are ancient relicts). Few of the supposed trends of oceanic island evolution are convincingly evident in the flora of St Helena. St Helena was formed in the Miocene, and it is likely that the relict genera colonized St Helena from Southern Africa before the wet forest flora in this area largely disappeared as the climate deteriorated from the late Miocene onwards.
EVIDENCE FOR CALCIUM‐MEDIATED REGULATION OF HETEROCYST FREQUENCY AND NITROGENASE ACTIVITY IN NOSTOC6720
Authors: SMITH R. J. , HOBSON S. , ELLIS I. R.
Published: April 2006

Summary

The proportion of heterocysts (heterocyst frequency) in cultures of Nostoc 6720 varied with the Ca2+ concentration present in the growth medium, the proportion increasing from 5 to 9 % as the Ca2+ concentration was increased from 10−5 to 10−4 M. The total nitrogenase activity of the culture declined by 20% whereas the activity per heterocyst decreased by 30%. In the 10−4 to 10−3 M range of Ca2+ concentrations, which includes those employed in most standard media, the heterocyst frequency and nitrogenase activity did not change. At Ca2+ concentrations above 2 × 10−3 M the heterocyst frequency decreased and this decrease was associated with a lower growth rate.

The presence of the calcium ionophore A23187 increased the heterocyst content of the culture and total nitrogenase activity, but still decreased the nitrogenase activity per heterocyst. The effects of A23187 and Ca2+ concentration were additive and demonstrated a positive interaction between Ca2+ and the ionophore. In contrast, the Ca2+ antagonist, lanthanum, decreased the heterocyst frequency and, at concentrations below 10−8 M, stimulated nitrogenase activity. At higher concentrations of lanthanum, growth, heterocyst frequency and nitrogenase activity were markedly diminished; cell degradation occurred at concentrations in excess of 10−4 M.

An analysis of the intra‐ and extracellular binding of 45Ca2+ showed that decreasing the Ca2+ concentrations in the medium from 10−4 to 10−5 M lowered the intracellular Ca2+ content. In the presence of A23187, intracellular Ca2+ increased. The lanthanum treatment was exceptional in that the intracellular Ca2+ was increased about four‐fold.

These results are interpreted as favouring the hypothesis that a Ca2+‐mediated regulatory process modulates heterocyst frequency and nitrogenase activity in Nostoc 6720.
COMPARATIVE STUDIES OF PLANT GROWTH AND DISTRIBUTION IN RELATION TO WATERLOGGING
Authors: WALDREN S. , DAVIES M. S. , ETHERINGTON J. R.
Published: April 2006

Summary

The tolerance of Geum rivale L. and G. urbanum L. to soil waterlogging was investigated by growing plants in a compost under a variety of flooding regimes. Chemical analyses showed that high concentrations of available ammonium, iron and manganese but almost no nitrate could be extracted from this compost if conditions became sufficiently reducing, though a considerable period of flooding was required for such reducing conditions to develop. This slow rate of reduction may be due to the absence of an anaerobic microflora in the loam base of the compost.

Waterlogging caused a reduction in root biomass and a change in the distribution of living roots of both species such that rooting was restricted to the surface zones of flooded soil. G. rivale possessed a rhizome‐like caudex, which is virtually absent in G. urbanum, and it is suggested that this may facilitate adventitious rooting which aids survival in waterlogged soil. The numbers of leaves, shoot apices and leaves per apex were reduced by flooding in G. urbanum but not in G. rivale. The responses of the two species to transient flooding, soil volume and soils with and without a previous flooding history are discussed in relation to their distribution.
SCANNING ELECTRON MICROSCOPY OF CRYOFIXED MYCORRHIZAS OF SITKA SPRUCE, PICE A SITCHENSIS (BONG.) CARR.: A COMPARISON WITH CRITICAL POINT‐DRIED MATERIAL
Authors: ALEXANDER CLARE , JONES D. , McHARDY W. J.
Published: April 2006

Summary

Critical point‐drying is the standard method of preparation for scanning electron microscope studies of mycorrhizas. However, a number of recent studies on various biological specimens have shown disadvantages associated with this technique. These can be overcome by cryofixation, where specimens are frozen in liquid nitrogen and viewed in the frozen‐hydrated state. Three distinct types of mycorrhiza of Sitka spruce [Picea sitchensis (Bong.) Carr.] were critical point‐dried or cryofixed, and were examined for differences in the preservation of both surface features and internal morphology after fracturing or cutting the material transversely.

Surface features were better preserved in frozen‐hydrated mycorrhizas than in critical point‐dried ones. Surface cementing matrix seen in the former was partially or entirely removed by critical point‐drying. Some distortion of both fungal and plant cells also occurred after the latter treatment, and cell contents retained in frozen‐hydrated specimens were often lost after critical point‐drying. Fractured frozen‐hydrated material revealed surface features of the cortical cells and lobed Hartig net hyphae.
MYCORRHIZAS IN GRASSLANDS: INTERACTIONS OF UNGULATES, FUNGI AND DROUGHT
Authors: WALLACE L. L.
Published: April 2006

Summary

Fumigated and non‐fumigated plots were planted with several species of either mycorrhizal or non‐mycorrhizal grasses. These plots were either inside or outside enclosures which allowed grazing by native North American ungulates and longhorn cattle. The morphology and growth of plants as well as the level of vesicular‐arbuscular mycorrhizal (VAM) development were followed over one growing season during which growing season precipitation was 50 % of the normal level. Grazing was the most important treatment effect, greatly reducing plant size and relative growth rates. Mycorrhizal colonization had no effect upon plant, growth rates or size. Even in grazed plots where nutrient content and soil moisture were higher, no mycorrhizal effect on plant growth was observed. Grazing did not influence VAM colonization, although grazed plants which were growing in non‐fumigated soil had significantly lower colonization levels than any other treatment.
GLUTAMINE SYNTHETASE AND AMMONIUM ASSIMILATION IN ROOTS OF ZINC‐TOLERANT AND NON‐TOLERANT CLONES OF DESCHAMPSIA CESPITOSA (L.) BEAUV. AND ANTHOXANTHUM ODORATUM L.
Authors: SMIRNOFF N. , STEWART G. R.
Published: April 2006

Summary

The influence of zinc on glutamine synthetase (GS) activity and ammonium assimilation in roots of zinc‐tolerant and non‐tolerant clones of Deschampsia cespitosa (L.) Beauv. and Anthoxanthum odoratum L. was studied. One isoform of GS was resolved by ion exchange chromatography in D. cespitosa roots. However, two isoforms were found in A. odoratum roots; one of these was attributed to fungal contamination and was not studied further. There was no difference in the sensitivity of partially purified GS from zinc‐tolerant and non‐tolerant clones to inhibition by zinc. Inhibition of GS by Zn was reversible and the kinetics of inhibition were studied. Methionine sulphoximine (an inhibitor of GS) induced ammonium accumulation in the roots of control and zinc‐grown roots of Zn‐tolerant D. cespitosa. Zinc inhibited incorporation of [15N]ammonium into free amino acids (determined by combined gas chromatography–mass spectrometry) in both clones of D. cespitosa, but higher concentrations were required to achieve inhibition in the tolerant clone. The labelling pattern was unaffected by zinc in both clones. The results suggest that zinc has no effect on the pathway of ammonium assimilation in zinc‐grown roots and that assimilation is via the glutamine synthetase–glutamate synthase pathway. There was no evidence to support the hypothesis that glutamate dehydrogenase functions preferentially in ammonium assimilation under conditions of environmental stress.
A 86Rb BIOASSAY TO DETERMINE THE POTASSIUM STATUS OF TREES
Authors: JONES HELEN E. , HARRISON A. F. , DIGHTON J.
Published: April 2006

Summary

A bioassay method has been developed to test for potassium deficiency in forest trees; it is based on the metabolic uptake by excised roots of 86Rb from 5 × 10−6 M rubidium chloride solution during a period of 15 min. Betula pubescens Ehrh. seedlings were grown in sand culture supplied with Hewitt's nutrient solution containing K at concentrations between 1 and 200 mg l−1. A negative curvilinear relationship was demonstrated between uptake by the seedlings of 86Rb from the rubidium chloride solution and the concentration of K previously supplied in the growth medium. When roots of intact plants were immersed in 86Rb‐labelled solution for 15 min, less than 1% of 86Rb taken up was translocated to the shoots. A similar response curve, relating 86Rb uptake to K supplied to growing plants, was demonstrated when Picea sitchensis (Bong.) Carr seedlings were grown in peat culture with K added at 0 to 400 mg l−1. In three field trials of the bioassay, roots were excised from P. sitchensis and Pinus contorta Douglas ex Loud, growing on peat or peaty gley plots where different amounts of fertilizer had been added to give a range of degrees of K deficiency or from unfertilized, control plots. The patterns of 86Rb uptake by excised roots indicated that the method is potentially useful for the rapid determination of K deficiency in forest trees.
INDUCTION OF PHOSPHOLIPASE ACTIVITY IN THE HYPERSENSITIVE RESPONSE OF WHEAT
Authors: OCAMPO C. A. , GRAMBOW H. J.
Published: April 2006

Summary

Phospholipase activity, probably C‐type, increased when cultivars of wheat were infected in incompatible, but not in compatible, combinations with wheat stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. & Henn) or with the non‐pathogenic crown rust of barley (P. coronata Cda. f. sp. avenae Fraser & Led.)‐ The effect was mimicked when an elicitor fraction isolated from germ tubes of the wheat stem rust was injected into leaves. The increase in phospholipase activity correlated with the hypersensitive response of leaves when infected with incompatible rust fungi or treated with fungal elicitor. This effect was not observed in healthy or in water‐injected control plants. The results suggest that membrane degradation is involved in the hypersensitive response.
THE GENETICS OF RACE‐SPECIFIC RESISTANCE IN GROUNDSEL (SENECIO VULGARIS L.) TO THE POWDERY MILDEW FUNGUS, ERYSIPHE FISCHERI BLUMER
Authors: HARRY I. B. , CLARKE D. D.
Published: April 2006

Summary

The inheritance of race specific resistance to five pathotypes of the powdery mildew fungus Erysiphe fischeri Blumer was studied in different lines of Senecio vulgaris L. With two exceptions, resistance was dominant over susceptibility. The exceptions were two crosses in which resistance to pathotype 3 behaved as the recessive character. The data from these crosses suggested the involvement of dominant inhibitor genes which suppressed the activity of the resistance gene. Seven different dominant resistance genes were identified in three resistant lines, two conditioned resistance to pathotype 4, two to pathotype 10 and one each to pathotypes 3, 5 and 11. The two genes conditioning resistance to pathotype 4 were unlinked, as were the two conditioning resistance to pathotype 10. The genes for resistance to pathotypes 3, 5 and 11 were clearly linked in one line and were probably linked in the other two lines. The linkage group also appears to contain one of each of the genes for resistance to pathotype 4 and 10. The expression of the two genes conditioning resistance to 4 and 10, not in the main linkage group, appeared to be reduced in one line, probably due to the activity of modifier genes. The results are consistent with the hypothesis that race‐specific resistance in S. vulgaris is controlled by a gene‐for‐gene interaction.