bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion transport and metal sensitivity of vacuolar channels from the roots of the aquatic plant Eichhornia crassipes
Authors: Paganetto A. , Carpaneto A. , Gambale F.
Published: March 2003

Using the patch‐clamp technique, we investigated the transport properties of vacuolar ion channels from the roots of water hyacinth, Eichhornia crassipes (Mart. Solms, Pontederiacae). Eichhornia crassipes vacuoles displayed large voltage‐dependent rectifying slow‐vacuolar (SV) currents, which activated in a few seconds at positive potentials and deactivated at negative voltages in a few hundreds of millseconds. Similarly to SV channel previously identified in the tonoplast of terrestrial plants, SV currents in E. crassipes were activated by micromolar concentrations of Ca2+ and current slightly increased (25%) on addition (10 mm) of the reducing agent dithiothreitol (DTT). Eichhornia crassipes SV channels were equally permeable to K+ and Na+. The permeability sequence derived from current values is: K+ ≈ Na+ > Rb+ > NH4+ ≈ Cs+ >> TEA+. Excised membrane patches displayed single channel transitions typical of SV‐type single channel openings with a conductance of (83·0 ± 5·6) pS; a smaller channel with a conductance of (31·0 ± 2·7) pS was also identified. Metals such as Ni2+ and Zn2+ decreased the vacuolar current in a reversible manner. However, although Zn2+ inhibition is comparable to that induced by the same metal in vacuoles from the main root of sugar beet (Beta vulgaris L.), the inhibition of the SV currents by Ni2+ is not as substantial in E. crassipes as in sugar beet. To our knowledge, this is the first electrophysiological characterization of ionic transport in E. crassipes, a pervasive troublesome aquatic weed, which has exceptional absorption properties of several water contaminants such as heavy metals, pesticides and phenols.
The effect of drought and ultraviolet radiation on growth and stress markers in pea and wheat
Authors: Alexieva V. , Sergiev I. , Mapelli S. , Karanov E.
Published: March 2003

It emerged recently that there is an inter‐relationship between drought and ultraviolet‐B (UV‐B) radiation in plant responses, in that both stresses provoke an oxidative burst. The purpose of this investigation was to compare the effects and interaction of drought and UV‐B in wheat and pea. The absence of changes in relative leaf water content (RWC) after UV‐B treatments indicate that changes in water content were not involved. RWC was the main factor resulting in reduced growth in response to drought. Increases in anthocyanin and phenols were detected after exposure to UV‐B. The increases do not appear to be of sufficient magnitude to act as a UV‐B screen. UV‐B application caused greater membrane damage than drought stress, as assessed by lipid peroxidation as well as osmolyte leakage. An increase in the specific activities of antioxidant enzymes was measured after UV‐B alone as well as after application to droughted plants. Proline increased primarily in drought‐stressed pea or wheat. Proline may be the drought‐induced factor which has a protective role in response to UV‐B. The physiological and biochemical parameters measured indicate the UV‐B light has stronger stress effectors than drought on the growth of seedlings of both species. The two environmental stresses acted synergistically to induce protective mechanisms in that pre‐application of either stress reduced the damage caused by subsequent application of the other stress.
N2 fixation by Acacia species increases under elevated atmospheric CO2
Authors: Schortemeyer M. , Atkin O. K. , McFarlane N. , Evans J. R.
Published: March 2003

In the present study the effect of elevated CO2 on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol−1) or elevated (700 µmol mol−1) CO2 concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO2. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO2, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO2 increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.
Cadmium causes the oxidative modification of proteins in pea plants
Authors: Romero‐Puertas M. C. , Palma J. M. , Gómez M. , Del Río L. A. , Sandalio L. M.
Published: March 2003

In pea (Pisum sativum L.) leaves from plants grown in the presence of 50 µm CdCl2 the oxidative production of carbonyl groups in proteins, the rate of protein degradation and the proteolytic activity were investigated. In leaf extracts the content of carbonyl groups measured by derivatization with 2,4‐dinitrophenylhydrazine (DNPH), was two‐fold higher in plants treated with Cd than in control plants. The identification of oxidized proteins was carried out by sodium dodecyl sulphate‐polyacrylamide gel electrophoresis of proteins derivatized with DNPH and immunochemical detection with an antibody against DNPH. The intensity of the reactive bands was higher in plants exposed to Cd than in controls. By using different antibodies some of the oxidized proteins were identified as Rubisco, glutathione reductase, manganese superoxide dismutase, and catalase. The incubation of leaf crude extracts with increasing H2O2 concentrations showed a progressive enhancement in carbonyl content and the pattern of oxidized proteins was similar to that found in Cd‐treated plants. Oxidized proteins were more efficiently degraded, and the proteolytic activity increased 20% due to the metal treatment. In peroxisomes purified from pea leaves a rise in the carbonyl content similar to that obtained in crude extracts from Cd‐treated plants was observed, but the functionality of the peroxisomal membrane was not apparently affected by Cd. Results obtained demonstrate the participation of both oxidative stress, probably mediated by H2O2, and proteolytic degradation in the mechanism of Cd toxicity in leaves of pea plants, and they appear to be involved in the Cd‐induced senescence previously reported in these plants.
Drought can be more critical in the shade than in the sun: a field study of carbon gain and photo‐inhibition in a Californian shrub during a dry El Niño year
Authors: Valladares F. , Pearcy R. W.
Published: March 2003

Diurnal courses of leaf water potential (Ψl), gas exchange and chlorophyll fluorescence were measured in natural sun and shade populations ofHeteromeles arbutifoliathroughout the seasons of an unusually dry El Niño year in Central California. The onset of drought resulted in decreased stomatal conductance and net photosynthesis in both sun and shade plants. However, the decline inΨlwas much greater and carbon gain was much more strongly limited by the development of drought stress in the shade than in the sun. Photorespiratory energy dissipation was significantly higher in the sun than in the shade in spring and autumn, but not during the summer. Pre‐dawn photochemical efficiency (Fv/Fm) was significantly higher in the shade than in the sun during the spring but the differences disappeared during the summer and autumn. The strong irradiance in the open field site studied led to a chronic but only mild reduction inFv/Fm, with values around 0·79. Summer sunflecks led to a sustained photo‐inhibition in shade plants, which exhibited a significant reduction in pre‐dawnFv/Fmof 10% with the onset of drought. Photo‐inhibition became relatively more important for carbon gain in the shade than in the sun due to the low photochemical efficiency under the low light that follows sunflecks. Sun plants ofH. arbutifoliaexhibited a rather efficient photoprotection against strong irradiance conferred by both the architecture of the crown and the physiology of the leaves. There is evidence that El Niño events and the associated droughts have become more frequent and severe. Counter‐intuitively, the effects on plant performance of such extreme droughts could be more critical in the shade than in the sun.
Improvement in ozone tolerance of tobacco plants with an antisense DNA for 1‐aminocyclopropane‐1‐carboxylate synthase
Authors: Nakajima N. , Itoh T. , Takikawa S. , Asai N. , Tamaoki M. , Aono M. , Kubo A. , Azumi Y. , Kamada H. , Saji H.
Published: March 2003

Antisense DNA for an ozone‐inducible 1‐aminocyclopropane‐1‐carboxylate (ACC) synthase (EC 4.4.1.14; LE‐ACS6) from tomato, under the control of the cauliflower mosaic virus 35S promoter, was introduced into tobacco to generate transgenic plants. Lower rates of ozone‐induced ethylene production were observed in three of seven transgenic plants than in the wild‐type plants. Ozone‐induced visible damage was attenuated in these three lines, and the extent of damage was positively related to the level of ozone‐induced ethylene production. In the most ozone‐resistant line, ozone‐induced accumulation of ACC and levels of transcripts for ozone‐inducible endogenous ACC synthases were suppressed compared with those in wild‐type plants, demonstrating that ozone‐inducible ACC synthases have a key role in the expression of leaf damage by ozone exposure. No significant differences in growth and morphology were observed between transgenic and wild‐type plants. Stomatal conductance of transgenic plants during ozone exposure was higher than that of wild‐type plants. These findings indicate that the introduction of antisense DNA for an ozone‐inducible ACC synthase can improve the ozone tolerance of plants without reducing their gas absorption and productivity.
A technique for measuring xylem hydraulic conductance under high negative pressures
Authors: Cochard H.
Published: March 2003

A technique for measuring hydraulic conductances of excised xylem segments exposed to high negative pressures is described. A centrifugal force is used to generate negative pressures (P) in the sample and to create a positive hydrostatic pressure difference (ΔP) between its two ends. ΔP forces water through the sample at a flow rate (F) determined optically during centrifugation. The sample hydraulic conductance k is derived from F and ΔP. The sample vulnerability curve is given by the dependence of k on P. Results for Cedrus atlantica Manetti and Laurus nobilis L. shoots are given. The technique is appropriate for the analysis of xylem refilling under negative pressure.
A computer‐operated routine of gas exchange and optical measurements to diagnose photosynthetic apparatus in leaves
Authors: Laisk A. , Oja V. , Rasulov B. , Rämma H. , Eichelmann H. , Kasparova I. , Pettai H. , Padu E. , Vapaavuori E.
Published: March 2003

Photosynthesis is a complex process whose rate is affected by many biochemical and biophysical factors. Fortunately, it is possible to determine, or at least estimate, many of the most important parameters using a combination of optical methods and gas transient analyses. We describe here a computer‐operated routine that has been developed to make detailed assessments of photosynthesis at a comprehensive level. The routine comprised the following measurements: steady‐state light and CO2response curves of net CO2assimilation at 21 and 2 kPa O2; transients from limiting to different saturating CO2concentrations at 2 kPa O2; post‐illumination CO2fixation transient; dark–light induction of O2evolution; O2yield from one saturating single‐turnover flash; chlorophyll fluorescenceF0,FsandFmduring the light and CO2response curves; leaf transmission at 820 nm (P700+) during the light and CO2response curves; post‐illumination re‐reduction time of P700+. The routine was executed on a two‐channel fast‐response gas exchange measurement system (A. Laisk and V. Oja:Dynamic Gas Exchange of Leaf Photosynthesis.CSIRO, Canberra, Australia). Thirty‐six intrinsic characteristics of the photosynthetic machinery were derived, including quantum yield of CO2fixation (YCO2), time constant of P700 re‐reduction (τ′), relative optical cross‐sections of PSII and PSI antennae (aII,aI), PSII and PSI density per leaf area unit, plastoquinone pool, total mesophyll resistance, mesophyll diffusion resistance,Vm,Km(CO2) and CO2/O2specificity of Rubisco, RuBP pool at CO2limitation (assimilatory charge). An example of the routine and calculations are shown for one leaf and data are presented for leaves of 8‐year‐old‐trees of two birch clones growing in Suonenjoki Forest Research Station, Finland, during summer 2000. ParametersYCO2, basicτ′,aII,aI,Km(CO2) andKsvaried little in different leaves [relative standard deviation (RSD) < 7%], other parameters scattered widely (RSD typically 10–40%). It is concluded that the little scattered parameters are determined by basic physico‐chemical properties of the photosynthetic machinery whereas the widely scattered parameters are adjusting to growth conditions. The proposed non‐destructive routine is suitable for diagnosing the photosynthetic machinery of leaves and may be applied in plant ecophysiology and in genetic engineering of plants.
Sapwood hydraulic conductivity and leaf area – sapwood area relationships following thinning of aEucalyptus nitensplantation
Authors: Medhurst J. L. , Beadle C. L.
Published: March 2003

This study tested the hypothesis that the relationship between leaf area (projected, or one‐sided) and sapwood area inEucalyptus nitens(Deane and Maiden) Maiden is affected by thinning treatment. However, no difference in the relationship between leaf area and sapwood area was found 8 years after thinning. This result suggests that a single regression equation can be used to predict the leaf area of trees in thinned and unthinned stands. The relationship was non‐linear, implying a causal relationship between growth rate and the ratio of leaf area to sapwood area (Al : As). Sapwood hydraulic conductivity increased by approximately 100% from breast height to crown base, whereas sapwood area decreased by 19%. This implies that the efficiency of water transport through the sapwood increased by 60% along this length. This conclusion is supported by theAl : Asrelationships which showed that the sapwood area at crown base supported, on average, close to 60% more leaf area than a similar amount of sapwood area at breast height. That large trees in this study had greater hydraulic conductivity and higherAl : Aslends support to the argument that resource capture, and hence growth rate, influence sapwood hydraulic conductivity.
Temperature response of parameters of a biochemically based model of photosynthesis. II. A review of experimental data
Authors: Medlyn B. E. , Dreyer E. , Ellsworth D. , Forstreuter M. , Harley P. C. , Kirschbaum M. U. F. , Le Roux X. , Montpied P. , Strassemeyer J. , Walcroft A. , Wang K. , Loustau D.
Published: March 2003

The temperature dependence of C3 photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity (Vcmax) and the potential rate of electron transport (Jmax). Original data sets were used for this review, as it is shown that derived values of Vcmax and its temperature response depend strongly on assumptions made in derivation. Values of Jmax and Vcmax at 25 °C varied considerably among species but were strongly correlated, with an average Jmax : Vcmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the Jmax : Vcmax ratio declined strongly with measurement temperature. The relative temperature responses of Jmax and Vcmax were relatively constant among tree species. Activation energies averaged 50 kJ mol−1 for Jmax and 65 kJ mol−1 for Vcmax, and for most species temperature optima averaged 33 °C for Jmax and 40 °C for Vcmax. However, the cold climate tree species had low temperature optima for both Jmax(19 °C) and Vcmax (29 °C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both Jmax and Vcmax, implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and plant type can influence the photosynthetic response to temperature. Based on these results, several suggestions are made to improve modelling of temperature responses.
Spatial separation of light perception and growth response in maize root phototropism
Authors: Mullen J. L. , Wolverton C. , Ishikawa H. , Hangarter R. P. , Evans M. L.
Published: March 2003

Although the effects of gravity on root growth are well known and interactions between light and gravity have been reported, details of root phototropic responses are less documented. We used high‐resolution image analysis to study phototropism in primary roots of Zea mays L. Similar to the location of perception in gravitropism, the perception of light was localized in the root cap. Phototropic curvature away from the light, on the other hand, developed in the central elongation zone, more basal than the site of initiation of gravitropic curvature. The phototropic curvature saturated at approximately 10 µmol m−2 s−1 blue light with a peak curvature of 29 ± 4°, in part due to induction of positive gravitropism following displacement of the root tip from vertical during negative phototropism. However, at higher fluence rates, development of phototropic curvature is arrested even if gravitropism is avoided by maintaining the root cap vertically using a rotating feedback system. Thus continuous illumination can cause adaptation in the signalling pathway of the phototropic response in roots.
The Ball–Berry–Leuning and Tardieu–Davies stomatal models: synthesis and extension within a spatially aggregated picture of guard cell function
Authors: Dewar R. C.
Published: March 2003

A new model of stomatal conductance is proposed which combines the essential features of the Ball–Berry–Leuning (BBL) and Tardieu–Davies (TD) models within a simple spatially aggregated picture of guard cell function. The model thus provides a coherent description of stomatal responses to both air and soil environments. The model also presents some novel features not included in either the BBL or TD models: stomatal sensing of intercellular (rather than leaf surface) CO2concentration; an explanation of all three observed regimes (A, B and C) of the stomatal response to air humidity (MonteithPlant, Cell and Environment18, 357–364, 1995); incorporation of xylem embolism; and maintenance of hydraulic homeostasis by combined hydraulic and chemical signalling in leaves (in which leaf epidermal hydraulic conductivity plays a key role). Significantly, maintenance of hydraulic homeostasis in the model does not require a direct feedback signal from xylem embolism, the predicted minimum leaf water potential being independent of xylem hydraulic conductivity. It is suggested that stomatal regulation through combined hydraulic and chemical signalling in leaves and/or roots provides a general mechanism enabling plants to maintain their water potentials above a minimum value. Natural selection of the key stomatal parameters would then set the minimum potential to a specific value determined by the most vulnerable plant process under water stress (e.g. cell growth, protein synthesis or xylem cavitation), depending on species and growth conditions.
Shoot P status regulates cluster‐root growth and citrate exudation in Lupinus albus grown with a divided root system
Authors: SHANE M. W. , DE VOS M. , DE ROOCK S. , LAMBERS H.
Published: March 2003

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster‐root growth and citrate‐exudation rate more so than low P concentration in the roots. In the split‐root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m−3 P), to the ‘privileged’ root halves. Roots ‘deprived’ of P invariably had the same low P concentrations, whereas those in the ‘privileged’ roots increased with increasing P supply (1, 25 or 75 mmol m−3 P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate‐exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the ‘deprived’ root halves when the ‘privileged’ half was exposed to 1 mmol m−3 P as compared with 25 or 75 mmol m−3 P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster‐root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester‐P and inorganic P among all ‘deprived’ root halves. The results indicate that cluster‐root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus.
Parameterization of the CO2 and H2O gas exchange of several temperate deciduous broad‐leaved trees at the leaf scale considering seasonal changes
Authors: KOSUGI Y. , SHIBATA S. , KOBASHI S.
Published: March 2003

A combined model to simulate CO2 and H2O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf‐scale observations of diurnal and seasonal changes in the CO2 and H2O gas exchange of four temperate deciduous broad‐leaved trees using a porometric method. The model consists of a Ball et al. type stomatal conductance submodel [Ball, Woodrow & Berry, pp. 221–224 in Progress in Photosynthesis Research (ed. I. Biggins), Martinus‐Nijhoff Publishers, Dordrecht, The Netherlands, 1987] and a Farquhar et al. type biochemical submodel of photosynthesis (Farquhar, von Caemmerer & Berry, Planta 149, 78–90, 1980). In these submodels, several parameters were optimized for each tree species as representative of the quantitative characteristics related to gas exchange. The results show that the seasonal physiological changes of Vcmax25 in the biochemical model of photosynthesis should be used to estimate the long‐term CO2 gas exchange. For Rd25 in the biochemical model of photosynthesis and m in the Ball et al. type stomatal conductance model, the difference should be counted during the leaf expansion period.
The phloem, a miracle of ingenuity
Authors: VAN BEL A. J. E.
Published: March 2003

This review deals with aspects of the cellular and molecular biology of the sieve element/companion cell complex, the functional unit of sieve tubes in angiosperms. It includes the following issues: (a) evolution of the sieve elements; (b) the specific structural outfit of sieve elements and its functional significance; (c) modes of cellular and molecular interaction between sieve element and companion cell; (d) plasmodesmal trafficking between sieve element and companion cell as the basis for macromolecular long‐distance signalling in the phloem; (e) diversity of sieve element/companion cell complexes in the respective phloem zones (collection phloem, transport phloem, release phloem); (f) deployment of carriers, pumps and channels on the plasma membrane of sieve element/companion cell complexes in various phloem zones; and (g) implications of the molecular‐cellular equipment of sieve element/companion cells complexes for mass flow of water and solutes in a whole‐plant frame.
The effect of resistance of rootstocks to crown gall (Agrobacterium spp.) on the susceptibility of scions in grape vine cultivars
Authors: Süle S. , Burr T. J.
Published: March 2003

The effects of two grapevine rootstocks on susceptibility of grafted scions to crown gall (Agrobacterium vitis and Agrobacterium tumefaciens) was investigated in greenhouse and field experiments. The incidence of crown gall on a susceptible grape scion cultivar (Vitis vinifera, cv.  Kiralyleanyka) was not affected by grafting it on a crown gall resistant rootstock (Vitis riparia, cv.  Gloire) or on a susceptible rootstock (V. berlandieri × V. riparia, cv.  Teleki 5C) when the inoculated vines were observed over a 2‐month period in the greenhouse. Weights of galls that were induced by six of eight strains of Agrobacterium (six A. vitis and two A. tumefaciens) were not significantly different, but A. tumefaciens strain 16/6 and A. vitis strain 2/3 produced significantly larger galls on Kiralyleanyka vines that were growing on their own roots or were grafted on 5C than on those grafted on Gloire. When the same vines were planted in the field and observed over a 4‐year period, no differences in incidence of crown gall were appa_rent until the third and fourth years, when vines grafted on Gloire had less crown gall than vines on 5C. By that time, the natural symptoms (galls along the trunk) of the disease appeared, coinciding with the decline of whole vines. In another field experiment, with naturally infected scions of V. vinifera, cv.  Blau Frankisch (susceptible), crown gall was appa_rent on vines grafted on both rootstocks after 2 years, but in the fifth year 79% of the vines on 5C had crown gall whereas only 9% on Gloire showed symptoms. After the appearance of typical natural symptoms, vines showed reduced vigor with substantial loss of trunks and entire vines. During this period, many vines on 5C died as compared with only a few on Gloire. By the end of the sixth year, 13% of the originally planted vines on 5C and 82% on Gloire were alive, and although many pathogenic strains of Agrobacterium spp. were isolated from the roots of 5C, none was isolated from Gloire.
Rhizoctonia species and anastomosis groups isolated from barley and wheat in Erzurum, Turkey
Authors: Demirci E.
Published: March 2003

Ninety‐eight isolates of Rhizoctonia spp. were obtained from barley and wheat grown in Erzurum, Turkey. Of these, 78% were Rhizoctoniasolani (AG‐2 type 1, AG‐3, AG‐4, AG‐5 and AG‐11), 10% were binucleate Rhizoctonia (AG‐I and AG‐K) and the remainder were Waitea circinata var circinata (Rhizoctonia sp.). Among the binucleate Rhizoctonia, AG‐I was not recovered from barley. In pathogenicity tests on barley and wheat, the highest disease severity was caused by isolates of AG‐4 and AG‐11, whereas isolates of AG‐2 type 1, AG‐3, AG‐5 and W. c. var circinata were moderately virulent. Isolates of binucleate Rhizoctonia were all nonpathogenic. This is the first report of R. solani AG‐11 and W. c. var circinata from Turkey.
The epidemiology, variability and control of the downy mildews of pearl millet and sorghum, with particular reference to Africa
Authors: Jeger  , Gilijamse  , Bock  , Frinking
Published: March 2003

Sorghum downy mildew (Peronosclerospora sorghi) infecting sorghum and maize, and pearl millet downy mildew (Sclerospora graminicola) infecting pearl millet can cause considerable yield loss in Africa. The last 15 years have witnessed an increase in knowledge of the biology, epidemiology and control of these two pathogens. Much information has been obtained on the effect of environmental factors on disease epidemiology, spore production and dispersal. Molecular techniques applied to study pathogenic variability have aided in defining relationships among these pathogens, although scope of the work is limited. Knowledge of the genetics and inheritance of resistance, and of resistance mechanisms, has also increased. This review presents the current state of knowledge of both downy mildew pathogens, with focus on their status on sorghum and pearl millet in Africa. Despite the advances in knowledge over the last 15 years, these downy mildews remain important constraints to sustainable crop production in the semi‐arid regions of Africa. In some cases information obtained in Asia and the Americas can be extrapolated to Africa but care must be taken in ensuring its applicability. Priorities for future research relevant for Africa are proposed and discussed.
Comparison of PCR, BIO‐PCR, DIA, ELISA and isolation on semiselective medium for detection of Xanthomonas albilineans, the causal agent of leaf scald of sugarcane
Authors: Wang  , Comstock  , Hatziloukas  , Schaad
Published: March 2003

Polymerase chain reaction (PCR) and newly designed primers, XAF1/XAR1, were tested for selective detection of the causal agent of leaf scald of sugarcane, Xanthomonas albilineans. The efficiency and reliability of PCR were compared with dot immunobinding assay (DIA), ELISA and classical isolation techniques for detecting X. albilineans in suspensions of pure cells and extracts of field‐collected stalk and leaf samples of sugarcane. In addition, classical PCR and BIO‐PCR (biological amplification followed by PCR) were compared with isolation on a semiselective agar medium. Classical PCR and BIO‐PCR techniques had the advantage of not requiring pathogenicity tests to confirm the identity of colonies tentatively identified as X. albilineans on modified semiselective XAM agar medium. The m‐XAM medium and BIO‐PCR techniques were the most sensitive; however, the former required seven days whereas the latter required only four days. The BIO‐PCR technique was as sensitive as the semiselective medium technique and eliminated the need to conduct any additional tests to confirm the identification.
Genetic diversity of Ganoderma in oil palm plantings
Authors: Miller  , Holderness  , Bridge  , Chung  , Zakaria
Published: March 2003

Isolates of Ganoderma spp. associated with basal stem rot of oil palm were obtained by sampling from two palm plantings in Malaysia. Members of a subset of these were compared using a combination of somatic incompatibility and molecular genetic analyses. Incompatibility interactions between heterokaryons in culture indicated that Ganoderma generally occurred as numerous distinct strains, with compatible strains usually confined to individual palms. Analysis of restriction fragment length polymorphisms (RFLPs) in presumptive mitochondrial DNA (mtDNA) revealed heterogeneity among isolates from neighbouring palms, and from within individual palms. The combination of mtDNA variability and somatic incompatibility suggests that Ganoderma BSR does not spread by direct root‐to‐root contact between palms. The results may indicate infection spread through basidiospore dispersal or through contact with a heterogeneous long‐term residual inoculum in debris.
Internal colonization pathways of potato plants by Erwinia carotovora ssp. atroseptica
Authors: Hélias  , Andrivon  , Jouan
Published: March 2003

Transmission of pectinolytic Erwinia species from infected mother tubers to daughter tubers has been studied mainly through detection tests, carried out at harvest, on limited samples of tubers produced by plants grown from artificially inoculated mother tubers. However, detection has not been performed on samples collected at different stages of crop development, in order to follow the contamination progress in different organs through the plants to the progeny tubers. In this study the bacterial contamination of progeny tubers was investigated by detecting Erwinia carotovora ssp. atroseptica in different symptomless plant organs (stolons, stems, progeny tubers) and in the parts with or without symptoms of diseased stems, collected at various stages of crop development. Infection levels in below‐ and above‐ground organs of plants of two cultivars differing in their resistance to Erwinia, infected by either vacuum infiltration or sand wounding, were monitored throughout the growing season and at harvest using DAS‐ELISA and PCR. Detection tests showed that healthy organs from symptomless plants were less frequently contaminated than symptomless organs from diseased plants, and that stolons were precociously and more frequently contaminated than stems and daughter tubers, irrespective of the health of the plant. Stem infections were shown to progress latently in the stem, bacteria usually being recovered 10–15 cm past visible lesions. In many cases, typical aerial stem‐rot symptoms could be related to this upward movement of bacteria from the infected mother tuber. Daughter tubers without symptoms were shown to be frequently contaminated, usually at heel ends, suggesting internal contamination from mother tuber to progeny.
Epidemiology of brown rot (Monilinia fructigena) on apple: infection of fruits by conidia
Authors: Xu X.‐M. , Robinson J.D.
Published: March 2003

Effects of fruit maturity, wound age, temperature and the duration of wetness periods on infection of apple fruits by conidia of the brown rot fungus, Monilinia fructigena, were studied. Inoculation of fruits on potted apple trees and harvested mature fruits showed that wounding was essential for infection by M. fructigena. On potted trees, there was a significant difference between the susceptibility of cvs Cox and Gala and this difference depended on wound age. The incidence of brown rot was affected greatly by fruit maturity and wound age. Wounds on younger fruits were more resistant to infection than those on older fruits, whilst the older the wound, the more resistant it was to infection. Furthermore, the degree of wound age‐related resistance was greater on younger fruits than on older fruits. These relationships were well described by regression models. The effect of the duration of wetness periods was very small: increasing the duration of wetness periods reduced the incidence of brown rot on older wounds. For detached fruits, all those wounded were rotted after inoculation, except for those in two treatments under 20°C on fruits with wounds which were 8 days old. The incubation period of the fungus was generally very short. Wound age was the single most important factor influencing the length of the incubation period; the incubation period increased as wound age increased.
Sensitivity of fungi from cereal roots to fluquinconazole and their suppressiveness towards take‐all on plants with or without fluquinconazole seed treatment in a controlled environment
Authors: Dawson W. A. J. M. , Bateman G. L.
Published: March 2003

The take‐all fungus, Gaeumannomyces graminis var. tritici, was highly sensitive to fluquinconazole (in‐vitro EC50 0·016–0·018 mg L−1), a fungicide developed for use as a seed treatment to control take‐all, and to prochloraz (EC50 0·006 mg L−1). Fungi of other genera that were commonly isolated from cereal roots were sensitive in varying degrees to prochloraz but were relatively insensitive (e.g. Fusarium culmorum, EC50 > 20 mg L−1) or slightly sensitive (e.g. Epicoccum purpurascens, EC50 0·514 mg L−1) to fluquinconazole. Gaeumannomyces graminis var. graminis and G. cylindrosporus, weak parasites that can protect roots against take‐all, and an unnamed Phialophora sp., all closely related to the take‐all fungus, were highly or moderately sensitive to fluquinconazole. Alternaria infectoria and E. purpurascens were most consistently effective in suppressing development of take‐all on pot‐grown wheat plants dual‐inoculated with G. graminis var. tritici and the nonpathogen. Take‐all was decreased more on dual‐inoculated wheat plants grown from seed treated with fluquinconazole or fluquinconazole plus prochloraz than when only an antagonistic fungus (A. infectoria, E. purpurascens, Fusarium culmorum or Idriella bolleyi) or a seed treatment was applied. These fungi were less effective in combination with seed treatments on barley. Gaeumannomyces graminis var. graminis and G. cylindrosporus, tested on wheat, suppressed take‐all only in the absence of fungicides. It is suggested that the performance of seed treatment containing fluquinconazole against take‐all may in some circumstances be enhanced by its partial specificity for the take‐all fungus.
Sensitive detection of Ralstonia solanacearum in soil: a comparison of different detection techniques
Authors: Pradhanang P. M. , Elphinstone J. G. , Fox R. T. V.
Published: March 2003

The sensitivity and specificity of various methods were compared for routine detection of Ralstonia solanacearum in a sandy loam soil. Populations fewer than 102 CFU per g soil were detected by dilution plating on a modified semiselective medium (SMSA). In comparison, a tomato bioassay was shown consistently to detect populations at or greater than 7·5 × 105 CFU per g soil. An indirect enzyme‐linked immunosorbent assay (ELISA) was as sensitive as the tomato bioassay, but detected as few as 104 CFU per g soil when the suspension was first incubated in SMSA broth prior to testing. Detection using a nested polymerase chain reaction (PCR) was equally as sensitive as that using culture on SMSA agar, but only when the infested soil sample was first enriched overnight in SMSA broth prior to the nested PCR. Longer incubation periods in SMSA broth also increased the sensitivity of pathogen detection using a conventional PCR method, permitting detection of as few as 102 CFU per g soil after 60 h enrichment in SMSA broth. When evaluated using naturally infected field soils in Nepal, isolation of R. solanacearum on SMSA was reliable only when pathogen populations were higher than those of saprophytic soilborne bacteria. As few as 5 × 102 CFU of R. solanacearum per g were recovered from naturally infested soil, whereas the sensitivity of indirect ELISA was 106 CFU g−1.
Seed transmission of maize downy mildew (Peronosclerospora sorghi) in Nigeria
Authors: Adenle V. O. , Cardwell K. F.
Published: March 2003

In an area of Nigeria where downy mildew of maize is present, histological assessment of maize seed revealed the presence of mycelium and oospores of Peronosclerospora sorghi in the kernels. Seed transmission of downy mildew of maize was demonstrated when grain purchased at local markets gave mean seedling infection rates of 12·3% (untreated seeds) and 10·0% (in metalaxyl‐treated seeds) within 7 days of emergence, after storage in a desiccator for 30 days. When untreated seeds taken from nubbin ears of systemically infected plants from four states in southern Nigeria were planted at 9 days (17–22% moisture content) and 27 days (9–22% moisture content) after harvest, 20·0% infected seedlings resulted in both trials. Seeds from Borno state in northern Nigeria had 26·6% systemic seedling infection after 9 months of storage at 11% moisture content. When seeds harvested from maize plants inoculated with P. sorghi through silks were examined histologically, hyphae of P. sorghi were observed mostly in the scutellum of the embryo. Transmission of disease to seedlings was observed when the silk‐inoculated seeds (9% moisture content) were planted in pots in a greenhouse; however, no disease transmission was observed when such seeds were planted in the field. The epidemiological significance of seed transmission is discussed with particular reference to survival of inoculum and development of epidemics. Also noteworthy is the overall significance of seed transmission in Nigeria, where the major source of seed is that saved by farmers from their grain crop, occasionally supplemented by seed bought from the local market.
Cell wall‐degrading enzymes produced in vitro by isolates of Phaeosphaeria nodorum differing in aggressiveness
Authors: Lalaoui F. , Halama P. , Dumortier V. , Paul B.
Published: March 2003

The relationships between in vitro production of cell wall‐degrading enzymes and aggressiveness of three Phaeosphaeria nodorum isolates were investigated. When grown in liquid medium containing 1% cell wall from wheat leaves as the carbon source, the isolates secreted xylanase, α‐arabinosidase, β‐xylosidase, polygalacturonase, β‐galactosidase, cellulase, β‐1,3‐glucanase, β‐glucosidase, acetyl esterase and butyrate esterase. Time‐course experiments showed different levels of enzyme production and different kinetics between isolates. A highly aggressive isolate produced more xylanase, cellulase, polygalacturonase and butyrate esterase than did the two weakly aggressive isolates. Xylanase was the most active polymer‐degrading enzyme produced, suggesting a key role during pathogenesis by P. nodorum.
Genetic variation among populations of Pythium irregulare in southern Australia
Authors: Harvey P. R. , Butterworth P. J. , Hawke B. G. , Pankhurst C. E.
Published: March 2003

Isolates of Pythium irregulare were sampled from seven cereal crops throughout South Australia to determine the extent of genetic diversity within this pathogen and the scale of genetic differentiation among populations. Data derived from 29 individual restriction fragment length polymorphism (RFLP) loci differentiated 54 DNA fingerprints among the 92 isolates analysed. Some isolates had two alleles at several RFLP loci and were scored as heterozygous. One such isolate was selfed in vitro and segregation ratios in the progeny were not significantly different from those expected for allelic variation in a diploid. These data provided evidence that outcrossing occurs within P. irregulare and may contribute to the high level of genetic variation within the species (DT = 0·502). Allelic frequencies were significantly different among all seven populations and GST values showed significant genetic differentiation between populations. The average genetic identity among populations was low and hierarchical cluster analysis provided no clear evidence that populations formed geographically related groups. These analyses indicate low levels of interpopulation gene flow within P. irregulare and imply that population differentiation results from genetic drift.
Epidemiology and management of Leptosphaeria maculans (phoma stem canker) on oilseed rape in Australia, Canada and Europe
Authors: West J. S. , Kharbanda P. D. , Barbetti M. J. , Fitt B. D. L.
Published: March 2003

Phoma stem canker (blackleg), caused by Leptosphaeria maculans, is an important disease on oilseed rape (canola, rapeseed, Brassica napus, Brassica juncea, Brassica rapa) causing seedling death, lodging or early senescence in Australia, Canada and Europe, but not in China. The two forms of L. maculans (A group and B group) that occur on oilseed rape are now considered to be separate species. The epidemiology and severity of phoma stem canker differs between continents due to differences in the pathogen population structure, oilseed rape species and cultivars grown, climate and agricultural practices. Epidemics are most severe in Australia, where only the A group occurs, and can be damaging in Canada and western Europe, where both A and B groups occur, although their proportions vary within regions and throughout the year. Epidemics are slight in China, where the A group has not been found. Dry climates (Australia, western Canada) lengthen the persistence of infected debris and may synchronize the release of airborne ascospores (after rain) with seedling emergence. L. maculans spreads from cotyledon and leaf infections down petioles to reach the stem, with infections on cotyledons and leaves early in the season producing the most damaging stem cankers at the stem base (crown). Development of both crown cankers and phoma stem lesions higher up stems is most rapid in regions with high temperatures from flowering to harvest, such as Australia and Canada. Breeding for resistance (genetic, disease escape or tolerance), stubble management, crop rotation and fungicide seed treatments are important strategies for control of phoma stem canker in all areas. Fungicide spray treatments are justified only in regions such as western Europe where high yields are obtained, and accurate forecasts of epidemic severity are needed to optimize their use.
Molecular characterization and detection of European isolates of Soil‐borne wheat mosaic virus
Authors: Clover G. R. G. , Ratti C. , Henry C. M.
Published: March 2003

Sequencing of a recently identified isolate of Soil‐borne wheat mosaic virus (SBWMV) from the UK confirmed its identity as a European strain of the species and provided further evidence for taxonomic divisions in the group. Two RT–PCR protocols were developed for the detection of all SBWMV strains and for the specific detection of the European SBWMV strain, and were tested successfully on 21 isolates of SBWMV from a range of countries. Both protocols worked well using either purified total RNA in one‐ or two‐step RT‐PCR, or immunocapture (IC) RT–PCR. The sensitivity of IC RT‐PCR was 100 times greater than ELISA. Neither set of primers produced any PCR product with either Wheat spindle streak mosaic virus or Wheat yellow mosaic virus which are frequently associated with SBWMV, or with the related viruses Indian peanut clump virus, Potato mop‐top virus, Beet soil‐borne virus and Beet necrotic yellow vein virus. This new diagnostic protocol will improve disease management by enabling correct identification of the causal pathogen and earlier detection than is possible serologically.
Genetic diversity in the mango malformation pathogen and development of a PCR assay
Authors: Zheng Q. , Ploetz R.
Published: March 2003

Malformation is a destructive disease of mango, Mangifera indica. Its causal agent possesses the morphological features of Fusarium subglutinans, a species whose taxonomy and nomenclature has recently been in a state of flux. Genetic diversity was examined among 74 F. subglutinans‐like isolates from malformed mango in Brazil, Egypt, Florida (USA), India, Israel and South Africa. With nitrate‐nonutilizing (nit) auxotrophic mutants, seven vegetative compatibility groups (VCGs) were identified. Three of the VCGs were found in a single country, and VCG diversity was greatest in Egypt and the USA where, respectively, four and three different VCGs were found. RAPD profiles generated with arbitrary decamer primers were variable among isolates in different VCGs, but were generally uniform for isolates within a VCG. In PCR assays, a 20‐mer primer pair that was developed previously to identify F. subglutinans from maize (mating population [MP]‐E of the Gibberella fujikuroi complex) also amplified a specific 448 bp fragment for isolates of F. sacchari from sugarcane (MP‐B) and what was probably F. circinatum (pine, MP‐H). With the exception of three isolates from Brazil, it did not amplify the fragment from F. subglutinans‐like isolates from mango. A second pair of 20‐mer primers was developed from a unique fragment in the RAPD assays. It amplified a specific 608 bp fragment for 51 of 54 isolates from mango (all but the three Brazilian isolates). It also amplified a smaller, 550 bp fragment from isolates of F. nygamai (MP‐G), but did not amplify DNA of isolates of any other taxon of Fusarium that was tested.