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Showing papers in "Plant and Soil in 1990"


Journal ArticleDOI
TL;DR: The major source of substrates for microbial activity in the ectorhizosphere and on the rhizoplane are rhizodeposition products, which are composed of exudates, lysates, mucilage, secretions and dead cell material, as well as gases including respiratory CO2 as discussed by the authors.
Abstract: The major source of substrates for microbial activity in the ectorhizosphere and on the rhizoplane are rhizodeposition products. They are composed of exudates, lysates, mucilage, secretions and dead cell material, as well as gases including respiratory CO2. Depending on plant species, age and environmental conditions, these can account for up to 40% (or more) of the dry matter produced by plants. The microbial populations colonizing the endorhizosphere, including mycorrhizae, pathogens and symbiotic N2-fixers have greater access to the total pool of carbon including that recently derived from photosynthesis. Utilization of rhizodeposition products induces at least a transient increase in soil biomass but a sustained increase depends on the state of the native soil biomass, the flow of other metabolites from the soil to the rhizosphere and the water relations of the soil. In addition, the phenomena of oligotrophy, cryptic growth, plasmolysis, dormancy and arrested metabolism can all influence the longevity of rhizosphere organisms. With this background, microbial growth in the rhizosphere will be discussed.

1,048 citations


Journal ArticleDOI
TL;DR: The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant poly phenol needs to be standardized.
Abstract: A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and 15N-recovery methods Mature tops of alfalfa (Medicago sativa L), round leaf cassia (Cassia rotundifolia Pers, var Wynn), leucaena (Leucaena leucocephala Lam, deWit), Fitzroy stylo (Stylosanthes scabra Vog, var Fitzroy), snail medic (Medicago scutellata L), and vigna (Vigna trilobata L, var verde) were incorporated in soil at the rate of 100 mg legume N kg-1 soil The medic and vigna were labeled with 15N Sorghum-sudan hybrid (Sorghum bicolor, L Moench) was used as the test crop A non-amended treatment was used as a control Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa With the two legumes that contained less than 20 g kg-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 005 level and at the 001 level at 6 weeks (r2=0866) Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r2=0692) The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized Estimates of legume-N mineralization by the difference and 15N recovery methods were significantly different at all sampling dates for both 15N-labeled legumes After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the 15N recovery method This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from 15N recovery experiments

383 citations


Journal ArticleDOI
TL;DR: The reduced investment of dry matter in the root system and thus the lower root:shoot ratio from early in the growing season may partly explain the increased HI and WUEgr of modern compared to old varieties.
Abstract: A field study tested the hypothesis that modern wheat varieties invest a lesser proportion of the total dry matter (root plus shoot) in the root system compared to old varieties. The study was carried out on a duplex soil (sand over clay) at Merredin, Western Australia in a Mediterranean type environment. We also compared the root:shoot dry matter ratios of near-isogenic lines for Rht dwarfing genes.

337 citations


Journal ArticleDOI
TL;DR: In this article, the processes responsible for maintaining cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed.
Abstract: The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H+ or OH− into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations (NH4++Ca2++ Mg2++K++Na+) and anions (NO3−+Cl−+SO42−+H2PO4−) and charge balance is maintained by excretion of an amount of H+ or OH− which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H+ and in particular OH− ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.

291 citations


Journal ArticleDOI
Abstract: Soil samples from mature and secondary forests and agricultural sites in three subtropical life zones of Puerto Rico and the US Virgin Islands were collected to determine the effects of forest conversion to agriculture and succession on soil organic carbon (C) and nitrogen (N) contents. Site characteristics that may affect soil C and N (slope, elevation, aspect, and texture) were as uniform as possible. Carbon contents (to 50 cm depth or bedrock) of cultivated sites, as a percent of corresponding mature forests, were lower in the wet (44%) and moist (31%) than in the dry (86%) life zones whereas N contents were relatively high regardless of life zone (60–130% of the mature forests). Conversion of forests to pasture resulted in less soil C and N loss than conversion to crops. The time for recovery of soil C and N during succession was approximately the same in all three life zones, about 40–50 yr for C about 15–20 yr for N. However, the rate of recovery of soil C was faster in the wet and moist life zone, whereas N appeared to recover faster in the dry life zone. Evidence for loss of soil C during cultivation and gain during succession to soil depths of 50–100 cm is presented.

236 citations


Journal ArticleDOI
TL;DR: The results indicate that release of PS and subsequent uptake of FePS are under different genetic control, and attention should be paid to the effects of iron nutritional status and daytime on PS release as well as on rapid microbial degradation of PS.
Abstract: Graminaceous species can enhance iron (Fe) acquisition from sparingly soluble inorganic Fe(III) compounds by release of phytosiderophores (PS) which mobilize Fe(III) by chelation. In most graminaceous species Fe deficiency increases the rate of PS release from roots by a factor of 10–20, but in some species, for example sorghum, this increase is much less. The chemical nature of PS can differ between species and even cultivars.

233 citations


Book ChapterDOI
TL;DR: The simulation results emphasized the complexity of selection for drought tranrance caused by the many plant processes involved, the contrast between instantaneous and cumulative reactions and the strong genotype × environment interaction for drought tolerance.
Abstract: Breeding strategies for drought tolerance in potato were evaluated by means of a crop growth model, in which seasonal courses of crop dry matter accumulation and soil moisture availability were simulated in dependence of plant characteristics and weather and soil data.

206 citations


Journal ArticleDOI
TL;DR: In this article, the effect of silicon on phosphorus uptake and on the growth of rice at different levels of P was investigated, and it was shown that +Si increased shoot weight more when P was low or high than when p was medium.
Abstract: A pot experiment was conducted to measure the effect of silicon on phosphorus uptake and on the growth of rice at different P levels. Rice (Oryza sativa L. cv. Akebono) was cultured in Kimura B nutrient solution without and with silicon (1.66 mM Si) and with three phosphorus levels (0.014 mM P, low; 0.21 mM, medium; and 0.70 mM, high). Shoot dry weight with Si (+Si) in solution increased with increasing P level, while shoot weight without Si (−Si) was maximum at 0.21 mM P, suggesting that +Si raised the optimum P level for rice. +Si increased shoot weight more when P was low or high than when P was medium. The concentration and amount of inorganic P in shoots increased with increasing P level. +Si did not significantly decrease P uptake by rice at 0.014 mM P, however, uptake at 0.21 and 0.70 mM P was 27 and 30 percent less than uptake with −Si, respectively. In −Si with 0.21 and 0.70 mM P, inorganic P in shoots was more than double the concentration in shoots grown in +Si solutions. The Si concentration in shoots decreased slightly with increasing P level, although Si uptake was not significantly affected by P. +Si decreased the uptake of Fe and Mn by an average of 20 and 50 percent, respectively, thus P/Mn and P/Fe ratios increased in the shoot when P was low. From the results above, the beneficial effect of Si on the growth of rice was clearly shown when P was low or high. This effect may have resulted from decreased Mn and Fe uptake, and thus increased P availability within P deficient plants, or from reduced P uptake when P was high.

200 citations


Journal ArticleDOI
TL;DR: In this article, the authors discussed the most likely process controls for net and potential rates of nitrification in coniferous forest soils, including substrate availability, pH, allelopathy, water potential, nutrient status and temperature.
Abstract: Net nitrification rates tend to be low or negligible in the forest floor of many coniferous forests of North-East Scotland. The most likely process controls are substrate availability, pH, allelopathy, water potential, nutrient status and temperature. These are discussed in relation to field and laboratory studies of net and potential rates of nitrification. Fungi make up by far the largest part of the nitrifier community in the coniferous forest floor. Very little is known about the distribution and activity of autotrophs in these systems, although it is certain that in vitro evidence suggesting autotrophs cannot nitrify at pH levels characteristic of coniferous forest soils is unrealistic. Because of the metabolic diversity of nitrifying fungi, a variety of organic and inorganic nitrification pathways may exist in coniferous forests. The possible involvement of free radicles in fungal nitrification in coniferous forest soils is also suggested. A complete understanding of nitrification in coniferous forest soils can only result from field characterisation of N flux such as through the use of 15N. This must be combined with ecophysiological characterisation of the organisms involved in order that the complexity of nitrification in coniferous forest soils can be resolved.

177 citations


Journal ArticleDOI
TL;DR: Three post-emergence herbicides were applied to samples of an Alberta agricultural soil and it was concluded that because changes in microbial variables only occurred at herbicide concentrations of much higher than that which occurs following field application, the side-effects of these chemicals is probably of little ecological significance.
Abstract: Three post-emergence herbicides (2,4-D, picloram and glyphosate) were applied to samples of an Alberta agricultural soil at concentrations of 0, 2, 20, and 200 μg g−1. The effects of these chemicals on certain microbial variables was monitored over 27 days. All herbicides caused enhancement of basal respiration but only for 9 days following application, and only for concentrations of 200 μg g−1. Substrate-induced respiration was temporarily depressed by 200 μg g−1 picloram and 2,4-D, and briefly enhanced by 200 μg g−1 glyphosate. It is concluded that because changes in microbial variables only occurred at herbicide concentrations of much higher than that which occurs following field application, the side-effects of these chemicals is probably of little ecological significance.

166 citations


Journal ArticleDOI
TL;DR: In this paper, a portable meter for measuring the intensity of the green color of a rice canopy in the field by using spectral reflectance at 550 and 800 nm was newly devised.
Abstract: A portable meter for measuring the intensity of the green color of a rice canopy in the field by using spectral reflectance at 550 and 800 nm was newly devised. The measurements were found to be affected by reflection of sunshine on the leaf surface. With such limitations being taken into account, rice canopy green color intensity could be evaluated with this meter.

Book ChapterDOI
TL;DR: The interaction of Pythium species with germinating seeds has served as a model system to answer questions about propagule behavior and the role of exudate stimulant molecules in establishing root-fungus interactions and the potential role of both volatile and water-soluble molecules in stimulating propagule germination are discussed.
Abstract: Plant pathogenic fungi survive in soils in a quiescent state. In order for many root-pathogen interactions to be initiated, dormant propagules must be activated by molecules present in seed and root exudates. Without the release of such stimulatory molecules, the majority of root infections do not occur. Currently, little is known about the specific molecules involved in stimulating propagule germination and initiating root-pathogen interactions. Although certain molecules can be shown to elicit germination responses in vitro, responses of propagules reared on conventional culture media do not always reflect the responses of those formed on plant tissues in soil. Consequently, it is not possible to extend conclusions from laboratory determinations of the role of specific exudate molecules in stimulating fungal propagule germination to soil systems. The interaction of Pythium species with germinating seeds has served as a model system to answer questions about propagule behavior and the role of exudate stimulant molecules in establishing root-fungus interactions. The potential role of both volatile and water-soluble molecules in stimulating propagule germination are discussed.

Journal ArticleDOI
TL;DR: The effects of nickel were studied in two serpentine species with different metal tolerance strategies: Silene italica L., which limits nickel uptake and translocation, and Alyssum bertolonii Desv, a serpentine endemic, which accumulates nickel mostly in the leaves.
Abstract: The effects of nickel were studied in two serpentine species with different metal tolerance strategies:Silene italica L., which limits nickel uptake and translocation, andAlyssum bertolonii Desv., a serpentine endemic, which accumulates nickel mostly in the leaves. InS. italica, nickel 7.5 μM inhibited root growth and depressed mitotic activity in root tips. Peroxidase activity and phenol concentration both in roots and shoots were increased; under the same conditions nickel did not produce any relevant effect onA. bertolonii.

Journal ArticleDOI
TL;DR: In this paper, three soils which had been amended for several years with pig slurry, cattle slurry and sewage sludge were dry-sieved to obtain microaggregates in the size range of 250-125, 125-50, and <50 μm.
Abstract: Three soils which had been amended for several years with pig slurry, cattle slurry, and sewage sludge were dry-sieved to obtain microaggregates in the size range of 250–125, 125–50, and <50 μm. With amendments, aggregate size distribution of whole soils was shifted to larger sizes, especially for the most fragile soil, whereas percent content of microaggregates decreased except for the lower size aggregates of the fragile soil. Particle size distribution of microaggregates revealed an increase in percent sand and a reduction of percent silt and clay in the <50 μg size fraction for all soils. These results showed the aggregation effect induced by the organic waste additions. Aggregate stability of microaggregates revealed significant correlation with humic substances content (humic acids alone and humic plus fulvic acids) and non significant with total organic matter substantiating the belief that humic substances are the predominant binding agents in this aggregation range. Molecular weight distribution of humic acids extracted from microaggregates of unamended soils demonstrated that the lower the soil aggregate size distribution, the larger the contribution of the high molecular weight fraction. All microaggregates from amended soils showed a progressive increase of the high molecular weight humic acids with decreasing size, reaching a maximum in the <50 μm fraction. In this aggregate size a parallel enhancement of the aggregate stability was also evident. It is concluded that a close relationship exists between aggregate stability and high molecular weight humic substances. Additions to soils of organic material containing high molecular weight constituents would represent a useful management practice to improve aggregate stability.

Book ChapterDOI
TL;DR: It appears that genetic engineering of the lytic enzymes, such as chitinase, which play an important role in plant disease control, may improve the efficacy of biocontrol agents.
Abstract: Bacterial antagonism, responsible for biological control, may operate by antiobiosis, competition or parasitism. Parasitism relies on lytic enzymes for the degradation of cell walls of pathogenic fungi. Serratia marcescens was found to be an efficient biocontrol agent of Sclerotium rolfsii and Rhizoctonia solani under greenhouse conditions. Populations of 105 or 106 colony forming units g-1 soil were the most effective. Drench and drip application of S. marcescens suspension were more effective in controlling S. rolfsii than spraying, mixing in soil or seed coating. The highest population density of the bacteria in the rhizosphere was found on the proximal portion of the root, decreasing significantly until the tips, where it increased again. The isolated Serratia, found to possess chitinolytic activity, was able to release N-acetyl D-glucosamine from cell walls of S. rolfsii. The gene coding for chitinase was cloned into Escherichia coli and the enzyme was uniquely excreted from the bacterium into its growth medium. When S. rolfsii was sprayed by partially purified chitinase produced by the cloned gene, rapid and extensive bursting of the hyphal tips was observed. This chitinase preparation was effective in reducing disease incidence caused by S. rolfsii in beans and R. solani in cotton, under greenhouse conditions. A similar effect was obtained when a viable E. coli cell, containing the plasmid with the chitinase gene (pLCHIA), was applied. It appears that genetic engineering of the lytic enzymes, such as chitinase, which play an important role in plant disease control, may improve the efficacy of biocontrol agents.

Journal ArticleDOI
TL;DR: This paper discusses interspecific differences and phenotypic responses to nitrogen supply in various root parameters of five perennial grasses from contrasting habitats with significant between-species variation.
Abstract: This paper discusses interspecific differences and phenotypic responses to nitrogen supply in various root parameters of five perennial grasses from contrasting habitats. The following root parameters were studied: root:shoot ratio, specific root length, specific root area, mean root diameter, frequency of fine roots, and the length and density of root hairs. Significant between-species variation was found in all of these features. Species from fertile sites had higher root:shoot ratios at high nitrogen supply than species from infertile habitats. All species growing at low nitrogen supply showed a significant increase in root:shoot ratio. Specific root length, specific root area, mean root diameter and frequency of fine roots were not affected significantly by nitrogen supply. Species from infertile sites responded to low nitrogen supply by a significant increase in root hair length and root hair density.

Journal ArticleDOI
TL;DR: In this article, the influence of slurry application rate, wind speed and applying slurry in narrow bands on ammonia volatilization from cattle slurry surface-applied to grassland was examined.
Abstract: Three experiments were conducted to examine the influence of slurry application rate, wind speed and applying slurry in narrow bands on ammonia (NH3) volatilization from cattle slurry surface-applied to grassland. The experiments were conducted in the field using a system of small wind tunnels to measure NH3 loss. There was an inverse relationship between slurry application rate and the proportion of NH4 +-N volatilized. From slurry applied at 20, 40, 60, 80, 100 and 120 m3 ha-1, the respective proportions of NH4 +-N lost by NH3 volatization in 6 days were 60, 56, 49, 40, 44 and 44%. The negative relationship was most pronounced in the first 24 hours after application when 57–77% of the total loss for 6 days occurred. Wind speed had a positive effect on NH3 volatilization, although the effect was small in relation to the total loss; increasing the wind speed from 0.5 to 3.0 m s-1 increased the total 5 day loss by a factor of 0.29. The effect of wind speed was also most pronounced in the first 24 hours when much of the NH3 loss took place. The effect of reducing the surface area of the applied slurry was examined by comparing NH3 volatilization from slurry broadcast across plots with that applied in narrow bands. Although the rate of NH3 volatilization was considerably smaller from the banded application immediately after the slurry was applied, the difference between the treatments progressively narrowed until 2 days after application, after which a higher rate was maintained from the banded slurry. After 5 days the total loss from the banded application was 83% of that from broadcast slurry.

Journal ArticleDOI
TL;DR: Differences between the plant species were found with respect to growth improvements due to VAM inoculation and/or phosphorus fertilization under drought stress conditions.
Abstract: The effect of vesicular-arbuscular mycorrhizal (VAM) fungi on growth and drought resistance of Acacia nilotica and Leucaena leucocephala seedlings was studied in a glasshouse experiment. The experimental design was a 2·2·2 factorial: ± mycorrhizal inoculation, ± application of phosphorus fertilizer and ± repeated drought treatment. The growth promoting effect of VAM fungi equalled the effect of phosphorus fertilization after 12 weeks. The drought treatment reduced seedling biomass and nodulation. Differences between the plant species were found with respect to growth improvements due to VAM inoculation and/or phosphorus fertilization under drought stress conditions. The results are discussed in relation to plant drought resistance and reforestation in the subhumid to arid tropics.

Journal ArticleDOI
TL;DR: In this paper, the effect of low root medium pH on growth and proton release of field beans (Vicia faba L. cv) was studied in soil and nutrient solution experiments.
Abstract: The effect of low root medium pH on growth and proton release of field beans (Vicia faba L. cv. Kristall) was studied in soil and nutrient solution experiments. Decrease of soil pH due to proton release by roots strongly depended on the proton buffer capacity of 8 different soil types tested in a pot experiment. Whereas in soils of high proton buffer capacity no pH decrease during the growth period was detectable, in soils of low buffer capacity pH in the bulk soil dropped from about pH 7.3 to 6.5, 6.3 or 5.8 during growth until maturity. This decrease in pH was closely correlated with an inhibition of plant dry weight production (Y= 1.06 x +3.33, r = 0.94***). Growth reduction was not due to direct inhibition of nitrogen fixation. In short term experiments vegetative growth and proton release were inhibited at p H < 6. At pH 5 or lower proton uptake was observed in 1 mM CaSO4. Low pH (4.0 relative to pH 7.0) decreased uptake of all major ions except for CI the exclusion of which was disturbed. It is concluded that the sensitivity of field beans to low pH is related to a lack of capability to release protons by ATPase activity. This sets limits to nutrient uptake and possibly cytoplasmic pH regulation.

Book ChapterDOI
TL;DR: Depending on environmental factors and plant species, certain strains of rhizosphere Pseudomonas spp.
Abstract: Rhizobacteria live around roots but also inside the cortical root tissues by utilizing organic substances released from root cells into the intercellular spaces and the root environment. The effects of metabolites of these rhizosphere-inhabiting bacteria on root physiology and plant development have hardly been studied. However, recent studies indicate that, depending on environmental factors and plant species, certain strains of rhizosphere Pseudomonas spp. and some of their metabolites such as HCN may inhibit or enhance plant establishment or inhibit development of plant disease. Cultural practices such as cropping frequency, no tillage, and soilless cultivation, as well as edaphic factors seem to determine these rhizosphere interactions.

Journal ArticleDOI
TL;DR: The results confirm the importance of Ca interaction with salinity stress, and indicate differences in species response.
Abstract: Salinity-calcium interactions, which have been shown to be important in plants grown in dryland saline soils of the Canadian prairies, were studied in two species differing in salt tolerance. In solution culture, wheat showed a greater reduction in growth and a higher incidence of foliar Ca deficiency symptoms than barley when grown under MgSO4 or Na2SO4 plus MgSO4 salt stress. Amendment of the saline solution with Ca to increase the Ca/(Na+Mg) ratio ameliorated the effects of salt, but more so in wheat than in barley. At least part of the difference in salt tolerance between the two species must therefore relate to species differences in the interaction of salinity and Ca nutrition. The greater response of wheat to Ca was not due to a lower Ca status in leaf tissue; on the contrary, although Ca amendments improved tissue Ca/(Na+Mg) ratios in both species, salinized wheat had equivalent or higher Ca content, and higher Ca/(Na+Mg) ratios than did barley. The higher Ca requirement of wheat is apparently specific to a saline situation; at low salinity, wheat growth was not reduced as extensively as that of barley as Ca/(Na+Mg) ratio was decreased. High night-time humidity dramatically improved wheat growth under saline conditions, but increasing the Ca concentration of the saline solution had no effect on growth in the high humidity treatment. Membrane leakage from leaf tissue of wheat grown under saline conditions was increased compared to tissue from non-saline plants. Plants grown in Ca-amended saline solutions showed no increase in membrane leakage. These results confirm the importance of Ca interaction with salinity stress, and indicate differences in species response.

Journal ArticleDOI
TL;DR: A number of glucosinolates and isothiocyanates were tested for their allelopathic potential against wheat and the most active compound, 2-phenethyl ITC completely inhibited wheat germination at 500 ppm.
Abstract: A number of glucosinolates and isothiocyanates were tested for their allelopathic potential against wheat. Most of the glucosinolates showed no activity against wheat, with the exception of glucobrassicin which was moderately active, as was sinapine thiocyanate. Isothiocyanates showed high activity against wheat germination and seedling growth. The most active compound, 2-phenethyl ITC completely inhibited wheat germination at 500 ppm. Allyl ITC showed high activity whereas other isothiocyanates tested were only moderately active. The data is discussed in relation to the possible use of some mustard species for effective weed control.

Journal ArticleDOI
TL;DR: In this article, the effects of loosening a pronounced traffic pan which existed on the site (LS)versus unloosened (US) were compared for two years.
Abstract: Spring wheat cv. ‘Gutha’ was grown in continuous wheat (W/W) and narrow-leafed lupin (L. angustifolius L. cv. Yandee)-wheat (L/W) rotation on a yellow earth over mottled clay (Arenic Fragiudult) in a mediterranean climate for two years. The first year had a higher than average rainfall with adequate soil water until anthesis. The second year was very dry (only 232 mm total rainfall) and soil water contents were low throughout the growing season. Nitrogen fertilizer (+N) treatments were included in both years. In the first year an adjacent experiment compared the effects of loosening a pronounced traffic pan which existed on the site (LS)versus unloosened (US).

Journal ArticleDOI
TL;DR: The discovery that cells shed from the cap express hos-specific genes suggests that some microorganisms may have greater access to constituents of the cells than others, which could have a significant impact on rhizosphere populations.
Abstract: A group of cells at the periphery of the root cap separate from plant roots as they move through the soil. Experimentally, these cells can be separated from the root by gentle agitation in water. Detached root cap cells provide a convenient system for comparing cellular and whole-plant responses to fungal and bacterial pathogens. Results with several host-parasite combinations indicate that these cells express host-specific traits with respect to chemotaxis, binding, and infection. Mutants of Agrobacterium tumefaciens were used to test the hypothesis that recognition of the cells plays a role in establishment of microbial-plant associations. Tn5 mutants deficient in chemotaxis to the cells exhibit reduced ability to colonize the rhizosphere and to induce crown gall tumorigenesis on pea plants grown in soil. The discovery that cells shed from the cap express host-specific genes suggests that some microorganisms may have greater access to constituents of the cells than others. This cellular selectivity could have a significant impact on rhizosphere populations.

Journal ArticleDOI
TL;DR: It is concluded that Al-tolerance genes located in A genome control the expression of other Al- tolerance genes Located in the D genome, which has implications for chromosome and gene manipulations in cereals.
Abstract: Preliminary studies indicated that aluminium-tolerance in wheat (Triticum aestivum L. Thell.) is a dominant character controlled by several genes. The present paper describes further work on localization and characterization of some of these genes in the genome of the medium Al tolerant wheat cultivar Chinese Spring (C.S.), using an aneuploid series (ditelosomics). Aluminium-tolerance of seedlings was assessed using the modified ‘pulse’ method; the aluminium concentration in the nutrient solution causing irreversible damage to the root apical meristems on exposure for 24 h at 25°C was the measure of Al-tolerance. At least three different factors controlling Al-tolerance in the C.S. cultivar were located on chromosomes 5As, 2D1 and 4D1. Significant differences were found in Al-uptake and accumulation in roots of the respective ditelosomic lines and euploid seedlings of C.S. Genes controlling Al-tolerance located in the D genome (2D1 and 4D1) were not expressed in solution culture when genes located on 5As were missing, whereas some tolerance was observed in aneuploid lines in which genes from 5As were present while genes from 2D1 and 4D1 were missing. It is concluded that Al-tolerance genes located in A genome control the expression of other Al-tolerance genes located in the D genome. The implications of the obtained results for chromosome and gene manipulations in cereals are discussed.

Journal ArticleDOI
TL;DR: Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects, and may be accounted for by increased biomass.
Abstract: Sorghum [Sorghum bicolor (L.) Moench] plants were grown in growth chambers at 20, 25 and 30°C in a low P Typic Argiudoll (3.65 µg P g−1 soil, pH 8.3) inoculated with Glomus fasciculatum, Glomus intraradices, and Glomus macrocarpum to determine effects of vesicular-arbuscular mycorrhizal fungi (VAMF) species on plant growth and mineral nutrient uptake. Sorghum root colonization by VAMF and plant responses to Glomus species were temperature dependent. G. macrocarpum colonized sorghum roots best and enhanced plant growth and mineral uptake considerably more than the other VAMF species, especially at 30°C. G. fasciculatum enhanced shoot growth at 20 and 25°C, and mineral uptake only at 20°C. G. intraradices depressed shoot growth and mineral uptake at 30°C. G. macrocarpum enhanced shoot P, K, and Zn at all temperatures, and Fe at 25 and 30°C above that which could be accounted for by increased biomass. Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects.

Journal ArticleDOI
TL;DR: In both techniques, roots and their rhizosphere organisms can be observed at the soil-wall boundary.
Abstract: This paper discusses two glass-wall techniques-rhizotrons and minirhizotrons. Rhizotrons are covered underground walkways with clear windows on one or both sides. Their design varies somewhat with the type of research to be conducted in them. A minirhizotron consists of a clear tube inserted into the soil then some type of viewing arrangement is lowered into the tube. In both techniques, roots and their rhizosphere organisms can be observed at the soil-wall boundary.

Journal ArticleDOI
TL;DR: In this article, the characteristics of nitrification in 41 humus samples of Dutch heathlands were studied and it was shown that nitrification was probably of a chemolithotrophic nature.
Abstract: Some characteristics of nitrification in 41 humus samples of Dutch heathlands were studied Most of the acid humus samples (30) showed accumulation of nitrate during a 4-week incubation of field-moist material In these samples net nitrate production was completely blocked by 006% acetylene indicating that nitrification was probably of a chemolithotrophic nature From a comparison of the net production of nitrate in humus suspensions at pH 4 and pH 6 a differentiation into four patterns could be made:

Journal ArticleDOI
TL;DR: Barley (Hordeum vulgare L. cv. "Onda" as mentioned in this paper ) plants were grown in nutrient solutions supplied either 0 (no Ni added), 0.6, or 1.0 μM NiSO4.
Abstract: Barley (Hordeum vulgare L. cv. ‘Onda’) plants were grown in nutrient solutions supplied either 0 (no Ni added), 0.6, or 1.0 μM NiSO4. Plants supplied 0 μM Ni developed Ni deficiency symptoms; Ni deficiency resulted in the disruption of nitrogen metabolism, and affected the concentration of malate and various inorganic anions in roots, shoots, and grain of barley.

Book ChapterDOI
TL;DR: Two potato cultivars differing in their nitrogen acquisition from the soil were used in nutrient solutions to study the effect of increasing concentrations of nitrate particularly on root growth and morphology; in each variety increasing nitrogen concentrations stimulated shoot growth more than root growth.
Abstract: Two potato (Solanum tuberosum L.) cultivars (‘Astrid’ and ‘Bodenkraft’) differing in their nitrogen acquisition from the soil (Hunnius, 1981) were used in nutrient solutions to study the effect of increasing concentrations of nitrate (0.05; 0.5; 5.0mol m-3) particularly on root growth and morphology. In each variety increasing nitrogen concentrations stimulated shoot growth more than root growth. At all nitrate concentrations, the variety with higher nitrogen acquisition (‘Astrid’) had a significantly larger root system. The larger root system of ‘Astrid’ compared to ‘Bodenkraft’ was particularly evident when surface area and total length of the roots, instead of root dry weight were used as parameters. The results stress the importance of root length and surface area for nitrogen acquisition from soils.