Field calibration of soil-core microcosms: Ecosystem structural and functional comparisons

Microcosms containing intact soil-cores are a potential biotechnology risk assessment tool for assessing the ecological effects of genetically engineered microorganisms before they are released to the field; however, microcosms must first be calibrated to ensure that they adequately simulate key field parameters. Soil-core microcosms were compared with the field in terms of ecological response to the introduction of a large inoculum of a rifampicin-resistant rhizobacterium,Pseudomonas sp. RC1. RC1 was inoculated into intact soil-core microcosms incubated in the laboratory at ambient temperature (22°C) and in a growth chamber with temperature fluctuations that mimicked a verage field values, as well as into field lysimeters and plots. The effect of the introduced bacterium on ecosystem structure, including wheat rhizoplane populations of total and fluorescent pseudomonads, total heterotrophic bacteria, and the diversity of total heterotrophic bacteria, was determined. Fluorescent pseudomonads were present on the rhizoplane in significantly lower numbers in soil inoculated with RC1, in both microcosms and the field. Conditions for microbial growth appeared to be most favorable in the growth chamber microcosm, as evidenced by higher populations of heterotrophs and a greater species diversity on the rhizoplane at the three-leaf stage of wheat growth. Ecosystem functional parameters, as determined by soil dehydrogenase activity, plant biomass production, and¹⁵N-fertilizer uptake by wheat, were different in the four systems. The stimulation of soil dehydrogenase activity by the addition of alfalfa was greater in the microcosms than in the field. In general, growth chamber microcosms, which simulated average field temperatures, were better predictors of field behavior than microcosms incubated continuously at 22°C.     

Rhizoplane colonization of pea seedlings by Rhizobium leguminosarum and a deleterious root colonizing Pseudomonas sp. and effects on plant growth

Some pseudomonads produce a toxin that specifically inhibits winter wheat (Triticum aestivum L.) root growth and the growth of several microorganisms. The toxin does not inhibit pea (Pisum sativum) root growth, but the organisms are aggressive root colonizers and their effect on Rhizobium leguminosarum growth, colonization, and nodulation of peas was not known. Peas were grown in Leonard jars in the greenhouse. Pea roots were inoculated with R. leguminosarum, a toxin-producing Pseudomonas sp., both, or neither (control). The Pseudomonas sp. colonized pea roots more rapidly and in greater number than R. leguminosarum after ten days. In the presence of the Pseudomonas sp., the R. leguminosarum population on the rhizoplane was less at ten days. When the roots were inoculated with both R. leguminosarum and Pseudomonas sp., the number of nodules were greater than when R. leguminosarum was inoculated alone, but nodule dry weight and pea shoot biomass were similar to plants inoculated with only R. leguminosarum. Although these results need confirmation with non-sterile soil and field studies, these preliminary results indicate that peas will not be affected by wheat root-inhibitory rhizobacteria.     

Survival of native <Emphasis Type="Italic">Pseudomonas</Emphasis> in soil and wheat rhizosphere and antagonist activity against plant pathogenic fungi

Survival of Pseudomonas sp. SF4c and Pseudomonas sp. SF10b (two plant-growth-promoting bacteria isolated from wheat rhizosphere) was investigated in microcosms. Spontaneous rifampicin-resistant mutants derived from these strains (showing both growth rate and viability comparable to the wild-strains) were used to monitor the strains in bulk soil and wheat rhizosphere. Studies were carried out for 60 days in pots containing non-sterile fertilized or non-fertilized soil. The number of viable cells of both mutant strains declined during the first days but then became established in the wheat rhizosphere at an appropriate cell density in both kinds of soil. Survival of the strains was better in the rhizosphere than in the bulk soil. Finally, the antagonism of Pseudomonas spp. against phytopatogenic fungi was evaluated in vitro. Both strains inhibited the mycelial growth (or the resistance structures) of some of the phytopathogenic fungi tested, though variation in this antagonism was observed in different media. This inhibition could be due to the production of extracellular enzymes, hydrogen cyanide or siderophores, signifying that these microorganisms might be applied in agriculture to minimize the utilization of chemical pesticides and fertilizers.     

Biological control of anthracnose of chilli with rhizosphere and rhizoplane fungal isolates from grasses

The fungal species from rhizosphere and rhizoplane of perennial grasses of the Western Ghats of India were studied for their pathogenicity, antagonism in vitro, substrate and root colonization abilities, rhizosphere competence, growth in different soil pH and inoculum shelf-life. Out of 138 non-pathogenic fungal isolates tested, 85 were antagonistic in vitro to chilli anthracnose pathogen Colletotrichum capsici. Fifteen isolates with >60% inhibition zone to pathogen culture had saprophytic and root and rhizosphere colonization abilities. The sorghum grain inocula of test antagonistic fungi- Fusarium oxysporum, Chaetomium globosum and Trichoderma harzianum had the shelf-life of 90 days at 20?±?2?°C and required optimum soil pH of 6.5. The above fungal isolates when tested for biocontrol of anthracnose disease in greenhouse and field caused reduction in seedling mortality and decreased disease incidence and severity at various plant growth stages and significant reduction in chilli fruit and seed infection. The test antagonistic fungi promoted seedling and mature plant growth and increased fruit and seed yield. Populations of these antagonistic fungi were fairly high in chilli rhizosphere at harvest. The present study indicated that antagonistic fungi from grass rhizosphere and rhizoplane could be used to control anthracnose and promote plant growth, and increase yield of chilli in field.     

<Emphasis Type="Italic">Pseudomonas</Emphasis> bacteria and phosphorous fertilization,affecting wheat (<Emphasis Type="Italic">Triticum</Emphasis><Emphasis Type="Italic">aestivum</Emphasis> L.) yield and P uptake under greenhouse and field conditions

We have recently indicated the plant growth promoting activities of Pseudomonas sp. as well as their alleviating effects on some soil stressors such as salinity. This is because in recent years, biological fertilizers have received special attention by scientists in sustainable agriculture. Accordingly, it is pertinent to specify the beneficiary level of such soil bacteria on plant growth including phosphorous (P) uptake. Hence, the objectives were to determine: (1) the plant growth promoting effects of the tested Pseudomonas sp., and (2) its combined effects with different P fertilization rates on the nutrient uptake (N, P, and K) and yield of wheat (Triticum aestivum L.) under greenhouse and field conditions. The experiments were factorially arranged on the basis of a completely randomized block design with three replicates and were conducted at the Research Farm of Agriculture and Natural Resources Research Center of Khorasan, Mashhad, Iran. P was fertilized at three levels including 0, 25 and 50 kg/ha P₂O₅. Pseudomonas sp. including Pseudomonas fluorescens 153, P. fluorescens 169, P. putida 4, and P. putida 108 were tested. Activities such as production of ACC deaminase and IAA-like products, as well as P solubilization were among the most important activities of the tested Pseudomonas sp. Such bacterial effects greatly enhanced wheat growth and yield under greenhouse and field conditions. The results also showed that the effects of Pseudomonas sp. on wheat nutrient uptake and the effects of bacteria as well as P fertilization on wheat yield were significant. P. putida 108 was the most effective strain enhancing wheat P uptake and grain yield under greenhouse (96 and 58%) and field (80 and 37%) conditions, respectively. Hence, although Pseudomonas sp. could be a suitable replacement for high P fertilization, however, the optimum wheat yield resulted when the bioinoculants are combined with 50% (25 kg/ha P₂O₅) P fertilization. This finding has great agricultural and environmental implications.     

Identification of rhizobacteria from maize and determination of their plant-growth promoting potential

During the growing season of 1986, the rhizobacteria (including organisms from the ectorhizosphere, the rhizoplane and endorhizosphere) of 20 different maize hybrids sampled from different locations in the Province of Quebec were inventoried by use of seven different selective media. Isolates were characterized by morphological and biochemical tests and identified using the API20E and API20B diagnostic strips.Pseudomonas spp. were the prominent bacteria found in the rhizoplane and in the ectorhizosphere.Bacillus spp. andSerratia spp. were also detected, but in smaller numbers. In the endorhizosphere,Bacillus spp. andPseudomonas spp. were detected in order of importance. Screening for plant growth-promoting rhizobacteria was carried out in three soils with different physical and chemical characteristics. The results depended on the soil used, but two isolates (Serratia liquefaciens andPseudomonas sp.) consistently caused a promotion of plant growth.Contribution no. 350 of the Research Station, Agriculture Canada, Sainte-Foy, Quebec.     

Production of Exo-polysaccharide by <Emphasis Type="Italic">Rhizobium</Emphasis> sp.

Two fold increase in the yield of glucose and maltose containing exo-polysaccharide (EPS) by Rhizobium sp. was observed during its growth in modified YEMB. EPS production, plant growth promotion activity and root colonization of Rhizobium sp. studies showed enhanced EPS synthesis, more seed germination and over all improvement in plant growth over control and R. meliloti treatment. Groundnut seeds bacterized with Rhizobium sp. resulted in 69.75% more root length, 49.51% more shoot height, 13.75% more number of branches and 13.60% more number of pods over the control and R. meliloti treatment. Bacterization of wheat seeds increased the dry matter yield of roots (1.7-fold), and roots adhering soil (RAS) (1.5) and shoot mass (1.9-fold). Rhizobium sp. inoculation also increased the population density of EPS-producing bacteria on the rhizoplane. Roots of plants inoculated with Rhizobium sp. maintained a higher K⁺/Na⁺ ratio and K⁺–Na⁺ selectivity.     

External and Internal Root Colonization of Maize by Two Pseudomonas Strains: Enumeration by Enzyme-Linked Immunosorbent Assay (ELISA)

Maize root colonization by two fluorescent Pseudomonas strains M.3.1. and TR335, isolated respectively from maize and tomato roots, were studied in hydroponic conditions. Each bacterium was inoculated separately, and three different colonization areas were studied: nutrient solution, rhizoplane, and endorhizosphere. The two Pseudomonas strains established large rhizosphere populations, and rhizoplane colonization of the entire root system was similar for both strains. However, strain M.3.1. colonized the endorhizosphere more efficiently than strain TR335. Seminal root cuttings from the tip to the seed allowed the assessment of colonization of three different root areas (i.e., root cap and elongation area, root-hair zone, and mature zone). Rhizoplane colonizations of all these three areas by M.3.1. were significantly the same, whereas strain TR335 colonized the rhizoplane of the root cap and elongation area more actively than the root-hair zone and mature zone. Population size of the strain M.3.1. in the internal tissue of these areas was greater than that of strain TR335. Co-inoculations of the two strains indicated a stimulation of the population size of strain M.3.1. regardless of root area studied, whereas population size of strain TR335 remained unchanged. These results demonstrated that external and internal maize root tissues were colonized to a greater extent by a strain isolated from a maize rhizosphere than by one isolated from another rhizosphere. Received: 26 September 1996 / Accepted: 1 November 1996     

Effect of Tilemsi phosphate rock-solubilizing microorganisms on phosphorus uptake and yield of field-grown wheat (Triticum aestivum L.) in Mali

With the broad aim of biologically improving P uptake by wheat fertilized with Tilemsi phosphate rock (TPR), we investigated the effect of inoculation with TPR-solubilizing microorganisms isolated from Malian soils and with a commercial isolate of the arbuscular mycorrhizal (AM) fungus Glomus intraradices (Gi). AM root length colonization, and growth yield and P concentration of the cultivar Tetra of wheat were measured under field conditions in Mali. Experimental plots were established in Koygour (Diré) during the 2001–2002 cropping season. Inoculation treatments included two fungal isolates, Aspergillus awamori (C1) and Penicillium chrysogenum (C13), and an isolate of Pseudomonas sp. (BR2), used alone or in fungus-bacterium combinations in the presence or absence of the AM fungus Gi. In fertilized treatments, 0 or 30 kg P ha⁻¹ was applied as TPR or diammonium phosphate (DAP). In 45-day-old wheat plants, the highest root length AM colonization (62%) was observed with TPR fertilized wheat inoculated with Gi and BR2. Our results suggest that BR2 is a mycorrhizal-helper bacteria and a good plant growth-promoting rhizobacteria. In fact, inoculation of wheat Tetra fertilized with TPR with a combination of Gi, BR2 and C1 produced the best grain yield with the highest P concentration. This work shows that by inoculating seeds with TPR-solubilizing microorganisms and AM fungi under field conditions in Mali it is possible to obtain wheat grain yields comparable to those produced by using the expensive DAP fertilizer.     

Interactions between freshwater bacteria andAnkistrodesmus braunii in batch and continuous culture

Batch and continuous cultures ofAnkistrodesmus braunii were established in an inorganic medium with growth rate limited by P. In batch culture, inoculation of lake water bacterial isolates ofPseudomonas sp. andFlavobacterium sp. showed that thePseudomonas isolate was capable of more rapid growth on algal exudates of lytic products than was theFlavobacterium isolate. When inoculated singly into a continuous culture (D=0.267 day^–1; P level, 2M), theFlavobacterium isolate initially caused a decrease in the population density of the alga, but then steady states for both organisms were obtained. ThePseudomonas isolate under the same conditions caused a rapid washout of the algal culture, and steady-state conditions were never obtained. When thePseudomonas isolate was added to the two-member, steady-state system ofA. braunii andFlavobacterium, the algal population again washed out of the vessel, followed by theFlavobacterium and then thePseudomonas isolate. A transient increase in the P concentration to 200M in the culture vessel caused the low algal population level to increase, followed by increases in the bacterial isolates when the algal population was high enough to supply the required organic carbon source. The system demonstrated that competition for P between the alga and the bacteria can occur, and the results were dependent on the algal and bacterial relative growth rates. The bacterial growth rates were limited initially by organic substrates produced by the alga, and the different bacterial isolates competed for these substrates.     

Only C-2 specific glucose oxidase activity is expressed in ligninolytic cultures of the white rot fungus Phanerochaete chrysosporium

Hydroxylamine oxidation was measured in four recently isolated heterotrophic nitrate-reducing bacteria belonging to the generaPseudomonas, Moraxella, Arthrobacter andAeromonas. A hydroxylamine-cytochromec oxidoreductase activity was detected in periplasmic fractions of thePseudomonas andAeromonas spp. and in total soluble fractions of theArthrobacter sp. A monomeric 19-kDa non-haem iron hydroxylamine-cytochromec oxidoreductase was purified from thePseudomonas species and shown to be similar to hydroxylaminecytochromec oxidoreductase ofParacoccus denitrificans.Abbreviations AMO Ammonia monoxygenase - HAO Hydroxylamine-cytochromec oxidoreductase     

Root Colonization by Pseudomonas sp. DSMZ 13134 and Impact on the Indigenous Rhizosphere Bacterial Community of Barley

Over the last few decades, the ability of rhizosphere bacteria to promote plant growth has been considered to be of scientific, ecological, and economic interest. The properties and mechanisms of interaction of these root-colonizing bacteria have been extensively investigated, and plant protection agents that are based on these bacterial strains have been developed for agricultural applications. In the present study, the root colonization of barley by Pseudomonas sp. DSMZ 13134, that is contained in the commercially available plant protection agent Proradix®, was examined using the fluorescence in situ hybridization method with oligonucleotide probes and specific gfp-tagging of the inoculant strain in combination with confocal laser scanning microscopy. In the first phase of root colonization, the inoculant strain competed successfully with seed and soil-borne bacteria (including Pseudomonads) for the colonization of the rhizoplane. Pseudomonas sp. DSMZ 13134 could be detected in all parts of the roots, although it did not belong to the dominant members of the root-associated bacterial community. Gfp-tagged cells were localized particularly in the root hair zone, and high cell densities were apparent on the root hair surface. To investigate the impact of the application of Proradix® on the structure of the dominant root-associated bacterial community of barley, T-RFLP analyses were performed. Only a transient community effect was found until 3 weeks post-application.     

Intact soil-core microcosms compared with multi-site field releases for pre-release testing of microbes in diverse soils and climates

Intact soil-core microcosms were used to compare persistence of Pseudomonas chlororaphis 3732RN-L11 in fallow soil and on wheat roots with field releases at diverse sites. Parallel field and microcosm releases at four sites in 1996 were repeated with addition of one site in 1997. Microcosms were obtained fresh and maintained at 60% soil water holding capacity in a growth chamber at 70% relative humidity, a 12-hour photoperiod, and constant temperature. Persistence of 3732RN-L11 was measured at each site in field plots and microcosms at 7-21 day intervals, and in duplicate microcosms sampled at an independent laboratory. Linear regression slopes of field plot and microcosm persistence were compared for each site, and between identical microcosms sampled at different sites, using log10 transformed plate counts. Microcosm persistence closely matched field plots for wheat roots, but persistence in fallow soil differed significantly in several instances where persistence in field plots was lower than in microcosms. Analysis of weather variations at each site indicated that rainfall events of 30-40 mm caused decreased persistence in fallow soil. Cooler temperatures enhanced persistence in field plots at later time points. Inter-laboratory comparison of regression slopes showed good agreement for data generated at different sites, though in two instances, longer sampling periods at one site caused significant differences between the sites. Soil characteristics were compared and it was found that fertility, namely the carbon to nitrogen ratio, and the presence of expanding clays, were related to persistence. These microcosm protocols produced reliable data at low cost, and were useable for pre-release risk analyses for microorganisms.     

Root colonization and growth promotion of winter wheat and pea by Cellulomonas spp. at different temperatures

Plant-growth-promoting bacteria isolated from the rhizosphere andphyllosphere were analysed for their colonization and growth-promoting effectson winter wheat and pea at different temperatures. The investigations werecarried out in pot experiments using loamy sand in Germany. The colonization ofstrains Cellulomonas sp. 21/2 andCellulomonas sp. 43 in the rhizosphere of winter wheat andpea were much better at 16 °C than that at 26°C. The inoculation with effective bacterial strainssignificantly increased the root and shoot growth of winter wheat and pea at 16more than at 26 °C. Bacterial inoculation also resulted insignificantly higher amount of N, P, and K contents of plant components.     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

Production of alkali-tolerant cellulase-free xylanase by <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. WLUN024 with wheat bran as the main substrate

An alkali-tolerant cellulase-free xylanase producer, WLI-11, was screened from soil samples collected from a pulp and paper mill in China. It was subsequently identified as a Pseudomonas sp. A mutant, WLUN024, was selected by consecutive mutagenesis by u.v. irradiation and NTG treatment using Pseudomonas sp. WLI-11 as parent strain. Pseudomonas sp. WLUN024 produced xylanase when grown on xylosidic materials, such as hemicellulose, xylan, xylose, and wheat bran. Effects of various nutritional factors on xylanase production by Pseudomonas sp. WLUN024 with wheat bran as the main substrate were investigated. A batch culture of Pseudomonas sp. WLUN024 was conducted under suitable fermentation conditions, where the maximum activity of xylanase reached 1245 U ml⁻¹ after incubating at 37 °C for 24 h. Xylanase produced by Pseudomonas sp. WLUN024 was purified and the molecular weight was estimated as 25.4 kDa. Primary studies on the characteristics of the purified xylanase revealed that this xylanase was alkali-tolerant (optimum pH 7.2–8.0) and cellulase-free. In addition, the xylanase was also capable of producing high quality xylo-oligosaccharides, which indicated its application potential in not only pulp bio-bleaching processes but also in the nutraceutical industry.     

Studies on the genus Fusarium in Egypt

Five Fusarium species were recovered from the rhizoplane of healthy and damped-off cotton, pea, tomato, maize and wheat seedlings raised in the field during a 12-month experiment: F. solani, F. oxysporum, F. moniliforme, F. acuminatum and F. equiseti. F. solani and F. oxysporum were the most common species in the rhizoplane of healthy and damped-off seedlings of cotton, pea, wheat and tomato. In the case of maize, they were surpassed by F. moniliforme which was very scarce in the roots of the other test plants. There was some regular periodicity in the occurrence of Fusarium species in the rhizoplane of test plants. F. oxysporum showed its highest records usually in winter months, F. solani usually in moderate and high temperature months, and F. moniliforme, in maize rhizoplane, in winter months.     

Suppression of maize root diseases caused by Macrophomina phaseolina, Fusarium moniliforme and Fusarium graminearum by plant growth promoting rhizobacteria

A plant growth-promoting isolate of a fluorescent Pseudomonas sp. EM85 and two bacilli isolates MR-11(2) and MRF, isolated from maize rhizosphere, were found strongly antagonistic to Fusarium moniliforme, Fusarium graminearum and Macrophomina phaseolina, causal agents of foot rots and wilting, collar rots/stalk rots and root rots and wilting, and charcoal rots of maize, respectively. Pseudomonas sp. EM85 produced antifungal antibiotics (Afa⁺), siderophore (Sid⁺), HCN (HCN⁺) and fluorescent pigments (Flu⁺) besides exhibiting plant growth promoting traits like nitrogen fixation, phosphate solubilization, and production of organic acids and IAA. While MR-11(2) produced siderophore (Sid⁺), antibiotics (Afa⁺) and antifungal volatiles (Afv⁺), MRF exhibited the production of antifungal antibiotics (Afa⁺) and siderophores (Sid⁺). Bacillus spp. MRF was also found to produce organic acids and IAA, solubilized tri-calcium phosphate and fixed nitrogen from the atmosphere. All three isolates suppressed the diseases caused by Fusarium moniliforme, Fusarium graminearum and Macrophomina phaseolina in vitro. A Tn5:: lac Z induced isogenic mutant of the fluorescent Pseudomonas EM85, M23, along with the two bacilli were evaluated for in situ disease suppression of maize. Results indicated that combined application of the two bacilli significantly (P = 0.05) reduced the Macrophomina-induced charcoal rots of maize by 56.04%. Treatments with the MRF isolate of Bacillus spp. and Tn5:: lac Z mutant (M23) of fluorescent Pseudomonas sp. EM85 significantly reduced collar rots, root and foot rots, and wilting of maize caused by Fusarium moniliforme and F. graminearum (P = 0.05) compared to all other treatments. All these isolates were found very efficient in colonizing the rhizotic zones of maize after inoculation. Evaluation of the population dynamics of the fluorescent Pseudomonas sp. EM85 using the Tn5:: lac Z marker and of the Bacillus spp. MRF and MR-11(2) using an antibiotic resistance marker revealed that all the three isolates could proliferate successfully in the rhizosphere, rhizoplane and endorhizosphere of maize, both at 30 and 60 days after seeding. Four antifungal compounds from fluorescent Pseudomonas sp. EM85, one from Bacillus sp. MR-11(2) and three from Bacillus sp. MRF were isolated, purified and tested in vitro and in thin layer chromatography bioassays. All these compounds inhibited R. solani, M. phaseolina, F. moniliforme, F. graminearum and F. solani strongly. Results indicated that antifungal antibiotics and/or fluorescent pigment of fluorescent Pseudomonas sp. EM85, and antifungal antibiotics of the bacilli along with the successful colonization of all the isolates might be involved in the biological suppression of the maize root diseases.     

Colonization potential of bacteria in the rhizosphere

The effect of inoculum density on growth and steady-state populations of aPseudomonas sp., aMycoplana sp., and aCurtobacterium sp. in the rhizosphere if gnotobiotic barley plants was studied. Inoculation of sterile barley seedling at concentrations of about 1×10³, 1×10⁵ and 1×10⁷ viable cells (mg dry wt root)^–1 resulted in rapid colonization; maximum populations of about 5×10⁷ viable cells (mg dry wt root)^–1 developed in each case. We define this maximum population as the colonization potential. Measurement of growth of known rhizosphere bacteria might be a useful index of the amount of available carbon and energy lost by growing roots.     

The Effect of Glumes on Fungal Infection of Germinating Seed of Spelt (Triticum spelta L.) in Comparison to Wheat (Triticum aestivum L.)

The influence of fungi on seedling emergence of naked and hulled spelt (Triticum spelta L.) and winter wheat (Triticum aestivum L.) was investigated. Seeds were sown in flat trays and placed in a growth chamber under stress conditions (low temperature and water logging) for four weeks, followed by favourable growth conditions. At weekly intervals, 150 seeds were removed, surface sterilized, and investigated for fungi. Pythium aristosporum Vanterpool, a causal agent of damping-off, was found to be best adapted to the unfavourable conditions and to be a major cause of pre-emergence damping-off. The frequency of seed-borne fungi decreased during the stress period, whereas soil-borne fungi survived and could be isolated from seeds and glumes in the following period with more favourable growth conditions. Seedlings from hulled seeds of spelt emerged more frequently than from naked kernels of spelt and both emerged at a higher rate than from seeds of winter wheat. This is good evidence that glumes protect seeds against fungal colonization and therefore increase the fitness of spelt under unfavourable germinating conditions.