Rhizosphärenprozesse, Umweltstreß und Ökosystemstabilität

Mycorrhization of nursery seedlings can be improved by artificial inoculation with selected ectomycorrhizal fungal strains. Thus, seedlings are more tolerant against abiotic and biotic stress factors resulting in a higher survival rate after outplanting. Two year old nursery seedlings of Pinus sylvestris L. have been inoculated with different strains of Paxillus involutus (Batsch) Fr. using ash from of a lignite power plant rinsed into a basin near Trattendorf, Brandenburg, as substrate. One strain, isolated from a fruitbody grown in the ash basin, induced the highest total amount of root tips, the highest mycorrhizal frequency and the highest Paxillus involutus-index. Dry weight of fine roots was shown to be an improper parameter for description of inoculation success. Growth of root collar was enhanced after inoculation, whereas stem growth wasuneffected. It is concluded that the high mycorrhizal potential of this strain was caused by its ecological adaptation to the ash substrate.

In this investigation the influence of Pseudomonas fluorescens on AM (isolate 49 Glomus intra-radices) has been tested by means of a cuvette system. AM development (root colonization rate) as well as root and shoot fresh weights and shoot lenght were determined six times over a period of two months.

The effect of the Pseudomonas fluorescens application was particularly apparent at the beginning of mycorrhizal development. Tagetes-African-Marigold did benefit from the symbiosis. The root and shoot fresh weights were significantly higher with combined inoculated plants.

A VAM inoculation with Glomus intraradices could not stimulate the growth of one year old Panax ginseng plants in a pot experiment, but it rather diminished the shoot and root fresh matter and the protein content. However mycorrhiza led to a better P, K, Fe and Mn uptake into the shoots, but also increased the content of Al. The concentration of prolin and arginin was decreased in mycorrhizal ginseng roots compared to uninoculated ones. That could be caused by the stress reducing activity of VAM.

In P. fluorescens Pf 2-79RL lux genes are activated by a ribosomal promoter, thus exponential growth is associated with bioluminescence. Starved cells are characterized by a low physiological status and have a longer length of lag after transfer into rich medium than actively growing cells. Due to the coupling of bioluminescence with exponential growth, the length of the lag phase of bioluminescence can be used to assess its physiological status of P. fluorescens 2-79RL. The length of the lag phase of bioluminescence was increased with increasing starvation in vitro. Mycorrhizal infection decreased both population density and physiological status of P. fluorescens 2-79RL in the rhizosphere. In P. fluorescens Pf-5, pyoverdine production is reported by ice nucleation. Thus ice nucleation activity increases with increasing Fe stress. Using this reporter, Fe stress of P. fluorescens Pf-5 was determined in the rhizosphere of barley and rice. Fe-stress was greater in rice than in barley and in barley, foliar Fe treatment increased Fe stress of P. fluorescens Pf-5. By incorporation of the ice reporter into P. fluorescens 2-79RL it is now possible to monitor the physiological status and Fe stress simultaneously. Low N availability reduced population density but increased Fe stress of P. fluorescens 2-79RLI in the rhizosphere of wheat. However, the physiological status was not affected by N-availability.

Eine Rhizobien-Impfung erhöhte bei Ackerbohnen den N-Ertrag (mg N/Gefäß) gegenüber einer ungeimpften Kontrollvariante signifikant. Eine Mo-Gabe führte zu einer drastischen Verbesserung des Impferfolges (Trockenmasseertrag, N-Menge pro Gefäß) bei verringerter Knöllchenanzahl. Mo steigerte also offensichtlich die spezifische N₂-Fixierungsleistung.

Weizenpflanzen nahmen nach ¹⁵NH₃-Begasung der Sprosse mehr als 50% des angebotenen ¹⁵N auf. Davon inkorporierten sie unter nicht sterilen Bedingungen ca. 87% in die Sprosse und 6–7% in die Wurzeln. 6–7% des aufgenommenen ¹⁵N wurden an den Boden abgegeben. Mikrobenbesiedlung erhöhte im Vergleich zu Sterilvarianten die Netto-N-Abgabe signifikant. Dabei nahm der wasserlösliche Anteil ab und der wasserunlösliche Teil zu.

The length density of pine (Pins silvestris L.) fine root systems in different soil layers was measured in pine forest ecosystems at three sandy sites with different pollution levels. In spite of an increased nutrient supply in the soils, the root density at the more heavily polluted sites Rösa and Taura was clearly lower than that at Neuglobsow (background). The Taura site which was predominantly acid polluted showed a very low fine root development in the humus layer and a relatively constant length density of fine roots down to a depth of 55 cm. Differences in soil chemical soil properties, especially in the N saturation of the ecosystems, originating from the different pollution levels, were identified as a cause of the different development of fine root systems.

In a greenhouse pot experiment investigations were conducted to study the uptake of different amounts of the chlorinated hydrocarbons (lindane and methoxychlor) and the polycyclic aromatic hydrocarbons (fluoranthene and benzo(a)pyrene) by maize from chernozem soil. There were substantial differences in the uptake and translocation into maize (shoots and roots). With increasing time the concentration of all tested substances in plant decreased. This effect is due to growth dilution. Increasing concentrations of lindane (γ-HCH) and methoxychlor in soil lead to an increase of their concentrations in maize, whereas benzo(a)pyrene and fluoranthene showed no directly effect.

The use of higher plants for soil bioremediation processes is an emerging technology. The beneficial effects of plants on the elimination of hazardous organic pollutants comprise an overall increase of microbial numbers and activities within the rhizosphere, an enhanced microbial metabolism of pollutants by providing root exudates for cometabolic oxidation and the release of unspecific oxidative enzymes which can detoxify aromatic compounds by oxidative binding to soil organic matter. In laboratory studies with soil columns contaminated with hydrocarbons, in the rhizosphere of ryegrass the organic pollutant disappeared faster than in the unplanted control columns. In the planted soil system microbial numbers and soil respiration rates were higher compared to the bulk soil.

The fallow vegetation of the eastern Amazon region, which is an integral part of the regional agro-ecosystem, usually regenerates vegetatively from stumps and roots of the former vegetation. Successional regrowth highly depends on root growth dynamics. The below ground productivity is directly influenced by seasonal changes of rainfall as well as human impact by agricultural land preparation. Two field experiments were established in order to investigate the impact of- 4 different land preparation methods on the regeneration capacity of the fallow vegetation. It was observed, that the traditional slash-and-burn treatment did not considerably affect the root and above-ground regrowth. Contrary to that, mechanized land preparation decreased the regeneration capacity of the fallow vegetation. In order to develop a sustainable agricultural land-use system it is recommended to use land preparation with low impact on the coarse root system. Ploughing, harrowing and repeated stump removal should to be avoided because they negatively affect fallow vegetation regrowth.

In an incubation experiment the mineralization behaviour of shoots and roots of fallow weeds was tested. The differences between the species were small. Carbon mineralization was only 32–43% of the C input. That means after an incubation of 90 days under optimal conditions (25 °C, 60% WK_max) 57–68% of the C input remained in the soil. Nitrogen content, carbon nitrogen ratio and cellulose content influenced significantly the height of mineralization at the 10. incubation day, but not at the end of incubation. The carbon mineralization was connected with a nitrogen immobilization of 110–180 kg/ha.

The effect of plants on the N transformation processes and on the emission of radiatively active trace gases (N₂O, CH₄) in the system reed canarygrass (Phalaris arundinacea L.) — fen soil were determined in laboratory experiments with an automated gas analysis equipment and the ¹⁵N tracer technique. Under dry soil conditions (pF=4) plants reduced, and under wet soil conditions (pF=2) plants enhanced soil and fertilizer N transformation and N₂O emission. The loss of fertilizer N was reduced by plant cover in any case (22, 8/29,1% without plants versus 4,7/10,5% with plants under dry/wet conditions).

Since 1982 a pot experiment with 3 different soil types and two extrem C-levels each has been carried out without nitrogen fertilization under field and greenhouse conditions. The N-uptake and changes of carbon in the soil were investigated. After 12 years the N-uptake amounts between 1030 and 2960 kg/ha. After 14 years in the average of all treatments the difference between field and greenhouse conditions was 0,36 % carbon. Under comparable experimental conditions changes depending on soil type and C-level amounts under field conditions between +0.01 and +0.02% C annualy, in greenhouse between + 0.004 and − 0.059 % C. The C/N ratio increases with a decreasing mineralisable part of carbon.

Different supplies of nutrients result morphological changes of the plant roots and in changes of activity. Often it is related with root death. Share of inactive roots in a total root system of tomatoes grown in hydroponics should be investigated for two cultivars (Lycopersicon lycopersicum cv. Counter, Karina) dependent on plant age in three variations of the nitrogen amount supplied (between 7 and 51 mmoll⁻¹).

First step of investigations was to produce evidence of the triphenyl tetrazolium chloride method to detect inactive roots of tomatoes grown in hydroponics. With this method the inactive part was determined in a fresh sample of about 0.5 g during a growing period of about 170 days at 4 in 1994 and 6 in 1995 sampling dates.

The best position to draw a representative sample was the middle of a root system and not the border. In the first 60 days of the growing period the inactive part of the roots stayed constant, lower than 10 %. After this time the part increased significantly until the end of the growing time and summed up to almost 70 %. The results were the same for both cultivars investigated. The influence of the increasing N supply was in 1994 not significant but it was in 1995. In general with increasing N in the nutrient solution the inactive part increased, too. The results showed that especially at the end of the growing period with high plant age growers have to pay attention to the roots to save the uptake of nutrients and water with a reduced active root system.

In order to get information about reactions of wetland plants on a changing climate we studied some growth parameters and the pattern of free amino acids in roots of Agrostis stolonifera under increasing drought stress conditions, simulated by root application of polyethylene glycol. Fresh weight of shoots and roots was reduced with increasing intensity of stress, while dry weight was enhanced in roots under mild stress but not in shoots. Proline was found to accumulate in roots in high amounts with increasing strength and duration of stress. The pattern of free amino acids was considerably changed. Especially, the portion of aspartate and asparagine increased in roots under stress. The results suggest, that aspartate and asparagine play an important role in stress metabolism of Agrostis stolonifera.

Growth parameters, proline accumulation and α-amino nitrogen content in germinating maize seedlings were investigated under osmotic stress conditions. Polyethylene glycol (PEG) caused a decrease in fresh weight and length of all seedlings organs. The dry weight of coleoptiles was also reduced under stress while it was enhanced in roots. An increase in total α-amino nitrogen and a rapid accumulation of proline was observed in intact seedlings but not in isolated roots. In decapitated seedlings proline content of roots increased under stress conditions. The pattern of free amino acids estimated by HPLC-analysis was changed by PEG. The importance of these stress reactions are discussed.

Lupin production in Germany is restricted by poor growth of this genus on neutral or alkaline soils. This intolerance towards higher pH values may be attributed to several soil chemical and physical factors. There is indication in the literature that OH⁻ activity in the root medium negatively affects root growth of lupins even at moderate pH values. The aim of the experiments reported here was to examine whether variation in root growth exists among lupin genotypes regarding OH⁻ activity in the root medium. The experiments were set up as nutrient solution cultures with seedlings of L. luteus L., L. angustifolius L., L. albus L., L. mutabilis L., Pisum sativum L. and Vicia faba L. The seedlings were grown for 48 h in unbuffered nutrient or test solutions. Their pH was adjusted to values between 5 and 8. Root growth within 48 h was determined and the rate of root growth calculated. There was no significant reduction in root elongation at pH 6. pH 7 caused a decrease of root elongation by less than 20%. Lupins and pea did not differ in this respect. No consistent species or cultivar differences were observed. The results indicate that OH⁻ activity itself has no strong inhibitory effect on root elongation and thus seems not to be the major cause for the poor performance of lupins on neutral or alkaline soils.

The aim of the presented experiments was to determine the ability of corn roots to take up nutrients after drought and during recovery after rewatering of the soil and to examine root characteristics which possibly limit nutrient uptake under these conditions. Two different experimental approaches were used:

In the split root experiment neither the shoot or root growth nor the ability to take up ¹⁵N-nitrate after a drought period of 4 days (6 weight% soil water content) was impaired.

In the second experimental system, the roots of control plants which were watered continuously, took up ¹⁵N-nitrate immediately after ¹⁵N application, and ¹⁵N in the shoot was already detected 24 h later. In severely stressed plants ¹⁵N-nitrate was also found in the roots after 24 h but in contrast to the split root experiments there was no translocation into the shoot within the first 4 days. Obviously after severe drought stress the ability of the roots to take up nutrients and particularly to translocate ¹⁵N into the shoot, was severly decreased. Under these conditions it seems that nutrient uptake in the roots and translocation into the shoot is limited by a low nutrient demand of the shoot but also due to impaired hydraulic conductivity of the roots and reduced root pressure.

After severe drought stress root growth recovered within one day after rewetting. This new root growth may have contributed to the recovery of hydraulic conductivity of the root system and thus to translocation of ¹⁵N into the shoot.

High affinity to the soil organic matter and extreme hydrophobicity (log K_o/w > 4) are the main physicochemical characteristics of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) limiting their mobility in soils, and the translocation within plants. As an exception, uptake of PCDD/PCDF via the roots and subsequent translocation to the shoots was demonstrated for zucchini (Cucurbita pepo L.). Root exudates of zucchini are able to mobilize PCDD/PCDF from contaminated soils. Probably, certain compounds in the root exudates attach to PCDD/PCDF, forming polar adducts, thus facilating root uptake as well as transport within the plant of the per se extremely hydrophobic PCDD/PCDF. A chemical characterization of these compounds was achieved by differential fractionation of root exudates, xylem sap and plant extracts, using ¹⁴C-tetrachlorodibenzo-p-dioxin (TCDD) as a probe to detect the PCDD/PCDF-binding fractions. Insolubility in methanol and in dilute trichloroacetic acid (TCA) as well as UV absorption at 280 nm suggest that PCDD/PCDF binding compounds in root exudates of zucchini are polypeptides. Corresponding electrophoretic polypeptide patterns in the TCDD-binding fractions of root exudates and xylem sap separated by gel permeation chromatography, suggest that roots of zucchini release TCDD-binding polypeptides into the rhizosphere which are subsequently taken up by the roots and translocated to the shoot via xylem transport.

Die Wachstumsbedingungen an Fichtenstandorten haben sich durch Säure- und Stickstoffeinträge aus der Atmosphäre stark verändert. Folgen davon sind Störungen im Mineralstoahaushalt der Bäume, Bodenversauerung und Elementausträge mit dem Sickerwasser. Die Ammonium- und Nitrataufnahme der Fichten sind in diesem Zusammenhang Schlüsselprozesse, die das Ausmaß der Störungen beeinflussen können. Zum einen ist die Stickstoffaufnahme der Fichten eine wichtige Senke für Stickstoff im Ökosystem. Andererseits sind die Ammonium- und Nitrataufnahme die wichtigsten Stickstoffquellen für Fichten, die das Wachstum steuern. Weiterhin hemmt Ammoniumaufnahme die Kationenaufnahme, während Nitrataufnahme diese fördert. Daraus ergeben sich wichtige Konsequenzen für den Kationenhaushalt der Bäume. Schließlich können die Aufnahme und Assimilation von Ammonium und Nitrat, die bei der Fichte vorwiegend in der Wurzel stattfinden, den pH-Wert des Wurzelraumes verändern: Ammoniumaufnahme fahrt zum Absinken, Nitrataufnahme zum Anstieg des pH.

A new approach for non destructive monitoring of soil solution chemistry in high spatial and temporal resolution for rhizosphere studies is presented. In a 5×10mm grid, 30 micro suction cups (ø1mm) were installed in a rhizotron with Norway spruce (Picea abies [L.] Karst.) growing in low pH B-horizon soil. Roots grew through the grid, closely passing the suction cups. Soil solution composition before, during and after root passage was determined. For K⁺ and Mg²⁺ a significant decrease of soil solution concentration near root tips and elongation zones was observed, indicating a marked uptake of these elements. Mg²⁺ concentration was also significantly lowered when the root system aged, suggesting that this ion might also be taken up in older parts of the root system. No influence of growing roots was found on Na⁺-concentrations.

In pot experiments with two chinese rice soils we studied the availability of nonexchangeable ammonium for rice plants after flooding and the influence of redox potential (E_h) on fixation and release of NH ₄ ⁺ in the vincinity of the roots.

The results show that the amount of ammonium fixed after submergence was completely released by rice plants in the rhizosphere during the experiments. Mobilization of fixed NH ₄ ⁺ decreased with growing distance from the roots.

The redox potentials of the soils were greatly raised by the influence of the rice roots. In one soil the oxidizing effect reached to more than 5 mm away from the roots.

Fixation and release of nonexchangeable NH ₄ ⁺ appeared to be influenced by the height of the E_h. A close correlation between the content of fixed NH ₄ ⁺ and E_h in the soils was observed when the redox potential of fallow pots was changed by aeration.