TY  - THES
T1  - Rhizodeposition and biotic interactions in the rhizosphere of Phaseolus vulgaris L. and Hordeum vulgare L.
A1  - Haase,Susan
Y1  - 2008/04/07
N2  - Biochemical processes at the soil-plant interface are largely regulated by organic and inorganic compounds released by roots and microorganisms. Several abiotic and biotic factors are suspected to stimulate rhizodeposition and, thus, contribute to enriching of the rhizosphere with plant-derived compounds. This thesis focused on the effects of two factors, (i) the elevation of atmospheric CO2 concentration accompanied by nutrient limitation in the soil and (ii) low-level root infestation by plant-parasitic nematodes, on the quantity and quality of rhizodeposits with consequences for plant-nutrient acquisition and plant-microbial interactions in the rhizosphere. Experiments were largely conducted in mini-rhizotrones, which allowed a localized collection of rhizodeposits and rhizosphere soil along single roots.
Since the beginning of the industrial revolution atmospheric CO2 concentrations have been steadily increasing. This probably impacts terrestrial ecosystems by stimulating plant photosynthesis and belowground allocation of the additional fixed C. Increased root exudation, promoting rhizosphere microbes, has been hypothesized as a possible explanation for the lower plant N nutritional status under elevated CO2, due to enhanced plant-microbial N competition. Legumes may counterbalance the enhanced N requirement by increased symbiotic N2 fixation. The effects of elevated CO2 on factors determining this symbiotic interaction were assessed in Phaseolus vulgaris L. grown under limited or sufficient N supply and ambient or elevated CO2 concentration. Elevated CO2 reduced N tissue concentrations but did not affect plant biomass production. 14CO2 pulse-labelling revealed no indication for a general increase in root exudation by the whole root system, which might have forced N-competition in the rhizosphere under elevated CO2. However, a CO2-induced stimulation in the exudation of sugars and malate, a chemoattractant for rhizobia, was detected in apical root zones, as potential infection sites. In nodules, elevated CO2 increased the accumulation of malate as a major C source for the microsymbiont and of malonate, with functions in nodule development. Nodule biomass was also enhanced. Moreover, the release of nod-gene-inducing flavonoids was stimulated under elevated CO2, suggesting a selective stimulation of factors involved in establishing the Rhizobium symbiosis.
Since elevated-CO2-mediated effects on exudation by Phaseolus vulgaris L. are restricted to root apices, the abundance and function of the soil microbial community were investigated at two levels of spatial resolution to assess the response of microorganisms in the rhizosphere of the whole root system and in apical root zones to elevated CO2 and different N supply. At the coarser resolution, the microbial community did not respond to CO2 elevation because the C flux from the whole root system into soil did not change. At the higher spatial resolution, the CO2-mediated enhanced root exudation from root apices led to higher enzyme activities of the C and N cycle in the adhering soil at an early stage of plant growth. At later stages, however, enzyme activities decreased under elevated CO2. This might reflect a shift in microbial C usage from the decay of polymers towards soluble carbohydrates derived from increased root exudation. CO2 elevation or N supply did not affect the abundance of total and denitrifying bacteria in rhizosphere soil of apical root zones. Thus, the microbial community in the rhizosphere of bean plants responded to elevated CO2 by altered enzyme regulation and not by enhanced growth. 
Beyond N, plants and microorganisms may also compete for micronutrients such as Fe in the rhizosphere. Hordeum vulgare L., a model plant with high secretion of phytosiderophores (PS) under Fe limitation, was investigated to assess the effects of elevated CO2 on PS release, Fe acquisition and potential impacts on rhizosphere microbial communities. Experiments were conducted in hydroponics and soil culture with or without Fe-fertilization and ambient or elevated CO2 concentration. Elevated CO2 stimulated biomass production of Fe-sufficient and Fe-deficient plants in both culture systems. Secretion of PS in apical root zones of N deficient plants increased strongly under elevated CO2 in hydroponics, but no PS were detectable in root exudates from soil-grown plants. However, higher Fe shoot-contents of plants grown in soil culture without Fe supply suggest an increased efficiency for Fe acquisition under elevated CO2. Despite the evidence for altered PS secretion under elevated CO2, no significant influence on rhizosphere-bacterial communities was detected.
Low-level herbivory by parasitic nematodes is thought to induce leakage of plant metabolites from damaged roots, which can foster microorganisms. Other factors such as alterations in root exudation or morphology in undamaged roots, caused by nematode-host interactions were almost not considered yet. Hordeum vulgare L. was inoculated with 0, 2000, 4000 or 8000 root-knot nematodes (Meloidogyne incognita) for 4 weeks. In treatments with 4000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. One week after inoculation, greater release of sugars, carboxylates and amino acids from apical root zones indicates leakage from this main nematode penetration site. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. This probably contributed to the increased sugar exudation in uninfected roots in all nematode treatments at three weeks after inoculation. Root-knots formed a separate microhabitat within the root system. They were characterised by decreased rhizodeposition and an increased fungal to bacterial ratio in the surrounding soil. This study provides evidence that, beside leakage, low-level root herbivory induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance and competition.
In conclusion, this thesis extends our knowledge about the potential impact of two different plant-growth-affecting factors on rhizosphere processes, particularly at the small scale and is, thus, interesting for future assessment of management strategies in agriculture under global climate change.

KW  - Rhizosphäre
KW  - Gerste
KW  - Bohne
KW  - Herbivorie
KW  - Nematoden 
CY  - Hohenheim
PB  - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
AD  - Garbenstr. 15, 70593 Stuttgart
UR  - http://opus.uni-hohenheim.de/volltexte/2008/248
ER  -