Abstract
Abstract The paper summarizes the results of a series of experiments on enumeration of N 2 ‐fixing bacteria (diazotrophs) and hormonal effects of Azospirillum on root development. Numbers of N 2 ‐fixing and N‐heterotrophic bacteria were determined on the root (rhizoplane plus “inner” root surface) and in the rhizosphere soil (0–3 mm from the root surface) of Arrhenatherum elatius , other forage grasses and some herbaceous plant species. Pot experiments involved freshly collected soil from an unfertilized grassland area containing its natural population of N 2 ‐fixing bacteria. The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil, suggesting that diazotrophs were not selectively advantaged at the root surface. Supply of mineral nitrogen (NH 4 NO 3 ) decreased the proportion of N 2 ‐fixing bacteria at the rhizoplane as well as in the rhizosphere soil. Similar results were obtained when N was supplied via the leaves. The data suggest that N 2 ‐fixing bacteria in the rhizosphere are poor competitors once they loose their competitive advantage of binding dinitrogen. Correspondingly, the increase in the MPN of the diazotrophs found during plant development was interpreted as a result of decreased available combined N in the rhizosphere. The proportion of N 2 ‐fixing bacteria relative to the total number of bacteria was generally below 1%. Considering the potential amount of substrate released from the roots and the substrate requirement of the bacterial population, N 2 ‐fixation was considered insignificant for plant growth under the given conditions. For the investigations on possible beneficial effects on plant development by bacterial hormones, Azospirillum brasilense was chosen because evidence suggests that amongst the soil bacteria releasing hormones, especially IAA, certain strains of this species are more important than other bacteria. Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs. Similar results were obtained after application of IAA. This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense . The increase in root surface may improve acquisition of nutrients and enhance growth of plants. Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar. Again pure IAA resulted in a similar increase in nodule number. Increases in nodule number were only in part associated with a change in root morphology. Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely.
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Claims 12
Claim · model-written summary
The most probable number of diazotrophs relative to the total bacterial population was always lower on the root than in the rhizosphere soil of Arrhenatherum elatius and other plants.
- Taxon
- Arrhenatherum elatius
Arrhenatherum elatiuse0 - Interactor
- diazotrophse1other
- Outcome
- MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial populatione1
- Direction
- lowere1
- Setting
- on the root than in the rhizosphere soile1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Arrhenatherum elatius”
e1“The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil”
Claim · model-written summary
Supply of mineral nitrogen (NH4NO3) decreased the proportion of N2-fixing bacteria at the rhizoplane and in the rhizosphere soil.
- Treatment
- Supply of mineral nitrogen (NH 4 NO 3 )e0
- Interactor
- N 2 ‐fixing bacteriae0other
- Outcome
- proportion of N 2 ‐fixing bacteriae0
- Direction
- decreasede0
Not reportedTaxonComparator
Verbatim
e0“Supply of mineral nitrogen (NH 4 NO 3 ) decreased the proportion of N 2 ‐fixing bacteria at the rhizoplane as well as in the rhizosphere soil”
Claim · model-written summary
The proportion of N2-fixing bacteria relative to the total number of bacteria was generally below 1%.
- Interactor
- N 2 ‐fixing bacteriae0other
- Outcome
- proportion of N 2 ‐fixing bacteria relative to the total number of bacteriae0
- Value
- below 1%e0
Not reportedTaxon
Verbatim
e0“The proportion of N 2 ‐fixing bacteria relative to the total number of bacteria was generally below 1%”
Claim · model-written summary
The increase in the MPN of the diazotrophs during plant development was interpreted as a result of decreased available combined N in the rhizosphere.
- Interactor
- diazotrophse0other
- Outcome
- increase in the MPN of the diazotrophse0
- Direction
- increasee0
- Qualifier
- found during plant developmente0
Not reportedTaxon
Verbatim
e0“the increase in the MPN of the diazotrophs found during plant development”
Claim · model-written summary
N2-fixation was considered insignificant for plant growth under the given conditions.
- Outcome
- N 2 ‐fixation was considered insignificant for plant growthe0
- Qualifier
- under the given conditionse0
Not reportedTaxon
Verbatim
e0“N 2 ‐fixation was considered insignificant for plant growth under the given conditions”
Claim · model-written summary
Application of A. brasilense Cd onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs.
- Taxon
- wheat
Triticum aestivume0 - Organ / stage
- young wheat plantse0
- Treatment
- Application of A. brasilense Cd (ATCC 29710) onto the rootse0
- Interactor
- A. brasilense Cd
Azospirillum brasilensee0symbiont - Outcome
- the number of lateral roots, the total root length and the number of root hairse0
- Direction
- increasede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs”
Claim · model-written summary
Application of IAA gave similar results to A. brasilense on root development.
Not reportedTaxonComparator
Verbatim
e0“Similar results were obtained after application of IAA”
Claim · model-written summary
The authors suggest that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense.
Not reportedTaxon
Verbatim
e0“This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense”
Claim · model-written summary
Application of A. brasilense increased nodulation of Medicago sativa grown on agar.
- Taxon
- Medicago sativa
Medicago sativae0 - Treatment
- A. brasilensee0
- Interactor
- A. brasilenseAzospirillum brasilensee0symbiont
- Outcome
- nodulatione0
- Direction
- increasee0
- Setting
- agare0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar”
Claim · model-written summary
Pure IAA resulted in a similar increase in nodule number.
Not reportedTaxonComparator
Verbatim
e0“Again pure IAA resulted in a similar increase in nodule number”
Claim · model-written summary
Increases in nodule number were only in part associated with a change in root morphology.
Not reportedTaxonDirection
Verbatim
e0“Increases in nodule number were only in part associated with a change in root morphology”
Claim · model-written summary
An effect of IAA on the plant regulation system for nodulation is likely.
Not reportedTaxon
Verbatim
e0“Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely”
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See all 110 papers →Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.