Rhizosphere 2 Flashcards
Rhizodeposition
is the release of organic carbon from the root system to the rhizosphere
- annual species-40% of C allocated to roots
- Forest trees-70% or more of C
- increases when plants are under stress
- increases microorganism activity as more C substrate is available
- low molecular weight organic exudates include organic acids.
root exudates
- high molecular weight solutes
- low molecular weight solutes
high molecular weight solutes
- mucilage
- ectoenzymes
low molecular weight solutes
- organic acids
- sugars
- phenolic compounds
- amino acids
Major components of plant root exudates
Sugars: Glucose, Fructose, Maltose,
Amino Acids: Valine, Leucine, serine
Organic acids: Oxalate, Malate, acetate
Others: flavones, adenine, Guanine
Proteins/Enzymes: Invertase, Amylase, Protease
Quantities of Exudates
- Increase when plant roots are experiencing high bulk densities (cereal plants: 5% of root dry weight were exudates when plants grown in nutrient solution, 9% when grown in glass bead substrate)
- Increases when plant experiences mineral nutrient deficiencies
Release of Organic material (rhizodeposition): Amounts and composition are affected by:
- Plant species and age
- Soil type and properties
- Nutritional status of the plant
- temperature
- light intensity and duration
- Presence of microorganisms
Main sites of root exudation
-spike at apex of root system and at point where lateral roots come off of a root system we have increases in root exudates
Mucilage def:
high-molecular weight gelatinous material that covers roots in the apical zones
Mucilage
- consists mainly of polysaccharides
- positively correlated with root growth rate
- secreted by the root cap cells and released by epidermis cells
- colonized by microorganisms
Mucigel
mixture of gelatinous material, microorganisms, and soil particles
Mucilage functions
- protection of root apical zones from desiccation
- lubrication of the root as it moves through soil
- influence ion uptake
- improving the soil-root contact
- aggregation of soil particles
Low-molecular weight root exudates: Main compounds
- sugars
- organic acids
- phenolic compounds
Low-molecular weight root exudates
principle reactions involve mineral nutrient mobilization in the rhizosphere
-insoluble nutrient containing compounds in soil are solubilized
Plant exudates and plant nutritional status
Plants are nutrient deficient:
- quantity of root exudates increases
- composition of exudates changes
ex. K deficiency of maize
- amount of exudates increases
- organic acids increase to solubilize and exchange limited soil K.
Ectoenzymes
root borne enzymes such as acid phosphatases (increase in relation to deficiencies, i.e. P deficiency)
- important to extraction and P uptake on low P soils
- Many enzymes located in the root apoplasm and influence mineral nutrition of plants in low nutrient status soil situations
Mycorrhizas
- most widespread association between microorganisms and plants-80-90%
- fungus is strongly or wholly dependent on the higher plant (Mutualistic, neutral, or parasitic)
Non-mycorrhizal plants
- very dry or saline soils
- waterlogged (submerged soils)
- severely disturbed soils
- severely disturbed soils
- soil fertility is extremely high or low
Symbiosis Plant: Plant interactions
- Nitrogen Fixing (certain species)
- Mycorrhizas (nearly all plants)
- Endophyte (antifeedant in grasses) (Acremonium on fescue, and ryegrass)
Roots: Ectomycorrhizas
- swollen, forked fine roots,
- fungal mantle
- harting net
- no intracellular hyphae
Roots: Endomycorrhizas
- no obvious external root modification
- no mantle or harting net
- Extensive intracellular hyphae
- Presence of arbusucles, vesicles, and chlamydospores
Mycorrhizas and mineral nutrition: Vesicular-Arbuscular Mycorrhizas (VAM)
- improve supply of low mobility mineral nutrients, mostly phosphorus
- External hyphae absorb P outside the root depletion zone of nonmycorrhizal roots
- uptake rate of P per unit root length in 2-3 times greater than nonmycorrhizal plants
Ectomycorrhizas
-structurally different from VAM
-Fungal Sheath surrounding the apical root zone (varies with plant species, season, and environmental conditions)
-important for P nutrition at low soil P levels
Several specific differences and plant responses are discussed in text
Mycorrhizas: Practical Implications
- Enhanced P availability in low P soils
- Feasible to inoculate plants, i.e., forest tree seedlings, to improve early growth (Possible with ECM fungi, limited VAM inoculation success)
- high fertility level agronomic and horticultural crop production, not of great importance
- Natural ecosystem studies, always consider
Strigolactone is produced from ____
carotenoids
these hormones and signals are produced in the roots in response to low phosphate conditions or high auxin flow from the shoot
Strigolactones is produced….
at the base of the root and sent throughout the roots to attract fungi and prevent lateral growth
Strigolactones roles
- involved in promoting germination in root-parasitic plants
- plays a role in attracting mycorrhizal fungi to the root
- suppress lateral shoot branching and promotes apical dominance
Roots: Symbiont Inoculation
- spores
- mycelium
- inoculated roots
Nitrogen Fixation
- Rhizobium + legumes
- Frankia + various families
- Russian Olive
- Alder
- Mt. mahogany
Nitrogen Fixation
- Globally, 140 billion kg N fixed/yr by symbiotic organisms
- atmosphere=78% N2
- Convert N2 gas into amino acids
- Legumes + Bacteria (rhizobium)
- Alder + Actinomycete (Frankia)
- Cycads + Blue green algae (Nostoc)
