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
