Transcript 投影片 1

Rhizosphere Microbiology
What is Rhizosphere?
The region of the soil in contact with the roots
of a plant. It contains many microorganisms
and its composition is affected by root
activities. In another word, it is an
environment under the influence of plant.
The Rhizosphere is the zone surrounding the
roots of plants in which complex relations exist
among the plant, the soil microorganisms and
the soil itself.
The plant roots and the biofilm
associated with them can profoundly,
influence the chemistry of the soil
including pH and the transformation
of mineral compounds.
Rhizosphere can be divided into：
☆
Inner rhizosphere: the very root
surface
☆
Outer rhizosphere: embracing the
immediately adjacent soil
Rhizosphere effect
A stimulation that can be put on a quantitative
basis by the use of the R：S ratio
R：S ratio = number of microorganisms in the
rhizosphere soil：number of microorganisms in
the non-rhizosphere soil.
R/S > 1, good stimulation
R/S = 1, no stimulation
R/S < 1, inhibition
Importance of Rhizosphere:
1. Plant health
2. Crop yield
3. Nutrient cycling
Methodology:
Microscopy
Plate count
Molecular Biology technique
Plant affects the rhizosphere microorganisms
through the release of root exudates
What is root exudate?
Small molecules that are released from plant
roots, which include sugars, amino acids,
organic acids and amides. These molecules
influence soil nutrient availability both directly
and indirectly by stimulating the activities of
certain microbial and fungal components of the
soil biota.
The exudates contain carbohydrates,
organic acids, vitamins and many other
substances essential for life. From 5
percent to 40 percent of the total dry
matter production of organic carbon from
photosynthesis may be released as
exudates!
When plants begin to decline, the amount
of organic carbon released as exudates
increases. Mineral deficiencies, low
amounts of soil air and severe wounding
are major causes for the increase.
Another way to say this is that an increase
in exudates would be caused by overpruning, construction injury, planting too
deeply, over-watering, compaction and
planting plants in soils that have a pH too
high or too low for their optimal growth.
Factors affecting the amount and the
composition of root exudates
1. Plant species and developmental stage
2. Variety
3. Temperature
4. Light
5. Plant injury
6. Atmosphere
7. Foliar sprays
8. Plant nutrient
9. Moisture
10. Microbial activity
Translocation of microorganisms in the
rhizosphere
When seeds are used as the carrier for introducing
various bio-pesticide and bio-fertilizer. It is
important to understand the movement of these
agents in the rhizosphere.