Transcript Nitrogen and Plants

Roots and Root-Soil
Relations
SWES 316
Section G
Functions of Roots
• Absorption
• Anchorage
• Storage
Root Systems
• Tap vs. fibrous root systems
Root Systems
• Roots may occupy about 1% of total soil
volume (with actively growing crops)
• In wildlands (e.g. forest, grassland), roots
may represent up to 50% of total plant
mass (roots + shoots)
• In agriculture, roots may represent about
25% of total plant mass
Depth of Root Systems
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Lettuce
30 cm
Cotton
120-150 cm
Alfalfa
250-300 cm
Turf
60-90 cm
Mesquite 50 m ?????
• Regardless of root system depth, the most
important roots for nutrient and water uptake
are usually found in the top 15-20 cm of soil.
Root Morphology
• Longitudinal
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Meristematic zone
Elongation zone
Maturation zone
Mature zone
• Cross-sectional
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Epidermis
Cortex
Endodermis
Stele
Root Morphology
Mature
Zone
Maturation
Zone
{
Elongation
Zone
Meristematic
Zone
{
{
Cross-section of maturation zone
Functions
• Root cap
– Protection of meristem
– Secretion of mucigel
– Initiation of symbiotic relationships
• Elongation Zone
– Elongation of cells forces root through the
soil
• Maturation zone
– Root Hairs
– Major zone of water and nutrient uptake
Root Hairs
• Single-cell extensions of epidermal cells
• Tremendously important for providing
surface area for water and nutrient
uptake
• Produced in the maturation zone
• Sites of infection by pathogens, N fixing
bacteria, and mycorrhizae
Root Hairs
• Root hairs can account for 2/3 of total root
surface area.
Germinating
radish seedling
with root hairs.
• Root hairs are fragile and susceptible to
breakage as soils dry.
Size of Roots
• Fine lateral roots are 0.1 to 0.2 mm in diameter.
• Root hairs are 0.01 to 0.05 mm in diameter.
• Soil “micropores” are considered to be those
<0.08 mm in diameter.
• Therefore, root hairs are important for accessing
water and nutrients in micropores.
Mature Roots
• The root epidermis and endodermis become
covered with a waxy substance known as
“suberin”.
• Function: Protects the root from desiccation
Water and nutrient uptake rates are lower in
mature root zones than in immature:
– Lower permeability
– Formation of aerenchyma (air pockets)
Cross-section of mature zone
Aerenchyma
Eastern
Gamagrass
roots
Lettuce
Drip
tubing
Implications of Root Morphology
• The youngest part of the root is more permeable
to water and nutrients than is the older part
(behind the maturation zone).
• Most nutrients are taken up in the younger root
zone (‘feeder roots’) with abundant root hairs.
• Excessive soil drying and mechanical
disturbance will damage young roots, reduce
nutrient uptake.
The Rhizosphere
• Definition: a zone of soil where microbial
activity is influenced by roots
• Why? Border cells, root leakage and
secretion of organic compounds
• May extend to about 2 mm from root surface.
The Rhizosphere
Properties:
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Higher available C for microbial growth
Higher microbial population and activity
Lower pH than “bulk” soil
Lower O2 than “bulk” soil
Altered nutrient availability for plants
Organic C in the Rhizosphere
(Rhizodeposition)
• Plants release simple and complex
carbohydrates, nucleic acids, enzymes into the
rhizosphere
• These compounds are used by microorganisms
as sources of C and energy.
• Up to 30% of plant C fixed in photosynthesis may
be released from roots.
Importance of the Rhizosphere
• A healthy rhizosphere will help plants by:
– Increasing nutrient availability
– Suppressing pathogens
– Increasing water availability
• However, the effects of agricultural
management on the rhizosphere are still
largely unknown.
Mycorrhizae
Endomycorrhizae
Ectomycorrhizae
Mycorrhizae
Many of the mushrooms you
see in the forest are
mycorrhizal fruiting bodies.
Mycorrhizae
• A fungal-root symbiosis
– increase root length and surface area
– help plants to take up nutrients that are “immobile”
in soil (e.g. P)
– inhibit heavy metal uptake
– iIncrease water uptake by roots
– are vital for the growth and survival of plants where
nutrients are likely to be limiting, and/or where
heavy metals are problems
Managing Mycorrhizae
• About 80% of all plants have mycorrhizael
associations.
• Innoculation (addition) with mycorrhizae is often
used for nursery and forestry plantings.
• Addition of mycorrhizae to agricultural soils has
shown few benefits so far:
– Difficulty of establishing Endo fungi
Nitrogen Fixation
• Definition: The conversion of atmospheric N2 to
forms usable by plants.
• Certain bacteria and actinomycetes can carry
out N fixation.
• The most important N fixation occurs through a
symbiotic relationship between legume roots
and bacteria.
Legume Root Nodules
Within nodules, bacteria of the
genus Rhizobium reduce N2 to
NH3. The NH3 is then used by
The plant as an N source.
Nitrogen Fixation
• Legumes commonly form N-fixation
symbioses
• N fixation is not free
– may cost the plant up to 30% of the C it
captures from the atmosphere in
photosynthesis
• Rate of N fixation is inversely proportional
to available N in the soil.
Importance of N Fixation
• Legume crops can derive virtually all their N
requirement through N fixation.
• In crop rotations, legume crop residues can be
incorporated into the soil. Upon decomposition,
N is released.
• Legumes and non-legumes can be grown
together—in this case 20-50% of the N fixed in
the legume may become available to the nonlegume.
Final Thoughts on Roots
• Root system is responsible for water and
nutrient uptake.
• The youngest part of the root system is
responsible for much of this uptake
– Root hairs are critical, fragile and can be easily
damaged by compaction, tillage, etc.
• The root system is dynamic and will respond to
changes in soil conditions.
Final Thoughts (2)
• The root is part of a microbial community known
as the rhizosphere
• A healthy rhizosphere is important for plant
health
– There’s a lot about it we don’t know yet
• Mycorrhizae - water, nutrients, pathogen
protection
• Rhizobia - N fixation