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

• 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– Meristematic zone– Elongation zone– Maturation zone– Mature zone

• Cross-sectional– Epidermis– Cortex– Endodermis– Stele

Root Morphology

{MeristematicZone

{ElongationZone

{MaturationZone

MatureZone

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.

• Root hairs are fragile and susceptible to breakage as soils dry.

Germinatingradish seedlingwith root hairs.

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:Water and nutrient uptake rates are lower in mature root zones than in immature:– Lower permeability– Formation of aerenchyma (air pockets)

Protects the root from desiccation

Cross-section of mature zone

Aerenchyma

EasternGamagrassroots

Driptubing

Lettuce

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:– 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

Ectomycorrhizae

Endomycorrhizae

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 toNH3. The NH3 is then used byThe 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 non-legume.

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