Soil pH influences availability of soil nutrients.
Fine roots and root hairs “mine” the soil for
nutrients. Mycorrhizal hyphae do
this even better.
4. Roots and mineral nutrient acquisition
Roots
Provide large surface area for nutrient uptake
- Root hairs
Why are fine structures like hyphae and root hairsparticularly effective at nutrient absorption?
For a given volume (or mass) of roots, what size rootpresents the most surface area?
Surface area of a cylinderSA = circumference x length
SA = x diam x lengthSA = x 2r x length
Volume = area x lengthVol = x r2 x length
SA/Vol = ( x 2r x length)/( x r2 x length)
SA/Vol = 2/r
As the radius decreases, the surface area per volume increases.
Minirhizotron photos of yellow birch roots in the Hubbard Brook Experimental Forest (New Hampshire) in April (left) and June (right). 0.3 mm in diameter.
Fig. 5.7
Depletion zones - regions of lowernutrient concentration -developaround roots
A depletion zone of low concentration forms near the rootwhen the rate of nutrient uptake exceeds the rate of diffusion
Mycorrhizae“fungus -
root”Symbiosis with fungi
Nutrient Uptake and Mycorrhiza
Roots and Mycorrhiza – an old symbiosis
Mutual benefit• Carbohydrates for the fungus • P, Zn, Cu, water, N for plant
Different types
1. Vesicular-arbuscular mycorrhiza – VA-mycorrhiza2. EctomycorrhizaOther types
ericoidorchid endomycorrhiza
1. Vesicular arbuscular mycorrhiza (AM)
• Glomales (130 species – infects 300.000 plant species)
• Found on roots of herbaceous angiosperms, most trees, mosses, ferns…
• not present on Cruciferae, Chenopodiaceae, Proteaceae
• small biomass compared to roots
Vesicular Arbuscular Mycorrhiza
Inside root• Intercellular mycelium• Intracellular arbuscule
• tree-like haustorium• Vesicle with reserves
Outside root• Spores (multinucleate)• Hyphae
•thick runners•filamentous hyphae
Form extensive network of hyphaeeven connecting different plants
AM
Arbuscule of Glomus mosseae – branching provides large surface area
Outside of root network of hyphae and spores
2. Ectomycorrhiza (EM)
• Ascomycetes and Basiodiomycetes – form large fruiting bodies
• 5000 species interact with 2000 plant species
• Interaction with trees: angiosperms and all Pinaceae
Ectomycorrhiza
Inside root• Intercellular hyphae
• Does not enter cells
Outside root• Thick layer of hyphae around root
• Fungal sheath• Lateral roots become stunted
• Hyphae•Mass about equal to root mass
Forms extensive network of hyphaeeven connecting different plants
Ectomycorrhizal root tip
Mantle Hyphae
Hartig Net
Why mycorrhiza?
• Roots and root hairs cannot enter the smallest pores
Why mycorrhiza?
Root hair Smallest hyphae
• Roots and root hairs cannot enter the smallest pores
• Hyphae is 1/10th diameter of root hair
• Increased surface area
•Surface area/volume of a cylinder: SA/vol ≈ 2/radius
Inoculated with mycorrhizae
Not inoculated with
mycorrhizae
Why mycorrhiza?
• Roots and root hairs cannot enter the smallest pores
• Hyphae is 1/10th of root hair
• Increased surface area
• Extension beyond depletion zone
Why mycorrhiza?
• Roots and root hairs cannot enter the smallest pores
• Hyphae is 1/10th of root hair
• Increased surface area
• Extension beyond depletion zone
• Breakdown of organic matter
C – C – NH2 --> C – C + NH3
Summary on mycorrhizae
• Symbiosis with mycorrhiza allows greater soil exploration, and increases uptake of nutrients (P, Zn, Cu, N, water)
• Great SA per mass for hyphae vs. roots
• Mycorrhiza gets carbon from plant
• Two main groups of mycorrhiza – Ectomycorrhiza and VA-mycorrhiza
For usmore on nitrogen nutrition
•Why is N so important for plant growth?
•What percentage of the mass of plant tissues is N?
•What kinds of compounds is N found in?
•Why is there a strong relationship between the Nconcentration of leaves and photosynthesis?
Nitrogen - the most limiting soil nutrient
Evidence - factorial fertilization experiments (N, P, K, etc.)
show largest growth response to N.
1. Required in greatest amount of all soil nutrients
2. A component of proteins (enzymes, structural proteins,
chlorophyll, nucleic acids)
3. The primary photosynthetic enzyme, Rubisco, accounts
for a 25 to 50% of leaf N.Photosynthetic capacity is strongly correlated with leaf N concentration.
4. Availability in most soils is low
5. Plants spend a lot of energy on N acquisition - growing
roots, supporting symbionts, uptake into roots, biochemical assimilation into amino acids, etc.
The inorganic forms of nitrogen in soils.
1.NH4+, ammonium ion. A cation that is bound to clays.
2.NO3-, nitrate ion. An anion that is not bound to clays.
Nutrient “mobility” in soils refers to the rate of diffusion,which is influenced by nutrient ion interactions with soil particles.
Would you expect NH4+ or NO3- to diffuse more rapidly?
Would you expect a more pronounced depletion zone for NH4+ or NO3
-?
SoilOrganic N
NH4+
Plant N
NO3-
NH4+ uptake
Mineralization
NH4+
immobilization
Nitrification
NO3-uptake
NO3-
immobilization
NO, N2O
N2
Leaching
Denitrification
N Fixation
Atmospheric N2
The Nitrogen Cycle
SoilOrganic N
NH4+
Plant N
NO3-
NH4+ uptake
Mineralization
NH4+
immobilization
Nitrification
NO3-uptake
NO3-
immobilization
NO, N2O
N2
Leaching
Denitrification
N Fixation
Atmospheric N21. Emissions to
atmosphere
Dead organismsand tissues
Pathways of N loss from ecosystems
2.
