Chapter 29Soil Bacteria and Mycorrhizal Fungi

Concept 29.3: Plants roots absorb essential elements from the soil

• Water, air, and soil minerals contribute to plant growth.– 80–90% of a plant’s fresh mass is water.

– 96% of plant’s dry mass consists of carbohydrates from the CO2 assimilated during photosynthesis.

– 4% of a plant’s dry mass is inorganic substances from soil.

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Concept 29.4: Plant nutrition often involves relationships with other organisms

• Plants and soil microbes have a mutualistic relationship.

• Microbial activity within a plant’s rhizosphere is 10 to 100 times higher than in nearby soil.

• Up to 20% of the plants photosynthetic output is secreted into the soil.

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• Rhizobacteria known as plant-growth-promoting rhizobacteria can play several roles.

– Produce hormones that stimulate plant growth

– Produce antibiotics that protect roots from disease

– Absorb toxic metals or make nutrients more available to roots

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Bacteria in the Nitrogen Cycle

• Nitrogen can be an important limiting nutrient for plant growth.

• The nitrogen cycle transforms atmospheric nitrogen and nitrogen-containing compounds.

• Plants can only absorb nitrogen as either nitrate (NO3–)

or ammonium (NH4)

• Most usable soil nitrogen comes from actions of soil bacteria.

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ATMOSPHERE

SOIL

Nitrogen-fixingbacteria

N2 N2

NH3

(ammonia)

H+

(from soil)

Ammonifyingbacteria

Amino acids

NH4+

(ammonium) NO2

−

(nitrite)Nitrifying bacteria

NO3−

(nitrate)Nitrifying bacteria

Denitrifyingbacteria

Microbialdecomposition

Nitrate andnitrogenous

organiccompoundsexported in

xylem to shoot system

NH4+

Root

Proteins from humus(dead organic material)

Plants can only absorb nitrogen as either nitrate (NO3–) or

ammonium (NH4)

Roots

Nodules

Symbiotic relationships with nitrogen-fixing Rhizobium bacteria provide some legumes with a source of fixed nitrogen.

Fungi and Plant Nutrition

• Mycorrhizae are mutualistic associations of fungi and roots.

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Epidermalcell

(LM) 50 m

Fungalhyphaebetweencorticalcells Mantle

(fungal sheath)

1.5 mm

(Co

lori

zed

SE

M)

Figure 29.14

Experiment Results

Invaded Uninvaded Sterilizedinvaded

Sterilizeduninvaded

Soil type

Invaded UninvadedSoil type

White ash

Sugar maple

Seedlings

Red maple

Myc

orr

hiz

alco

lon

izat

ion

(%

)

Incr

ease

inp

lan

t b

iom

ass

(%) 300

200

100

0

30

20

10

0

40

garlic mustard slows growth of other plants by preventing the growth of mycorrhizal fungi.