37.3 Plant nutrition often involves relationships with other organisms.

37.3

Plant nutrition often involves relationships

with other organisms

Rhizobacteria• Rhizosphere: the soil layer that is bound to the

plant’s roots.• Rhizobacteria: Soil bacteria with especially large

populations in the rhizosphere– Roots of a plant’s rhizosphere secrete

nutrients; sugars, amino acids, organic acids• Plant-growth-promoting-rhizobacteria

• Have chemicals that induce plant growth• Produce antibiotics that protect against

disease• Absorb more nutrients

Nitrogen-Fixing Bacteria

• Plants cannot use free gaseous nitrogen in Earth’s atmosphere (N2)

• Nitrogen Fixation- process of reducing N2 to NH3 so a plant can use it use

• N2 + 8e- + 8H+ + 16ATP2NH3 + H2 + 16ADP + 16Pi

– N2 reduced to NH3 by adding electrons and H+

– Reaction catalyzed by nitrogenase enzyme• Ex. Bacteria Rhizobium and legume plant roots

– Rhizobium fixates N2 so it can be processed by the legume root

Nitrogen Fixation and Agriculture

• Crop rotation- crops rotate each year between a non-legume (ex. maize) and a legume (ex. Alfalfa).

– The legume is planted to restore the fixed nitrogen concentration of the soil

– Legume seeds get soaked in bacteria culture or dusted with bacterial spores to be sure that they connect to their correct Rhizobium strain.


• Mutualistic nitrogen fixation– Ex. Rice – Azolla

• The plant, Azolla is fixes nitrogen. The growing rice kills the Azolla. As Azolla decomposes, the fertility of the crop land is increased for the rice.

Fungi and Plant Nutrition• Mycorrhizae: A mutualistic relationship

between plants and fungi– Plants offer constant sugar supply for

fungus– Fungus increases surface area for the

plant to bring in water and nutrients, stimulates root growth, and secretes antibiotics to protect the plant

Types of Mycorrhizae• Ectomycorrhizae

– The fungus surrounds the root and its hyphae extend into the soil to absorb nutrients

– Fungus hyphae extends through the root cortex for an increased surface area to exchange nutrients• Hyphae: thread-like extentions of fungi that

form the mycelium

Types of Mycorrhizae

• Arbuscular Mycorrhizae– Fungus does not fully

surround plant root, but it hyphae still extend into the root cortex to form arbuscules to provide maximum surface area

Agricultural and Ecological Importance of Mycorrhizae

• Mycorrihzal symbiosis only occurs if the root is exposed to the right fungus speicies.

• Fungus is present in the soil of most ecosystems which allows seedlings to develop mycorrhizae.

• When seeds are replanted in foreign soil, the plants often become deficient in many nutrients due to the lack of a mycorrhizal partner.