Experiment 15: Identification and Study of Symbiotic Associations (Root Nodules of Leguminous Plants, Lichens) and Parasitism (Cuscuta on Host)

AIM

To observe model specimen showing symbiotic association in root nodules of leguminous plants, Cuscuta on host, lichens.

PRINCIPLE

Symbiosis is close association between two different organisms where at least one is benefited. It includes mutualism where both are benefited, parasitism where one is benefited and other is harmed, and lichen association.

Rhizobium in root nodules of leguminous plants shows mutualism. Bacteria fix atmospheric nitrogen into soluble nitrates for plant growth while plant provides nutrients and shelter to bacteria for continued growth.

Cuscuta shows total stem ectoparasitism with haustoria penetrating host vascular tissue. Lichen shows mutualism between alga (phycobiont) and fungus (mycobiont) where alga prepares food and fungus provides shelter and absorbs minerals.

REQUIREMENTS

• Model / specimen of symbiotic association in root nodules of leguminous plants
• Model / specimen of Cuscuta on a host
• Model / specimen of lichen
• Hand lens
• Dissecting microscope
• Practical record book, pencil and eraser

PROCEDURE

1. Observe the model / specimen of root nodules of leguminous plant showing Rhizobium association.
2. Observe the specimen of Cuscuta growing on host plant and identify stem nature, leaves, chlorophyll and haustoria.
3. Observe the model of lichen showing upper cortex, algal layer, medulla and substrate attachment.
4. Draw neat labelled diagrams of each association as seen and note down identification characteristics.
5. Compare the observations with standard diagrams and interpret the type of symbiotic relationship.

OBSERVATION

a) Rhizobium in root nodules of leguminous plant

• Rhizobium bacteria are present in root nodules of legume plants and form a symbiotic relationship, where both are benefited from each other.
• Rhizobium convert the atmospheric nitrogen into soluble nitrates which can be used by plant for growth and development.
• Bacteria receives nutrients to continue their growth.
• Root nodules are small, round, pinkish due to leghaemoglobin and present on roots of pea, gram and other legumes.

b) Cuscuta with host

• Cuscuta commonly called dodder or amarbel and lives as stem ecto parasite on other plants.
• Cuscuta has no fully expanded form of leaves and has no chlorophyll.
• The stem of Cuscuta is thin and slender shaped.
• Stem of Cuscuta fixes itself to the stem of host plant with special structures called haustoria.
• Haustoria absorbs the water and other solutes and penetrates host tissue, establishing connection with host xylem and phloem.
• Cuscuta can weaken / kill host plant by depriving nutrients.
• Diagram shows stem parasite on upper part and root parasite indication on lower part of host plant, with penetration of host cells by parasite through haustoria.

c) Lichen

• It represents the symbiotic relationship / association between fungus and alga.
• The algal component is known as phycobiont and fungal component is known as mycobiont.
• Algae prepare food for fungi, fungi provide shelter and absorb minerals, nutrients.
• They grow on lands, rocks, tree trunks.
• They are pollution indicators and sensitive to sulphur dioxide.
• Transverse section shows upper cortex, algal layer, medulla and substrate; algal cells surrounded by fungal hyphae.

RESULT

Root nodules of leguminous plants show mutualistic association with Rhizobium fixing atmospheric nitrogen. Cuscuta shows total parasitic association with haustoria absorbing host solutes. Lichen shows mutualism between phycobiont and mycobiont growing on rocks and tree trunks.

CONCLUSION

Rhizobium legume association is mutualism beneficial for nitrogen nutrition. Cuscuta is a stem ectoparasite lacking chlorophyll and harming host through haustoria. Lichen is composite mutualistic organism and pollution indicator where alga provides food and fungus provides shelter and mineral absorption.

PRECAUTIONS

• Observe the specimens carefully with hand lens and dissecting microscope.
• Do not damage the preserved models and specimens of root nodules, Cuscuta and lichen.
• Draw neat and correctly labelled diagrams showing haustoria, nodules, algal layer, medulla and cortex.
• Handle host plant with Cuscuta gently to see attachment clearly.