Blue Green Algal Association with Higher Plants

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Nitrogen Fixation by Symbiotic Blue-Green Algae Lichens Stages in Lichen RE Establishment Nitrogen Fixation and Assimilation

Blue Green Algal Association with Higher Plants

	Blue-green Algal Association with Higher Plants The only angiosperm to develop a symbiotic association and fix nitrogen with a nitrogen-fixing blue-green alga is Gunnera. This genus Diagrammatic Representation of Interrelationships
has about 40 species which are herbaceous and widely distributed in the southern hemisphere. The plants have mucilage-filled cavities called glands near the bases of petioles. Nostoc cells penetrate the cells of the gland when infection takes place naturally. Two glands known as apical papillate glands, just below the point of cotyledonary attachment are formed which produce mucilage enabling the growth of Nostoc. From the mucilage, Nostoc cells penetrate the interior of the glands and then the host cells. Other blue-green algal species are also capable of forming as sociations with Gunnera under artificial inoculated conditions.
An excellent example of algal association with higher plants is the occurrence of endophytes Anabaena or Nostoc in the coralloid masses on the roots of Cycadaceae. Cycads produce stubby apogeotropic profusely dichotomously branching tannin-rich coral-like (coralloid) root masses which become infected with cyanobacteria. These roots appear in addition to normal ordinary roots which are often very tuberous. The functions of coralloid roots are not clearly understood. It has been suggested that lenticels present on these roots facilitate gas exchange between the plant and the atmosphere. Cyanobacteria may enter through lenticels or through breaks in the root’s dermal layers.
The cyanobacteria are restricted to intercellular spaces of the cyanobacterial zone although occasional penetration f the cortex has been reported in Cycas revolute. Invariably the cyanobacteria are heterocystous belonging to the genus Nostoc. In some cases Anabaena and Calothrix have been encountered. Endophytes have been found in distinct zones in the cortex of the coralloid nodules on roots of genera such as Cycas, Encephalartos, Zamia, Ceratozamia, Macrozamia and Stangeria. The blue-green alga, Nostoc cycadeae isolated from Cycas, Encephalartos and Macrozamia have been shown to fix atmospheric nitrogen in the free state as well as in association with their host plants, as revealed by experiments using 15N.

When washed coralloid and normal roots were exposed to 15N labelled gas, coralloid roots showed enrichment with 15N while normal roots showed no such enrichment.

Blue Green Algal Association with Roots of Encephalartos

B-A. Mass of Coralloid Roots	C. Transverse Section of Coralloid Root

B. Transversely Sectioned Coralloid Root	C. Transverse Section of Uninfected Coralloid Root

D. Transverse Section of Infected Coralloid Root	E. Cyanobiont Trichome with a Terminal Heterocyst in the Coralloid Root

However, coralloid roots of two species of Encephalartos have been shown to reduce acetylene to ethylene rather weakly.

The speed with which the transfer of fixed nitrogen takes place from the site of fixation into different parts of the hosts varies from 11/2 hours (in Gunnera symbiosis) to 48 hours (in cycad symbiosis).

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Blue Green Algal Association with Bryophytes Blue Green Algal Association with Higher Plants Aazolla Anabaena Asociation Survial of Aazolla Limitations of Aazolla Anabaena Asociation

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