Studies of the Azolla-Anabaena Symbiosis Using Azolla mexicana, I. Growth in Nature and Laboratory

Abstract

The symbiosis of Azolla and Anabaena is the only known association of a pterophyte with a cyanophyte. Anabaena azollae is found in all species of the aquatic heterosporous fern Azolla. It is enclosed within a small, basal cavity of the upper green lobe of the bilobed, alternate leaves. Very rapid propagation of the intact Azolla is generally achieved by fragmentation, but may occur by germination of algal-infected sporocarps (Rao, 1935; Smith, 1955, pp. 371-383). In many. temperate and tropical regions, Azolla is considered a pest because it can cover a farm pond or rice paddy rapidly (Sculthorpe, 1967). However, it has long been cultivated as a green manure for rice crops in southeast Asia (Moore, 1969). This latter use provides the basis for the recent intense interest in this unique symbiosis. The algal symbiont is a nitrogen-fixing organism that behaves much like Rhizobium species do in their symbioses with legumes. Studies on the factors affecting the growth of the Azolla-Anabaena symbiosis are relevant to research on nitrogen-fixation and also weed control. Previous laboratory studies on nitrogen-fixation have employed A. filiculoides Lam. or A. caroliniana Willd. (Ahmad, 1941; Peters & Mayne, 1974). However, of the six New World species, A. mexicana Presl is the major representative of the central and western United States (Svenson, 1944). The present study was conducted to characterize the optimal environment for growth of A. mexicana. By virtue of environmental preferences or a more controllable or rapid growth rate, this species might provide a better and more convenient model than larger land plants do for studies of symbiotic nitrogen fixation. MATERIALS AND METHODS Studies in the natural environment.-Growth ofAzolla mexicana was studied in a small, 2 ha pond in southwestern Jackson County, Illinois. This pond, in the Mississippi River flood plain, is fed by runoff from low alluvial plains and swamps delimited by levees. The environmental factors recorded at each observation were light intensity (Weston photometer), light quality (IL 150 Photometer, International Light, Newburyport, MA.), photoperiod, air temperature in the shade and in the sun, and water pH and surface and bottom temperature. Total plant cover of the pond surface and the percentage contributed by A. mexicana were also recorded. Plant cover was determined by the line intercept method from photographs of the pond surface (Canfield, 1941). Laboratory studies. - Plant material was collected from the pond on 18 June 1975. It was separated and washed clean of other aquatic plants. All subsequent