A Scanning Electron Microscopic View of Sporulation in Azolla mexicana Presl

Abstract

The Azolla-Anabaena symbioses are widely recognized as a potentially important alternative nitrogen source for lowland rice. The realization of their full potential remains contingent, however, upon overcoming a variety of physio-ecological and managerial limitations. While ecotype selection programs, the development of improved managerial schemes and the design of specialized machinery all hold promise for improving Azolla utilization, the key to future maximal utilization would seem to lie in understanding the process and control of sporulation and subsequent life cycle development through the haploid gametophyte stage back to the sporophyte. The ability to control the life cycle will facilitate the development of breeding programs and spore inoculants. Few studies have been conducted on the basic reproductive biology of Azolla and to date the factors controlling sporulation remain unknown. As part of an ongoing research program aimed at improving our understanding of the sporulation process and identifying its controlling factors, we have further examined the morphology of sporulation in Azolla mexicana. The result is an improved pictorial record and general confirmation of structures described in earlier literature from other species. Through microdissection sequences the distinctive features within mega- and microsporocarps have been studied and micrographed. We have confirmed the presence of Anabaena in both megasporocarps and microsporocarps. In mixed sporocarp cultures we have followed the dehiscence of the proximal megasporocarp wall to reveal the perispore, rupture of microsporangia which releases the massulae, and attachment of massulae ro the megasporocarp by entaglement of the barbed glochidia in thread-like protuberances in the perispore. These cultures have yielded sporophytes. Their emergence and release from the megasporocarp apparatus have been documented pictorially. Taxonomically. our A. mexicana cannot be distinguished readily from A. caroliniana using the derails of perispore architecture reported in the literature for the latter species.