Developmental morphology of Acanthophora nayadiformis (Rhodophyta, Ceramiales): sympodial growth and propagule germination

The A. nayadiformis thallus is peculiar due both to its morphological organization and to the production of specialized vegetative propagules resembling turions of aquatic dicotyledons. Propagules have proved to be useful culture inocula for morphogenetic research. They germinate quickly when cultured under optimal conditions, growing in a polar fashion, producing uprights from their distal buds and axes with rhizoids from the proximal ones. The thallus growth pattern in A. nayadiformis at first involves a monopodial arrangement of determinate branches followed by ramisympodial growth at all thallus level with an unpredictable model. There is not, in fact, a fixed 'sympodial unit' which reiterates itself in the ramisympodia, thus giving rise to great morphological diversity. Both irradiance and temperature have a significant effect on propagule bud growth rate, but not on their developmental morphology. In contrast, changes in the apex morphogenetic trend, such as transformation from the stolon apex into the upright kind and sympodial growth, are promoted by long-day conditions and largely suppressed under short-day regimes. The short-day inhibition of upright growth seems to be a specialized mechanism for over-wintering in a resistant vegetative phase