Beyond Seeds and Fruits: Water-Impermeability in Sporocarps of Marsilea minuta (Marsileaceae) Expands the Taxonomic Limits of Physical Dormancy.

BACKGROUND AND AIMS: Occurrence of physical dormancy (PY) is thought to be limited to members of 18 angiosperm families belonging to 10 orders and is caused by a water-impermeable palisade cell layer(s) in seed or fruit coats. PY has not been reported in seeds of gymnosperms or in reproductive structures of non-seed plants. Sporocarps of the heterosporous fern family Marsileaceae are functionally analogous to seeds of angiosperms, and anatomy of the sporocarp wall and requirement of scarification to release spores suggest possible occurrence of PY. The primary aims of this study were to confirm (or not) the presence of PY in sporocarps of Marsileaceae and to morpho-anatomically characterize the water-gap complex with special reference to Marsilea minuta. METHODS: Water impermeability of M. minuta sporocarps was tested with an imbibition test. PY was broken by exposing sporocarps to wet and dry heat under laboratory conditions and storage under different moisture regimes. Water-gap region of the sporocarp was identified and characterized by a water-gap blocking test, microtome sectioning and light and scanning electron microscopy. KEY RESULTS: Water impermeability was confirmed in sporocarps of M. minuta. Both wet and dry heat treatments were effective in breaking PY (opening the water gap). Dry storage increased sensitivity for PY breaking treatments. Long term storage under various moisture regimes made the sporocarps permeable. Morphology and anatomy in the water-gap region differ from those in the rest of the sporocarp wall. CONCLUSIONS: Presence of a new kind of PY (sporocarp physical dormancy [ScPY]) was confirmed for the first time in Division Polypodiophyta. This is the first report of PY other than for seeds and fruits of Spermatophyta. The Type-II compound water-gap complex (Superior tooth gap or Russow's gap) is anatomically more complex and differs in both morphology and anatomy from water gaps previously described for Spermatophyta.