AOBPreview originally published online on May 2, 2008
Annals of Botany 2008 102(1):39-48; doi:10.1093/aob/mcn064
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Physical Dormancy in Seeds of the Holoparasitic Angiosperm Cuscuta australis (Convolvulaceae, Cuscuteae): Dormancy-breaking Requirements, Anatomy of the Water Gap and Sensitivity Cycling
1 Department of Biology
2 Department of Horticulture
3 Department of Plant and Soil Sciences, University of Kentucky, Lexington, Kentucky, 40506, USA
4 Division of Silviculture, Taiwan Forestry Research Institute, Taipei, Taiwan
* For correspondence. E-mail ccbask0{at}uky.edu
Received: 21 January 2008 Returned for revision: 17 March 2008 Accepted: 31 March 2008 Published electronically: 2 May 2008
Background and Aims: Dormancy in seeds of Cuscuta (Convolvulaceae, tribe Cuscuteae) is due to a water-impermeable seed coat (physical dormancy). In nondormant seeds of several species of this family, bulges adjacent to the micropyle have been identified as the initial route of water entry into seeds (water gap). However, there are claims that water enters seeds of Cuscuta spp. via the entire seed coat. Although several studies have been done on seed coat anatomy of Cuscuta, none has identified and/or characterized the morphology/anatomy of a water gap. Thus, the primary aim of this research was to identify and describe the morphology and anatomy of the water gap in seeds of Cuscuta australis. It was also determined if sensitivity cycling to dormancy-breaking treatments occurs in seeds of this species.
Methods: Light microscopy, scanning electron microscopy, tissue-sectioning and dye-tracking and blocking experiments were used to investigate the morphology and anatomy of the water gap. Treatments simulating natural conditions were used to break seed dormancy. Storage of seeds at different temperatures was tested for their effect on sensitivity to dormancy-breaking treatment.
Key Results: Dormancy-breaking treatments caused the tightly closed hilar fissure to open. Staining was observed in cells below the hilum area but not in those below the seed coat away from the hilum. Sensitivity to dormancy-breaking treatment was induced by storing seeds dry and reduced by storing them wet.
Conclusions: Whereas bulges adjacent to the micropyle act as the water gap in other species of Convolvulaceae with physical dormancy, the hilar fissure serves this function in Cuscuta. Cuscuta australis can cycle between insensitivity 
sensitivity to dormancy-breaking treatments.
Key words: Convolvulaceae, Cuscuta, hilar fissure, palisade layer, physical dormancy, seed coat anatomy, seed dormancy, seed germination, sensitivity cycling, water gap
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