Seeds of legumes are generally considered to have physical dormancy and to be orthodox, but most seed biologists are unaware of the various kinds and combinations of dormancy and storage behaviour in seeds of this family. The aim of our study was to document the dormancy and storage behaviour of seeds of 100 native and introduced tropical Fabaceae species in Sri Lanka and classify them into germination/storage behaviour categories. Moisture content (MC) was < 16% for fresh seeds of 94 species and >29% for those of six. Seeds of these six species had low tolerance for desiccation and for low temperatures. Thus, seeds of six species are non-orthodox and 94 species orthodox. Nine of the 100 species were non-dormant, and 2, 3 and 86 had physiological, physiological epicotyl and physical dormancy, respectively. Six germination/storage behaviour categories were identified among the 100 species. However, as in extratropical regions of the world, orthodox storage behaviour and physical dormancy are characteristic of seeds of the majority of species of Fabaceae in tropical Sri Lanka.

Convolvulaceae is the most advanced plant family (asterid clade) that produces seeds with physical dormancy (water-impermeable seed coat). There are several different opinions about the nature of the specialized structure ('water gap') in the seed coat through which water initially enters seeds of Convolvulaceae, but none of them has been documented clearly. The primary aim of the study was to identify the water gap in seeds of Ipomoea lacunosa (Convolvulaceae) and to describe its morphology, anatomy and function. Light microscopy, scanning electron microscopy, tissue-sectioning, dye-tracking and blocking experiments were used to describe the morphology, anatomy and function of the water gap in seeds of I. lacunosa. Dormancy-breaking treatments caused slits to form around the two bulges on the seed coat adjacent to the hilum, and dye entered the seed only via the disrupted bulges. Bulge anatomy differs from that of the rest of the seed coat. Sclereid cells of the bulges are more compacted and elongated than those in the hilum pad and in the rest of the seed coat away from the bulges. The transition area between elongated and square-shaped sclereid cells is the place where the water gap opens. Morphology/anatomy of the water gap in Convolvulaceae differs from that of taxa in the other 11 angiosperm plant families that produce seeds with physical dormancy for which it has been described.

Although a claim has been made that dormancy cycling occurs in seeds of Ipomoea lacunosa (Convolvulaceae) with physical dormancy, this would seem to be impossible since the water gap cannot be closed again after it opens (dormancy break). On the other hand, changes in sensitivity (sensitive ↔ non-sensitive) to dormancy-breaking factors have been reported in seeds of Fabaceae with physical dormancy. The primary aim of the present study was to determine if sensitivity cycling also occurs in physically dormant seeds of I. lacunosa. Treatments simulating conditions in the natural habitat of I. lacunosa were used to break seed dormancy. Storage of seeds at temperatures simulating those in spring, summer, autumn and winter were tested for their effect on sensitivity change. Seeds made non-dormant were stored dry in different temperature regimes to test for dormancy cycling. In addition, seeds collected on different dates (i.e. matured under different climatic conditions) were used to test for maternal effects on sensitivity to dormancy-breaking factors. Sensitivity was induced by storing seeds under wet conditions and reversed by storing them under dry conditions at low (≤5 °C) or high (≥30 °C) temperatures, demonstrating that seeds of I. lacunosa can cycle between sensitive and insensitive states. Sensitive seeds required ≥2 h at 35 °C on moist sand for release of dormancy. However, there is no evidence to support dormancy cycling per se. Conceptual models are proposed for sensitivity cycling and germination phenology of I. lacunosa in the field. Seasonal germination behaviour of physically dormant I. lacunosa seeds can be explained by sensitivity cycling but not by dormancy cycling per se. Convolvulaceae is only the second of 16 families known to contain species with physical dormancy for which sensitivity cycling has been demonstrated.

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. 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. 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. 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.

Cycling of physically dormant (PY) seeds between states insensitive and sensitive to dormancy-breaking factors in the environment has recently been demonstrated inFabaceaeandConvolvulaceae, and it may be a common phenomenon in seeds with water-impermeable seed coats. In contrast to seeds of many species with physiological dormancy (PD), those with PY cannot cycle between dormancy and non-dormancy (ND). In this paper, we evaluate the role of sensitivity cycling in controlling the timing of germination of seeds with PY in nature, and show that sensitivity cycling in seeds with PY serves the same ecological role as dormancy cycling in seeds with PD. Thus, sensitivity cycling in seeds with PY ensures that germination in nature occurs only at (a) time(s) of the year when environmental conditions for growth are, and are likely to remain, suitable long enough for the plant to complete its life cycle or to form a perennating structure. Further, we describe the experimental procedures necessary to determine whether sensitivity cycling is occurring, and discuss briefly the possible relevance of sensitivity cycling to dormancy classification.

The water-impermeable seeds of Ipomoea lacunosa undergo sensitivity cycling to dormancy breaking treatment, and slits are formed around bulges adjacent to the micropyle during dormancy break, i.e. the water gap opens. The primary aim of this research was to identify the mechanism of slit formation in seeds of this species. Sensitive seeds were incubated at various combinations of relative humidity (RH) and temperature after blocking the hilar area in different places. Increase in seed mass was measured before and after incubation. Scanning electron microscopy (SEM) and staining of insensitive and sensitive seeds were carried out to characterize these states morphologically and anatomically. Water absorption was monitored at 35 and 25 °C at 100 % RH. There was a significant relationship between incubation temperature and RH with percentage seed dormancy break. Sensitive seeds absorbed water vapour, but insensitive seeds did not. Different amounts of water were absorbed by seeds with different blocking treatments. There was a significant relationship between dormancy break and the amount of water absorbed during incubation. Water vapour seals openings that allow it to escape from seeds and causes pressure to develop below the bulge, thereby causing slits to form. A model for the mechanism of formation of slits (physical dormancy break) is proposed.