The present study assessed the desiccation sensitivity of the excised embryonic axes of the following Amaryllidaceae species: Scadoxus puniceus, Amaryllis belladonna, Nerine huttoniae, N. humulus, N. bowdenii, N. filifolia, Haemanthus humulus humulus, H. coccineus, H. deformis, H. bakerae, Brunsvigia gregaria, Brunsvigia orientalis, Boophane disticha, Strumaria discifera, Crinum macowanii and C. bulbispermum. Excised embryonic axes were rapidly dehydrated (flash-dried). Curves of water content (g g− 1) and viability (%) versus drying time (min) were used to interpolate the water content (WC) and drying time (DT) values corresponding to 80% viability (WC/DT, V = 80) for individual species, and used to compare desiccation sensitivity between developmental stages and among years, provenances, species and genera. The seeds of all 16 species were shed with high axis water contents (2.25 ± 1.34 to 6.95 ± 0.89 g g− 1) and were sensitive to desiccation, with WC V = 80 values ranging from 0.13 to 3.15 g g− 1. Germination in amaryllids is unusual in that part of the cotyledon, to which the embryonic axis is attached, grows right out of the seed. It is at this stage that the axes were found to be more desiccation-sensitive in six out of nine species investigated. Seed fresh mass (SFM), axis shedding water content (SWC) and the degree of desiccation sensitivity were species characteristic. SWC, SFM and drying time were not significantly related to desiccation sensitivity. Parent plants were not confined to any particular biome type, occurring in areas characterized by seasonal dry spells. Parent plant habitat and seed characteristics were not useful in predicting the degree of desiccation sensitivity across species belonging to the same family.

Lannea microcarpa, a multipurpose tree species from the dry African savanna, sheds seeds that often display inhibition of germination. The underlying mechanism was investigated using seeds processed from fully matured fruits collected from natural stands in Burkina Faso. Germination of fresh seeds was variable (16–28%), while they did not germinate after drying and rehydration. Mechanical scarification of the endocarp at the proximal end of the seeds increased germination to 83–94%. Scarification on the distal end led to delayed radicle emergence through the produced hole in c. 40% of the seeds. The endocarp was permeable to water and respiratory gases. Increased water content in scarified seeds was associated with radicle extension during germination. Intact and scarified non-germinated seeds displayed a moderate rate of respiration with respiratory quotient (RQ) values of c. 1. Respiration increased and RQ decreased to c. 0.7 with radicle emergence. Ethylene evolution peaked in both intact and scarified seeds at the beginning of incubation and then decreased to low values. Inhibition of ethylene production by 1–5 mM 2-amino-ethoxyvinylglycine (AVG) caused only a partial decrease of germination of the scarified seeds. Intact non-germinated seeds gradually lost viability during incubation at 30°C, but could be rescued by delayed scarification before day 15 of incubation. It is concluded that radicle emergence in dry L. microcarpa seeds is inhibited only mechanically. The mechanical properties of the endocarp are attributed to irreversible structural changes of the lignin–hemicellulose complex, which occur during drying.

Little is known about the seed ecology of annual holoparasites, such as Cuscuta (dodders), and how germination is timed to occur when seedlings can attach to a host. Seeds of Cuscuta epithymum, which is a rare and often threatened species in dry heaths of north-western Europe, were water-impermeable (physically dormant) at maturity in late summer. An 8-week period at 5°C, followed by incubation at 23, 15/6 or 30/20°C, resulted in 15–30% germination. Thus, in nature, only part of the seed population exposed to low winter temperatures and a subsequent warm spring potentially becomes water-permeable and can germinate in spring. When scarified seeds were cold-stratified at 5°C for 8 weeks and then incubated at 23°C, essentially all the viable seeds (c. 65%) germinated. It is concluded that seeds have a combination of physical (PY) and physiological (PD) dormancy (the first to be reported in the Convolvulaceae), which is interpreted as a double safety mechanism preventing germination at unfavourable times or places. After PY and PD were broken, seeds did not require light or fluctuating temperatures for germination. Some loss of PD (afterripening) can occur before PY is broken. These data indicate that a portion of the seed crop remains physically dormant in spring and potentially forms a persistent seed bank, a strategy that eliminates the risk of a total reproductive failure in a particular bad year.

To understand germination timing in an ecological context, the response to environmental events that affect seed dormancy is central, and has to be combined with knowledge of germination responses to different circumstances. In this study, seed dormancy, germination and emergence phenology of six annual co-occurring weedy Asteraceae species were investigated. Three pre-treatments (warm and cold stratification, and dry storage) were tested as possible dormancy-affecting environmental events. Seeds were also sown outdoors. Species-specific differences were revealed in analyses. To facilitate general descriptions of dormancy patterns and germination preferences separately, condensed responses to the different possible dormancy-affecting treatments and relative germination in different environments were plotted, giving species-specific patterns. Most species exhibited decreased dormancy to two or three pre-treatments. Dormancy was most effectively reduced by cold stratification for three species (Guizotia scabra, Parthenium hysterophorus, Verbesina encelioides), by warm stratification for two (Bidens pilosa, Galinsoga parviflora) and by dry storage for one (Tagetes minuta). All species germinated more when provided with light than in continuous darkness. Temperature levels most suitable for germination varied from low (15/5–20/10°C) for V. encelioides to high (25/15–30/20°C) for B. pilosa. It is concluded that, even though the species have different dormancy patterns and germination preferences that suggest different possible distribution ranges, they achieve similar emergence timing in the field in environments with a pronounced dry period after dispersal and small annual temperature fluctuations.

The soluble selenium (Se) compounds selenic acid, selenious acid, selenium dioxide, selenium tetrachloride, selenomethionine, selenourea, sodium selenate and sodium selenite broke the dormancy of scarified seeds of Townsville stylo (Stylosanthes humilis HBK). Given the chemical diversity of all these molecules, it could be deduced that Se may act as a dormancy-breaking agent of Townsville stylo seeds. Se-stimulated seeds produced great amounts of both ethylene and 1-aminocyclopropane-1-carboxylic acid, which correlated with the process of germination. As specific inhibitors of ethylene biosynthesis and action largely depressed this Se-stimulated germination, it was concluded that Se broke dormancy by triggering ethylene production by the seeds.