Germination of seeds of some summer annuals in Kentucky (eastern USA) in late-winter lead to the hypothesis that under present climate conditions the whole length of the winter cold stratification (CS) period is not required for dormancy-break of seeds of summer annuals with physiological dormancy (PD). We evaluated our data from germination phenology studies of 45 species (69 datasets) and buried-seed studies of 33 species (44 datasets). We determined time and temperature of germination after CS and percentage of the total number of hours of CS during winter (% of winter CS) seeds received prior to start of germination. In the phenology studies, mean temperature during the week of first germination for C3 and C4 species was 11.1 and 14.4°C, respectively, and % of winter CS was 80.8 and 87.4, respectively. In the buried-seed studies, % of CS for C3 and C4 species was 40.8 and 48.1, respectively, when they germinated to 25% at 20/10°C. For 32 of 33 species in the buried-seed studies, the minimum temperature at which seeds germinated decreased with increased CS; thus, seeds had Type 2 non-deep PD. The time of germination is controlled by a number of hours of CS, a decrease in minimum temperature at which seeds can germinate and a temperature increase in early spring. Seeds can germinate at relatively high temperatures as early as December and January, but they continue to be CS until spring. Temperature increases in eastern North America due to global warming are not likely to inhibit the germination of summer annuals with PD in spring.

Fire stimulates the germination of most seeds in fire-prone vegetation. Fruits of Leucadendron (Proteaceae) are winged achenes or nutlets that correlate with their requirements for smoke and/or heat in promoting germination. We describe five possible smoke–heat dormancy-release/germination syndromes among plants, of which Leucadendron displays three (no response, smoke only, smoke and heat). As seed-coat thickness varies with seed-storage location (plant or soil) and morphology (winged or wingless), we tested its possible role in water uptake and germination. Species with winged seeds achieved 100% germination in the absence of smoke/heat, seed coats were an order of magnitude thinner, and their permeability greatly exceeded that of nutlets. As seed-coat thickness increased (1) imbibitional water uptake declined at a decreasing rate, and (2) the response to smoke, and to a lesser extent heat, increased linearly to reach levels of germination approaching those of winged seeds. For species responsive to smoke and heat, there was no additive effect when applied together, suggesting that they may have promoted the same physiological process. We conclude that seed-coat thickness holds the key to germination requirements in this genus, independent of seed-storage location or morphology. By what mechanisms (1) the smoke response is greater the thicker the seed coat and (2) smoke chemicals might increase water permeability to explain the non-additive effect of smoke and heat, warrant further investigation.

The Brazilian Cerrado, a Neotropical savanna, is a fire-prone ecosystem where the ground layer biomass consists mainly of graminoids. However, as for other savannas, the effects of fire cues (such as smoke) on Cerrado grasses do not present a clear pattern, either for germination or seedling development. Smoke can stimulate different stages of the plant life cycle, which can alter the community and invasion processes. So far, most research on the subject focuses on germination, not addressing post-germinative phases, a sensitive stage of plant development. Here, we investigated the effect of smoke on a native (Echinolaena inflexa) and an invasive (Urochloa decumbens) grass species common in the Cerrado. We analysed germinative parameters and seedling mass and length after exposing the seeds to dry smoke for 5, 10, 15 or 20 min. Seedling development was assessed by measuring shoot and root systems after cultivating germinated seeds for 3, 7 or 15 d. Smoke did not affect germination percentages. However, fumigation reduced the mean germination time of both species and the germination onset of E. inflexa. U. decumbens had higher length values in all periods of cultivation, whereas mass values only surpassed that of E. inflexa at 15 d. Smoke exposure reduced the aboveground length of 7-d seedlings of U. decumbens, and mass of 15-d plants of both species. Also, smoke enhanced the root investment of the native and invasive species in different cultivation periods. Therefore, studying post-germinative parameters on seedling development may bring further insights into the smoke effects.

Cistus species have seeds with hard coats which impose physical seed dormancy that can be released after seed scarification. In fire-prone habitats, the break of physical seed dormancy is usually related to the heat produced during fires. It is commonly accepted that most hard-seeded species, including those of the genus Cistus, are able to germinate under a wide range of temperatures in light as well as in darkness, once the seed becomes permeable. However, although many studies have focused on the release of physical dormancy only, a few have done so on the effect of environmental factors once dormancy is released. In this research, through a factorial experiment, we analysed the effects of light (light and darkness) and a range of temperatures (10, 15, 20, 25 and 30°C) on the seed germination of eight Cistus species after a heat shock. On average, almost 60% of the seeds did not germinate despite being viable, and this lack of germination increased with higher temperatures during the treatment. Although an idiosyncratic germination response emerged, temperature had a significant effect in all the species, reaching the highest levels of germination between 10 and 20°C. Light interacted with temperature in four cases by increasing the germination, especially under the least favourable temperatures. Environmental factors, such as temperature and light, appear to modulate the germination of the studied Cistus species after the release of physical seed dormancy.

Plant community-level studies on seed dormancy traits are important to understand and determine the significance of seed dormancy in different ecosystems. Hence, we studied seed dormancy and other related seed biological traits of 42 selected tropical lowland rainforest tree species from Sri Lanka, aiming to address seed dormancy class(es) for a biodiverse tropical lowland wet zone forest community and the relationship between dormancy classes, forest strata and seed dispersal mechanisms. Seed germination, imbibition, embryo length:seed length ratio, embryo morphology and the effect of gibberrelic acid on seed germination were determined. Sixty-two percent of the species with T50 < 30 days were identified as having fast-germinating seeds and the remaining 38% with T50 > 30 possessed slow-germinating seeds. Seeds of 33 species had fully developed embryos, while nine species had underdeveloped embryos; three had morphological dormancy (MD) and six morphophysiological dormancy (MPD). Treatment with gibberellic acid revealed physiological dormancy (PD) in seeds of six species, and the response to manual scarification confirmed physical dormancy (PY) in seeds of Pericopsis moonina. The majority of tropical lowland rainforests had non-dormant (ND) species (62%), and 14.3, 14.3, 7 and 2.3% of the species had MPD, PD, MD, and PY, respectively. Non-dormancy decreased for taxa from the upper strata to the lower strata of the forest. ND seeds were dispersed during the rainy season. Thus, non-dormancy seems to be the most dominant germination behaviour among the tree species in the lowland rainforest of Sri Lanka with the class of dormancy related to forest strata and dispersal time.

The objective of this study was to test the tolerance of two species of Schinus and two species of Handroanthus cultivated in iron mining tailings from the rupture of the dam in Mariana, Brazil. Samples of mining tailings were collected 1 km away from the dam location and then dried, stored in plastic bags and further analysed for elemental composition. The seeds, later seedlings, were cultivated in the mining waste and in sand in two experiments separately and the experimental design was in a 2 × 3 factorial scheme (two substrates and three combinations of species), with six replications (n = 36). After 60 d of the establishment of the experiments, the germination data, biometric and anatomical measurements of the leaves were evaluated, in addition to the elemental characterization of the tailings. Mining tailings showed macro and micronutrients in addition to potentially toxic elements (As, Al, Cr, Pb and Ni). Seeds germinated and seedlings survived in the mining tailings. Mining tailings reduced the seedling emergence in Handroanthus, whereas it increased the emergence in S. molle and had no significant effect in S. terebinthifolia. Mining tailings reduced the number and length of roots in Schinus but increased these traits in Handroanthus species. Moreover, mining tailings reduced the fresh mass in Handroanthus but had no effect in the Schinus species. Mining tailings reduced the palisade and spongy parenchyma Handroanthus but only the spongy parenchyma was reduced in Schinus species. Therefore, mining tailings provided conditions for seed germination and seedling growth and Schinus species showed higher tolerance.