Fire is a frequent disturbance in the Cerrado and is one of the major factors affecting vegetation structure and diversity. Fire events open gaps within the herbaceous layer and increase temperature fluctuation in the soil surface. In addition to being an important environmental filter for germination, fire is a germination cue for species with physiological dormancy. This study aimed to evaluate the germination of native grasses, using daily temperature fluctuations and heat shock to overcome physiological dormancy in native grasses. We also evaluated seed longevity after dispersal for some species. We conducted the daily temperature fluctuation experiments on seeds of ten native grass species, which were collected and then placed in germination chambers simulating thermal fluctuation throughout the day (19–55°C). We also subjected seeds to different heat-shock treatments: 100°C for 1 min, 100°C for 3 min and 200°C for 1 min. To determine seed longevity, we stored seeds for 6 and 12 months after collection and then set them to germinate (27°C, 12/12 h light). Non-germinated seeds from all experiments were tested for viability. Most species had low longevity and germination percentages. Those that had physiological dormancy were stimulated to germinate when exposed to temperature fluctuations. One species resisted temperatures up to 200°C. For all other species, neither treatment affected germination percentages. Our results indicate the importance of these environmental filters for seedling recruitment of these species, considering the low longevity and the presence of physiological dormancy.

We present a germination and emergence model that can be used as a sub-model in an individual-based model of population dynamics. Seed germination and seedling elongation rates were measured in petri dishes in the laboratory for ivyleaf morningglory, velvetleaf, corn, and soybean seeds, as functions of temperature and water potential. The analysis yielded a set of indices: the germination temperature index (GTI), the germination water index (GWI), the emergence temperature index (ETI), and the emergence water index (EWI). The seed populations were divided into 100 discrete cohorts, with each cohort (i), having its own germination rate (GR) as the product of a reference rate and the germination indices, or GRi = GRiREF × GTI × GWI. After germination, the emergence rate (ER) was the product of a reference rate and emergence indices, or ERi = ERREF × ETI × EWI. The model was tested against the timing of emergence in the field for seeds planted 1, 2, 4, and 8 cm deep in natural or rain-excluded environments. The predictions were more accurate for all depths combined than for any particular depth.

Common waterhemp, giant foxtail, and velvetleaf seed germination in response to temperature was studied with a two-way thermogradient plate. Seeds were maintained under dark and wet conditions at 4 C for 12 wk, and velvetleaf seeds were scarified before the experiments were conducted. The seeds were germinated at 25 different temperature treatments. Minimum and optimum temperatures for velvetleaf germination were approximately 8 and 24 C, respectively. Temperature alternation did not affect the germination of this species. The minimum germination temperature was 10 C for common waterhemp and 14 C for giant foxtail. The optimum germination of giant foxtail occurred at approximately 24 C, but common waterhemp optimum germination was variable depending on temperature alternation. Increased amplitude of the diurnal temperature alternation increased percent germination of these two species, and this was more evident at lower temperatures. In the case of common waterhemp, the temperature required to reach specific germination percentages was reduced by increasing the amplitude of the temperature alternation.

Common waterhemp is an obligate outcrosser that has high genetic variability. However, under selection pressure, this weed shows population differentiation for adaptive traits. Intraspecific variation for herbicide resistance has been studied, but no studies have been conducted to determine the existence of variation for other adaptive traits that could influence weed management. The objective of this study was to examine the existence of different seed dormancy regulatory mechanisms in common waterhemp. Seed dormancy regulation, in response to different temperature and moisture regimes, was studied through germination experiments and proteome analysis using two common waterhemp biotypes (Ames and Everly) collected from agricultural fields in Iowa, and one biotype (Ohio) collected from a pristine area in Ohio. Without stratification, germination percentage among the different biotypes was 9, 29 and 88% for Ames, Everly, and Ohio respectively. The germination rate of seeds from Ames was dramatically increased after incubation at either 4 or 25 C under wet conditions, whereas germination of seeds from Everly was only increased at 25 C under wet conditions. The Ohio biotype showed no change in germination response to any of the incubation treatments. Germination studies indicated that the rate of seed dormancy alleviation differed between biotypes. Seed protein profiles obtained from the three biotypes differed in protein abundance, number, and type. A putative small heat-shock protein (sHSP) of 17.6 kDa and isoelectric point (pI) 6.1 increased whereas a putative glyceraldehyde-3-phosphate dehydrogenase (G3PDH) of 30.9 kDa and pI 6.4 decreased in abundance in the Ames biotype as seed dormancy was reduced in response to incubation at 4 C and wet conditions. These two proteins did not change in the Everly and Ohio biotypes, suggesting that these proteins changed their abundance in response to seed dormancy alleviation. The results of this study suggest that differences in seed dormancy levels between the biotypes were due to different physiological regulatory mechanisms.

Restoration of semi-natural grassland communities involves a combination of (1) sward disturbance to create a temporal window for establishment, and (2) target species introduction, the latter usually by seed sowing. With great regularity, particular species establish only poorly. More reliable establishment could improve outcome of restoration projects and increase cost-effectiveness. We investigated the abiotic germination niche of ten poorly establishing calcareous grassland species by simultaneously exploring the effects of moisture and light availability and temperature fluctuation on percentage germination and speed of germination. We also investigated the effects of three different pre-treatments used to enhance seed germination – cold-stratification, osmotic priming and priming in combination with gibberellic acid (GA3) – and how these affected abiotic germination niches. Species varied markedly in width of abiotic germination niche, ranging from Carex flacca with very strict abiotic requirements, to several species reliably germinating across the whole range of abiotic conditions. Our results suggest pronounced differences between species in gap requirements for establishment. Germination was improved in most species by at least one pre-treatment. Evidence for positive effects of adding GA3 to seed priming solutions was limited. In several species, pre-treated seeds germinated under a wider range of abiotic conditions than untreated seeds. Improved knowledge of species-specific germination niches and the effects of seed pre-treatments may help to improve species establishment by sowing, and to identify species for which sowing at a later stage of restoration or introduction as small plants may represent a more viable strategy.

Seed germination of the wetland emergent perennial Phragmites australis is stimulated by diurnally fluctuating temperatures. A germination experiment in darkness and light at different temperature regimes showed that P. australis germinated as well in darkness as in light over most of the temperature regimes tested. The germination requirements could partly explain why this species, despite a large annual production of small seeds, does not accumulate a persistent soil seed bank. A second experiment examined the effect of one to four diurnal temperature cycles with amplitude of fluctuations ranging from 0 to 30°C; diurnal mean temperature of 15°C. Germination in the absence of fluctuations was low, and logistic model estimates showed a positive effect of number of temperature cycles which was strongly influenced by amplitude size. For seeds that have fallen on moist ground during the winter, these laboratory results predict that a few large diurnal temperature fluctuations could be sufficient for onset of germination in the subsequently warmer spring period. Seeds that are located under water, however, are predicted to remain ungerminated until the water level falls. Hence, we suggest that the requirement for fluctuating temperature, in addition to being a sensor for ‘exposed’ seed sites, is also effectively acting as a germination timing mechanism.