Soldier thistle [Picnomon acarna (L.) Cass.] is widely distributed throughout rainfed fields across western Iran, where it decreases crop yields and interferes with harvest operations. This study was conducted to determine the influence of different factors on seed germination and seedling emergence of P. acarna. Freshly harvested seeds were dormant and required an after-ripening period for breaking dormancy. Seed germination was greatly promoted by light. Germination occurred over a wide spectrum of constant and fluctuating temperature regimes, ranging from 5 to 35 C, with highest germination at constant (74%) and fluctuating (94%) temperatures of 20 and 20/10 C. Seed germination of P. acarna was tolerant to osmotic potential, while salt stress significantly inhibited its germination percentage. pH was not an inhibiting factor for germination of P. acarna seeds. Seedling emergence decreased exponentially with an increase in seed burial depth in the soil; at soil burial depths of 4 cm or greater, no seedlings were able to reach the soil surface. The results suggest that significant seed germination of P. acarna in rainfed fields is possible, and the weed has great potential to spread throughout rainfed systems in western Iran. Based on these results, effective control of P. acarna can be achieved by applying interrow cultivation in row crops and deep tillage at seedbed preparation.

Controlled environment experiments were completed to determine the effect of temperature on giant foxtail and fall panicum germination, emergence, and growth. Giant foxtail seed germination decreased when exposed to a constant 30 C compared to 20 C. Germination also decreased in the alternating 20/30 C temperature regime when the hours of exposure to 30 C as compared to 20 C increased. Fall panicum required alternating temperatures of 14 C (9 h)/28 C (15 h) to germinate. Giant foxtail seed germination exceeded 60% 4 d after exposure to an alternating temperature of 7 C (9.4 h)/20 C (14.6 h). Conversely, fall panicum seed did not germinate at the 7 C (9.4 h)/20 C (14.6 h) temperature regime and required a minimum of 7 d exposure to alternating temperatures of 13 C (8.7 h)/26 C (15.3 h) for 88% of the seed to germinate. The greatest emergence of giant foxtail and fall panicum was from 1 cm and 1 to 2.5 cm, respectively. Less than 5% of the giant foxtail and fall panicum seed emerged from 7.5 cm. The growth of giant foxtail seedlings was five times greater than that of fall panicum at each temperature regime tested. Incorporation of this information into bioeconomic models could result in accurate predictions of weed germination for effective weed management strategies.

Caryopses (seeds) of fall panicum (Panicum dichotomiflorum Michx.) have a dormancy that is not easily overcome. Although accelerated after-ripening for 4 weeks was partially successful in overcoming dormancy, a dark imbibition at 35 C for about a week was more effective and also interacted strongly with an accelerated after-ripening. In either case, activation of phytochrome by red irradiation was required to induce germination. Similarly, alternating temperatures allowed germination of red-treated seeds but constant temperatures did not. More seeds germinated in sealed flasks than in dishes. The enhancement of germination in sealed flasks appeared to be related to increased CO2 levels. The data suggested a multiple dormancy in fall panicum seeds. One dormancy, the most intense, was relieved by high-temperature imbibition. The residual dormancy was relieved by active phytochrome and alternating temperatures.

Fall panicum (Panicum dichotomiflorum Michx.) seed were found to be dormant at harvest. This dormancy was overcome by an after-ripening period of 17 to 22 weeks, alternating germination temperatures (10 C for 16 h and 30 C for 8 h), stratification, and mechanical or chemical scarification. Soaking seed in gibberellic acid (GA3) or thiourea for 1 day, or washing seed in running tap water for up to 96 h before seed were placed in a germinator, had no effect on dormancy. In the greenhouse, fall panicum emergence was greatest (82%) when seed were placed on the growth media surface. The maximum number of seedlings (39%) emerged in the field when seed were placed 1.3 cm below the soil surface, although some emerged from as deep as 5.1 cm.

Experiments were conducted on Palmer amaranth seeds collected in 2004 and 2006 from a natural population near Pendleton, SC, to determine the temperature and light requirements for germination of seeds retrieved from soil surface or from 10-cm depth in the field. A cyclic change in seed germination of Palmer amaranth in response to temperature and light occurred during a 12-mo after-ripening period. Freshly matured seeds collected in November required mean temperatures ≥ 25 C, and natural or red (R) light for increased germination. Following after-ripening in winter, seeds experienced a reduction in dormancy and germinated higher at 25 to 35 C mean compared with 10 to 15 C mean. With after-ripening for an additional 3 mo in May, seeds experienced a broadening of thermal range (10 to 40 C mean), and germination in natural light or R light was more than twice the germination in the absence of light. Fluctuating temperatures (7.5 C amplitude) improved germination over constant temperatures, except in summer and fall (9 and 12 mo after seed maturation). Exposure of seeds to high temperatures during summer caused secondary dormancy induction. Averaged over thermal amplitudes, seeds retrieved in fall required mean temperatures > 25 C for increased germination. Burial in spring for 3 to 6 mo induced seed dormancy, and the relative germination in fall (12 mo after seed maturation) was at least 50% higher for seeds retrieved from soil surface compared to seeds exhumed from 10-cm soil depth. Seeds retrieved in late summer and fall required natural light or R light for promoting germination, whereas far-red (FR) light or darkness inhibited germination. Furthermore, the effect of R and FR light was reversible, indicating a partially phytochrome-mediated germination response of Palmer amaranth seeds following 9 to 12 mo of after-ripening in the field.

At the end of the cropping season in November 1994, Striga hermonthica seed populations were collected in northern Bénin (in the Atacora and Borgou departments). Host crops included Zea mays L. (corn), Pennisetum americanum (pearl millet), and Sorghum bicolor (L.) Moench (sorghum). The seed populations were kept dry in the laboratory, and germination was tested regularly after 2 wk conditioning in the laboratory. The seeds passed through a state of primary dormancy, which was not the same for seed populations collected from Z. mays and S. bicolor fields and those collected from P. americanum fields. The length of the primary dormancy was approximately 6 mo. After passing through primary dormancy (after-ripening), the seeds later went through annual, recurrent states of secondary dormancy. Primary dormancy coincided with the dry season directly after maturity (i.e., between December 1994 and April 1995), and secondary dormancy coincided more or less with the subsequent 1995/1996 and 1996/1997 dry seasons. It is concluded that the secondary dormancy pattern was endogenous. Germination percentages during the period that coincided with the first rainy season after collection were generally higher than during the period that coincided with the second rainy season.

Knowledge of the germination requirements of wild radish will help in determining the favorable conditions for germination and emergence and allow better management of this weed. Experiments were conducted during 2005 to 2006 and 2006 to 2007 to evaluate wild radish temperature and light requirements over a 12-mo period beginning in July on seeds placed on the soil surface and at a 10-cm depth. Germination response was influenced by temperature, light, duration of burial, and burial depth. Freshly harvested seeds (July) had no more than 18% germination whereas seeds allowed to after-ripen in the field for 3 to 6 mo (October to January) had up to 40% germination. The germination of wild radish retrieved from the soil surface was 1.2 to 1.5 times greater at alternating temperatures (2.5/17.5, 7.5/22.5, and 12.5/27.5 C) than at constant temperatures (10, 15, and 20 C) at 0, 3, and 6 mo after maturation. The light requirement for germination varied by time of year with no differences in germination between light and dark conditions for freshly harvested seeds. Far-red light inhibited germination of wild radish, indicating that wild radish may become sensitive to light following an after-ripening period.

Specific knowledge about the dormancy, germination, and emergence patterns of weed species aids the development of integrated management strategies. The after-ripening period for jointed goatgrass seed was quantified, and the effects of germination conditions and spikelet structures on jointed goatgrass seed germination were measured. As the duration of after-ripening increased, jointed goatgrass seed germinated earlier, at faster rates, and to greater final percentages compared to non–after-ripened seed. Both primary and secondary positioned seed within jointed goatgrass spikelets were nondormant after 16 wk after-ripening at 22 ± 2 C. Germination of dormant seed depended on incubation temperature and dark/light conditions. Sixty-seven percent of spikelets produced a radicle when exposed to low temperatures in the dark, and light at warm temperatures increased germination by 7%. The relationship between light and incubation temperature was similar also for germination of the primary positioned seed in nondormant spikelets; however, the magnitude of the effect increased. Light increased germination of seed incubated at warm temperatures by 18%. Coleoptile emergence was dependent on planting depth for three jointed goatgrass populations, winter wheat, and spring wheat. Under optimum conditions in the greenhouse, no planting depth selectively allowed wheat germination and emergence while preventing jointed goatgrass germination and emergence. Glume removal increased jointed goatgrass secondary positioned seed final germination percentage to 96%, increased the germination rate, and decreased the number of days required to reach 50% germination to 6 d. Glume removal also promoted germination of the primary seed within jointed goatgrass spikelets. Glume removal resulted in 80% of the spikelets having two coleoptiles, but did not alleviate dormancy completely in jointed goatgrass seed. Tillage and herbicide applications for jointed goatgrass control will be most effective in the fall when primary dormancy is lost, but before secondary dormancy is imposed.

The transition from the dormant to the non-dormant state of a viable and mature seed can take place at low hydration by exposure to air-dry storage conditions (dry afterripening; AR). The events occurring during this loss of dormancy are of considerable physiological, ecological and agricultural interest. AR may be attributable to increased sensitivity to germination-stimulating factors and a widening of the temperature window for germination. Genetic, –omics and physiological studies on this mode of dormancy breaking provide support for a key role of the balance between gibberellin (GA) and abscisic acid (ABA) metabolism and sensitivity. Recent evidence also supports a possible role for ethylene (ET) in this complex signalling network that is necessary for AR implementation. However, hormone-independent signals, such as reactive oxygen species (ROS), nitrate () or nicotinamide adenine dinucleotide (NAD+), also appear to be involved. The way in which hormone- and non-hormone-signalling pathways affects each other (cross-talk) is still under study. This review provides updated information on the programmes that overcome seed dormancy. Thus, we have reviewed: (1) the –omic status in dry seeds; (2) the relationship between temperature and nitrate signalling and AR; (3) alterations in ABA/GA synthesis and signalling; (4) the action of hormone molecules other than ABA and GA (i.e. ET, salicylic and jasmonic acids); and (5) participation of reactive oxygen species (ROS), NAD+ and protein carbonylation. Taken together, the acquisition and implementation of dry AR involve a complex signalling network that is difficult to disentangle.

The effects of maternal air temperature and soil moisture upon seed physiological dormancy (PD) alleviation of an Australian native Asteraceae were investigated. From the onset of flowering, Actinobole uliginosum plants growing ex situ were subjected to either a warm (mean 26°C) or cool (mean 17°C) temperature regime, with adequate or limited water availability. In the warm environment, the reproductive phase was accelerated, and plants yielded fewer seeds over a shorter, earlier harvest period, when compared to those in the cool environment. Initial germination of all seeds was low ( < 20% at 15°C) due to PD, which was gradually alleviated by a dry after-ripening (DAR) treatment (34/20°C, 40% relative humidity, in darkness). Seeds from plants grown in the warm environment were more responsive to DAR than seeds from the cool environment, but maternal plant water availability had little effect on dormancy status. Germination was higher at 15°C than at 25/15°C, reaching a plateau of c. 80% germination after 20 weeks DAR. Before DAR, application of GA3 had little impact on seeds, which would consequently be classified as having deep PD if tested at the time of dispersal. However, DAR caused seeds to become increasingly responsive to GA3, reaching 97% germination at 15°C following just 4 weeks of DAR, which would indicate non-deep PD if seeds were tested following a period of warm, dry storage. Maternal air temperature regulates PD status of A. uliginosum, such that seeds collected from a warmer environment are likely to be more responsive to DAR. Post-harvest storage in an environment suitable for DAR affects seed response to GA3, which has implications for germination stimulation and dormancy classification.

The effect of dry storage under ambient laboratory conditions on after-ripening and survivorship was tested on seeds of the geographically-widespread Solidago altissima and S. nemoralis and the narrow-endemic S. shortii. Freshly-matured seeds of S. altissima collected in 1991 and in 1992 germinated to low or moderate percentages in light at 15/6, 20/10 and 25/15°C and to high percentages at 30/15 and 35/20°C, whereas those of S. nemoralis and S. shortii germinated to low percentages over the range of temperature regimes. After 0.8–1.8 years of storage, 1991 seeds of S. altissima incubated in light germinated to high percentages at 25/15, 30/15 and 35/20°C, those of S. nemoralis did so at 30/15 and 35/20°C and those of S. shortii at 20/10, 25/15, 30/15 and 35/20°C; 1992 seeds of all three species germinated to high percentages at 20/10, 25/15, 30/15 and 35/20°C. Freshly-matured 1991 and 1992 seeds of the three species germinated to low percentages in darkness over the range of temperature regimes, and only seeds of S. shortii germinated to high percentages after 0.8–1.8 years of storage. Compared with cold stratification, dry storage was only moderately effective in breaking dormancy in these three species. The primary difference in after-ripening of seeds of the three species was that seeds of the narrow endemic germinated to higher percentages in darkness than those of its two geographically-widespread congeners. Survivorship curves for 1991 and 1992 seeds of S. altissima and S. nemoralis and for 1992 seeds of S. shortii were of Deevey Type I; the survivorship curve for 1991 seeds of S. shortii was closest to Type II. Longevity of 1991 seeds of S. altissima, 1992 seeds of S. nemoralis and 1991 and 1992 seeds of S. shortii was <4.0 years, whereas that of 1991 seeds of S. nemoralis was <2.3 years; 5% of 1992 seeds of S. altissima were viable after 4.0 years.

Capeweed (Arctotheca calendula) seeds were found to be dormant at harvest. Effects of duration and temperature of storage under ‘laboratory’ and ‘natural’ conditions, growth regulators, stratification and age of seeds, were studied on the germination of dormant seeds. Three factors imposed on seeds were found to promote germination of capeweed: (i) allowing seed to age, either in storage, buried in soil or during stratification (germination of 18-month-old seeds was up to 60% higher than that of fresh seeds); (ii) the presence of light (the average germination percentage of seeds exposed to light during storage was 3.2-fold greater than that of seed stored in the dark); and (iii) the application of growth regulators, particularly gibberellic acid, enhanced by scarification (GA3 and ethephon promoted germination by up to 58% when applied to scarified seeds). Other factors, e.g. temperature and depth of storage, influenced germination but were less critical to its success. These results provide a preliminary assessment of the importance of these factors in controlling dormancy in seeds of capeweed.

Lolium multiflorum Lam., a winter annual grass species originating from the Mediterranean region, has become widespread in native grasslands of the flooding pampa (Argentina). The dynamics of dormancy release of L. multiflorum seeds in simulated natural conditions was studied. Dormancy was overcome by exposing imbibed seeds to mild cool temperatures (15°C or below) for brief periods (pre-chilling). After-ripening progressively released dormancy, at a higher rate in seeds that remained under the canopy (30/20°C) than those that were dry-stored (20°C). Germination was promoted by red light only when the after-ripening requirement was partially fulfilled or after pre-chilling treatment. These results provide useful knowledge for designing grazing strategies aimed at obtaining well-developed ryegrass canopy of high forage quality during winter and spring.

Bromus tectorum L. is an invasive winter annual grass with seeds that lose dormancy through the process of dry after-ripening. This paper proposes a model for after-ripening of B. tectorum seeds based on the concept of hydrothermal time. Seed germination time course curves are modelled using five parameters: a hydrothermal time constant, the fraction of viable seeds in the population, base temperature, mean base water potential and the standard deviation of base water potentials in the population. It is considered that only mean base water potential varies as a function of storage duration and incubation temperature following after-ripening. All other parameters are held constant throughout after-ripening and at all incubation temperatures. Data for model development are from seed germination studies carried out at four water potentials (0, −0.5, −1.0 and −1.5 MPa) at each of two constant incubation temperatures (15 and 25°C) following different storage intervals including recently harvested, partially after-ripened (stored for 4, 9 or 16 weeks at 20°C) and fully after-ripened (stored for 14 weeks at 40°C). The model was fitted using a repeated probit regression method, and for the two seed populations studied gave R2 values of 0.898 and 0.829. Germination time course curves predicted by the model generally had a good fit when compared with observed curves at the incubation temperature/water potential treatment combinations for different after-ripening intervals. Changes in germination time course curves during after-ripening of B. tectorum can largely be explained by decreases in the mean base water potential. The simplicity and good fit of the model give it considerable potential for extension to simulation of after-ripening under field conditions.

Germination and survival of water-impermeable seeds of 11 species of Geraniaceae and Malvaceae were monitored during dry storage and during burial in soil for up to 2.5 years. During dry storage, seeds of annual Geraniaceae became permeable and also lost their physiological dormancy. However, during burial in natural conditions, most seeds remained impermeable and viable, with no seasonal change in germination capacity. Germination in only one species (Geranium robertianum) was enhanced by daily alternating temperatures when seeds were exhumed in spring. Drying of exhumed seeds broke physical dormancy. Seeds of the perennial Geranium pratense gradually became permeable in a prolonged germination test of 31 weeks. Most seeds of Malva remained impermeable during dry storage. Buried seeds gradually germinated in situ, and exhumed seeds had a low germination capacity in all seasons. We concluded that dormancy of hard seeds in natural conditions may be broken by drying during summer, by specific temperature regimes or by gradual softening of the seed coat, ensuring the spread of germination over many seasons.

This paper describes the dispersal–germination characteristics of seeds of 77 native tree species in a seasonal monsoon forest in Uttaranchal state, northern India. Results indicate that 50% of the species dispersed in the hot, dry summer months, 18% during the rainy season, 23% during the cold season and the remainder in late spring. Germination tests on fresh and laboratory-stored seeds revealed a relationship between morphological features of the fruit and both germination percentage and mean germination time (MGT). Highest mean germination (50%) was for dry-dehiscent fruits with winged wind-dispersed seeds, followed by dry-dehiscent fruits with non-winged seeds (38%) and seeds of dry-indehiscent fruits (37%). Lowest germination (29%) was for seeds from fleshy or pulpy fruits. MGT followed the reverse course. Germination data for seeds stored dry in the laboratory during one seeding cycle indicated six patterns of seed germination: (1) average germination percentage of fresh seeds lower than that of stored seeds, indicating an after-ripening requirement; (2) initial high germination percentage followed by low values, indicating a steep to moderate decline in viability following harvest; (3) no germination after 1 or 2 months of seed storage, due to complete loss of viability, indicating short seed longevity; (4) fresh seed germination in some species equalled the average germination value of stored seeds, indicating constant germination for one whole seeding cycle; (5) germination of both fresh and stored seeds remained consistently low throughout the season, indicating a requirement for some kind of pre-treatment or having poor quality of seeds; (6) initial high germination followed by decline and again increase, showing a seasonal rhythm of germination.

The breaking of dormancy in seeds can be elicited by many factors, including temperature and short exposure to low molecular weight amphipathic molecules such as primary alcohols, monocarboxylic acids and anaesthetics. Their action has been suggested to be mediated through effecting changes to membranes. Paradoxically, though, these molecules can inhibit the germination of some non-dormant seeds. Here, we review the structure–activity relationships between amphipathic molecules and dormancy breaking and, based on the known responses of membranes to them and to temperature changes, offer an alternative interpretation of the data and a new hypothesis to explain their action. We suggest that amphipathic molecules break dormancy by partitioning into the membrane, thereby increasing and optimizing phospholipid headgroup spacing. This, in turn, facilitates the binding and activation of a peripheral membrane protein component of a signal transduction pathway that is essential for the completion of germination. In cases where amphipathic molecules inhibit germination, it is predicted that they cause the optimal headgroup spacing to be exceeded, thus preventing subsequent association of the membrane with the peripheral protein component. The hypothesis is extended to explain membrane changes that can lead to dormancy breaking during dry after-ripening.

Experiments on the facultative winter annuals Carduus acanthoides and Anagallis arvensis were performed: (i) to determine thermal conditions that induce or release dormancy, (ii) to investigate to what extent changes in dormancy level resulting from those thermal conditions explain the seasonal pattern of emergence of these species, and (iii) to estimate required thermal time and base temperature for the germination of non-dormant seeds. Carduus acanthoides required high temperatures followed by decreasing temperatures for dormancy release; however, low winter temperatures did not induce secondary dormancy as expected for a winter annual. To the contrary, low temperatures stimulated dormancy release in the long term. In A. arvensis, dormancy relief was enhanced by dry storage at 25°C, and the response to low temperature was different depending on moisture conditions. Prolonged exposure to moist-chilling increased the dormancy level of the population, while dry storage at 4°C relieved dormancy. For both species, changes in the thermal range permissive for germination as a result of dormancy modifications explained to a large extent the timing of the emergence periods observed in the field. In neither species did base temperature for germination change with the dormancy level of the population. Thermal time required forgermination of C. acanthoides varied with dormancy, while for A. arvensis seeds it was constant.

Hydrothermal time (HTT) describes progress toward seed germination under various combinations of incubation water potential ( ) and temperature (T). To examine changes in HTT parameters during dormancy loss, seeds from two populations of the bunchgrass Elymus elymoides were incubated under seven temperature regimes following dry storage at 10, 20 and 30°C for intervals from 0 to 16 weeks. Fully after-ripened seeds were primed for 1 week at a range of s. Data on germination rate during priming were used to obtain a HTT equation for each seed population, while data obtained following transfer to water were used to calculate HTT accumulation during priming. HTT equations accurately predicted germination time course curves if mean base water potential, b(50), was allowed to vary with temperature. b(50) values increased linearly with temperature, explaining why germination rate does not increase with temperature in this species. b(50) showed a linear decrease as a function of thermal time in storage. Slopes for the T × b(50) relationship did not change during after-ripening. This thermal after-ripening time model was characterized by a single base temperature and a constant slope across temperatures for each collection. Because the difference between initial and final b(50)s was uniform across tempera-tures, the thermal after-ripening requirement was also a constant. When seeds were primed for 1 week at −4 to −20 MPa, accumulation of HTT was a uniform 20% of the total HTT requirement. When primed at 0 to −4 MPa, HTT accumulation decreased linearly with decreasing priming potential, and a hydrothermal priming time model using a constant minimum priming potential adequately described priming effects. Use of these simple HTT relationships will facilitate modelling of germination phenology in the field.

Seeds of Cenchrus ciliaris L. were produced under different hydro–photo–thermal environments with and without fertilizer. Dormancy loss of spikelets and extracted caryopses was tested during dry after-ripening at 40°C and 43% equilibrium relative humidity. Caryopses had higher initial germination and lost their dormancy faster than spikelets. Dormancy of both caryopses and spikelets generally decreased with an increase of maturation temperature and fertility, whereas dormancy increased if water stress was imposed during maturation. The latter effect was smaller when the mother plants were exposed to water stress after caryopses were fully formed than when water stress cycles were applied throughout maturation. Daylength extension (to 14 or 18 h d-1) by artificial light increased dormancy of both caryopses and spikelets. The effect of long days declined when plants were exposed to natural daylight for more than 10 h d-1. The after-ripening curves were consistent with the hypothesis that dormancy periods of individual seeds are normally distributed within each seed lot. Rates of loss of dormancy were quantified by the slopes of these curves. In a given experiment, these rates were identical for caryopses but not always for spikelets that matured in diverse environments. Even for caryopses, however, the slopes varied between experiments. Therefore, the results do not support the hypothesis that a dormancy model can be applied universally to all seed lots of Cenchrus ciliaris. Methods of predicting the period of after-ripening required to achieve desired levels of dormancy for reseeding degraded rangelands are discussed.