Effect of high temperatures and ash on germination of ten species from gorse shrubland

Seeds of ten species of Cistaceae, Ericaceae and Poaceae were examinated to compare their responses to high temperatures and ash, simulating the direct effect of fire on germination. A variable response to these factors between families and within species from the same family was detected.In Cistaceae, heat treatment pre-sowing stimulated germination. In Ericaceae, germination was stimulated by thermic shock, but a wide range of response was detected (between all four species), from Erica ciliari, very sensitive, to Daboecia cantabrica and Calluna vulgaris which showed no significant response. The range of reaction shown by the species of this family suggest that the size of the seeds is directly related to the difference in response, at least in species with a similar structure. In Poaceae, germination was not found to be dependent upon thermic shock.The effect of ash varies in magnitude. Although it decreased the germination percentage in all species, in Avenula marginata (Poaceae) the effect was not statistically significant, whilst in Ericaceae, particularly in those showing the highest levels of germination in the control (C. vulgaris and E. umbellata), the effect is to inhibit germination completely. In respect to the effect of ash the families could be arranged in the order Ericaceae>Cistaceae>Poaceae.Complex interactions between various dormancy types and seed size could explain these results. In species which depend on physical dormancy temperature has an important role. When other dormancy type is dominant in the control of germination, the medium characteristics could have a relevant role on seed response. In addition, the seed-size could determine the threshold of tolerance to temperature.     

Physiology of oil seeds: I. Regulation of dormancy in virginia-type peanut seeds

The germination and ethylene production by dormant Virginia-type peanut seeds were observed in relation to phytohormone treatments that could conceivably release the dormancy of these seeds. A comparison was made between the effects of these treatments on the less dormant apical seeds and the more dormant basal seeds. Indole-3-acetic acid did not stimulate ethylene production by, or germination of, the dormant seeds to any extent. Gibberellic acid at 5 × 10⁻⁴ M stimulated ethylene production by apical seeds to 17 millimicroliters per hour and germination to only 40% above the control. The more dormant basal seeds were affected even less by gibberellic acid than the seeds. Ethylene gas at 8 microliters per liter stimulated germination to 85% above the control for both apical and basal seeds. At this ethylene concentration the physiology of the more dormant basal seeds was altered, so that they behaved in a manner similar to the inherently less dormant apical seeds. 2-Chloroethylphosphonic acid at 10⁻³ and 5 × 10⁻⁴ M provided results similar to ethylene gas. Both apical and basal seeds germinated 100% at 48 hours. Among the phytohormones tested in this study, ethylene gas produced the greatest germination at low concentrations, and it appears must directly related to initiating the reactions required for converting the quiescent cells to an active state of growth.     

Effect of Temperatures on Dormancy and Germination in Three Species in the Lamiaceae Occurring in Northern Wetlands

In the temperate region temperature is the main factor influencing the germination period of plant species. The purpose of this study was to examine effects of constant and fluctuating temperatures on dormancy and germination under laboratory and field conditions in the three wetland species Lycopus europaeus, Mentha aquatica and Stachys palustris. The results should give indications if the temperature-dependent regulation of dormancy and germination is phylogenetically constrained. Tests for germination requirements showed a minimum temperature for germination of 9 °C in Mentha and 12 °C in Lycopus and Stachys, and a maximum temperature of 33 °C for Lycopus and 36 °C for Mentha and Stachys. Fluctuating temperatures promoted germination in all three species but the amplitude required for high germination (>50%) differed: it was 8 °C in Mentha, 10 °C in Stachys and 14 °C in Lycopus (mean temperature 22 °C). The effect of temperatures on the level of dormancy was examined in the laboratory by imbibing seeds at temperatures between 3 °C and 18 °C for periods between 2 and 28 weeks, as well as by a 30-month burial period, followed by germination tests at various temperatures, in light and darkness. In the laboratory only low temperatures (≤12 °C) relieved primary dormancy in seeds of Lycopus, while in Mentha and Stachys also higher temperatures lead to an increase of germination. Dormancy was only induced in Lycopus seeds after prolonged imbibition at 12 °C in the laboratory. Buried seeds of all species exhibited annual dormancy cycles with lower germination in summer and higher germination from autumn to spring. Exhumed seeds, however, showed considerable differences in periods of germination success. Dormancy was relieved when ambient temperatures were below 12 °C. Ambient temperatures that caused an induction of dormancy varied depending on species and test condition, but even low temperatures (8 °C) were effective. At high test temperatures (25 °C) in light, exhumed seeds of all three species showed high germination throughout the year. The three species showed various differences in the effects of temperatures on dormancy and germination. Similarities in dormancy and germination found among the species are in common with other spring-germinating species occurring in wetlands, so it seems that the temperature dependent regulation of dormancy and germination are related to habitat and not to phylogenetic relatedness.     

Role of temperature in the germination ecology of three summer annual weeds

Summary Ambrosia artemisiifolia L., Chenopodium album L., and Amaranthus retroflexus L. are three summer annual weeds that occur in disturbed habitats. In nature, the peak germination season for A. artemisiifolia and C. album is in early to mid-spring, while in A. retroflexus the peak germination season is late spring to early summer. Furthermore, seeds of A. artemisiifolia germinate only in spring, while seeds of C. album and A. retroflexus germinate throughout the summer. In an attempt to explain the differential germination behavior of these three species in nature, changes in their germination responses to temperature during burial in a non-heated greenhouse from October 1974 to October 1975 were monitored. A high percentage of the seeds of all three species after-ripened during winter. Seeds of A. artemisiifolia and C. album germinated at temperatures characteristic of those in the field in early and mid-spring, but seeds of A. retroflexus required the higher temperatures of late spring and early summer for germination. Seeds of all three species germinated to higher percentages in light than in darkness. Non-dormant seeds of A. artemisiifolia that did not germinate in spring entered secondary dormancy. On the other hand, seeds of C. album and A. retroflexus that did not germinate when temperatures first became favorable for germination, did not enter secondary dormancy and, thus, retained the ability to germinate at summer field temperatures during summer. Thus, temporal differences in the germination behavior of these three species are caused by the differential reaction of the seeds to temperature during the annual temperature cycle.     

Heat pre-treatment and the germination of soil- and canopy-stored seeds of south-western Australian species

The role of heat (shock) in stimulating the germination of soil-stored hard seeds from fire-following species is well known. However, the effects of high temperatures on germination of canopy-stored (serotinous) seeds are less well understood. In this study, we examined the effect of heat shock at four temperatures (60, 80, 100 and 120 °C) applied for 10 min on the germination of ten co-occurring Western Australian fire-following species (five hardseeded, five serotinous). Unlike previous studies, we distinguished between the effects of heat shock on germination rate, as well as total seedling emergence. In comparison with unheated controls, a heat pulse at one or more temperatures increased total germination and germination rate for three of the hardseeded species (Acacia pulchella, Daviesia cordata and Trymalium ledifolium). The precise pattern of germination response for D. cordata was influenced by whether we examined total germination or germination rate. Germination of four serotinous species (Calothamnus quadrifidus and three Hakea species) was unaffected by pre-treatments at one or more above-ambient temperatures. Only Allocasuarina humilis displayed both increased rate and total germination at higher temperatures. Our results show that germination in some serotinous species may respond favourably to, or at least be unaffected by, the passage of fire.     

Effects of forest fire ash on germination and early growth of four pinus species

Fire modifies the germination of seeds of numerous species. One of the fire factors that cause these modifications is the ash. This study analysed the germination of seeds of Pinus sylvestris L., Pinus nigra Arn. Pinus radiata D. Don and Pinus pinaster Aiton, subjected to different ash treatments, and the development of seedlings that grew in these conditions for 14 weeks. We obtained the ash by completely burning leaves and small twigs from the most abundant woody species in populations of pines. The ash treatments applied were as follows: Control (without ash), Low (half of the amount registered in a fire), Medium (equal to the amount registered in a fire) and High (double the amount registered in a fire). Germination took place on paper inside Petri dishes and in soil, and growth was only analysed for the plants that grew in the soil. For all species, and especially P. nigra, germination rate decreased as the amount of ash applied increased. The negative effects of the ash were more apparent following treatments in Petri dishes than in the soil. In the dishes, the average germination time varies little between the four species of pine. In soil, the average germination time is more prolonged, and only some differences were observed between treatments in P. sylvestris and P. nigra. In these species the high treatment significantly increased the average germination time. We found a greater sensitivity of P. sylvestris and P. nigra to the addition of ash, showing a higher seedling mortality rate. However, it appears that seedling development (length and weight) was not affected in any of the species through the addition of ash. These tests allow us to deduce that, in P. sylvestris, P. nigra, P. radiata and P. pinaster, the ash produced by forest fires has an inhibiting effect on germination and little effect on the development of seedlings in the first months of life. This revised version was published online in June 2006 with corrections to the Cover Date.     

Constraints on tree seedling establishment after fires: passing the germination bottlenecks

• Persistence and colonization by tree species in an environment following a fire depends on the effects on seed germination and seedling development. We used seeds of Kielmeyera coriacea and Qualea parviflora as a model to test the effects of high temperatures on germination and initial development of tree seedlings.
• We exposed the seeds to heat flow (70, 100, 130, 150 or 170 °C) for 2 or 5 min and compared the germination with that of unheated seeds (control). Seedlings were then harvested after 3, 7 or 15 days to evaluate aerial and root mass, root:shoot ratio, presence of cotyledon opening, true leaves, and secondary roots.
• We found no effect on germination for seeds exposed to temperatures ≥150 °C. However, germination was significantly reduced for seeds exposed to 100 °C for both 2 and 5 min. The mass of 15-day-old K. coriacea seedlings was smaller when seeds were heated at 70 °C for 5 min or at temperatures higher or equal to 100 °C. Qualea parviflora seedlings did not show any difference in mass, but there were marginal differences in the presence of roots and the opening of cotyledons. Kielmeyera coriacea seedlings allocated biomass faster than Q. parviflora.
• High temperatures affect both quantity and quality of germinable seeds, as well as biomass allocation during initial seedling development. These factors may explain the decrease in seedlings observed after fire, suggesting a bottleneck effect that influences population dynamics and species persistence in systems with frequent fires.

     

The effect of stratification temperatures on the level of dormancy in primary and secondary dormant seeds of two <Emphasis Type="Italic">Carex</Emphasis> species

The effect of temperature on the level of dormancy of primary and secondary dormant Carex pendula and Carex remota seeds was investigated. Primary dormant and secondary dormant seeds were stratified for 4 weeks at 5, 11, 13, and 15 °C, respectively, and tested for germination at 15/5 °C in light. To obtain secondary dormant seeds, primary dormant seeds were stratified at 5 °C and afterwards at 25 °C for 4 weeks. Germination tests were carried out in water and in 25 μmol KNO₃-solution to examine differences in sensitivity to nitrate between seeds relieved from primary and secondary dormancy. In both species, seeds with primary and with induced secondary dormancy showed no significant differences in germination. The two sedges showed significant differences in the effect of stratification temperatures between primary and secondary dormant seeds. Primary dormant seeds of C. pendula showed high germination (>80%) in nitrate-solution after stratification at all temperatures, while only temperatures of 5, 11, and 13 °C led to higher germination in nitrate-solution in secondary dormant seeds. Germination percentages of primary and of secondary dormant C. pendula seeds in water increased to a higher extent only after stratification at 5 and 11 °C; stratification of 11 °C was more effective in secondary than in primary dormant seeds. The only temperature that relieved primary dormancy in C. remota seeds was 5 °C where germination in water and nitrate-solution was >90%. Germination of secondary dormant seeds was increased by stratification at 11 °C independent of the test solution but higher germination after stratification at 13 °C occurred only in nitrate-solution. The results support the existence of physiological differences in the regulation of primary and secondary dormancy by temperature, and in the reaction of nitrate, at least in C. remota.     

The impact of temperature regimes on development,dormancy breaking and germination of dwarf shrub seeds from arctic,alpine and boreal sites

It has been suggested that the infrequent sexual reproduction of arctic dwarf shrubs might be related to the harsh environmental conditions in which they live. If this is the case, then increases in temperature resulting from global climate change might drastically affect regeneration of arctic species. We examined whether recruitment of Empetrum nigrum ssp. hermaphroditum and Vaccinium uliginosum (hereafter E. nigrum and V. uliginosum) was affected by temperature during three reproductive stages: seed development, dormancy breakage and germination. Seeds were collected from an arctic, an alpine (only E. nigrum) and a boreal site with different climates; stored at different winter temperatures and incubated for germination at different temperatures. Seeds of V. uliginosum developed in the boreal region had a higher percentage germination than did seeds developed in the Arctic. In contrast, seeds of E. nigrum from the arctic site had a higher or similar percentage germination than did seeds from the alpine and boreal sites. Increased winter temperatures had no significant effect on resulting germination percentage of E. nigrum. However, V. uliginosum seeds from the arctic site suffered increased fungal attack (and thus decreased germination) when they were stratified under high winter temperatures. Seeds of both species increased germination with increased incubation temperatures. Our results suggest that both species would increase their germination in response to warmer summers. Longer summers might also favour the slow-germinating E. nigrum. However, increased winter temperatures might increase mortality due to fungal attack in V. uliginosum ecotypes that are not adapted to mild winters.     

Factors affecting the dormancy and germination of bleeding heart [Lamprocapnos spectabilis (L.) Fukuhara] seeds

• Information on the optimal conditions to promote the germination of Lamprocapnos spectabilis (L.) Fukuhara seeds is limited; consequently, this study was conducted to establish the requirements to break seed dormancy and promote germination.
• The selected seeds had morphophysiological dormancy and had not begun embryo development. To study the dormancy breaking and embryo development processes, seeds were subjected to constant or changing temperature treatments during moist stratification.
• High temperature and humidity resulted in vigorous embryo growth, with the longest embryos occurring after 1 month of incubation at 20 °C. At 4 °C, the seeds required incubation period of at least 3 months to germinate. Embryo growth and germination were higher with changing high and low temperatures than under a constant temperature, and changing temperatures also considerably changed the endogenous hormone levels, embryo development and germination. Bioactive gibberellin (GA) content was higher in seeds incubated at 20 °C for 1 month, then at 4 °C for 2 months. The content of endogenous abscisic acid in seeds subjected to the same treatment decreased by 97.6% compared with that of the untreated seeds.
• Embryo growth and seed germination require changing high and low temperatures; however, exogenous GA₃ could substitute for high temperatures, as it also causes accelerated germination. In this study, the seeds of L. spectabilis were identified as an intermediate simple type, a sub‐level of morphophysiologically dormant seeds.

     

Photoperiod buffers responses to salt and temperature during germination of two coastal salt marsh colonizers Juncus acutus and Juncus maritimus

Juncus acutus and J. maritimus are two colonizers of coastal marsh rangeland with worldwide distribution. We tested the effect of salt, temperature, and photoperiod on the germination capacity of seeds of the two Juncus from the Rhône delta (south of France). We measured the first day of germination, the mean time, speed, and rate of germination on seeds subjected to five salinity levels, three Dark–Light temperatures and two D-L photoperiods (12–12 and 10–14). The 10–14 D-L photoperiod, although it corresponds to one of the two main germination periods of J. acutus and J. maritimus in the northern Mediterranean, has never been previously used to study their germination capacity. Analyses showed significant effects of salinity and temperature on the germination parameters tested, and for these two factors we found results comparable to those of previous studies. Salinity slowed down and reduced the germination process while spring temperatures had a positive effect. A surprising result was obtained by changing the D-L photoperiod from 12–12 to 10–14 which then largely buffered the negative effect of salt and high temperatures on germination. This capacity of J. acutus and J. maritimus to withstand salty conditions during the germination phase due to the spring photoperiod could be decisive in the ability of both species to colonize saline environments.     

Application of a probabilistic model for analysing the abortion of seeds and pods in winter oilseed rape (Brassica napus)

In winter oilseed rape (WOSR), only a subset of ovules can develop into seeds in the majority of pods. Any difficulty during the process of seed production may result in seed or pod abortion. This study aimed to reproduce the process of seed development in WOSR based on a limited number of parameters. As a result of the complexity of the developmental patterns of WOSR, it is challenging to identify the roles of various factors that influence seed production using an experimental approach. Here, we present a stochastic probabilistic model of seed development. The generalised least squares method was implemented to estimate the model parameters using the experimental data. Experiments were done in Grignon (France) in 2008 and 2009. The variations in the parameters were analysed according to the following four factors: year, pod rank, inflorescence position and ramification‐clipping treatment. The year had no effect on the number of ovules per ovary (μ) and the probability of seed viability (p). The proportion of effective pollen grains (k) significantly decreased with pod rank at the end of the main stem. Inflorescence position influenced the number of ovules per ovary (μ: 30.8–33.8 from top to bottom) and the parameter k. The mean number of seeds per pod on the main stem and the bottom ramification along the stem was larger than the other ramifications within one plant. Ramification‐clipping treatment increased the number of ovules per ovary (μ: 31 for control plants and 32 for clipped plants), the parameter k and the number of seeds per pod (p). This effect could be due to the competition for assimilates between the pods and seeds. Furthermore, the distribution parameters of the pollen number per stigma (m) remained stable, and the probability of pod survival (Bo) varied with different factors, including the year, pod rank and inflorescence position. Our results indicate that pollen germination is a factor that determines final seed number. This model can identify the impact of each of the factors that lead to the abortion of seeds and pods in WOSR, which include the position effect, assimilate competition and pollination limitation. However, further studies on the pollination process in WOSR should help to refine this model.     

Seed germination of Pilosocereus arrabidae (Cactaceae) from a semiarid region of south‐east Brazil

We evaluated the effect of temperature regimes (six constant and four alternating temperatures), light qualities (five red : far red ratios) and water potentials (ΨW; seven NaCl and polyethylene glycol 6000 [PEG] solutions) on the percentage and germination rate, as well as the post‐seminal development morphology, that allow Pilosocereus arrabidae seeds to germinate in a hot semiarid climate on the south‐eastern Brazilian coast. The results showed that seeds germinated similarly between constant and alternating temperatures, with an optimal germination at 25/20°C and 20°C. Pilosocereus arrabidae seeds were photoblastic positive and the final germination percentage was inhibited at low red : far red ratios. Maximum germination was obtained in distilled water (0 MPa) and decreases of Ψ_W in the solutions reduced the germination, which was lower in NaCl than in iso‐osmotic PEG solutions. Germination inhibition appears to be osmotic because the recovery response was high when non‐germinated seeds from both iso‐osmotic solutions were transferred to water. Seeds of P. arrabidae are small and germination is phaneroepigeal. Despite the slow growth typically seen in seedlings and adults of Cactaceae, germination in this species depends on the ability of the seeds to appropriately sense and react to environmental cues that correlate with times and places under low‐risk growth conditions.     

Comparative seed ecology of the endangered shrub, Pimelea spicata and a threatening weed, Bridal Creeper: Smoke, heat and other fire-related germination cues

Summary Pimelea spicata R. Br. is a nationally listed endangered Australian shrub threatened with extinction by habitat fragmentation and environmental weed invasion. Bridal Creeper (Asparagus asparagoides L. W. Wight) is the primary weed threat to the largest remaining populations of P. spicata in the Cumberland Plain. Fire, as part of an integrated pest management program, offers the potential to stimulate P. spicata populations while controlling Bridal Creeper. It is important, therefore, to understand how the components of fire affect the germination and growth of both species. Using laboratory experiments we investigated the effects of smoke, heat, ash and/or light on the germination of P. spicata and Bridal Creeper. We found a significant promotive effect of smoke and indication of an inhibitory heat shock (90°C for 10 min) effect on the germination of P. spicata seeds. The response of Bridal Creeper seeds to the same factors was complex; while the results of one experiment suggested an inhibitory effect of smoke and a promotive effect of heat, subsequent trials were contradictory, implying that Bridal Creeper, like many weeds, is able to germinate under a wide range of environmental conditions. Other experiments investigated the optimal germination temperature and innate dormancy of P. spicata in the absence of fire‐related germination cues. Of the incubation temperatures investigated, the optimal diurnally fluctuating regime for P. spicata germinations was 10°C and 20°C in the night and day, respectively. The innate dormancy of freshly produced seeds disappeared after 3 months. In contrast to Bridal Creeper, we found a persistent germinable seed bank of about 97 P. spicata seeds/m² located in the top 5 cm of the soil profile. While fire alone is unlikely to kill Bridal Creeper plants, fire may help to manage local infestations of the weed by limiting germination and providing opportunity for herbicide treatment of regrowth.     

Germination ecology of the weed Parthenium hysterophorus in eastern Ethiopia

Germination ecology of Parthenium hysterophorus, recently introduced to Ethiopia, was studied in a series of experiments. Viability of the seeds was greater than 50% after 26 months of burial in the soil indicating the potential build‐up of a substantial persistent soil seed bank. A short period of dry storage was sufficient to overcome a light requirement for germination in a minor fraction of the seeds. Likewise, seeds exhumed from burial showed an increase in germination ability in darkness over time, with a weak tendency for seasonal cyclicity in dormancy level at one of two sites. Germination occurred at the mean minimum (10°C) and maximum (25°C) temperatures of the collection sites, as well as over a wide range of fluctuating (12/2°C‐35/25°C) temperatures in light. No germination of P. hysterophorus seeds occurred at osmotic potentials < ‐0.52 MPa (at 27°C), the species being less tolerant to moisture stress than sorghum grains. Most seedlings emerged from shallowly buried (< 0.5 cm) seeds and none from more than 5 cm depth. Naturally dispersed seeds required about 60 days, at a hot lowland site, to start emergence despite the presence of adequate rainfall, and higher number of seedlings emerged in undisturbed plots than in hand hoed plots. These experiments and field observations suggest that there are no obvious climatic conditions that may limit the germination of Parthenium hysterophorus in Ethiopia, but a high moisture requirement of the seeds for germination could be the major factor limiting germination during the dry season.     

Effect of high temperature on moisture loss,imbibition and germination of seeds ofTrianthema Govindia Buch.-Ham. ex DC

Summary Seeds of erect and prostrate plants ofTrianthema govindia Buch. ham. ex DC., growing in shade and open respectively, differed significantly in seed weight and percentage germination. Effect of high temperature exposure to these seeds has been studied in view of water depletion, imbibition and seed germination. The seeds of both the types were subjected to temperatures of 40, 50, 60, and 70° C for 24, 48, 96, and 144 hours. The three factors viz., loss of water, water imbibition and germination of seeds were positively correlated to the duration of treatment at different temperatures. A highly significant positive correlation was also observed between moisture depletion and imbibition, and between imbibition and germination. The percentage germination was favoured at 40° C in both the types of seeds and was increased with the increase of treatment duration. However, at higher temperatures (50 and 60° C) the percentage declined while at 70° C the seeds lost their vitality.     

GERMINATION ECOLOGY OF PHACELIA DUBIA VAR. DUBIA IN TENNESSEE GLADES

The germination characteristics of a population of the winter annual Phacelia dubia (L.) Trel. var. dubia from the middle Tennessee cedar glades were investigated in an attempt to define the factor(s) regulating germination in nature. Factors considered were changes in physiological response of the seeds (after-ripening), temperature, age, light and darkness, and soil moisture. At seed dispersal (late May to early June), approximately 50 % of the seeds were non-dormant but, would germinate only at low temperatures (10–15 C). As the seeds aged from June to September, there was an increase in rate and total percent of germination at 10, 15, and 20 C, and the maximum temperature for germination increased to 25 C. Little or no germination occurred at the June, July, and August temperatures in 0- to 2-month-old seeds, even in seeds on soil that was kept continuously moist during this 3-month period. At the September, October, and November temperatures 3- to 5-month-old seeds germinated to high percentages. In all experiments seeds germinated better at a 14-hr photoperiod than in constant darkness. Inability of 0- to 2-month-old seeds to germinate at high summer temperatures allows P. dubia dubia to pass the dry summer in the seed stage, while increase in optimum and maximum temperatures for germination during the summer permits seeds to germinate in late summer and early fall when conditions are favorable for seedling survival and eventual maturation.     

Heat shock effects on seed germination of five Brazilian savanna species

Fire is considered an important factor in influencing the physiognomy, dynamics and composition of Neotropical savannas. Species of diverse physiognomies exhibit different responses to fire, such as population persistence and seed mortality, according to the fire frequency to which they are submitted. The aim of this study is to investigate the effects of heat shocks on seed germination of Anadenanthera macrocarpa (Benth.) Brenan, Dalbergia miscolobium Benth., Aristolochia galeata Mart. & Zucc., Kielmeyera coriacea (Spreng.) Mart. and Guazuma ulmifolia Lam., which are native species of the Brazilian savanna. The temperatures and exposure times to which the seeds were submitted were established according to data obtained in the field during a prescribed fire: 60 °C (10, 20 and 40 min), 80 °C (5, 10 and 20 min) and 100 °C (2, 5 and 10 min). Untreated seeds were used as controls. Seeds of A. galeata and K. coriacea showed high tolerance to most heat treatments, and seeds of A. macrocarpa showed a significant reduction in germination percentage after treatments of 80 °C and 100 °C. Treatments of 100 °C for 10 min reduced germination percentage for all species except G. ulmifolia, which has dormant seeds. For this species, germination was accelerated by heat treatments. The high temperatures applied did not interfere with the time to 50% germination (T₅₀) of the tolerant seeds. Seeds of the savanna species K. coriacea and A. galeata were more tolerant to heat shocks than seeds of the forest species A. macrocarpa. Guazuma ulmifolia, the forest species with seeds that germinate after heat shock, also occurs in savanna physiognomies. Overall, the high temperatures applied did not affect the germination rate of the tolerant seeds.     

A butenolide, isolated from smoke, can overcome the detrimental effects of extreme temperatures during tomato seed germination

The butenolide, 3-methyl-2H-furo[2, 3-c]pyran-2-one, is an highly active compound isolated from plant-derived smoke. This compound is known to stimulate seed germination in a wide range of plants akin to smoke or aqueous extracts of smoke. The present study attempted to elucidate the role of the butenolide in overcoming detrimental effects of low and high temperatures on tomato seed germination and seedling growth. The germination percentage followed a parabolic curve for temperatures ranging from 10 to 40°C, with 25°C being the optimum for all treatments. Control seeds showed radicle emergence at two extreme temperatures (10 and 40°C) and seedlings failed to develop further, even upon prolonged incubation. By comparison the butenolide-treated seeds grew into phenotypically normal seedlings at these non-optimum temperatures. The smoke–water-treated seeds had an intermediate response as only a fraction of germinated seed developed into normal seedlings. Seedling vigour indices as well as seedling weight were significantly higher (p ≤ 0.05) for butenolide-treated seeds at all temperatures. Furthermore, seedlings developed in the presence of the butenolide had about a 1:1 correspondence between root and shoot length. Butenolide-treated seeds grew better than the control seeds in the temperature shift experiments. A gradual decline in the vigour index values was recorded with an increased duration of incubation at the extreme temperatures. Results of the present study are very important from an horticultural point of view as they indicate the potential use of the butenolide compound in restoring normal seed germination and seedling establishment in tomato below and above optimum temperatures.     

THE GERMINATION STRATEGY OF OLDFIELD ASTER (ASTER PILOSUS)

At maturity in November, a high percentage of Aster pilosus Willd. seeds germinated in light at high temperatures (30/15, 35/20 and 40/25 C). Stratification during winter lowered the temperature requirement for germination, and high percentages of germination were obtained in light at 15/6 and 20/10 C., as well as at 30/15, 35/20 and 40/25 C. Stratification in darkness was completely ineffective, but stratification in light was partially effective in overcoming the light requirement for germination. Inability of seeds to germinate at low temperatures prevents germination after dispersal in late autumn and winter, when freezing temperatures could kill the seedlings. The lowering of the temperature requirement for germination during winter stratification allows the seeds to germinate and the resulting vegetative rosettes to become well established before the onset of the periodic summer droughts that occur in habitats occupied by A. pilosus.