Germination of <Emphasis Type="Italic">Sedum oxypetalum</Emphasis> (Crassulaceae) in a primary lava-field shrubland

Sedum oxypetalum is one of the dominant species in the xerophilous shrublands in the lava fields of the Basin of Mexico. Germination of this species was evaluated to understand its ecological response. We tested the effects of different pre-germination treatments (cold, and dry and wet heat) and storage time, as well as those of natural priming in two microhabitats with different disturbance levels. Experiments were performed in laboratory conditions under constant (25 °C) and fluctuating (20/30 °C) temperatures. Seeds did not germinate during burial and proved to be positively photoblastic. Under pre-germination treatments, final germination percentage was higher at 20/30 °C in seeds after four or more months of storage. None of the pre-germination treatments favored germination. Seeds can survive for more than 1 year and form a seed bank. Thus, seeds underwent natural priming that favored final germination percentage; however, germination rate and lag time were not affected by this process. In natural conditions, germination is delayed until the rainy season, improving the success of seedling establishment and growth. We discuss the role of fluctuating temperature in germination processes and the adaptations of seeds to their seasonal environment.     

Hormonal and temperature regulation of seed dormancy and germination in <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Fluctuating temperature plays a critical role in determining the timing of seed germination in many plant species. However, the physiological and biochemical mechanisms underlying such a response have been paid little attention. The present study investigated the effect of plant growth regulators and cold stratification in regulating Leymus chinensis seed germination and dormancy response to temperature. Results showed that seed germination was less than 2 % at all constant temperatures while fluctuating temperature significantly increased germination percentage. The highest germination was 71 % at 20/30 °C. Removal of the embryo enclosing material of L. chinensis seed germinated to 74 %, and replaced the requirement for fluctuating temperature to germinate, by increasing embryo growth potential. Applications of GA₄₊₇ significantly increased seed germination at constant temperature. Also, inhibition of GA biosynthesis significantly decreased seed germination at fluctuating temperatures depending upon paclobutrazol concentration. This implied GA was necessary for non-dormant seed germination and played an important role in regulating seed germination response to temperature. Inhibition of ABA biosynthesis during imbibition completely released seed dormancy at 20/30 °C, but showed no effect on seed germination at constant temperature, suggesting ABA biosynthesis was important for seed dormancy maintenance but may not involve in seed germination response to temperature. Cold stratification with water or GA₃ induced seed into secondary dormancy, but this effect was reversed by exogenous FL, suggesting ABA biosynthesis during cold stratification was involved in secondary dormancy. Also, cold stratification with FL entirely replaced the requirement of fluctuating temperature for germination with seeds having 73 % germination at constant temperature. This appears to be attributed to inhibition of ABA biosynthesis and an increase of GA biosynthesis during cold stratification, leading to an increased embryo growth potential. We suggest that fluctuating temperature promotes seed germination by increasing embryo growth potential, mainly attributed to GA biosynthesis during imbibitions. ABA is important for seed dormancy maintenance and induction but showed less effect on non-dormant seed germination response to temperature.     

Germination responses of Discopodium penninervium Hochst. to temperature and light

The germination responses of Discopodium penninervium were tested at different constant and alternating temperature regimes as well as under various light conditions both in the laboratory and glasshouse. Seeds incubated at 10, 15, 20, 25 and 30 °C failed to germinate. When the seeds were incubated at alternating temperatures of 20/12 °C and 30/12 °C under continuous light, germination was 89 and 61%, indicating that the species requires alternating temperatures as a cue for germination. However, germination declined as the amplitude of alternating temperatures increased from 8 °C and was completely inhibited at an amplitude of 23 °C, suggesting that the optimum amplitude is around 8 °C. Germination was less than 10% in light and nil in darkness at 20 °C in the laboratory. In contrast, seeds incubated at 20/12 °C germinated to 96 and 86% in light and darkness, respectively. Seeds incubated under leaf shade in the glasshouse failed to germinate whereas those incubated under direct daylight and darkness germinated to 44 and 50%, respectively, 30 days after sowing. When seeds incubated under leaf shade and in darkness were exposed afterwards to light, final percent germination was 83% from seeds incubated initially under direct daylight, 79% from those incubated under leaf shade and 86% from those incubated in darkness. The requirement for alternating temperatures and light rich in red:far red ratio to break the dormancy of seeds of D. penninervium could restrict germination to gaps in the vegetation. The results conform with the ecology of the species.     

Environmental cues for germination of the invasive bunch grass <Emphasis Type="Italic">Pennisetum ciliare</Emphasis> (L.) Link

Responses of seed germination to air temperature, water potential, light, and smoke were studied in the laboratory for seeds of the invasive bunch grass Pennisetum ciliare (L.) Link (syn. Cenchrus ciliare L.; buffel grass). First introduced to North America during the mid-twentieth Century for establishing pastures, this African bunch grass has become an invasive species of concern. Across all the experiments conducted, a low germination was observed for P. ciliare fascicles that never exceeded 30 % at 21 days after sowing. Optimal day/night air temperatures for germination, controlled with an environmental chamber, were 25/15 and 30/20 °C, while extreme temperatures of 15/5 and 45/35 °C inhibited germination. By sowing seeds of P. ciliare under different water potentials, created with aqueous solutions of polyethylene glycol, an optimum of ?0.03 MPa led to the highest germination, while no germination was observed at ?1.0 MPa. Monochromatic optical filters were utilized to germinate seeds under various wavelengths, of which red (650 nm) and far red (730 nm) led to the highest germination. In addition, seeds that were incubated in the dark had higher germination than those incubated under white light. Incubation in smoke water, which can stimulate germination of pyrophytic species, resulted in a marginal inhibition of germination compared with imbibition with distilled water.     

Germination behaviour of Conyza bonariensis to constant and alternating temperatures across different populations

Conyza bonariensis is one of the most problematic weed species throughout the world. It is considered highly noxious due to its interference with human activities, and especially the competition it poses with economically important crops. This research investigated the temperature requirements for seed germination of four populations of C. bonariensis with distinct origin and the influence of daily alternating temperatures. For this, a set of germination tests were performed in growth chambers to explore the effect of constant and alternating temperatures. Seeds of the four populations (from Lleida, Badajoz and Seville, Spain and Bahía Blanca, Argentina) were maintained at constant temperatures ranging from 5 to 35°C. The final germination and cardinal temperatures (base, optimum and maximum) of each population were obtained. We also tested the influence of daily alternating temperatures on final germination. To do so, seeds were exposed to two temperature regimes: 5/15, 10/20, 15/25, 20/30 and 25/35°C night/day temperature (intervals increasing 5°C, with constant oscillation of 10°C) and to 18/22, 16/24, 14/26, 12/28 and 10/30°C night/day temperature (intervals with average of 20°C, but increasing the oscillation in 4°C between intervals). In general, all populations behaved similarly, with the highest germination percentages occurring in the optimum temperature range (between 21.7°C and 22.3°C) for both constant and alternating temperatures. In general, climatic origin affected germination response, where seeds obtained from the coldest origin exhibited the highest germination percentage at the lowest temperature assayed. In addition, we observed that the alternating temperatures can positively affect total germination, especially in oscillations that were further from the average optimum temperature (20°C), with high germination percentage for the oscillations of 15/25, 20/30, 18/22, 16/24, 14/26, 12/28 and 10/30°C in all populations. The cardinal temperatures obtained were significantly different across the populations. These results provide information that will facilitate a better understanding of the behaviour of Conyza and improve current field emergence models.     

Response of Amaranthus retroflexus L. seeds to gibberellic acid, ethylene and abscisic acid depending on duration of stratification and burial

Freshly harvested, dormant seeds of Amaranthus retroflexus were unable to germinate at 25 and 35 °C. To release their dormancy at the above temperatures, the seeds were stratified at a constant temperature (4 °C) under laboratory conditions or at fluctuating temperatures in soil or by outdoor burial in soil. Fully dormant, or seeds stratified or buried (2006/2007 and 2007/2008) for various periods were treated with exogenous gibberellic acid (GA₃), ethephon and abscisic acid (ABA). Likewise, the effects of these regulators, applied during stratification, on seed germination were determined. The results indicate that A. retroflexus seed dormancy can be released either by stratification or by autumn–winter burial. The effect of GA₃ and ethylene, liberated from ethephon, applied after various periods of stratification or during stratification, depends on dormancy level. GA₃ did not affect or only slightly stimulated the germination of non-stratified, fully dormant seeds at 25 and 35 °C respectively. Ethylene increased germination at both temperatures. Seed response to GA₃ and ethylene at 25 °C was increased when dormancy was partially removed by stratification at constant or fluctuating temperatures or autumn–winter burial. The response to GA₃ and ethylene increased with increasing time of stratification. The presence of GA₃ and ethephon during stratification may stimulate germination at 35 °C. Thus, both GA₃ and ethylene can partially substitute the requirement for stratification or autumn–winter burial. Both hormones may also stimulate germination of secondary dormant seeds, exhumed in September. The response to ABA decreased in parallel with an increasing time of stratification and burial up to May 2007 or March 2008. Endogenous GA_n, ethylene and ABA may be involved in the control of dormancy state and germination of A. retroflexus. It is possible that releasing dormancy by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones.     

Gap-detecting mechanism in the seed germination ofMallotus japonicus (Thunb.) Muell. Arg., a common pioneer tree of secondary succession in temperate Japan

Germination responses ofMallotus japonicus (Thumb). Muell. Arg. seeds to temperature revealed a gap-detecting mechanism in the seed germination of the species. Among various constant and alternating temperatures examined in the range from 12–40°C, only very limited temperature regimes were found to be favourable for seed germination, specifically, alternating temperatures between 18–32°C and 28–40°C. A single several-hour higher-temperature (32–40°C) treatment could also induce the germination of seeds which had been imbibed for several days at a constant temperature in the range of 20–26°C, suggesting that there is a process requiring higher temperature among the overal germination processes. Seeds located at or near the surface of denuded soil would have a good chance of experiencing such a temperature change when several rainy days are followed by fine weather, while seeds beneath close vegetation would not. On the other hand, the pressence or absence of light or a simulated ‘canopy ligh’ had little effect on the germination. Therefore, it was concluded that the seeds ofM. japonicus have a ‘gapdetecting mechanism’ in the form of a higher-temperature requirement of a certain process involved in the overall germination processes.     

九里香种子脱水耐性和萌发生理的研究

九里香种子自花后 42～ 77d,含水量和电导率逐渐降低 ,种子干重、发芽率、发芽指数和活力指数逐渐增加。硅胶脱水 1～ 6d后 ,种子含水量下降 1 0 %～ 3 5 % ,发芽率、发芽指数和活力指数均有不同程度的降低 ,不同发育时期九里香种子的脱水耐性有别 ,花后 42～ 70d不断增强 ,77d有所减弱。花后 70d的种子含水量降至 1 0 % ,种子发芽率无明显降低 ;含水量为 9%的种子在 4°C和 2 0°C的低温条件贮存 3 0d和 42d ,多数种子仍能萌发 ,这表明九里香种子是一种正常型种子。光照能促进种子的萌发 ;在 2 0～ 3 0°C、室温和 2 0 /3 0°C变温条件下种子萌发较好 ;光照和温度对种子萌发有单独影响 ,但又相互作用 ,同时光照对萌发的影响还与种子含水量有关。     

The effect of Sea Water and Temperature on the Germination Behaviour of Crithmum maritimum

The seeds of Crithmmm maritimum L. were germinated floating on various concentrations of sea water up to 50% at constant temperatures of 5, 10, 15, 20, and 25°C and at alternating temperatures of 5 and 15°C. 5 and 25°C. and 15 and 25°C. Significantly higher germination was obtained at alternating than at constant temperature. When two constant temperatures at which no germination occurred were alternated, good germination was obtained. There was reduced germination and increase in time of first germination as sea water concentration increased, in the absence of sea water, high temperature caused not only severe inhibition of germination but also permanent injury to the seeds. The results help to explain the germination behaviour of the species in nature.     

Effects of light, temperature and water stress on germination of Artemisia sphaerocephala

Artemisia sphaerocephala is widely used for vegetation rehabilitation, but its germination is very low after air seeding of achenes. We explored effects of light, temperature and water stress on germination. Results show that both final percent germination and germination rate were increased by temperature increment, with the highest values occurring at 15: 25°C (night: day) in dark and 20: 30°C under light. Light inhibited germination, especially at lower alternating temperatures (5: 15°C and 10: 20°C). The alternating temperature window for germination was slightly narrower under light than in dark, and germination was slower under light than in dark across the temperature range. Achenes incubated in the dark and at constant temperatures had over 80% germination at 10 to 25°C, with an optimum at 20°C. Under dark and 25μmol m^‐2 s^‐1 light flux density at 10: 20°C, final percent germination was over 94%, but if the light flux density was increased to 100 and 400 μmol m^‐2 s^‐1, final percent germination was significantly lower (64% and 38% respectively). However, achenes could keep their germination capacity for a long time (over 50 days) and germinate well after going back to the dark. Germination was also lower under water stress and few achenes germinated at ‐1.4 MPa. This was more pronounced at high and low temperatures. Given these findings and the prevailing climatic characteristics, the most suitable time for air seeding of achenes may be mid‐May.     

Salinity and Temperature Effects on Germination for Three Salt-resistant Euhalophytes, Halostachys caspica, Kalidium foliatum and Halocnemum strobilaceum

The effects of three salinities (0, 100 and 500 mM NaCl) and four constant temperatures (10, 20, 30 and 35 °C) on seed germination of Halostachys caspica (M. B.) C. A. Mey., Kalidium foliatum (Pall.) Mop. and Halocnemum strobilaceum (Pall.) Bieb. were investigated. After seeds were treated with different concentrations of NaCl at constant temperatures of 10–35 °C for 16 days, ungerminated seeds were transferred to distilled water for 10 days to investigate the total germination; after this time, the ungerminated seeds from the 10 and 20 °C treatments were then moved to 35 °C for another 5 days to determine the final germination. The three plant species in the present experiment are salt-resistant euhalophytes growing in high saline soils in the Zhungur Basin in Xinjiang, a northwest province of China.Compared with germination under control conditions, germination percentages of all three species were not affected by 100 mM NaCl at 10–35 °C, while severely inhibited by 500 mM NaCl; germination percentages were very low at 10 °C up to 100 mM NaCl for all species; the optimum temperature for germination of H. caspica and K. foliatum was 20–30 °C, while 35 °C for H. strobilaceum, up to 100 mM NaCl; seeds did not suffer ion toxicity for all species, as evidenced by the high total germination after ungerminated seeds pretreated with 500 mM NaCl were transferred to distilled water at constant temperatures of 10–35 °C for 10 days, and the high final germination after the ungerminated seeds from the 10 and 20 °C treatments were subsequently moved to 35 °C for another 5 days; Halostachys caspica had greater sensitivity to increasing temperatures from 10 and 20 °C to 35 °C compared with the other two species.     

The Effects of Light and Temperature on Germination and Growth of Luffa aegyptiaca

The effects of light and temperature on the germination and growth of Luffa aegyptiaca were investigated both in the laboratory and in the field. The seeds germinated in both darkness and light but germination was better in the light. At constant temperatures germination was best at 21°C, while alternating temperatures of 21 and 31°C and 15 and 41°C caused higher germination than the most favourable constant temperature. Constant temperatures of 15 and 31°C and alternating temperatures of 21 and 41°C resulted in very low germination, whereas no germination occurred at 41°C and at alternating temperatures of 31 and 41°C. Soil depth caused only a delay in seed germination, as it did not affect the total germination. High temperature and high light intensity resulted in good seedling growth in terms of dry weight, leaf area and relative growth rate. High temperature and low light intensity caused increased plant height and high shoot weight ratio, both of which manifested in seedling etiolation. They also caused high leaf area ratio. Under low temperatures, irrespective of light intensity, growth was generally poor, but it was significantly poorer under low light intensity, which also caused high root weight ratio. High light intensity was principally responsible for high leaf weight ratio. The results help to explain the abundance of the species in newly cleared areas in Lagos and its environs.     

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.     

Germination characteristics ofDiamorpha cymosa seeds and an ecological interpretation

Summary Laboratory-stored seeds ofDiamorpha cymosa (Nutt.) Britton (Crassulaceae) were germinated at monthly intervals starting shortly after maturity in late May and ending at approximately the time germination is completed in the field (November). Seeds were placed at 5, 10, 15, 20, 25, 30, 15/6, 20/10, 30/15 and 35/20°C at a 14-hr photoperiod (12/12 hr thermoperiods at the alternating temperature regimes) and in constant darkness. In June, seeds were almost completely dormant and thus germinated poorly or not at all under all conditions. As seeds aged from late May to November 1. germination at the 14-hr photoperiod increased in rate and total percentage, 2. the maximum germination temperature increased from 15 to 25°C at constant temperatures and from 20/10 to 30/15°C at the alternating temperature regimes and 3. the optimum temperature for germination increased from 15 to 15–20°C at constant temperatures but remained at20/10°C at alternating temperature regimes throughout the study. During the same period germination in constant darkness was negligible at constant and alternating temperature regimes. This pattern of physiological after-ripening apparently is an adaptation to summer-dry,winter-wet habitats such as rock outcrops of southeastern United States.A short period of illumination with white light given after a 12-hr imbibition period in darkness promoted germination in the dark at 25/10°C but not at 15 or 25°C. A short period of illumination given during the imbibition period was much less effective in promoting germination in the dark. Drying up to 7 days did not cause light-stimulated seeds to lose their ability to germinate in darkness. The light requirement for seed germination probably does not play a role in restrictingD. cymosa to its well-lighted habitats on granite and sandstone outcrops.This research was supported by funds from the University of Kentucky Research Foundation and by an NIH Biomedical Sciences Support Grant to the University of Kentucky.     

Geographic variation in the flood-induced fluctuating temperature requirement for germination in Setaria parviflora seeds

Our aim was to search for specific seed germinative strategies related to flooding escape in Setaria parviflora, a common species across the Americas. For this purpose, we investigated induction after floods, in relation to fluctuating temperature requirements for germination in seeds from mountain, floodplain and successional grasslands. A laboratory experiment was conducted in which seeds were imbibed or immersed in water at 5°C. Seeds were also buried in flood-prone and upland grasslands and exhumed during the flooding season. Additionally, seeds were buried in flooded or drained grassland mesocosms. Germination of exhumed seeds was assayed at 25°C or at 20°C/30°C in the dark or in the presence of red light pulses. After submergence or soil flooding, a high fraction (>32%) of seeds from the floodplain required fluctuating temperatures to germinate. In contrast, seeds from the mountains showed maximum differences in germination between fluctuating and constant temperature treatment only after imbibition (35%) or in non-flooded soil conditions (40%). The fluctuating temperature requirement was not clearly related to the foregoing conditions in the successional grassland seeds. Maximum germination could also be attained with red light pulses to seeds from mountain and successional grasslands. Results show that the fluctuating temperature requirement might help floodplain seeds to germinate after floods, indicating a unique feature of the dormancy of S. parviflora seeds from floodplains, which suggests an adaptive advantage aimed at postponing emergence during inundation periods. In contrast, the fluctuating temperature required for germination among seeds from mountain and successional grasslands show its importance for gap detection.     

Environmental factors affecting seed germination of gray birch (Betula populifolia) collected from abandoned anthracite coal mine spoils in northeast Pennsylvania

Germination of gray birch (Betula populifolia) seed collected from anthracite mine spoils in northeastern Pennsylvania was studied. Environmental conditions of the spoil banks are such that high mortality may occur at seed and germination stages because of low moisture availability and thermal stress. The mine spoil banks are harsh environments with respect to key seed germination factors: percent soil moisture as low as 1.8% and soil surface temperatures reaching 59°C. In the field, gray birch typically germinated in mid-April prior to severe environmental stress. Trends in germination success were inversely related to rising soil temperature and decreasing soil moisture availability. Although seeds were capable of survival and germination under laboratory conditions of constant temperatures in excess of 55°C, dramatic decline in germination was observed under fluctuating temperature regimes likely to be experienced in the field. No germinations occurred under fluctuating temperatures in excess of 30°C. Germinations in the field were seen to end after mid-June when substrate temperatures exceeded 30°C.     

Seed germination ecology of Viola calaminaria,an endangered metallophyte with a narrow distribution

Viola calaminaria is an endangered metallophyte endemic to a small area close to the border between Belgium, Germany and the Netherlands, where it grows on rock outcrops rich in heavy metals (zinc, lead and cadmium). Because V. calaminaria reproduces mainly by seeds, it is of crucial importance to understand its germination requirements. Germination percentage and speed at constant (11–25°C) and alternating (23/09°C) temperatures were investigated in five large populations. Germination percentage was positively correlated to seed weight. Germination was low (<25%) at 11 and 16°C, intermediate (around 65%) between 20 and 25°C and the highest (93%) at the alternating temperature regime (23/09°C). V. calaminaria is a slow germinator requiring 41 days on average to germinate at 23/09°C and considerably more at 20 to 25°C (105 days on average). Our results also highlighted that the species is desiccation tolerant and can therefore be safely conserved under standard seed bank conditions.     

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.