Effects of temperature,light and gibberellic acid on the germination of seeds of 43 species native to Western Australia

Abstract. Species native to the southwest of Western Australia, representing a range of plant families, life-history strategies, fire-response syndromes, seed-store types and seed weights, were tested for viability using tetrazolium chloride and for germination under combinations of constant temperatures of 15 °C or 23 °C, constantly dark or 12 h diurnal whitelight conditions, and with, or without, addition of gibberellic acid (GA₃, 50 mg/l). Species previously known to require a heat-shock treatment to overcome dormancy due to an impervious testa were pre-treated prior to imposition of temperature, light and GA₃ conditions. The test environmental conditions related to differences between winter and autumn temperatures and surface and buried seed germination positions of post-fire habitats. The viability of the selection of native Western Australian species ranged from 0 to 100 %, averaging 71 %. For all taxa, no combination of temperature, light and gibberellic acid treatment induced all viable seeds to germinate. The greatest percentage germination achieved in any combination of treatments averaged 71 % of all viable seeds for all species. Larger seeds (> 10 mg seed weight) tended to have greater viability percentages, but no overall patterns of viability or germinability were attributable to seed storage syndrome, strategy of fire recovery response or life-form type. Germination of most of the dominant tree representatives (Eucalyptus calophylla, E. diversicolor, E. erythrocorys, E. gomphocephala, and E. patens) was indifferent to the trial conditions of temperature, light and GA₃. However, Eucalyptus marginata showed reduced germination in the light, which was overcome with GA₃. GA₃ also overcame the inhibition resulting from exposure to light in some understorey species (e.g. Allocasuarina campestris, Regelia ciliata, Xanthorrhoea gracilis and X. preissii). Germination of many hard-seeded, understorey shrub and herbaceous perennial species, especially those with small (< 10 mg) seeds (e.g. Bossiaea ornata, B. aquifolium and Acacia drummondii ssp. candolleana) was greater at the lower trial temperature and in the dark. Some large (> 10 mg) seeded, understorey species (e.g. Acacia extensa, Kennedia coccinea, K. prostrata, Hovea trisperma and Hardenbergia comptoniana) germinated in high percentages in both temperatures, but maximum germination percentages still tended to be at 15 °C. Large-seeded species were less sensitive to exposure to light compared to the smaller seeded species. The largest seeded species tested, Paraserianthes lophantha, germinated best in the warmer incubation temperature and in the light. The ecological significance of the tests would be that species which have seed dormancy mechanisms capable of delaying germination until the cool temperature, winter rainy period of this mediterranean-type climate would be more likely to survive than if germination followed summer rain showers or the first, intermittent rains of autumn. Burial of seeds becomes more important if germination occurs when rains first begin as this period has less available soil moisture and temperatures are high. Also survival of seedlings could be enhanced if germination of seed was restricted to the positions protected from high light, higher temperatures and lower soil moisture by the presence of a forest canopy. Therefore, seeds which have an ability to sense the presence of a previous fire in the habitat, conditions in light environment and appropriate temperature level have an adaptive advantage to time emergence to situations of time and space where survival is maximized. Variation in viability and germination percentages were apparent in some cases where more than one seed collection of available for testing, indicating that further aspects, such as seed age, maturity at collection, storage conditions and depth of seed dormancy, remain to be considered.     

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.     

Gibberellin-biosynthesis and -sensitivity mediated stimulation of seed germination of Sisymbrium officinale by red light and nitrate

Light stimulated seed germination of Sisymbrium officinale (L.) Scop, (hedge mustard) by means of two different mechanisms. Light effect I was absolutely dependent on the simultaneous presence of nitrate. Without nitrate, red (R) irradiated seeds did not escape from the antagonizing action of far-red (FR) irradiation. The data indicated that nitrate acted as a cofactor at the level of the FR absorbing form of phy-tochrome (P_fr). The combined action of R and nitrate could be replaced by addition of the gibberellins 4 and 7 (GA,₄₊₇). This action could be inhibited by the growth re-tardant tetcyclacis, an inhibitor to GA biosynthesis in cell free systems and intact plants. The action of tetcyclacis was fully neutralized by GA₄₊₇. It is concluded that the combination of R and nitrate stimulated GA biosynthesis. Omission of nitrate from the incubation medium enabled the study of light effects apart from G A biosynthesis. In such conditions R stimulated the sensitivity to GA₄₊₇, (light effect II). The two light effects could also be distinguished by their different reactions to the temperature of a pre-treatment in water and darkness. The sensitivity to R and nitrate was subject to breaking and induction of dormancy. Both processes were stimulated at rising temperatures. Due to a different optimum, breaking of dormancy prevailed at lower temperatures and induction of secondary dormancy at more elevated temperatures. The sensitivity to GA₄₊₇ rose and fell in a comparable way during dark incubation at a broad range of temperatures. The capacity of light to stimulate GA₄₊₇, action did not diminish at higher temperatures, it even tended to rise. The study indicated that seed germination is regulated by an increase in both the levels of GAs and the sensitivity to GAs.     

The use of population parameters in the analysis of germination of lettuce seed

Probit analysis was used to determine the parameters of the germinating population of lettuce seed. Seeds were induced to germinate under different conditions and it was shown that with light induced germination the germinating populations were the same. However, when gibberellic acid (GA₃) was the inductive agent then the germinating populations were not the same being concentration dependent. – This type of analysis offers a rigorous comparison of different inductive treatments.     

Morphological side effects of using gibberellic acid to induce germination: consequences for the study of seed dormancy

To assess the evolutionary significance of persistent seed banks, phenotypes of naturally germinating seeds must be compared with those that remain dormant under the same environmental conditions. Dormant seeds can often be induced to germinate by application of gibberellic acid (GA). However, this method is valid only if there are no phenotypic “side effects” of GA that could confound comparisons between dormant and naturally germinating seeds. We examined this assumption in Lesquerella fendleri, a short-lived perennial mustard of the desert Southwest. We exposed 3840 seeds from 16 maternal sibships to two different GA treatments (0 or 1 g/L GA) in two different germination environments (greenhouse and growth chamber), and measured germination and postgermination traits. As expected, application of GA increased germination. GA also had strong and long-lasting effects on seedling morphology. Seeds that received GA developed into seedlings that were taller, with fewer but longer leaves, than seeds that did not receive GA. Effects of GA on both dormancy and postgermination traits varied among maternal sibships. Our results indicate that for this species and this concentration of GA, morphological effects can be substantial. Further study is required to determine whether such side effects are found for lower concentrations of GA, or under conditions that encourage faster seedling growth. Nonetheless, the present results illustrate the importance of testing potential confounding effects of GA in studies of the evolution of seed dormancy and its influence on postgermination traits.     

Effect of light on seed germination of eight wetland Carex species

BACKGROUND AND AIMS: In wetland plant communities, species-specific responses to pulses of white light and to red : far-red light ratios can vary widely and influence plant emergence from the seed bank. Carex species are the characteristic plants of sedge meadows of natural prairie wetlands in mid-continental USA but are not returning to restored wetlands. Little is known about how light affects seed germination in these species-information which is necessary to predict seed bank emergence and to develop optimal revegetation practices. The effects of light on germination in eight Carex species from prairie wetlands were investigated. METHODS: Non-dormant seeds of eight Carex species were used to determine the influence of light on germination by examining: (a) the ability of Carex seeds to germinate in the dark; (b) the effect of different lengths of exposures to white light on germination; (c) whether the effect of white light can be replaced by red light; and (d) whether the germination response of Carex seeds to white or red light is photoreversible by far-red light. KEY RESULTS: Seeds of C. brevior and C. stipata germinated >25 % in continuous darkness. Germination responses after exposure to different lengths of white light varied widely across the eight species. Carex brevior required <15 min of white light for > or =50 % germination, while C. hystericina, C. comosa, C. granularis and C. vulpinoidea required > or =8 h. The effect of white light was replaced by red light in all species. The induction of germination after exposure to white or red light was reversed by far-red light in all species, except C. stipata. CONCLUSIONS: The species-specific responses to simulated field light conditions suggest that (a) the light requirements for germination contribute to the formation of persistent seed banks in these species and (b) in revegetation efforts, timing seed sowing to plant community development and avoiding cover crops will improve Carex seed germination.     

Abscisic acid regulates seed germination of Vellozia species in response to temperature

• The relationship between the phytohormones, gibberellin (GA) and abscisic acid (ABA) and light and temperature on seed germination is still not well understood. We aimed to investigate the role of the ABA and GA on seed germination of Vellozia caruncularis, V. intermedia and V. alutacea in response to light/dark conditions on different temperature.
• Seeds were incubated in GA (GA₃ or GA₄) or ABA and their respective biosynthesis inhibitors (paclobutrazol – PAC, and fluridone – FLU) solutions at two contrasting temperatures (25 and 40 °C). Furthermore, endogenous concentrations of active GAs and those of ABA were measured in seeds of V. intermedia and V. alutacea during imbibition/germination.
• Exogenous ABA inhibited the germination of Vellozia species under all conditions tested. GA, FLU and FLU + GA₃ stimulated germination in the dark at 25 °C (GA₄ being more effective than GA₃). PAC reduced seed germination in V. caruncularis and V. alutacea, but did not affect germination of V. intermedia at 40 °C either under light or dark conditions. During imbibition in the dark, levels of active GAs decreased in the seeds of V. intermedia, but were not altered in those of V. alutacea. Incubation at 40 °C decreased ABA levels during imbibition in both V. caruncularis and V. alutacea.
• We conclude that the seeds of Vellozia species studied here require light or high temperature to germinate and ABA has a major role in the regulation of Vellozia seed germination in response to light and temperature.

     

Diversity of germination strategies and seed dormancy in herbaceous species of campo rupestre grasslands

The effects of fire on the vegetation vary across continents. However, in Neotropical fire‐prone grasslands, the relationship between fire and seed germination is still poorly understood, while their regeneration, especially after strong anthropogenic disturbance, is challenging for their conservation. In the present study, we assessed diversity of germination strategies in 15 dominant herbaceous species from Neotropical altitudinal grasslands (locally known as campos rupestres). We exposed seeds to several fire‐related treatments. We also compared germination between regularly and post‐fire fruiting species. Finally, we investigated the diversity of dormancy classes aiming at better understanding the biogeography and phylogeny of seed dormancy. Germination strategies varied among families. Velloziaceae and Xyridaceae produced non‐dormant, fast‐germinating seeds. Cyperaceae and Poaceae showed an extremely low or null germination due to a high proportion of unviable or embryo‐less seeds. The seeds of campo rupestre grasslands are fire resistant, but there is no evidence that fire triggers germination in this fire‐prone ecosystem. Although heat and charred wood did not promote germination, smoke enhanced germination in one grass species and decreased the mean germination time and improved synchrony in Xyridaceae and Velloziaceae. Fire had a positive effect on post‐fire regeneration by stimulating fruit set in some Cyperaceae and Poaceae species. These species produced faster germinating seeds with higher germination percentage and synchrony compared to regularly fruiting Cyperaceae and Poaceae species. This strategy of dispersion and regeneration seems to be an alternative to the production of seeds with germination triggered by fire. Physiological dormancy is reported for the first time in several clades of Neotropical plants. Our data help advance the knowledge on the role of fire in the regeneration of Neotropical grasslands.     

光对种子萌发的影响机理研究进展

种子萌发是植物成功实现天然更新的关键环节, 需要适宜的温度、水分或光照条件。对于需光性种子, 光照是决定其萌发与否或萌发率高低的主要因素。光对植物种子萌发的影响不仅是一个复杂的生理过程, 也是受到调控的信号传递和基因表达过程。该文系统总结了影响种子萌发的光照属性、光与水/热耦合作用和种子的光属性(光敏色素)与种子萌发的关系, 明确了光调控种子萌发的生态意义; 重点综述了种子内光敏色素调控种子萌发的生理反应模式和光敏色素的光信号转导途径。试图为全面评估光对种子萌发的影响和将来开展更深入的研究提供参考。     

Relationships between seed germination requirements and ecophysiological characteristics aid the recovery of threatened native plant species in Western Australia

Summary One of the foremost technical issues addressed in reintroduction and restoration projects is the feasibility of establishing living plants. To advance the recovery process, the germination requirements of 201 threatened Western Australian seed‐bearing taxa representing a range of life forms, families and ecophysiological characteristics were studied. Procedures used to stimulate germination in otherwise dormant seed involved pretreatment using thermal shock, scarification, seed coat removal, soaking in an aqueous smoke solution and/or additions of the growth hormone gibberellic acid (GA₃). Sixty‐one taxa germinated under the basic trial conditions of light (12‐ h photoperiod), temperature (constant 15°C) and moisture, without additional pretreatments. These taxa were generally those with canopy‐stored seeds in the families Proteaceae and Casuarinaceae, and small‐seeded taxa in Myrtaceae. Taxa with soil‐stored seeds required single or multiple cues to stimulate germination. Seeds in the families Fabaceae and Mimosaceae were dependent on cracking of the seed coat, mechanically through nicking of the testa or through thermal shock, as were several non‐leguminous species of the Sterculiaceae and Liliaceae. Complete or partial removal of seed coats, in conjunction with GA₃ enhanced germination percentage in some taxa of the Myoporaceae, Lamiaceae and Myrtaceae. Application of GA₃ also enhanced germination percentage in members of the Epacridaceae. Several taxa previously stimulated by aqueous smoke solutions were equally responsive to additions of GA₃ after complete seed coat removal. In general, species with seed weights greater than 10 mg germinated better under a range of conditions than those with lighter seeds. There was no difference in germinability between resprouter and seeder species, and no obvious relationship between seed weight and germination rate. In the light of previous studies these results indicate that the relationship between germination requirements and ecophysiological characteristics is similar for both threatened and common species. These data will enable better prediction of likely dormancy breaking cues for other related species and will greatly assist the process of recovery and restoration work for mining operations and community bushland regeneration as well as single species reintroductions.     

Study on the mechanism of cerium nitrate effects on germination of aged rice seed

Attempts were made to promote germination of naturally and artificially aged rice seeds by treating them with cerium nitrate. The germination rate, germination index, and vigor index of aged rice seed were significantly increased by cerium. It was because the treatments of aged rice seed with cerium nitrate enhanced respiratory rate and activities of superoxide dismutase, catalase, and peroxidase, and decreased superoxide O₂ ⁻ and malondialdehyde contents that plasma membrane permeability was reduced. It was suggested that cerium be used for the seed treatment before sowing.     

Independent mode of action of cyanide and light on lettuce seed germination

Gaseous hydrogen cyanide stimulated subsequent lettuce seed ( Lactuca sativa L. cv. Grand Rapids) germination in darkness when applied for 1 or 22 h. Optimum concentrations were 5 × 10-5 M and 10-6 M, respectively. However, seeds did not germinate in the presence of HCN even at 10-8 M. The effects of unsaturating red light (R) and HCN (1 μM) showed a slight synergism. On the other hand, there was no difference between the effects of the sequences HCN – R and R – HCN. Stimulation of lettuce germination by an HCN pulse was practically not affected by far-red illumination, independently of the sequence of treatments. It was concluded that the primary stimulatory effect of HCN is of a regulatory character. Cyanide controls a regulatory point different from that affected by activated phytochrome.     

The role of light in regulating seed dormancy and germination

Seed dormancy is an adaptive trait in plants. Breaking seed dormancy determines the timing of germination and is, thereby essential for ensuring plant survival and agricultural production. Seed dormancy and the subsequent germination are controlled by both internal cues (mainly hormones) and environmental signals. In the past few years, the roles of plant hormones in regulating seed dormancy and germination have been uncovered. However, we are only beginning to understand how light signaling pathways modulate seed dormancy and interaction with endogenous hormones. In this review, we summarize current views of the molecular mechanisms by which light controls the induction, maintenance and release of seed dormancy, as well as seed germination, by regulating hormone metabolism and signaling pathways.     

The impact of experimental fire regimes on seed production in two tropical eucalypt species in northern Australia

Abstract This study investigated the effect of three experimental fire regimes on the fecundity, ovule development and seedfall of two common wet-dry tropical savanna eucalypts, Eucalyptus minima and Eucalyptus tetrodonta, in northern Australia. Both species flower early in the dry season and ovule development occurs during the dry season. This coincides with a period of frequent fires. The three fire regimes considered were applied for four years between 1990 and 1994. These regimes were (i) Unburnt, (ii) Early, fires lit early in the dry season, and (iii) Late, fires lit late in the dry season. The treatments were applied to nine catchments (15–20 km²) with each fire regime replicated three times. Fire intensity typically increases as the dry season proceeds. Therefore, early dry season fires generally differ from late dry season fires in both their intensity and their timing in relation to the reproductive phenology of the eucalypts. Late dry season burning significantly reduced the fecundity of both species, whereas Early burning had no significant effect. Ovule success was significantly reduced by the Early burning for both species. The Late burning significantly reduced ovule success in E. tetrodonta, but not in E. miniata. The results suggest that fire intensity and fire timing may both be important determinants of seed supply. Fire intensity may be a determinant of fecundity, whereas fire timing in relation to the reproduction phenology may have a significant impact on ovule survival. Both fire regimes resulted in a substantial reduction in seed supply compared with the Unburnt treatment. This may have a significant impact on seedling regeneration of these tropical savanna eucalypts.     

Penetration of light into soil and its role in the control of seed germination

Abstract Light transmitted through soil, and the leaf litter of two tree species, was measured using a spectroradiometer. In general, a greater penetration by longer wavelengths, especially far-red, was noted. This was most marked in a dry sand sample. The effect was less in the case of clay-loam aggregates. Moisture in the samples had an opposite effect in these two cases, causing an increase in transmission through sand but a decrease through clay loam aggregates. Reduction in particle size was found to reduce both the total light transmitted and the red/far-red ratio (R : FR). Red wavelengths were also more attenuated by a layer of freshly fallen oak (Quercus rotrur) leaves than were far-red wavelengths. A layer of Corsican pine (Pinus nigramaritima) needles however, was found to act as a neutral density filter over the 400 800 nm range. Prolonged exposure to soil-filtered light was found to affect the germination of seven species tested. Plantago major demonstrated an approximately linear decrease in germination with increasing depth. Rumex obtusifotius showed an apparent threshold response at 4 6 mm depth, as did Cecropia obtusifolia but at a slightly greater depth. Digitalis purpurea germinated very poorly in darkness, yet was extremely photosensitive with very high germination even at 10 mm depth. Galiutn aparine and Chenopodium album showed a two-phase response with germination reaching a peak at 2 mm depth. The implications for the function of the photoreceptor phytochrome in the control of germination are discussed in relation to soil-transmitted light and with regard to the results of the germination tests.     

Study of the inheritance of seed primary dormancy and the ability to enter secondary dormancy in Petunia: influence of temperature, light and gibberellic acid on dormancy

Abstract. The germination behaviour of two Petunia hybrida lines. M₃₀ and Th₇, and their reciprocal hybrids was studied. Two sets of experimental conditions appeared helped to distinguish between dormant and non-dormant parental lines: (1) 25 and 35 °C in the dark, in the latter case after 2 months of dry storage at 20 °C; (2) 35 and 40 °C in the light. Photosensitivity was tested in the first case and sensitivity to GA₃ in the second case. The predominance of paternal control over dormancy was evident. A maternal or tegumentary control of photosensitivity and of sensitivity to GA₃ was also shown. Transferring the seeds, originally imbibed in conditions expressing primary dormancy, to conditions which previously supported their germination, allowed us to show that secondary dormancy could be easily induced when a deeper primary dormancy had already developed in the seeds.     

Seed production and germination in two rare and three common co‐occurring Acacia species from south‐east Australia

Abstract Seed set, size, viability and germination requirements were investigated for two rare (Acacia ausfeldii and A. willianisonii) and three common (A. pycnantha, A. genistifolia and A.paradoxa) co‐occurring congeners in box‐ironbark eucalypt forests near Bendigo, south‐east Australia to investigate correlates of rarity. Seed size was significantly smaller for the two rare species and germinants were less able to emerge from deeper sowing depths than were the larger seeded common congeners. All species had a strong heat‐stimulated germination response. While the rare A. ausfeldii showed strong germination only at the highest temperature treatment (100°C), the common and widespread A.pycnantha showed strong germination across a broad range of temperatures (60‐100°C), likely to be experienced by soil‐stored seeds during a fire. Seed viability, number of seeds per plant, and number of firm, aborted and eaten seeds per pod varied between species, but the pattern of variation was not related to rarity. Small seed size and a very specific temperature requirement for germination may help to explain rarity in A. ausfeldii, and to a lesser extent in A. willianisonii. Fires are often patchy and heating of the soil is likely to be highly spatially variable, so species with germination responses to a broad range of temperatures have an advantage over those that respond only to a narrow range. A narrower range of soil depths from which seeds can emerge will further reduce the proportion of the seed bank that might recruit following fire. Human impacts on species habitats, such as fragmentation, loss of topsoil through mining, timber harvesting, grazing and urbanization, and consequent reduction in fire intensity, are likely to have further contributed to rarity in these species. The role of pollination and other factors in relation to population size is the subject of further investigation.