Polyploidy,flowering phenology and climatic adaptation in Heteropogon contortus (Gramineae)

Chromosome numbers of 2n = 39, 40, 50, 60, 69, 70, 80 and 90 are reported for Heteropogon contortus from a world collection. The numbers 2n = 39, 69, 70 and 90 are new to the literature. The cytogeographic distribution indicates that the tropical latitudes are almost exclusively occupied by tetraploids while the sub-tropical latitudes are characterized by a wide range of ploidy levels from tetraploid to nonaphid. Observations on the time of flowering of these accessions using uniform grass garden techniques indicate that pan-tropical populations are made up almost exclusively of late flowering lines and those from subtropical areas are of mixed lines showing great diversity from early to late flowering types. The association of polyploidy and flowering time are discussed in relation to the possible origin of H. contortus, to its adaptation and migration. It seems reasonable to suggest that polyploidy and the development of an earlier flowering response are both adoptively advantageous and have been selected for the species’migration into sub-tropical latitudes.     

Seed germination in Gactaceae

The mode of germination of representatives of 89 genera of the Cactaceae, 4 genera of Portulacaceae and 1 genus of Phytolaccaceae was studied. Most of the species of the Cactaceae germinate by means of a seed lid (operculum). In the Cactaceae studied 11 kinds of germination could be distinguished, 3 of which were with, and 8 without, operculum formation.
Opercula are restricted in their occurrence to the subfamilies Cactoideae (Cereoideae) and Pereskioideae and are not found in the subfamily Opuntioideae. Within the subfamily Cactoideae operculum formation was found to occur in all tribes and in all investigated subtribes. Opercula were also found in two genera of the related family of the Portulacaceae. In the Phytolaccaceae no operculum formation was observed.     

Seed germination as a thermobiological problem

Summary Thermal effects on seed germination are considered through the changes brought about by temperature in the germination capacity, in the germination rate, and in the distribution of the relative frequency of germination along the incubation time. A number of questions of general thermobiological interest are thus raised, entailing the need of an analysis of the temperature dependence of the seed germination rate.A treatment of these rates by the activation-energy approach cannot be general, for their Arrhenius plots are not always linear. Moreover, it is shown that any process displaying a temperature optimum (as happens in the germination of most seed species) cannot follow one of the fundamental tenets of the collision rate theory. The need of a theoretical treatment stressing the essential role of the partition of energy within the seed system has led to an analysis using the absolute reaction rate theory. New experimental prospects for the physiology of seed germination are thus raised, concerning the meaning of the temperature cardinal points, the growth pattern of the embryo in germinating seeds, the dual effect of protein thermodenaturation, the effects of high hydrostatic pressures, and a whole pharmacological line of work.The cybernetic counterpart of the thermodynamic view of seed germination appears in the study of the distribution of the relative frequency of germination along the isothermal incubation time. In some species of seeds the thermal communication between the environment and the seed growth effector can be shown to proceed by molecular collisions at all germination isotherms. In the seeds ofDolichos biflorus this communication through random thermal noise prevails only at temperatures close to both extreme limits of germination. Both in this species and inCalotropis procera there is a temperature range (encompassing the optimum) within which a temperature signal is superimposed upon the gaussian noise. An interpretation is proposed according to which the temperature signal is transduced in a protein-conformation code.     

Seed germination traits of desert perennials

While understanding that seed germination is crucial for ecological restoration activities, the seed traits of desert perennials are understudied. We experimentally determined germination traits of 43 species from 14 families from Hummock grasslands in the Great Sandy Desert, Australia. We defined morphological and physiological seed traits of framework species required for restoration and investigated the effects of fire and temperature on seed germination. We classified dormancy and explored the effect of Karrikinolide, a fire cue derived from smoke, on germination. Seeds of 38 (88%) out of 43 species were dormant: 13 (30%) with physical and 25 (58%) with physiological dormancy. Karrikinolide promoted seed germination of 9 (21%) species across all life-forms except trees, and widened the range of germination temperatures and increased germination rate of one species. Although high germination percentages were obtained over a wide temperature range, germination rate was affected by temperature. Non-dormant seeds and seeds pre-treated to overcome physical dormancy germinated quickly, with times to 50% germination of 1–5 days. Dormancy class differed between life-forms and families. Fast germination of non-dormant seeds is a trait that allows seeds to germinate during short periods of moisture availability. An absence of under-developed embryos is consistent with the global trends for hot deserts. A response to Karrikinolide shows that seed germination is related to a fire cue. These results will inform land managers of effective seed pre-treatments prior to seed broadcasting for restoration, and information on seed germination temperatures and rates will improve the understanding of when and where seeds could germinate in restored sites.     

Seed germination temperatures of eight Mexican Agave species with economic importance

The genetic diversity of Agave plants is threatened by clonal commercial reproduction and climatic change. Sexual reproduction is uncommon and research on seed germination is scarce. The present study evaluated the seed germination of Agave lechuguilla, Agave striata, Agave americana var. marginata, Agave asperrima, Agave cupreata, Agave duranguesis, Agave angustifolia ssp. tequilana and Agave salmiana at constant temperatures (10, 15, 20, 25, 30, 35 and 40°C). Initial imbibition (after the first 12 h) was significantly variable among species, positively correlated with seed weight (r = 0.6560, P < 0.001) and increased with temperature (from 35% at 10°C to 66% at 40°C). Temperature affected maximum imbibition (83–150%) for A. asperrima, A. lechuguilla, A. salmiana and A. striata; other species averaged 110%. Most germination kinetics best fitted a logistic model, whereas only a few treatments fit a Weibull model. The time to germination onset diminished (P < 0.05) from 125–173 h at 15°C to 68–84 h at 25°C, and then ascended to 84–196 h at 35°C. The mean germination rate and seed germination percentage after 312 h peaked at 25°C (0.50–0.95% seeds/h and 85–99%, respectively) and fell (P < 0.05) to near zero at 10 and 40°C. Temperatures of 10, 35 and 40°C were partially lethal to A. asperrima, A. duranguensis and A. salmiana seeds. The time to germination onset, seed germination percentage after 312 h and mean germination rate are best described by a Gaussian distribution, with its optimum at approximately 25°C. Thus, optimum temperatures are related to the ecological characteristics of each species area.     

Seed germination strategies of five perennial weeds

Summary The germination responses of seeds of Achillea millefolium L., Artemisia vulgaris L., Cirsium arvense (L.) Scop., Taraxacum officinale Weber, sensu lato, and Tussilago farfara L. to light, nitrate, alternating temperatures, chilling, light quality, and water availability were studied in laboratory tests, using fresh seed and seed stored for 6 months at 27°C and buried in the soil. A factorial experiment with light, nitrate, alternating temperatures, and seed age as factors found that all four affected germination except in T. farfara. All three external factors were stimulatory, especially in combinations. Fresh seed of A. vulgaris and C. arvense showed a light x alternating temperature synergism, responded to chilling, and after-ripened in cold dry storage. That of T. farfara had no dormancy and rapid germination, and germinated well on substrates with a water content too low for the other species. Seed of A. millefolium and A. vulgaris had good survival in both experimental storage conditions, while that of C. arvense and T. officinale did not, and that of T. farfara did not survive. Longevity in both conditions was associated with depth of initial dormancy. The two conditions caused different changes in dormancy in both A. vulgaris and A. millefolium. The germination behaviour, and the size, morphology and dispersal of the seeds of the species are discussed as strategies adapted to intermittently available situations for seedling establishment.     

Seed germination ecology in southwestern Western Australia

Germination responses of species from the native plant communities of southwestern Western Australia can be related to syndromes of life history, fire response, and seed storage, and also to factors related to environmental stress. The Mediterranean-type climate of the region with periodic drought and recurrent fires affects the production of viable seeds in plants of limited stature and rooting depth. Fire response ephemerals and species cued to flower by fire tend to produce viable, readily germinable seeds, but there are instances where seed production is aborted in these predominantly herbaceous life forms. Clonal, rhizomatous species often produce mainly inviable seeds. Production of viable seeds in woody species of these highly diverse communities may also be restricted by limitations to cross pollination. Obligate post-fire seeding species tend to produce a greater proportion of viable seeds than species which are capable of resprouting following fire. Serotinous species, whether post-fire re-seeders or post-fire resprouting species, produce mainly viable seeds, which germinate readily once freed from protective fruits. Species of the legume families and a few others of the soil seed bank produce innately dormant seeds which can be germinated following heat shock treatments which simulate the effects of fire. Heat shock in these species appears mainly as a mechanism to crack the hard seed coats, but the effect of heat to denature seed coat inhibitors has not been eliminated. Western Australian species do not seem to break dormancy when exposed to leachates from burned wood as has been observed in comparable habitats in California and South Africa, but further research is advised. Germination in many native southwestern Australian species is cued by temperatures that correspond to the winter rainfall period. There are also indications that an after-ripening period of warm, dry storage increases percentage of germinable seeds. Stimulation of germination by hormones is almost unresearched in Western Australia, but germination percentages have been increased in a small number of species of horticultural potential. Stimulation of germination by soil nutrient concentrations is almost unresearched in Western Australia, except for the inhibitory effect of excess sodium chloride levels inEucalyptus andMelaleuca. These species only germinate when osmotic effects are reduced to lower levels as would occur when winter rains dilute soil salts. Application of research on seed germination has already enhanced the establishment of seedlings in the restoration of mine sites and is becoming important in aspects of the breeding and selection of native plants for the cut flower, bedding plant and essential oil industries.     

Field studies on inhibition of Haemonchus contortus.

The epidemiology of Haemonchus contortus was studied in north east England in lambs reared under field conditions with infected ewes. The parasite population was monitored by pasture larval counts, faecal egg counts and lamb postmortems. From 25 June to 11 November, eight lambs were slaughtered each month, together with eight worm-free tracer lambs. Postmortem worm counts showed that on 25 June, small numbers of adult H. contortus were present but no inhibited larvae. However, subsequently the number of worms recovered rose rapidly to 1000-5000 and the percentage inhibition increased to 57% in July, 75% in August and virtually 100% in September, October and November. On each sampling occasion no significant differences in worm count or percentage inhibition were recorded between experimental and tracer lambs. These results suggest that neither autumn climatic effects nor host immunity were responsible for inhibition in this strain of H. contortus.     

Seed dormancy and the control of germination

Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.     

种子萌发对策:理论与实验

植物种子的萌发／休眠现象有复杂的原因和机制,综述了理论生态学家的研究结果。应用的理论基础是最优化理论和进化稳定对策(Ess)理论。当环境条件随机波动,种群受非密度依赖因素调节时,采用最优化理论的两头下注对策预测休眠一定会得到进化且萌发率与环境条件直接相关。环境条件稳定时采用进化稳定对策理论可得到在亲属竞争,种子扩散,基因冲突等等因素影响下的进化稳定休眠／萌发率,预测了休眠／萌发与它们之间的相互关系。以上各种环境条件影响种子萌发行为的方式可以表述为若种子立即萌发会遭遇到不良环境使适合度下降,那么就会推迟萌发,出现休眠,形成土壤种子库。萌发率应使种群适合度最优或具有进化稳定性。一些实验也部分验证了理论预测。     

Long‐distance dispersal of Black Spear Grass (Heteropogon contortus) seed on socks and trouser legs by walkers in Kakadu National Park

Humans can contribute to the long‐distance dispersal of many plants, including weeds. We assessed the distance for which seed remained attached to the socks and trouser legs of walkers. The experiment in Kakadu National Park, Australia used seed of the Australian native Black Spear Grass (Heteropogon contortus) as a surrogate for the potential dispersal of weed seed. Two models were fitted to the data, with a double exponential model fitting the data slightly better than a power exponential model. Although 19% of seed were dispersed within the first 5 m of walking, most (55%) seed remained attached to socks and trousers at 5 km. Humans may, therefore, unintentionally carry and eventually disperse this grass and other invasive grasses with similar long awns and pointed seeds over long distances. Implementing strategies to reduce the potential for seed to attach to walkers (gaiters) and reduce potential dispersal (removal and careful disposal of seed) would reduce the risks posed by this type of human‐mediated seed dispersal.     

Seed germination ofGaleola altissima,an achlorophyllous orchid,with aphyllophorales fungi

Seed germination test ofGaleola altissima was carried out with five aphyllophorales fungi:Erythromyces crocicreas, Ganoderma australe, Loweporus tephroporus, Microporus affinus andPhellinus sp.. All five species were effective for seed germination of the orchid.Erythromyces crocicreas, which has hitherto been regarded as the only endomycorrhizal fungus of the orchid, was confirmed to be effective for further development of the orchid.     

Aids to seed germination studies

Summary Two aids to seed germination studies are described. The first, a closed unit, enables rapid and effective surface sterilisation of small fragile seeds to be carried out. The second, for use in field situations, facilitates a given number of seeds being placed under natural conditions. The design of the apparatus causes minimal interference with the microclimate and prevents seed loss by natural vectors and seed input from indigenous species adjacent to experimental areas.     

假酸浆种子发芽特性研究

在不同温度、不同贮藏时间和方法下,对来源于印度尼西亚的假酸浆种子发芽特性进行研究。结果表明:不同温度对假酸浆种子萌发的影响差异极显著,在15/25℃变温条件下种子发芽率最高,为84.3%,发芽快且整齐;假酸浆种子采收后立即播种其发芽率非常低,室温下贮藏6～9个月时在25℃恒温或15/25℃变温条件下均有较高发芽率,说明假酸浆种子有休眠性,通过延长贮藏时间能打破休眠,促进种子发芽;但室温贮藏15个月后,种子活力下降非常明显。低温冷藏在一定条件下能提高假酸浆种子萌发能力,能延长种子寿命。     

Seed dimorphism and germination behaviour in Salicornia patula

One of the annual species of the genus Salicornia from the Mediterranean coast, Salicornia patula presents a seed dimorphism. This morphological seed dimorphism is linked with a physiological ‘dimorphism’ with regard to some requirements for germination. Central seeds, in the inflorescence, are larger, are dispersed attached to the perianth, have no light requirement for germination and seem to have a low germination response to salinity. The lateral seeds, which are free from perianth envelopes, require light and cold pretreatment and seem to have a high sensitivity to salinity. Salicornia patula colonizes unstable transition zones between permanently flooded muds and perennial vegetation. These zones have a winter flooding period. The beginning of the flooding period and of the dry period may vary from year to year, depending on the erratic distribution of autumn and winter rains on the Mediterranean coast. So, in some years, a seedling population may be destroyed before reaching the stage of seed production. Seed dimorphism might cause germination to the more distributed in time, thus reducing the chances of extinction of a complete generation. In the present paper salinity and temperature response are described more precisely, are investigated, some aspects of the longevity of the two types of seed, and the adaptative significance of the corresponding dispersal and germination syndromes are discussed in more detail.     

Comments on Seed germination as a thermobiological problem

In the above mentioned article [1] the notion of a time-dependent Gibbs free energy has been introduced to explain the observed time-pattern of embryo growth in seeds. Furthermore, the notion of a 'non-random thermal communication' has been inferred from an inspection of the shapes of germination time distributions. It can be shown, however, that the reasoning leading to the time-dependent thermodynamic potential is based on inappropriate interpretation of kinetic equations, and that the shape of the distributions of germination time might be a natural consequence of the initial distribution of embryo size in the seeds.     

濒危植物合柱金莲木种子萌发特性

研究了合柱金莲木种子萌发的生物学特性,以了解该物种种群自然更新困难的原因和机制。结果表明:合柱金莲木种子萌发的适宜温度为25℃,萌发率为56.67%,在20℃和30℃条件下,萌发速度和萌发率均较低,且在30℃条件下,幼苗不能正常生长,萌发在15℃受到抑制。土壤含水量在20%～30%,种子正常萌发,随着土壤含水量的升高,萌发开始时间缩短,萌发速度加快。种子萌发需要光照,为需光种子。种子在河沙、粘质壤土、沙土3种不同的基质中萌发率并无显著差异,但在粘质壤土和沙土中生长的幼苗长势较好。不同地理种源的种子,其种子质量存在差异,融水种群种子萌发率高,幼苗长势好。合柱金莲木种子萌发速度慢,萌发不整齐,幼苗生长缓慢,使得其在种间竞争中处于不利地位,且种子萌发对温度的适应范围狭窄,再加上林下光照不足限制种子萌发,这些因素是导致该物种自然更新困难的重要原因。