Photo-thermal regulation of seed germination in natural populations of Eruca sativa Miller (Brassicaceae)

Populations of the winter annual Eruca sativa in Israel are distributed along a climatic gradient, from Mediterranean mesic habitats in the north to semi-arid and arid habitats in the south. Seed cohorts of five populations sampled across the gradient were produced in a common-garden experiment, and their germination was tested under continuous light or darkness at 5–35 °C, as well as with alternating 30/20 and 25/15 °C temperatures under a 12/12 h photoperiod. Germination was enhanced by light and by increasing temperature. Under constant temperatures maximal germination of arid and semi-arid populations was lower than in the mesic populations, suggesting on higher levels of primary dormancy with increasing aridity. Seeds that did not germinate in darkness or at low temperatures in light, germinated almost fully when exposed to GA₃ at 25 °C, indicating that imbibition under less favorable conditions induces secondary dormancy in E. sativa. Daily alternating temperatures did not affect or even reduced seed germination compared to constant 25 and 30 °C in continuous light. Exposure of dry seeds to 30 °C before imbibition increased germination to ≥80% in seed cohorts of all populations. These responses to light and temperature indicate that adaptation of E. sativa to the Mediterranean environment is associated with the ability to germinate under the relatively warm autumn temperatures, and is enhanced by exposure to high temperature during the dry summer.     

Environmental control of seed dormancy and germination in the short-lived Olimarabidopsis pumila (Brassicaceae)

The purpose of our study was to better understand seed germination ecology of the spring annual short-lived Olimarabidopsis pumila, which grows in the Gurbantonggut Desert, China. Seeds underwent after-ripening at 4, 20 ± 2 (room temperature) and 30 °C. After dormancy was broken, germination capacity was a function of temperature and presence of light. For the temperature range studied (4–30 °C), germination capacity was significantly higher between 15 and 25 °C than at temperatures above or below them. Alternating temperatures of 20/10, 25/10 and 25/15 °C were favorable for germination. Although GA₃ did promote dark germination of seeds, GA₃ did not replace light for germination. Seeds germinated to >75% if light exposure time was over 8 h and temperature and moisture conditions were favorable. Seeds were able to germinate at relatively low water potentials (83% at ?0.41 MPa), but decreasing water potentials produced detrimental effects on germination percentage and rate. Thus, dormancy characteristics and germination behavior of O. pumila seeds ensure that germination occurs in the desert only when soil moisture conditions are favorable for seedling establishment and survival.     

Seed biology of Australian arid zone species: Germination of 18 species used for rehabilitation

Revegetation of disturbed land, particularly in arid environments, is often hindered by low seedling establishment. Information on seed biology and germination cues of keystone species is lacking, particularly in arid Australia; a major zone for mining developments. This study investigated seed characteristics and germination of 18 common species required for rehabilitation of disturbed areas at Shark Bay Salt in the Shark Bay World Heritage Area, Western Australia. Untreated seeds of seven species (Aphanopetalum clematideum, Atriplex bunburyana, Austrostipa elegantissima, Melaleuca cardiophylla, Pembertonia latisquamea, Rhagodia baccata, Salsola tragus) exhibited high germination percentages. Seeds of two species (Acacia tetragonophylla, Stylobasium spathulatum) had low imbibition, which increased with hot-water treatment, hence require scarification for germination. Gibberellic acid, smoke water and karrikinolide (a butenolide isolated from smoke) substantially increased germination percentages of three species (Anthocercis littorea, Diplolaena grandiflora, Solanum orbiculatum). Seeds of the remaining six species (Dioscorea hastifolia, Eremophila oldfieldii, Nitraria billardierei, Ptilotus exaltatus, Thryptomene baeckeacea, Zygophyllum fruticulosum) had low germination percentages regardless of treatment. Most species germinated equally well at 26/13 °C and 33/18 °C, however seven species had improved performance at 26/13 °C. This study is significant to land managers and conservation agencies with an interest in optimising germination of arid zone seeds for restoration.     

Effect of burial on the germination of Opuntia tomentosa's (Cactaceae,Opuntioideae) seeds

Opuntia tomentosa seeds have physiological dormancy and a permeable but hard funicular envelope that restricts water uptake and embryo growth. Dormancy break, germination, and changes in the funiculus were compared in laboratory-stored seeds and in those buried for 7 months at different microsites and later exhumed and stored in the laboratory. The funicular envelopes of both lots were examined with a scanning electron microscope (SEM). Seeds in both lots were: scarified with H₂SO₄ (0, 45, 60 and 90 min), treated with gibberellins (0, 1000 and 2000 ppm) and germinated at the same ages (0, 2, 4, 7, and 10 months after exhumation) at 24 °C. Light effect was determined at 24 °C and 20–35 °C. In laboratory-stored seeds germination was low. Exhumed seeds germinated in ～50%, the remaining ones required scarification and gibberellins, but lost dormancy two months later. Gibberellins reduced germination heterogeneity, and scarification reduced germination; the adverse effect of scarification was offset by gibberellins. Exhumed seeds germinated mainly in darkness at 20–35 °C and partially in far red light at both temperatures. SEM showed that fungi eroded the funiculus reducing its resistance. O. tomentosa could form a seed bank, ensuring its survival under heterogeneous environments.     

Heliotropium europaeum only germinates following sufficient rainfall to allow reproduction

Regions with Mediterranean climates are seasonally arid and provide a niche for ephemeral species which germinate following discrete and variable rainfall during summer. These species must be able to detect when conditions are suitable for completion of their life cycle. Common heliotrope (Heliotropium europaeum) is one such species. It is considered a weed in its naturalised habitat in southern Australia as it uses resources that could be used by ensuing crops, and is toxic to livestock.We examined common heliotrope's germination responses to temperature and water potential, the effect of simulated rainfall on seedling emergence and plant growth in lysimeters.Fresh seeds of common heliotrope have relatively high optimum temperatures and water potentials for germination (around 35 °C and 0 MPa). Germination percentage of seeds extracted from the soil seed bank varies seasonally. In the soil types and climate of the study area, 24.2 mm is the minimum amount of rainfall resulting in emergence. Evapotranspiration required to reach minimal reproductive output was 19.4 mm.Common heliotrope's germination requirements have been selected to ensure that it will only germinate after sufficient rainfall to allow reproductive output. This is the mechanism through which common heliotrope detects its temporal niche.     

Water requirements for germination and early seedling establishment in four African savanna woody plant species

The aim of this study was to determine the water requirements for germination and early seedling establishment of four African savanna tree species, namelyAcacia karroo, A. nilotica, A. tortilisandMundulea sericea. The acacias are characteristic of nutrient-rich, andM. sericeaof nutrient-poor savannas. Imbibition times of scarified seeds ofA. karroo(4 h),M. sericea(6 h) andA. tortilis(8 h) were rapid relative toA. nilotica(28 h) and were inversely correlated with seed size. Imbibed water is lost in about 2 h after drying at 25°C for all species exceptA. nilotica(only 70% moisture loss). Seeds ofA. karrooandM. sericeakept at 40°C dried to below their original moisture contents within 2 h. Seeds ofA. niloticaandA. tortilisdried at 40°C lost viability relative to (undried) control seeds, while there was no significant loss of viability forA. karrooandM. sericea. For fully imbibed seeds to germinate in sandy savanna soils, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungerminated seeds were still viable despite drying-out from a fully imbibed state. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedlings ofA. niloticaandM. sericeato continue growth for a further 5 weeks under greenhouse conditions, althoughA. niloticamostly survived 25% FC with one seedling surviving 12·5% FC. Root penetration was rapid in sandy soils, withA. niloticaandM. sericeaattaining a depth of 40 cm within 15 days. Frequent, but not necessarily high, rainfall appears to be essential for germination and seedling survival over the first 7 weeks.     

Critical conditions for successful regeneration of an endangered annual plant,Cryptantha minima: A modeling approach

Tiny cryptanthe (Cryptantha minima Rydb.) is an endangered annual plant adapted to prairie sand dunes of Canada. Thermal time and hydrothermal time models were used to quantify the effect of temperature and water potential on seed germination and to relate it to field conditions. Tiny cryptanthe seeds had a base temperature of ?3.9 °C for germination. Final germination percentage was highest at temperatures near the freezing point. Seeds required approximately 130.6 °C d to reach 50% germination under optimal water conditions. Seed germination was sensitive to water stress and large seeds had lower germination than small ones especially at reduced water potentials and higher temperatures. The high potential germination and low seed dormancy indicate that tiny cryptanthe lacks a dormancy mechanism, but the unique germination response to temperature enables it to fully utilize water from spring snowmelt before drought in the summer. Future climate change with higher temperatures could reduce regeneration success of this species unless there is an associated increase in winter and spring precipitation.     

荒漠植物苦豆子和苦马豆种子硬实特性与活力关系研究

 以荒漠边缘采集的苦豆子和苦马豆种子为材料,研究其硬实特性及其与活力的关系。结果表明,每日内吸胀的种子数量随时间推移,以一定比列下降,苦豆子非硬实种子吸胀速率较缓慢,第10天后吸胀率降到1%,苦马豆非硬实种子第3天后下降到1%,第13—16天突然上升后又下降到1%。两种豆类种子都显示出硬实种子发芽率、发芽指数、活力指数、脱氢酶、呼吸速率、超氧化物歧化酶(SOD)均高于非硬实种子,而电导率、浸出液可溶性糖和丙二醛(MDA)低于非硬实种子,缓慢吸胀的硬实种子活力指标高于快速吸胀的硬实种子,这表明硬实种子吸胀过程中存在吸胀损伤。而在非硬实种子中,根据以上活力指标判断,晚吸胀的种子比早吸胀的种子活力高。     

Germination responses of Medicago ruthenica seeds to salinity,alkalinity, and temperature

To determine the effects of and the interactions between salinity and temperature, and alkalinity and temperature, we conducted three germination tests of Medicago ruthenica (L.) Sojak cv. ‘Zhilixing’ seeds in growth chambers. In experiment 1, the seeds were placed at a range of temperatures (5, 10, 15, 20, 25, 30, 35, or 40 °C), and in experiments 2 and 3, we varied the salinity (0, 50, 100, 150, 200, or 250 mM NaCl) and alkalinity (1, 5, 10, 15, 20, or 25 mM Na₂CO₃), respectively, within three ranges of alternating temperatures (10–20, 15–25, and 20–30 °C). The seeds of M. ruthenica (L.) Sojak cv. ‘Zhilixing’ showed high percentage germination at all temperatures, except for 5 °C. Seeds germinated well at low NaCl and Na₂CO₃ concentrations under all three alternating temperature regimes. Approximately half the seeds germinated at high salinity and alkalinity at 15–25 °C, whereas the ungerminated seeds germinated when the high-saline and alkaline stresses were removed. These results suggest that M. ruthenica (L.) Sojak cv. ‘Zhilixing’ has potential utility as a forage legume in saline and alkaline environments.     

Effect of sand burial depth on seed germination and seedling emergence of Calligonum L. species

A greenhouse study was conducted to determine the effects of sand burial on seed germination and seedling emergence of ten Calligonum L. species. The results suggested that the deeper the seeds in sand, the lower and slower their germination and seedling emergence. Seedling emergence occurred from a maximum depth of 12 cm with most seedlings emerging from 2 and 4 cm depths. The percent seedling emergence, number of days of first emergence, percent seed germination and percent of dormant seeds were significantly affected by different species and burial depth. There was a significant positive correlation between the number of days to first emergence and burial depth. Both mean percent germination and percent seedling emergence were negatively correlated with depth significantly for each species.     

Microclimate, freezing tolerance, and cold acclimation along an elevation gradient for seedlings of the Great Basin Desert shrub, Artemisia tridentata

Vegetation, microclimate, seedling frequency, freezing tolerance, and cold acclimation were compared for seedlings of Artemisia tridentata collected from 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada, California. Data were used to test the hypothesis that ecotypic differences in stress physiology are important for seedling survival along gradients from desert to montane ecosystems. The vegetation canopy cover and A. tridentata seedling frequency were greatest at 2575 m, compared to 1775 and 2175 m. Snow cover ameliorated temperatures near the soil surface for part of the winter and depth varied across elevations. Freezing tolerance was compared for seedlings maintained in growth chambers at day/night air temperatures of 25°C/15°C. The temperature at which electrolyte leakage and Photosystem II function (F_V/F_M) from leaves were half-maximum was approximately −13·5°C for leaves of seedlings from all three elevations. Shifting day/night air temperatures from 25°C/15°C to 15°C/5°C initiated about 1·5° of acclimation by plants from all three altitudes, with seedlings from the highest elevation exhibiting the greatest acclimation change. Measurements of ambient air and canopy temperatures at the three elevations indicated that wintertime average low temperatures were consistent with the measured degree of freezing tolerance. At small spatial scales used in this study, pollen and seed dispersal between study sites may have precluded resolution of ecotypic differences. Patterns of freezing tolerance and cold acclimation may depend on a combination of mesoclimate and microclimate temperatures, canopy cover, snow depth, and snow melt patterns.     

Avalanche prediction in Scotland: II. Development of a predictive model

The paper presents a method for predicting avalanche activity from meteorological data which is suitable for Scottish snow conditions. Two main types of avalanche are distinguished. Direct-action avalanches are the result of fresh snow accumulation and may release after approximately 200 mm of fresh snow has fallen over a period of a few days. They become extremely likely if a further 100 mm falls in a single day. Climax avalanches are the result of strength loss in the snow cover, due either to thawing or persistent cold. Thaws may produce an avalanche after only three or four days with maximum temperatures around 2°C. Alternatively, if maximum temperatures remain below ?4°C for over a week, and especially if cold weather persists for two weeks, slab avalanches of dry snow are likely to occur. Between these two types are several possible avalanche situations in which fresh snow accumulation is combined with high or low temperatures. Topography plays a passive role, and determines where avalanche activity is most likely. Free faces, smooth surfaces and slopes in the lee of major storms respond most rapidly to the onset of avalanche conditions.     

Influences of fine-scale disturbance on germinant success in a treeline ecotone

Fine-scale disturbance can increase seed access to suitable substrates, facilitating germinant emergence and survival, which are necessary elements for treeline advance. We conducted an experiment to test this hypothesis in a white spruce (Picea glauca) treeline ecotone in southwest Yukon, Canada. Sixty seed germination quadrats were established at two elevations (treeline and alpine tundra) and subjected to three levels of simulated disturbance. We sowed 125 seeds in half of the quadrats (30) and measured their emergence and survival over 3 years. Soil temperature, moisture, and organic depth were recorded in all treatments. Treeline quadrats had significantly greater seedling emergence and survival than alpine tundra quadrats. Mean soil temperature, moisture, and organic layer depth were all greater in treeline quadrats. Partially scarified quadrats had the highest germinant emergence compared to unscarified and completely scarified quadrats. Completely scarified quadrats had the highest temperature range and the lowest soil moisture. The results indicate that moderate levels of disturbance can positively influence seedling emergence, while more severe disturbance can lead to high temperature ranges and moisture loss that negate the benefits of lower interspecific competition. Collectively, our findings suggest that fine-scale disturbance can play a significant role in influencing seedling presence in treeline ecotones.     

Seed germination of five Helianthemum species: Effect of temperature and presowing treatments

The germination behaviour of five Helianthemum species (H. almeriense, H. appeninum, H. cinereum, H. hirtum, H. squamatum) has been studied under controlled conditions. Constant 15, 20 or 25 °C and alternating 25/15 °C temperature regimes and 16/8 h light/dark photoperiod conditions were used. Presowing treatments applied were manual scarification, boiling water, hot water, dry heat and sulphuric acid. Germination values recorded were final germination percentage and germination rate expressed as days to reach 50% of the final germination percentage (T₅₀). Incubation temperature had no significant effect on final germination percentage for untreated seeds of the five Helianthemum species. However, variation due to temperature was significant for scarified seeds, with the lowest germination percentage attained at 25 °C. In all Helianthemum species studied, the highest germination percentages were obtained by manual scarification of seeds. Germination rate of scarified seeds decreased as germination temperature increased. The different presowing treatments investigated allowed some germination in some species, but none were any better than manual scarification. The high germination among most species studied, following mechanical rupture of the seed coat, shows that the mechanism of dormancy lies in the seed coat. The physical dormancy caused by impermeable seed-coat appears to be the main reason of poor germination of untreated seeds of Helianthemum species studied.     

播种深度和土壤水分对黄花补血草种子萌发的影响

 采集来自腾格里沙漠的黄花补血草种子,分别设置7个水分梯度（3.0%, 5.0%, 8.0%, 10.0%, 15.0%,18.0%,20%) 和6个播种深度（0、0.5、1.0、1.5、2.0、2.5 cm）,进行黑暗（10～15 ℃）和光照(20～25 ℃)处理,研究水分、埋藏深度和温度对黄花补血草种子萌发和出苗的影响。结果表明：黄花补血草种子吸涨速率很快,在6 h种子吸水达到饱和;在土壤水分为3.0%～10.0%时,种子萌发率较高,当土壤水分达到20%时,萌发受到抑制;种子埋藏越深,萌发率越低,土壤水分太低或者太高,都不利于种子萌发。黄花补血草种子萌发和幼苗生长的最佳条件是：白天温度在25 ℃,夜间温度在15 ℃,土壤水分为10.0%,埋藏深度为0.5 cm时种子萌发率和出苗率最高。     

超旱生小灌木——红砂大田育苗技术研发

采用对比试验,研究了红砂（Reamuria songarica （Pall.））大田容器育苗的容器选择、种子处理、覆土厚度、覆盖保护和灌水方式等措施对出苗率、成苗率和容器苗质量的影响。结果表明：红砂适宜的育苗容器为底部多孔的大规格容器,适宜的覆土厚度为0.3～0.6 cm,覆盖材料的适宜遮光率应控制在60%以下;以遮光率60%的遮荫网为例,适宜的覆盖时间为20~30 d左右;细质地的河沙表面覆盖是较好的覆盖方式。育苗容器和覆盖方式对幼苗质量有显著影响。覆土厚度和覆盖时间对出苗率和成苗率均有显著影响;覆盖方式仅对出苗率有显著影响;种子处理和灌水方式对出苗率、成苗率均无显著影响,但从生产的经济实用性及容器苗质量方面综合评价,种子机械处理和漫灌+喷灌灌水方式更符合生产推广要求。     

蒙古韭(Allium mongolicum)的分布和结实特性

为掌握蒙古韭(Allium mongolicum)在中国的分布现状和种群发展趋势,采用野外踏查和栽培实验相结合的方法,对其地理分布范围进行了界定;以结实量调查数据为基础,分析了不同分布区域蒙古韭的结实特点以及影响因素。结果显示:(1)蒙古韭在中国的分布区约在36°28′—46°14′N、88°39′—116°05′E之间,包括陕西、甘肃、宁夏、内蒙古、新疆、青海6省(区)。(2)由于生境差异,蒙古韭不同群落中物种组成差异很大,蒙古韭多为伴生种或偶见种,很少为优势种。(3)不同生境条件下蒙古韭的结实特性差异较大,多数地区没有结实或结实率很小,主要是因为干旱导致其生殖生长受阻,加之人为因素干扰,使蒙古韭不能正常完成其生活史。     

种子预处理对唐古特白刺出苗和幼苗生长的影响

 采用不同方法对唐古特白刺种子预处理,以不处理为对照,再行60 ℃浸种24 h后育苗,通过测定出苗动态和生长指标,探寻促进白刺种子出苗和幼苗生长的技术途径。结果表明,唐古特白刺种子出苗率变化动态均呈“S”型曲线,符合Logistic曲线方程,出苗质量随种子处理不同而异,出苗期为38～55 d,极限出苗率为30.2%～57.8%。沙搓和沙碾均可显著提高白刺种子出苗质量,为培育壮苗奠定良好基础,出苗率较对照分别提高15.6%和13.4%,出苗势分别提高60.1%和6.9%,出苗指数分别提高35.2%和13.2%,活力指数分别提高41.5%和23.3%。尤其沙搓种子出苗集中,较对照缩短5 d。而掐尖、20%醋酸和5%碱水浸泡均抑制发芽。将种子与沙子混合后手搓和棍碾压均可破坏白刺种子坚硬致密的内果皮结构,破除休眠,促进出苗和幼苗生长,但需注意的是沙碾操作不当会造成种子内部结构被破坏。     

Seed dispersal by domestic goats in a semiarid thornscrub of Mexico

Previous studies conducted in the semiarid thornscrub of the Biosphere Reserve of Tehuacán-Cuicatlán have reported a high consumption of cactus and legume fruits by domestic goats. To test the hypothesis that goats may effectively disperse seed, we collected domestic goat feces from different microhabitats (bare ground, under shrub or under tree). The area occupied by each microhabitat was measured in the field. We then quantified the presence of viable seeds in goat feces and their deposition in microhabitats suitable for seedling establishment. We collected 209 seeds and using a seed reference collection, we identified these as belonging to three endemic cactus species, two legumes and several unidentified species. A high proportion of the cactus seeds germinated. Although most of the feces were found in open areas, more than 30% were recorded beneath the canopies of perennial plants, a suitable microhabitat for seedling establishment and survival. Contrary to the general assumption that goats cause only degradation of vegetation, these findings show that goats can effectively disperse viable seeds of diverse species. Studies that evaluate the ecological role of goats as important elements of biotic interactions are needed in order to generate an adequate management program for livestock.     

Effect of environmental heterogeneity on field germination of Opuntia tomentosa (Cactaceae,Opuntioideae) seeds

We studied in Opuntia tomentosa the effect of plant shade (orientation and distance), and the place where the seeds remained before germination. Seeds were collected in 1998 and a) sown immediately on soil (non-buried seeds), b) stored in the laboratory and sown on soil during the 1999 rainy season (control) or c) buried after collection (under the shade of a tree, a rock and in an open space), exhumed 7 months later and sown in 1999 (exhumed seeds). Seeds were sown under or outside the shade of several trees; or on the northern, eastern and western sides of a 14-m-tall tree. In 1999, germination was high in all samples, but the control seeds had delayed germination. After two rainy seasons, non-buried seeds had the lowest germination rate. Germination was not affected by the distance from the tree. Outside of the plant's shade, germination was low. Germination was affected more by the environmental heterogeneity than by the individual plant shade. Germination probabilities were the highest at 16–19 °C; field germination was zero at 20.6 °C. Seed polymorphism and microenvironmental heterogeneity spread germination across two rainy seasons creating a permanent seed bank.