白沙蒿种子萌发特性的研究Ⅰ．粘液瘦果的结构和功能

白莎蒿（Ａrtemisia sphaerocephala Krasch.)是中国西北部沙漠的流动及固定沙丘上广泛分布的优势种灌木。瘦果小而轻,借助显微镜和扫描电镜对瘦果的形态结果进行了观察,其种子的种质与果皮愈合,要皮外层为很厚的粘液层,其粘液层在遇湿后迅速吸水膨胀,其重量增至原来的５８９倍。在自然生境中,粘液物质将沙粒粘附于瘦果的使瘦果的重量发生变化。瘦果能长时间在水上漂浮,粘液物质有助于种子的萌发和苗的发育。白沙蒿产生三种不同颜色的瘦果,其种子具有不同的萌发速率但最终达到相的萌发率。研究表明：瘦果的粘液物质对白沙蒿种子的传播与萌发具有重要的生态意义。     

白沙蒿粘液瘦果萌发中外切糖苷酶活动特性

白沙蒿 (ArtemisiasphaerocephalaKrasch .)是中国西北沙区分布的多年生植物。瘦果遇湿后其粘液层开始膨胀。对其种子萌发不同阶段的种胚提取物中几种可能降解果皮外层粘液物质的外切糖苷酶进行了检测。结果表明 :β 吡喃半乳糖苷酶在干种子中的活性很高 ,但是随着种子的吸涨而逐渐降低 ,在已萌发的种子中无法探测到。α和 β 吡喃阿拉伯糖苷酶、β 吡喃葡萄糖苷酶以及 β 吡喃甘露糖苷酶在种子萌发过程的胚提取物中也显示出了一些活性 ,但是这些活性在种胚外却无法探测到。所有胚提取物中的外切糖苷酶都无法降解果皮粘液物质。所有的证据都表明果皮外层的粘液物质无法在种子萌发的早期被水解。因此果皮粘液物质在干旱沙质条件下的生态功能被限制在吸收水分以及延长胚吸水的时间。     

白沙蒿粘液瘦果萌发中外切糖苷酶活动特性

白沙蒿(Artemisia sphaerocephala Krasch.)是中国西北沙区分布的多年生植物.瘦果遇湿后其粘液层开始膨胀.对其种子萌发不同阶段的种胚提取物中几种可能降解果皮外层粘液物质的外切糖苷酶进行了检测.结果表明:β-吡喃半乳糖苷酶在干种子中的活性很高,但是随着种子的吸涨而逐渐降低,在已萌发的种子中无法探测到.α和β-吡喃阿拉伯糖苷酶、β-吡喃葡萄糖苷酶以及β-吡喃甘露糖苷酶在种子萌发过程的胚提取物中也显示出了一些活性,但是这些活性在种胚外却无法探测到.所有胚提取物中的外切糖苷酶都无法降解果皮粘液物质.所有的证据都表明果皮外层的粘液物质无法在种子萌发的早期被水解.因此果皮粘液物质在干旱沙质条件下的生态功能被限制在吸收水分以及延长胚吸水的时间.     

白沙蒿粘液瘦果萌发中聚糖内切水解酶的活动特性

对白沙蒿 (ArtemisiasphaerocephahaKrasch .)种子萌发不同阶段的种胚提取物中几种可能降解果皮外层粘液物质的多糖内切酶进行了检测。研究结果表明 :多聚半乳糖醛酸酶在干燥胚中已存在并具较高的活性 ,但其活性随着种子吸涨及萌发而降低。此酶并不释放到种胚外。β 甘露聚糖内切酶随着种子的吸涨而增加并被萌发的种胚释放到种子外。纤维素酶在种子萌发的过程中不表现出活性。提取物中的所有多糖内切酶都无法降解果皮外层的粘液物质。     

白沙蒿粘液瘦果萌发中聚糖内切水解酶的活动特性

对白沙蒿( Artemisia sphaerocephaha Krasch.)种子萌发不同阶段的种胚提取物中几种可能降解果皮外层粘液物质的多糖内切酶进行了检测.研究结果表明:多聚半乳糖醛酸酶在干燥胚中已存在并具较高的活性,但其活性随着种子吸涨及萌发而降低.此酶并不释放到种胚外.β-甘露聚糖内切酶随着种子的吸涨而增加并被萌发的种胚释放到种子外.纤维素酶在种子萌发的过程中不表现出活性.提取物中的所有多糖内切酶都无法降解果皮外层的粘液物质.     

油蒿与中国和以色列沙漠中的两种蒿属植物种子萌发策略的比较

油蒿（Ａｒｔｅｍｉｓｉａ ｏｒｄｏｓｉｃａ Ｋｒａｓｃｈ．）是中国戈壁荒漠的固定沙丘上分布的优势种灌木,属蒙古植被地理分布区成分。油蒿的瘦果在光下萌发,萌发在暗中受到抑制光是调节其在沙土不同深度萌发的限制因素。瘦果在春天或秋天萌发,适宜温度为２０℃。瘦果的粘淮物质能吸收大量水分,具有重要的生态功能。不同浓度的盐溶液能影响瘦果的萌发。土壤湿度也是调节瘦果在沙中萌发的因素。尽管油蒿、白沙蒿（Ａ．ｓｐｈ     

朱唇种子吸水特性及其在干旱胁迫下萌发特性

种子粘液质是植物在长期适应环境过程中形成的,该物质对于种子的扩散、定居、生存力的改善、萌发、幼苗生存乃至抵御有毒化学物质毒害等都具有重要的生态学意义。朱唇为唇形科鼠尾草属多年生草本植物,原产美洲热带地区,现已广泛栽植于世界各地。为了理解朱唇种子表面的粘液物质吸水特性和种子在干旱胁迫下的萌发特性,该研究以朱唇种子为材料,运用光学显微镜和扫描电子显微镜观察以及种子萌发试验的方法,对种子和粘液层的形态结构、粘液质对种子萌发的影响进行了研究。结果表明:朱唇种子为卵形,表面为负网状结构,千粒重为(1.611±0.0084)g,无粘液种子吸水倍数为3,粘液种子吸水倍数为25,粘液层吸水倍数为122。粘液和无粘液种子及粘液层的重量都随吸水时间的延长而增长,但脱水过程要远长于吸水过程。朱唇种子吸水2 h达到饱和,经过36 h可干燥失水恢复原重。不同浓度PEG对朱唇种子的萌发均有影响,发芽势随PEG浓度升高而显著降低。朱唇种子在5%PEG胁迫下发芽率最高达(90.00±8.66)%,20%PEG胁迫下发芽率最低为(76.67±10.41)%,低浓度PEG对朱唇种子萌发有一定促进作用。这说明朱唇种子为速萌型种子,其粘液质在种子吸水过程中起到举足轻重的作用,能保证短时间内有充足的水分供其萌发。     

不同降水量处理时5种蒿属植物瘦果粘液溶出

用粘沙法对科尔沁沙地西部5种蒿属植物冷蒿(Artemisiafrigida)、差巴嘎蒿(A.halodendron)、大籽蒿(A.sieversiana)、黄蒿(A.scoparia)、乌丹蒿(A.wudanica)进行了瘦果粘液溶出及粘沙量的比较。主要模拟埋深5mm、降水量为1、2、4、8mm时瘦果的粘液溶出。5种供试植物均有粘液瘦果。在降水量1mm时,供试植物的粘液瘦果就能溶出粘液。当降水量达到2～4mm时,典型的沙生植物乌丹蒿和差巴嘎蒿的粘液瘦果粘沙量平均值分别达到7.31～13.56mg、0.63～2.55mg。除冷蒿外,其它4种植物均有随降水增多、粘液溶出量加大的趋势。粘液瘦果比例从大到小的排序是冷蒿>乌丹蒿>差巴嘎蒿>大籽蒿>黄蒿。在各种处理时,粘沙瘦果百粒瘦果粘沙量由大到小的顺序是乌丹蒿>差巴嘎蒿>大籽蒿>冷蒿>黄蒿。溶出粘液粘沙后,冷蒿瘦果所形成沙团重量达到瘦果重量的5～7倍,差巴嘎蒿2～9倍,大籽蒿3～5倍,黄蒿5～8倍,乌丹蒿5～29倍。流沙先锋植物乌丹蒿和差巴嘎蒿百粒瘦果粘沙量较其它植物大、在较低降水量时这两种植物粘液瘦果粘沙量较大表明瘦果溶出粘液粘沙是植物针对沙地流动性的进化适应之一。     

蔷薇种子的休眠及解除方法

分析了蔷薇(Rosa L.)种子休眠原因、解除休眠方法以及环境条件对休眠与萌发的影响.蔷薇种子休眠的主要原因有瘦果果皮和种皮的限制作用,胚生理休眠以及果肉、瘦果果皮、种皮和胚中的抑制物质.解除休眠的方法包括去除瘦果果皮限制、解除胚的生理休眠、去除抑制物质等.种子发育过程中及成熟后,环境因子,如温度、水分和光照,对种子休眠和萌发有影响.此外,微生物、果实采集时间也对种子休眠及萌发有较大影响.蔷薇种子的休眠机制复杂,且种间差异很大.     

七种蒿属植物种子重量形状及萌发特性的比较研究

在实验室条件下 ,对 7种蒿属植物种子 (差巴嘎蒿、乌丹蒿、万年蒿、大籽蒿、黄蒿、野艾蒿和冷蒿 )进行重量、形状及萌发特性的比较研究。沙生先锋植物乌丹蒿和差巴嘎蒿的种子重量较大、形状扁平 ,这些特征是植物对流沙环境进化的适应机制之一。黄蒿种子小且呈圆形 ,具有持久土壤种子库 ,因此黄蒿抗干扰能力较强。 7种蒿属植物有 3种萌发格局 :大籽蒿、万年蒿、差巴嘎蒿和冷蒿的萌发前期快 ,后期平缓 ;野艾蒿和黄蒿整个萌发过程平缓 ;乌丹蒿早期和后期萌发平缓 ,中间快。乌丹蒿推迟萌发高峰是它比差巴嘎蒿更适应流沙环境的机制之一。从种子萌发格局分析 ,黄蒿种子具有生理后熟或休眠机制 ,大籽蒿种子萌发是典型的机会主义。黄蒿、野艾蒿和冷蒿种子具有风险分摊的萌发机制。种子重量和形状与发芽率之间无相关性 ,重量和形状则显著相关。     

The achene mucilage hydrated in desert dew assists seed cells in maintaining DNA integrity: adaptive strategy of desert plant Artemisia sphaerocephala

Despite proposed ecological importance of mucilage in seed dispersal, germination and seedling establishment, little is known about the role of mucilage in seed pre-germination processes. Here we investigated the role of mucilage in assisting achene cells to repair DNA damage during dew deposition in the desert. Artemisia sphaerocephala achenes were first treated γ-irradiation to induce DNA damage, and then they were repaired in situ in the desert dew. Dew deposition duration can be as long as 421 min in early mornings. Intact achenes absorbed more water than demucilaged achenes during dew deposition and also carried water for longer time following sunrise. After 4-d dew treatment, DNA damage of irradiated intact and demucilaged achenes was reduced to 24.38% and 46.84%, respectively. The irradiated intact achenes exhibited much higher DNA repair ratio than irradiated demucilaged achenes. Irradiated intact achenes showed an improved germination and decreased nonviable achenes after dew treatment, and significant differences in viability between the two types of achenes were detected after 1020 min of dew treatment. Achene mucilage presumably plays an ecologically important role in the life cycle of A. sphaerocephala by aiding DNA repair of achene cells in genomic-stressful habitats.     

Significance of achene characteristics and within-achene resource allocation in the germination strategy of tetraploid Aster pilosus var. pilosus (Asteraceae)

The interrelationships among achene weight, allocation to embryo and pericarp, and germination time were determined for 500 stratified achenes of tetraploid Aster pilosus Willd. var. pilosus. Only 52.6% of the achenes germinated. Germinated achenes were significantly heavier than ungerminated achenes. Germination time was independent of achene weight and embryo weight, but varied inversely with pericarp weight. Variable achene weight is evolutionarily advantageous. Heavy achenes are at an advantage in that their proportionately larger embryos and thinner pericarps facilitate germination, promoting competitive establishment of seedlings. Lighter achenes are also at an advantage through increased dispersibility, and their relatively thick pericarp provides a persistent seed bank. Evolutionary pressures presumably maintain the variability in achene weight of var. pilosus. These results are discussed in the context of the early midsuccessional ecology of var. pilosus.     

Role of mucilage in the germination of Artemisia sphaerocephala (Asteraceae) achenes exposed to osmotic stress and salinity

Artemisia sphaerocephala (Asteraceae) is one of the pioneer species in moving and semi-stable sand dunes in the deserts of northwest China. The outer surface of A. sphaerocephala achenes contains a pectinaceous mucilage layer that can imbibe a large amount of water when wetted. We hypothesized that the mucilage can aid achene germination in heterogeneous environments. Germination of both intact achenes and those from which the mucilage had been removed (demucilaged) declined with increasing osmotic potential and NaCl concentration. However, the germination percentage of intact achenes was significantly higher than that of demucilaged achenes. The early seedling growth of intact achenes did not differ significantly from that of demucilaged achenes in either osmotic potential or NaCl solutions. Achene mucilage presumably plays an ecologically important role in the life cycle of A. sphaerocephala by aiding germination in osmotically- and saline-stressful habitats of the cold desert environment.     

Possible role of pectin-containing mucilage and dew in repairing embryo DNA of seeds adapted to desert conditions

BACKGROUND AND AIMS: Repair of damage to DNA of seed embryos sustained during long periods of quiescence under dry desert conditions is important for subsequent germination. The possibility that repair of embryo DNA can be facilitated by small amounts of water derived from dew temporarily captured at night by pectinaceous surface pellicles was tested. These pellicles are secreted during early seed development and form mucilage when hydrated. METHODS: Seeds of Artemisia sphaerocephala and Artemisia ordosica were collected from a sandy desert. Their embryos were damaged by gamma radiation to induce a standard level of DNA damage. The treated seeds were then exposed to nocturnal dew deposition on the surface of soil in the Negev desert highlands. The pellicles were removed from some seeds and left intact on others to test the ability of mucilage to support repair of the damaged DNA when night-time humidity and temperature favoured dew formation. Repair was assessed from fragmentation patterns of extracted DNA on agarose gels. KEY RESULTS: For A. sphaerocephala, which has thick seed pellicles, DNA repair occurred in seeds with intact pellicles after 50 min of cumulative night dew formation, but not in seeds from which the pellicles had been removed. For A. ordosica, which has thin seed pellicles, DNA repair took at least 510 min of cumulative night dewing to achieve partial recovery of DNA integrity. The mucilage has the ability to rehydrate after daytime dehydration. CONCLUSIONS: The ability of seeds to develop a mucilaginous layer when wetted by night-time dew, and to repair their DNA under these conditions, appear to be mechanisms that help maintain seed viability under harsh desert conditions.     

新疆短命植物抱茎独行菜种子粘液质特性的研究

以新疆荒漠植物抱茎独行菜为材料,运用光镜与扫描电镜观察以及紫外吸收光谱法、化学反应及种子萌发实验等方法,对粘液质的形态和结构,物理化学特性,粘液质对种子萌发及萌发后的影响进行了研究.结果显示:(1)完整干种子表面覆盖着一层膜状物质(完全脱水的粘液质),并呈同一走向的山脊状突出的网状结构,遇水后粘液物质呈射线状向外发射出来,化学反应实验结果表明,粘液质的组成可能是某种多糖,如β-葡聚糖.(2)粘液质约占干种子重量的1/4,有很强的吸水能力,完全浸润10 min后,种子重量增加约30~40倍,种子长度、宽度、厚度的增加分别多于1倍、2倍、4倍;完全润湿的种子能够粘附相当于其干种子重量68倍的沙粒.(3)种皮粘液质对于不同土壤基质中的种子萌发有重要作用,但是对萌发后幼苗的生长没有作用.     

Germination requirements and dispersal timing in two heterocarpic weedy asteraceae

In SW Spain the winter annuals Anacyclus radiatus and Chrysanthemum coronarium (Asteraceae) are found as weeds in diverse crops. Both plant species are heterocarpic, i.e. the peripheral and central achenes of the capitulum are morphologically distinctive. In heterocarpic and heterospermic species the different fruit or seed morphs usually have differential ecological behaviour. In this work we have studied the morphometry, germination and dispersal timing of t he different achene morphs in A. radiatus and C. coronarium. Laboratory germination tests were carried out to evaluate the influence of incubation temperature (light/dark, 27/27, 24/18, 20/10, 15/5, 10/4 degrees C), chilling period (0, 1, 7, 21 and 60 days at 2 degrees C), GA3, nitrate and the dark on the germination percentage and rate (t50). The peripheral achenes of A. radiatus have a significantly broader wing than the central achenes. In this species, germination was inhibited in the dark but viable achenes of both types germinated completely under light exposure irrespective of test conditions. Nevertheless, the peripheral achenes germinated significantly faster compared to the central achenes (t5o 1.04 vs. 1.55 days at 24/18 degrees C). In C. coronarium peripheral achenes have three-winged ribs and are significantly longer, wider and thicker than the central achenes, which have only one- or two-winged ribs. In this species the peripheral achenes showed a much lower germination compared to the central achenes under all conditions tested (0.3-3.5% vs. 41.5-58.0%). Embryos isolated from the pericarp of peripheral achenes germinated quickly in a great extent, and when placed in close contact with a pericarp fragment they did not show any inhibition of germination, suggesting physical dormancy. To determine differences in dispersal timing between achene morphs, we monitored dispersal in a stand of both plant species. In A. radiatus the peripheral achenes were shed first, at the onset of the rainy season in late summer, whereas the central achenes dispersed from late summer to early winter. In contrast, in C. coronarium the central achenes were shed first, from early summer to early autumn, whereas the peripheral achenes dispersed mainly in early autumn. Thus, in both weed species the more dispersible and readier-to-germinate achene morph is dispersed first. The differences in the germination and dispersal behaviour between achene morphs represent an opportunistic, mixed strategy which is favourable under environmental uncertainty, and should be accounted for in order to understand the population dynamics of both weed species.     

DIFFERENTIAL GERMINATION OF RAY AND DISC ACHENES IN HEMIZONIA INCRESCENS (ASTERACEAE)

The differential germination responses of ray and disc achenes of Hemizonia increscens (Asteraceae) were compared in field and laboratory investigations in order to gain insight into the ecological and evolutionary significance of heterocarpy. In the field, 200 ray and 200 disc achenes were placed in native, sterilized soil in a series of cleared, randomized, replicated plots. In a nearby plot a similar number of achenes were placed in plastic petri dishes in which high moisture conditions were maintained. Disc achene germination occurred under relatively minimal moisture conditions (<1 cm rainfall for 19 days) and relatively mild temperature regimes (21–7 C). Disc achene germination began three days after planting in the field plot and four days after they were put into the field petri dishes. In contrast, the onset of ray achene germination occurred 21 days after planting in the field plot and 19 days after planting in the field petri dishes. Averages of 2.05 and 2.71 disc achenes/day germinated in the field plot and field petri dishes, respectively. These contrasted with averages of 0.57 and 0.50 ray achenes/day germinated in the field plot and in the field petri dishes, respectively. A total of 67.5% and 69% disc achenes germinated in the field plot and the field petri dishes, but only 18% and 16.5% ray achenes germinated in the field plot and field petri dishes, respectively. Three separate treatments, using 100 ray and 100 disc achenes in each, were performed in laboratory growth chambers: 1) nicking the fruit coat, 2) excising the embryo, and 3) leaving the fruit coat intact. Onset of germination for all disc achene treatments occurred after three days. No significant differences were found among the three disc achene treatments in timing, rate, or germination percentage. All three disc treatments in the laboratory closely paralleled those for disc achenes in the field plots in time and germination percentage, but rates of germination were not as high. Germination of the nicked and excised ray achenes treatments began after four days, while germination of the untreated ray achenes began after 27 days. Untreated ray achenes in the laboratory paralleled the ray achenes in both field experiments in rate and germination percentage, but were delayed in time of germination. The nicked and excised ray achene treatments, however, were similar to the disc achene treatments in time of germination, and were not significantly different from disc achenes in rate of germination. These data suggest 1) that ray and disc achenes are markedly different in germination under identical conditions in field and laboratory experiments, and 2) this difference in germination response may be due to the thicker pericarp of the ray achenes.     

Variability and Cryptic Heteromorphism of Ambrosia artemisiifolia Seeds: What Consequences for its Invasion in France?

BACKGROUND AND AIMS: Ambrosia artemisiifolia is a ruderal weed introduced from North America to Europe. It produces large amount of achenes which are highly heterogeneous in size. Due to the preponderant role of propagules in invasive plant processes, the achene mass variability related to germination, dispersal strategy and life history traits of offspring were investigated within this species. METHODS: The variability in achene mass was quantified among six populations sampled in different habitats. The effects of achene mass variation on germination were studied. The percentages of floating and non-floating achenes were evaluated in the studied populations. The consequences of floatability on the growth and traits of the offspring were studied. KEY RESULTS: Mean achene mass ranged from 1.72 to 3.60 mg, depending on the populations, and was highly variable. Variation among achenes within plants accounted for 63.9 % of the variance, whereas variances among plants within each population (22.2 %) and among populations (13.9 %) were lower. Achene masses were also positively correlated to the total germination percentage for four populations out of six. Two kinds of achenes were distinguished: floating and non-floating. The majority of floating achenes (90 %) sank 24 h after water immersion. Whatever the population, floating achenes were lighter, more dormant and germinated faster than non-floating achenes. Plants which issued from floating achenes had better growth than those from non-floating achenes. CONCLUSIONS: The capacity of A. artemisiifolia to be invasive in Europe appears to be high, possibly due to its huge plasticity in seed mass which may help it to cope with a wide range of conditions and to establish in disturbed habitats. Furthermore, the recent invasion of southern France by A. artemisiifolia could be partially explained by water dispersal of achenes through rivers and has pinpointed its colonization potential along French rivers.     

Dormancy and dispersal in dimorphic achenes of tansy ragwort,Senecio jacobaea L. (Compositae)

Summary Marginal and central florets of the capitula of tansy ragwort Senecio jacobaea yield different kinds of fruit. The central (disk) achenes are lighter (x±SE=199±5g), more numerous (x±SE=58±0.6 achenes per head), and are equipped with a pappus aiding wind transport and rows of trichomes aiding animal transport. The marginal (ray) achenes are heavier (x±SE=286±7g), less numerous (virtually invariant at 13 achenes per head), and lack dispersal structures. Whereas disk achenes are relased shortly after they mature, ray achenes are retained by the parent for a period of months following maturity.Germination at constant temperature (20°C) and with alternating light (12 h light: 12 h dark) demonstrated that disk and ray achenes exhibit different germination syndromes. Germination percentage increases linearly with achene fresh weight in both types; for a given weight, disk achenes have a higher germination percentage than ray achenes. Germination time decreases with increasing achene weight in disk achenes, but increases with achene weight in ray achenes.The germination percentages and germination times for disk and ray achenes diverge progressively with increasing achene weight. The divergence in behavior is a result of diverging patterns of dry matter allocation in the two achene types. Increase in the size of disk achenes favors the embryo fraction, thereby speeding germination while reducing protection. Increase in the size of the ray achenes favors the pericarp fraction, thereby increasing protection while delaying germination.Reduced germination percentage and germination speed of the ray achenes were shown by experimental manipulation to be caused by physical inhibition by their thicker pericarps.Dimorphism in ragwort likely speards germination out in space and time, thereby increasing the number of safe sites an individual parent can exploit in disseminating offspring. The syndrome in other heteromorphic composites resembles that of ragwort, generally combining reduced dispersal-delayed germination in the outer achenes and distance dispersal-quick germination in the central achenes. The outer achenes are generally less numerous and larger. Dispersal traits (large numbers, early release and light wieght) are the direct opposite of dormancy traits (small numbers, delayed release and heavier weight). Thus conflicts between the properties determining dormancy and dispersal appear to require separate dormancy and dispersal phenotypes.