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.     

吉林省豚草瘦果形态及微结构特征

【目的】研究吉林省不同地区豚草瘦果形态及微结构特征,为豚草的识别及防控提供参考。【方法】采集吉林省吉林市磨盘山、吉林市龙潭山、桦甸市、蛟河市、长春市、四平市、公主岭市、通化市、白山市、白城市、松原市和乾安县12个地区不同豚草种群的瘦果进行扫描电镜观察、描述和拍照,分析豚草瘦果表型性状与地理-气候因子的相关性,并对瘦果形态及微结构特征进行聚类分析。【结果】豚草果皮纹饰有条状纹饰、网状纹饰和穴状纹饰;瘦果的喙长度随年均降水量增加而增加、随纬度升高而减少;果形系数随经度升高及年均降水量的增加而增加。根据形态和微结构性状聚类分析结果,将豚草瘦果分为3类（Ⅰ、Ⅱ、Ⅲ）。其中,Ⅰ类瘦果果皮纹饰网状、中果形、短喙类,种源主要来自蛟河市、通化市、公主岭市、松原市和乾安县;Ⅱ类瘦果果皮纹饰条状、小果形、中长喙类,种源主要来自吉林市磨盘山、吉林市龙潭山、桦甸市、长春市和四平市;Ⅲ类瘦果果皮纹饰穴状、大果形、长喙类,种源主要来自白山市和白城市。【结论】吉林省豚草瘦果形态及微结构存在差异,低纬度、高降水量有利于豚草瘦果生长发育。     

Achene anatomy and stomatal characteristics of eighteen Crepis L. (Asteraceae) taxa from Turkey with notes on their systematic significance

Achene anatomy and stomatal characteristics of eighteen Crepis taxa from Turkey are here described for the first time. In all taxa examined the pericarp is composed of several layers of sclerenchymatous and parenchymatous cells. As for the achene, differences among taxa mainly concern the pericarp structure and its thickness and width. Stomata are present on both surface of the leaf in all studied taxa and all taxa have anomocytic type stomata. However, the dimensions (length and width) and density of the stomata differ significantly among the studied taxa. In addition, the dimensions of stomata are negatively correlated with stomata density. It is concluded that achene anatomy and stomatal characteristics are useful for delimitation of Crepis taxa and a key to taxa based on these characters is provided. However, based on achene anatomy and stomatal characteristics, we found no argument for an exclusion of the Lagoseris group from Crepis as has previously been proposed.     

东北地区蒲公英属瘦果微形态特征及其分类学意义

通过对东北地区10种蒲公英瘦果形态及微形态特征进行观察比较,为蒲公英属（Taraxacum F.H.Wigg.）分类研究提供瘦果形态学证据。利用体式数码光学解剖镜和电子探针显微镜,对蒲公英瘦果进行观察比较,并根据瘦果大小、形状、喙基比例、颜色及微形态表面纹饰特征进行聚类检索。结果表明,（1）瘦果微形态特征为全身具刺,具白色斑点,刺状突起成行且规则排列,但刺大小及密度存在显著差异;（2）检索结果表明,以蒲公英瘦果形态及微形态特征进行检索的结果与《中国植物志》中的分类结果一致。该实验通过对东北地区10种蒲公英瘦果形态及微形态特征进行描述,确定了蒲公英瘦果形态特征差异可作为分类学表征证据,并将喙基长度（B）与瘦果长度（A）（不包括喙基长）之比命名为喙基比（B/A）,该比值可增补为分类要点。研究结果支持《中国植物志》中对蒲公英属的分类结果。     

Role of within-individual variation in capitulum size and achene mass in the adaptation of the annual Centaurea eriophora to varying water supply in a Mediterranean environment

To clarify the adaptive value of variation in capitulum size and achene mass, plants of Centaurea eriophora were studied in a glasshouse and in a natural population. C. eriophora plants consist of a basal leaf rosette from which an erect stem grows, with lateral branches of various orders ending in capitula of different orders. Primary, secondary and tertiary capitula are comparable in size and they produce similar numbers of achenes, which are similar in weight (large achenes). These capitula are formed during April, May and early June, and constitute the normal or primary flowering. Following ripening of tertiary capitula, leaves senesce, but, later during June and the first half of July, a secondary flowering of a variable number of smaller capitula may occur if wet conditions persist for longer than usual. Plants that have almost senesced develop small lateral branches 1-2 cm long bearing a few small leaves and ending in a capitulum about half the diameter of capitula from the primary flowering period. The number of achenes produced in these capitula (small achenes) and their weight are 70 and 30% less, respectively, than those of capitula formed during primary flowering. These reductions appear to result from restricted availability of resources. Large and small achenes have similar dispersal characteristics and possess similar germination potential. However, large achenes produce seedlings that are capable of emerging from greater burial depths, providing the resulting plants with a potential advantage. The normal flowering period coincides with the optimum time of year for flowering and fruiting in the south of Spain, and only if rainfall lasts longer than usual does secondary flowering occur. Secondary flowering extends the normal flowering and fruiting periods, thereby providing a supplementary crop of smaller, yet viable, fruits. It can be considered to be an adaptive response to the unpredictable Mediterranean climate, optimizing the use of available resources.     

Germination Response Patterns during Dormancy Loss in Achenes of Six Perennial Asteraceae from Texas, USA

Abstract When subjected to simulated habitat temperatures, achenes of six perennial Asteraceae from southcentral Texas came out of dormancy during summer. In the early stages of dormancy loss, achenes of Erigeron modestus, Gaillardia suavis and Hymenoxys scaposa germinated (to ≥ 10%) in light at 12/12 hr daily thermoperiods of 15/6, 20/10 and 25/15°C and those of Pinaropappus roseus at 15/6 and 20/10°C. After additional dormancy loss, achenes of these four species also germinated at 30/15 and 35/20°C. Achenes of these four species had a Type 1 germination response pattern, which heretofore has not been reported in perennial Asteraceae. Achenes of Chaptalia nutans first germinated (to ≥ 10%) at 20/10 and 25/15°C and those of Hymenopappus scabiosaeus at 20/10°C, but with further loss of dormancy achenes of both species also germinated at 15/6, 30/15 and 35/20°C. Thus, achenes of these two species had a Type 3 pattern. This is the first report of perennials in any family with a Type 3 response pattern whose seeds come out of dormancy during summer. Presence of annual and perennial Asteraceae with Type 2 in temperate eastern North America and annual and perennial Asteraceae with Type 1 in southcentral Texas causes us to conclude that climate is more important than the type of life cycle in determining the type of germination response pattern.     

莴苣属6种近似种瘦果形态的扫描电镜观察

【目的】研究莴苣属6种近似种瘦果超微形态,确定鉴别特征,为口岸检疫鉴定提供参考。【方法】利用扫描电子显微镜观察莴苣属6个近似种瘦果的表面特征、冠毛、种脐、衣领状环等超微形态特征,测量瘦果表面刺长、刺宽、刺间距、冠毛节间长度等,并应用单因素方差分析进行显著差异评价。【结果】野莴苣脐口无环形边,其他5种均具环形边;野莴苣瘦果表面突起呈锐三角状物,乳苣突起呈耳状物;乳苣与莴苣衣领状环呈圆形或近圆形,其他4种呈椭圆形;乳苣、刺毛莴苣冠毛节间长度最小,毒莴苣的最长;野莴苣、莴苣、山莴苣次之;毒莴苣瘦果冠毛上的刺基部稍肿大,而其他5种未见肿大;瘦果超显微特征性状测量值在种之间存在差异。【结论】莴苣属种脐的形状和环形边的有无、种脐中央的突起物形状、衣领状环形状、瘦果表面纹饰特征及突起物的形状等超微形态特征可以作为莴苣属种类判定依据;瘦果表面的刺长度和宽度、刺间距、冠毛节间长度数值差异显著可以用于莴苣属种类鉴定。     

荨麻科楼梯草属植物瘦果的补充描述

楼梯草属植物的瘦果具有较高的形态多样性,可作为种类鉴定的重要依据.然而,中国超过半数楼梯草属植物缺少瘦果形态信息.该研究通过野外考察、标本查阅以及光镜和扫描电镜的观察,对七种楼梯草属植物的瘦果进行了补充描述,并提供了墨线图和照片.该研究结果对该属的进一步分类修订提供了有价值的信息.     

A nodding capitulum enhances the reproductive success of Cremanthodium campanulatum (Asteraceae) at high elevations in the Sino–Himalayan Mountains

Background: Flower orientation is considered an evolutionary response to pollinators and abiotic factors. Several members of the genus Cremanthodium (Asteraceae) growing in alpine habitats in the Sino–Himalayas have conspicuously nodding capitula, the function of which is not understood.

Aims: We investigated the influences of nodding capitula on floral thermal conditions, pollinator visitation rate, pollen viability and achene production in Cremanthodium campanulatum.

Results: (1) the nodding capitulum did not modify internal thermal conditions, but the elongated involucral bracts could shelter pollen from UV-B radiation; (2) water and UV-B radiation seriously reduced the viability of pollen grains; (3) capitula artificially held erect set significantly fewer achenes than nodding ones; and (4) no pollinator preference was observed between capitula that were artificially held erect and natural nodding ones.

Conclusions: This study indicated that the nodding capitulum of C. campanulatum may protect pollen from being damaged by UV-B radiation, being washed away from the anther and stigma, and enable it to avoid rain damage during the wet monsoonal period. Our results suggest that nodding capitula in C. campanulatum possibly confer a selective advantage and may be driven by non-biological agents rather than pollinator attractors.