The balance between sexual and asexual reproduction in plants living in variable environments

The balance between sexual and asexual propagule production is studied in an evolutionary model where plants produce the two kinds of propagules in genetically determined proportions. The male function of plants producing asexual propagules can be varied, and the sexual and asexual propagules carry different probabilities to turn into new reproductive individuals. These fitnesses may vary over years. The evolution of the population’s reproductive system is studied assuming modifier alleles with small effects. In this setting a balanced, mixed reproductive system can evolve, but only if the difference in fitness between the sexual and asexual propagules varies over years. When the two kinds of propagules are very similar to each other, as is often the case with sexual and asexual seed formation, evolution will tend towards a state dominated by the one or the other reproductive system.     

Propagule bank buildup of Chara aspera and its significance for colonization of a shallow lake

Several aspects of the propagule bank dynamics including germination, burial and storage characteristics, and the relationship between the distribution of the propagule bank and cover of an expanding Chara vegetation (dominated by C. asperaDeth. ex Willd.) was studied in the shallow lake Veluwemeer. The density of oospores in the sediment was positively correlated with the number of years that Chara was present at that particular site. After six years of Chara presence, at least 1.7 × 10⁶ oospores m^–2 had accumulated in the sediment. Oospores in sediment were evenly distributed in the 15 cm top layer, which was very similar to the foraging depth of Bewick's Swans (Cygnus columbianus bewickii Yarr). Burial in the sediment may be an important mechanism by which oospores are stored. On the other hand, at shallow areas about 50% of the biomass was consumed enhancing the potential dispersal of oospores by water birds. In a laboratory experiment, 100% of the tested bulbils of C. aspera emerged and thus may be important for short time survival of established vegetation. In contrast, oospore germination varied between 1 and 15% depending on light level and burial depth. The low germination and the high accumulation of oospores suggest that oospores are adapted to long time survival in a dormant state. Charophytes colonized Veluwemeer step by step in the course of about nine years. Not all the sites with suitable light conditions were colonized at the same speed. C. aspera established a dense vegetation only at sites with high oospores densities (> c. 1 × 10⁴ m^–2). The results indicate that the high density of oospores needed for attaining complete vegetation cover may be an important limiting factor during colonization.     

怀山药微型块茎愈伤组织的诱导形成及高频率再生

对怀山药微型块茎愈伤组织的诱导及高频率再生进行了研究。结果表明 :(1)光下诱导铁棍山药脱分化形成愈伤组织 ,6 -BA2 mg/L NAA2 mg/L 为最佳激素组合。KT2 mg/L 2 ,4 - D2 mg/L 有利于铁棍山药愈伤组织的增殖。附加 2 mg/L NAA能缩短 4 7号山药愈伤组织的诱导时间。KT2 mg/L NAA2 m g/L 有利于 4 7号山药愈伤组织增殖 ;(2 )不同光照条件对愈伤组织的诱导和增殖影响不同。光照是缩短 4 7号山药愈伤组织诱导时间的另一因素。暗培养有利于愈伤组织的增殖 ,对 4 7号山药来说 ,暗培养下诱导率也较高 ;(3)基因型不同 ,愈伤组织类型不同 ,诱导率和不定芽分化率也有差异 ,4 7号山药高于铁棍山药 ;(4 )KT对 4 7号山药愈伤组织分化形成不定芽起主要作用 ,2 ,4 - D2 mg/L KT2 mg/L 为最佳激素组合 ;(5 )光培养有利于不定芽的分化     

Response to competition of bulbous geophyte Allium oleraceum differing in ploidy level

Experimental studies that explore the possible causes of ploidy distributions and niche differentiation are rare. Increased competitive ability may be advantageous for survival in dense vegetation and may strongly affect local and regional abundances of cytotypes and potentially contribute to invasion success. We compared survival, growth and reproduction of plants originating from bulbils of three cytotypes (2n = 4x, 5x, 6x) of Allium oleraceum growing with and without a competitor (Arrhenatherum elatius). There was a strong negative effect of competition but no effect of ploidy or ploidy × competition on survivorship, height and total dry mass of A. oleraceum, i.e. no support for different competitive abilities of the ploidy levels. However, slightly different responses of populations to competition treatments within all cytotypes suggest differentiation within cytotypes. Under competition, plant survivorship was low, surviving plants were small, had low dry mass and produced neither sexual nor asexual propagules. Without competition, plant survivorship was high, and cytotypes differed in three traits after 2 year's growth: dry mass of flowers, number of flowers and ratio of the dry mass of sexual to asexual propagules all decreased with increasing ploidy level. We additionally tested tetra‐ and pentaploids as to whether plants originating from different types of propagule (bulbils, seeds) differ in survivorship, growth and reproduction when growing with and without a competitor. Plants originating from bulbils had higher survivorship, were more robust, flowered earlier and produced more propagules when compared to plants originating from seeds and grown without competition. Under competition, differences in performance between plants originating from seeds and bulbils mostly disappeared, with higher survivorship only for plants originating from bulbils.     

The occurrence of batatasins in the dioscoreaceae

The tubers and/or aerial bulbils of 13 species of Dioscorea, mostly of tropical origin, and one species each of Rajania and Tamus, were examined for the presence of batatasins. These phenolics have previously been reported in the temperate species D. opposite. One or more batatasins were found in either tubers or bulbils of most species examined, including taxa from both Old and New Worlds, and with annual or perennial tubers. Batatasin I was the most widespread in occurrence, followed by B-II, while B-III, B-IV and B-V were comparatively rare. Only the buibils of D. opposite contained all five batatasins.     

A Landscape Perspective of the Stream Corridor Invasion and Habitat Characteristics of an Exotic (Dioscorea oppositifolia) in a Pristine Watershed in Illinois

The spatial distribution of exotics across riparian landscapes is not uniform, and research elaborating the environmental constraints and dispersal behavior that underlie these patterns of distribution is warranted. This study examined the spatial distribution, growth patterns, and habitat constraints of populations of the invasive Dioscorea oppositifolia in a forested stream corridor of a tributary of Drury Creek in Giant City State Park, IL. The distribution of D. oppositifolia was determined at the watershed scale mainly by floodplain structure and connectivity. Populations of D. oppositifolia were confined to the floodplain, with overbank flooding from the stream. Dioscorea oppositifolia probably originates in disturbed areas upstream of natural corridors, and subsequently, the species disperses downstream into pristine canyons or ravines via bulbils dispersing in the water. In Giant City State Park, populations of D. oppositifolia were distributed on the floodplain across broad gradients of soil texture, light, slope, and potential radiation. The study also examined the longevity of bulbils in various micro-environments to illuminate strategies for the management of the species in invaded watersheds. After 1 year, the highest percentages of bulbils were viable under leaves, and much lower percentages were viable over leaves, in soil, and in the creek (76.0±6.8, 21.2±9.6, 21.6±3.6, and 5.2±5.2%), respectively. This study suggests that management procedures that reduce leaf litter on the forest floor (e.g., prescribed burning) could reduce the number of bulbils of D. oppositifolia stored in the watershed.     

Understanding the germination of bulbils from an ecological perspective: a case study on Chinese yam (Dioscorea polystachya)

Background and Aims

Bulbils serve as a means of vegetative reproduction and of dispersal for many plants; this latter aspect making them analogous to seeds. However, germination of bulbils may differ considerably from seeds due to dissimilar anatomical structures and perhaps environmental cue perception. The few laboratory studies done on bulbils suggest that their germination is similar to that of seeds in the same habitats and to vegetative buds of winter-dormant plants. The present study is the first to examine how bulbil germination is controlled in nature in relation to dispersal (before vs. after winter of the same cohort) and to ambient temperatures.

Methods

Under laboratory conditions, temperature and light requirements for root and shoot emergences from bulbils of Dioscorea polystachya collected in September, 2005, February, 2006 (produced in 2005) and July, 2006 were determined. Effects of cold stratification and dry storage for releasing dormancy were tested on September and July bulbils. The phenology of dormancy release and of root and shoot emergences and the persistence of bulbils in soil were followed over time under field conditions.

Key Results

Although a low percentage of bulbils collected in July or in September produced roots, but no shoots, in the laboratory and field, these roots died within approx. 1 month. Regardless of collection date, cold stratification markedly increased root and shoot emergences. Bulbils sown outdoors in October produced roots and shoots the following March and April, respectively. The soil bulbil bank is short lived.

Conclusions

Bulbils of D. polystachya are similar to seeds of many temperate plants being mostly dormant when dispersed in summer or autumn and overcoming dormancy with cold stratification during winter. Adaptively, bulbil germination primarily occurs in spring at the beginning of a favourable period for survivorship and growth.     

铁棍山药组织培养快繁及试管珠芽离体再生体系研究

以河南温县铁棍山药带腋芽的茎段为外植体进行铁棍山药种苗快繁及珠芽离体再生体系的研究。结果表明:（1）75%乙醇浸泡30 s和5% NaClO消毒15 min配合使用灭菌效果最好;腋芽诱导最适培养基为MS+0.5 mg·L^-1 6-BA+0.1 mg·L^-1 NAA,培养20 d后诱导的多芽体倍数最高,为2.22,高度最高为3.3 cm;继代增殖最适培养基为MS+1.5 mg·L^-1 6-BA,增殖倍数可达4.1;生根培养最适培养基为1/2MS+0.2 mg·L^-1 6-BA+1.0 mg·L^-1 NAA+0.02%活性炭,平均生根天数为12 d,生根率达100%,根系最长为1.04 cm。（2）用单芽带外植体的接种方式,其珠芽诱导率及诱导的珠芽数显著高于只接单芽的接种方式,珠芽诱导率达88.9%,平均珠芽数为1.50,大小为0.38 cm×0.54 cm;蔗糖浓度为1%～3%有利于珠芽诱导,珠芽整齐度好,形状规则,试管苗叶色浓绿;离体珠芽芽诱导的最适培养基为MS+1.5 mg·L^-1 6-BA+0.2 mg·L^-1 NAA,珠芽在18～22 d发芽,30 d后诱导率最高为83.3%。该实验结果为铁棍山药试管苗的工厂化生产奠定了技术基础。