一年生短命植物疏齿千里光果实异形性的生态学意义

疏齿千里光(Senecio subdentatus)是分布在新疆北部古尔班通古特沙漠中的一种具异形瘦果的菊科一年生短命植物。将野外观测与室内实验相结合, 对该物种异形瘦果的形态、扩散和萌发特性, 以及异形瘦果产量与植株大小的关系进行了研究, 并对其生态学意义进行了探讨。结果表明: 疏齿千里光果序中的外围果和中央果均为柱形, 但前者为淡黄色, 后者为褐色, 且二者在大小、冠毛长度及果皮微形态等方面均存在明显差异。两种瘦果均以单个果实为扩散单元, 且在静止空气中的降落速度和在1 m·s^-1与2 m·s^-1风速下的扩散距离无显著差异, 说明虽然果实大小和冠毛长度对瘦果扩散具有不同的影响, 但对其整体扩散能力无明显影响。在各温变周期(5/2、15/2、20/10、25/15和30/15 ℃)处理中, 淡黄色外围果的萌发率均高于褐色中央果的, 且不同温度间两种瘦果的萌发率均存在显著差异, 但光照条件对其无显著影响。果序中的中央瘦果数明显多于外围果的, 且植株中外围果所占比例与植株大小间呈显著负相关关系, 而中央果所占比例与植株大小间呈显著正相关关系。这些特点说明, 该物种的小植株倾向于产生较多较易萌发的外围果, 大植株倾向于产生较多不易萌发的中央果。在古尔班通古特沙漠不可预测的极端环境中, 疏齿千里光可通过异形瘦果间的萌发差异及调节其不同大小植株中异形瘦果的比例, 来减少同胞后代之间的竞争, 增加其对不同微环境条件的生态适应性。     

Inbreeding effects on fitness traits in the heterocarpic herb Leontodon autumnalis L. (Asteraceae)

Heterocarpic plants are characterized by the production of distinct types of fruits that usually differ in their ecological behavior. In the Asteraceae, differences are mainly found between peripheral non-dispersal and central dispersal achenes (single-seeded fruits). Inbreeding depression is considered as an evolutionary force as it may reduce several fitness traits, and in the case of heterocarpic plants, it could influence fitness traits (e.g., seed set, germination rate, growth rate) of each fruit morph, which may have important ecological and evolutionary consequences. In particular, differential effects on fitness traits and dispersal of selfed and outcrossed progeny can strongly determine the viability of extant populations and the potential to colonize new habitats. We conducted a hand-pollination experiment in greenhouse conditions to test whether inbreeding affects the fitness of achene morphs in the heterocarpic herb Leontodon autumnalis (Asteraceae). Results show that achene morphs significantly differ in their ecological behavior, peripheral achenes germinating more and faster than central achenes. The significant interaction between pollination treatment and achene morph for germination probability might indicate a link between dormancy and mating system in L. autumnalis: germination was higher for outcrossed achenes in central achenes whereas the opposite pattern was exhibited by peripheral achenes. Selfing dramatically reduced seed set, probably as a consequence of strong self-incompatibility mechanisms rather than inbreeding effects. Inbreeding depression significantly affected late life-cycle traits, such as growth rate and biomass at flowering. Overall, results suggest that inbreeding depression seems to be an important selective force maintaining outcrossing in L. autumnalis.     

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.     

Influence of achene heteromorphism on life-cycle traits in the annual weed gallant soldier (Galinsoga parviflora Cav.)

Galinsoga parviflora (gallant soldier), Asteraceae, produces two morphologically distinct achenes in a single capitulum: peripheral and central. The morphological, phenological and generative reproduction traits of the progeny derived from peripheral and central achenes that were cultivated in greenhouse conditions were analyzed. Differences between the progeny of various morph types were manifested at different stages of life. The plants of both morph types developed at a similar rate and they entered key phenological phases at the same time except for the flowering stage. The average height of the studied plants was similar on the same days of the experiment. The study showed that plants from peripheral achenes realize the generative reproduction in other pattern than plants from central achenes. The plants from central achenes produce more capitula per plant at the beginning of the fruiting stage, whereas plants from peripheral seeds achieve the highest number of capitula per plant at later stages. Finally, on day 130 of the experiment, the number of capitula per plant of both morphs equalized. At the early fruiting stage, the number of achenes per capitulum of plants from the two morphs did not differ significantly. The number of achenes within one capitulum decreases with plant age in both populations, but the rate of that decline is greater in the progeny of peripheral seeds. The number of peripheral achenes per capitulum in plants from both achene types decreases at a comparable rate. In contrast, the number of central achenes is reduced at a faster rate in plants germinated from peripheral achenes. After day 83, the individuals from peripheral diaspores intensified their reproductive effort; they produced more capitula but with fewer number of achenes than in individuals from central diaspores. On day 130, the number of dispores per plant of the two morphs equalized. The changes of fecundity of peripheral and central progeny with the age of the plant (at different stages of the life cycle of a single plant) contribute to an intensive seed rain throughout the fruiting period. By producing a large number of less fertile capitula with a stable number of peripheral achenes, plants derived from peripheral achenes are able to supply a higher share of peripheral achenes than if they would produce fewer but more fertile capitula. This strategy increases the pool of peripheral achenes.     

短命植物异喙菊异形果实的两头下注策略

异喙菊(Heteracia szovitsii)是准噶尔荒漠中常见的一种具异形果实的菊科一年生早春短命植物。该文对其同一果序中3种果实的形态结构与扩散特征进行了比较研究, 并对其生态学意义进行了探讨。结果表明: 1)异喙菊3种果实的形态特征存在明显差异, 导致其扩散特性明显不同, 避免了同胞子代在生态位上的竞争。其中, 中央果具冠毛, 成熟后以果实为散布单元借风力扩散到较大范围; 外围果与过渡果均无冠毛, 分别以果序和果实为单元扩散至母株附近。2) 3种果实胚大小的差异导致其幼苗大小和竞争能力不同。外围果与过渡果的胚比中央果的大, 萌发后可产生较大和竞争力较强的幼苗。3)与中央果相比, 外围果和过渡果果皮颜色较深, 所含酚类物质较多, 厚度较大且厚壁组织较多, 对胚的保护能力强, 且种子休眠程度深。4)异形痩果在数量上的差异, 分摊了扩散和定居中的风险。数量较多的中央果增加了随机散布的机会, 可为其扩大种群开拓更大的范围, 而数量较少的外围果和过渡果可有效地减少近距离扩散所造成的同胞子代间竞争。异喙菊异形果实的产生及其生态学行为属于典型的两头下注策略: 外围果和过渡果代表了“谨慎”或“低风险”策略, 中央果则代表了“冒险”或“高风险”策略。通过这些策略, 异喙菊可将其在准噶尔荒漠极端环境中的生存风险进行有效地分摊, 以保障其繁衍成功。     

Germination of dimorphic seeds of the desert annual halophyte Suaeda aralocaspica (Chenopodiaceae), a C4 plant without Kranz anatomy

Background and Aims

Suaeda aralocaspica is a C₄ summer annual halophyte without Kranz anatomy that is restricted to the deserts of central Asia. It produces two distinct types of seeds that differ in colour, shape and size. The primary aims of the present study were to compare the dormancy and germination characteristics of dimorphic seeds of S. aralocaspica and to develop a conceptual model of their dynamics.

Methods

Temperatures simulating those in the natural habitat of S. aralocaspica were used to test for primary dormancy and germination behaviour of fresh brown and black seeds. The effects of cold stratification, gibberellic acid, seed coat scarification, seed coat removal and dry storage on dormancy breaking were tested in black seeds. Germination percentage and recovery responses of brown seeds, non-treated black seeds and 8-week cold-stratified black seeds to salt stress were tested.

Key Results

Brown seeds were non-dormant, whereas black seeds had non-deep Type 2 physiological dormancy (PD). Germination percentage and rate of germination of brown seeds and of variously pretreated black seeds were significantly higher than those of non-pretreated black seeds. Exposure of seeds to various salinities had significant effects on germination, germination recovery and induction into secondary dormancy. A conceptual model is presented that ties these results together and puts them into an ecological context.

Conclusions

The two seed morphs of S. aralocaspica exhibit distinct differences in dormancy and germination characteristics. Suaeda aralocaspica is the first cold desert halophyte for which non-deep Type 2 PD has been documented.Key words: Borszczowia, cold desert halophyte, physiological seed dormancy, seed germination, Suaeda     

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.     

Comparison of germination and seed bank dynamics of dimorphic seeds of the cold desert halophyte Suaeda corniculata subsp. mongolica

Background and Aims

Differences in dormancy and germination requirements have been documented in heteromorphic seeds of many species, but it is unknown how this difference contributes to maintenance and regeneration of populations. The primary aim of this study was to compare the seed bank dynamics, including dormancy cycling, of the two seed morphs (black and brown) of the cold desert halophyte Suaeda corniculata and, if differences were found, to determine their influence on regeneration of the species.

Method

Seeds of the two seed morphs were buried, exhumed and tested monthly for 24 months over a range of temperatures and salinities, and germination recovery and viability were determined after exposure to salinity and water stress. Seedling emergence and dynamics of the soil seed bank were also investigated for the two morphs.

Key Results

Black seeds had an annual dormancy/non-dormancy cycle, while brown seeds, which were non-dormant at maturity, remained non-dormant. Black seeds also exhibited an annual cycle in sensitivity of germination to salinity. Seedlings derived from black seeds emerged in July and August and those from brown seeds in May. Seedlings were recruited from 2·6 % of the black seeds and from 2·8 % of the brown seeds in the soil, and only 0·5 % and 0·4 % of the total number of black and brown seeds in the soil, respectively, gave rise to seedlings that survived to produce seeds. Salinity and water stress induced dormancy in black seeds and decreased viability of brown seeds. Brown seeds formed only a transient soil seed bank and black seeds a persistent seed bank.

Conclusions

The presence of a dormancy cycle in black but not in brown seeds of S. corniculata and differences in germination requirements of the two morphs cause them to differ in their germination dynamics. The study contributes to our limited knowledge of dormancy cycling and seed bank formation in species producing heteromorphic seeds.     

POPULATION DYNAMIC CONSEQUENCES AND EVOLUTION OF SEED TRAITS OF HETEROSPERMA PINNATUM (ASTERACEAE)

We summarize research on variation in achene morphology of Heterosperma pinnatum. Each flowering head has a broad range of achene morphologies. There is no between-year seed bank. There are no ecologically relevant differences in achene size, seedling size, or seedling growth rates of the different morphs. Achenes located centrally in the heads lose dormancy earlier during the period between autumn achene production and the onset (May-July) of the rainy season. This results in relatively more early emergence of central achenes. Awned central achenes also have greater adhesive dispersibility. Emergence, survival, and fecundity were measured for achenes sown into natural populations in central Mexico. When harsh conditions result in few seeds surviving to reproduce, the advantage tends to go to the more “conservative” peripheral achenes. Greenhouse experiments show a strong genetic component to the determination of achene proportions within and among populations and a variety of genetic correlations. Populations with a high proportion of awned achenes tend to be found in vegetation types where they are likely to be ephemeral. A high proportion of central achenes tend to be found in sites with little pre-rainy season precipitation. These patterns are interpreted in terms of selection for dispersal and the within-year timing of germination.     

Genetic and Non-genetic Variation in the Number of Pappus Parts in Microseris douglasii Strain D37 (Asteraceae, Lactuceae)

The pappus of the annual species of Microseris usually consistsof a constant number of five paleaceous, awn-tipped scales.In some populations of Microseris douglasii, plants with variablenumbers of pappus parts, always five or fewer, are frequent.Here we study inbred lines derived from a plant with an averageof slightly less than five pappus parts (strain D40), from aplant with an average near three pappus parts (strain B14),and from F2 plants of a hybrid (D37) between these two strains.The average deviation from five pappus parts is subject to varioussources of non-genetic variation. The peripheral achenes ineach head have more pappus parts than the central ones, andthe overall average number decreases in later-formed capitulaof a plant. Both effects are strongly affected by environmentalfactors. Against this background, we show (polygenic) geneticvariation for the average deviation from five, and additionalheritable differences in the strength of the gradient in pappuspart numbers from the periphery to the centre of the capitulum,while the decrease of pappus part numbers in later-formed capitulais essentially identical in both parental strains, even in itsresponse to environmental factors. Visible phenotypic variationdue to all sources decreases as the average nears a constantnumber of five pappus parts Asteraceae, canalized characters, developmental genetics, inflorescence, Lactuceae, Microseris douglasii, phenotypic plasticity     

Effects of seed colour heterogeneity on germination behaviour of the desert plant Lotononis platycarpa (Fabaceae)

We aimed to determine the ecological role of three seed morphs observed for the first time in a desert population of Lotononis platycarpa (Fabaceae), with respect to their germination requirements. Seeds sorted by seed coat colour (olive green, orange and brown) were germinated under laboratory conditions under two photoperiods (12/12‐h light and continuous dark) and three alternating temperature regimes (15/25, 20/30, 25/35°C). We found that the three distinct seed types differ in their seed mass, germination percentage and speed of germination. Overall, the light‐incubated seeds germinated with higher percentages than seeds in the total darkness. Furthermore, seeds with orange coat germinated with higher percentages at 15/25 and 25/35°C (up to 60%, for both) and significantly faster than the other two colour morphs. Our results suggest an adaptive significance of seed colour heterogeneity in the harsh desert habitat inhabited by the study species.     

Variability of the inflorescence ofMicroseris laciniata (Compositae: Cichorieae)

Quantitative characters of the flowering head of a garden population ofMicroseris laciniata were scored during the second, third, and fourth season of growth. Number of achenes per head, number of phyllaries per head and the average number of pappus parts per achene in single heads show significant plant to plant variation. Achenes per head and pappus parts per achene were scored in identical plants in two subsequent seasons. The number of pappus parts per achene varies freely between five and ten. This contrasts with annual species ofMicroseris in which either five or ten pappus parts are found, depending on the species. In spite of a clear plant-specific average of pappus parts, both high and low pappus part determination can be demonstrated in all specimens. The number of pappus parts depends on the position of an achene on the receptacle, marginal achenes usually having fewer pappus parts than central ones. This gradient is not closely correlated with the position of an achene on the genetic spiral.     

Role of mucilage during achene germination and sprout growth of the endangered Tibetan medicinal herb Mirabilis himalaica (Nyctaginaceae) exposed to abiotic stresses

Aim Mirabilis himalaica (Nyctaginaceae) is an endangered medicinal plant mainly distributed in the plateau region of northern Tibet, China. The outer surface of M. himalaica achenes is covered by a pectinaceous mucilaginous layer upon hydration. However, the role of the achene mucilage is poorly understood. In this study, we investigated the effects of mucilage on achene germination and sprout growth under abiotic stress to explain how M. himalaica survive the alpine environment.Methods We investigated the effect of mucilage on achenes germination by contrast the capacity of water absorption, dehydration and respiration of intact achene and the achene with mucilage removal. We performed abiotic stresses experiments including drought stress, salt stress, cold stress and high temperature stress, and quantified the effects of mucilage removal on achene germination rate, root and shoot lengths of seedlings.Important findings Mucilage is extremely hydrophilic, and the mass of intact achenes can be 9-fold greater than that of demucilaged achenes. The removal of the mucilaginous layer did not significantly change final germination percentages under ideal conditions, but intact achenes (i.e. with mucilage) took longer to germinate. The mucilage significantly decreased seed respiration rates by acting as a physical barrier that prevented oxygen diffusion. Germination rates, shoot and root growth of intact achenes were higher than those of demucilaged ones during exposures to cold, heat, osmotic and salt stresses. Achene mucilage presumably plays an ecologically important role in the life cycle of M. himalaica by aiding the critical achene germination and early seedling growth in the stressful habitats of the plateau region of northern Tibet.     

The role of the soil seed bank in the unpredictable habitat of Anthemis chrysantha J. Gay,a critically endangered species

The ability of Anthemis chrysantha to form a soil seed bank (SB) was investigated in order to understand better the adaptation of this rare annual species to its arid and unpredictable natural habitat. The natural seed bank population was analyzed in five consecutive plant cohorts (2006–2010) by sampling at two different times: in May, after the germination period, and in October, after the first dispersal episodes due to the beginning winter rainfall. In addition, to determine the persistence in the soil of the two achene morphs of this species’ (white and dark achenes), an artificial SB was created where achenes were buried and exhumed successively after each season, during two years. In all the cohorts studied, seedling emergence from May samples indicated the existence of a persistent seed bank (PSB). Moreover, the dark achenes were largely responsible for the permanent fraction because, after two years of burial in the artificial SB, 85.0% of them remained apparently healthy without having germinated, versus 9.9% of white achenes. Both types of buried achenes exhibited an annual conditional dormancy/non-dormancy cycle, induced by low winter temperatures. The PSB dynamics appeared to oscillate between the minimum values at the end of the germination period in spring (up to 2000 achenes per square meter) and the maximum values of the dispersal episodes in early autumn (up to 6000 achenes per square meter), with fluctuations of achene density due to the variability in annual rainfall. Hence, the SB showed a decline due to the failure of fruiting in the 2008 cohort, caused by drought, although the low value of ca. 560 achenes per square meter was able to establish the following population. Our study highlights the importance of the PSB, which, in “bad” years, may be critical for the persistence of this species.     

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.     

The Relationship Between Achene Weight,Embryo Weight and Germination inTaraxacumApomicts

Apomicts provide the opportunity to investigate the variation in achene size and performance that is due to factors other than genetic differences between achenes. Achene characteristics of a single capitulum from each of 15 biotypes of apomicticTaraxacumwere investigated. Within a capitulum, achene fresh weight ranged from 4.45-fold to 13.74-fold. Overall, 59.8% of achenes germinated. The probability of any one achene germinating was influenced both by its source and by its weight and ranged from 0.049 to 0.902 between capitula and from 0.028 to 0.873 between weight classes. An investigation of the relationship between achene parts in three biotypes identified a linear trend for embryos to acquire a greater proportion of total achene weight as achene weight increases. Although the heaviest achenes showed the best germination, it is predicted that, for a given investment of maternal resources, seedling number would be maximized by producing achenes of a single intermediate weight.     

盐生植物角果碱蓬种子二型性对环境的适应策略

角果碱蓬(Suaeda corniculata)是藜科一年生盐生植物, 在我国分布于北方盐碱滩涂和盐碱荒漠地区。角果碱蓬具有棕色和黑色两种异型体种子(简称棕色和黑色种子)。对采自内蒙古鄂托克前旗盐渍化生境的角果碱蓬二型种子的形态、休眠和萌发特性开展对比研究, 测定了二型种子休眠和萌发行为对温度、光照和盐分(NaCl)的响应, 以揭示盐生植物异型种子对温带盐漠生境的适应对策。结果表明: (1)二型性种子在大小、种皮特性和结实比例方面有显著差异。与黑色种子相比, 棕色种子个体较大, 种皮透水性强。黑色种子与棕色种子的结实比例约为5.6 : 1。(2)新成熟的棕色种子的萌发对各温度梯度和光照条件不敏感, 萌发率较高(84%-100%); 而新成熟的黑色种子萌发率较低(8%-78%), 萌发对光照敏感。(3)黑色种子具有浅度生理休眠, 种皮划破、赤霉素处理和低温层积均可有效地提高种子的萌发率。(4)二型种子萌发对土壤盐分的胁迫具有不同的响应。与黑色种子相比, 棕色种子对盐分胁迫不敏感, 在较高的盐分浓度下仍有较高的萌发率, 低温层积处理能够降低黑色种子对盐胁迫的敏感性, 有效地提高种子的初始萌发率、萌发恢复率和最终萌发率。角果碱蓬二型种子不同的形态、休眠和萌发特性, 提高了该物种在高度异质性生境中的适合度, 对种群成功地适应温带盐漠环境具有重要的意义。     

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.     

Reproductive traits associated with invasiveness in Conyza sumatrensis

Conyza sumatrensis (Retz.) E. Walker, a member of Asteraceae, is a highly invasive species. However, its reproduction biology remains poorly known. To understand the role of reproductive traits in successful invasion of the species, we studied several traits of its reproductive system: the miniature capitulum and gynomonoecious sexual system, the biology and phenology of capitula and florets, pollen/ovule ratio, the mating system (selfcompatibility), flower visitors, physical traits and dispersal potential of achenes, germination potential of achenes from manually pollinated capitula, and the association of these traits with invasiveness. Our study showed that the reproductive traits of autonomous seed production, versatile mating system of self- and cross-pollination, and generalized pollination system might contribute to the species' successful invasive capability. The invasiveness was further enhanced by the high and rapid production of achenes, as well as the high percentage, rapid germination rate and high dispersal capability of achenes. It was concluded that in annual or winter-annual weeds, autonomous seed production contributed significantly to the invasiveness of the species.