Intramolecular uncoupling of chromophore photoconversion from structural signaling determinants drive mutant phytochrome B photoreceptor to far-red light perception

The phytochrome (phy) photoreceptor family regulates almost all aspects of plant development in a broad range of light environments including seed germination, onset of the photomorphogenic program in seedling stage, the shade avoidance syndrome in competing plant communities, flowering induction and senescence of adult plants. During evolution two clearly distinct classes of phy-s emerged covering these very different physiological tasks.¹ PhyA is rapidly degraded in its activated state. PhyA functions in controlling seed germination at very low light intensities (very low fluence response, VLFR) and seedling establishment under photosynthetic shade conditions (high irradiance response, HIR) where the far-red portion of the transmitted light to understorey habitats is substantially enhanced. Arabidopsis phyB together with phyC, D and E belongs to the relatively stable sensor class in comparison to the light labile phyA. PhyB functions at all stages of development including seed germination and seedling establishment, mediates classical red/far-red reversible low fluence responses (LFR) as well as red light high irradiance responses, and it is considered to be the dominating phytochrome sensor of its class.     

Positive relationship between fruit removal by animals and seedling recruitment in a tropical forest

Fleshy-fruited plants rely on animal frugivores to disperse their seeds, and seed removal by frugivores may leave an imprint on seedling recruitment. However, to what extent plant–frugivore interactions are related to seedling recruitment has rarely been quantified at the community level, especially in species-rich tropical forests. In this study, we tested the effect of different plant traits on fruit removal by frugivores and tested the relative importance of fruit removal, plant traits and abiotic factors for seedling recruitment. We quantified plant–frugivore interactions of 22 fleshy-fruited plant species consumed by 56 diurnal frugivore species, and counted the number of seedlings that emerged along an elevational gradient in the Colombian Andes. We measured a set of plant traits (i.e., crop size; fruit size; seed load and mass; fruit nutritional contents), estimated the density of adult plants and recorded relevant abiotic factors (light, temperature and humidity). We found that fruit removal by frugivores was positively associated with crop size, but negatively associated with fruit length and unrelated to seed load and fruit nutritional content. Seedling densities were positively related to the density of adult plants, seed mass and fruit removal by animals. We found no relationship between abiotic factors and seedling recruitment. Our results indicate that fruit abundance and morphology are important determinants of fruit removal and that fruit removal is positively associated with seedling recruitment accounting for effects of species abundance and plant traits. We conclude that plant traits shape fruit removal and seedling recruitment at the community level, while these two crucial processes of forest regeneration are directly linked by seed dispersal of animals.     

Light limitation and litter of an invasive clonal plant,Wedelia trilobata,inhibit its seedling recruitment

Background and Aims

Invasive clonal plants have two reproduction patterns, namely sexual and vegetative propagation. However, seedling recruitment of invasive clonal plants can decline as the invasion process proceeds. For example, although the invasive clonal Wedelia trilobata (Asteraceae) produces numerous seeds, few seedlings emerge under its dense population canopy in the field. In this study it is hypothesized that light limitation and the presence of a thick layer of its own litter may be the primary factors causing the failure of seedling recruitment for this invasive weed in the field.

Methods

A field survey was conducted to determine the allocation of resources to sexual reproduction and seedling recruitment in W. trilobata. Seed germination was also determined in the field. Effects of light and W. trilobata leaf extracts on seed germination and seedling growth were tested in the laboratory.

Key Results

Wedelia trilobata blooms profusely and produces copious viable seeds in the field. However, seedlings of W. trilobata were not detected under mother ramets and few emerged seedlings were found in the bare ground near to populations. In laboratory experiments, low light significantly inhibited seed germination. Leaf extracts also decreased seed germination and inhibited seedling growth, and significant interactions were found between low light and leaf extracts on seed germination. However, seeds were found to germinate in an invaded field after removal of the W. trilobata plant canopy.

Conclusions

The results indicate that lack of light and the presence of its own litter might be two major factors responsible for the low numbers of W. trilobata seedlings found in the field. New populations will establish from seeds once the limiting factors are eliminated, and seeds can be the agents of long-distance dispersal; therefore, prevention of seed production remains an important component in controlling the spread of this invasive clonal plant.     

Arabidopsis thaliana for testing the phytotoxicity of volatile organic compounds

Geosmin and C-8 hydrocarbons are among the major volatile organic compounds (VOC) responsible for the distinctive, musty odor of filamentous fungi. In this study, we developed a plant bioassay for testing the possible toxicity of these compounds, as well as four air freshener products sometimes used to mask their odor. Seeds and vegetative plants of Arabidopsis thaliana were exposed to 1 ppm of 14 different volatile treatments (both single compounds and mixtures) for 72 h and monitored for germination rate, seedling formation, vegetative plant vigor and chlorophyll concentration. All VOCs tested had some inhibitory effect on seed germination or seedling formation; 1-octen-3-one was the most active, giving almost complete inhibition of germination. Geosmin did not prevent germination (radicle protrusion) but seedling formation was arrested 90 %. Of solvents and fragrance products tested, only the scented oil product was as active as the C-8 biogenic compounds in inhibiting seed germination and seedling formation. Two-week-old plants exposed to 1 ppm of individual fungal VOCs for 72 h all exhibited some degree of stress symptoms including smaller leaf size and weight, discoloration, leaf curling, small necrotic lesions, and reduced chlorophyll concentration. Two-week-old vegetative plants exposed to solvents and air freshener products were generally smaller in size. The single most phytotoxic compound tested was 1-octen-3-one which almost completely inhibited seed germination and was lethal to vegetative phase plants. Formaldehyde at 1 ppm killed 2-week-old seedlings but had little effect on seed germination. In conclusion, the A. thaliana bioassay provides an inexpensive approach for testing the toxicity of gas phase molecules.     

Variation in mycorrhizal performance in the epiphytic orchid <Emphasis Type="Italic">Tolumnia variegata in vitro</Emphasis>: the potential for natural selection

Symbiotic seed germination is a critical stage in orchid life histories. Natural selection may act to favor plants that efficiently use mycorrhizal fungi. However, the necessary conditions for natural selection – variation, heritability, and differences in fitness – have not been demonstrated for either orchid or fungus. With the epiphytic orchid Tolumnia variegata as a model system, we ask the following questions: (1) Do seeds from different individuals in a population differ in germination and seedling development in the presence of the same fungi? (2) Do different mycorrhizal fungi (Ceratobasidium spp.) differ in ability to stimulate seed germination and growth in T. variegata? And (3) are the Ceratobasidium isolates that best induce seed germination and seedling development more closely related to each other than to isolates that are less effective? We performed symbiotic seed germination experiments in vitro. The experiments were done using mycorrhizal fungi isolated from T. variegata; relationships among the fungi were inferred from nuclear ribosomal ITS sequences. We found significant variation for both symbiotic germination and seedling growth among biparental seed crops obtained from a population of T. variegata plants. Differences among Ceratobasidium fungi in seed germination were significant. The fungi that induced highest seed germination and seedling development belonged to two of four clades of Ceratobasidium. The two experiments show that there is potential for natural selection to act on orchid–fungus relationships. Given that orchids vary in performance, and that mycorrhizal fungi are not geographically distributed homogeneously, mycorrhizae may affect population size, distribution and evolution of orchids.     

果肉和埋土深度对新疆野杏种子萌发与幼苗生长的影响

为探明新疆野杏（Armeniaca vulgaris）种子萌发与幼苗生长对果肉和埋土深度的响应,以期为新疆野杏的天然更新与实生苗培育提供理论参考。通过2种果皮结构（有果肉和无果肉）的种子在不同埋土深度（地表至18.0 cm的14个梯度）对新疆野杏种子萌发和幼苗生长进行研究,旨在揭示果皮结构和埋土深度对新疆野杏种子萌发与成苗能力的影响。结果表明：果肉和埋土深度显著影响野杏种子的萌发、幼苗生长与质量（P<0.05）。埋土深度<3.0 cm不利于成苗,埋土深度>6.0 cm时,萌发能力与幼苗生长量随埋土深度的增加而降低,3.0～6.0cm为适宜埋土深度。无果肉种子萌发优于有果肉种子,萌发率、萌发指数、成苗率、活力指数分别增长了37.18%、3.88%、37.18%、26.59%,幼苗高、基径、叶片数量、根冠比、幼苗质量指数分别增长了36.99%、7.48%、68.69%、20.61%、14.29%,其萌发能力与幼苗生长量显著高于有果肉种子（P<0.05）。有无果肉种子的萌发和幼苗生长指标与埋土深度呈显著负相关（P<0.05）。研究结果表明,无果肉处理对新疆野杏种...     

How detrimental are seed galls to their hosts? Plant performance,germination, developmental instability and tolerance to herbivory in Inga laurina,a leguminous tree

• Gall inducers use these structures as shelters and sources of nutrition. Consequently, they cause multiple physiological changes in host plants.
• We studied the impact caused by seed coat galls of a braconid wasp on the performance of fruits, seeds and seedlings of tree Inga laurina. We tested whether these seed galls are ‘nutrient sinks’ with respect to the fruit/seed of host plant, and so constrain the reproductive ability and reduce seedling longevity. We measured the influence of such galls on the secondary compounds, fruit and seed parameters, seed viability and germination and seedling performance.
• Inga laurina has indehiscent legumes with polyembryonic seeds surrounded by a fleshy sarcotesta rich in sugars. The galls formed inside the seed coat and galled tissues presented higher phenol concentrations, around 7‐fold that of ungalled tissues. Galls caused a significant reduction in parameters such as fruit and seed size, seed weight and the number of embryos. Fluctuating asymmetry (a stress indicator) was 31% higher in leaves of galled seed plants in comparison to ungalled seed plants. However, the negative effects on fruit and seed parameters were not sufficient to reduce seed germination (except the synchronization index) or seedling performance (except leaf area and chlorophyll content).
• We attributed these results to the ability of I. laurina to tolerate gall attack on seeds without a marked influence on seedling performance. Moreover, because of the intensity of seed galling on host plant, we suggest that polyembryony may play a role in I. laurina reproduction increasing tolerance to seed damage.

     

Responses of Primula vulgaris to light quality in the maternal and germination environments

• In the model species Arabidopsis thaliana phytochromes mediate dormancy and germination responses to seasonal cues experienced during seed maturation on the maternal plants. However, the effect of the maternal light environment on seed germination in native wild species has not been well studied. This is particularly important given its practical application in the context of environmental restoration, when there can be marked changes in the canopy.
• Plants of Primula vulgaris were grown in the field over two vegetative seasons under four shading treatments from low to high ratio of red to far‐red light (R:FR). Leaf and seed traits were assessed in response to the light treatments. The germination of seeds from these four maternal environments (pre‐dispersal) was investigated at seven light and five temperature treatments (post‐dispersal).
• Thinner leaves, larger leaf area and greater chlorophyll content were found in plants growing in reduced R:FR. Shading in the maternal environment led to increased seed size and yield, although the conditions experienced by the maternal plants had no effect on seed germination. Seeds responded strongly to the cues experienced in their immediate germination environment. Germination was always enhanced under higher R:FR conditions.
• The observed phenotypic trait variation plays a major role in the ability of P. vulgaris to grow in a wide range of light conditions. However, the increased germination capacity in response to a higher R:FR for all maternal environments suggests potential for seedling establishment under vegetative shade only in the presence of canopy gaps.

     

Resource allocation in an annual herb: Effects of light,mycorrhizal fungi,and defoliation

Concurrent interactions and the availability of resources (e.g., light) affect the cost/benefit balance during mutualistic and antagonistic interactions, as well as plant resource allocation patterns. Mycorrhizal interactions and herbivory concur in most plants, where mycorrhizae can enhance the uptake of soil nutrients by plants as well as consuming a large fraction of the plant's carbon, and defoliation usually reduces light interception and photosynthesis, thereby causing direct losses to the hosts of mycorrhizal fungi. Both types of interactions affect the carbon budget of their host plants and thus we predict that the relative costs of herbivory and mycorrhizal colonization will increase when photosynthesis is reduced, for instance in light limited environments. We conducted a greenhouse experiment using Datura stramonium to investigate the effects of defoliation and mycorrhizal inoculation on the resource allocation patterns in two different light environments. Defoliated plants overcompensated in terms of leaf mass in both light environments, but total seed mass per fruit was negatively affected by defoliation in both light environments. Mycorrhizal inoculation had a positive effect on vegetative growth and the leaf nitrogen content, but defoliation negates the benefit of mycorrhizal interactions in terms of the leaf nitrogen content. In general, D. stramonium compensated for the relative costs of concurrent mycorrhizal interactions and defoliation; plants that lacked both interactions exhibited the same performance as plants with both types of interactions.     

Habitat-specific responses of seed germination and seedling establishment to soil water condition in two Rheum species in the high Sino-Himalayas

Knowledge of how germination and seedling establishment respond to soil water condition is crucial for plant conservation under global warming and land-use changes. We tested the flooding and drought tolerance of two plant species with different occurrences along a soil water gradient by assessing seed germination, seedling survival, seedling growth, and root characteristics. In the high Sino-Himalayas, Rheum alexandrae typically occurs in wetlands, R. nobile in scree or open slope with well-drained soils. Seeds and seedlings of the two species were subjected to different soil water conditions in controlled greenhouse experiments. Seed germination in both species was inhibited by high soil water content; however, seeds of R. alexandrae were more tolerant to flooding, especially to submergence. Seedling survival, biomass accumulation, root diameter, and root porosity of R. alexandrae increased significantly with increased soil water content, but submergence was lethal for seedlings. Seedling survival, biomass accumulation, and root length of R. nobile increased significantly in response to reduced soil water content. These results indicate that in the two species, seed germination and seedling establishment in response to different soil water condition are habitat-specific. Because both species are susceptible to moderate changes in soil water condition, their species-specific requirements with respect to this factor should be a consideration when planning their conservation.     

The Distribution of Fruit and Seed Toxicity during Development for Eleven Neotropical Trees and Vines in Central Panama

Secondary compounds in fruit mediate interactions with natural enemies and seed dispersers, influencing plant survival and species distributions. The functions of secondary metabolites in plant defenses have been well-studied in green tissues, but not in reproductive structures of plants. In this study, the distribution of toxicity within plants was quantified and its influence on seed survival was determined in Central Panama. To investigate patterns of allocation to chemical defenses and shifts in allocation with fruit development, I quantified variation in toxicity between immature and mature fruit and between the seed and pericarp for eleven species. Toxicity of seed and pericarp was compared to leaf toxicity for five species. Toxicity was measured as reduced hyphal growth of two fungal pathogens, Phoma sp. and Fusarium sp., and reduced survivorship of brine shrimp, Artemia franciscana, across a range of concentrations of crude extract. I used these measures of potential toxicity against generalist natural enemies to examine the effect of fruit toxicity on reductions of fruit development and seed survival by vertebrates, invertebrates, and pathogens measured for seven species in a natural enemy removal experiment. The seed or pericarp of all vertebrate- and wind-dispersed species reduced Artemia survivorship and hyphal growth of Fusarium during the immature and mature stages. Only mature fruit of two vertebrate-dispersed species reduced hyphal growth of Phoma. Predispersal seed survival increased with toxicity of immature fruit to Artemia during germination and decreased with toxicity to fungi during fruit development. This study suggests that fruit toxicity against generalist natural enemies may be common in Central Panama. These results support the hypothesis that secondary metabolites in fruit have adaptive value and are important in the evolution of fruit-frugivore interactions.     

In situ seed baiting to isolate germination-enhancing fungi for an epiphytic orchid,Dendrobium aphyllum (Orchidaceae)

Orchid conservation efforts, using seeds and species-specific fungi that support seed germination, require the isolation, identification, and germination enhancement testing of symbiotic fungi. However, few studies have focused on developing such techniques for the epiphytes that constitute the majority of orchids. In this study, conducted in Xishuangbanna Tropical Botanical Garden, Yunnan, China, we used seeds of Dendrobium aphyllum, a locally endangered and medicinally valuable epiphytic orchid, to attract germination promoting fungi. Of the two fungi isolated from seed baiting, Tulasnella spp. and Trichoderma spp., Tulasnella, enhanced seed germination by 13.6 %, protocorm formation by 85.7 %, and seedling development by 45.2 % (all P?Epulorhiza, another seed germination promoting fungi isolated from Cymbidium mannii, also enhanced seed germination (6.5 %; P?P?Trichoderma suppressed seed germination by 26.4 % (P?Tulasnella was the only treatment that produced seedlings. Light increased seed imbibition, protocorm formation, and two-leaved seed development of Tulasnella inoculated seeds (P?Tulasnella be introduced for facilitating D. aphyllum seed germination at the protocorm formation stage and that light be provided for increasing germination as well as further seedling development. Our findings suggest that in situ seed baiting can be used to isolate seed germination-enhancing fungi for the development of seedling production for conservation and reintroduction efforts of epiphytic orchids such as D. aphyllum.     

Does endophyte symbiosis resist allelopathic effects of an invasive plant in degraded grassland?

Allelopathic effects and plant associated systemic endophytic fungi are often thought to play a role in the invasion of exotic plant species. Here, we tested the inhibitory effects of the aqueous extracts of the hemiparasitic weed Pedicularis kansuensis on seed germination and seedling growth of endophyte-free (E−) and -infected (E+) grass species, Stipa purpurea and Elymus tangutorum. The weed extracts significantly inhibited both seed germination and seedling growth of the target grass species. Extracts from the inflorescences gave greater inhibition than those from the stems or roots, while the concentration of the extract had a direct effect on the extent of inhibition. The E+ target plants were less susceptible to the extracts than their E-counterparts. Our results suggest that the allelopathic potential of P. kansuensis will lead to increased frequencies of endophyte infected plants in grass populations.     

Do novel weapons that degrade mycorrhizal mutualisms promote species invasion?

Non-native plants often dominate novel habitats where they did not co-evolve with the local species. The novel weapons hypothesis suggests that non-native plants bring competitive traits against which native species have not adapted defenses. Novel weapons may directly affect plant competitors by inhibiting germination or growth, or indirectly by attacking competitor plant mutualists (degraded mutualisms hypothesis). Japanese knotweed (Fallopia japonica) and European buckthorn (Rhamnus cathartica) are widespread plant invaders that produce potent secondary compounds that negatively impact plant competitors. We tested whether their impacts were consistent with a direct effect on the tree seedlings (novel weapons) or an indirect attack via degradation of seedling mutualists (degraded mutualism). We compared recruitment and performance using three Ulmus congeners and three Betula congeners treated with allelopathic root macerations from allopatric and sympatric ranges. Moreover, given that the allelopathic species would be less likely to degrade their own fungal symbiont types, we used arbuscular mycorrhizal (AMF) and ectomycorrhizal (ECM) tree species to investigate the effects of F. japonica (no mycorrhizal association) and Rhamnus cathartica (ECM association) on the different fungal types. We also investigated the effects of F. japonica and R. cathartica exudates on AMF root colonization. Our results suggest that the allelopathic plant exudates impact seedlings directly by inhibiting germination and indirectly by degrading fungal mutualists. Novel weapons inhibited allopatric seedling germination but sympatric species were unaffected. However, seedling survivorship and growth appeared more dependent on mycorrhizal fungi, and mycorrhizal fungi were inhibited by allopatric species. These results suggest that novel weapons promote plant invasion by directly inhibiting allopatric competitor germination and indirectly by inhibiting mutualist fungi necessary for growth and survival.     

Pasture seed dynamics in a dry monsoonal climate,II The effect of water availability,light and temperature on germination speed and seedling survival of Stylosanthes humilis and Digitaria ciliaris

Environmental factors affecting speed of germination and survival of naturally occurring seed of the legume Stylosanthes humilis and the grass Digitaria ciliaris were studied. Previous studies on the seedbed environment had shown that germination speed, defined as the proportion of seeds capable of germinating in the first 12 h period, was likely to be the most relevant laboratory measurement. Ten studies were conducted to compare species performance and to devise a standard technique for the measurement of germination speed under laboratory conditions. For S. humilis, the level and duration of submergence in water in the germination test had a large effect on germination speed (from 0–70% in 12 h). The absence of light delayed germination (7% in 12 h compared to 52% under light), but did not prevent complete germination over 7 days. Optimum temperature was 25°C with a decrease to 0% at 30°C. A standard technique was developed for measuring potential germination speed which gave reproducible results allowing differences in seed lots to be described. D. ciliaris seeds sampled just prior to field germination in the early storm period were less sensitive to environmental control. Temperature optimum for germination decreased from 40 to 30°C with the duration of the germination test. Seeds sampled at the time of seed dispersal (early dry season) had a high fraction (80%) requiring light for germination but this requirement had been lost by the early storm period. Investigations on the role of leaching (water movement) showed that long periods of water movement (12–18 h) increased the speed of germination of both species and in the case of S. humilis overcame environmental blocks to rapid germination (absence of light or 30°C temperature). Root elongation rates in S. humilis were not greatly affected by temperature over the range of seedbed temperatures when moisture is available (25–40°C) in the field. However, D. ciliaris was delayed in development at 25°C, the most likely overnight surface soil temperature. D. ciliaris was more susceptible to death by desiccation once germination had occurred compared to S. humilis. In both species seedling death due to desiccation increased with stage of development. The better relative performance of S. humilis in the germination-establishment phase of the life cycle, which had been observed in a previous study, could be explained by a qualitative comparison of the species' attributes. This suggested that D. ciliaris was more likely to germinate at rainfall events following which there was a greater chance of seedling death     

Proteome changes in Oncidium sphacelatum (Orchidaceae) at different trophic stages of symbiotic germination

Mutualistic symbioses between plants and fungi are a widespread phenomenon in nature. Particularly in orchids, association with symbiotic fungi is required for seed germination and seedling development. During the initial stages of symbiotic germination, before the onset of photosynthesis, orchid protocorms are fully mycoheterotrophic. The molecular mechanisms involved in orchid symbiotic germination and development are largely unknown, but it is likely that changes in plant energy metabolism and defense-related responses play a central role in these processes. We have used 2D-LC-MS/MS coupled to isobaric tagging for relative and absolute quantification to identify proteins with differential accumulation in Oncidium sphacelatum at different stages of mycorrhizal protocorm development (achlorophyllous and green protocorms) after seed inoculation with a Ceratobasidium sp. isolate. We identified and quantified 88 proteins, including proteins putatively involved in energy metabolism, cell rescue and defense, molecular signaling, and secondary metabolism. Quantitative analysis showed that the expected changes in carbon metabolism in green protocorms were accompanied by enhanced accumulation of proteins involved in the modulation of reactive oxygen species homeostasis, defense-related responses, and phytoalexins and carotenoid biosynthesis. Our results suggest profound metabolic changes in orchid protocorms during the switch from the fully mycoheterotrophic to the photosynthetic stage. Part of these changes may be also related to the obligatory nature of the interaction with the endomycorrhizal fungus.     

光和不同打破种子休眠方法对紫茎泽兰种子萌发及幼苗状态的影响

紫茎泽兰是著名的外来入侵植物,作为入侵的第一步,发芽及其幼苗生长应该与其强入侵能力有关.基于此,通过不同光照强度处理和不同打破休眠方法的双因素实验,旨在探讨紫茎泽兰种子是否具有需光萌发特性以及低温、水杨酸、聚乙二醇,硝酸钾等常规打破休眠方法和光照如何共同影响其萌发、幼苗生长等问题.结果表明:在全光照条件下,不同处理的紫茎泽兰种子的萌发率均大于63％,铝箔纸覆盖的遮光条件(0.23％光照)萌发率均大于60％,而在完全黑暗条件下,其萌发率较低(均小于30％),这表明紫茎泽兰种子具有需光萌发的特性.有别于以往对其它植物种子的报道,低温处理、水杨酸处理、聚乙二醇处理和硝酸钾处理不能代替光照打破种子休眠,显示紫茎泽兰种子可能处于一种强迫休眠状态(种子静态).全光照与水杨酸处理、PEG处理对幼苗生长具有交互影响:黑暗下水杨酸处理浓度与幼苗生物量成正相关(P＜0.05),但全光照和加铝箔下不相关(P＞0.05);全光照下PEG处理浓度与根长显著正相关(P＜0.05),而加铝箔和黑暗下不相关(P＞0.05).紫茎泽兰种子需光萌发特征及其幼苗生长特点是人为破坏表土壤、深层土壤种子库地表化导致快速入侵的基础.结果也为通过引入适宜树种造林来控制光照因子对紫茎泽兰进行生态控制提供了理论依据.     

入侵植物小花山桃草种群构件生物量结构及种子萌发特征

通过野外设置样方调查和室内萌发试验,研究小花山桃草种群各构件生物量的结构特征和它们之间的关系模型、繁殖分配以及种子萌发特点。结果表明:(1)小花山桃草根、茎、叶、花(果)序生物量与植株高度之间以及各构件生物量之间均呈正相关关系,可用幂函数模型或线性函数模型较好地表达;(2)各构件生物量在个体生物量中所占的比率表现为茎>花序>叶>根;(3)小花山桃草的繁殖投入和繁殖分配都随植株个体的增大而增加;(4)小花山桃草个体大小和繁殖投入之间为线性关系,而个体大小和繁殖分配之间为幂函数关系;(5)小花山桃草存在一个较小的繁殖阈值(0.6043g);(6)小花山桃草种子在有光照(12h)和黑暗条件下发芽率均可达到85%以上;未经贮藏的种子不萌发,低温沙藏(1～2℃)和室温干藏(14～32℃)一个半月种子萌发率分别可达92.5%和79%;低温沙藏时种子即可发芽,且发芽率可达61%。在研究地区,小花山桃草几乎整个生长季都可萌发,甚至初冬还有幼苗产生。小花山桃草构件生物量结构和繁殖分配特征、种子萌发特点等都有助于其入侵能力的提高,是其成功入侵我国的重要原因。     

THE EFFECT OF SEED SIZE AND MATERNAL SOURCE ON INDIVIDUAL SIZE IN A POPULATION OF LUDWIGIA LEPTOCARPA (ONAGRACEAE)

Seed size is normally distributed for many annual species, while mature plant size is frequently positively skewed. A study was conducted to determine the influence of seed size and the role of genetic differences in determining relative seedling size for Ludwigia leptocarpa. Seed size had a significant effect on percentage germination and time of seed germination but no effect on dry weight or leaf area of seedlings. Seed size and spacing had a significant effect on seedling dry weight for plants grown under competition, while relative day of emergence had no effect. Familial (genetic) differences were found in average seed weight between maternal plants, but not in average number of days to germination, average weight of seeds which germinated, or shoot dry weight. It is concluded that neither seed size alone nor genetic differences between plants are directly responsible for the development of size hierarchies in Ludwigia leptocarpa populations. Large seed size does convey an advantage in growth when plants from seeds of differing initial size interact.     

Germination of salt-stressed seeds as related to the ethylene biosynthesis ability in three Stylosanthes species

Stylosanthes, a genus of tropical forage legume, is known to exhibit good persistence in saline soils, yet mechanisms for regulation of seed germination under salt stress are poorly understood. This study was carried out to evaluate the mode of action of salt stress on seed germination of Stylosanthes. 1-Aminocyclopropane-1-carboxylic acid (ACC) increased ethylene biosynthesis and germination of NaCl-inhibited seeds in a dose-dependent manner. Contents of ACC and germination of Stylosanthes humilis seeds increased following transfer from NaCl solution to deionised water, but not after transfer to l-α-(2-aminoethoxyvinyl)-glycine (AVG) solution, an inhibitor of ethylene biosynthesis. Ethylene biosynthesis was much larger in NaCl-treated seeds of Stylosanthes guianensis than in seeds of S. humilis and Stylosanthes capitata, a fact which was reflected in higher germination rates. S. guianensis seedlings also displayed higher growth and survival rates than S. humilis and S. capitata under salt stress. Moreover, smaller ACC levels, as well as reduced ethylene biosynthesis of S. capitata seeds were accompanied by lower germination under salt stress. In addition, S. capitata seedlings treated with NaCl solutions exhibited relatively lower growth and survival rates in comparison with S. humilis and S. guianensis. Thus, different abilities to synthesize ethylene by S. guianensis, S. humilis and S. capitata seeds explain the differences in tolerance to salt stress of the three species.