Seed Rain and Seed Bank Reveal that Seed Limitation Strongly Influences Plant Community Assembly in Grasslands

Dispersal is an important factor in plant community assembly, but assembly studies seldom include information on actual dispersal into communities, i.e. the local propagule pool. The aim of this study was to determine which factors influence plant community assembly by focusing on two phases of the assembly process: the dispersal phase and the establishment phase. At 12 study sites in grazed ex-arable fields in Sweden the local plant community was determined and in a 100-m radius around the centre of each site, the regional species pool was measured. The local seed bank and the seed rain was explored to estimate the local propagule pool. Trait-based models were then applied to investigate if species traits (height, seed mass, clonal abilities, specific leaf area and dispersal method) and regional abundance influenced which species from the regional species pool, dispersed to the local community (dispersal phase) and which established (establishment phase). Filtering of species during the dispersal phase indicates the effect of seed limitation while filtering during the establishment phase indicates microsite limitation. On average 36% of the regional species pool dispersed to the local sites and of those 78% did establish. Species with enhanced dispersal abilities, e.g. higher regional abundance, smaller seeds and dispersed by cattle, were more likely to disperse to the sites than other species. At half the sites, dispersal was influenced by species height. Species establishment was however mainly unlinked to the traits included in this study. This study underlines the importance of seed limitation in local plant community assembly. It also suggests that without information on species dispersal into a site, it is difficult to distinguish between the influence of dispersal and establishment abilities, and thus seed and microsite limitation, as both can be linked to the same trait.     

Seed Number, Seed Size and Seed Diversity in Washington Lupin (Lupinus polyphyllus Lindl.)

Over 20 000 seeds of the Washington lupin (Lupinus polyphyllusLindl.) were examined and measured in an experiment carriedout over a 10 year period (1989–1999). Four differentgroups of Washington lupin seeds were found: dark, patterned,grey and light seeds. During the 10 year experiment, the totalaverage number of seeds per plant decreased from 2654 (1990)to 1220 (1999), there was a slight decrease in seed weight perplant and an increase in the average weight per seed. Therewas a clear seed size/number trade-off at the intraspecificlevel. The relative proportions of each seed group also changedwith patterned seeds becoming dominant (50% at the beginningand 90% at the end of the experiment), grey seeds remained constant(constituting 10% of the total seeds) and the proportion ofboth dark (33% at the beginning and 5% at the end) and light(10% at the beginning and 5% at the end of the experiment) seedsdecreased. Six hundred and sixty nine seeds were found to havea different testa ornamentation; they were distributed amongthe different groups as follows: 48% patterned seeds, 29% darkseeds, 12% grey seeds and 11% light seeds. There were no significantdifferences in the physical dimensions of the ornamented seedscompared with all other seeds. The results suggest that theWashington lupin is a mixture of different species and botanicalforms; this is discussed in relation to possible selection pressuresto produce both smaller and larger seeds. The possibility thatchanges in testa ornamentation are influenced by genes controllingthe synthesis of the seed coat pigment is discussed. Copyright2001 Annals of Botany Company Lupinus polyphyllus Lindl., lupine, plant biotechnology, plant parameters, selection pressure, seed size/number trade-off, testa ornamentation     

Seed Rain and Seed Limitation in a Planted Gallery Forest in Brazil

With seeds collected monthly during one year from 53 1‐m² seed traps, we investigated the seed rain and seed limitation in a gallery forest planted in 1994 in SE Brazil. Contrasting animal‐ (zoochorous) and wind‐dispersed (anemochorous) plants we investigated (1) which aspects of the composition and structure of the vegetation influence the abundance and species richness of the seed rain; (2) if such influences differ between zoochorous and anemochorous seeds; (3) if the abundance and richness of the seed rain sampled under zoochorous and nonzoochorous plant species differ; and (4) if seed limitation (given by the proportion of sites to which seeds were not dispersed) differs between zoochorous and anemochorous plant species, and also between species that have been planted and those that further colonized the area (colonists). Seed rain was intense and dominated by anemochorous species. The overall seed rain was not influenced by the vegetation parameters we analyzed (canopy height and cover, plant size, abundance, and richness) or by the plant species above the seed trap. The abundance and richness of zoochorous seeds in a given spot was influenced by the abundance and richness of zoochorous plants in its immediate vicinity. Seed limitation was higher for anemochorous than zoochorous species and higher for planted than for colonist species. We concluded with recommendations for the initial establishment of a planted forest, including the homogeneous distribution of zoochorous plants to permit a spatially homogeneous zoochorous seedfall, which will likely enhance the chances of survival and successful establishment of seeds.     

Seed dispersal in fens

Question: How does seed dispersal reduce fen isolation and contribute to biodiversity? Location: European and North American fens. Methods: This paper reviews the literature on seed dispersal to fens. Results: Landscape fragmentation may reduce dispersal opportunities thereby isolating fens and reducing genetic exchange. Species in fragmented wetlands may have lower reproductive success, which can lead to biodiversity loss. While fens may have always been relatively isolated from each other, they have become increasingly fragmented in modern times within agricultural and urban landscapes in both Europe and North America. Dispersal by water, animals and wind has been hampered by changes related to development in landscapes surrounding fens. Because the seeds of certain species are long‐lived in the seed bank, frequent episodes of dispersal are not always necessary to maintain the biodiversity of fens. However, of particular concern to restoration is that some dominant species, such as the tussock sedge Carex stricta, may not disperse readily between fens. Conclusions: Knowledge of seed dispersal can be used to maintain and restore the biodiversity of fens in fragmented landscapes. Given that development has fragmented landscapes and that this situation is not likely to change, the dispersal of seeds might be enhanced by moving hay or cattle from fens to damaged sites, or by reestablishing lost hydrological connections.     

Seed germination in Gactaceae

The mode of germination of representatives of 89 genera of the Cactaceae, 4 genera of Portulacaceae and 1 genus of Phytolaccaceae was studied. Most of the species of the Cactaceae germinate by means of a seed lid (operculum). In the Cactaceae studied 11 kinds of germination could be distinguished, 3 of which were with, and 8 without, operculum formation.
Opercula are restricted in their occurrence to the subfamilies Cactoideae (Cereoideae) and Pereskioideae and are not found in the subfamily Opuntioideae. Within the subfamily Cactoideae operculum formation was found to occur in all tribes and in all investigated subtribes. Opercula were also found in two genera of the related family of the Portulacaceae. In the Phytolaccaceae no operculum formation was observed.     

Seed development in Pisum

The Ih and lh _i alleles have been shown previously to reduce the level of endogenous gibberellin A₁ (GA₁) in shoots of pea (Pisum sativum L.), resulting in a dwarf phenotype compared with the wild type, cv. Torsdag (Lh). In addition, plants homozygous for the lh ⁱ allele have reduced seed yield compared with Lh (tall, wild type) and lh (dwarf) plants. In this paper we show that the lh ⁱ mutation is expressed in developing seeds and pods. Comparison of GA levels in young shoots and developing seeds of genotypes lh and lh ⁱ demonstrates that the relative severity of the two mutations varies in different tissues. Homozygous h ⁱ seeds have reduced GA levels, weigh less, and are less likely to develop to maturity when compared with Lh seeds. However, fertilization of lh ⁱ plants with Lh pollen increases seed GA levels, seed weight and seed survival, indicating that an increase in seed GA levels due to the presence of the Lh allele can restore normal seed growth. Pods developing on self-pollinated lh ⁱ plants are shorter than pods on Lh (wild type) plants, although this may be an indirect effect of the increased seed abortion of lh _i plants. Based on these results we suggest that endogenous GAs play an important role in the development of seeds of P. sativum L.Abbreviations GA_(n) gibberellin A_n We wish to thank Katherine McPherson, Peter Newman, Leigh Johnson and Peter Bobbi for technical assistance, Professor L. Mander (ANU, Canberra) and Professor B.O. Phinney (UCLA, USA) for labelled GA standards, and the Australian Research Council for financial support.     

种子超干贮藏技术应用面临的问题和研究方向

种子超干贮藏技术对种质资源保存具有很大的应用前景,但鉴于目前的研究进展,其应用还存在诸多技术上的问题。本文对种子超干贮藏技术在种子库中应用所面临的挑战以及需要开展的研究内容进行了如下几个方面的讨论：种子贮藏安全含水量下限的多样性决定了超干技术应用的复杂性;超干贮藏含水量安全下限的确定尚未有可靠的方法;超干贮藏的种子吸胀前需要恰当的预处理以避免吸胀损伤;超干处理前外源抗氧剂预处理可能改善超干贮藏效果;无氧条件可能对超干贮藏具有增效作用;糖类物质以及两性物质对种子在超干处理及超干贮藏过程中的保护作用。     

Source-sink Relationships, Seed Sucrose Levels and Seed Growth Rates in Soybean

Field experiments using two soybean (Glycine max L. Merrill)cultivars (‘Elgin 87’ and ‘Essex’) wereconducted for 2 years near Lexington, KY, USA to evaluate theeffect of source-sink alterations on seed carbohydrate statusand growth. Sucrose concentrations in developing cotyledonsof control plants were consistently low (<50 m M) early inseed development, but they increased to 100–150 m M byphysiological maturity. The concentrations increased in bothyears by 47 to 59% when 90% of the pods were removed from ‘Elgin87’, but the increase had no effect on individual seedgrowth rate (SGR). Shading (80%) reduced cotyledon sucrose levelsand SGR in both years. The critical cotyledon sucrose concentration(the concentration providing 80% of the maximum cotyledon growthrate) was estimated fromin vitro cotyledon growth at sucroseconcentrations of 0–200 m M. These critical concentrationsvaried from 72–124 m M;in planta control cotyledon sucroseconcentrations were below this critical level during the firsthalf of seed growth but exceeded it in the later stages of growthin all experiments. The estimated critical concentration wasconsistent with the failure of in planta SGR to respond to anincrease in assimilate supply and with the reduction in SGRassociated with a decrease in assimilate supply. The resultssuggest that soybean SGR is generally sink limited if photosynthesisincreases during seed filling, but source limited if photosynthesisis reduced. Copyright 2001 Annals of Botany Company Glycine max(L.) Merrill, soybean, source-sink ratios, sucrose, starch, depodding, shade, in vitro culture     

种子胎萌机制研究进展

种子胎萌是内在的遗传基础和外部环境共同作用的结果,受许多基因的调控和植物激素的影响。近些年来,随着分子生物学的快速发展,种子胎萌研究已经深入到分子水平。分子生物学技术的运用,特别是基因的克隆与表达、植物激素的生物合成与信号转导和分子遗传学等手段已成为研究种子胎萌的新工具和新方向。现从种皮色泽基因R、矮杆基因Rht3以及Viviparous(Vp)基因家族等方面就种子胎萌相关基因与胎萌关系进行了综述;并对植物激素脱落酸(ABA)和赤霉素(GA)的生物合成或信号转导在种子胎萌的调控中的作用等方面进行综述。     

种子细胞——组织工程连载之三

种子细胞也是组织工程的核心研究内容,获得足够数量和质量的种子细胞是开展体外组织工程的必要基础。用于组织工程的种子细胞必须具有形成新组织结构的能力,主要来源于自体、同种异体或异种,在具体应用时各有利弊。一些成体干细胞由于不存在伦理争议以及发育分化条件相对简单等优势是重要的种子细胞,包括造血干细胞、骨髓干细胞、神经干细胞、脂肪干细胞、皮肤干细胞。人胚胎干细胞及其组织工程要真正在临床医学中得到应用,还有很长的一段路要走。其他一些细胞也可以作为组织工程种子细胞,包括内皮细胞、上皮细胞、成纤维细胞、骨细胞、成骨细胞、角质细胞、前脂肪细胞、脂肪细胞、肌腱细胞等。这些细胞已分化,分裂能力有限,但仍应用于组织工程。理想的种子细胞具有一定标准。     

Seed dormancy in alpine species

In alpine species the classification of the various mechanisms underlying seed dormancy has been rather questionable and controversial. Thus, we investigated 28 alpine species to evaluate the prevailing types of dormancy. Embryo type and water impermeability of seed coats gave an indication of the potential seed dormancy class. To ascertain the actual dormancy class and level, we performed germination experiments comparing the behavior of seeds without storage, after cold-dry storage, after cold-wet storage, and scarification. We also tested the light requirement for germination in some species. Germination behavior was characterized using the final germination percentage and the mean germination time. Considering the effects of the pretreatments, a refined classification of the prevailing dormancy types was constructed based on the results of our pretreatments. Only two out of the 28 species that we evaluated had predominantly non-dormant seeds. Physiological dormancy was prevalent in 20 species, with deep physiological dormancy being the most abundant, followed by non-deep and intermediate physiological dormancy. Seeds of four species with underdeveloped embryos were assigned to the morphophysiologial dormancy class. An impermeable seed coat was identified in two species, with no additional physiological germination block. We defined these species as having physical dormancy. Light promoted the germination of seeds without storage in all but one species with physiological dormancy. In species with physical dormancy, light responses were of minor importance. We discuss our new classification in the context of former germination studies and draw implications for the timing of germination in the field.     

Seed hemagglutinins

Substances which agglutinate red blood cells are widely distributed in nature, particularly in the seeds of leguminous plants. Some of these have been isolated in a high state of purity and carefully studied with respect to their chemical and physical properties, immunological behavior, and toxicological effects on animals. Of these, the hemagglutinins of the castor bean, jack bean, soybean, and varieties of Phaseolus vulgaris have received the most attention. The possible nutritional significance of those hemagglutinins which are found in edible legumes is discussed. A mechanism is proposed to explain the nature of the interaction of hemagglutinins with red blood cells, and their possible function in the plant.     

Seed biology

Seed biology is an intensive area of study,a reflection of the significance of seeds for several scientific areas.From an evolutionary perspective,the ability of plants to make seeds has conf erred major selective advantages,accounting,in part,for the success of seed plants as the largest and most species-rich group of land plants.The seed habit facilitates fertilization in non-aqueous environments,provides a level of physical and biological protect!on for the female gametophyte and embryo,allows for the massive accumulation of storage proteins,lipids,and/or carbohydrates in the embryo and/or endosperm to serve as a nutrient reserve for the germinating seed and developing seedling,and permits the embryo to remain in a developmentally arrested and metabolically quiescent state until environ mental conditions permit germination to occur(Steeves 1983).