Spatial genetic structure in populations of the terrestrial orchid Orchis cyclochila (Orchidaceae)

Orchid seeds are minute, dust-like, wind-borne and, thus, would seem to have the potential for long-distance dispersal. Based on this perception, one may predict near-random spatial genetic structure within orchid populations. In reality we do not know much about seed dispersal in orchids and the few empirical studies of fine-scale genetic structure have revealed significant genetic structure at short distances (< 5m), suggesting that most seeds of orchids fall close to the maternal plant. To obtain more empirical data on dispersal, Ripley’s L(d)-statistics, spatial autocorrelation analyses (coancestry, f_ij analyses) and Wright’s F statistics were used to examine the distribution of individuals and the genetic structure within two populations of the terrestrial orchid Orchis cyclochila in southern Korea. High levels of genetic diversity (H_e = 0.210) and low between-population variation were found (F_ST = 0.030). Ripley’s L(d)-statistics indicated significant aggregation of individuals, and patterns varied depending on populations. Spatial autocorrelation analysis revealed significant positive genetic correlations among individuals located <1 m, with mean f_ij values expected for half sibs. This genetic structure suggests that many seeds fall in the immediate vicinity of the maternal plant. The finding of significant fine-scale genetic structure, however, does not have to preclude the potential for the long distance dispersal of seeds. Both the existence of fine-scale genetic structure and low F_ST are consistent with a leptokurtic distribution of seed dispersal distances with a very flat tail.     

Fine-scale genetic structure of life history stages in the food-deceptive orchid Orchis purpurea

In natural plant populations, fine-scale spatial genetic structure can result from limited gene flow, selection pressures or historical events, but the role of each factor is in general hard to discern. One way to investigate the origination of spatial genetic structure within a plant population consists of comparing spatial genetic structure among different life history stages. In this study, spatial genetic structure of the food-deceptive orchid Orchis purpurea was determined across life history stages in two populations that were regenerating after many years of population decline. Based on demographic analyses (2001-2004), we distinguished between recruits and adult plants. For both sites, there was no difference in the proportion of polymorphic loci and expected heterozygosity between life history stages. However, spatial autocorrelation analyses showed that spatial genetic structure increased in magnitude with life history stage. Weak or no spatial genetic structure was observed for recruits, whereas adult plants showed a pattern that is consistent with that found in other species with a predominantly outcrossing mating system. The observed differences between seedlings and adults are probably a consequence of changes in management of the two study sites and associated demographic changes in both populations. Our results illustrate that recurrent population crashes and recovery may strongly affect genetic diversity and fine-scale spatial genetic structure of plant populations.     

Effects of temperature and light on germination and early seedling development of the pine pink orchid (Bletia purpurea)

Orchid seed physiology is a poorly understood phenomenon owing to an emphasis on production and the challenges associated with propagating orchids from minute seed. We investigated the role of simulated south Florida temperatures and illumination (dark and 12 h photoperiod) in regulating germination and seedling development using asymbiotic seed germination assays of Bletia purpurea. Our objectives were to determine whether in situ germination is limited by seasonal temperatures and to determine whether temperature alters responses to illumination. Bletia purpurea seeds were able to germinate to > 90% under all treatments. The greatest germination after 3 weeks was observed at 29/19°C under continual darkness and at 25°C under dark and illuminated conditions. The slowest germination was observed at simulated winter temperatures (22/11°C). Illumination initially inhibited germination and development, but resulted in equal or greater development by week six. Germination under 22/11°C was strongly inhibited by illumination, indicating an interaction between temperature and light sensing systems.     

The underground phase: a special challenge in studies of terrestrial orchid populations

Orchid seeds are minute and the first seedling stages of terrestrial species are underground, so there is little knowledge about the biology of the early life history and the size of seed/seedling populations relative to the number of plants that eventually emerge above ground. A recently developed field sowing technique makes it possible to explore a variety of topics such as the length of time that seeds spend as part of the soil seed bank, the phenology of seed germination, substrate requirements for germination, dependence of germination on fungal availability, substrate requirements and growth rate of seedlings, seedling phenology and development, and seedling-fungus specificity patterns in the soil. Several of these applications are relevant for conservation issues. Studies of Goodyera pubescens, Corallorhiza odontorhiza and Liparis lilifolia show considerable variation in germination strategies, patterns of seedling development and establishment, substrate requirements, and levels of specificity with the fungi.     

Asymbiotic seed germination,in vitro seedling development,and greenhouse acclimatization of the threatened terrestrial orchid <Emphasis Type="Italic">Bletia purpurea</Emphasis>

Procedures for asymbiotic seed germination and seedling acclimatization were developed for Bletia purpurea, a threatened North America native terrestrial orchid. Six asymbiotic orchid seed germination media (Knudson C, PhytoTechnology Orchid Seed Sowing Medium, Malmgren Modified Terrestrial Orchid Medium, Vacin &; Went Modified Orchid Medium, ½-strengh Murashige &; Skoog, and BM-1 Terrestrial Orchid Medium) were examined for their effectiveness in promoting seed germination and protocorm development of B. purpurea in either a 0/24 h or 16/8 h L/D photoperiod. Germination occurred regardless of medium or photoperiod treatment. However, advanced seedling development (Stage 6) only occurred on Vacin &; Went Modified Orchid Medium in the 16/8 h L/D photoperiod. Further effects of photoperiod on in vitro seedling development were also examined. Shoot length, leaf width, root number and length, and fresh weight and dry weight in the 16/8 h L/D photoperiod were all significantly different when compared to the 8/16 h and 12/12 L/D photoperiods. In vitro seedlings were readily acclimatized to greenhouse conditions. Seedlings showed high survival all potting media. Seedlings acclimatized in Fafard Mix 4 potting medium developed significantly longer roots. Corm formation occurred regardless of potting media used.     

Quantitative genetic analysis of seed vigour and pre-emergence seedling growth traits in Brassica oleracea

The ability of seeds to germinate and establish seedlings in a predictable manner under a range of conditions has a direct contribution to the economic success of commercial crops, and should therefore be considered in crop improvement. We measured traits associated with seed vigour and pre-emergence seedling growth in a segregating population of 105 doubled haploid Brassica oleracea lines. The germination traits measured were: mean germination times for unstressed germination; germination under water stress or germination after a heat treatment; and conductivity of seed leachate. The seedling growth traits measured were: seed weight; seedling growth rate; and seedling size at the end of the exponential growth phase. There were some correlations, notably among germination traits, and between seed weight and pre-emergence seedling growth. Heritability of the various traits was typically in the 10–15% range, with heritability of conductivity and mean germination time under water stress 25 and 24% respectively. Collectively the results indicate that germination and pre-emergence seedling growth are under separate genetic control. Quantitative trait loci analyses were carried out on all measurements and revealed significant loci on linkage groups O1, O3, O6, O7 and O9. We suggest that genes at these loci are important in determining predictable seed germination and seedling establishment in practice.     

A novel dual-phase culture medium promotes germination and seedling establishment from immature embryos in South African <Emphasis Type=Italic>Disa</Emphasis> (Orchidaceae) species

In vitro germination as a conservation aid demands propagation systems that yield increasingly larger numbers of plants as rapidly as possible. However, summer-rainfall Disa species are germinated infrequently under asymbiotic conditions. Attempts to bypass impermeable testae in these species resulted in the formulation of a dual-phase protocol—with the specific aim of increasing water availability to the embryo. Dual-phase cultures were initiated using seed from green capsules and comprised a solid, charcoal-rich medium overlaid with a reduced strength, liquid medium fraction of the same type. The former served to negate the influence of leached phenols and allowed protocorms to establish polarity, whilst the latter increased water availability. The dual-phase protocol germinated nine Disa species (including D. cooperi, D. nervosa, D. pulchra and D. woodi); representing five first time reports (D. brevicornis, D. crassicornis, D. patula, D. stachyoides and D. versicolor). Furthermore, germination was comparatively rapid and percentages for most species approached their estimated germination potential (∼viability). Congruency here suggests that dual-phase conditions are amenable to germinating a number of seeds that reflects the maximum number of viable embryos—critical for conservation efforts.     

Spatial aggregation and association in different resource-patch distributions: experimental analysis with Drosophila

1. Laboratory experiments using homogeneous resources were conducted to examine intra- and interspecific spatial egg distribution of D. simulans, D. auraria and D. immigrans in three different resource-patch distribution patterns: patchy, even and clustered. 2. Individuals of each species were introduced separately or simultaneously into the cage, into which artificial substrates were placed and allowed to oviposit for 24 h. Spatial analyses were performed with indices of intraspecific aggregation (J), interspecific association (C) and L-function based on Ripley's K-function. 3. Eggs were always spatially aggregated irrespective of species and the resource-patch distribution patterns. Spatial egg aggregation was influenced significantly by the resource-patch distribution pattern and tended to be weaker in the clustered resource-patch distribution than in the patchy or even resource-patch distribution. 4. Spatial extent of egg aggregation was always beyond the single resource patch scale, indicating aggregation of ovipositing females. 5. Interspecific association of egg distribution was absent or very weak. Thus, these results present experimental evidence of independent egg aggregation among drosophilids.     

陕北黄土丘陵沟壑区14种植物的萌发特性

在实验室条件下观测了陕北黄土丘陵沟壑区14种植物的萌发特性,结果发现：萌发率在80%以上的植物有杠柳、铁杆蒿和茭蒿;在60%～80%之间的物种有白羊草、紫丁香、阿尔泰狗娃花、大针茅、沙棘和猪毛蒿;长芒草为37.3%;小于20%的植物有达乌里胡枝子、刺槐、虎榛子和狼牙刺。14种植物的萌发时滞最短的为2 d,最长的为8 d;几乎所有植物种子达到T50的时间都在其萌发开始后的7 d以内,且大部分物种在萌发开始后的15 d完成总萌发量的90%;萌发持续时间最短的为6 d,有刺槐、狼牙刺、虎榛子、沙棘4种,最长的为猪毛蒿,达32 d。这些植物种子的萌发特性及生态适宜性决定了它们在陕北黄土丘陵沟壑区植被演替中的角色与地位。     

扩展点格局分析方法在灌木种群空间分布格局研究中的应用

当研究对象个体差异很大或者研究尺度与植物大小属于一个数量级时,传统的以种群空间分布的坐标点图为基础的点格局分析方法存在局限性.本文引入一种在传统点格局分析方法基础上发展起来的以栅格数据为分析对象的新方法——扩展点格局分析方法,用以分析具有一定面积和形状对象的空间格局和生态过程,该方法适用于灌木研究对象.以狭域特有种四合木(Tetraena mongolica)种群的空间分布格局分析为例,采用摄影定位法,用图像处理软件及地理信息系统软件将样地信息数字化后,利用扩展点格局分析方法分析了群落中四合木单种的空间分布格局及其与群落中霸王(Zygophyllum xanthoxylum)种群的关系,四合木种群在69～ 99 cm尺度上表现为均匀分布,四合木与霸王在350 ～570cm尺度上表现出显著的负关联关系.结果揭示了灌木种群的空间结构特征.