Seed germination,seedling inoculation and establishment ofAlnus spp. in containers in greenhouse trials

Methods for production of containerized seedlings ofAlnus species were developed which permit nitrogen-fixing nodules to form on the root systems prior to outplanting, in order to provide an early nitrogen input during seedling establishment. The methods are based on procedures for inoculating root systems with suspensions ofFrankia (Actinomycetales), applied either directly in the container cell as a soil drench at the time of seeding, or as a root dip for seedlings transplanted into the containers. Germination of dried, stored seed was enhanced by light and by presoaking for 16 h in water. Pretreatments to overcome seed dormancy or to eliminate fungal pathogens did not further enhance germination. Some loss of seedlings was recorded in the early stages of growth shortly after germination, which is a factor in calculating projected seedling yield. Nodulation and seedling growth were evaluated in terms of growth media characteristics. Seedlings performed well in peat-vermiculite, at soil pH between 5.5 and 8.0.     

On the ecology and evolution of annual plants in disturbed environments

This paper is concerned with the effect of disturbance on some crucial characteristics of annual plants. The theoretically optimal life-history traits that maximize individual fitness in disturbed environments are described and critically evaluated. It seems that none of them holds for all annual species.Self-pollination and especially seed polymorphism are considered important adaptations to life in unpredictable environments. The thesis is put forward that amphicarpic annuals, which exhibit both self-pollination and extreme seed polymorphism, are best adapted to life in hazardous habitats. The hypothetical course of the evolution of amphicarpy is demonstrated on the grounds of the comparison of contemporary annual species producing chasmogamous and cleistogamous flowers on a single individual.     

Sucrose utilization during ovule and seed development ofGasteria verrucosa (Mill.) H. Duval as monitored by sucrose synthase and invertase localization

Summary The development ofGasteria verrucosa ovules and seeds seems to follow a pattern of growth in which the majority of carbohydrates is first used in the sporophytic tissue (nucellus, integuments, and arillus) around the gametophyte-derived cells. After fertilization the carbohydrates are used for further development of the arillus and seed coat. During the next stage carbohydrates are directed to develop the endosperm, followed by carbohydrate investment in the developing embryo and in storage products. This utilization pattern is deducted from a localization study on sucrose synthase and invertase. These two enzymes break down imported sucrose and are in that perspective used as markers for carbohydrate transport since diffusion is expected to be induced towards cells and tissues with high sucrose-hydrolyzing activities.     

Role of plant parasitic nematodes and abiotic soil factors in growth heterogeneity of sugarcane on a sandy soil in South Africa

A feature of many sugarcane fields is the patchy growth. This is often thought to be due to physical or chemical differences in the soil. In this paper we investigate the causes of growth heterogeneity of sugarcane on a sandy soil in KwaZulu-Natal, South Africa. To identify the factors that were associated with the good and poor areas, soil texture, pH, organic matter content and a number of soil chemical elements and nematode community data were subjected to principal component analysis (PCA). The numbers of each of the nematode species (Meloidogyne sp., Pratylenchus zeae, Helicotylenchus dihystera, Xiphinema elongatum and Paratrichodorus sp.) were first converted to relative proportions of ectoparasites and endoparasites. The data were collected from the 2nd ratoon crop of a nematicide trial, where half of the plots had been treated with aldicarb in the preceding two crops. Yields of control plots varied from 34.7 to 126.8 t cane ha^–1 and from 85.4 to 138.7 for the treated plots. The yield data were centred and normalised separately for the treated and control plots and the values projected on the trial map to study spatial distribution. Plots with above-average yields, whether treated or untreated, occurred in the lower part of the trial site. The PCA factorial values were also projected onto the map of the trial. According to the first factor of the analysis of the abiotic soil characteristics in the 0–20 cm surface layer, the trial site could be divided into two areas, one on the left and one on the right. PCA of the soil data from the 0–20 and 20–40 cm layers showed that there was little or no difference between the two that might explain the two growth areas. However, analysis of the nematode community distinguished two main areas that largely corresponded to the distribution of the plots of low and high yielding cane. Correlation analysis confirmed the relationship between nematodes and yield. H. dihystera was positively correlated with yield of cane whereas the reverse was true for the Meloidogyne species.     

Floral meristems as a source of enhanced yield and quality of DNA in grasses

Floral meristems of Lolium and Festuca grasses give a 5- to 19-fold enhancement in yield of extracted DNA in comparison with leaves. Meristems also provide highly pure DNA samples. The method could be useful for applications in molecular genetics in many species of the Gramineae. Received: 14 April 1998 / Revision received: 8 June 1998 / Accepted: 10 July 1998     

Why there are photodamages to photosystem II at low light intensities

An explanation of photodamages to PS II at low light intensities is proposed. The explanation is based on the two following postulates: 1. Intermediate states of the water-oxidizing complex can undergo parasitic reduction to lower degrees of oxidation. 2. Photosynthetic apparatus can minimize the unfavorable effect of such reduction by changes in the number of active reaction centers of PS II. Some steady-state characteristics were calculated under these assumptions. The results obtained show that the frequency of P680 excitations at low light intensities becomes as high as that at saturating light provided that the rate of parasitic reduction is comparable with the rate of the slowest reaction in the cycle of the water- oxidizing complex conversions.     

Pollen and gene flow in fragmented habitats

Abstract. Habitat fragmentation affects both plants and pollinators. Habitat fragmentation leads to changes in species richness, population number and size, density, and shape, thus to changes in the spatial arrangement of flowers. These changes influence the amount of food for flower-visiting insects and the quantity and quality of pollinations. Seed set in small populations is often reduced and genetic variation is expected but not always found to be low. The majority of studies show that low flower densities have reduced pollination success and higher inbreeding. Density effects are stronger than size effects. Most studies concluded that species richness in flower-visiting insects is directly related to richness in plant species. However, the consequences of low insect species richness for pollination are not always clear, depending on the studied pollinator-plant relationship. The effects of the presence of simultaneously flowering species are highly dependent on the circumstances and may range from competition to facilitation. Other flowering plant species may play a role as stepping stones or corridor in the connection between populations. In the absence of stepping stones even short distances between populations act as strong barriers for gene flow. We illustrate the present review paper with own data collected for three plant species, rare in The Netherlands: Phyteuma spicatum ssp. nigrum (Campanulaceae), Salvia pratensis (Labiatae) and Scabiosa columbaria (Dipsacaceae). The species differ in their breeding systems and in the assemblage of visitor species. Data are shown on the effects of population size on species richness with consequences for seed set. Effects of flower density and isolation on pollen exchange are given. Since plant reproduction depends on the behaviour of individual insects and not on the overall behaviour of the species, the examples all point to individual insects and extrapolate to effects at the species level.     

Strategies in Mediterranean grassland annuals in relation to stress and disturbance

Abstract. Morphological (size and shape) and functional (growth, reproduction and phenology) attributes are used to characterize 42 annual species of Mediterranean grasslands according to their strategy. Principal component analysis of the matrix of 42 species x 9 attributes shows that the main trend of variation is related to plant size. Larger species have larger seeds, lower relative growth rates and lower reproductive output. The second and third trends of variation are related to plant shape. Ordination of species shows differences in shape between taxa and growth forms (grasses, legumes, forbs). The relative abundance of species with different attributes vary with the level of stress (water and nutrient availability) and disturbance (grazing and ploughing). Size is related to stress, with larger plants dominating in productive habitats and smaller ones in the most unproductive. Disturbance is related to shape and phenology, since grazing favours species with low canopies and ploughing favours species with shorter life cycles, that are usually small in size. Relations between plant attributes and habitat characteristics are examined within the more general framework of plant strategy theory.     

Disproportionately large ecological role of a recently mass-culled flying fox in native forests of an oceanic island

Human-wildlife conflicts pose a growing threat to many species worldwide and require increasingly innovative and multi-disciplinary resolutions. Because of their apparent simplicity and political appeal, lethal approaches like culling are often favoured, decisions to cull are typically poorly supported by scientific evidence and the limitations and drawbacks of culls minimised. As natural habitats decline and fruit crop production expands, fruit-eating bats in the Old World (family Pteropodidae) are increasingly in conflict with fruit farmers. This conflict is exemplified on Mauritius where the government has implemented two mass culls of a threatened flying fox (Pteropus niger) since 2015 in response to fruit-grower concerns. The culls and illegal hunting reduced the bat population by >50%. In this context, we sought to investigate the ecological role and service provided by the targeted flying fox through seed dissemination to gauge what may be lost as the species becomes rarer or extinct. We randomly sampled the woody native community in six of the best-preserved moist-to-wet native forests of Mauritius using 90 plots of 100 m² each and identified and measured the stem size of all woody plants ≥1 cm in diameter. Species were classified by whether their fruits occur in the diet of the flying fox in an exclusive, confirmed or likely manner and we assumed that fruit consumption is equivalent to potential seed dissemination. The relative importance of these three categories to the total woody plant community was then quantified as the proportion of species richness, stem density and basal area (as a surrogate of biomass) that they represent. We also investigated whether the main traits of species (seed size, fruit size, and maximum stem diameter) vary among bat-dispersal categories and differ from those of species whose fruits are not currently known to be eaten by the flying fox. On average, although about a quarter of the native woody species (24.6%) have fruits confirmed as eaten and seeds dispersed by the flying fox, these species comprise about half (53.1%) of the stems sampled and the majority (63.1%) of the basal area of native woody plant in the island’s native forests. About half of that latter biomass figure comprises species for which the flying fox is the exclusive native vertebrate frugivore and seed disperser. Plants disseminated by the flying fox are typically large trees with large fruit and seeds. These trees are often key components of the canopy, therefore fulfilling structural engineer roles that provide resources and conditions for the survival of many forest species. The flying fox plays a disproportionally large ecological role in maintaining forest structure and biodiversity in the long-term. Consequently, lethal approaches to the conflict with fruit-farmers threaten not only an endangered species, but ecological processes central to the viability of native forests. Our findings highlight the importance of including the ecological costs of culling in decision-making processes intended to resolve human-wildlife conflicts.