Showing papers in "Perspectives in Plant Ecology Evolution and Systematics in 2001"

Functional and ecological xylem anatomy

Abstract: Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure ( P x ), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.

Plant allelochemical interference or soil chemical ecology

Abstract: While allelopathy has been defined as plant-plant chemical interference, there has been much confusion about what the concept encompasses and how important it is in nature. We distinguish between (1) direct plant-plant interference mediated by allelochemicals, and (2) the effects of secondary compounds released by plants on abiotic and biotic soil processes that affect other plants. It very difficult to demonstrate direct effects of chemicals released by a plant on nearby plants. Although soil ecology-mediated effects of secondary plant compounds do not fit the classical concept of allelopathy, we find support in the literature for the hypothesis that the most important effects of compounds released into the soil environment by plants on other plants occur through such indirect effects. The emphasis on, and skepticism of, direct plant-plant allelopathic interference has led some researchers to demand unreasonably high standards of evidence for establishing even the existence of allelopathic interactions, standards that are not demanded for other plant-plant interactions such as resource competition. While the complete elucidation of the mechanisms by which allelochemicals function in the field is many years away, such elucidation is not necessary to establish the existence of allelopathic interactions. We propose that most of the phenomena broadly referred to as allelopathic interference are better conceptualized and investigated in terms of soil chemical ecology. Even when direct plant-plant allelochemical interference occur, the levels of allelochemicals in the environment and their effects on plants are heavily influenced by abiotic and biotic components of the soil ecosystem. Putting allelopathy in the context of soil ecology can further research and reduce some of the less fruitful controversy surrounding the phenomenon.

Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands

Abstract: The presence of diverse and species-rich plant lineages on oceanic islands is most often associated with adaptive radiation. Here we discuss the possible adaptive significance of some of the most prominent traits in island plants, including woodiness, monocarpy and sexual dimorphisms. Indirect evidence that such traits have been acquired through convergent evolution on islands comes from molecular phylogenies; however, direct evidence of their selective value rarely is obtained. The importance of hybridization in the evolution of island plants is also considered as part of a more general discussion of the mechanisms governing radiations on islands. Most examples are from the Hawaiian and Canarian floras, and in particular from studies on the morphological, ecological and molecular diversification of the genus Aeonium , the largest plant radiation of the Canarian Islands.

Implications of transgenic, insecticidal plants for insect and plant biodiversity

Abstract: Genetically modified plants are widely grown predominantly in North America and to a lesser extent in Australia, Argentina and China but their regions of production are expected to spread soon beyond these limited areas also reaching Europe where great controversy over the application of gene technology in agriculture persists. Currently, several cultivars of eight major crop plants are commercially available including canola, corn, cotton, potato, soybean, sugar beet, tobacco and tomato, but many more plants with new and combined multiple traits are close to registration. While currently agronomic traits (herbicide resistance, insect resistance) dominate, traits conferring “quality” traits (altered oil compositions, protein and starch contents) will begin to dominate within the next years. However, economically the most promising future lies in the development and marketing of crop plants expressing pharmaceutical or “nutraceuticals” (functional foods), and plants that express a number of different genes. From this it is clear that future agricultural and, ultimately, also natural ecosystems will be challenged by the large-scale introduction of entirely novel genes and gene products in new combinations at high frequencies all of which will have unknown impacts on their associated complex of non-target organisms, i.e. all organisms that are not targeted by the insecticidal protein. In times of severe global decline of biodiversity, pro-active precaution is necessary and careful consideration of the likely expected effects of transgenic plants on biodiversity of plants and insects is mandatory. In this paper possible implications of non-target effects for insect and plant biodiversity are discussed and a case example of such non-target effects is presented. In a multiple year research project, tritrophic and bitrophic effects of transgenic corn, expressing the gene from Bacillus thuringiensis (Bt-corn) that codes for the high expression of an insecticidal toxin (Cry1Ab), on the natural enemy species, Chrysoperla carnea (the green lacewing), was investigated. In these laboratory trials, we found prey-mediated effects of transgenic Bt-corn causing significantly higher mortality of C. carnea larvae. In further laboratory trials, we confirmed that the route of exposure (fed directly or via a herbivorous prey) and the origin of the Bt (from transgenic plants or incorporated into artificial diet) strongly influenced the degree of mortality. In choice feeding trials where C. carnea could choose between Spodoptera littoralis fed transgenic Bt-corn and S. littoralis fed non-transgenic corn, larger instars showed a significant preference for S. littoralis fed non-transgenic corn while this was not the case when the choice was between Bt- and isogenic corn fed aphids. Field implications of these findings could be multifold but will be difficult to assess because they interfere in very intricate ways with complex ecosystem processes that we still know only very little about. The future challenge in pest management will be to explore how transgenic plants can be incorporated as safe and effective components of IPM systems and what gene technology can contribute to the needs of a modern sustainable agriculture that avoids or reduces adverse impacts on biodiversity? For mainly economically motivated resistance management purposes, constitutive high expression of Bt-toxins in transgenic plants is promoted seeking to kill almost 100% of all susceptible (and if possible heterozygote resistant) target pest insects. However, for pest management this is usually not necessary. Control at or below an established economic injury level is sufficient for most pests and cropping systems. It is proposed that partially or moderately resistant plants expressing quantitative rather than single gene traits and affecting the target pest sub-lethally may provide a more meaningful contribution of agricultural biotechnology to modern sustainable agriculture. Some examples of such plants produced through conventional breeding are presented. Non-target effects may be less severe allowing for better incorporation of these plants into IPM or biological control programs using multiple control strategies, thereby, also reducing selection pressure for pest resistance development.

Induced systemic resistance (ISR) against pathogens – a promising field for ecological research

Abstract: Putative fitness costs provide an explanation for why ISR is induced instead of constitutive, and they might constrain the use of ISR as preventative protection of cultivated plants. Though ISR is mainly elicited by and effective against pathogens, further biotic agents such as leaf-chewing herbivores, leaf miners, aphids and even non-pathogenic root-colonising bacteria can induce systemic pathogen resistance, while some ISR traits can have a defensive effect against herbivores. ‘Cross-resistance’ elicited by and effective against non-microbial plant enemies thus might add significantly to the function of ISR. On the other hand, ‘trade-offs” have been reported, i.e. increased susceptibility to herbivores in ISR-expressing plants. Finally, ISR is a rather unspecific response, being active against different microbes. It thus might have effects on mutualistic bacteria and fungi, too. The question of how expression of ISR affects the large variety of mutualistic and antagonistic plant-microbe and plant-insect interactions cannot yet be answered. This knowledge is, however, needed to obtain a risk assessment for the use of chemically induced or genetically engineered ISR in crop protection. This review aims to provide an overview and to highlight some of the many open questions which require intensive ecological research.

Ectomycorrhizas: their role in forest ecosystems under the impact of acidifying pollutants

Abstract: The physiologically active lateral rootlets of all main trees in temperate forests are colonised by ectomycorrhizal fungi, forming so-called ectomycorrhizas. These symbiotic organs are the sites of exchange of nutrients, mainly P and N, provided from the fungal partner, and C from the host. Emerging from the ectomycorrhizas, fungal hyphae exploit the soil for the mobilisation and absorption of water and nutrient elements. By doing so, they connect the tree roots intimately with the soil and provide anchorage. The deposition of acidifying pollutants into forest ecosystems is a potential threat to the health and vitality of forest trees because it leads to the acidification and eutrophication of forest soils. Pollutants are also a threat to the functioning of ectomycorrhizas. Increased N concentrations in the soil lead to enhanced fungal N uptake and storage, and to enhanced N transfer to the host plants, and therefore to higher plant biomass of above ground parts. In consequence, there is a decrease of C allocation to the plant roots. This in turn leads to reduced ectomycorrhization, and to reduced production of external mycelia and fruiting bodies. Soil acidification leads to enhanced availability of Al, heavy metals, and radionuclides in the soil, all of which can be toxic to plants and fungi. Reduced growth of roots and hyphae are amongst the first symptoms. In ectomycorrhizas, the hyphae of the fungal tissues contain vacuolar polyphosphates which have the ability to bind Al, heavy metals, radionuclides and N. These electronegative polymers of phosphates represent an effective storage and detoxifying mechanism which otherwise is lacking in roots. Therefore, ectomycorrhizas have the potential to increase the tolerance of trees to acidifying pollutants and to the increased availability in the soil of toxic elements.

Conceptual problems and scale limitations of defining ecological communities: a critique of the CI concept (Community of Individuals)

Abstract: Recently, Looijen & van Andel (1999) proposed a new definition of an ecological community by using two criteria: (1) restricting membership by taxonomic relatedness, and (2) defining boundaries by the intersection of the area of population range boundaries. I analyze the implications of their definition and explore the limitations of the approach. Overall, I show this definition to be highly scale-limited, to not encompass many ecological concepts developed for the community level, and to have hidden assumptions that are not met in natural systems. An alternative model of the ecological community is proposed as a contrast, a model based on the community of an individual, in which individuals and interactions are used to develop the larger entity of an ecological community. This alternative model illustrates that the principal problems Looijen & van Andel (1999) discussed about previous community concepts with respect to application to vegetation classification are not ‘problems’ but are characteristics of ecological communities. Any definition of an ecological community must be able to incorporate these characteristics as well as current ecological concepts used at the community level.

Cintractiella diplasiae – a second species of Cintractiella (Ustilaginales) with sori in adventitious spikelets on Hypolytreae (Cyperaceae)

Abstract: Only two species of smut fungi are known to infect members of Hypolytreae (Mapanioideae, Cyperaceae), Cintractiella lamii on Hypolytrum sp. from New Guinea, the only species of this poorly known genus, and Ustanciosporium diplasiae on Diplasia karataefolia from Brazil. Comparison of recently discovered complete herbarium material of U. diplasiae (infecting D. karataefolia from Venezuela) to the description of C. lamii , indicates that the former is a species of Cintractiella . Both species develop relatively large teliospores in dark masses which are covered by a thin layer of sterile fungal hyphae. These develop in sterile spikelets in an adventitious position on leaves or peduncles. Spikelets with sori of U. diplasiae often ramify and form dense witches brooms. Based on descriptions, illustrations and photographs, the genus Cintractiella is emended and the combination Cintractiella diplasiae is made.