Introduction: Plant–herbivore interactions

This special feature resulted from a symposium entitled "Interactions Between Plants and Their Herbivores," held during the Meeting of the Society of Population Ecology in Ohmi-Shirahama, Shiga, Japan, in October 1999 (Chairperson of Symposium: Professor Emeritus E. Kuno of Kyoto University). This theme emerged from discussions by the organizing committee for this symposium: N. Yamamura, J. Takabayashi, T. Nishida, and N. Ohsaki. Professor Mark D. Rausher of Duke University was invited as a special lecturer. In this series of reports, five of the seven participants illustrate the variation found in plant–herbivore interactions and address some problems inherent in current theory.     

Uni-directional Interaction and Plant–Pollinator–Robber Coexistence

A mathematical model for the plant-pollinator-robber interaction is studied to understand the factors leading to the widespread occurrence and stability of such interactions. In the interaction, a flowering plant provides resource for its pollinator and the pollinator has both positive and negative effects on the plant. A nectar robber acts as a plant predator, consuming a common resource with the pollinator, but with a different functional response. Using dynamical systems theory, mechanisms of species coexistence are investigated to show how a robber could invade the plant-pollinator system and persist stably with the pollinator. In addition, circumstances are demonstrated in which the pollinator's positive and negative effects on the plant could determine the robber's invasibility and the three-species coexistence.     

Mathematical Modeling of the Dynamics of Plant Mineral Nutrition in the Fertilizer–Soil–Plant System

A numerical simulation of the spatial–temporal dynamics of a multi-parameter system has been developed. The components of this system are plant biomass, the mobile and stationary forms of mineral nutrition elements, rhizosphere microorganisms, and environmental parameters (temperature, humidity, and acidity). Parametric identification and verification of the adequacy of the model were carried out based on the experimental data on the growth of Krasnoufimskaya-100 spring wheat on peat lowland soil. The results are represented by temporal distributions of biomass from agricultural crops and the findings on the contents of the main nutrition elements within the plant (nitrogen, phosphorus, and potassium). An agronomic assessment and interpretation of the results are given.

     

Cyanobacterial–Bacterial Complexes in Plant Syncyanoses

The morphology and ultrastructure of associative microsymbiont complexes (AMC) isolated from the ferns Azolla pinnata and Azolla sp. and the apogeotropic roots of the cycad Cycas revoluta were studied. The composition of the AMC obtained includes the cyanobionts (symbiotic cyanobacteria) and satellite bacteria (SB). It was found that two types of cyanobacteria that substantially differ in their morphological organization are likely present as cyanobionts in the coralloids of C. revoluta. The isolated cyanobiont strains exhibited the morphological traits and regularities of development typical of the genus Nostoc; they were characterized by the ability of their cells to divide in mutually perpendicular planes. When isolating AMC from different morphological zones of C. revoluta apogeotropic roots, SB growth was revealed only around the pieces corresponding to the coralloid apical zone. No AMC components were revealed around the segments of the basal growth zone. Pure cyanobiont cultures were obtained from the AMC of C. revoluta coralloids. The AMC isolated from the ferns A. pinnata and Azolla sp. are characterized by obligate mutual dependence of the partners (the cyanobiont and SB).     

Negative Plant–Soil Feedback and Positive Species Interaction in a Herbaceous Plant Community

Increasing evidence shows that facilitative interaction and negative plant–soil feedback are driving factors of plant population dynamics and community processes. We studied the intensity and the relative impact of negative feedback on clonal growth and seed germination of Scirpus holoschoenus, a ‘ring’ forming sedge dominant in grazed grassland, and the consequences for species coexistence. The structure of aboveground tussocks was described. A Lithium tracer assessed belowground distribution of functional roots. Seed rain and seedling emergence were compared for different positions in relation to Scirpus tussocks. Soil bioassays were used to compare growth on soil taken from inside and outside Scirpus tussocks of four coexisting species (Mentha acquatica, Pulicaria dysenterica, Scirpus holoschoenus and Dittrichia viscosa). We also compared plant performance of dominant plant species inside and outside Scirpus tussocks in the field. The ‘ring’ shaped tussocks of S. holoschoenus were generated by centrifugal rhizome development. Roots were functional and abundant under the tillers and extending outside the tussocks. The large roots mats that were present in the inner tussock zone were almost all dead. Seedling emergence and growth both showed a strong negative feedback of Scirpus in the inner tussock zone. Scirpus clonal development strongly reduced grass biomass. In the degenerated tussock zone, Pulicaria and Mentha mortality was lower, and biomass of individual plants and seed production were higher. This positive indirect interaction could be related to species-specific affinity to soil conditions generated by Scirpus, and interspecific competitive release in the degenerated tussock zone. We conclude that Scirpus negative feedback affects its seedling emergence and growth contributing to the development of the degenerated inner tussock zone. Moreover, this enhances species coexistence through facilitative interaction because the colonization of the inner tussock zone is highly species-specific.     

Plant–plant interactions in tropical alpine environments

Plant–plant interactions are increasingly recognized as a key driver of community organization and ecosystem processes in alpine environments. However, patterns and mechanisms of plant–plant interactions remain largely uncharacterized in tropical alpine ecosystems (TAE) which represent as much as 10% of the total surface area of alpine ecosystems worldwide. In this paper, we review (1) the ecological and environmental features that are specific to TAE in comparison with other alpine ecosystems, (2) the existing literature on plant–plant interactions in TAE, and (3) whether patterns and mechanisms of plant–plant interactions established in extratropical alpine zones can be extended to TAE. TAE are located predominantly in South America, East Africa, and South-East Asia where they show a unique combination of environmental characteristics, such as absence of persisting snow cover, high frequency of diurnal freeze–thaw cycles and needle-ice activity, and a decrease in precipitation with increasing altitude. These environmental characteristics result in the presence of giant growth forms with a great architectural diversity. These biotic and abiotic characteristics influence the outcome of plant–plant interactions by imposing other types of environmental constraints than those found in extratropical alpine environments, and by potentially generating distinctive patterns of niche differentiation/complementarity between species and populations. To generalize the conceptual framework of plant–plant interactions in alpine environments, we advocate that TAE should be investigated more thoroughly by applying designs, methods and hypotheses that are used currently in temperate areas and by conducting studies along large latitudinal gradients that include tropical regions.     

Plant–rhizobia interactions alter aphid honeydew composition

Both above- and below-ground interspecific interactions contribute to ecosystem functioning in terrestrial systems, and the integration of below- and above-ground interactions is crucial for deepening our knowledge of nutrient cycling and community dynamics in terrestrial ecosystems. The present study explored the effects of plant–microbe interactions on aphid honeydew quality and quantity and important factors mediating ant–aphid mutualisms and below-ground nutrient dynamics. Soybean aphids (Aphis glycines) were inoculated onto two closely related strains of soybean plants: a nodulating strain that associates with rhizobia and a non-nodulating strain that does not harbor any nitrogen-fixing bacteria. As expected, prior to aphid inoculation, nodulating plants were significantly taller and had more leaves than non-nodulating plants. Aphids feeding on nodulating strains were found to reach slightly larger colony sizes and produce honeydew with significantly different sugar profiles than those feeding on non-nodulating plants. The honeydew collected from aphid colonies feeding on nodulating plants contained 160 % more total sugars than honeydew collected from colonies feeding on non-nodulating plants, but there was no difference in total amino acid-N content in honeydew from colonies feeding on the different plant strains. We discuss the implications of honeydew composition for nutrient cycling and community dynamics and suggest areas of future research to elucidate the consequences of altered aphid honeydew composition on ecosystem properties.     

Molecular Adaptation in Plant Hemoglobin, a Duplicated GeneInvolved in Plant–Bacteria Symbiosis

The evolutionary history of the hemoglobin gene family in angiosperms is unusual in that it involves two mechanisms known for potentially generating molecular adaptation: gene duplication and among-species interaction. In plants able to achieve symbiosis with nitrogen-fixing bacteria, class 2 hemoglobin is expressed at high concentrations in nodules and appears to be a key factor for the achievement and regulation of the symbiotic exchange. In this study, we make use of codon models of DNA sequence evolution with the goal of determining the nature of the selective forces which have driven the evolution of this gene. Our results suggest that adaptive evolution occurred during the period of time following the duplication event (functional divergence) and that a change in the selective pressures arose in class 2 hemoglobin in relation to the acquisition of a symbiotic function.This article contains online supplementary material.Reviewing Editor: Dr. Rasmus Nielsen     

Prey Habitat Location by the Cassava Mealybug Predator Exochomus flaviventris: Olfactory Responses to Odor of Plant, Mealybug, Plant–Mealybug Complex, and Plant–Mealybug–Natural Enemy Complex

Exochomus flaviventris Mader is considered to be the most active predator of the cassava mealybug Phenacoccus manihoti Matile–Ferrero in Central Africa. The response of experienced gravid female coccinellids to the odor of cassava plant (var. Zanaga), unparasitized mealybugs, plant–mealybug complex with or without feeding prey (parasitized or not), and plant–mealybug complex with or without conspecific coccinellids was investigated in a Y-tube olfactometer. The odor of uninfested cassava plants was not more attractive than clean air. Dual-choice tests revealed that mealybug-infested plants were preferred to mealybugs alone and mealybug-damaged plants and were the major sources of volatiles that attract females coccinellids to the microhabitat of its prey. The emission of volatile chemicals did not appear to be limited to the infested parts of the plant but did occur systemically throughout the plant. The presence of conspecific coccinellid larvae or adult males did not modify the attractiveness of the mealybug-infested plants. However, when an infested plant with conspecific predator females (alone or with conspecific males) was compared to an infested plant or infested plant with conspecific males, E. flaviventris females showed a preference for the last two sources of odor. The uninfested plant with conspecific males was also preferred to the uninfested plant with conspecific females. In addition, the odor of conspecific males was preferred over that of conspecific females. Female predators preferred the plant infested with unparasitized mealybugs over the plant infested with mealybugs previously parasitized. These results showed that E. flaviventris females use herbivore-induced plant volatiles during foraging and can detect via olfaction the presence of conspecific gravid females and parasitized prey, thus assessing patch suitability from a distance.     

Invasibility of Nectarless Flowers in Plant–Pollinator Systems

This paper considers plant–pollinator systems in which plants are divided into two categories: The plants that secret a substantial volume of nectar in their flowers are called secretors, while those without secreting nectar are called nonsecretors (cheaters). The interaction between pollinators and secretors is mutualistic, while the interaction between pollinators and nonsecretors is parasitic. Both interactions can be described by Beddington–DeAngelis functional responses. Using dynamical systems theory, we show global dynamics of a pollinator–secretor–cheater model and demonstrate mechanisms by which nectarless flowers/nonsecretors can invade the pollinator–secretor system and by which the three species could coexist. We define a threshold in the nonsecretors’ efficiency in translating pollinator–cheater interaction into fitness, which is determined by parameters (factors) in the systems. When their efficiency is above the threshold, non-secretors can invade the pollinator–secretor system. While the nonsecretors’ invasion often leads to their persistence in pollinator–secretor systems, the model demonstrates situations in which the non-secretors’ invasion can drive secretors into extinction, which consequently leads to extinction of the nonsecretors themselves.     

A Regional Assessment of Changes in Plant–Plant Interactions Along Topography Gradients in Tunisian Sebkhas

Ecosystems - Facilitation among plants in dry ecosystems is crucial for diversity and ecosystem functioning and stability. However, the importance of facilitation in extremely stressful conditions...     

Plant–bacteria interactions in the removal of pollutants

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Highlights► Rhizobacteria degrade a wide range of pollutants and efficiently colonize plant roots. ► Plants have an effect on the selection of their own rhizospheric microorganisms. ► Catabolic pathways can be induced by natural secondary plant products. ► Horizontal gene transfer has an important role in bioremediation. ► Manipulation of plant/microbe interactions could improve rhizoremediation outcomes.