Differentiation between native and exotic plant species in a dry grassland: realized responses to perturbation,and comparison with fundamental responses

Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short‐term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre‐adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.     

Induction of plant responses to oviposition and feeding by herbivorous arthropods: a comparison

Plants may respond both to feeding and oviposition by herbivorous insects. While responses of plants to feeding damage by herbivores have been studied intensively during the past decades, only a few, but growing number of studies consider the reactions of plants towards egg deposition by herbivorous insects. Plants showing defensive response to oviposition by herbivores do not `wait' until being damaged by feeding, but may instead react towards one of the initial steps of herbivore attack, the egg deposition. Direct plant defensive responses to feeding act directly against the feeding stages of the herbivores. However, a plant may also show direct defensive responses to egg deposition by (a) formation of neoplasms, (b) formation of necrotic tissue (= hypersensitive response), and (c) production of oviposition deterrents. All these plant reactions have directly negative effects on the eggs, hatching larvae, or on the ovipositing females. Indirect plant defensive responses to feeding result in the emission of volatiles (= synomones) that attract predators or parasitoids of the feeding stages. A few recent studies have shown that plants are able to emit volatiles also in response to egg deposition and that these volatiles attract egg parasitoids. Studies on the mechanisms of induction of synomones by egg deposition show several parallels to the mechanisms of induction of plant responses by feeding damage. When considering induced plant defence against herbivores from an evolutionary point of view, the question arises whether herbivores evolved the ability to circumvent or even to exploit the plant's defensive responses. The reactions of herbivores to oviposition induced plant responses are compared with their reactions to feeding induced plant responses.     

Interactive responses of old‐field plant growth and composition to warming and precipitation

As Earth's atmosphere accumulates carbon dioxide (CO₂) and other greenhouse gases, Earth's climate is expected to warm and precipitation patterns will likely change. The manner in which terrestrial ecosystems respond to climatic changes will in turn affect the rate of climate change. Here we describe responses of an old‐field herbaceous community to a factorial combination of four levels of warming (up to 4 °C) and three precipitation regimes (drought, ambient and rain addition) over 2 years. Warming suppressed total production, shoot production, and species richness, but only in the drought treatment. Root production did not respond to warming, but drought stimulated the growth of deeper (> 10 cm) roots by 121% in 1 year. Warming and precipitation treatments both affected functional group composition, with C₄ grasses and other annual and biennial species entering the C₃ perennial‐dominated community in ambient rainfall and rain addition treatments as well as in warmed treatments. Our results suggest that, in this mesic system, expected changes in temperature or large changes in precipitation alone can alter functional composition, but they have little effect on total herbaceous plant growth. However, drought limits the capacity of the entire system to withstand warming. The relative insensitivity of our study system to climate suggests that the herbaceous component of old‐field communities will not dramatically increase production in response to warming or precipitation change, and so it is unlikely to provide either substantial increases in forage production or a meaningful negative feedback to climate change later this century.     

A contribution to the mathematical biophysics of cell growth and shapes: II

A theoretical study of the growing nerve cell filopodium is made using the assumptions of volume constancy, cylindrical shape, and substrate track of an earlier paper, but assuming additionally that a retarding force per unit area proportional to the rate of elongation is also acting. Equations of elongation for two different cases are derived.     

A contribution to the mathematical biophysics of cell growth and shapes: I

Under the assumption that the elongated shape assumed by a growing nerve cell is caused by surface tension forces between the cell, its fluid medium, and a fibrous substrate track along which the cell grows, equations of elongation and conditions insuring elongation are derived. One specific type of cell-substrate contact is considered. Equations of elongation and conditions for elongation are treated in both the nonfrictional and frictional types of motion of the cell-process.     

Macromolecular biophysics of the plant cell wall: Concepts and methodology

Plant cell walls provide form and mechanical strength to the living plant, but the relationship between their complex architecture and their remarkable ability to withstand external stress is not well understood. Primary cell walls are adapted to withstand tensile stresses while secondary cell walls also need to withstand compressive stresses. Therefore, while primary cell walls can with advantage be flexible and elastic, secondary cell walls must be rigid to avoid buckling under compressive loads. In addition, primary cell walls must be capable of growth and are subjected to cell separation forces at the cell corners. To understand how these stresses are resisted by cell walls, it will be necessary to find out how the walls deform internally under load, and how rigid are specific constituents of each type of cell wall. The most promising spectroscopic techniques for this purpose are solid-state nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) and Raman microscopy. By NMR relaxation experiments, it is possible to probe thermal motion in each cell-wall component. Novel adaptations of FTIR and Raman spectroscopy promise to allow mechanical stress and strain upon specific polymers to be examined in situ within the cell wall.     

The nucleolonema of plant and animal cells: a comparison

Depending on the author and the animal or plant origin of the material under study, the term “nucleolonema” is used in different contexts and thus indicates nucleolar ultrastructures that are different. In this paper, we attempt to clarify this state of affairs and to propose a definition for the plant cell nucleolonema     

Growth of a hypercycle and comparison with conventional autocatalysis

The hypercycle is a chemical model for reproduction which has been advocated for early stages of biological evolution. Its advantage is thought to lie in the high rate of growth conferred by hyperbolic kinetics. Earlier work has also indicated a saturation phase at large constituent concentrations. The present paper shows that both the saturation phase and the phase of hyperbolic growth have been introduced into the kinetics by making some of the reactions reversible. Reversibility is not essential to the operation of the hypercycle and the system with irreversible reactions grows faster. However, even the fastest hypercycle does not grow as fast as the simple auto-catalyst obtained by removing that reaction which is characteristic of the hypercycle. Also, both because the hypercycle is not a coherent system and because its growth requires reactions among separated constituents in the medium, it is more subject to decay than a simple autocatalytic particle. With greater complexity, slower growth, and more severe decay, the hypercycle is not a satisfactory alternative to conventional models of reproduction.     

The nucleolonema of plant and animal cells: a comparison

Depending on the author and the animal or plant origin of the material under study, the term "nucleolonema" is used in different contexts and thus indicates nucleolar ultrastructures that are different. In this paper, we attempt to clarify this state of affairs and to propose a definition for the plant cell nucleolonema.     

Virtual issue: cell wall functions in plant growth and environmental responses

Journal of Plant Research - Plant cell walls have multiple functions, including determining cell shape and size, cell–cell adhesion, controlling cell differentiation and growth, and promoting...     

Heat stress responses in cultured plant cells : development and comparison of viability tests

The response of suspension-cultured pear (Pyrus communis cv Bartlett) cells to heat stress was studied using three viability tests: regrowth (culture growth during 10 days after stress); triphenyltetrazolium chloride reduction; and electrolyte leakage. Critical (50% injury) temperatures for a 20-minute exposure were 42°, 52°, and 56°C, respectively, for these viability tests. Electrolyte leakage had the lowest temperature coefficient. Heat stress inhibition of triphenyltetrazolium chloride reducing capacity was much greater if the viability test was conducted 3 days, rather than immediately, after the stress treatment. Consistent with a major role for indirect metabolic strain in heat injury, treatment with 3.6 micromolar cycloheximide and heat stress (20 minutes at 43°C) affected culture regrowth similarly. We conclude that the measurements of direct response are not adequate substitutes for regrowth tests in assessing heat injury to cultured plant cells.     

No escape: The influence of substrate sodium on plant growth and tissue sodium responses

1. As an essential micronutrient for many organisms, sodium plays an important role in ecological and evolutionary dynamics. Although plants mediate trophic fluxes of sodium, from substrates to higher trophic levels, relatively little comparative research has been published about plant growth and sodium accumulation in response to variation in substrate sodium. Accordingly, we carried out a systematic review of plants'' responses to variation in substrate sodium concentrations.
2. We compared biomass and tissue‐sodium accumulation among 107 cultivars or populations (67 species in 20 plant families), broadly expanding beyond the agricultural and model taxa for which several generalizations previously had been made. We hypothesized a priori response models for each population''s growth and sodium accumulation as a function of increasing substrate NaCl and used Bayesian Information Criterion to choose the best model. Additionally, using a phylogenetic signal analysis, we tested for phylogenetic patterning of responses across taxa.
3. The influence of substrate sodium on growth differed across taxa, with most populations experiencing detrimental effects at high concentrations. Irrespective of growth responses, tissue sodium concentrations for most taxa increased as sodium concentration in the substrate increased. We found no strong associations between the type of growth response and the type of sodium accumulation response across taxa. Although experiments often fail to test plants across a sufficiently broad range of substrate salinities, non‐crop species tended toward higher sodium tolerance than domesticated species. Moreover, some phylogenetic conservatism was apparent, in that evolutionary history helped predict the distribution of total‐plant growth responses across the phylogeny, but not sodium accumulation responses.
4. Our study reveals that saltier plants in saltier soils proves to be a broadly general pattern for sodium across plant taxa. Regardless of growth responses, sodium accumulation mostly followed an increasing trend as substrate sodium levels increased.

     

The biophysics of leaf growth in salt-stressed barley. A study at the cell level

Biophysical parameters potentially involved in growth regulation were studied at the single-cell level in the third leaf of barley (Hordeum vulgare) after exposure to various degrees of NaCl stress for 3 to 5 d. Gradients of elongation growth were measured, and turgor pressure, osmolality, and water potentials (psi) were determined (pressure probe and picoliter osmometry) in epidermal cells of the elongation zone and the mature blade. Cells in the elongation zone adjusted to decreasing external psi through increases in cell osmolality that were accomplished by increased solute loads and reduced water contents. Cell turgor changed only slightly. In contrast, decreases in turgor also contributed significantly to psi adjustment in the mature blade. Solute deposition rates in the elongation zone increased at moderate stress levels as compared with control conditions, but decreased again at more severe NaCl exposure. Growth-associated psi gradients between expanding epidermal cells and the xylem were significant under control and moderate stress conditions (75 mM NaCl) but seemed negligible at severe stress (120 mM NaCl). We conclude that leaf cell elongation in NaCl-treated barley is probably limited by the rate at which solutes can be taken up to generate turgor, particularly at high NaCl levels.     

The role of nitric oxide in plant growth regulation and responses to abiotic stresses

Nitric oxide (NO) has received much attention in the recent two decades, equally from human, animal and plant biologists. It was found to play a crucial role in human and animal physiology, immunological reactions and signal transduction. Its ubiquity and versatile properties caught the attention of plant physiologists and biochemists. This work presents an extensive review on the NO presence and action in plants. Various modes of NO synthesis are discussed and the most novel approaches to the elucidation of plant nitric oxide synthase (NOS) structure are presented. This review focuses on the physiological role of NO in regulation of plant growth and development, as well as in the process of gene expression. Special attention is given to the action of NO during abiotic stress and the antioxidant properties of the molecule.     

Cerium-based demonstration of phosphatase activity in plastic-embedded sections: a comparison with conventional methods

Cerium-based methods have been used for the demonstration of several phosphatases at alkaline, acid and neutral pH in low temperature acetone-fixed, plastic-embedded sections. At alkaline pH calcium is used as capturing agent and the precipitated calcium phosphate converted to cerium phosphate. At neutral and acid pH cerium is used directly as capturing agent. Cerium phosphate is subsequently visualized using the H2O2-DAB method. A comparison has been made with conventional calcium-cobalt and lead methods. It appeared that calcium-cobalt methods are more susceptible to improvement than lead methods.