Modelling the effect of autotoxicity on density-dependent phytotoxicity

An established method to separate resource competition from chemical interference is cultivation of monospecific, even-aged stands. The stands grow at several densities and they are exposed to homogenously spread toxins. Hence, the dose received by individual plants is inversely related to stand density. This results in distinguishable alterations in dose-response slopes. The method is often recommended in ecological studies of allelopathy. However, many plant species are known to release autotoxic compounds. Often, the probability of autotoxicity increases as sowing density increases. Despite this, the possibility of autotoxicity is ignored when experiments including monospecific stands are designed and when their results are evaluated. In this paper, I model mathematically how autotoxicity changes the outcome of dose-response slopes as different densities of monospecific stands are grown on homogenously phytotoxic substrata. Several ecologically reasonable relations between plant density and autotoxin exposure are considered over a range of parameter values, and similarities between different relations are searched for. The models indicate that autotoxicity affects the outcome of density-dependent dose-response experiments. Autotoxicity seems to abolish the effects of other phytochemicals in certain cases, while it may augment them in other cases. Autotoxicity may alter the outcome of tests using the method of monospecific stands even if the dose of autotoxic compounds per plant is a fraction of the dose of non-autotoxic phytochemicals with similar allelopathic potential. Data from the literature support these conclusions. A faulty null hypothesis may be accepted if the autotoxic potential of a test species is overlooked in density-response experiments. On the contrary, if test species are known to be non-autotoxic, the method of monospecific stands does not need fine-tuning. The results also suggest that the possibility of autotoxicity should be investigated in many density-response bioassays that are made with even-aged plants, and that measure plant growth or germination.     

药用牡丹的自毒作用及其防治措施

自毒作用是同种植物个体之间的一种化学作用,是植物经过长期环境适应与进化选择而获得的一种避免种内竞争的机制。药用牡丹皮是中国著名的道地中药材,连作障碍会影响药用牡丹皮的产量、质量及产品安全,在药用牡丹皮的GAP生产中明确提出禁止连作和重茬。综述了自毒作用的定义、特点、自毒物质的种类、释放途径及作用机理,并结合药用牡丹的生物学特征及研究现状等问题分析了药用牡丹连作障碍中的自毒作用。认为药用牡丹的自毒作用是连作障碍的主要原因之一,自毒作用可以通过牡丹根系分泌产生自毒物质、植株残体分解释放有害物质及专性或兼性病虫害产生的毒素来实现,并在此基础上提出连作障碍的防治措施和研究展望,以期实现药用牡丹皮生产的可持续发展。     

Density and reproductive success in wild populations of Diplotaxis erucoides (Brassicaceae)

Summary One possible consequence of low population density, particularly in self-incompatible plants, is reproductive failure. I surveyed seed set per flower in two populations of the self-incompatible annual Diplotaxis erucoides (Brassicaceae) in Jerusalem, Israel. Widely spaced plants had lower fruit set and fewer seeds per filled silique than did plants growing close to conspecific neighbors. Such density-dependent reproductive success could help explain the maintanence of spatial patchiness in plant populations, and could also have implications for population dynamics of rare species.     

植物自毒物质剂量与效应的机理模型研究

植物自毒现象在自然和农业生态系统中广泛存在.该研究针对植物自毒作用强弱与自毒物质浓度相关的低促高抑赫米斯(Hormesis)特性,在An-Johnson-Lovett Hormesis模型中引入生态限制因子米式(Mitscherlich)模型,建立了自毒物质作用的剂量/自毒效应规律的机理数学模型.模型的演示与文献的报道结果一致,用已经发表的多种植物的自毒研究数据进行检验显示很强的拟合效果,说明以生态限制因子为核心建立的自毒模型不仅从植物生长的生物生态学属性上进一步揭示了自毒作用随自毒物质浓度变化的效应规律,同时在应用上具普遍性,这为今后更深入地研究自毒作用提供了一理论平台.     

Soil microbial biomass C and symbiotic processes associated with soybean after sulfentrazone herbicide application

Resource competition and chemical interference are mechanisms of interaction among plants that may occur simultaneously. However, both mechanisms are rarely considered together when modelling plant growth. We propose a new empirical model that estimates biologically significant parameters on both plant competition and chemical interference. The model is tested with data sets from different density-dependent experiments done with two species (the grass Lolium rigidum Gaud. and the legume Glycine max soya L.) subjected to a noxious chemical environment when growing (allelochemicals and herbicides, respectively). Hypotheses on the effect of allelochemicals and its interaction with density are tested using maximum likelihood ratio tests in order to ask, for these species, whether chemical interference is playing a significant role in the interactions among plants or on the contrary, whether interactions among plants are sufficiently explained by the resource competition. In all cases a significant interaction between chemicals and density is observed. This interaction is inconsistent with the hypothesis of only resource competition having an influence of plant biomass and suggests a significant density-dependent effect of chemicals on plant growth.     

Autotoxicity in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) seedlings is alleviated by silicon through an increase in the activity of antioxidant enzymes and by mitigating lipid peroxidation

Autotoxicity in plants limits their growth and that of nearby plants of the same species, which has obvious implications in crop yield and quality. Silicon (Si) has been shown to increase plant tolerance to autotoxic stress. However, the physiological mechanisms underlying the effects of Si in alleviating autotoxicity during germination in cucumber (Cucumis sativus L.) are unknown. Cinnamic acid derivatives, such as 3-phenylpropionic acid (PA), are a class of autotoxins present in cucumber root exudates. Our objective was to investigate Si-induced autotoxic stress tolerance in cucumber seedlings by focusing on the effects of Si on the induction of antioxidant defense pathways. We found that PA treatment significantly reduced seed germination, radicle length, lateral root number, fresh weight, AsA and GSH contents, and the activities of SOD, CAT, and APX in cucumber seedlings, while it increased membrane permeability and levels of MDA, proline, O₂^-, and H₂O₂. Application of Si enhanced growth of PA-treated plants and significantly increased germination rate, radicle length, lateral root number, fresh weight, AsA and GSH levels, and SOD, CAT, POD, and APX activities. These results suggest that exogenous Si alleviates autotoxicity caused by PA during seed germination by increasing antioxidant enzyme activities and mitigating lipid peroxidation.     

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

Plants may experience reduced reproductive success at low densities, due to lower numbers of pollinator visits or reduced visit quality. Co-occurring plant species that share pollinators have the potential to facilitate pollination by either increasing numbers of pollinator visits or increasing the quality of visits, but also have the potential to reduce plant reproductive success through competition for pollination. I used a field experiment with a common distylous perennial (Piriqueta caroliniana) in the presence and absence of a co-flowering species (Coreopsis leavenworthii) in plots with one of four different distances between conspecific plants. I found strong negative effects of increasing interplant distance (related to conspecific density) on several components of P. caroliniana reproductive success: pollinator visits to plants per plot visit, visits received by individual plants, conspecific pollen grains on stigmas, outcross pollen grains on stigmas, and probability of fruit production. Although P. caroliniana and C. leavenworthii share pollinators, the co-flowering species did not affect visitation, pollen receipt or reproductive effort in P. caroliniana. Pollinators moved very infrequently between species in this experiment, so floral constancy might explain the lack of effect of the co-flowering species on P. caroliniana reproductive success at low densities. In co-occurring self-incompatible plants with floral rewards, reproductive success at low density may depend more on conspecific densities than on the presence of other species.     

Pollen and seed dispersal among dispersed plants

The ecological significance of spacing among plants in contributing to the maintenance of species richness, particularly in tropical forests, has received considerable attention that has largely focussed on distance- and density-dependent seed and seedling mortality. More recently it has become apparent that plant spacing is also relevant to pollination, which often constrains seed production. While seed and seedling survival is reduced at high conspecific densities, pollination success, by contrast, is positively correlated to local conspecific density. Distance-dependent mechanisms acting on pollination and seed production have now been described for a variety of plants, with relatively isolated plants or fragmented populations generally suffering reduced fecundity due to pollen limitation. Yet there is considerable variability in the vulnerability of plant species to pollination failure, which may be a function of breeding system, life history, the pollination vector, the degree of specialisation among plants and their pollinators, and other indirect effects of habitat change acting on plants or pollinators. As reduced tree densities and population fragmentation are common outcomes of anthropogenically altered landscapes, understanding how pollination processes are affected in such degraded landscapes can inform effective conservation and management of remaining natural areas.     

Sexual conflict and ecology: Species composition and male density interact to reduce male mating harassment and increase female survival

Sexual conflict is a pervasive evolutionary force that can reduce female fitness. Experimental evolution studies in the laboratory might overestimate the importance of sexual conflict because the ecological conditions in such settings typically include only a single species. Here, we experimentally manipulated conspecific male density (high or low) and species composition (sympatric or allopatric) to investigate how ecological conditions affect female survival in a sexually dimorphic insect, the banded demoiselle (Calopteryx splendens). Female survival was strongly influenced by an interaction between male density and species composition. Specifically, at low conspecific male density, female survival increased in the presence of heterospecific males (C. virgo). Behavioral mating experiments showed that interspecific interference competition reduced conspecific male mating success with large females. These findings suggest that reproductive interference competition between con‐ and heterospecific males might indirectly facilitate female survival by reducing mating harassment from conspecific males. Hence, interspecific competitors can show contrasting effects on the two sexes thereby influencing sexual conflict dynamics. Our results call for incorporation of more ecological realism in sexual conflict research, particularly how local community context and reproductive interference competition between heterospecific males can affect female fitness.     

Identification of autotoxic compounds in fibrous roots of Rehmannia (Rehmannia glutinosa Libosch.)

Rehmannia is a medicinal plant in China. Autotoxicity has been reported to be one of the major problems hindering the consecutive monoculture of Rehmannia. However, potential autotoxins produced by the fibrous roots are less known. In this study, the autotoxicity of these fibrous roots was investigated. Four groups of autotoxic compounds from the aqueous extracts of the fibrous roots were isolated and characterized. The ethyl acetate extracts of these water-soluble compounds were further analyzed and separated into five fractions. Among them, the most autotoxic fraction (Fr 3) was subjected to GC/MS analysis, resulting in 32 identified compounds. Based on literature, nine compounds were selected for testing their autotoxic effects on radicle growth. Seven out of the nine compounds were phenolic, which significantly reduced radicle growth in a concentration-dependent manner. The other two were aliphatic compounds that showed a moderate inhibition effect at three concentrations. Concentration of these compounds in soil samples was determined by HPLC. Furthermore, the autotoxic compounds were also found in the top soil of the commercially cultivated Rehmannia fields. It appears that a close link exists between the autotoxic effects on the seedlings and the compounds extracted from fibrous roots of Rehmannia.     

Spatial density-dependent survival and development at different larval stages of the tiger beetle <Emphasis Type="Italic">Cicindela japonica</Emphasis> (Thunberg)

Conspecific competition is an important component of the ecological processes of many species. In the case of sessile consumers, high population densities lead to competition within conspecific populations that, in turn, affect the survival, growth, and reproduction of the individuals involved. This study quantified neighborhood crowding and evaluated the extent of density effects on a tiger beetle (Cicindela japonica Thunberg) population by monitoring individually identified larvae at regular intervals. As an index of conspecific competition, the neighborhood density (the number of other larvae within a given radius for each larva) of each individual was measured. The radius size representing the highest mean coefficient of variation of density was determined as a suitable scale for detecting the density effects. Multiple logistic regression analysis was carried out to evaluate the effects of three factors (neighborhood density, prey abundance, and environmental influences) on larval survival and development. The analysis revealed that the neighborhood density significantly influenced the survival and development of larvae through every larval stage from the first to the third-instars. Moreover, the neighborhood density had a stronger influence on larvae of the same instar as compared to that on those of different instars. Our results suggest that density-dependent mortality affects the tiger beetle larvae due to the lifestyle pattern of these sedentary, ambushing predators that exhibit an aggregated spatial distribution.     

Experimental excursions on adaptive landscapes: density-dependent selection on egg size

Abstract. Theories of density-dependent natural selection suggest that intraspecific competition will favor juveniles of high competitive ability. Empirical evidence has been provided from laboratory selection experiments, but field studies are lacking due to the logistical difficulties of experimentally manipulating population densities in natural settings. Here, we present data from a decade-long experimental field study of side-blotched lizards, Uta stansburiana that overcomes these difficulties. We tested the hypothesis that density-dependent natural selection causes egg size to increase from early to late clutches in this and many other species. Using a novel combination of environmental manipulations of hatchling density and phenotypic manipulations of egg size, we demonstrate that the nature of selection on egg size changes dramatically in the absence of older competitors. The strength of selection on egg size among later-clutch hatchlings released in areas without competitors from early clutches became almost doubled in magnitude, compared to that among hatchlings released in the presence of older competitors. These experimental findings demonstrate density-dependent natural selection on egg size; however, they contradict the classical idea that egg size increases during the reproductive season because of competition between early and late hatchlings. The results indicate that competitive age or size asymmetries between early and late hatchlings can override within-cohort asymmetries due to egg size. We suggest that competition could be an important mediator of oscillating selection pressures in this and other systems. Finally, we discuss the utility of "double-level," simultaneous experimental manipulation of both phenotypic traits that are targets of selection (e.g., egg size) as well the environmental agents of selection (e.g., population density).     

Structural blueprint and ontogeny determine the adaptive value of the plastic response to competition in clonal plants: a modelling approach

Local competitive interactions strongly influence plant community dynamics. To maintain their performance under competition, clonal plants may plastically modify their network architecture to grow in the direction of least interference. The adaptive value of this plastic avoidance response may depend, however, on traits linked with the plant’s structural blueprint and ontogeny. We tested this hypothesis using virtual populations. We used an Individual Based Model to simulate competitive interactions among clones within a plant population. Clonal growth was studied under three competition intensities in plastic and non-plastic individuals. Plasticity buffered the negative impacts of competition at intermediate densities of competitors by promoting clone clumping. Success despite competition was promoted by traits linked with (1) the plant’s structural blueprint (weak apical dominance and sympodial growth) and (2) ontogenetic processes, with an increasing or a decreasing dependence of the elongation process on the branch generation level or length along the competition intensity gradient respectively. The adaptive value of the plastic avoidance response depended on the same traits. This response only modulated their importance for clone success. Our results show that structural blueprint and ontogeny can be primary filters of plasticity and can have strong implications for evolutionary ecology, as they may explain why clonal plants have developed many species-specific plastic avoidance behaviours.     

Inclusive fitness,asymmetric competition and kin selection in plants

The findings that some plants alter their competitive phenotype in response to genetic relatedness of its conspecific neighbour (and presumed competitor) has spurred an increasing interest in plant kin‐interactions. This phenotypic response suggests the ability to assess the genetic relatedness of conspecific competitors, proposing kin selection as a process that can influence plant competitive interactions. Kin selection can favour restrained competitive growth towards kin, if the fitness loss from reducing own growth is compensated by increased fitness in the related neighbour. This may lead to positive frequency dependency among related conspecifics with important ecological consequences for species assemblage and coexistence. However, kin selection in plants is still controversial. First, many studies documenting a plastic response to neighbour relatedness do not estimate fitness consequences of the individual that responds, and when estimated, fitness of individuals grown in competition with kin did not necessarily exceed that of individuals grown in non‐kin groups. Although higher fitness in kin groups could be consistent with kin selection, this could also arise from mechanisms like asymmetric competition in the non‐kin groups. Here we outline the main challenges for studying kin selection in plants taking genetic variation for competitive ability into account. We emphasize the need to measure inclusive fitness in order to assess whether kin selection occurs, and show under which circumstances kin selected responses can be expected. We also illustrate why direct fitness estimates of a focal plant, and group fitness estimates are not suitable for documenting kin selection. Importantly, natural selection occurs at the individual level and it is the inclusive fitness of an individual plant – not the mean fitness of the group – that can capture if a differential response to neighbour relatedness is favoured by kin selection.     

Competitive asymmetry, foraging area size and coexistence of annuals

Individuals allocate resources to the expansion of their foraging area and those resources are no longer available for the traits that determine how well those individuals are able to protect their foraging area against competitors. The resulting trade‐off between foraging area size and the traits associated with the ability to compete for the resources within the foraging area applies to ecological scenarios as different as territorial defence by individuals and colonies, and light competition in plants. Whether the trade‐off affects species performance in competition for resources at the area of overlap between foraging areas depends on the symmetry of resource division. In symmetric competition resources are divided equally between the competitors, while in asymmetric competition the individual with the smallest foraging area, and consequently the greatest competitive ability, gains all the resources. Competition may also be a combination of the symmetric and asymmetric processes. I studied the effects of competitive asymmetry on population dynamics and coexistence of two annual species with different sized foraging areas using an individual‐based spatially explicit simulation model. Symmetric competition favoured the species with the larger foraging area and did not allow coexistence. Competitive asymmetry favoured the species with smaller foraging area and allowed coexistence, which was due to the consequences of losing an asymmetric competition being more severe than losing a symmetric competition. The mechanism of coexistence is the larger foraging area's superiority in low population densities (little competition) and the smaller foraging area's ability to win a large foraging area when competition was intense. Competitive asymmetry and small size of both foraging areas led to population dynamics dominated by long‐term fluctuations of small intensity. Symmetric competition and large size of the foraging areas led to large short‐term fluctuations, which often resulted in the extinction of one or both of the species due to demographic stochasticity.     

Rosa multiflora’s performance under water stress: the role of positive and negative density-dependent intraspecific interactions

It is well known that biotic interactions may significantly alter plant population and community dynamics in natural ecosystems. Multiple studies have reported that density-dependent positive interactions can modify the effect of extreme stress on plant performance (in line with predictions by the stress gradient hypothesis, SGH). However, the performance of invasive species and the role of intraspecific density dependence, either negative or positive, in successful invasions are not well understood. In the present study, we tested if monocultures of Rosa multiflora, a highly invasive shrub, experience a group advantage under extremely low water availability. Using a manipulative greenhouse study, we tested the growth response of R. multiflora seedlings to a 4-level conspecific density treatment and a 4-level water availability gradient. Overall, our results provide preliminary evidence that biotic interactions under stressful conditions between R. multiflora individuals do not experience a group advantage, as predicted. Instead, our study indicates that seedling performance was strongly driven by negative density dependence, in which individuals grown at higher conspecific densities displayed lower plant performance, regardless of water availability. This demonstrates that R. multiflora experiences intense intraspecific competition even under a stressful, low resource environment. Thus, management efforts of R. multiflora need to ensure that re-sprouting of individuals after treatment is eliminated, as release from negative density dependence could lead the invader population to rebound.

     

Effects of intraspecific and interspecific density on the demography of a perennial herb, Sanicula europaea

We examined the effects of intraspecific and interspecific competition on demographic processes in the perennial herb Sanicula europaea by manipulating the density of neighbouring plants. We followed the response in terms of survival, growth and reproduction and in terms of seedling recruitment. The demographic data from all phases of the life cycle enabled us to assess also the overall effects of treatments on population growth rate (λ) by transition matrix models. We also decomposed the differences in λ between control and treatments, using life table response experiments (LTRE). To study the effects of competition on recruitment in more detail and to evaluate the role of seed availability, we sowed seeds at different densities with or without vegetation removal.
Vegetative growth and flowering frequency of established individuals was not significantly affected by removal treatments, which suggest no, or a delayed response to released competition. Neighbour removal had no effect on seedling emergence but enhanced recruitment through a higher seedling survival. Conspecific and simultaneous conspecific and heterospecific removal of plants led to an increase in population growth rate (λ), whereas heterospecific removal alone led to a decrease. Emergence of seedlings and fate of vegetative established individuals contributed most to differences in λ between the control and the different treatments. Seed addition enhanced seedling emergence but, as seedling and juvenile survival were density dependent, densities of established individuals appear not to be seed limited.
In S. europaea removal treatments had different effects on established individuals and recruitment. This suggests that studies quantifying the effects of competition over the entire life cycle and performed in a natural environment are necessary to assess the importance of competition in perennial plant populations.     

An ecological perspective of allelochemical interference in land–water interface communities

Allelochemical interactions among aquatic macrophytes and between macrophytes and attached microbial assemblages (epiphyton) influence a number of ecological processes. The ecological importance of these interactions, however, is poorly understood; we hypothesize that paucity has resulted, in part, from (1) a narrow focus on exploration for herbicidal plant products from aquatic macrophytes, (2) the difficulties in distinguishing resource competition from allelopathic interference, and (3) a predominance of approaching aquatic allelopathy from a terrestrial perspective. Based upon recent thorough investigations of allelopathy among aquatic vascular plants, chemical compounds that influence competitive interactions among littoral organisms are amphiphilic compounds that tend to remain near the producing organism (e.g., polyphenolic compounds and volatile fatty acids). Production of these compounds may be influenced by relative availability of nutrients (particularly phosphorus and nitrogen), inorganic carbon, and light. Macrophyte strategies of clonal reproduction, in an effort to persist in these highly productive and competitive habitats, have contributed to reduced reliance upon sexual reproduction that is correlated with allelopathic autotoxicity among several dominant wetland plant species. Although few studies document the importance of allelochemical interactions in the wetland and littoral zones of aquatic ecosystems, abundant evidence supports the potential for significant effects on competition and community structure; effects of altered nutrient ratios and availability on plant chemical composition; and resultant effects on trophic interactions, particularly suppression of herbivory, competitive attached algae and cyanobacteria, and heterotrophic utilization of organic matter by bacteria and fungi.     

Autotoxicity: Concept,Organisms, and Ecological Significance

The present review deals with the phenomenon of autotoxicity — a type of intraspecific allelopathy, where a plant species inhibits the growth of its own kind through the release of toxic chemicals into the environment. This phenomenon has been reported to occur in a number of weeds and crop plants in agroecosystem and wastelands causing the soil sickness. Besides, it plays a significant role in the orchards (of apple, pear, grapes, etc.) where it is the major reason of the replant problem, natural forests and coffee and tea plantations causing the regeneration problems. Not only the higher plants, but even some ferns and algae are also reported to show this phenomenon. Some plants have even developed extensive mechanisms to overcome this phenomenon, whereas the others have adapted to it by making structural and ecological changes providing to them a competitive ecological advantage over the others. Although autotoxicity is a natural phenomena providing selective benefit to the plant, yet the chemicals responsible for this have good potential for weed and pest management.     

Heterogeneous interspecific interactions in a host-parasite system

Macroparasites of vertebrates usually occur in multi-species communities, producing infections whose outcome in individual hosts or host populations may depend on the dynamics of interactions amongst the different component species. Within a single co-infection, competition can occur between conspecific and heterospecific parasite individuals, either directly or via the host's physiological and immune responses. We studied a natural single-host, multi-parasite model infection system (polystomes in the anuran Xenopus laevis victorianus) in which the parasite species show total interspecific competitive exclusion as adults in host individuals. Multi-species infection experiments indicated that competitive outcomes were dependent on infection species composition and strongly influenced by the intraspecific genetic identity of the interacting organisms. Our results also demonstrate the special importance of temporal heterogeneity (the sequence of infection by different species) in competition and co-existence between parasite species and predict that developmental plasticity in inferior competitors, and the induction of species-specific host resistance, will partition the within-host-individual habitat over time. We emphasise that such local (within-host) context-dependent processes are likely to be a fundamental determinant of population dynamics in multi-species parasite assemblages.