Electrical signals in avocado trees: Responses to light and water availability conditions

Plant responses to environmental changes are associated with electrical excitability and signaling; automatic and continuous measurements of electrical potential differences (ΔEP) between plant tissues can be effectively used to study information transport mechanisms and physiological responses that result from external stimuli on plants. The generation and conduction of electrochemical impulses within plant different tissues and organs, resulting from abiotic and biotic changes in environmental conditions is reported. In this work, electrical potential differences are monitored continuously using Ag/AgCl microelectrodes, inserted 5 mm deep into sapwood at two positions in the trunks of several Avocado trees. Electrodes are referenced to a non polarisable Ag/AgCl microelectrode installed 20 cm deep in the soil. Systematic patterns of ΔEP during absolute darkness, day-night cycles and different conditions of soil water availability are discussed as alternative tools to assess early plant stress conditions.Key words: plant electrical potential, light and dark cycles, water availability, plant signaling, avocado trees, plant sensors, irrigation automation     

Evidence for the transmission of information through electric potentials in injured avocado trees

Electrical excitability and signaling, frequently associated with rapid responses to environmental stimuli, have been documented in both animals and higher plants. The presence of electrical potentials (EPs), such as action potentials (APs) and variation potentials (VPs), in plant cells suggests that plants make use of ion channels to transmit information over long distances. The reason why plants have developed pathways for electrical signal transmission is most probably the necessity to respond rapidly, for example, to environmental stress factors.We examined the nature and specific characteristics of the electrical response to wounding in the woody plant Persea americana (avocado). Under field conditions, wounds can be the result of insect activity, strong winds or handling injury during fruit harvest. Evidence for extracellular EP signaling in avocado trees after mechanical injury was expressed in the form of variation potentials. For tipping and pruning, signal velocities of 8.7 and 20.9 cm/s, respectively, were calculated, based on data measured with Ag/AgCl microelectrodes inserted at different positions of the trunk. EP signal intensity decreased with increasing distance between the tipping and pruning point and the electrode. Recovery time to pre-tipping or pre-pruning EP values was also affected by the distance and signal intensity from the tipping or pruning point to the specific electrode position. Real time detection of remote EP signaling can provide an efficient tool for the early detection of insect attacks, strong wind damage or handling injury during fruit harvest.Our results indicate that electrical signaling in avocado, resulting from microenvironment modifications, can be quantitatively related to the intensity and duration of the stimuli, as well as to the distance between the stimuli site and the location of EP detection. These results may be indicative of the existence of a specific kind of proto-nervous system in plants.     

Signal transduction in Mimosa pudica: biologically closed electrical circuits

Biologically closed electrical circuits operate over large distances in biological tissues. The activation of such circuits can lead to various physiological and biophysical responses. Here, we analyse the biologically closed electrical circuits of the sensitive plant Mimosa pudica Linn. using electrostimulation of a petiole or pulvinus by the charged capacitor method, and evaluate the equivalent electrical scheme of electrical signal transduction inside the plant. The discharge of a 100 µF capacitor in the pulvinus resulted in the downward fall of the petiole in a few seconds, if the capacitor was charged beforehand by a 1.5 V power supply. Upon disconnection of the capacitor from Ag/AgCl electrodes, the petiole slowly relaxed to the initial position. The electrical properties of the M. pudica were investigated, and an equivalent electrical circuit was proposed that explains the experimental data.     

A dynamical model of environmental effects on allocation to carbon-based secondary compounds in juvenile trees

BACKGROUND AND AIMS: Patterns and variations in concentration of carbon-based secondary compounds in plant tissues have been explained by means of different complementary and, in some cases, contradictory plant defence hypotheses for more than 20 years. These hypotheses are conceptual models which consider environmental impacts on plant internal demands. In the present study, a mathematical model is presented, which converts and integrates the concepts of the 'Growth-Differentiation Balance' hypothesis and the 'Protein Competition' model into a dynamic plant growth model, that was tested with concentration data of polyphenols in leaves of juvenile apple, beech and spruce trees. The modelling approach is part of the plant growth model PLATHO that considers simultaneously different environmental impacts on the most important physiological processes of plants. METHODS: The modelling approach for plant internal resource allocation is based on a priority scheme assuming that growth processes have priority over allocation to secondary compounds and that growth-related metabolism is more strongly affected by nitrogen deficiency than defence-related secondary metabolism. KEY RESULTS: It is shown that the model can reproduce the effect of nitrogen fertilization on allocation patterns in apple trees and the effects of elevated CO(2) and competition in juvenile beech and spruce trees. The analysis of model behaviour reveals that large fluctuations in plant internal availability of carbon and nitrogen are possible within a single vegetation period. Furthermore, the model displays a non-linear allocation behaviour to carbon-based secondary compounds. CONCLUSIONS: The simulation results corroborate the underlying assumptions of the presented modelling approach for resource partitioning between growth-related primary metabolism and defence-related secondary metabolism. Thus, the dynamical modelling approach, which considers variable source and sink strengths of plant internal resources within different phenological growth stages, presents a successful translation of existing concepts into a dynamic mathematical model.     

Mycorrhizal colonization does not affect tolerance to defoliation of an annual herb in different light availability and soil fertility treatments but increases flower size in light-rich environments

Heterogeneous distribution of resources in most plant populations results in a mosaic of plant physiological responses tending to maximize plant fitness. This includes plant responses to trophic interactions such as herbivory and mycorrhizal symbiosis which are concurrent in most plants. We explored fitness costs of 50% manual defoliation and mycorrhizal inoculation in Datura stramonium at different light availability and soil fertility environments in a greenhouse experiment. Overall, we showed that non-inoculated and mycorrhiza-inoculated plants did not suffer from 50% manual defoliation in all the tested combinations of light availability and soil fertility treatments, while soil nutrients and light availability predominately affected plant responses to the mycorrhizal inoculation. Fifty percent defoliation had a direct negative effect on reproductive traits whereas mycorrhiza-inoculated plants produced larger flowers than non-inoculated plants when light was not a limiting factor. Although D. stramonium is a facultative selfing species, other investigations had shown clear advantages of cross-pollination in this species; therefore, the effects of mycorrhizal inoculation on flower size observed in this study open new lines of inquiry for our understanding of plant responses to trophic interactions. Also in this study, we detected shifts in the limiting resources affecting plant responses to trophic interactions.     

The ecological significance of long-distance water transport: short-term regulation, long-term acclimation and the hydraulic costs of stature across plant life forms

Plant hydraulic conductance, namely the rate of water flow inside plants per unit time and unit pressure difference, varies largely from plant to plant and under different environmental conditions. Herein the main factors affecting: (a) the scaling between whole‐plant hydraulic conductance and leaf area; (b) the relationship between gas exchange at the leaf level and leaf‐specific xylem hydraulic conductance; (c) the short‐term physiological regulation of plant hydraulic conductance under conditions of ample soil water, and (d) the long‐term structural acclimation of xylem hydraulic conductance to changes in environmental conditions are reviewed. It is shown that plant hydraulic conductance is a highly plastic character that varies as a result of multiple processes acting at several time scales. Across species ranging from coniferous and broad‐leaved trees to shrubs, crop and herbaceous species, and desert subshrubs, hydraulic conductance scaled linearly with leaf area, as expected from first principles. Despite considerable convergence in the scaling of hydraulic properties, significant differences were apparent across life forms that underlie their different abilities to conduct gas exchange at the leaf level. A simple model of carbon allocation between leaves and support tissues explained the observed patterns and correctly predicted the inverse relationships with plant height. Therefore, stature appears as a fundamental factor affecting gas exchange across plant life forms. Both short‐term physiological regulation and long‐term structural acclimation can change the levels of hydraulic conductance significantly. Based on a meta‐analysis of the existing literature, any change in environmental parameters that increases the availability of resources (either above‐ or below‐ground) results in the long‐term acclimation of a less efficient (per unit leaf area) hydraulic system.     

植物抗旱性中的补偿效应及其在农业节水中的应用

在论述植物补偿效应存在类型和研究范畴的基础上,详细评述了植物抗旱性中根系形态结构功能及地上部干物质积累、产量和水分利用效率方面的补偿效应及其影响因素,并对植物抗旱作用中补偿生长的可能生理学机制作了探讨。同时,对补偿效应在提高农业水分利用效率中的应用进行了讨论     

Formation and possible roles of nitric oxide in plant roots

Nitric oxide has been reported to act as a signalling molecule in different plant tissues and to participate in a variety of physiological processes. It is produced by different enzymes and sources. The root-specific plasma membrane-bound enzymes forming NO from the substrates nitrate and nitrite are of particular interest because roots serve as interfaces between plants and the soil. The co-ordinated activity of the root-specific plasma membrane-bound nitrate reductase (PM-NR) and nitrite:NO reductase (NI-NOR) suggests that NO might also be involved in root signalling and development. The rate of enzymatic production of this NO depends largely on the environmental conditions, mainly the availability of nitrate and oxygen and it is proposed that this NO plays a role during anoxia as an indicator of the external nitrate availability and in regulating symbiotic interactions at the root surface.     

Plant behavioural ecology: dynamic plasticity in secondary metabolites

Behaviour is in part the ability to respond rapidly and reversibly in response to environmental stimuli during the lifetime of an individual. Plants and animals both exhibit behaviour, but plant behaviour is most often examined in the context of morphologically plastic growth. Rapid and reversible secondary metabolite production and release is also a key mechanism by which plants behave. Here, we review plant biochemical plasticity as plant behaviour, and explicitly focus on evidence for responses that display rapid induction, reversibility and ecological relevance. Rapid induction and attenuation of plant secondary metabolites occur as chemically mediated root foraging, plant defence, allelochemistry and to regulate mutualistic relationships. We describe a wealth of information on the induction of various plant biochemical responses to environmental stimuli but found a limited body of literature on the reversibility of induced biochemical responses. Understanding the full cycle of dynamic plasticity in secondary metabolites is an important niche for future research. Biochemical behaviours extend beyond the plant kingdom; however, they clearly illustrate the capacity for plants to behave in ways that closely mirror the classic definitions and research approaches applied to behaviour in animals.     

Electrical signals and their physiological significance in plants

Electrical excitability and signalling, frequently associated with rapid responses to environmental stimuli, are well known in some algae and higher plants. The presence of electrical signals, such as action potentials (AP), in both animal and plant cells suggested that plant cells, too, make use of ion channels to transmit information over long distances. In the light of rapid progress in plant biology during the past decade, the assumption that electrical signals do not only trigger rapid leaf movements in 'sensitive' plants such as Mimosa pudica or Dionaea muscipula, but also physiological processes in ordinary plants proved to be correct. Summarizing recent progress in the field of electrical signalling in plants, the present review will focus on the generation and propagation of various electrical signals, their ways of transmission within the plant body and various physiological effects.     

Tree physiological responses to above-ground herbivory directly modify below-ground processes of soil carbon and nitrogen cycling

Above‐ground herbivory is ubiquitous in terrestrial ecosystems, yet its impacts on below‐ground processes and consequences for plants remain ambiguous. To examine whether physiological responses of individual trees may potentially modify soil nutrient availability, we subjected Fagus sylvatica L. (European beech) and Abies alba Mill. (silver fir) to simulated foliar herbivory over two growing seasons. Above‐ground herbivory enhanced N mineralization and inorganic N availability in the soil. The total input of C from the plant roots to the soil is not known; however, carbon sequestration in the soil, measured using stable isotopic techniques, was unaffected by herbivory. Fagus responded to herbivory by producing larger leaves, with increased photosynthetic capacity and N content, which largely compensated for the loss of biomass; Abies exhibited no such response. We conclude that despite large interspecific differences in the growth response, tree physiological responses to foliar herbivory are capable of directly modifying soil biological processes.     

Functional dependence underlies a positive plant-grasshopper richness relationship

A central focus of ecology is identifying the factors that shape spatial patterns of species diversity and this is particularly relevant in an era of global change. Positive relationships between plant and consumer diversity are common, but could be driven by direct responses of each trophic level to underlying environmental gradients, or indirectly where changes in environmental conditions propagate through food webs. Here we use structural equation modeling to examine the relative importance of soil resource availability and disturbance (fire) in mediating relationships between plant and grasshopper richness in insular grasslands. We found a positive relationship between plant and consumer richness that became stronger after accounting for disturbance, despite unique responses of plants and consumers to the two environmental gradients. Plant richness responded to an underlying gradient in soil resource availability. Time since the last fire had a direct positive effect on grasshopper richness but had no effect on plant richness. This work supports that plant and consumer richness are functionally linked, rather than having similar responses to environmental gradients. By disentangling the direct and indirect processes underlying a positive relationship between plant and consumer diversity in a natural system that spans multiple environmental gradients, we demonstrate the importance of investigating biodiversity through explicit multivariate models.     

连作障碍与根际微生态研究Ⅰ.根系分泌物及其生态效应

作物、蔬菜、果树以及苗木长期连作后,皆出现生长衰退和产量降低．许多研究结果表明,连作条件下土壤生态环境对植物生长有很大的影响,尤以植物残体与病原微生物的分解产物,对植物有致毒作用,并影响植物根系分泌物正常代谢,以致于发生自毒作用．本文围绕根系分泌物与根际微生态的相互关系,系统地介绍连作障碍条件下,影响根系分泌物的环境因素（土壤空气、湿度、养分与微生物）、活性物质（自身毒素、残体分解物、微生物产生毒素）、土壤病原菌等的根际效应,为深入研究根系分泌物与连作障碍的相互作用机制提供启示．     

连作障碍与根际微生态研究 Ⅰ. 根系分泌物及其生态效应

作物、蔬菜、果树以及苗木长期连作后,皆出现生长衰退和产量降低。许多研究结果表明,连作条件下土壤生态环境对植物生长有很大的影响,尤以植物残体与病原微生物的分解产物,对植物有致毒作用,并影响植物根系分泌物正常代谢,以致于发生自毒作用。本文围绕根系分泌物与根际微生态的相互关系,系统地介绍连作障碍条件下,影响根系分泌物的环境因素(土壤空气、湿度、养分与微生物)、活性物质(自身毒素、残体分解物、微生物产生毒素)、土壤病原菌等的根际效应,为深入研究根系分泌物与连作障碍的相互作用机制提供启示。     

Long-term changes in vegetation and soil chemistry in a calcareous and sandy semi-natural grassland

Calcicole plant species are vulnerable to acidification and fertilization, caused by deposition and changes in land use, since they are adapted to nutrient-poor calcareous conditions. In this study we used vegetation data (vascular plants, bryophytes and lichens) from 1964 and 1985 and stored soil samples from 1966 to investigate long-term soil chemistry and vegetation changes in a semi-natural, sandy calcareous grassland in southern Sweden. In the re-investigation in 2008 we found that increased decalcification due to acidification could not be verified. The plant community had changed from stress-tolerant calcareous grassland towards a community promoted by higher nutrient availability. Furthermore, the cover of species indicating calcareous conditions had decreased. A decline in the cover of species adapted to alkaline, phosphorus-poor conditions may be due to increased nutrient availability, but there were also indications that the vegetation had changed due to overgrowth by woody plants. This long-term impoverishment of the plant community highlights the need for appropriate management of calcareous grasslands, in order to limit the nutrients available in the soil and prevent overgrowth by shrubs and trees.     

Interplay between ethylene and nitrogen nutrition: How ethylene orchestrates nitrogen responses in plants

The stress hormone ethylene plays a key role in plant adaptation to adverse environmental conditions.Nitrogen(N) is the most quantitatively required mineral nutrient for plants,and its availability is a major determinant for crop production.Changes in N availability or N forms can alter ethylene biosynthesis and/or signaling.Ethylene serves as an important cellular signal to mediate root system architecture adaptation,N uptake and translocation,ammonium toxicity,anthocyanin accumulation,and prem...     

Patterns in a species‐rich tropical understory plant community

Understory plants are an important component of the high plant species diversity characteristic of neotropical rain forests. Herbs, shrubs, understory trees, and saplings of canopy trees occupy a broadly uniform environment of abundant rainfall, low light levels, and high humidity. We asked whether this community at the La Selva Biological Station in the Caribbean lowlands of Costa Rica was structured by environmental filters such as soil origin, topographic position, and understory light availability. We used nested quadrats to assess effects of soil origin (recent alluvium, weathered alluvium, residual volcanic soil) and topographic position (ridges, mid‐slopes and flats) on species composition, density, and diversity and measured six edaphic and understory light parameters. Non‐metric multidimensional scaling ordinations were based on frequency of occurrence in 20 quadrats for 272 species in the shrub size class and 136 species in the small‐tree size class for 17 sites. Three axes were correlated with composite environmental variables produced by principal component analysis representing slope, extractable phosphorus, and light. NMS site positions also reflected soil origin, topographic position, and geographic location. The analyses illustrated a complex community structured by species responses to environmental filters at multiple, interdigitated spatial scales. We suggest that light availability affected by canopy dynamics and dispersal limitation provides additional sources of variation in species distributions, which interact with edaphic patterns in complex ways. Abstract in Spanish is available with online material.     

Conservation between higher plants and the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis>in response to the phytohormone abscisic acid: a proteomics analysis

Background  

The plant hormone abscisic acid (ABA) is ubiquitous among land plants where it plays an important role in plant growth and development. In seeds, ABA induces embryogenesis and seed maturation as well as seed dormancy and germination. In vegetative tissues, ABA is a necessary mediator in the triggering of many of the physiological and molecular adaptive responses of the plant to adverse environmental conditions, such as desiccation, salt and cold.     

Effects of previous plant infestation on sieve element acceptance by two aphids

Probing behaviour as affected by a previous infestation was studied in two aphid species on their respective host plants. Probing behaviour by Aphis fabae (reported to have beneficial effects when living in colonies) and Rhopalosiphum padi (without known beneficial effects) was studied using the electrical penetration graph (EPG) technique. In A. fabae the main effects were longer and more continuous sap ingestion, and less salivation into sieve elements before sap ingestion. These suggest phloem factors. Nevertheless, mesophyll and non-vascular tissues are likely to be involved to a lesser extent, as reflected by fewer non-probing periods before the first phloem phase on previously colonised leaves as compared to clean leaves. Total honeydew production increased on a previously colonised leaf due to the prolonged sap ingestion periods but the excretion rate was not affected, indicating that the ingestion rate remained unaltered. R. padi did not show responses to previous colonisation. It is hypothesized that the changes in probing behaviour are due to changed plant properties, chemical contents of sieve element sap and/or physiological changes induced by the saliva from the colony.