Plants as environmental biosensors

Editorial for Special Issue on Biosensors for Biomedical and Environmental Applications

A sensor is typically defined as a device able to transform a physical quantity of interest into a different kind of signal that can be easily measured and recorded [...].

Organisms as sensors in biohybrid entities as a novel tool for in-field aquatic monitoring

Rapidly intensifying global warming and water pollution calls for more efficient and continuous environmental monitoring methods. Biohybrid systems connect mechatronic components to living organisms and this approach can be used to extract data from the organisms. Compared to conventional monitoring methods, they allow for a broader data collection over long periods, minimizing the need for sampling processes and human labour. We aim to develop a methodology for creating various bioinspired entities, here referred to as 'biohybrids', designed for long-term aquatic monitoring. Here, we test several aspects of the development of the biohybrid entity: autonomous power source, lifeform integration and partial biodegradability. An autonomous power source was supplied by microbial fuel cells which exploit electron flows from microbial metabolic processes in the sediments. Here, we show that by stacking multiple cells, sufficient power can be supplied. We integrated lifeforms into the developed bioinspired entity which includes organisms such as the zebra musselDreissena polymorphaand water fleaDaphniaspp. The setups developed allowed for observing their stress behaviours. Through this, we can monitor changes in the environment in a continuous manner. The further development of this approach will allow for extensive, long-term aquatic data collection and create an early-warning monitoring system.

Can Plants Sense Humans? Using Plants as Biosensors to Detect the Presence of Eurythmic Gestures

This paper describes the preliminary results of measuring the impact of human body movements on plants. The scope of this project is to investigate if a plant perceives human activity in its vicinity. In particular, we analyze the influence of eurythmic gestures of human actors on lettuce and beans. In an eight-week experiment, we exposed rows of lettuce and beans to weekly eurythmic movements (similar to Qi Gong) of a eurythmist, while at the same time measuring changes in voltage between the roots and leaves of lettuce and beans using the plant spikerbox. We compared this experimental group of vegetables to a control group of vegetables whose voltage differential was also measured while not being exposed to eurythmy. We placed a plant spikerbox connected to lettuce or beans in the vegetable plot while the eurythmist was performing their gestures about 2 m away; a second spikerbox was connected to a control plant 20 m away. Using t-tests, we found a clear difference between the experimental and the control group, which was also verified with a machine learning model. In other words, the vegetables showed a noticeably different pattern in electric potentials in response to eurythmic gestures.

Detecting stress caused by nitrogen deficit using deep learning techniques applied on plant electrophysiological data

Plant electrophysiology carries a strong potential for assessing the health of a plant. Current literature for the classification of plant electrophysiology generally comprises classical methods based on signal features that portray a simplification of the raw data and introduce a high computational cost. The Deep Learning (DL) techniques automatically learn the classification targets from the input data, overcoming the need for precalculated features. However, they are scarcely explored for identifying plant stress on electrophysiological recordings. This study applies DL techniques to the raw electrophysiological data from 16 tomato plants growing in typical production conditions to detect the presence of stress caused by a nitrogen deficiency. The proposed approach predicts the stressed state with an accuracy of around 88%, which could be increased to over 96% using a combination of the obtained prediction confidences. It outperforms the current state-of-the-art with over 8% higher accuracy and a potential for a direct application in production conditions. Moreover, the proposed approach demonstrates the ability to detect the presence of stress at its early stage. Overall, the presented findings suggest new means to automatize and improve agricultural practices with the aim of sustainability.

Structural and Physiological Traits of Compound Leaves of Ceratonia siliqua Trees Grown in Urban and Suburban Ambient Conditions

Ceratonia siliqua L. (carob tree) is an endemic plant to the eastern Mediterranean region. In the present study, anatomical and physiological traits of successively grown compound leaves (i.e., the first, third, fifth and seventh leaves) of C. siliqua were investigated in an attempt to evaluate their growth under urban and suburban environmental conditions. Chlorophyll and phenolic content, as well as the specific leaf area of the compound leaves were determined. Structural traits of leaflets (i.e., thickness of palisade and spongy parenchyma, abaxial and adaxial epidermis, as well as abaxial and adaxial periclinal wall) were also investigated in expanding and fully expanded leaflets. Fully expanded leaflets from urban sites exhibited increased thickness of the lamina and the palisade parenchyma, while the thickness of the spongy parenchyma was thicker in suburban specimens. The palisade tissue was less extended than the spongy tissue in expanding leaflets, while the opposite held true for the expanded leaflets. Moreover, the thickness of the adaxial and the abaxial epidermises, as well as the adaxial and abaxial periclinal wall were higher in suburban leaflets. The chlorophyll content increased concomitantly with the specific leaf area (SLA) of both expanding and expanded leaflets, and strong positive correlations were detected, while the phenolic content declined with the increased SLA of expanding and expanded leaflets. It is noteworthy that the SLA of expanding leaflets in the suburban site was comparable to the SLA of expanded leaflets experiencing air pollution in urban sites; the size and the mass of leaf blades of C. siliqua possess adaptive features to air pollution. These results, linked to the functional structure of expanding and expanded successive foliar tissues, provide valuable assessment information coordinated with an adaptive process and yield of carob trees exposed to the considered ambient conditions, which have not hitherto been published.

Biological and Molecular Components for Genetically Engineering Biosensors in Plants

Plants adapt to their changing environments by sensing and responding to physical, biological, and chemical stimuli. Due to their sessile lifestyles, plants experience a vast array of external stimuli and selectively perceive and respond to specific signals. By repurposing the logic circuitry and biological and molecular components used by plants in nature, genetically encoded plant-based biosensors (GEPBs) have been developed by directing signal recognition mechanisms into carefully assembled outcomes that are easily detected. GEPBs allow for in vivo monitoring of biological processes in plants to facilitate basic studies of plant growth and development. GEPBs are also useful for environmental monitoring, plant abiotic and biotic stress management, and accelerating design-build-test-learn cycles of plant bioengineering. With the advent of synthetic biology, biological and molecular components derived from alternate natural organisms (e.g., microbes) and/or de novo parts have been used to build GEPBs. In this review, we summarize the framework for engineering different types of GEPBs. We then highlight representative validated biological components for building plant-based biosensors, along with various applications of plant-based biosensors in basic and applied plant science research. Finally, we discuss challenges and strategies for the identification and design of biological components for plant-based biosensors.

Electrical Responses of Pinus halepensis Mill. as an Indicator of Wildfire Risk in Mediterranean Forests by Complementing Live Fuel Moisture

Pinus halepensis forests, as Mediterranean-type ecosystems, are subject to high levels of wildfire risk in times of drought, with meteorological conditions of water stress and very high temperatures, mainly in summer. Considering the difficulty of knowing the phenological state of this species, the objective of this research was to evaluate the possibility of implementing the electrical responses (voltage and short-circuit current) as a variable in fire risk management models, compared to live fuel moisture. On the one hand, the obtained results demonstrate non-significant differences between the moisture content of the different fractions of the living branches (base and half of the branch and live fuel), even in times of drought with hydric stress and very high temperatures. Live fuel moisture of Pinus halepensis does not show significant seasonal variations under the influence of extreme fire risk factors. For this reason, it should be complemented with other variables for fire risk management models. On the other hand, the differences registered in the electrical signal show oscillations with significant variations, which are strongly correlated with the periods of extremely favourable meteorological conditions for wildfires. So, the voltages measured show ranges that correspond with great accuracy to the FWI. Voltage variation is dependent on the hydraulic dynamic plant behaviour and a result of the physiological response of pine trees to abiotic stress of drought. It is an easy-to-measure electrical parameter as well as a very reliable indicator with a high correlation with wildfire risk. Thus, electrical responses could add more knowledge about the phenological state of the trees in dependence on stress climatic conditions, allowing integration of these variables in the preventive wildfire modelling and management.

Field-Effect Transistor-Based Biosensors for Environmental and Agricultural Monitoring

The precise monitoring of environmental contaminants and agricultural plant stress factors, respectively responsible for damages to our ecosystems and crop losses, has nowadays become a topic of uttermost importance. This is also highlighted by the recent introduction of the so-called "Sustainable Development Goals" of the United Nations, which aim at reducing pollutants while implementing more sustainable food production practices, leading to a reduced impact on all ecosystems. In this context, the standard methods currently used in these fields represent a sub-optimal solution, being expensive, laboratory-based techniques, and typically requiring trained personnel with high expertise. Recent advances in both biotechnology and material science have led to the emergence of new sensing (and biosensing) technologies, enabling low-cost, precise, and real-time detection. An especially interesting category of biosensors is represented by field-effect transistor-based biosensors (bio-FETs), which enable the possibility of performing in situ, continuous, selective, and sensitive measurements of a wide palette of different parameters of interest. Furthermore, bio-FETs offer the possibility of being fabricated using innovative and sustainable materials, employing various device configurations, each customized for a specific application. In the specific field of environmental and agricultural monitoring, the exploitation of these devices is particularly attractive as it paves the way to early detection and intervention strategies useful to limit, or even completely avoid negative outcomes (such as diseases to animals or ecosystems losses). This review focuses exactly on bio-FETs for environmental and agricultural monitoring, highlighting the recent and most relevant studies. First, bio-FET technology is introduced, followed by a detailed description of the the most commonly employed configurations, the available device fabrication techniques, as well as the specific materials and recognition elements. Then, examples of studies employing bio-FETs for environmental and agricultural monitoring are presented, highlighting in detail advantages and disadvantages of available examples. Finally, in the discussion, the major challenges to be overcome (e.g., short device lifetime, small sensitivity and selectivity in complex media) are critically presented. Despite the current limitations and challenges, this review clearly shows that bio-FETs are extremely promising for new and disruptive innovations in these areas and others.

Century Impact of Macromolecules for Advances of Sensing Sciences

Impact of macro molecular theory on the progress of sensing sciences and technology has been presented in the light of materials developments, advances in physical and chemical properties. The chronological advances in the properties of macromolecules have significantly improved the sensing performances towards gases, heavy metals, biomolecules, hydrocarbon, and energetic compounds in terms of unexplored sensing parameters, durability, and working lifetime. In this review article, efforts have been made to correlate the advances in structure and interactivity of macro-molecules with their sensing behavior and working performances. The significant findings on the macromolecules towards advancing the sensing sciences are highlighted with the suitable illustration and schemes to establish it as a potential “microanalytical technique” along with existing challenges.

Companion and Smart Plants: Scientific Background to Promote Conservation Biological Control

To attain sustainable agricultural crop protection, tools such as host plant resistance, enhanced ecosystem services (i.e. conserving natural enemies) and the deployment of companion plants should be promoted in pest management programmes. These agro system manipulations could be based on chemical ecology studies considering the interactions with natural enemies and pests, regarding specifically plant defence signalling. Further, new crop protection strategies might rise from widening the knowledge regarding how herbivore-induced plant volatiles can govern a multifaceted defence response including natural enemy recruitment, pest repellence or induced defence in neighbouring plants. It is crucial to use a multitrophic approach to understand better the interactions involving companion plants, herbivores and natural enemies in the field, increasing the knowledge to build more efficient and sustainable pest management strategies. In this review, we explore the perspectives of companion plants and their semiochemicals to promote conservation biological control according to the 'smart plants' concept. Further, we discuss the advantages and disadvantages of using companion plants and explore the application of companion plants in different agroecosystems using several case studies.

Seasonal variations of electrical signals of Pinus halepensis Mill. in Mediterranean forests in dependence on climatic conditions

The temporal evolution of the electrical signal generated by Pinus halepensis was measured in a sample of 15 trees. Weekly experiments were carried out during a long-term campaign lasting over a year, while trials with a high frequency of measurements were also performed during several days. In the latter case, day-night oscillations of the electrical magnitudes were observed. Additionally, punctual meteorological events such as rainfall and electrical storms affect the electrical signal as well.The measured electrical intensity grows exponentially with the voltage. In fact, no electrical intensity that exceeds the threshold of 0.01 μA is gathered when voltage values are lower than 0.6 V. In general, higher electrical signals were gathered during the rainy seasons with moderate temperatures; while very low signals, including few measures of zero intensity, were obtained during the most stressful periods over the year, mainly by mid-summer.There is a strong correlation between the rainfall and the electrical signal. The rain-intensity correlation, together with sustained intensity values during the reproductive period in spring, suggests that this electrical magnitude could be an indicator of the physiological state of the tree and thus used for in situ and minimally invasive forest monitoring.

Identifying General Stress in Commercial Tomatoes Based on Machine Learning Applied to Plant Electrophysiology

Automated monitoring of plant health is becoming a crucial component for optimizing agricultural production. Recently, several studies have shown that plant electrophysiology could be used as a tool to determine plant status related to applied stressors. However, to the best of our knowledge, there have been no studies relating electrical plant response to general stress responses as a proxy for plant health. This study models general stress of plants exposed to either biotic or abiotic stressors, namely drought, nutrient deficiencies or infestation with spider mites, using electrophysiological signals acquired from 36 plants. Moreover, in the signal processing procedure, the proposed workflow reuses information from the previous steps, therefore considerably reducing computation time regarding recent related approaches in the literature. Careful choice of the principal parameters leads to a classification of the general stress in plants with more than 80% accuracy. The main descriptive statistics measured together with the Hjorth complexity provide the most discriminative information for such classification. The presented findings open new paths to explore for improved monitoring of plant health.

Classification of Plant Electrophysiology Signals for Detection of Spider Mites Infestation in Tomatoes

Herbivorous arthropods, such as spider mites, are one of the major causes of annual crop losses. They are usually hard to spot before a severe infestation takes place. When feeding, these insects cause external perturbation that triggers changes in the underlying physiological process of a plant, which are expressed by a generation of distinct variations of electrical potential. Therefore, plant electrophysiology data portray information of the plant state. Analyses involving machine learning techniques applied to plant electrical response triggered by spider mite infestation have not been previously reported. This study investigates plant electrophysiological signals recorded from 12 commercial tomatoes plants contaminated with spider mites and proposes a workflow based on Gradient Boosted Tree algorithm for an automated differentiation of the plant’s normal state from the stressed state caused by infestation. The classification model built using the signal samples recorded during daylight and employing a reduced feature subset performs with an accuracy of 80% in identifying the plant’s stressed state. Furthermore, the Hjorth complexity encloses the most relevant information for discrimination of the plant status. The obtained findings open novel access towards automated detection of insect infestation in greenhouse crops and, consequently, more optimal prevention and treatment approaches.

Plant behaviour: an evolutionary response to the environment?

Plants are not just passive living beings that exist in nature. They are complex and highly adaptable species that react sensitively to environmental forces/stimuli with movement, morphological changes and through the communication via volatile molecules. In a way, plants mimic some traits of animal and human behaviour; they compete for limited resources by gaining more area for more sunlight and spread their roots underground. Furthermore, in order to survive and thrive, they evolve and 'learn' to control various environmental stress factors in order to increase the yield of flowering, fertilization and germination processes. The concept of associating complex behaviour, such as intelligence, with plants is still a highly debatable topic among researchers worldwide. Recent studies have shown that plants are able to discriminate between positive and negative experiences and 'learn' from them. Some botanists have interpreted these behavioural data as a form of primitive cognitive processes. Others have evaluated these responses as biological automatisms of plants determined by adaptation to the environment and absence of intelligence. This review aims to explore adaptive behavioural aspects of various plant species distributed in different ecosystems by emphasizing their biological complexity and survival instincts.

Evaluation of electrical signals in pine trees in a mediterranean forest ecosystem

Electric potential differences in living plants are explained by theories based on sap flow. In order to acquire more advanced knowledge about the spatial distribution of these electric potential measures in trees, this research aims to analyze electrical signals in a population of Aleppo pines (Pinus halepensis Mill.) in a representative Mediterranean forest ecosystem. The specific research objective is to assess some of the most significant factors that influence the distribution pattern of those electric signals: tree age, measurement type and electrode placement. The research has been conducted in representative forest stands, obtaining measurements of different representative trees. After a statistical evaluation of the obtained results, the main conclusions of our research are: A.Tree maturity influences directly on electric potential. B.Maximum electrical signals can be measured in young pines showing values of 0.6 V and 0.6 µA for voltage and current, respectively. C.The distribution patterns of both voltage and short-circuit current depending on electrode placement are uniform.

Electrode insertion generates slow propagating electric potentials in Myriophyllum aquaticum plants

The insertion of microelectrodes into plants to record electric potentials can generate electric potential responses due to disturbance of plant tissues. Here, the electric potential triggered by Ag/AgCl glass microelectrode insertion into the stele of Myriophyllum aquaticum (parrot feather) plants was recorded. A system potential was triggered upon the electrode insertion and was propagated along the stele of the stem. The microelectrode detected this electric potential that was triggered by its own insertion and the electric potential was identical among the plants assessed. The temporal variation in electric potential registered two prominent peaks at 31.9 ± 1.8 and 17.1 ± 4.3 mV. The electric potential was repolarized after approximately 50-70 min and the stabilized electric potential was 6.5 ± 2.5 mV higher than the initial electric potential of plants. Control experiments conducted using a non-biological spongy rod wetted with distilled water or 1 M KCl confirmed that the peaks were solely due to the electric potential in the stem. These signals can be recognized as system potentials. The systematic EP could develop stimuli responses in distant locations, which is to be tested in further studies.

Constructing living buildings: a review of relevant technologies for a novel application of biohybrid robotics

Biohybrid robotics takes an engineering approach to the expansion and exploitation of biological behaviours for application to automated tasks. Here, we identify the construction of living buildings and infrastructure as a high-potential application domain for biohybrid robotics, and review technological advances relevant to its future development. Construction, civil infrastructure maintenance and building occupancy in the last decades have comprised a major portion of economic production, energy consumption and carbon emissions. Integrating biological organisms into automated construction tasks and permanent building components therefore has high potential for impact. Live materials can provide several advantages over standard synthetic construction materials, including self-repair of damage, increase rather than degradation of structural performance over time, resilience to corrosive environments, support of biodiversity, and mitigation of urban heat islands. Here, we review relevant technologies, which are currently disparate. They span robotics, self-organizing systems, artificial life, construction automation, structural engineering, architecture, bioengineering, biomaterials, and molecular and cellular biology. In these disciplines, developments relevant to biohybrid construction and living buildings are in the early stages, and typically are not exchanged between disciplines. We, therefore, consider this review useful to the future development of biohybrid engineering for this highly interdisciplinary application.

The basic roles of indoor plants in human health and comfort

Humans have a close relationship with nature, and so integrating the nature world into indoor space could effectively increase people's engagement with nature, and this in turn may benefit their health and comfort. Since people spend 80-90% of their time indoors, the indoor environment is very important for their health. Indoor plants are part of natural indoor environment, but their effect on the indoor environment and on humans has not been quantified. This review provides a comprehensive summary of the role and importance of indoor plants in human health and comfort according to the following four criteria: photosynthesis; transpiration; psychological effects; and purification. Photosynthesis and transpiration are important mechanisms for plants, and the basic functions maintaining the carbon and oxygen cycles in nature. Above all have potential inspiration to human's activities that people often ignored, for example, the application of solar panel, artificial photosynthesis, and green roof/facades were motivated by those functions. Indoor plants have also been shown to have indirect unconscious psychological effect on task performance, health, and levels of stress. Indoor plants can act as indoor air purifiers, they are an effective way to reduce pollutants indoor to reduce human exposure, and have been widely studied in this regard. Indoor plants have potential applications in other fields, including sensing, solar energy, acoustic, and people's health and comfort. Making full use of various effects in plants benefit human health and comfort.

Are plants sentient?

Feelings in humans are mental states representing groups of physiological functions that usually have defined behavioural purposes. Feelings, being evolutionarily ancient, are thought to be coordinated in the brain stem of animals. One function of the brain is to prioritise between competing mental states and, thus, groups of physiological functions and in turn behaviour. Plants use groups of coordinated physiological activities to deal with defined environmental situations but currently have no known mental state to prioritise any order of response. Plants do have a nervous system based on action potentials transmitted along phloem conduits but which in addition, through anastomoses and other cross-links, forms a complex network. The emergent potential for this excitable network to form a mental state is unknown, but it might be used to distinguish between different and even contradictory signals to the individual plant and thus determine a priority of response. This plant nervous system stretches throughout the whole plant providing the potential for assessment in all parts and commensurate with its self-organising, phenotypically plastic behaviour. Plasticity may, in turn, depend heavily on the instructive capabilities of local bioelectric fields enabling both a degree of behavioural independence but influenced by the condition of the whole plant.

High-Temperature Tolerance of Photosynthesis Can Be Linked to Local Electrical Responses in Leaves of Pea

It is known that numerous stimuli induce electrical signals which can increase a plant's tolerance to stressors, including high temperature. However, the physiological role of local electrical responses (LERs), i.e., responses in the zone of stimulus action, in the plant's tolerance has not been sufficiently investigated. The aim of a current work is to analyze the connection between parameters of LERs with the thermal tolerance of photosynthetic processes in pea. Electrical activity and photosynthetic parameters in pea leaves were registered during transitions of air temperature in a measurement head (from 23 to 30°C, from 30 to 40°C, from 40 to 45°C, and from 45 to 23°C). This stepped heating decreased a photosynthetic assimilation of CO2 and induced generation of LERs in the heated leaf. Amplitudes of LERs, quantity of responses during the heating and the number of temperature transition, which induced the first generation of LERs, varied among different pea plants. Parameters of LERs were weakly connected with the photosynthetic assimilation of CO2 during the heating; however, a residual photosynthetic activity after a treatment by high temperatures increased with the growth of amplitudes and quantity of LERs and with lowering of the number of the heating transition, inducing the first electrical response. The effect was not connected with a photosynthetic activity before heating; similar dependences were also observed for effective and maximal quantum yields of photosystem II after heating. We believe that the observed effect can reflect a positive influence of LERs on the thermal tolerance of photosynthesis. It is possible that the process can participate in a plant's adaptation to stressors.

Responses of tropical legumes from the Brazilian Atlantic Rainforest to simulated acid rain

We investigated the morphological and anatomical effects of simulated acid rain on leaves of two species native to the Brazilian Atlantic Rainforest: Paubrasilia echinata and Libidibia ferrea var. leiostachya. Saplings were subjected to acid rain in a simulation chamber during 10 days for 15 min daily, using H₂SO₄ solution pH 3.0 and, in the control, deionized water. At the end of the experiment, fragments from young and expanding leaves were anatomically analyzed. Although L. ferrea var. leiostachya leaves are more hydrophobic, rain droplets remained in contact with them for a longer time, as in the hydrophilic P. echinata leaves, droplets coalesce and rapidly run off. Visual symptomatology consisted in interveinal and marginal necrotic dots. Microscopic damage found included epicuticular wax flaking, turgor loss and epidermal cell shape alteration, hypertrophy of parenchymatous cells, and epidermal and mesophyll cell collapse. Formation of a wound tissue was observed in P. echinata, and it isolated the necrosis to the adaxial leaf surface. Acid rain increased thickness of all leaf tissues except spongy parenchyma in young leaves of L. ferrea var. leiostachya, and such thickness was maintained throughout leaf expansion. To our knowledge, this is the first report of acidity causing increase in leaf tissue thickness. This could represent the beginning of cell hypertrophy, which was seen in visually affected leaf regions. Paubrasilia echinata was more sensitive, showing earlier symptoms, but the anatomical damage in L. ferrea var. leiostachya was more severe, probably due to the higher time of contact with acid solution in this species.

Data Acquisition System for In Situ Monitoring of Chemoelectrical Potential in Living Plant Fuel Cells

Photosynthesis process in plants generates numerous sources of bioenergy. However, only a small fraction is readily exploited for electrical energy. The impact of environmental factors is one of the significant physiological influences on the electrical potential of the plants. Hence, we developed a data acquisition (DAQ) system for instantaneous monitoring of electrical potential in plants and Aloe vera was used as a plant sample. The static response characterization, capability index (P/T), and Pearson's coefficient of correlation procedures were applied to assess the reliability of the obtained data. This developed system offers the capability of in situ monitoring and detecting gradual changes in the electrical potential of plants up to a correlational strength of greater than 0.7. Interpretation of the electrical signal mechanisms in the Aloe vera plant and the optimization of the electricity can be achieved through the application of this monitoring system. This system, therefore, can serve as a tool to measure and analyze the electrical signals in plants at different conditions.

The effects of Ni(2+) on electrical signaling of Nitellopsis obtusa cells

The effect of nickel (Ni) on the generation of plant bioelectrical signals was evaluated in Nitellopsis obtusa, a Characean model organism. Conventional glass-microelectrode technique and K(+)-anaesthesia method in current-clamp and voltage-clamp modes were used for the measurement and analysis of electrical parameters. Ni(2+) treatment rapidly influenced the action potential (AP) parameters namely, excitation threshold, AP peak and duration, membrane potential at various voltages and dynamics of ion currents. We conclude that altered electrical signaling pathway in the test organism constituted the early target for Ni toxicity imposition. The observed Ni interference could be ascribed to disturbed [Ca(2+)]cyt content, impaired Cl(-) and K(+) channels activity resulting in decreased excitability and repolarization rate in generated AP.

Exploring strategies for classification of external stimuli using statistical features of the plant electrical response

Plants sense their environment by producing electrical signals which in essence represent changes in underlying physiological processes. These electrical signals, when monitored, show both stochastic and deterministic dynamics. In this paper, we compute 11 statistical features from the raw non-stationary plant electrical signal time series to classify the stimulus applied (causing the electrical signal). By using different discriminant analysis-based classification techniques, we successfully establish that there is enough information in the raw electrical signal to classify the stimuli. In the process, we also propose two standard features which consistently give good classification results for three types of stimuli--sodium chloride (NaCl), sulfuric acid (H₂SO₄) and ozone (O₃). This may facilitate reduction in the complexity involved in computing all the features for online classification of similar external stimuli in future.

Sunflower exposed to high-intensity microwave-frequency electromagnetic field: electrophysiological response requires a mechanical injury to initiate

We have monitored the electrical potential variations (EPV) of sunflower plants illuminated by a high-intensity microwave-frequency (2.5 GHz, 1.5 kV/m) electromagnetic field (EMF). We have designed an appropriate set-up that allows parallel temperature and EPV measurements while part of the plant is being exposed to the field. The results show that the considered EMF does not induce plant EPV directly. This electrophysiological response appears only when the EMF leads to a mechanical injury of the tissues via a thermal effect (dielectric heating). Once the plant inner temperature reached a threshold, we systematically observed burn-like lesions associated with the bending of the stem or leaf-stalks. Theses mechanical constraints were rapidly followed by EPVs, moving through the stem.

Use of plant woody species electrical potential for irrigation scheduling

The electrical response of plants to environmental stimuli can be measured and quantitatively related to the intensity of several stimulating sources, like temperature, solar radiation, soil water content, evapotranspiration rates, sap flow and dendrometric cycles. These relations can be used to assess the influence of different environmental situations on soil water availability to plants, defined as a steady state condition between leaf transpirative flow and soil water flow to plant roots. A restricted soil water flow due to soil dryness can trigger water stress in plants, if the atmospheric evaporative demand is high, causing partial stomata closure as a physiological response to avoid plant dehydration; water stressed and unstressed plants manifest a differential electrical response. Real time plant electrical response measurements can anticipate actions that prevent the plant reaching actual stress conditions, optimizing stomata gas exchange and photosynthetic rates. An electrophysiological sensor developed in this work, allows remote real-time recording information on plant electrical potential (EP) in the field, which is highly related to EP measurements obtained with a laboratory Keithley voltmeter sensor used in an highly controlled experimental setup. Our electrophysiological sensor is a wireless, autonomous devise, which transmits EP information via Internet to a data server. Using both types of sensors (EP electrodes with a Keithley voltmeter and the electrophysiological sensor), we measured in real time the electrical responses of Persea americana and Prunus domestica plants, to induced water deficits. The differential response for 2 scenarios: irrigation and water restriction is identified by a progressive change in slope on the daily maximal and minimal electric signal values in stressed plants, and a zero-slope for similar signals for well-watered plants. Results show a correspondence between measured signals obtained by our electrophysiological sensor and the EP electrodes connected to the Keithley voltmeter in each irrigation stage. Also, both sensors show a daily cyclical signal (circadian cycle).

Charophyte electrogenesis as a biomarker for assessing the risk from low-dose ionizing radiation to a single plant cell

The impact of low-dose ionizing radiation on the electrical signalling pattern and membrane properties of the characea Nitellopsis obtusa was examined using conventional glass-microelectrode and voltage-clamp techniques. The giant cell was exposed to a ubiquitous radionuclide of high biological importance - tritium - for low-dose irradiation. Tritium was applied as tritiated water with an activity concentration of 15 kBq L(-1) (an external dose rate that is approximately 0.05 μGy h(-1) above the background radiation level); experiments indicated that this was the lowest effective concentration. Investigating the dynamics of electrical excitation of the plasma membrane (action potential) showed that exposing Characeae to tritium for half an hour prolonged the repolarization phase of the action potential by approximately 35%: the repolarization rate decreased from 39.2 ± 3.1 mV s(-1) to 25.5 ± 1,8 mV s(-1) due to tritium. Voltage-clamp measurements showed that the tritium exposure decreased the Cl(-) efflux and Ca(2+) influx involved in generating an action potential by approximately 27% (Δ = 12.4 ± 1.1 μA cm(-2)) and 64% (Δ = -5.3 ± 0.4 μA cm(-2)), respectively. The measured alterations in the action potential dynamics and in the chloride and calcium ion transport due to the exogenous low-dose tritium exposure provide the basis for predicting possible further impairments of plasma membrane regulatory functions, which subsequently disturb essential physiological processes of the plant cell.

Electrophysiological assessment of water stress in fruit-bearing woody plants

Development and evaluation of a real-time plant water stress sensor, based on the electrophysiological behavior of fruit-bearing woody plants is presented. Continuous electric potentials are measured in tree trunks for different irrigation schedules, inducing variable water stress conditions; results are discussed in relation to soil water content and micro-atmospheric evaporative demand, determined continuously by conventional sensors, correlating this information with tree electric potential measurements. Systematic and differentiable patterns of electric potentials for water-stressed and no-stressed trees in 2 fruit species are presented. Early detection and recovery dynamics of water stress conditions can also be monitored with these electrophysiology sensors, which enable continuous and non-destructive measurements for efficient irrigation scheduling throughout the year. The experiment is developed under controlled conditions, in Faraday cages located at a greenhouse area, both in Persea americana and Prunus domestica plants. Soil moisture evolution is controlled using capacitance sensors and solar radiation, temperature, relative humidity, wind intensity and direction are continuously registered with accurate weather sensors, in a micro-agrometeorological automatic station located at the experimental site. The electrophysiological sensor has two stainless steel electrodes (measuring/reference), inserted on the stem; a high precision Keithley 2701 digital multimeter is used to measure plant electrical signals; an algorithm written in MatLab(®), allows correlating the signal to environmental variables. An electric cyclic behavior is observed (circadian cycle) in the experimental plants. For non-irrigated plants, the electrical signal shows a time positive slope and then, a negative slope after restarting irrigation throughout a rather extended recovery process, before reaching a stable electrical signal with zero slope. Well-watered plants presented a continuous signal with daily maximum and a minimum EP of similar magnitude in time, with zero slope. This plant electrical behavior is proposed for the development of a sensor measuring real-time plant water status.

Mechanisms and physiological roles of K+ efflux from root cells

Potassium is the most abundant macronutrient, which is involved in a multitude of physiological processes. Potassium uptake in roots is crucial for plants; however, K(+) efflux can also occur and has important functions. Potassium efflux from roots is mainly induced by stresses, such as pathogens, salinity, freezing, oxidants and heavy metals. Reactive oxygen species (ROS) and exogenous purines also cause this reaction. The depolarisation and activation of cation channels are required for K(+) efflux from plant roots. Potassium channels and nonselective cation channels (NSCCs) are involved in this process. Some of them are 'constitutive', while the others require a chemical agent for activation. In Arabidopsis, there are 77 genes that can potentially encode K(+)-permeable channels. Potassium-selective channel genes include 9 Shaker and 6 Tandem-Pore K(+) channels. Genes of NSCCs are more abundant and present by 20 cyclic nucleotide gated channels, 20 ionotropic glutamate receptors, 1 two-pore channel, 10 mechanosensitive-like channels, 2 mechanosensitive 'Mid1-Complementing Activity' channels, 1 mechanosensitive Piezo channel, and 8 annexins. Two Shakers (SKOR and GORK) and several NSCCs are expressed in root cell plasma membranes. SKOR mediates K(+) efflux from xylem parenchyma cells to xylem vessels while GORK is expressed in the epidermis and functions in K(+) release. Both these channels are activated by ROS. The GORK channel activity is stimulated by hydroxyl radicals that are generated in a Ca(2+)-dependent manner in stress conditions, such as salinity or pathogen attack, resulting in dramatic K(+) efflux from root cells. Potassium loss simulates cytosolic proteases and endonucleases, leading to programmed cell death. Other physiological functions of K(+) efflux channels include repolarisation of the plasma membrane during action potentials and the 'hypothetical' function of a metabolic switch, which provides inhibition of energy-consuming biosyntheses and releasing energy for defence and reparation needs.

Plant intelligence and attention

This article applies the phenomenological model of attention to plant monitoring of environmental stimuli and signal perception. Three complementary definitions of attention as selectivity, modulation and perdurance are explained with reference to plant signaling and behaviors, including foraging, ramet placement and abiotic stress communication. Elements of animal and human attentive attitudes are compared with plant attention at the levels of cognitive focus, context and margin. It is argued that the concept of attention holds the potential of becoming a cornerstone of plant intelligence studies.

Lead stress disrupts the cytoskeleton organization and cell wall construction during Picea wilsonii pollen germination and tube growth

Lead is a widespread pollutant and has been reported to inhibit pollen tube development, but the mechanism of toxicity involved remains unclear. Here, we report that lead stress significantly prevented Picea wilsonii pollen germination and tube growth and also dramatically altered the tube morphology in a concentration-dependent manner. Fluorescence labeling with JIM 5 (anti-acidic pectin antibody) and Calcofluor white revealed the lead-induced decline of acidic pectin and cellulose, especially in the subapical region. Decolorized aniline blue staining showed the marked accumulation of callose in the apical and subapical regions of lead-treated tubes. Fluorescence labeling with Alexa Fluor 568 phalloidin and anti-tubulin antibody revealed that the distribution of the cytoskeleton in P. wilsonii pollen grains and tubes were developmentally regulated and that lead disturbed the cytoskeleton organization, especially in the shank of the pollen tubes. Taken together, our experiments revealed a link between the dynamics of cytoskeleton organization and the process of P. wilsonii pollen tube development and also indicated that lead disturbed the cytoskeleton assembly and, consequently, cell wall construction. These findings provide new insights into the mechanism of lead toxicity in the tip growth of pollen tubes.

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.

Herbivore-induced plant volatiles allow detection of Trichoplusia ni (Lepidoptera: Noctuidae) infestation on greenhouse tomato plants

BACKGROUND

Monitoring of insect populations is an important component of integrated pest management and typically is based on the presence and number of insects in various development stages. Yet plants respond to insect herbivory and release herbivore-induced plant volatiles (HIPVs), which could be exploited in monitoring systems. The present objective was to investigate whether the information associated with HIPVs has potential to become part of advanced technologies for monitoring pest insect populations.

RESULTS

In a laboratory experiment, it was determined that tomato plants, Lycopersicon esculentum Mill cv. clarence, each infested with 20 caterpillars of the cabbage looper, Trichoplusia ni (Hübner), emit HIPVs, of which (Z)-3-hexenyl acetate, (E)-beta-ocimene and beta-caryophyllene were selected as chemicals indicative of herbivory. Using an ultrafast portable gas chromatograph (zNose()) in a research greenhouse and in a commercial greenhouse, it was possible (i) to reveal differential emissions of these three indicator chemicals from plants with or without herbivory, (ii) to detect herbivory within 6 h of its onset, (iii) to track changes in indicator chemical emissions over time and (iv) to study the effect of environmental and crop-maintenance-related factors on the emission of indicator chemicals.

CONCLUSION

HIPVs appear to be promising as reliable indicators of plant health, but further studies are needed to fully understand the potential of this concept.

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 (DeltaEP) 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 DeltaEP during absolute darkness, day-night cycles and different conditions of soil water availability are discussed as alternative tools to assess early plant stress conditions.

Electrical signaling in Aloe vera induced by localized thermal stress

Action potentials in higher plants are theorized as the information carriers in intercellular and intracellular communication in the presence of environmental stressors. Among the most common stressors is heat shock. Under stressful conditions, the response reactions of plant tissues and organs can be local or transmitted over long distances. In this article, the speeds of propagation of thermally induced action potentials in green plants are discussed, and their speeds were found to be comparable to those occurring in various mammalian species. These rapid action potentials in green plants were recorded in real time using modern data acquisition techniques. According to our measurements, a single application of localized heat stress induces fast action potentials in Aloe vera (67 m/s). Electrical signals propagated along all leaves of the A. vera plants were studied. Possible pathways for electrical signal propagation in vascular plants are also discussed.

Closing of venus flytrap by electrical stimulation of motor cells

Electrical signaling and rapid closure of the carnivorous plant Dionaea muscipula Ellis (Venus flytrap) have been attracting the attention of researchers since XIX century, but the exact mechanism of Venus flytrap closure is still unknown. We found that the electrical stimulus between a midrib and a lobe closes the Venus flytrap leaf by activating motor cells without mechanical stimulation of trigger hairs. The closing time of Venus flytrap by electrical stimulation of motor cells is 0.3 s, the same as mechanically induced closing. The mean electrical charge required for the closure of the Venus flytrap leaf is 13.6 microC. Ion channel blockers such as Ba(2+), TEACl as well as uncouplers such as FCCP, 2,4-dinitrophenol and pentachlorophenol dramatically decrease the speed of the trap closing. Using an ultra-fast data acquisition system with measurements in real time, we found that the action potential in the Venus flytrap has a duration time of about 1.5 ms. Our results demonstrate that electrical stimulation can be used to study mechanisms of fast activity in motor cells of the plant kingdom.