Plant perception (physiology)
In botany, plant perception is the ability of plants to sense the environment and adjust their morphology, physiology and phenotype accordingly. Research draws on the fields of plant physiology, ecology and molecular biology. Examples of stimuli which plants perceive and can react to include chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, physical disruption, sound, and touch. Plants have a variety of means to detect such stimuli and a variety of reaction responses or behaviors.
Plant perception occurs on a cellular level. Research published in September 2006 has shown, certainly in the case of Arabidopsis thaliana, the role of cryptochromes in the perception of magnetic fields by plants. Mechanical perturbation can also be detected by plants. Poplar stems can detect reorientation and inclination (equilibrioception).
Plant response strategies depend on quick and reliable recognition-systems.
Wounded tomatoes are known to produce the volatile odour methyl-jasmonate as an alarm-signal. Plants in the neighbourhood can then detect the chemical and prepare for the attack by producing chemicals that defend against insects or attract predators.
Plants produce several proteins found in the animal neuron systems such as acetylcholine esterase, glutamate receptors, GABA receptors, and endocannabinoid signaling components. They also use ATP, NO, and ROS like animals for signaling.
Plant cells can be electrically excitable and can display rapid electrical responses (action potentials) to environmental stimuli. These action potentials can influence processes such as actin-based cytoplasmic streaming, plant organ movements, wound responses, respiration, photosynthesis and flowering.
These electrical responses can cause the synthesis of numerous organic molecules including ones that act as neuroactive substances in other organisms. Thus, plants accomplish behavioural responses to environmental, communicative, and ecological contexts.
While many cells in nearly all living organisms can be electrically excitable, this is not evidence of neurons, or of intelligence.
A plant's concomitant reactive behavior is mediated by phytochromes, kinins, hormones, antibiotic or other chemical release, changes of water and chemical transport, and other means. These responses are generally slow, taking at minimum a number of hours to accomplish, and can best be observed with time-lapse cinematography, but rapid movements can occur as well. Plants respond to volatile signals produced by other plants. Jasmonate levels also increase rapidly in response to mechanical perturbations such as tendril coiling.
Plants have many strategies to fight off pests. For example, they can produce different toxins (phytoalexins) against invaders or they can induce rapid cell death in invading cells to hinder the pests from spreading out.
Some plants are capable of rapid movement: the mimosa plant (Mimosa pudica) makes its thin leaves point down at the slightest touch and carnivorous plants such as the Venus flytrap snap shut by the touch of insects.
Adaptive responses include:
- Active foraging for light and nutrients. They do this by changing their architecture[vague], physiology and phenotype.
- Leaves and branches are positioned and oriented in response to light source.
- Ability to detect soil volume and adapt growth accordingly independently of nutrient availability.
- Adaptively defend against herbivores.
Aspects of perception
Many plant-organs contain photo-sensitive compounds (phototropins, cryptochromes and phytochromes) each reacting very specifically to certain wavelengths of light. These light-sensors tell the plant if it's day or night, how long the day is, how much light is available and from where the light comes. Shoots grow towards light and roots usually grow away from light. These responses are called phototropism and skototropism respectively. They are brought about by light sensitive pigments like phototropins and phytochromes and the plant hormone auxin. Many plants exhibit certain phenomena at specific times of the day, for example certain flowers open only in the mornings. Plants keep track of the time of the day with a molecular clock. This internal clock is set to the solar clock every day using sunlight. The internal clock coupled with the ability to perceive light also allows plants to measure the time of the day and so find the season of the year. This is how many plants know when to flower. (see photoperiodism) The seeds of many plants sprout only after they are exposed to light. This response is carried out by phytochrome signalling. Plants are also able to sense the quality of light and respond appropriately, for example in low light conditions plants produce more photosynthetic pigments whereas when the light is very bright and/or if the levels of harmful UV increase, plants produce more of their protective pigments that act as sunscreens.
Plants do not have a brain or neuronal network, but reactions within signalling pathways may provide a biochemical basis for learning and memory. Controversially, the brain is used as a metaphor in plant intelligence to provide an integrated view of signalling.
Plants are not passive entities merely subject to environmental forces, nor are they 'automata'-like organisms based only on reflexes and optimised solely for accumulation of photosynthate. Plants respond sensitively to environmental stimuli by movement and changes in morphology. They signal and communicate within and among themselves as they actively compete for limited resources, both above and below ground. In addition, plants accurately compute their circumstances, use sophisticated cost–benefit analysis and take tightly controlled actions to mitigate and control diverse environmental stressors. Plants are also capable of discriminating positive and negative experiences and of 'learning' (registering memories) from their past experiences. Plants use this information to update their behaviour in order to survive present and future challenges of their environment. Plants are also capable of refined recognition of self and non-self, and are territorial in behaviour.
Plant calculation and response claims to study the role of signalling, communication and behaviour to integrate data obtained at the genetic, molecular, biochemical and cellular levels, with the physiology, development and behaviour of individual organisms, plant ecosystems and evolution.
The neurobiological view sees plants as information-processing organisms with rather complex processes of communication occurring throughout the individual plant organism. Plant neurobiology researches how environmental information is gathered, processed, integrated and shared (sensory plant biology) to enable these adaptive and coordinated responses (plant behaviour); and how sensory perceptions and behavioural events are 'remembered' in order to allow predictions of future activities upon the basis of past experiences. Plants, it is claimed by some plant physiologists, are as sophisticated in behaviour as animals but this sophistication has been masked by the time scales of plants' response to stimuli, many orders of magnitude slower than animals'.
It has been argued that although plants are capable of adaptation, it should not be called intelligence, as neurobiologists rely primarily on metaphors and analogies to argue that complex responses in plants can only be produced by intelligence."A bacterium can monitor its environment and instigate developmental processes appropriate to the prevailing circumstances, but is that intelligence? Such simple adaptation behaviour might be bacterial intelligence but is clearly not animal intelligence." However, plant intelligence fits with the definition of intelligence proposed by David Stenhouse in a book he wrote about evolution where he described it as "adaptively variable behaviour during the lifetime of the individual".
It is also argued that a plant cannot have goals because operational control of the plant's organs is devolved.
It is hardly an exaggeration to say that the tip of the radicle thus endowed [..] acts like the brain of one of the lower animals; the brain being situated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.
In philosophy, there are few studies of the implications of plant perception. Michael Marder put forth a phenomenology of plant life based on the physiology of plant perception. Paco Calvo Garzon offers a philosophical take on plant perception based on the cognitive sciences and the computational modeling of consciousness.
Comparison to neurobiology
A plant's sensory and response system has been compared to the neurobiological processes of animals. Plant neurobiology, an unfamiliar misnomer, concerns mostly the sensory adaptive behaviour of plants and plant electrophysiology. The Indian scientist J. C. Bose is credited as the first person to research and talk about neurobiology of plants. Many plant scientists and neuroscientists, however, view this as inaccurate, because plants do not have neurons.
The ideas behind plant neurobiology were criticised in a 2007 article published in Trends in Plant Science by Amedeo Alpi and 35 other scientists, including such eminent plant biologists as Gerd Jürgens, Ben Scheres, and Chris Sommerville. The breadth of fields of plant science represented by these researchers reflects the fact that the vast majority of the plant science research community reject plant neurobiology. Their main arguments are that:
- "Plant neurobiology does not add to our understanding of plant physiology, plant cell biology or signaling".
- "There is no evidence for structures such as neurons, synapses or a brain in plants".
- The common occurrence of plasmodesmata in plants which "poses a problem for signaling from an electrophysiological point of view" since extensive electrical coupling would preclude the need for any cell-to-cell transport of a ‘neurotransmitter-like’ compounds.
The authors call for an end to "superficial analogies and questionable extrapolations" if the concept of "plant neurobiology" is to benefit the research community.
There were several responses to the criticism clarifying that the term "plant neurobiology" is a metaphor and metaphors have proved useful on several previous occasions. Plant ecophysiology describes this phenomenon.
- Auxin - A plant hormone which mediates responses
- Chemotropism - Plant response to chemicals
- Cryptochrome - A light receptor pigment
- Ethylene - A plant hormone which mediates responses
- Gravitropism - Behavior associated with gravitic perception
- Heliotropism - Behavior associated with sunlight perception
- Hormonal sentience - Plant information processing theory
- Hydrotropism - Plant response to moisture
- Hypersensitive response - Local reaction produced in response to infection by microbes
- Kinesis (biology) - Movement
- Nastic movements - A type of rapid response to non-directional stimulus
- Osmosis - A means of water transportation on the cellular level
- Phototropin - A light receptor pigment
- Phototropism - A behavior associated with light perception
- Phytochrome - A light receptor pigment
- Phytosemiotics - Analysis of vegetative processes on the basis of semiotic theory
- Plant defense against herbivory - Some plant responses to physical disruption
- Plant evolutionary developmental biology
- Plant hormone - A mediator of response to stimuli
- Plant perception (paranormal)
- Plant physiology - The science of plant function
- Plant tolerance to herbivory
- Rapid plant movement - Description of rapid plant movements
- The Secret Life of Plants
- Sensory receptors - Discussion of organs of perception in organisms
- Statolith - An organ of gravity perception
- Stoma - A plant pore which responds to stimulus and which regulates gas exchange
- Systemic acquired resistance - A "whole-plant" resistance response to microbial pathogens that occurs following an earlier, localized response
- Taxis - A type of response to a directional stimulus seen in motile developmental stages of lower plants
- Thermotropism - Plant response to heat
- Thigmotropism - Plant response to touch
- Tropism - A type of response to a directional stimulus
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- Plant Neurobiology Society
- Plant Signaling and Behavior Scientific journal for Plant Neurobiology
- Study hints plants have sensibilities
- Plants cannot "think and remember," but there's nothing stupid about them: They're shockingly sophisticated
- How Does a Venus Flytrap Work?