Are Plants Comfortably Numb? – Part Two

numb (adjective): “devoid of sensation; insensible”

Pain in the Ash?

In the previous post, I posed the question whether or not plants experienced something analogous to pain when physically wounded.

I concluded that they did….depending on how one defines “pain”.

At the present time, when even academic biologists are expanding the meanings of “know”, “feel”, “smell”, “think”, “talk”, “hear”, “see”, “remember”, etc., to include plants (e.g., see Refs. 1-5 below), I think it’s fair to also broaden the concept of “pain” to include plants.

But before I proffer my definition of “plant pain”, let’s take a look at the classic definitions, specifically as it’s related to physical injury.

What Is Pain?

According to the online dictionary.com, the two leading definitions are 1. “physical suffering or distress, as due to injury, illness, etc.” and 2. “a distressing sensation in a particular part of the body“.

A medical definition, provided by International Association for the Study of Pain: “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

More interesting definitions, however, and perhaps more germane to the discussion at hand, have to do with the biological functions of pain.

Here are a few examples found on the internet that I think most people would agree with:

  • “Pain is a universal experience that serves the vital function of triggering avoidance.
  • Pain motivates the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future.
  • Pain is a vital function of the nervous system in providing the body with a warning of potential or actual injury.

    And the definition of “pain” gets even more interesting when one delves into biological mechanisms of pain.

    Although it’s difficult to succinctly describe the complex neurobiology of pain, the beginning of pain is the perception of injury, which starts with nociceptors.

    “Nociceptors are the specialised sensory receptors responsible for the detection of noxious (unpleasant) stimuli, transforming the stimuli into electrical signals, which are then conducted to the central nervous system.” (from Ref. 5 below)

    Also, as Dr. Chamovitz points out, “…“touch and pain are biologically not the same phenomena. Pain does not simply result from an increase in the signals emanating from touch receptors. Our skin features distinct receptor neurons for different types of touch, but it also has unique receptor neurons for different types of pain. Pain receptors (called nociceptors) require a much stronger stimulus before they send action potentials to the brain.” (From Ref. 2 below)

    As I mentioned in the previous post, how plants perceive and respond to mechanical stimulation (e.g., wind, touch, etc.) is also different and distinct from how plants perceive and respond to physical wounding (i.e, cellular damage).

    There are several well-known “wound signals” produced by physical damage to plant tissue that can travel from the wound site all throughout the plant, which are called “systemic” wound signals. And some of these figure prominently in what I’ll refer to as “plant pain”.

    Plant “Pain” = Waves of Systemic Wound Signals?

    I should first mention that this and the previous post were prompted, in part, by a couple of reviews by Dr. Simon Gilroy and colleagues regarding rapid systemic signaling in plants (see Refs. 7 & 8 below).

    Briefly, when plants are exposed to abiotic (physical) stress, most of the scientific evidence supports the idea that waves of hydraulic, chemical, and/or electrical long-distance signals may propagate – within minutes – throughout the plant to initiate systemic stress responses.

    Therefore, I think it’s not too much of stretch to say that there is a plant analog to what we commonly call “pain” and that this analog is this wave of systemic wound signals.

    Though this plant “pain”, and the plant’s responses to it, occur much more slowly compared to animals, we should remember that plants function within a different time framework than animals, what I call “plant time”.

    Bottom Line: Plants are not “comfortably numb” when it comes to physical wounding, because waves of systemic wound signals – what I call “plant pain” – alert the plant that it has been physically damaged (or what others have referred to as “damaged-self recognition”).

    References

    1. Pollan, M. (2013) “The intelligent plant.” The New Yorker, December 23 & 30, 2013 issue. (Full Text)

    2. Chamovitz, D. (2012) What A Plant Knows.

    3. Ananthaswamy, A. (2014) “Smarty Plants: They think. They react. They remember. It’s time we rethought intelligence.” New Scientist, 6 December 2014 issue, pp. 34-37. (Full Text PDF)

    4. Leopold, A. C. (2014) “Smart plants: Memory and communication without brains.” Plant Signaling & Behavior, Vol. 9, e972268. DOI: dx.doi.org/10.4161/15592316.2014.972268 (Full Text)

    5. Popova, M. “The Secret Life of Trees: The Astonishing Science of What Trees Feel and How They Communicate.” Brain Pickings, 9/26/2016. (Full Text)

    6. Reddi, D. “An introduction to pain pathways and mechanisms.” (Full Text PDF)

    7. Gilroy, S., et al. (2016) “ROS, Calcium, and Electric Signals: Key Mediators of Rapid Systemic Signaling in Plants.” Plant Physiology, Vol. 171, pp. 1606-1615. (Full Text)

    8. Choi, W.-G, R. Hilleary, S. J. Swanson, S.-H. Kim, and S. Gilroy (2016) “Rapid, Long-Distance Electrical and Calcium Signaling in Plants.” Annual Review of Plant Biology, Vol. 67, pp. 287-307. (Abstract)

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    David Gilmour & David Bowie – Comfortably Numb

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