Through the
scientific study of plant behaviour, we continue to discover new ways in which
plants interact with their environment in animal-like ways. Words like “listening”,
“foraging”, and “parenting” may seem odd to associate with plants, and yet
plants show evidence of all of these behaviours. Here are some of the many ways
in which plants behave.
Listening
As it turns
out, plants are listening! A study by Heidi Appel and Rex Cocroft published in
2014 describes their discovery that plants can detect their predators
acoustically and ramp up their defenses as a response. Appel and Cocroft
recorded the sound vibrations of caterpillars chewing on Mouse‑ear Cress
leaves, and played these recorded vibrations to previously unaffected Mouse-ear
Cress plants over several hours before allowing caterpillars to attack the
plants. The researchers found that, compared to plants that were primed with
recordings of silence, those who were primed with recordings of chewing
produced much higher levels of the oils glucosinolate and anthocyanin, which
are toxic to caterpillars. Furthermore, Appel and Cocroft found that the plants
were able to distinguish the sound vibrations caused by chewing from recordings
of wind or other insect noises.
In addition
to listening for their predators, plants can also listen for water. In 2017, Monica
Gagliano and her colleagues published a paper showing that the Garden Pea was
able to locate water by sensing the vibrations generated by water moving inside
pipes. Garden Pea seedlings were planted in pots shaped like an upside-down Y,
and each arm of the pot was treated with different experimental conditions.
When one arm was placed in a tray of water and the other was dry, plant roots
grew towards the arm with water. Seems obvious enough. However, when one arm
was placed above a tube with flowing water and the other was dry, the plant
roots still grew towards the water, even though there was no moisture. When
given a choice between a tray of water and the tube with flowing water, the
Garden Pea seedlings chose the tray of water. This led Gagliano to hypothesize
that plants may use sound vibrations to detect water at a distance, but that
moisture gradients allow the plants to reach their target at close proximity.
Garden Pea
water acoustics experimental set-up (Gagliano et al., 2017)
Foraging
Plant roots
forage for food in a similar way to animals. In his 2011 review, James Cahill
explores plant root responses to varying nutrient cues in the soil. Cahill
explains that plant roots are responsive to both spatial and temporal nutrient
availability. For example, when a nutrient patch is placed in the soil at a
distance from the plant, there is a substantial acceleration in root growth in
the following days. This growth is directed precisely towards the nutrient
patch, and as the root approaches its target, the rate of growth slows as the
nutrient patch is consumed. Furthermore, plants develop greater root biomass in
richer nutrient patches, and they allocate more root biomass to patches with
increasing nutrient levels.
Other
foraging plants include the parasitic Dodder vine. This plant has no roots and
lives off a host plant. In 2006, Consuelo De Moraes and her team published a
study demonstrating that the Dodder plant uses scent, or volatile chemical
cues, to locate and select its host plant. De Moraes experimentally planted
Dodder seedlings between a Tomato and Common Wheat plant, the Tomato being its
preferred host. Using a time lapse camera, De Moraes captured the circling
movement of the Dodder plant as it approached both host options repeatedly,
before settling on the Tomato plant 90% of the time. Through further
experimentation by giving the Dodder seedlings a choice between the condensed
chemical odour of the Tomato plant and a live Tomato that has been covered to
prevent giving off odour, it was determined that the Dodder uses the chemical
signals to select its host. Without being able to “smell” the live Tomato
plant, the Dodder chose to attach to the vile containing the condensed chemical
odour.
Dodder
vine attaching to a Tomato plant
(PBS, 2014: https://www-tc.pbs.org/wnet/nature/files/2014/09/Mezzanine_485.jpg)
Parenting
One of the
most fascinating aspects of plant behaviour is parental care and kin
recognition. Suzanne Simard’s work studying forests as a complex,
interconnected organism has been featured on several popular media outlets
including the TED
Talk series and the Radiolab podcast. Through
her research, Simard discovered that through a network of mycorrhizal fungi,
adult trees were nurturing their young with a targeted exchange of nutrients
such as carbon and nitrogen, as well as defense signals and hormones. Through
experimental plantings of Douglas Fir seedlings that were directly related to
the adult trees and unrelated Douglas Fir seedlings, she found that the “mother
trees” recognized and colonized their kin with larger networks of mycorrhizal
fungi, and sent more carbon to these seedlings. Furthermore, there was a
reduction in root competition with the related seedlings. When injured, the
adult trees sent large amounts of carbon and defense signals to their young,
which increased the seedlings’ stress resistance.
Tree
mycorrhizal network schematic
(Medium, 2017: https://medium.com/ideo-colab/fungal-networks-connected-businesses-b38025ca7171)
Similar
recognition of kin was observed by Susan Dudley and Amanda File in a 2007 paper.
In this study, Dudley and File planted related “sibling” Sea Rocket plants
together in pots, as well as unrelated “stranger” Sea Rocket plants together in
pots. After several weeks, the roots were cleaned and assessed. The study found
that kin groups allocated less biomass to their fine roots, while stranger
groups grew larger roots in order to compete for resources. The same responses
were not observed when kin and stranger groups were grown in isolated pots,
which suggests that the mechanism for kin recognition was through root
interactions.
Although we
still don’t fully understand the mechanism by which plants process information,
it is clear that the way plants interact with their environment is far more
complicated than we previously thought. The concept of plants as inanimate
organisms, blindly competing for resources is now outdated. Continued discoveries
in plant behaviour demonstrate, once again, how little we understand about the
natural environment—a humbling thought in an age when humankind thinks itself superior
to our fellow species.
Bibliography
Appel, H.M.
& Cocroft, R.B. (2014). Plants respond to leaf vibrations caused by insect
herbivore chewing. Oecologia, (2014)175:
1257–1266.
Cahill, J.F.
& McNickle, G.G. (2011). The behavioral ecology of nutrient foraging by
plants. Annual Review of Ecology,
Evolution, and Systematics, 2011(42): 289–311.
Dudley, S.A.
& File, A.L. (2007). Kin recognition in an annual plant. Biology Letters (2007)3: 435–438.
Gagliano, M.,
Grimonprez, M., Depczynski, M. & Renton, M. (2017). Tuned in: plant roots
use sound to locate water. Oecologia
(2017)184: 151–160.
Runyon, J.B.,
Mescher, M.C. & De Moraes, C. (2006). Volatile chemical cues guide host
location and host selection by parasitic plants. Science, 313(5795): 1964–1967.
Simard, S.
(2016). Suzanne Simard: How trees talk to each other [Video file]. Retrieved
from
https://www.ted.com/talks/suzanne_simard_how_trees_talk_to_each_other#t-18444
