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Inside information

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Dr Dario Stefanelli, plant physiologist from Agriculture Victoria, continues to discuss the behaviours of plants and how plants protect themselves. Plant physiology is the study of the internal workings of all the different living plants. This encompasses survival, growth and the propagation of the species. Plants are anchored in soil so their relative speed in terms of movement is slow compared to other organisms. Plants have small receptors that can identify and measure the intensity of sunlight so plants can redirect themselves when that changes which is why some plants move their leaves to chase the sun. Sometimes plants do have very quick reactions, the carnivorous Venus flytrap being a good example.
Think of the outer section of the plant as jaws. The plant keeps its jaws open by siphoning out the content of the cells that control these jaws. The jaws collapse very quickly when prey enters and stay shut until the live organism is digested using proteins and acids. When the plant wants to open again it pumps liquid and chemicals to expand the control cells in the jaw, essentially reopening it, ready for the next prey.
A Venus fly trap can perceive when it is touched. A change in weight and air movement is sensed through small hairs and the plant also differentiates between smells, especially of different live organisms. They only trap live organisms, not things like fallen leaves.

Venus flytraps use “teeth,” triggers, and toxic juices to catch prey. Illustration by Jenny Wang, National Geographic https://blog.education.nationalgeographic.com/2016/01/25/for-venus-flytraps-catching-prey-is-as-easy-as-1-2-3/     



The plant’s “trap” is a single, foldable leaf with trigger hairs.

When a fly or ant brushes against one of the leaf’s trigger hairs two times, the plant folds its leaf quickly, trapping the prey inside.

Then, the Venus flytrap secretes a digestive fluid that helps the plant absorb nutrients from the trapped insect. It takes three to five days for the plant to digest the insect.

Each leaf-trap can open and close about three times before dying and falling off the plant.

The old trap is replaced by a new one from the Venus flytrap’s underground stems.

Plants have behaviours that come into effect when they are attacked, similar to a sound emitted by an animal to warn others there is a predator around. Since plants can’t make sound they emit odours that are perceived by other plants which can then start to protect themselves.
The major substance emitted, methyl jasmonates, is emitted by 90% of plants but there are also molecules attached to that volatile that is specific to different insects. There is also a partial ability of other species of plants to perceive those odours.
Plants can also change their leaf metabolism as a form of protection. As soon as a plant feels the entrance of a stylus or the mandibles of a chewing insect the plant reacts by killing all the cells around the point of entry. This reaction also happens when cells are penetrated by fungi. Humans have learnt to stimulate the plants by spraying them with volatiles (methyl jasmonates and its derivatives) to protect them, similar to boosting immune systems. In agriculture this does not kill the insects so synthetic pesticides are still needed. Organic agriculture would spray volatiles but using only natural pesticides. Plant physiology really does consist of a lot of useful inside information.

Dr Dario Stefanelli was interviewed for A Question of Balance by Ruby Vincent. Summary text by Victor Barry. November 2016.

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