Botany – Plant Movement

David Beeson, July 2021

Many people feel that plants ‘do not do anything’. Clearly, that is far from the truth, they often just work at a different speed. The clearest example could be a giant US redwood – they generate more height, more bulk and more potential offspring then me. But it takes them a thousand years. At a cellular level their metabolism is complicated and incorporates many of the biochemical pathways we employ – respiration, lipid metabolism and production of DNA. They also generate a wonderful range of family-specific toxins.

Plant systems are quite different from those evolution donated to higher animals. Their water transportation relies, mainly, on leaf evaporation drawing up soil water. Organic materials are transported in the phloem using an energy-linked pump that has some comparison with a heart.

But they go nowhere.

Well, seeds spread widely and can potentially move  from one continent to others. Wind dispersal of seeds is cheap and effective.

Top 5 Seed Dispersal Wonders - Soil Advocates Inc.
Wind dispersal
Seed dispersal — Science Learning Hub
Animal dispersal

Oh, come on! They are not intelligent. Humans can do things.

Ah, a good point. Yet have you seen how wheat, rice, sitka spruce and rubber plants have moved around the globe? How, by getting humans do it for them! Intelligent I think. Why do something yourself when others will do it for you?

That’s cheating. I move and can even run (when forced).

I’m glad you said that, because movement is what this article is all about.

Football to the Face 1000x Slower - The Slow Mo Guys - YouTube
… and then the reason hit me!

Organisms need to sense what happens around them. I bought some new glasses the other day, and their optical properties seemed a tad strange initially. I went into the sports centre and they were playing volleyball. I watched for a while, looked away, then glanced back. My glasses were playing tricks, it looked as though the ball was getting bigger. Then the reason hit me. Ouch, that hurts. (Joke? Well, I thought it was okay.)

Plants need to know where to send their roots and shoots on germination. Roots towards gravity (and water and nutrient resources) and shoots towards their energy supplies – light.

Plants can potentially respond to light direction, day length, gravity, touch, temperature and water. Air carried chemicals may cause them to manufacture chemicals to combat potential insect infestations. Carnivorous plants, such as sundews, will curl their leaves in a response to movement.

Sundews | National Wildlife Federation
Long-leaved sundew

Plants recognise their environment and can potentially respond to those stimuli. For example, beet. If a beet plant is protected from cold temperatures it remains in a juvenile state and will never flower. Cold is the stimulus that induces it to respond to a later flowering stimulus. It has become vernalised.

Lettuce and poppies need light before they will germinate.

However, many responses involve growth – TROPIC RESPONSES or tropisms. Others, work by changes in cell turgor (water pressure) – NASTIC RESPONSES.

TROPIC RESPONSES are growth related. Positive when growth is towards the stimulus, negative when away. So, growing towards light is a positive phototropism (phototropic response). Of course, some responses are in between, eg an oak’s branches growing parallel to the soil level.

A tropism to gravity = gravitropism; touch = thigmotropism; chemicals = chemotropism.

How Plants Climb - Climbing Plants & Vines | Gardener's Supply
Tendrils twist in response to touch. This enables the plant to grow upwards without the huge investment in lignifying their cellulose cells in the stem.

Roots grow (mostly) down but will be influenced by water and chemical resources needed. Shoots grow up, but influenced by light direction. Runner beans shoots grow up to light and twine when they touch a support. Germinating pollen on a stigma has a positive chemotropic response and grows the pollen tube towards the ovary.

How do bimetallic strips work? - EDN
Bimetallic strip being heated.

If cells on one side of a shoot or root grow longer than the other, or have more cells, the structure will bend. (Do you remember the bimetallic strips your physics teacher had at school? The two metals rivetted together expanded at different rates, so placed in a Bunsen flame it bent. Same idea.) How can it be achieved? Increased water pressure inside a cell or an unequally thickened cellulose cell wall, would do the job. (Think a balloon with tape on one side. What happens if it is blown up with more air? It bends. Think about stomata opening and closing!)

Now you are thinking, how is this controlled? It cannot be an animal-like nervous response, so it must be  … hormones. And that is where we could come to: AUXINS, CYTOKININS, GIBERELLINS, ABSCISIC ACID, ETHYLENE – the main plant hormones currently partly understood. A hormone being an organic compound produced in one location but having its effect elsewhere.

How these plant movement responses are triggered, and how precisely they operate is still a partial mystery.

Gravity is reasonably understood. There are dense starch grains (statocysts) in root cap cells, and these slowly drift downwards because of gravity. Somehow, do not ask details here, that triggers the bending reaction. It needs time for this stimulation to take effect. The hormones presumably join a receptor site on the target cell’s membrane and are either imported or trigger a change in the cell’s metabolism through an intermediary. At that stage a possible routeway is for that chemical to switch on a gene, with it causing the final response.

Gravireceptors in eukaryotes—a comparison of case studies on the cellular  level | npj Microgravity
A = statocyst of starch in a growing root tip. ER is the endoplasmic reticulum. In the upper insert the ‘line’ going across (blue in lower diagram) is the cellulose cell wall.

So, an environmental factor affects the plant’s sensor, produces an organic chemical (possibly a hormone), which moves to its DNA target and it then influences that call’s metabolism and the response occurs. Isn’t biology wonderful, and plants do move and do things.

Plant Hormones: Describing the Big 5
Plant hormones – go to the homepage and scroll down for 100+ articles on botany, wildlife, eco-gardening and how to mend a dripping tap. Well, perhaps I’m lying with one of them.

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