Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Saturday: 14 June 2008

Eppur si muove  -  @ 06:48:52
Yesterday I showed the thigmonastic response (to touch) of leaf folding in sensitive-briar.

It turns out that the same mechanism lies back of a lot of other plant movement behaviors. Seismonastic responses (to vibration) are a subset of thigmonasty, seen in the active traps of Venus flytrap plants. And nyctinastic behavior (night, or sleep movements) are also controlled by a similar mechanism.

That mechanism, ultimately, is the movement of potassium ions (K+) into and out of cells, with water following by osmosis. The result is that cells become more turgid, larger, if K+ and water move into them, and flaccid, collapsed, smaller, if K+ and water move out of them. Many of the movements of plants occur when two layers of tissue alternate simultaneously in this way, and the location of those tissues are in swollen structures called pulvini (singular = pulvinus).

In sensitive-briar, not only do the leaflets collapse, but the entire leaf branch will fall, if the stimulus is strong enough (it wasn't in the above photo). This is controlled by a swelling similar to the pulvinus, called the major pulvis. But it's the same stuff going on.

We really get a better handle on the plant behavior if we talk a bit about the analogous animal behavior, which is much more sophistocated and rich in possibilities. But there are similarities - it's just that plants don't have nervous or muscle systems and have to accomplish the same thing through different means.

As in animals, plants have to go through several stages to effect movement. They have to detect a stimulus - in animals this is done through specialized sensory neurons. In plants, it looks like certain cells have stretchy membranes that undergo membrane potential changes when mechanically stressed. In both cases action potentials are generated, and this starts everything off.

If you took an organismal biology course, or an anatomy or physiology course, then you might remember that in animals an action potential is a sudden rise in positive ions inside a neuron. It involves both potassium and sodium ions, but it's sodium (Na+) that is the major component of the action potential. In plants it's K+.

It's always tempting to view the action potential as a bolt of electricity pulsing down an axon, that lengthy bit of cell that connects one neuron to the next, but it's really a reflection of a sequential opening and closing of protein gates that specifically allow ion movements into and out of cells. This is an active and sophistocated process in animals. In plants, the movement of K+ is probably completely by diffusion, and so it's slower and passive (but it gets the job done).

Once the signal has been generated and propagated, it then has to effect a response. In animals, the signal will proceed to the central nervous system, the spinal cord or brain, and a return signal will be sent to the appropriate muscle cell to cause it to contract.

In plants, the diffusion of K+ causes a limited number of cells (those in the pulvini) to respond by either taking water up or expelling it. The process is complicated and may include plant hormones called turgorins, which in turn cause gene expression changes, and I'm not going to go into all that. I'm going to limit myself here now to a very simple diagram that shows how a leaflet can fold up.

The top panel shows the situation pre-stimulus. The extensor cells on top of the pulvinus (or pulvis) are flaccid and small. They contain lots of sugar, and this becomes important in the next step. The flexor cells on the bottom are full of K+ and water, and are large and turguid. They're holding the petiole up against gravity.

In the middle panel, the stimulus has been received, and the flexor cells are dumping their K+ loads out into the cell walls. Water follows by osmosis. This causes the flexor cells to become flaccid and small.

The water moves into the extensor cells by osmosis, drawn by the high concentrations of sugar. The extensor cells become large and turgid.

The result is that the petiole is depressed actively downward by the extensor cells, and this is helped along by the sudden collapse of the flaccid cells. The petiole drops and the leaflet appears to have folded up.

Meanwhile the K+ is diffusing outward along the stem and petiole branch. This is the plant's action potential, and when it encounters the next pulvinus along the way it will stimulate that to release *its* K+, drop its leaflet, and so on down the line. How fast is this? K+ diffuses along the branch at about 5 cm per second so for one of these leaves, it takes about 2 seconds for complete foldup. This is slow compared to an animal - imagine wanting to open a door and having your hand respond 15 seconds later - but it's a frenetic pace for a plant.

You can actually see this progressive action if you delicately touch the leaf at the base of the petiole - the leaflets will gradually drop one by one from the base to the tip of the leaf.

These are movements easily seen, but viewed really up close a plant is always in motion. The guard cells that define the stomata, those breathing holes in the leaf, are constantly collapsing and expanding, thereby closing and opening the holes upon need. The stomata are open when there's plenty of water available, and close when the plant is water stressed, to prevent loss. The K+ mechanism controls this too.

One last thing - in most of these movements, only setting the thing up requires energy. The idea is this: for plants, it would be foolish in an emergency situation to expect that energy would be available to respond to it. So plants make sure that the response is set up to happen spontaneously, and that setup requires energy.

It takes chemical energy to pump those bottom cells full of K+ and water, and similarly for pumping up the guard cells to open up the stomata. It takes energy to make the sugar that those top pulvinar cells carry.

The actual movement, though, the closing of the stomata or the folding of the leaf, is spontaneous and rapid once it's triggered. This is a very clever way of making sure the plant can respond to an emergency, whether it be a nibbling herbivore or a deficit of water, without having to expend energy to make that response happen.

Now most of this is my interpretation of a number of readings, but I think it's generally accurate. There are those who know *much* more than I do about all this, and I'm certainly willing to modify the post for corrections! Some of the information came from Wikipedia's entry on the pulvinus, a pdf on plant motor cell function and action potentials, and an article on plant cell action potentials, among several others.

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