Plants and their Responses to Environmental Cues
The sessile nature of plants has made them rely greatly upon
their surrounding environment. Over the course of their evolution,
plants have adapted internal mechanisms that take the fullest
advantage of whatever conditions they happen to be stuck in. Probably
the greatest adaption that a plant has is its ability to respond (in
a relatively small amount of time) to immediate and long term changes
in its habitat. This is no small feat, considering that plants don't
have a brain, nervous system, or a bunch of muscles like we do.
Instead they have to rely on other mechanisms to promote mechanical
or motor responses.
Tropisms
One way a plant can respond to its environment is by tropisms. A tropism is a growth response that results from a certain stimulus. It can either be away from (negative tropism) or towards (positive tropism) the stimulus. The different types of tropisms are classified by the stimulus that spurs their growth. That is, the tropism's stimulus can be found in its prefix. Tropisms are caused by differential growth. Differential growth is the term used when one side of something grows faster than the other, thus causing it to bend into the direction of the slower moving side.
PHOTOTROPISM:
DEFINITION: Looking at this word's prefix, we can figure out that phototropisms are tropisms induced by light. Light shining on one side of a plant (i.e. unidirectional light) causes the plant's unlighted side to grow faster than the lighted side, and thus the plant grows in towards the direction of the light.
HISTORY: The reason why light induces a plant's differential growth has taken many years and many scientists to figure out. First up, Fritz Went noticed that there are greater amounts of IAA (see V.J.'s web site) in the shaded side of a coleoptile as opposed to the side which receives unidirectional light. There are two hypothesis that explain this phenomena. The first one says that light destroys the IAA in the lighted side of the plant. This is a reasonable hypothesis, but it was disproved when Winslow Briggs showed that light does not destroy IAA. He did this by showing that the IAA levels of plants grown in the light are the same as those grown in the dark. The second hypothesis proves to be a more adequate one. It says that IAA moves from the lighted side to the shaded side of a coleoptile. Briggs proved this by showing that differential growth ceases when an impenetrable barrier is placed between the lighted and shaded sides of a coleoptile. Carbon 14 experiments in recent years have shown that IAA does in fact move towards the shaded side of a coleoptile in response to unidirectional light.
Scientists have also shown that the most effective wavelengths of light that induce phototropism are blue ones with wavelengths less than 500nm.
GRAVITROPISM:
DEFINITION: Once again, you can figure out this tropism's definition by looking at its prefix. A gravitropism is a growth response in response to gravity. Roots have positive gravitropism, that is, they grow towards the direction that gravity is moving (i.e. down). Therefore, it doesn't matter if you stick seeds in the ground "the wrong way." Gravitropism will make sure the roots grow down. Stems on the other hand have negative gravitropism. If you put it on its side, the stem will curl so it is moving against gravity (i.e. up).
HISTORY: Charles Darwin and son (Francis) discovered way back when that gravitropisms cease in roots when their root caps are lopped off. They still grow, mind you, but they don't "pay attention" to the environmental cue of gravity. Therefore, one can safely assume that the root cap is an integral part of a plant's gravitropism. Later scientists discovered that there are starch-containing amyoplasts inside the root cap which drop to the lowest side of cells in response to gravity. (Think of a handful of mud in a coffee can. It's going to drop to whichever side is the bottom whenever you turn the can.) After fifty years of tinkering with this enticing discovery no hard evidence came out of it to accurately explain the true mechanisms behind gravitropism. In fact, Timothy Casper proved that plants bioengineered to have no amyloplasts (or starch) still undergo gravitropism. Therefore, starch containing amyloplasts probably aren't as important in gravitropism as one would like to think. A more recent hypothesis therefore states that plants respond to gravity by sensing its effects by protoplasts, not amyloplasts.
RESPONSE TO GRAVITROPISM: Imagine horizontally oriented roots in the soil. We would expect differential growth to make the roots curve downward, but how does this happen? First of all Ca+ moves to and accumulates in the lower side of the root cap, which triggers an accumulation of IAA in this same area. IAA is known to inhibit cell growth in the roots, and thus, the bottom side grows slower than the upper side, causing the root to grow downward. When the root is growing parallel to gravity, there is no more "upper" and "lower" side, and therefore Ca+ and IAA are not accumulated in the "lower" side, and therefore the roots grow straight. Remember that IAA facilitates cellular elongation in stems, and therefore the lower side grows faster than the upper side, thus causing the negative gravitropism.
HYDROTROPISM: Roots also display hydrotropism, which is growth in response to water gradients in the soil. Other factors besides moisture gradients which cause hydrotropism are gravity and light. Finally, decapitated roots don't seem to show hydrotropism, which hints at the root caps playing a major role in this phenomena.
THIGMOTROPISM: For those of you that can't figure out what "thigmo" means, don't worry about it. The last of the tropisms that I'll be talking about are thigmotropisms, or responses to touch. Remember in the stem unit how we discussed searcher shoots and tendrils? They usually move through the air in a spiraling pattern (circumnutation) to increase their chances of touching something. When they do touch something, contact is perceived by certain epidermal cells that then initiate differential growth.The differential growth is thought to be controlled by ethylene and IAA. Light is also required for a thigmotropism, because ATP is needed for the process to happen. An interesting fact is that tendrils can remember the sensory "touch" information that they received in the dark and then start responding to it as soon as they receive light.
SEISMONASTY: Seismonasty is the term used to describe the nastic action of a plant in response to contact with one of its leaves or shaking the entire plant (like in that video we watched). As mentioned earlier, plants don't have muscles or nerves or specialized brain parts to help respond to stimuli, so how does Seismonastic activity occur? The basic idea has to do with changes in turgor pressures (which are reversible):
1. Contact with a leaf causes an electrical signal to be sent up the petiole.
2. The electrical signal turns into a chemical signal that makes certain cell membranes become more permeable to K+ ions (among others). These affected cells are known as motor cells, and are located at the base of each leaflet in a structure collectively known as the pulvinis.
3. The increased permeability of the Motor cell's membranes causes ions to move out which in turn causes water to leave the cells via osmosis.
4. This plasmolysis causes the cells to shrink, which makes the leaflets droop.
The most popular kind of a Seismonastic plant is the Venus fly trap. It has a different mechanism for seismonasty, which relies on an insect stepping on hairs which trigger the release of H+ that in turn acidifies the outer part of the leaf so much that it expands rapidly and snaps shut. Another hypothesis says that the VFT's leaves close due to a release of tension.
NYCTINASTY: Nyctinasty is a "sleep movement " or a nastic response to the changing from night to day and vice versa. Like seismonasty, nyctinasty is caused by a change in turgor pressure in the motor cells of the pulvini. The difference lies in the stimulus, but the action is the same, that is, a preprogrammed mechanical motor response. An example is the prayer plant, whose leaves nyctinasticly shoot up vertically, or "pray" during the night.
The fact that certain plants only bloom in response to certain photoperiods helps us classify them as day neutral, long,short or intermediate day plants.
1.Day Neutral Plants-- bloom independent of photoperiod cues, that is, daylength has no affect on them.
2.Short-Day plants-- need a photoperiod shorter than some critical length.
3.Long-Day plants-- need a photoperiod longer than some critical length(approx.9-16 hrs.)
4.Intermediate-Day plants-- need a photoperiod that can't be too long or too short, rather it must be of an intermediate length.
It must be noted however, that a more appropriate name for these plants would be short night (as opposed to long day) or long night (instead of short day) plants, since plans only flower if the dark period is of a certain length (they could care less about the light period). This data was found by Hamner and Bonner, who found another interesting fact. Long night plants (i.e. short day) require an uninterrupted dark period past a critical amount. If this dark period is interrupted by just one minute of light, the plant won't flower.
PHYTOCHROME: The pigment which controls photoperiodism is phytochrome. Phytochrome occurs in two forms, one that absorbs red light (Pr) and one that absorbs far-red light (Pfr). To stop a long night plant from flowering, it is most effective to interrupt it's dark period with a red light. This process however, can be reversed by immediately bombarding it with far red light. The biologically active form of phytochrome is Pfr, and is synthesized when Pr is hit with red light.