A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant.
- Positive = the plant, or a part of it, grows in the direction from which the stimulus originates.
- Negative = growth away from the stimulus.
Plants respond to:
- Light = phototropism
- Stems are positively phototropic.
- Roots are negatively phototropic.
- Gravity = gravitropism
- Stems are negatively gravitropic while
- roots are positively gravitropic.
The adaptive value of these tropisms is clear.
- Roots growing down and/or away from light are more likely to find the soil, water, and minerals they need.
- Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur.
The Mechanism of Phototropism
The Darwin Experiments
Charles Darwin and his son Francis discovered (in 1880) that the phototropic stimulus is detected at the tip of the plant.
Fig. 220.127.116.11 Coleoptile
The Darwins used grass seedlings for some of their experiments. When grass seeds germinate, the primary leaf pierces the seed coverings and the soil while protected by the coleoptile, a hollow, cylindrical sheath that surrounds it. Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it.
The Darwins found that the tip of the coleoptile was necessary for phototropism but that the bending takes place in the region below the tip.
Fig. 18.104.22.168 Darwins experiment
If they placed an opaque cover over the tip, phototropism failed to occur even though the rest of the coleoptile was illuminated from one side. However, when they buried the plant in fine black sand so that only its tip was exposed, there was no interference with the tropism — the buried coleoptile bent in the direction of the light.
From these experiments, it seemed clear that
- the stimulus (light) was detected at one location (the tip)
- the response (bending) was carried out at another (the region of elongation)
This implied that the tip was, in some way, communicating with the cells in the region of elongation.
Fig. 22.214.171.124 Boysen - Jensen experiment
The Danish plant physiologist Boysen-Jensen showed (in 1913) that the signal was a chemical passing down from the tip of the coleoptile.
- cut off the tip of the coleoptile
- covered the stump with a layer of gelatin
- replaced the tip.
Phototropism took place normally. However, when he used a flake of impervious mica between the tip and the stump, phototropism was prevented. Furthermore, this interference occurred only when the sheet of mica was inserted on the shady side of the preparation. When a horizontal incision was made on the illuminated side and the mica inserted in it, phototropism was normal.
This suggested that the chemical signal was a growth stimulant as the phototropic response involves faster cell elongation on the shady side than on the illuminated side.
The Discovery of Auxin
Fig. 126.96.36.199 Went experiment
F. W. Went extracted the growth stimulant.
He removed the tips of several coleoptiles of oat, Avena sativa, seedlings. He placed these on a block of agar for several hours. At the end of this time, the agar block itself was able to initiate resumption of growth of the decapitated coleoptile. The growth was vertical because the agar block was placed completely across the stump of the coleoptile and no light reached the plant from the side.
The unknown substance that had diffused from the agar block was named auxin. The amount of auxin in coleoptile tips was far too small to be purified and analyzed chemically. Therefore, a search was made for other sources of auxin activity.
The Avena Test - a Bioassay
Fig. 188.8.131.52 Avena test
This search was aided by a technique developed by Went for determining the relative amount of auxin activity in a preparation. The material to be assayed is incorporated into an agar block, and the block is placed on one edge of a decapitated Avena coleoptile. As the auxin diffuses into that side of the coleoptile, it stimulates cell elongation and the coleoptile bends away from the block.
The degree of curvature, measured after 1.5 hours in the dark, is proportional to the amount of auxin activity (e.g., number of coleoptile tips used).
Fig. 184.108.40.206 Photographic record of an Avena test courtesy of Dr. Kenneth V. Thimann
The use of living tissue to determine the amount of a substance is called a bioassay.
The Avena test soon revealed that substances with auxin activity occur widely in nature. One of the most potent was first isolated from human urine. It was indole-3-acetic acid (IAA) and turned out to be the auxin actually used by plants.
Fig. 220.127.116.11 IAA
Went also discovered that it is the unequal distribution of auxin that causes the bending in phototropism
Fig. 18.104.22.168 Phototips
When a coleoptile tip that has previously been illuminated from one side is placed on two separated agar blocks, the block on the side that had been shaded accumulates almost twice as much auxin as the block on the previously lighted side. Hence the more rapid cell elongation on the shady side of the plant.
Gravitropism of Shoots
Gravitropism also involves the unequal distribution of auxin.
Fig. 22.214.171.124 Gravi tips
When an oat coleoptile tip is placed on two separated agar blocks, as shown here, there is no difference in the auxin activity picked up by the two blocks. When the preparation is placed on its side, however, the lower block accumulates twice as much auxin activity as the upper block. Under natural conditions, this would cause greater cell elongation on the underside of the coleoptile and the plant would bend upward.
Shoots versus Roots
Unequal distribution of auxin is also the key to the negative phototropism and positive gravitropism of roots.
Fig. 126.96.36.199 Roots vs Shoots
The graph (based on the work of K. V. Thimann) shows the effect of auxin concentration on root and stem growth. The difference between the behavior of roots and stems lies in the difference in the sensitivity of their cells to auxin. Auxin concentrations high enough to stimulate stem growth inhibit root growth.
Possible Mechanism of Gravitropism in Roots
When a root is placed on its side,
- Statoliths (organelles containing starch grains) settle by gravity to the bottom of cells in the root tip.
- This causes PIN proteins to redistribute to the underside of the cell where they pump auxin out of the cell.
- The auxin then accumulates along the under side of the root.
- This INHIBITS root cell elongation (see graph).
- So the cells at the top surface of the root elongate, causing the root to grow down.