(Tropisms)
Tropisms are growth curvatures in response to stimuli of unequal intensity on different sides of a plant organ.
Phototropism - growth toward the light. When light is diffuse, i.e. coming from all directions equally, then the plant just grows straight up, but if the light is stronger in one direction in particular then the seedling should move toward the light in order to expose its leaves to the maximum light for photosynthesis. Phototropism is due to the differential growth of the seedling caused by lateral migration of the growth hormone auxin in the shoot tip under the influence of light and the subsequent downward transport on the nonilluminated side.
Link to view of radish seedlings responding to light
Gravitropism - the downward curvature of the root tip toward the center of gravity. If a plant part bends down, it is positive gravitropism (A2); if it bends up it is negative gravitropism (A3). Corn seeds were germinated sideways, upside down and right side up. In every case the root tip bent downwards and the shoot tip bent upwards. Gravitropism is due to the movement of an auxin to the lower surface of the shoot where it stimulates cell division and causes the shoot to bend upwards. In roots, a growth inhibitor, possibly auxin or abscissic acid moves to the lower surface of the root where it actually inhibits root cell elongation. Therefore the upper (uninhibited) cells of the root expand at a greater rate than the lower cells, and the root grows downward. Apparently, calcium also plays a role in coordinating the roots response to gravity. The root cap is the region of the root which perceives the gravitational stimulus.
Link to views of negative gravitropism
link to view of positive gravitropism
Thigmotropism - Thigmotropism is a response to contact with a solid object. One of the most common examples of thigmotropism is seen in tendrils, which wrap around any object with which they come in contact. There is some evidence that auxin plays a role in this response.
Link to view of tigmotropic response in tendril of passion flower
Growth of Beans in Light and Dark - Those grown in the dark were thin and pale with longer internodes and smaller leaves, than normal seedlings. This combination is called etiolation. The plant needs exposure to red light in order to trigger the change of the phytochrome pigment from P660 to P730, and "normal" stem development results.
Link to view of etiolated and light-grown bean seedlings
C. Photoperiodism - is the biological response to a change in the proportions of light and dark in a 24 hour daily cycle. In the greenhouse you saw two tables - one set up for a short day and long night (there was a black curtain that could be rolled over after 8 hours of light) and one set up for a long day and short night with spotlights to keep it bright for 16 hours. Long day plants flower when the dark period is shorter than a critical length, short day plants flower when the dark period is greater than a critical length, and day neutral plants flower in response to other parameters (e.g., number of nodes in tomato). Phytochrome, a pigment often present in small amounts in higher-plant tissue, is sensitive to the transitions between light and darkness. The pigment can exist in two forms, P660 which absorbs 660nm light and P730 which absorbs 730nm light. P730 is the active form of the pigment; it promotes flowering in long day plants and inhibits flowering in short day plants. An excellent example of a short day plant is Poinsettia, flowering normally in the winter. An example of a long day plant would be Alfalfa flowering normally in the summer. An example of a day neutral plant would be buckwheat which will flower in any proportions of light and darkness.
Link to view of a short day plant
Link to view of a long day plant
Link to view of a day neutral plant
D. Touch Responses - Thigmonastic Movements.
One part of a leaf was touched on the Mimosa and the whole leaf responded by folding up, each leaflet in turn. The same thing happened when a lit match was brought near. The effect was to reduce the amount of exposed leaf surface area in the presence of disturbance or extreme heat. This folding up of the leaf is due to a sudden change in turgor pressure within certain cells of pulvini at the base of the leaflets and leaves. The loss of water from these cells follows the migration of potassium ions from them.
Link to view of sensitive plant before and after being touched