The tropic movement of the part of plant in response to stimulus of light is called phototropism. Almost all part of the plant shows phototropism.
I. Stem: A potted plant is placed in a dark room. Light is coming
though a window. Its stem bends towards that window. Young stems are generally positively phototropic. They turn towards the light. The curvature is produced due to greater growth on the shaded side.
- Roots: Most of the roots do not show phototropism. But some roots show negative phototropism e.g. mustard
- Leaves: Leaves also show response to light. They twist their petioles and place their faces (upper sides) at right angles to the light. They are diaphototropic or transversely phototropic.
- Flowers: Many flowers show positive phototropism. In Helianthus (sunflower) the flower heads face the east in the morning. It moves with the sun during the day and face west in the evening.
Demonstration of phototropism
A young seedling is grown in a phototropic chamber. It consists of a tight box. It has a-removable- lid. Its inner side is painted black. It has a glass window one side. Therefore, light can enter only from this side. Phototrophic behaviour is studied on coloured screens. It shows that the maximum phototropic effect is produced in the blue violet end of the spectrum and the least in the yellow rays.
MECHANISM OF PHOTOTROPISM
(a) Role of coleoptile tip in phototropism
The germinating coleoptile of seedlings receives light from one direction. Their stems grow towards the light source. The side adjacent to the light source grows slowly. But the other side grows rapidly. This movement is called phototropism. The stem shows positive phototropism.
- Charles Darwin: He performed experiment on oat coleoptiles. He exposed it to unilateral light. He found that tip of the coleoptile receive the stimulus of unilateral light. But growth response occurred from lower part of the stem. He concluded that stimulus is transmitted from the tip to the growth region.
- Boysen Jenson (1913): They suggested chemical message moves from tip of coleoptile. They removed the tip of coleoptile. They placed a piece of gelatin between the tip and coleoptile stump. The stump curved towards light. Then they placed mica plate between the tip and stump. It is impermeable. The growth response was stopped. Now they placed mica sheet below the tip on shaded side. It developed a curvature. Then he placed mica sheet on illuminated side. The curvature did not develop.
- A. Paal (919): He removed coleoptile tip. He placed it on one side of the cut stump. He found that rapid growth occurred beneath the tip. It produces a curvature. Paal concluded that hormone diffused from the tip. It accelerated the growth.
- Fritz Went (1928): He carried out a series of experiments. He removed tips of dark-grown oat coleoptiles. He placed these tips on agar gel blocks. He kept these blocks in dark for several hours. lie placed agar blocks on one side of the coleoptile stumps. It citused curvature in the coleoptile. Went concluded that apt block absorbed growth hormone from the coleoptile tip. When It was ‘pTace on stump, it passed the hormone to the stump. Went named this _ hormone auxin (IAA). Auxin is synthesized in apical meristems and young leaves and then transported to the regions orgrowth in plant. It has highest concentrations at the tips of stems and roots, in young growing leaves, and in flowers and fruits.(b) Role of auxin in phototropism (Cholodny -Went Theory)Cholodny and Went theory suggested that the phototropic response occur due to unequal distribution of auxin (IAA). Went removed oat coleoptile root tip. He placed it in two agar blocks. These blocks were arranged in such a way that one block received auxin from illuminated side and the other block received from shaded sides. He passed unilateral light on the excised tip. He found that more auxin was collected on the illuminated side.
Later experiments were performed on maize. It gives following observations:
I. The tips of colepptiles were grown in dark light. They were cut and placed on agar blocks. It was found that 40 units of IAA diffused into the blocks in each case.
2. In other experiment the coleoptiles were split. Its two halves were separated by a thin glass cover slip. Same amounts of IAA were collected from the illuminated and darkened halves (20 units of IAA from each side).
Then the coleoptile was partially separated at the base by a thin piece of glass. Now more auxin diffused from the darkened half than the illuminated. About 30 IAA units collected from darkened half and 10 IAA units were collected from the illuminated half. These results suggest that unequal distribution of auxin causes phototrOpism.
(c) ROle of Photoreceptor in Phototropism
Blue and yellow lights are most effective for phetotropism. Thus two pigments are involved in the absorption of these lights. It is found that carotene and riboflavin receives light stimulus for phototropism. They are activated by light. Then they cause unequal distribution of IAA in stem or coleoptile tips.