Question

Phototropic curvature is the result of uneven distribution of
(a) Auxin
(b) Gibberellin
(c) Starch
(d) Phytochrome

Answer 1

This is a plant hormone produced in the stem tip that promotes cell elongation. This hormone makes the plant move to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant.

Complete Step by Step answer:
F. W. Went found that when the tip of Avena coleoptile is cut, its elongation is reduced. However, when auxin was added externally, its growth was back to normal. The experiments suggested that auxin is synthesized in the coleoptile tip and is translocated downwards towards the gravity. It moves from the illuminated side to the shaded side. More auxin is present on the shaded side than on the illuminated side. Due to this, the plant grows more in the shaded side than the illuminated side and bends in the direction of the light source. It shows positive phototropism because cell elongation is observed to be greater in the shaded side and less on the illuminated side.
So, the correct answer is, ‘Auxin.’

Additional Information:
- Auxin develops at the tip of the stem. Its movement is largely basipetal. Auxin travels towards the shadier side of the plant making the cells in that region grow longer than the other cells on the brighter side of the plant known as the Basipetal movement. When auxin comes into contact with plant cells, it creates drastic changes in the expression of genes and the elongation occurs in the direction of basipetal movement. Active research is still going on to know the precise mechanisms by which this occurs, and two auxin signalling pathways are almost completely investigated.
- In 1920 the auxin was first reported by a Dutch botanist Frits Warmolt Went and explained its part in plant development. The auxin was first isolated and its chemical structure was first analyzed as indole acetic acid (IAA) by Kenneth V. Thimann who collaborated with Frits Warmolt Went to publish a book on plant hormones in 1937.
- The key part of plant development is the pattern of auxin distributed inside the plant and its behaviour to the environment, and mainly for the growth of parts of plants like leaves and flowers. It is done through a very complex and well-coordinated process called polar auxin transporting.
- Due to polar auxin transporting, a plant can react to environmental conditions and adapt to them without having a nervous system. Auxins usually act in counter to other plant hormones, such as the certain ratio of auxin and cytokinin in several plants determines the starting of root and shoot formation.

Note:
- The first important plant hormone discovered was the auxin. The name auxin was taken from the Greek word ‘auxein’ which means ‘to grow/develop’. Auxin is present in all parts of a plant but the concentration in each part varies.
- Five major types of auxins that are endogenous are indole acetic acid, phenylacetic acid, indole butyric acid, indole propionic acid, and 4-chloroindole-3-acetic acid.
- Auxins are synthesized artificially and applied to plants. This is usually done with the combination of other hormones which plays a major role in plant tissue culture.