Back to 9.1 Structure of Plants

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9.1.7 The role of auxin in phototropism as an example of the control of plant growth.

- Tropism is a bending-growth movement either toward or away from a directional stiumulus

•      Tropisms may be positive (towards the stimulus) or negative (away from the stimulus).

-Phototropism means plant growth in response to light. Furthermore in order for a plant to grow toward the sunlight a hormone called Auxin is necessary. 

•      Generally, plant stems exhibit positive phototropism and roots demonstrate negative phototropism

-Auxin is a plant hormone that causes the positive phototropism of plant shoots and seedlings. Thus without Auxin, a plant would be unable to bend toward the sunlight, because Auxin increases the flexibility of plant cell walls in young developing shoots. 

•      Auxins are found in the embyos of seed, the meristems of apical buds and young leaves. These hormones work on plant cells that have auxin receptors.

-One thing to note about Auxin is that Auxin is unevenly distributed. There is more concentration of auxin to the stem side away from the light source. Thus the area of the stem with high concentration of Auxin (IAA; indoleacetic acid) grows and elongates, which results in a bend of the stem.

  

-In this drawing, it shows equal distribution of Auxin when the sunlight is right above the plant. In contrast it shows a plant that has higher concentration of auxin on the left side, away from the sun. 

•      Growth response does not appear to be due to an increased production of auxin on one side of the shoot. Rather it seems to be due to a redistribution of available auxin especially to the stem side away from the light source. The result of this uneven distribution of auxin is a greater elongation of cells on the stem side away from the light and therefore curvature toward the light source.



http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantHormones/Nutrition/3224.GIF

Questions

Objective lvl 1-Define positive Phototropism and negative phototropism (2)

Objective lvl 2-Outline the role of Auxin in plant growth. (6)

Objective lvl 3-Analyze the cause of uneven distribution of Auxin in plants. (4)