Growth Factors in Flowering Plants
Phototropism in Shoots: When light is unilateral (from one side), IAA produced at the tip moves toward the shaded side of the shoot. This higher concentration of IAA stimulates faster cell elongation on the shaded side compared to the light side, causing the shoot to bend toward the light.
Gravitropism in Roots: In a horizontally placed root, IAA moves toward the lower side due to the influence of gravity and the settling of dense organelles called amyloplasts. In roots, high concentrations of IAA inhibit cell elongation, meaning the upper side elongates faster than the lower side, causing the root to bend downward.
Gravitropism in Shoots: Similar to roots, IAA accumulates on the lower side of a horizontal shoot. However, because IAA stimulates elongation in shoots, the lower side grows faster than the upper side, resulting in the shoot bending upward (negative gravitropism).
| Feature | Shoot Response | Root Response |
|---|---|---|
| IAA Effect | Stimulates cell elongation | Inhibits cell elongation |
| Phototropism | Positive (towards light) | Negative (away from light) |
| Gravitropism | Negative (away from gravity) | Positive (towards gravity) |
Terminology Precision: Always use the term growth factors rather than 'plant hormones' to align with advanced biological standards. Ensure you specify that these factors affect cell elongation, not cell division, as this is a common point of confusion in exam questions.
Directional Logic: When describing a tropism, always identify the stimulus, the direction of IAA movement (e.g., to the shaded side or lower side), and the specific effect on elongation in that organ. This step-by-step logical chain is what examiners look for in long-answer responses.
Sanity Check: If a question asks about a root, remember that high IAA means less growth. If it asks about a shoot, high IAA means more growth. Mentally verify if your predicted bending direction matches the survival needs of the plant (e.g., roots must go down for water).
Growth vs. Division: A frequent error is stating that IAA causes cells to divide more rapidly. In reality, the primary mechanism for the rapid bending observed in tropisms is the stretching and lengthening of existing cells (elongation).
Uniform vs. Unilateral Stimuli: Students often forget that if a stimulus like light is uniform (from all sides), IAA is distributed evenly. In this case, the plant grows straight upward rather than bending, as there is no concentration gradient to cause uneven elongation.
Inhibition in Roots: It is counter-intuitive to many that a 'growth factor' can inhibit growth. It is vital to memorize that in roots, IAA acts as an inhibitor to facilitate the downward bending required for positive gravitropism.