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Q: What is the inactive form of phytochrome, and what is it triggered by?
A: Pr (phytochrome red); triggered by red light (660 nm) which converts it to the active form Pfr.
Q: How do plants use the phytochrome system?
A: To detect light quality, duration (photoperiod), and direction; helps regulate germination, flowering, and growth.
Q: What is phototropism a response to?
A: Light; specifically blue light.
Q: What ensures that roots grow into soil and shoots grow toward sunlight?
A: Gravitropism and phototropism, regulated by auxin (IAA) distribution.
Q: Define Phototropism.
A: Growth response of a plant toward or away from light.
Q: Define Thigmotropism.
A: Directional growth response to touch or physical contact (e.g., vines coiling).
Q: Define Gravitropism.
A: Growth response to gravity; roots grow downward, shoots grow upward.
Q: Define Thigmonastic response.
A: Rapid, non-directional movement in response to touch (e.g., Venus flytrap closing).
Q: Define Thigmomorphogenetic response.
A: Long-term structural changes due to mechanical stress like wind or repeated touch.
Q: What controls the response to gravity?
A: Statoliths—starch-filled organelles that settle with gravity and signal auxin redistribution.
Q: What does a high concentration of IAA in response to gravity stimulate? Where?
A: In shoots: stimulates cell elongation on the lower side → bends upward.
In roots: inhibits elongation on the lower side → bends downward.
Q: What is the first line of defense in plants?
A: Structural barriers such as the cuticle, bark, and cell walls.
Q: What structures have been modified to become thorns and spines?
A: Thorns = modified stems; Spines = modified leaves.
Q: What are alkaloids? What are they used for? Know some examples.
A: Nitrogen-containing compounds used in plant defense; toxic or bitter to herbivores.
Examples: morphine, caffeine, nicotine, quinine.