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• Mutant plants wilt unless grown
in constantly humid conditions. Gibberellins (p. 650; 613). Kurosawa (1926) “Foolish seedling disease” caused
by a parasitic fungus: Gibberella fujikuroi.
Other Japanese scientists isolated the factor:
Synthesized in young shoot tissues & developing
Transported via xylem & phloem. Action of Gibberellins
Action Stimulates cell division &
Induces seed germination.
Stimulates bolting & flowering in
biennials. Brassinosteroides: (P. 652).
Growth promoting steroid hormones.
Produced in all parts of plant:
Produced Usually act locally near site of synthesis. At least 70 related compounds known.
At Brassinolide most widespread.
Required for normal growth: Wide range of developmental roles. Cell division & elongation, differentiation of vascular
tissue, photomorphogenesis, flower & fruit
development, External Factors & Plant Growth (Ch 28)
The Tropisms Plant growth responses: (bending curving) because of
external stimulus (p. 660).
Phototropism: Growth in response to directional light Elongation on shaded side of growing shoot tip: Went’s Experiment: Auxin-permeated agar blocks
cause seedling tips to bend.
• Auxin migrates to shaded side, stimulating elongation. Gravitropism (p. 662; 623)
623 Growth in response to gravity.
Calcium involved: Ca++ moves to upper side of shoots,
lower side of roots.
Auxin & cytokinin also involved.
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- Spring '14