1. Plant hormones such as auxin, cytokinins, gibberellins, abscisic acid, and ethylene coordinate growth and development processes in plants through tropisms, photoperiodism, and other responses. 2. Auxin promotes cell elongation and growth, apical dominance, and gravitropism, while cytokinins promote cell division and inhibit senescence. Gibberellins promote stem elongation and flowering. 3. Abscisic acid inhibits growth and maintains seed dormancy, while ethylene promotes fruit ripening and leaf abscission.

1. Plant Control Systems
 It’s a
Hormonal
Thing!

2. Tropisms: movement toward or away from a stimulus
 Phototropism- growth/movement in
response to light.
 Growth on darker side is greater due to
assymetric auxin concentrations.
 Especially true for coleoptiles
 Photoreceptors present at shoot tip.
 Gravitropism- growth/movement in
response to gravity.
 Roots display positive gravitropism. (goes down)
 Shoots display negative gravitropism. (goes up)
 Mechanism not fully understood:
 Some evidence for “Statolith Theory”:
 Statoliths: starch grains which accumulate on the
low parts of plant cells
 Also Thigmotropism (response to touch)
 All these usually resulting from the effect of….
 HORMONES!!!

3. Hormone: chemical signals that coordinate
parts of an organism; produced in one part of the
system and then transported to other parts; low
concentrations
Anyway… there are 5 main classes of plant hormones.
They are…..

4. 1. Auxin:
 AKA: IAA (indoleacetic acid)
 Location: seed embryo; meristems of apical buds and
young leaves
 Function: cell elongation;
 Branching; fruit development; apical dominance;
tropisms
 Moves “down” a stem by polar transport.
 Uses “acid growth” to loosen cell walls for expansion
 Synthetic versions are used for herbicides (2-4-D)

5. 2. Cytokinins:
 Antagonistic to Auxin (and vis-versa)
 Location: roots (and actively growing tissues)
 Function: root growth and differentiation
 cell division (cytokinesis..get it??)
 germination; delay senescence (aging)
 Stimulates axillary growth in low auxin concentrations.

6. 3. Gibberellins
 GA3
 Location: meristems of apical buds and roots,
young leaves, embryo
 Function: germination of seed and bud; stem
elongation; leaf growth; flowering and…
 BOLTING! (rapid stem/flower growth)

7. 4. Abscisic acid
 ABA
 Location: leaves, stems, roots, green fruit
 Function: inhibits growth; closes stomata during
stress; maintains seed dormancy

8. 5. Ethylene
 Gaseous hormone
 Location: ripening fruit tissue; stem nodes; aging
leaves and flowers
 Function: fruit ripening (breaks down starches into
simpler sugars); oppositional to auxin (leaf
abscission); inhibits growth/development of plant;
promotes senescence

9. Photoperiodism:
physiological responses to day length
 Circadian rhythm (24 hour periodicity)
 Short-day plant: light period shorter than a critical length to flower
(flower in late summer, fall, or winter; poinsettias, chrysanthemums)
 Long-day plant: light period longer than a critical length to flower
(flower in late spring or early summer; spinach, radish, lettuce, iris)
 Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)
 Critical night length controls flowering

10. Phytochromes
 (definition) -Plant pigment that
measures length of darkness in a
photoperiod (red light)
 Pr (red absorbing) 660nm
 Pfr (far-red absorbing) 730nm
 Red light turns Pr to Pfr
 Far Red turns Pfr to Pr
 Plants continuously synthesize Pr but
degrade Pfr. Soooooo…….
 After sunset, Pfr reverts to Pr
 At sunrise Pfr levels increase
 It’s theorized that plants reset their
biological clocks in this way…