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…