1. Plant Responses to Internal & External Signals

2. Plant Hormones
Hormones are chemical signals that coordinate the various parts of an organism
A hormone is a compound produced in one part of the body which is then transported to other parts of the body, where it triggers responses in target cells and tissues
Examples of human hormones:
Adrenaline, testosterone, estrogen, epinephrine…

3. Plant Hormones
There are 5 major classes of plant hormones, each with specific functions:
Auxin
Cytokinins
Gibberellins
Abscisic acid
Ethylene

4. Auxin Stimulates stem elongation Stimulates development of fruit
Involved in phototropism and gravitropism

5. Cytokinins Stimulate cell division and growth
Stimulate cytokinesis
Stimulate germination and flowering

6. Gibberellins Trigger seed and bud germination
Promote stem elongation and leaf growth
Important in the growth of fruit

7. Ethylene Promotes fruit ripening
Senescence (aging) is a progression of irreversible change that eventually leads to death
Caused, at least in part, by ethylene
“One bad apple spoils the whole bunch”

8. Abscisic Acid Induces seed dormancy Inhibits cell growth
Anti-gibberellin
Inhibits cell growth
Anti-cytokinin
Inhibits fruit ripening
Anti-ethylene
Closes stomata during water stress, allowing many plants to survive droughts

9. Tropisms
Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli
There are three major stimuli that induce tropisms
Light (Phototropism)
Gravity (Gravitropism)
Touch (Thigmotropism)

10. Phototropism Phototropism is the growth of a shoot towards light
This is primarily due to the action of auxin
Auxin elongates the cells on the non-light side

11. Biological Clocks/Circadian Rhythms
A physiological cycle with a frequency of about 24 hours is called a circadian rhythm
Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes
Example: jet lag in humans

12. Photoperiodism
A physiological response to day length (differs in winter, summer, spring, and fall) is known as photoperiodism
Short-day plants
Require a shorter light period
Flower in later summer/fall/winter
Example: poinsettias
Long-day plants
Require a longer light period
Flower in late spring/early summer
Example: spinach
Day-neutral plants
Are unaffected by photoperiod
Example: tomatoes
But it’s actually the night that matters!!

13. Plant Defenses
Plants defend themselves against herbivores in several ways
Physical defenses, such as thorns
Chemical defenses, such as
producing distasteful/toxic
compounds