A Quick Discussion of External Factors and Plant Growth
and 
Plant Hormones
Kinds of Plant Responses
Tropisms
•	A tropism is a bending or curving growth response toward or away from a stimulus
–	Phototropism
–	Gravitropism
•	Shoots vs. roots
–	Thigmotropism

Circadian Rhythms
•	24 hour cycles of plant activity
–	e.g. flower opening
Photoperiodism
•	3 Kinds of flowering scenarios:
•	Long day (summer)
•	Short day (spring or fall)
•	Day neutral
•	Response of plants to day length
–	Flowering is controlled by phytochrome, which has two forms that interconvert in response to light
•	PredPfar red 
–	Far red form is the active one, and can act to stimulate or suppress flowering
–	Red exposure converts Pred to Pfar red 
–	Over time far red decays to red form

Dormancy
•	Seed dormancy
–	Environmental cues are important, but cannot pivot on short term temperature
–	Stratification--typical of temperate zone
–	Scarification--animal dispersed species
–	Soluble inhibitors--desert ephemerals
•	Bud dormancy
–	Onset late in growing season, decreasing daylength is important
–	Cold needed to break dormancy
Nastic Movements
•	Movements in response to stimulus
–	Thigmonasty--touch sensitivity
–	Solar tracking
Plant hormones work in concert to orchestrate plant responses
•	Hormones may be inhibitory or stimulatory
•	The cell determines response
–	e.g. Auxin may stimulate root growth or inhibit branch growth	
•	One hormone may stimulate the production of another
Auxin
•	Produced in shoot tips, suppresses elongation of buds below, providing apical dominance

•	Movement of auxin away from light in shoots causes bending of stems, allowing elongation of shaded side
Auxin 
•	Stimulates adventitious root growth (rooting powders)
•	Stimulates fruit growth; seeds a source
•	Auxins can be herbicides (2,4-D, common herbicide for lawns)

Cytokinin
•	Stimulates cell division
•	Works in conjunction with auxin to regulate root and shoot production in tissue culture

Ethylene
•	A gas produced by ripening fruits, artificial sources
•	Stimulates fruit ripening

Abscisic Acid (ABA)
•	Suppresses seed germination
•	Root-shoot signaling
Gibberellins
•	Stimulates cell enlargement and division
•	Stimulate seed germination