How Plants Work

dormancy

A period after seeds are dispersed from the parent plant, during which they will not germinate

abscisic acid (ABA)

A plant hormone that enforces dormancy

dormancy-breaking stimuli

Water – needed by all seeds
Warm temperatures after an extended cold period – ensures winter is
over
Light – needed by small seeds

phytochrome

A molecule that perceives the light and initiates the signal to germinate

far-red reversibility

A flash of far-red light reverses the effect of a flash of red light

Pr, Pfr

Pr absorbs red light to become Pfr and Pfr absorbs far red light to become Pr

photomorphogenesis

The control of plant form and development by light

seedling shoot elongation

The light reaching the ground in shady areas is enriched for far red wavelengths, and this converts the phytochrome in the seedlings to the Pr form, which causes elongation in of the seedling stems

auxin

A hormone that travels down the plant from the site of its synthesis in the shoot apexes and promotes branch root formation in the root system

cytokinin

Travels up the plant from its site of synthesis in the root apexes and promotes lateral branch formation in the shoot system

hormone balance

The balance of auxin and cytokinin in the plant determines the relative degrees of shoot and root branching

root-shoot balance

A plant with too much root growth compared to shoot growth, or too much shoot growth compared to root growth, has partitioned its carbon resources inefficiently and will be at a disadvantage in nature

root-shoot balance

A plant with too much root growth compared to shoot growth, or too much shoot growth compared to root growth, has partitioned its carbon resources inefficiently and will be at a disadvantage in nature

cohesion-tension theory

Water moves into and up the plant due to solute concentration differences or differences in negative pressure (tension)

negative pressure (tension)

The water column in the xylem is under tension

cohesion

The weight of the water column is supported by cohesion among water molecules and adhesion to the xylem walls

adhesion

transpiration

The evaporation creates tension on the chain of water molecules that run from the roots to the leaves

pressure flow model of phloem transport

Sucrose is transported into phloem cells by the process of phloem loading then water follows the sugar into the phloem cell then sugar is pushed along the phloem cell by bulk flow and finally sugar is unloaded from the phloem at the sink

source

sink

H+-ATPase

Provides the energy by setting up a proton gradient

sucrose / H+ symport

Uses the gradient to import sucrose against its concentration gradient

resource partitioning

how a plant decides where to send its photosynthate

tropism

A

positive phototropism

IAA

An auxin that is the transportable signal for bending

cell wall acidification

IAA causes cell elongation by acidifying cell walls on the nonilluminated side, making the walls more stretchy and extensible.

positive gravitropism

The bending of a root or shoot toward the pull of gravity

negative gravitropism

The bending of a root or shoot away from the pull of gravity

thigmotropism

The directional growth stimulated by touch, such as the responses of tendrils on climbing plants

photoperiod

The daylength (length of the light period in a day)

short-day plants (long night plants)

Flowering occurs only when the plant is exposed to a continuous dark period exceeding a critical length

long-day plants (short night plants)

Flowering occurs when the night length is shorter than a critical length

ethylene

Released to hasten the ripening of neighbouring fruit

senescence
senescence
senescence and abscission

Fruit, flowers, and leaves