Plant Hormones and Plant Tropisms

Plant Hormones

Remember that hormones are signals that can travel long distances to affect the actions of another cell.

Examples include abscisic acid, auxin, cytokinins, ethylene, and gibberellins.

Abscisic acid:-"babysitter hormone"

-makes sure seeds do not germinate too early -inhibits cell growth

-stimulates closing of the stomata (prevents water loss)

Auxin

elongation of stems

plays a role in phototropism and gravitropism

Cytokinins

promote cell division and leaf enlargement

slow down aging of leaves

used by supermarkets to keep veggies fresh

Ethylene

initiates fruit ripening

causes flowers and leaves to drop from trees

Gibberellins

stem elongation

growth (dormant seeds, buds, and flowers)

Plant TropismsTropism: growth that occurs in response to an environmental

stimulus such as sunlight or gravity

Gravitropism

plants response to gravitational force

stems grow up and roots grow down

auxin and gibberellins play a role

Phototropism

a plant's response to light

auxin is in charge

when light is received on all sides, auxin is distributed evenly and growth is even

when 1/2 of plant is in the sun, auxin focuses on the darker side which leads to unequal growth of the stem (side with less light grows faster causing movement of plant toward the light source)

Photoperiodism

Circadian Rhythm

Plants have a biological clock that maintain a circadian rhythm: a physiological cycle that occurs in time increments that are roughly equivalent to the length of of a day.

Photoperiodism is the response by a plant to the change in the length of days.

Hormones such as florigen and phyotchrome help flowers bloom at the appropriate times.

Short-Day Plants

Example : Poinsettias

Flowering requirements: exposure longer night hours

Have flowers from the end of summer to end of winter

Long-day Plants

Ex: Spinach

Flowering requirements--must be expose to a night shorter than day

Thigmotropism

a plant's response to touch or contact

ex: vines that wind around objects as they grow