Plant Responses to Internal & External Signals

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…

Plant Hormones

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

Auxin

Stimulates stem elongationStimulates development of fruitInvolved in phototropism and gravitropism

Cytokinins

Stimulate cell division and growth Stimulate cytokinesis

Stimulate germination and flowering

Gibberelins

Trigger seed and bud germination

Promote stem elongation and leaf growth

Important in the growth of fruit

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”

Abscisic Acid

Induces seed dormancy Anti-gibberellin

Inhibits cell growth Anti-cytokinin

Inhibits fruit ripening Anti-ethylene

Closes stomata during water stress, allowing many plants to survive droughts

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)

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

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

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!!

Plant Defenses

Plants defend themselves against herbivores in several ways Physical defenses, such as thorns Chemical defenses, such as

producing distasteful/toxic

compounds