Plant Responses to Internal & External Signals. Plant Hormones Hormones are chemical signals that...
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Transcript of Plant Responses to Internal & External Signals. Plant Hormones Hormones are chemical signals that...
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