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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 elongation Stimulates development of fruit Involved 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