Plant Responses to Internal & External Signals Chapter 39

Tropism  A plant growth response from hormones that results in the plant growing either toward or away from a stimulus  Hormones – chemical messengers that coordinate the different parts of a multicellular organism  Types of tropisms: Phototropism – growth toward or away from light Gravitropism – growth toward or away from a gravitational source Thigmotropism – growth toward or away from a touch Vines grow toward a support and then grow toward (around) that support

Phototropism video

Gravitropism Video

Thigmotropism Video

Positive v. Negative Tropisms  Positive is growth toward a stimulus Positive Phototropism – growth toward light Positive Gravitropism – growth toward a gravitational source Positive Thigmotropism – growth toward a tactile stimulus  Negative is growth away from a stimulus Replace the above terms’ “Positive” with “Negative” and then replace “toward” with “away”

Hormones  Auxins Stimulate cell elongation (remember that zone?) Cause proton pumps to activate  Lower pH  weakening of cell wall  turgor pressure expands & elongates cell wall High concentrations of synthetic auxins KILL certain plants, usually weeds Synthetically produced auxins in high concentration = Herbicides

How do you make a …?  Cytokinins Stimulate cell division or cytokinesis Proper ratio of auxins & cytokinins  cell division & differentiation  Gibberellins Work with auxins to stimulate stem elongation Loosen cell walls allowing cellular expansion  stem expansion Signal the seed to cease dormancy and germinate Many dwarf plant varieties have non-functional gibberellins

Hormones (Page 3)  Abscisic acid SLOWS Growth Antagonistic to previously mentioned hormones Promotes seed dormancy, but gibberellins cease it Causes stomata to close to conserve water  Ethylene Gas Plays crucial role in programmed cell death (apoptosis) Promotes ripening of fruit Ripening in one fruit  ripening in other fruits Positive feedback mechanism = rapid ripening of fruit One bad apple does spoil the lot.

Plants respond to Light  Plants can detect presence, direction, intensity, and wavelength of light  Red & Blue wavelengths are most important Red light is sensed by phytoreceptors  Blue light is most important for phototropisms and light-induced opening of stomata Sensed by photoreceptors

Phytochromes  Photoreceptors for red light (mainly)  Exist in 2 isomer forms which can switch forms depending on wavelength available  Phytochrome form triggers plant’s developmental responses to light  Responsible for Circadian Rhythms 24 hour cycle & not paced by environmental variables Chickens & Summer in Nome, Alaska

Photoperiodism  Physiological response to a photoperiod (relative lengths of day and night) Night is always the more important of the 2!! But we (humans) focus on the daylight, we’re dumb.  Controls Flowering  Short-day plants Require a period of continuous darkness exceeding a critical measure in order to flower Early Spring or Fall flowering AP Biology students call them “Long-Night” plants

Besides Short-Day plants,  Long-Day Plants Flower only if a period of continuous darkness is less than a critical value Flower in Late Spring or early Summer Considered “short-night” plants  Day-Neutral Plants No length of continual darkness is needed for flowering

Plants respond to things other than light  Gravitropism – growth toward or away from light Auxin plays key role If root is plased horizontally, then gravity causes an accumulation of auxins in root’s (radicle’s) lower side Remember HIGH [auxin] = inhibition of growth, so lower side = no growth Upper side = growth Allows the root to grow down into the ground

Auxin Accumulation

Plants respond to other things…  Drought Stomata close Leaves will cease growth Leaves roll into shape that reduces transpiration Deep roots (where H2O is) will speed their growth, but shallow roots will stop growing  Predators Thorns, chemicals, distasteful compounds Some plants even attract parasites First layer of defense - Epidermis

Coordinated Plant-Parasite Defense