Plant Responses to Internal & External Signals Chapter 39

Plant Responses to Stimuli Plants have receptors that inform the plant of specific stimuli Just as we have learned, there are 3 steps to the response to these stimuli: Reception – First to identify stimulus Transduction Second Messengers – chemical messengers that amplify the signal from the receptors to induce specific responses Response

Hormone Action in Plants Hormone-producing cells Plants produce hormones Chemical messengers that travel throughout the plant causing target cells to respond In plants, hormones control: Plant growth and development Plant responses to environment Movement of hormone Target cells Cells in one blooming flower signals other blooms using hormones to open.

Plant Hormones Auxins Produced in embryo, apical meristems Stimulate root growth, stem elongation

Plant Hormones Cytokinins Made in root, transported to other parts Stimulates cell division, plant growth

Plant Hormones Gibberelins Made in meristematic tissues Promotes bud/seed germination, stem elongation

Plant Hormones Brassinosteroids Produced in seeds, fruit, shoots, leaves, floral buds Inhibits root growth, prevent loss of leaves, promote xylem differentiation

Plant Hormones Abscisic acid Produced in leaves, stems, roots, green fruit Slows growth, promotes seed dormancy, closes stomata during water stress

Plant Hormones Ethylene Gas produced in tissues of ripening fruit, nodes of stems, aging leaves/flowers Hastens fruit ripening “one bad apple spoils the bunch”

Control of Flowering Key Concepts Phytochromes: pigments that regulate plant response to light Critical night length: the minimum amount of night time required for regulation of plant growth

Short-day plants dark Critical night length If light exceeds or interrupts critical night length, no flowering happens. light

Long-day plants dark Critical night length Night can exceed critical night length or be interrupted, flowering can still happen. light

Tropism The way a plant grows in response to stimuli in the environment Types of tropisms: Phototropism: growth response to light Plants bend towards light Geotropism: growth response to gravity Plant roots grow down with gravity, shoots (stems) grow up against gravity and out of soil Thigmotropism: growth response to touch Vines grow up around trees, Venus flytrap closes when leaves are touched

Plant Defenses Produce chemicals in response to predation (secondary compounds) Antioxidants to protect against UV rays