Lecture #17 Date _______ n Chapter 39 ~ Plant Responses to Internal and External Signals

SEEDLING (no light) No Need for Expanded Leaves Or Chlorophyll No Need for Major Root System All Energy Expended in Stem Elongation Shoot Reaches Sunlight Stem Elongation Slows Down Leaves Expand and Chlorophyll is Produced Roots Elongate

Plant hormones n Hormone: chemical signals that coordinate parts of an organism; produced in one part of the body and then transported to other parts of the body; low concentrations n Tropism: movement toward or away from a stimulus n Went experiments (phototropism) n Hormone: auxin n Others: gravitropism, thigmotropism

Auxin n IAA (indoleacetic acid) n Location: seed embryo; meristems of apical buds and young leaves n Function: stem elongation; root growth, differentiation, branching; fruit development; apical dominance; tropisms

Cytokinins n Zeatin n Location: roots (and actively growing tissues) n Function: root growth and differentiation; cell division and growth; germination; delay senescence (aging); apical dominance (w/ auxin)

Gibberellins n GA 3 n Location: meristems of apical buds and roots, young leaves, embryo n Function:germination of seed and bud; stem elongation; leaf growth; flowering (bolting); fruit development; root growth and differentiation TreatedUntreated

Abscisic acid n ABA n Location: leaves, stems, roots, green fruit n Function:inhibits growth; closes stomata during stress; counteracts breaking of dormancy

Ethylene n Gaseous hormone n Location:ripening fruit tissue; stem nodes; aging leaves and flowers n Function:fruit ripening; oppositional to auxin (leaf abscission); promotes/inhibits: growth/development of roots, leaves, and flowers; senescence

Daily and Seasonal Responses n Circadian rhythm (24 hour periodicity) – internal clock that measures time of day and night n Photoperiodism (phytochromes) n Short-day plant: light period shorter than a critical length to flower (flower in late summer, fall, or winter; poinsettias, chrysanthemums) n Long-day plant: light period longer than a critical length to flower (flower in late spring or early summer; spinach, radish, lettuce, iris) n Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) n Critical night length controls flowering

n Photoperiodism u Day and night cycles u Determines blooming patterns u “short-day” plants – when nights get longer u “long-day” plants – when nights get shorter u “neutral” plants – blooming due to other environmental factors u Controlled by pigment (phytochrome) responding to red wavelength of light

Phytochromes n Plant pigment that measures length of darkness in a photoperiod (red light) n P r (red absorbing) 660nm n P fr (far-red absorbing) 730nm