Presentation on theme: "Plant Control Systems n It’s a Hormonal Thing!. Tropisms: movement toward or away from a stimulus n Phototropism - growth/movement in response to light."— Presentation transcript:
Plant Control Systems n It’s a Hormonal Thing!
Tropisms: movement toward or away from a stimulus n Phototropism - growth/movement in response to light. n Growth on darker side is greater due to assymetric auxin concentrations. n Especially true for coleoptiles n Photoreceptors present at shoot tip. n Gravitropism - growth/movement in response to gravity. n Roots display positive gravitropism. (goes down) n Shoots display negative gravitropism. (goes up) n Mechanism not fully understood: n Some evidence for “Statolith Theory”: n Statoliths: starch grains which accumulate on the low parts of plant cells n Also Thigmotropism (response to touch) n All these usually resulting from the effect of…. n HORMONES!!!
Hormone : chemical signals that coordinate parts of an organism; produced in one part of the system and then transported to other parts; low concentrations n Anyway… there are 5 main classes of plant hormones. They are…..
1. Auxin: n AKA: IAA (indoleacetic acid) n Location: seed embryo; meristems of apical buds and young leaves n Function: cell elongation; n Branching; fruit development; apical dominance; tropisms n Moves “down” a stem by polar transport. n Uses “acid growth” to loosen cell walls for expansion n Synthetic versions are used for herbicides (2-4-D)
2. Cytokinins: n Antagonistic to Auxin (and vis-versa) n Location: roots (and actively growing tissues) n Function: root growth and differentiation n cell division (cytokinesis..get it??) n germination; delay senescence (aging) n Stimulates axillary growth in low auxin concentrations.
3. 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 and… n BOLTING! (rapid stem/flower growth)
4. Abscisic acid n ABA n Location: leaves, stems, roots, green fruit n Function:inhibits growth; closes stomata during stress; maintains seed dormancy
5. Ethylene n Gaseous hormone n Location:ripening fruit tissue; stem nodes; aging leaves and flowers n Function:fruit ripening (breaks down starches into simpler sugars); oppositional to auxin (leaf abscission); inhibits growth/development of plant; promotes senescence
Photoperiodism: physiological responses to day length n Circadian rhythm (24 hour periodicity) 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
Phytochromes n (definition) - 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 n Red light turns P r to P fr n Far Red turns P fr to P r n Plants continuously synthesize P r but degrade P fr. Soooooo……. n After sunset, P fr reverts to P r n At sunrise P fr levels increase n It’s theorized that plants reset their biological clocks in this way…