2 Tropisms: movement toward or away from a stimulus Phototropism- growth/movement in response to light.Growth on darker side is greater due to assymetric auxin concentrations.Especially true for coleoptilesPhotoreceptors present at shoot tip.Gravitropism- growth/movement in response to gravity.Roots display positive gravitropism. (goes down)Shoots display negative gravitropism. (goes up)Mechanism not fully understood:Some evidence for “Statolith Theory”:Statoliths: starch grains which accumulate on the low parts of plant cellsAlso Thigmotropism (response to touch)All these usually resulting from the effect of….HORMONES!!!
3 Hormone: chemical signals that coordinate parts of an organism; produced in one part of the system and then transported to other parts; low concentrationsAnyway… there are 5 main classes of plant hormones. They are…..
4 1. Auxin: AKA: IAA (indoleacetic acid) Location: seed embryo; meristems of apical buds and young leavesFunction: cell elongation;Branching; fruit development; apical dominance; tropismsMoves “down” a stem by polar transport.Uses “acid growth” to loosen cell walls for expansionSynthetic versions are used for herbicides (2-4-D)
5 2. Cytokinins: Antagonistic to Auxin (and vis-versa) Location: roots (and actively growing tissues)Function: root growth and differentiationcell division (cytokinesis..get it??)germination; delay senescence (aging)Stimulates axillary growth in low auxin concentrations.
6 3. GibberellinsGA3Location: meristems of apical buds and roots, young leaves, embryoFunction: germination of seed and bud; stem elongation; leaf growth; flowering and…BOLTING! (rapid stem/flower growth)
7 4. Abscisic acid ABA Location: leaves, stems, roots, green fruit Function: inhibits growth; closes stomata during stress; maintains seed dormancy
8 5. Ethylene Gaseous hormone Location: ripening fruit tissue; stem nodes; aging leaves and flowersFunction: fruit ripening (breaks down starches into simpler sugars); oppositional to auxin (leaf abscission); inhibits growth/development of plant; promotes senescence
9 Photoperiodism: physiological responses to day length Circadian rhythm (24 hour periodicity)Short-day plant: light period shorter than a critical length to flower (flower in late summer, fall, or winter; poinsettias, chrysanthemums)Long-day plant: light period longer than a critical length to flower (flower in late spring or early summer; spinach, radish, lettuce, iris)Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)Critical night length controls flowering
10 Phytochromes(definition) -Plant pigment that measures length of darkness in a photoperiod (red light)Pr (red absorbing) 660nmPfr (far-red absorbing) 730nmRed light turns Pr to PfrFar Red turns Pfr to PrPlants continuously synthesize Pr but degrade Pfr. Soooooo…….After sunset, Pfr reverts to PrAt sunrise Pfr levels increaseIt’s theorized that plants reset their biological clocks in this way…