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Plant hormones • 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 • Tropism: movement toward or away from a stimulus
Auxin Affects Plant Growth and Form • Phototropism is the tendency for plants to grow toward light sources. • In the 1800s, Charles Darwin and his son Francis experimented with canary grass seedlings grown in the dark. • They found that when the top millimeter of the coleoptile of a grass plant is covered, the plant cannot respond to the direction of light. • The photoreceptors are in the coleoptile tip. However, the bending takes place in the growing region below the tip. A signal must pass from the tip to the growing region.
Auxin Affects Plant Growth and Form • The movement of auxin is polar—it travels in just one direction along a line from apex to base. • This movement is not due to gravity. The apex to base direction is not reversed by inverting plants. • Polar transport depends on auxin anion efflux carriers, membrane proteins found only at the basal ends of cells. • At pH 7 in the cytoplasm, auxin exists as an anion. Auxin anions can leave the cell only by way of the protein carriers.
Auxin Affects Plant Growth and Form • The lateral redistribution of auxin is involved in both phototropism and gravitropism. • Redistribution occurs when the carrier proteins move to one side of the cell and allow exit of auxin only on that side. • When light strikes a coleoptile from one side, the auxin moves to the shaded side, growth on that side is increased, and the seedling bends towards the light. • If a shoot is tipped over, auxin moved to thelower side and causes more rapid growth there. The seedling bends upward.
Auxin Affects Plant Growth and Form • Auxin affects plant growth in many ways: • Initiating root growth • Inhibiting leaf abscission • Maintaining apical dominance • Promoting stem elongation and inhibiting root elongation • Controlling fruit development
Auxin Affects Plant Growth and Form • Shoot cuttings of many plant species develop profuse roots when the cut surfaces are dipped into an auxin solution. • This observation suggests a role for auxin in the initiation of lateral roots. • Commercial rooting powders usually contain synthetic auxin.
Auxin Affects Plant Growth and Form • Apical dominance is the tendency for lateral buds to remain dormant. Apical buds inhibit the growth of lateral buds. • Removing apical buds stimulates lateral bud growth. • If auxin is applied to the cut surface in place of the apical bud, the lateral buds are inhibited.
Auxin Affects Plant Growth and Form • Synthetic auxins have been produced and studied. • One of them, called 2,4-D, is lethal to eudicots at concentrations that are harmless to monocots. • This auxin has been used as a selective herbicide on lawns—grasses are monocots, and most of the “weeds” in lawns are eudicots. • 2,4-D takes a long time to break down, however, so it pollutes the environment.
Auxin Affects Plant Growth and Form • Auxin stimulates stem elongation but inhibits root elongation. Why different organs respond differently to the same hormone is a subject of current research. • In many species, treatment of unfertilized ovaries with auxin or gibberellins causes fruit formation. • This process is called parthenocarpy and is useful in the production of seedless fruits.
Gibberellins • Location: meristems of apical buds and roots, young leaves, embryo • Function: germination of seed and bud; stem elongation; leaf growth; flowering (bolting); fruit development; root growth and differentiation
Cytokinins • Zeatin • Location: roots (and actively growing tissues) • Function: root growth and differentiation; cell division and growth; germination; delay senescence (aging); apical dominance (w/ auxin)
Daily and Seasonal Responses • Circadian rhythm (24 hour periodicity) • Photoperiodism (phytochromes) • 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
Phytochromes • Plant pigment that measures length of darkness in a photoperiod (red light) • Pr (red absorbing) 660nm • Pfr (far-red absorbing) 730nm