Ch 39: Plant Responses to Internal and External Signals

Figure 38.4 Embryos Mobilize Their Reserves

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.

Figure 38.7 The Darwins’ Phototropism Experiment (Part 1)

Figure 38.7 The Darwins’ Phototropism Experiment (Part 2)

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 the lower side and causes more rapid growth there. The seedling bends upward.

Figure 38.10 Plants Respond to Light and Gravity

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.

Figure 38.12 Auxin and Apical Dominance

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)

Figure 38.5 The Effect of Gibberellins on Dwarf Plants

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

Figure 39.12 The Effect of Interrupted Days and Nights (Part 1)

Figure 39.15 Evidence for a Flowering Hormone (Part 1)

Figure 39.15 Evidence for a Flowering Hormone (Part 2)