Exam 4: Chapter 25

Plant Responses

cannot get up and move away from a predator or poor environment

• If a resource becomes available, plants have to grow towards it

Two ways plants get larger

Cell division and Cell elongation

All growth is regulated by

hormones

Auxins

stimulate elongation, division, and differentiation

Promote elongation of cells in

coleoptiles and stems

Heavily concentrated in (Auxins)

Shoot and root apex, Expanding leaves and buds, and Developing seeds and fruits

Auxin Effects on Whole Plant

influence growth, cell differentiation, and responses to light and gravity

Apical dominance driven by

auxin production

If apical shoot has poor dominance,

Then the plant can have “poor” form

Polar transport

from tip to base of auxin within the plant

Gibberellins stimulate

elongation

High concentrations in (Gibberellins)

Stem and root apex, Buds, embryos, cotyledons, Flowers and fruits, Vascular cambium

Promote stem elongation help seeds and buds

break dormancy

Effects of Gibberellins

include promoting stem elongation, breaking seed dormancy, and influencing flowering

Cytokinins

stimulate division

• Inhibits elongation

Synthesized mostly in (Cytokinins)

root tips, also in embryos, young fruits, young leaves, root tips

Stimulate cell division and retard leaf aging

Can stimulate auxin production, Inhibits elongation, but stimulates enlargement

Ratios between cytokinins and auxins will determine the

differentiation of tissue within the plant

Ethylene

gas stimulates aging, senescence

Ethylene promotes

fruit ripening, senescence and abscission

Modifies effects of auxin on growing organs

Inhibits stem and root elongation

Abscisic acid

is a general inhibitor of growth

Abscisic Acid Counteracts auxins and gibberellins

at higher concentrations it promotes seed dormancy and stress responses

Promotes stomatal closure (Abscisic Acid)

May trigger seed and bud dormancy and also promotes abscission

Roots experiencing drought produce abscisic acid and load into xylem (Abscisic Acid)

Signals stomata in leaves to close, reduce transpiration

All growth responses to the environment rely on

hormone production

Some environmental responses/movements are not

related to growth or hormones

Tropisms

are directional growth related to an external stimulus

Nastic movements

are non-directional and not related to an external stimulus

Phototropisms

Responses to light

• Growth responses to directional light source

Blue light receptors trigger auxin transport

Auxin triggers differential cell elongation

Gravitropism

Orients plant parts to the pull of gravity

Mechanical stimulus (Gravitropism)

from amyloplasts (statoliths)

• Triggers auxin transport

• Differential elongation

Stems (Gravitropism)

negative gravitropism as they grow upwards away from gravity.

Roots (Gravitropism)

positive gravitropism as they grow downwards towards gravity.

Thigmotropism

Responses to physical contact

• Usually see it with stems or tendrils

  Cells in contact with the object shorten, opposite cells elongate - Not clear if auxin gradients are involved

how roots navigate around rocks in soil

Thigmotropism

Photoperiodism

Responses to change in relative length of daylight and darkness

Photoperiodism also allows

biological clock to be “reset” for seasonal adjustments to day length

Photoperiodism is controlled by

phytochrome pigment

Phytochrome

Converts to active form (Pfr) during daylight, or when exposed to red wavelengths and helps plants detect changes in light duration

Phytochrome physically interacts with

phytochromeinteracting factors (PIFs) to regulate gene expression in response to light conditions

Nastic Movements

Reversible responses to nondirectional stimuli

• Movements in leaves, leaflets, flowers