Download Chapter 39 - Kohli Science

39.1- Plant Hormones
Hormones
Where its produced or found in plant
Major Functions
Auxin (IAA)
Embryo of seed, meristems of apical buds,
young leaves
Cytokinins
Synthesized in roots and transported to other
organs
Gibberellins
Meristems of apical buds and roots, young
leaves, embryo
Brassinosteroids
Seeds, fruit, shoots, leaves, and floral buds
Abscisic acid
Leaves, stems, roots, green fruit
Ethylene
Tissues of ripening fruit, nodes of stems, aging
leaves and flowers
Stimulates stem elongation (low
concentration only), root growth, cell
differentiation, and branching; regulates
development of fruit; enhances apical
dominance; functions in phototropism
and gravitropism; promotes xylem
differentiation
Affect root growth and differentiation;
stimulate cell division and growth;
stimulate germination; delay senescence
Promote seed and bud germination, stem
elongation, and leaf growth; stimulate
flowering and development of fruit; affect
root growth and differentiation
Inhibit root growth; retard leaf abscission;
promote xylem differentiation
Inhibits growth; closes stomata during
water stress; promotes seed dormancy
Promotes fruit ripening, opposes some
Auxin effects; promotes or inhibits
growth and development of roots, leaves,
and flowers, depending on species
39.2- Plant hormones help coordinate growth, development, and responses to stimuli
 Hormones are defined as chemical messengers that coordinate the different parts
of a multicellular organism. They are produced by one part of the body and
transported to another.
 A tropism is a plant growth response that results in the plant growing either
toward or away from a stimulus.
 Phototropism is the growth of a shoot in a certain direction in response to light.
Positive phototropism is the growth of a plant toward light; negative
phototropism is the growth of a plant away from light.
39.3- Responses to light are critical for plant success
 Photomorphogenesis is the term used to describe the effects of light on plant
morphology.
 Blue light has the greatest effect on plant growth and movement. Plants use three
different pigments to detect blue light; cryptochromes, phototropin, and
zeaxanthin.
 Phytochromes are pigments that are involved in many of a plant’s responses to
light.
 Many plant processes are in response to changes in light, temperature, and
humidity.

Circadian rhythms are physiological cycles that have a frequency of about 24
hours and that are not paced by a known environmental variable.
 A physiological response to a photoperiod (the relative lengths of night and day),
such as flowering, is called photoperiodism.
 Short-day plants require a long period of light shorter than a certain critical
length in order to flower. Long-day plants flower in late spring or early summer;
they require the most daylight to flower. Day-neutral plants can flower in days
of any length. (Note: It is night length—not day length—that controls flowering
and certain other responses to photoperiod.
39.4- Plants respond to a wide variety of stimuli other than light
 Gravitropism is a plant’s response to gravity. Roots show positive
gravitropism, and grow toward the source of gravity, whereas shoots show
negative gravitropism and grow away from gravity.
 Thigmomorphogenesis is the change in form of a plant as a response to touch.
 Plants have various responses to stresses. In times of drought, the guard cells lose
tugor. This causes stomata to close; young leaves will stop growing, and the will
roll into a shape that slows transpiration rates. Also, deep roots continue to grow,
while those near the surface (where there isn’t much water) do not grow very
quickly.
 In times of flooding, certain cells in the root cortex die, which creates are tubes
that bring in oxygen and enable the plant to continue cellular respiration.
 Plants respond to salt stress by producing organic compounds that keep the water
potential of cells more negative than that of the soil solution. Most plants cannot
survive salt stress for very long periods of time.
 In heat stress environments, plants produce heat-shock proteins, which are
thought to prevent other proteins from denaturing.
 In cold stress situations, plants respond by altering the composition of their cell
membranes.
39.5- Plants defend themselves against herbivores and pathogens
 Some physical defenses plants have against predators (herbivores) are thorns,
chemicals such as distasteful or poisonous compounds, and airborne attractants
that attract other animals to kill the herbivores