Chapter 11:
Growth / Hormones
In this chapter you will learn about
growth phenomena with a
discussion of the distinctions among growth, differentiation, and development.
This is followed by a discussion of plant hormones (auxins, gibberellins,
cytokinins, abscisic acid, ethylene) and their roles in plant growth and
development. The chapter explores plant movements, including spiraling, twining,
contraction, nastic, tropic, turgor, taxic, and miscellaneous other movements.
The discovery and functions of phytochrome and photoperiodism are briefly
surveyed. The chapter concludes with a discussion of the relationship of
temperature to growth and dormancy.
At the end of this chapter the successful student will be able to
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- Nutrients -
- Substances that furnish the elements and energy necessary to produce
organic molecules.
- Vitamins -
- Most are organic molecules of varied structure that participate in
catalyzed reactions, mostly by functioning as an electron acceptor or donor.
- Synthesized in cell membranes and cytoplasm.
- Genes also dictate the production of hormones.
- Because some effects of vitamins are similar to those of hormones, they
are sometimes difficult to distinguish.
- The term Growth Regulator has been applied to compounds that have
effects on plant development similar to those of hormones and vitamins.
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| Summary of Functions of Major Plant Hormones |
| Hormone | Function | Location |
| Auxins (IAA)* | stem elongation
apical dominanace
root formation | produced in shoot
apical meristem |
|
Cytokinins | cell division
differentiation | produced in roots |
|
Gibberellins (GA)* | stem & intemode
elongation
seed germination | produced in apical portion of root & shoot |
|
Ethylene* | abscission
fruit ripening | produced in leaves, stems & young fruits |
|
Abscisic Acid | supression of bud growth
stomatal opening
leaf senescence | mature leaves, fruits & root caps |
|
*most horticultural/ agricultural applications |
-
- Auxin production occurs mainly in apical meristems, buds, and young
leaves.
- Plant response varies according to concentration, location, and other
factors
- Promotes cell enlargement, stem growth, and delays development processes
such as fruit and leaf abscission and fruit ripening.
- Plant Hormones
Movement of auxins from the cells where they originate requires energy
expenditure.
- Movement is polar.
- Several Form
- Indoleacetic Acid (IAA)
- Phenylacetic Acid (PAA)
- 4-chloroindoleacetic Acid (4-chloroIAA)
- Indolebutyric Acid (IBA)
-
- Named after the fungus that produced it (Gibberella fujikuroi).
- Most GA produced by plants are inactive, apparently functioning as
precursors to active forms.
- Most dicots and a few monocots grow faster with an application of GA.
- Gibberellins are involved in nearly all the same regulatory processes in
plant development as auxins.
- Appears to lower the threshold of growth.
- Several commercial growth retardants can be used to block GA synthesis.
- Click here to interact with the Gibberellin
mode of action
-
- Regulate cell division.
- Synthesized in root tips and germinating seeds.
- If present during the cell cycle, cytokinins promote cell division by
speeding up the progression from the G2 phase to the mitosis phase.
- Can prolong the life of vegetables in storage.
-
- Has inhibitory effect on the stimulatory effects of other hormones, and
thus on plant growth.
- Synthesized in plastids.
- Particularly common in fleshy fruits.
- Has little influence on abscission.
-
- Produced by fruits, flowers, seeds, leaves, and roots.
- Produced from amino acid methionine.
- Used to ripen green fruits.
- Production almost ceases in the presence of oxygen.
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- Apical Dominance
- Apical dominance is the suppression of the growth of lateral or axillary
buds.
- Believed to be brought about by an auxin-like inhibitor in a terminal
bud.
- If cytokinins are applied in appropriate concentration to axillary
buds, they will begin to grow, even in the presence of a terminal bud.
- Senescence
- Senescence is the breakdown of cell components and membranes, eventually
leading to the death of the cell.
- Some studies have suggested certain plants produce a senescence
factor.
- Other Interactions
- Root and shoot development regulated by auxins and cytokinins.
- Seed germination regulated by gibberellins.
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- Movements From Internal Stimuli
- Nutations - Slight spiraling
- Nodding - Side-to-side oscillations
- Twining - Very defined spiraling
- Contraction - Contractile roots
- Nastic - Non-directional
- Epinasty - Permanent downward bending
- Movements From External Stimuli
- Tropisms can be divided into three phases:
- Initial Perception
- Transduction
- Asymmetric Growth
- Phototropism:
Click here to see a demonstration of
phototropism
- Positive - Growth towards a light source.
- Negative - Growth away from a light source.
- Different light intensities bring about different phototrophic
responses.
- Gravitropism
- Growth responses to the stimulus of gravity.
- Primary plant roots are positively gravitropic, while shoots forming
the main axis are negatively gravitropic.
- Other Tropisms
- Thigmotropism - Physical Contact.
- Chemotropism - Chemicals
- Thermotropism - Temperature
- Traumotropism - Wounding
- Electrotropism - Electricity
- Skototropism - Dark
- Aerotropism - Oxygen
- Turgor Movements
- Turgor movements result from changes in internal water pressures and are
often initiated by contact with objects outside of the plant.
- Turgor contact movements are not confined to leaves.
- Many flowers exhibit movements of stamens and other parts
facilitating pollination.
- Circadian Rhythms
- Members of the Legume Family exhibit movements in which leaves or petals
fold in regular daily cycles.
- Fold in the evening and unfold in the morning.
- Controlled by a biological “clock” on approximately 24 hours cycles.
- Appear to be controlled internally.
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Photoperiodism refers to the fact that day length is directly related to the
onset of flowering in many plants.
- Short-Day Plants will not flower unless the day
length is shorter than a critical period.
- Long-Day Plants will not flower unless periods
of light are longer than a critical period.
- Intermediate-Day Plants will not flower
if the days are too short, or too long.
- Day-Neutral Plants - Will flower under any
day-length, provided they have received the minimum amount of light necessary
for normal growth.
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- Phytochromes - Pale blue proteinaceous pigments associated with
light absorption.
- Two stable forms:
- Pr - Absorbs red light.
- Pfr - Absorbs far-red light.
- When either form absorbs light, it is converted to the other form.
- Cryptochromes - Blue, light-sensitive pigments that play a role in
circadian rhythms and help control reactions to light.
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- Temperature and Growth
- Each plant species has an optimum temperature for growth which may vary
with a plant’s growth stage, and a minimum temperature, below which growth
will not occur.
- Lower night temperatures often result in higher sugar content in
plants and may also produce greater root growth.
- Growth of many field crops is roughly proportional to prevailing
temperatures.
- Dormancy and Quiescence
- Dormancy - Period of growth inactivity in seeds, buds, bulbs, and
other plant organs even when environmental requirements are met.
- Quiescence - Sate in which a seed cannot germinate unless
environmental conditions normally required for growth are present.
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Last modified:
October 08, 2004 by
Cynthia Herbrandson 
© Copyright 1999, Kellogg Community College.
All rights reserved.