Plant Growth and Development

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Plant Growth and Development

• SBI3U

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Plant growth regulators (PGRs) plant hormones
that affect the rate of division, elongation and
differentiation of plant cells Five
well-characterized groups of PGRs auxins,
gibberellins, cytokinins, abscisic acid and
ethylene
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Auxins

• Produced in plants at the apical meristem
• Cause apical dominance plant grows upward with
  few to no side branches

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Auxins promote elongation of cells, stimulate
growth and ripening of fruit, and also inhibit
the dropping of fruit and leaves. Synthetic
auxins can be used to stimulate growth of fruit
from unpollinated plants, resulting in, for
example, seedless tomatoes.
Seedless tomatoes!
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Gibberellins promote cell division and
elongation. Sometimes they cause a process
called bolting where the stem of a plant rapid
grows taller before flowering. Many dwarf
varieties of plants stay small because they do
not produce normal levels of gibberellin.
Lettuce bolting after it has been treated with
gibberellins.
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Cytokinins stimulate cell divison and leaf
growth. Commercially, cytokinins are used to
extend the life of cut flowers.
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Abscisic acids main role is to coordinate
responses to stress in plants. Usually it
inhibits growth. It can induce dormancy of seeds
to protect them from harsh conditions and also
regulates rate of transpiration.
Maple tree seeds. Some seeds can remain dormant
for years and still be viable in newfound
hospitable conditions.
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Ethylene is a gas produced in many of the plants
tissues. It plays a role in flower death, fruit
ripening and fruit loss. Commercially, growers
can delay ripening of fruit until they are about
to be sold by controlling the levels of ethylene
in the fruit.
Tomatoes before ripening. A spray of ethylene
would have these ripe and red in no time.
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External Factors that Regulate Plant Development
Tropism a change in the growth pattern or
movement of a plant in response to an external
stimulus
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Nastic Response

• Plant response to a stimulus independent of the
  direction of the stimulus
• Examples
• Flower opening petals during the day and closing
  them at night
• Mimosa plant
• http//www.youtube.com/watch?vg0LFBM3hOLs

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Thigmotropism a change in the growth pattern or
movement of a plant in response to touch For
example, the tendrils of pea plants will grow
around a supporting structure at the point of
contact.
http//www.youtube.com/watch?vdTljaIVseTc
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Phototropism a change in the growth pattern or
movement of a plant in response to light For
example, a plant growing in uneven light will
lean and bend its stem in order to become better
exposed to the light.
http//www.youtube.com/watch?vzHe7y8cy-7Y
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Gravitropism a change in the growth pattern or
movement of a plant in response to gravity If
plants could not detect and respond to gravity,
their roots would be as likely to grow up into
the air as down into the ground.
This plant has been left to grow upside down. You
can see in the picture that the stem is trying to
grow back upwards and fight gravity.
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Photoperiodism plants in regions where there
are two or more seasons control their
reproductive cycles based on the the duration of
daylight, a.k.a the photoperiod. Plants flowering
in the spring means the most time available for
seed and fruit development before winter. Trees
dropping their leaves in the fall means
protection from the cold, dry conditions of
winter.
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NUTRIENTS IN ORDER OF PRIORITY

• CO2 and H2O ? needed for photosynthesis!
• N2 (from organic material) ? needed to produce
  proteins, nucleic acids, chlorophyll and leaf
  growth
• NH3 from animal waste
• NO2 and NO3 from decayed materials
• Nitrogen fixing bacteria contain N2 gas and
  convert it to nitrates
• Others... (next slide)

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NITROGEN CYCLE (DONT MEMORIZE!)
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OTHER ESSENTIAL NUTRIENTS
Nutrient Function Deficiency Symptoms
K Water balance, protein synthesis Poor growth, weak stems, yellowing leaves
P ATP (energy) syntesis, mitosis, cell division Stunted growth, poor seed/fruit development
Ca Part of cell walls, membrane permeability Stunted growth
Mg Part of chlorophyll and coenzymes for photosynthesis Yellowing leaves
S Part of proteins Stunted growth, yellowing leaves
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FERTILIZERS
Natural Synthetic
Examples Manure, compost, sludge, crop rotation Man-made chemical cocktails that contain ammonia and other nutrients
Advantage No harmful chemicals Get exact nutrients
Disadvantage May not always get the correct amount of each nutrient Costly, prone to leaching and runoff which alters chemical balance in water and soil