Download Unit 4 - Lesson 7 - Plant Growth Regulators

Grade 11 College Biology – Unit 4
Factors affecting Plant Growth – Plant Growth Regulators
Auxins
• Auxins are the best known growth regulators. They are found in all plant tissue. Indole-3-acetic
acid (IAA) is the most common auxin.
• Different concentrations have different effects, and different cells respond differently to same
concentration of auxins
• Many functions –
• Primarily involved in cell elongation
• e.g., phototrophic response – When light shine on one side of the growth, auxins
move to the shaded side to stimulate elongation of the cells on the shade side
causing the growth to bend towards the light
• e.g., geotrophism in root and stem – In stem cells, auxins concentrate on the
lower side causing the cells on the bottom to elongate and the stem to bend
upwards. In the root, higher concentrations of auxins on the downward side
inhibit cell elongation, and elongation of cells at the top cause the root to bend
downward.
• Stimulates apical bud growth, as well as growth and ripening of fruit
• Inhibits lateral bud growth
• Regulates cell division in the vascular cambium, as well as dropping of fruit and leaves
• Regulates lead abscission, root formation and fruit growth
•
Synthetic auxins cause unwanted plants to grow very fast so they exhaust their carbohydrate
supply and die (e.g., 2, 4-D or 2, 4-dichloro-phenoxyacetic acid – commonly used to eliminate
broad leafed plants such as dandelions).
Gibberellins
• Best known for promoting cell division and elongation in plant shoots
• 40 different types have been found
• Cause shoot to elongate just before the plant flowers (called BOLTING) so a long stem is
produced to raise the flower up for pollinators and wind seed dispersal
• Stimulates seed germination (e.g., added to barley malt in brewing beer to promote synthesis of
enzymes in the protein layer of the barley endosperm. The enzymes break down the starch,
causing release of energy-rich sugars that nourish the growing barley embryos and the growing
yeast).
•
Used commercially to stimulate seed germination
Cytokinins
• Hormones that stimulate cell division and leaf mesophyll growth
• Concentrated in the endosperm tissue and young fruit
•
Used synthetically in field of biotechnology
Ethylene (CH2=CH2)
• Role in fruit ripening. Many fruits begin to produce ethylene gas just before the rise in cellular
respiration that corresponds to –
• The mass conversion of starch to sugar (increases sweetness of the fruit)
• Colour change on surface of fruit causing chlorophyll to break down and new red and
orange pigments to form
• The softening of fruit tissue by breaking down the substance holding adjacent cells
together.
• Interacts with other growth hormones
• To keep fruit from ripening, keep it in air and temperature conditions that keep ethylene
concentrations low
• To ripen fruit, do the opposite
Abscisic Acid
• Inhibitor of growth
• Promotes the closure of stomata, inducing seed and bud dormancy, and provides resistance to
water stress
NOTE: Environmental conditions will determine the amounts of regulators that a plant will
synthesize, as well as the plant tissues’ sensitivity to the regulator. This ensures the
plants will respond to the environment in a way that ensures the plant’s survivial
External Regulation of Plant Growth
Geotropism
• Regardless of the orientation of the seed, the seedling root grows downward and the seedling
shoot grows upward. The growth of the root toward gravity is POSITIVE GEOTROPISM, while
the growth of the seedling away from gravity is NEGATIVE GEOTROPISM.
• To grow downward from a horizontal position, the cells of the root on the upper side elongate
faster that the cells on the lower side. Microscopically, starch-containing plastids called
AMYLOPLASTS and the endoplasmic reticulum are always found on the side of the cells of the
root cap toward the direction of gravity regardless of the orientation of the root suggesting these
organelles have some involvement with the perception of gravity.
Phototropism
• The bending of plants towards light is called PHOTOTROPISM. The COLEOPTILE is very light
sensitive. When it pushed its way through the seed, it immediately bends towards the source of
light. The coleoptile is able bend because the cells of the side away from the light elongate more
than the cells facing the light.