Lesson 4: Plant Growth and Development Flashcards
Plant Growth
The growing parts of angiosperms
are called meristems.
In eudicot plants, there are two
types of meristem.
Apical Meristem – found at the
tip of the root and the shoot
Lateral Meristem – found in
the vascular bundles of the
stem.
Primary Plant Growth: Apical Meristem
Growth in the main stem occurs as cells divide and by mitosis.
These new cells provide the tissues required to elongate the stem and
form new leaves.
This allows plants to grow upwards from the soil towards the light.
Growth in the root occurs as cells divide by mitosis.
This allows the root to extend further into the soil
Secondary Plant Growth: Lateral Meristem
Secondary plant growth occurs at lateral meristems that are found in the
vascular cambium.
Secondary growth makes stems and roots thicker.
Side growth may develop from the main stem as shoots or branches
If the apical meristem is damaged, the first lateral meristem will take over
its role.
If a flower on the apical meristem is cut off, the lateral meristems will
switch to producing flowers.
Growth Regulation in Plants
Plants can alter their growth in response to stimuli in the environment. This
is known as tropism.
Plant responses that are not growth related are called nastic responses.
The most easily observed tropism is the bending of plants to light
(phototropism).
The opening and closing of flowers is a nastic response to light. (photonasty)
Phototropism
Plants will direct the their growth towards the sunlight in order to maximize photosynthesis.
Cells on the dark side of the plant will elongate more rapidly than those on the bright side causing
the plant to curve rather than grow
straight upwards.
Plant Hormones Regulate Growth and
Development
There are five major types of plant hormones
Auxins
Cytokinins
Gibberelins
Abscisic Acid
Ethylene
Auxins
Observations of phototropism led to the discovery of plant hormones, Auxins
being the first.
Auxins regulate stem elongation, cell differentiation, branching, development
of fruit, apical dominance, phototropism and gravitropism.
Auxins are found in embryos of seeds and in apical meristems.
Auxins influence and coordinate the development of plants by influencing the
expression of genes.
Auxins may activate or inhibit genes.
Auxin affects both cell division and cell enlargement.
In a shoot auxin promotes the elongation of cells at the shoot apex
Auxin is not produced in all cells and has to be moved to where it is needed.
Auxin generally moves form roots to shoot but can move from one side of
the shoot to the other using polar auxin transport.
Movement only occurs in one direction and is controlled by auxin efflux
pumps (protein channels unevenly distributed in the plasma membrane).
This uneven distribution causes auxin build up in certain areas of the plant.
Auxins Control
Phototropism
Auxin moves to the side of the plant that is
not exposed to light.
This uneven auxin distribution causes the cells
on the shaded side to elongate faster, thereby
bending the shoot towards the light.
Auxin and Apical
Dominance
Growth of the shoot apex (terminal shoot) usually inhibits the
development of the lateral buds on the stem beneath. This
phenomenon is called apical dominance.
Apical dominance seems to result from the downward transport
of auxin produced in the apical meristem.
If the terminal shoot of a plant is removed, the inhibition is lifted,
and lateral buds begin growth.
Cytokinins
A group of hormones that promotes cell
division, stimulate germination and delay
aging.
Made in the roots and transported
throughout the plant.
Cytokinins interact with auxins to stimulate
cell division and differentiation.
In the absence of cytokinins, a piece of
parenchyma tissue grows large in
culture, but the cells do not divide
If the ratio of cytokinins and auxins is
balanced, then the mass of growing
cells, called a callus, divides but remains
undifferentiated
If cytokinin levels are raised, shoot buds
form from the callus; if auxin levels are
raised, roots form
Gibberellins
Gibberellins promote seed
germination, stem elongation
and leaf growth and stimulate
flowering and fruit development.
Found in embryos and meristems
of apical buds and roots.
Abscisic Acid
Abscisic acid (ABA) inhibits growth,
closes stomata during low water
conditions and maintains dormancy.
Found in leaves, stems, roots and green
fruits.
ABA counteracts the action of growth
hormones, the ratio of ABA to growth
hormones determines the growth
outcome of the plant.
ABA controls seed dormancy to ensure
that germination occurs during
favourable conditions.
Ethylene
Ethylene promotes fruit ripening.
Ethylene is produced in the fruit
and works by positive feedback.
Ripening involves the
breakdown of cell walls and
the conversion of starches to
sugars.
Ethylene is a gas so the ripening
effect can spread from fruit to
fruit.
