Transcript Resource Acquisition and Transport in Vascular Plants

Topic 9: Plant
Science
Heirarchy
• Plants are still
composed of cells,
tissues, organs and
organ systems
• 3 major plant organs
– Roots (root system)
– Stems (shoot system)
– Leaves (shoot
system)
Roots
• Roots have a variety of
jobs, including:
• Anchoring to the soil,
• absorbing minerals and
water,
• often storing
carbohydrates
• Taproots often hold
carbohydrates for use by the
plant during flowering
Roots
• Types
– Taproot: penetrate deep into
the soil. This is a “main root”
with lateral roots growing off
from the main
– Adventitious roots: tend to
have a fibrous root system
with no one main root.
These tend to be seen in
shallow soils
– Also a variety of modified
roots: prop roots, storage
roots, aerial roots, buttress
roots, pneumatophores
Stems
• Stem jobs include: growth,
food storage and asexual
reproduction
– Some of these jobs are
represented in modified stems
such as Rhizomes, Bulbs,
Stolons, and Tubers
• A stem is made up of nodes
(point at which leaves are
attached) and internodes
(space in between)
Stems
• The angle between each
leaf and a stem is a region
known as the axillary bud.
The axillary bud can give
rise to a branch.
– Most axillary buds are
dormant on young plants
because elongation is
concentrated at the shoot
tip (aka apical bud or
terminal bud)
Leaves
• Main photosynthetic organ in most
vascular plants
• The petiole joins the leaf to the
stem at the node
• There can be several different
arrangements of leaves: simple,
compound, doubly compound
– Veination will also vary (monocot /
eudicot)
• Modified leaves often act in
defense or reproduction, or may
protect against desiccation
– Tendrils, spines, storage leaves,
reproductive leaves, bracts
Plant Tissues
• Dermal Tissue – outer
covering
– Ex: epidermis, peridermis
(woody plants) or cuticle
• Vascular Tissue – carries
out long distance
transport of materials
– Ex: xylem, phloem, stele
• Ground Tissue – part of
neither the dermal or
vascular tissue
– Ex: Pith (internal to
vascular), cortex (external
to vascular)
Plant Cells
• Parenchyma
– “typical” plant cell with large vacuole
• Collenchyma
– offer support without limiting growth
– often found in young parts of plant
• Sclerenchyma
– Offer support, but cannot elongate
– Occur in parts of the plant that are
not growing
• Tracheids and vessel elements
– Water conducting of xylem
– Dead at functional maturity, leaving
behind the supportive cell wall
• Sieve Cells
– Sugar Conducting cells of phloem
– Alive, but lacking many cell parts
(such as nucleus, ribosomes, etc.)
allowing for easier passage of
materials
Meristems
• Plants have indeterminate
growth, meaning they grow
throughout their lives
• This is possible because of
meristems (embryonic
tissues)
– Apical (primary growth) –
located at the tips of roots
and shoots and in axillary
buds
– Lateral (secondary growth) –
growth of vascular cambium
and cork cambium
Primary Growth
(Roots)
– Root cap protects the
apical meristem and
secretes a polysaccharide
slime to lubricate the soil
– 3 zones above the root tip
• Zone of cell division
– New root cells produced
• Zone of elongation
– Cells increase in size
• Zone of differentiation
– Cells become distinct cell
types
Primary Growth
(Shoots)
• Leaf primordia give rise to leaves
along the side of the apical
meristem. Axillary buds then
develop from the remains of the
apical meristems.
• Stems – arrangement of vascular
tissue dependent on monocot
(scattered) or dicot (ring)
• Leaves
– stomata and guard cells interrupt the
epiderm
– Mesophyll is ground tissue of the leaf
• Dicots have palisade and spongy
mesophyll
Monocots vs. Dicots
Resource Acquisition and
Transport: Overview
• Leaves:
– CO2 is taken and O2 leaves through
stomata
– Sugars are produced
– Transpiration causes xylem sap to
be pulled upward
• Roots:
– Gases are exchanged through soil
– Water and minerals are absorbed
• Stem:
– Xylem transports water and
minerals from roots to shoots
– Phloem moves sugar from sites of
production to storage (can move up
or down within plant)
Shoot Architecture
• Leaf size is dependent on environmental conditions
– Ex: Largest leaves are found in tropical rain forests; smallest are
found in cold or dry areas
• Phyllotaxy (arrangement of leaves) is related to sun
exposure for the leaf
– May be to increase sun exposure or to allow shade in certain
environments
• Leaf orientation may be horizontal or vertical for similar
reasons
Root Architecture
• Plants with taproots tend to be taller because of the
strength of the root system in anchoring.
• Approximately 80% of extant land plants are involved in
mutualistic mycorrhizae relationships (roots and fungi)
– Better able to absorb water and minerals (esp. phosphate)
Transport: A Review
• Solutes can be moved from
cell to cell via several
methods both actively and
passively
Osmosis and Water
Potential
• In animal cells, water moves from an area of
lower solute concentration to an area of
higher solute concentration
• In plant cells, have to also take into account
the physical pressure of the cell wall pushing
back against protoplast
– This is water potential (both solute concentration
and physical pressure)
– Free water (not bound to surfaces or solutes)
moves from regions of higher water potential to
regions of lower water potential
Water Potential
• Ψ=ψs+ψp
– ψS= solute potential, always negative unless pure water (0)
• Those water molecules that are bound to solute have less potential to move
and do work
– Adding solute always lowers water potential
– ψP= pressure potential, can be positive or negative
Pathways of Transport
• Water generally moves
using the apoplastic,
symplastic or
transmembrane routes
– Apoplastic: water and solutes
move along the continuum of
cell walls and extracellular
space
– Symplastic: water and solutes
move along the continuum of
cytosol within the cell tissue
– Transmembrane route: water
and solutes move out of one
cell, across the cell wall and
into the neighboring cell
Ascent of Xylem Sap
• Xylem sap is pulled
upward via
transpiration
• Cohesion aids in this
by keeping water
molecules sticking
together
• Adhesion aids in this
by water sticking to
the xylem walls,
offsetting gravity
Stomata
• Stomata are the major source of water loss in
plants
– Mechanisms that control when they open and close can
help to control this water loss
– Solute concentration and water movement control the
guard cells which open and close the stomata
• Open because of light, CO2 depletion and an internal
clock in the guard cells
• May close because of environmental conditions (drought)
or hormonal reactions
Phloem
• Sucrose/Sugars
will go from their
source (where
they are made) to
a sink (where they
are stored) until
they are needed
• Movement is
caused by positive
pressure (sugar
concentration is
higher at sources
than at sinks)
Transpiration
• Levels of transpiration will vary depending on
environmental conditions
– This includes light, humidity, wind & temperature
• Example related to extremes is xerophytes
– These are plants adapted to live in dry environments
• Tend to have specially adapted leaves (waxy, spines,
smaller or fewer stomata)
• Many use a different metabolic pathway for
photosynthesis (C4 or CAM)
Angiosperm Life Cycle
•Angiosperms have structures unique to their life
cycles, including flowers and fruit
•Flowers allow for fertilization to occur which then give
rise to fruit that includes seeds
•Seeds then give rise to the next generation
Development of
Gametophytes
•Males:
–Pollen grains develop within
the microsporangia of
anthers via meiosis
–Becomes mature when it
generative nucleus divides,
forming 2 sperm (generally
when it lands on the stigma)
•Females:
–Embryo sac develops within
an ovule, enclosed by the
ovary at the base of the
carpel
–Megasporocyte divides via
meiosis, generally only 1
survives as the megaspore
–Megaspore undergoes 3
mitotic divisions to form
embryo sac
Flower Pollination
•Flowers can be
pollinated in a
variety of ways
including
–Abiotic: Wind,
Water
–Biotic: Bees,
Moths and
butterflies, Flies,
Birds, Bats
Seed Development
•As seeds develop, they will
include certain parts including
endosperm, cotyledons and the
embryo. The exact structure
varies between monocots and
dicots
•Late in development, the seed
dehydrates and enters
dormancy
•The seed will exit dormancy
when conditions are right for
development
Germination and
Development
•To germinate, the seed must
imbibe water, expanding and
breaking the seed coat. This
also triggers metabolic changes
that resume growth.
•Other needs for germination
include oxygen, particular
temperatures, and the presence
of gibberellin, a plant hormone
that stimulates mitosis.
•The first organ to emerge is the
radicle, followed by the shoot
breaking through the soil
surface.
Fruit Development
•As seeds develop
from ovules, fruit
develops from the
ovary
•Fruits and seeds
are then dispersed in
a variety of ways
including wind, water
and animals
Plant Hormones
Auxin
• Auxins are related to a
variety of plant activities
including:
– Cell elongation
– Lateral and adventitious
root formation
– Increased cambrial
activity related to
secondary growth
– Fruit growth
– Can also be used as
herbicides via hormonal
overdoes
Cytokinins
• Cytokinins are
related to
– Control of cell
division and
differentiation
– Control of apical
dominance
– Anti-aging effects
Gibberellins
• Gibberellins are
associated with:
– Stem elongation
– Fruit growth
– Germination
Brassinosteroids
• These are also
related to growth
• Were originally
thought to be an
auxin
• Chemically
similar to
cholesterol and
sex hormones
Abscisic Acid
• ABA is responsible
for slowing growth
• Also responsible for
seed dormancy.
Often ABA must be
removed to allow
for germination to
occur
Ethylene
• Ethylene plays a role in a variety
of important events
– Triple response to mechanical
stress
– Senescence
– Leaf Abscision
– Fruit Ripening
Light and Plants
• Plants can detect the direction,
intensity and wavelength of
light
• Reception of light allows plants
to measure the passage of
days and seasons
• Red and blue light are
important in
photomorphogenesis; thus,
there are two photoreceptors
– Blue-light photoreceptors
• Phototropism, light-induced
opening of stomata, light-induced
slowing of hypocotyl elongation
– Phytochromes
• Seed germination, shade
avoidance
Light and Plants
• Plant processes vary throughout the day in response
to variation in light, temperature and relative humidity
– In perfectly maintained conditions, some processes still
occur on a cycle of 21-27 hour frequency, such as opening
and closing of stomata and “sleeping” in legumes
– These are circadian rhythms
Light and Plants
• Short Day vs. Long Day Plant:
– Short: flowers when the dark
period is long enough. If the dark
period is interrupted, the plant will
not flower.
– Long: flowers when the light
period is long enough. If the dark
period is interrupted, the plant will
flower even if the light period is not
long enough.
• If red light and far red light are
used to effect phototropism, far
red light will offset the affects of
red light