Lecture 26 - Plant Nutrition (part 1) Flashcards
What did plant evolution result from?
adaptations to obtain food
What are the 2 systems that land plants have?
- above ground shoot system
2. below ground root system
above ground shoot system
absorb light for photosynthesis
below ground root system
water and minerals
Plant ancestors absorbed nutrients directly from their…
aquatic environment
Over time as plants moved onto land…
competition for resources resulted in taller plants, with broader leaves (more SA) for more light absorption
What is the cost of plants moving onto land?
More SA for light also means more SA for water loss
• Larger plants also need larger roots for anchorage
• Greater distance to transport nutrients between the root and shoot systems
What are the 2 vascular tissues that made the move to land all possible for plants?
- Xylem
* Phloem
Xylem
transports water and nutrients from roots to shoots
Phloem
transports photosynthates from source to sink
What varies greatly amongst land plants?
shoot architecture (i.e. morphology)
What are examples of shoot architecture (i.e. morphology) that varies greatly amongst land plants?
- Stem length and widths
* Branching pattern
What does stem length and widths allow for?
allows plants to grow tall
What does branching pattern allow for?
allows better light harvesting
What is the function of stems?
support leaves and act as conduits for water and nutrient transport
Leaves vary in size & structure & account for much of the diversity we see in land plants so DESCRIBE THIS
- Various adaptations for specific habitats, PREVENTING water loss and MAXIMIZING photosynthesis
- Most large leafed plants are tropical, smaller leafed plants are temperate, and very small leafed plants occur in harsh habitats
Phyllotaxy
is the arrangement of leaves on a plant
What is Phyllotaxy determined and genetically controlled by?
the SAM
1 leaf per node –>
alternate, spiral
2 leaves per node –>
opposite
Multiple leaves per node –>
whorled
What does total leaf area affect?
the productivity of each plant
How does total leaf area affect the productivity of each plant?
- Leaves can SHADE other leaves on the same plant
* Plants can SELF-PRUNE leaves (and branches!)
Leaves can shade other leaves on the same plant…
reducing photosynthetic capacity (ie. They respire more than photosynthesise!)
Plants can self-prune leaves (and branches!)…
to optimising leaf area index
Leave orientation also affects…
photosynthetic capacity
In low light –>
horizontal leaves are best
In high light –>
vertical leaves prevent water loss and burning
What is the photosynthesis - water loss compromise?
- Broad leaves absorb more light, have more stomata
* Stomata open to exchange gas, but there is water loss!
What is the roots function?
maximize morphology & growth to absorb nutrients
Roots won’t branch in areas of _______. But branches more in areas of _______.
low N availability
high nutrient loads, makes more proteins to take up more nitrogen
Roots associate with…
mycorrhizae to increase surface area of more nutrient uptake
What are the 2 main “compartments” of plant systems?
- Apoplast
2. Symplast
Apoplast
everything external to the plasma membrane (including cell walls and internal
spaces of dead tracheids and vessels)
Symplast
consists of the cytosol, plasmodesmata, & cytoplasmic interconnections
What are 3 main transport “routes”?
- Apoplastic
- Symplastic
- Transmembrane
Apoplastic
water and solutes move along the cell walls and extracellular spaces (like water through a sponge)
Symplastic
water and solutes move through the cytosol
Transmembrane
water and solutes move out of one cell, across the cell walls, and into the next cell
Both ____ & _____ transport occur in plant cells
active
passive
Primary Active Transport
Uses H+ for membrane potentials resulting from proton pumps
• Animals used Na+
Secondary Active Transport
H+ is also co-transported in plants (ex. sugar in phloem cells)
• Animals co-transport Na+
What does gated ion channels produce?
produces electrical signals similar to action potentials in animals
• K+ ions for the opening/closing of guard cells
What absorption or loss of water occurs by…
Osmosis (passive transport)
Water potential
(solute concentration + pressure) will direct the flow of water
Free water
(not bound to ions/solutes) moves from higher water potential to lower water potential
Active & passive transport in plant cells
- Uses ATP-dependent proton pumps to pump H+ OUT of the cell, establishing a membrane potential
- H+/sucrose cotransporters play a key role in sugar-loading INTO the PHLOEM
- H+/NO3 - cotransporters important for nitrate uptake BY the roots
- Opening and closing of the guard cells in the stomata
What is water potential determined by?
solute potential and pressure potential
Solute potential
is directly proportional to its molarity
• Also called osmotic pressure (solutes affect direction of osmosis)
Solutes in plants are mostly…
ions & sugars
Pure water has a solute potential =
0
As solutes are added, ions bind to water –>
less free water
• Has a negative effect on water potential, so always written negative
Pressure potential
is the PHYSICAL pressure on the solution
• Can be + or -
• Water inside plant cells is usually under + pressure due to water uptake
The protoplast (living part of the cell)…
exerts (+) pressure on the cell wall, creating turgor pressure
Water inside tracheids and vessels is often under…
negative pressure (tension)
Flaccid cells (limp)
are from a loss of water
- The cell has a pressure potential = 0
If put into a solution that has a lower water potential (hypertonic)…
water leaves the flaccid cell, resulting in plasmolysis
If put into a solution of pure water (hypotonic)…
water moves inside the cell since the cell has more solutes than the pure water, exerting turgor pressure and becoming turgid
Once the direction of water movement is determined…
aquaporins aid in the transport of water across the plasma membrane
Aquaporins
They affect the rate in which water moves through the membrane
Permeability of aquaporin channels is decreased by…
increases in cytosolic [Ca2+] or decreases in cytosolic pH
Short-distance transport occurs…
across plasma membranes
Long distance transport occurs through…
bulk flow
Bulk flow
Movement of a liquid in response to a pressure gradient
Bulk flow always occurs from…
high to low pressure
Bulk flow is independent of…
solute concentration
Where does bulk flow occur?
Occurs in tracheids and vessel elements in xylem and sieve-tube elements of the phloem
Mature xylem cells have no ______ & sieve-tube elements lack bulky _____
cytoplasm
organelles
Both kinds of transport can occur to move the same stuff around. Describe short-distance active transport & long distance transport
Short-distance active transport loads sugar into phloem cells
Long distance transport moves it from shoot to root
Healthy/not wiltered plant =
turgid
Wiltered plant =
flaccid
Where does most of the absorption of nutrients & water start?
at the root hairs
Root hairs are…
Permeable to water and its associated ions
Where does water pass freely through (during transpiration)?
the cell walls and extracellular spaces (apoplastic transport)
Pass through the cortex until it reaches the ________ surrounding the vascular stele (during transpiration)
endodermis
Selective transport
forces water + minerals into the symplastic route due to the Casparian strip
Casparian strip
highly suberized layer impervious to water
Water and minerals then returns to the ______ route in the tracheids and vessel elements
apoplastic
Once in the xylem, the xylem sap ______
move up the stem via bulk flow and into the leaf veins
Leaf cells get water and essential nutrients, but water is lost due to…
transpiration -> water vapour exits the leaf
What must the roots do continuously to counteract the loss through transpiration?
The roots must be continuously absorbing water to counteract the loss through transpiration
If water loss in the leaves > root absorption…
the plant wilts
How does xylem sap travel to the leaves?
- Root pressure pushes it up the plant
- Root cells are continuously actively pumping mineral ions in the xylem sap
- The Casparian strip prevents mineral ions for going back into the root (selectively permeable, remember?)
- Accumulation of minerals lowers water potential inside the vascular cylinder, causing water the flow from the root into the vascular stele, generating root pressure
- This pushes the xylem sap up the stem
Root pressure
pushes xylem sap up the plant
Too much pressure of xylem sap being pushed up the stem can cause what?
guttation, water droplets exude (forced out) from the leaves (NOT the same as dew)
Root pressure only accounts for a small fraction of what?
bulk transport in xylem
Cohesion-tension Hypothesis
- Transpiration provides the pull
* Cohesion of water molecules transmits this pull from shoots to roots
Transpiration (Cohesion-tension Hypothesis)
provides the pull
Cohesion (Cohesion-tension Hypothesis)
of water molecules transmits this pull from shoots to roots
Xylem sap is normally under…
negative pressure (tension)
Transpirational pull
- Stomata on the leaves lead to internal air pockets, exposing mesophyll cells to CO2 for photosynthesis
- Air is saturated with water vapour. Air outside the leaf is drier, and therefore has lower water potential than air inside the leaf
- The negative water potential develops on the mesophyll cells, resulting in tension
Stomata on the leaves lead to internal air pockets…
exposing mesophyll cells to CO2 for photosynthesis
Air is saturated with water vapour. Air outside the leaf is drier, and
therefore has…
lower water potential than air inside the leaf
• Causes water to leave from inside of leaf to outside
The negative water potential develops on the…
mesophyll cells, resulting in tension
Cohesion
attractive force between the same kind of molecules
• Very strong due to hydrogen bonds in water
• Can pull water molecules up without them separating
Adhesion
attractive force between different molecules
• Water molecules on hydrophilic cell wall molecules
Upward pull on the sap creates…
tension
Secondary cell walls prevent the…
tracheids and vessels from collapsing
Air bubbles (embolisms) can disrupt the bulk flow of…
xylem sap
Prolonged hydraulic failure can cause…
plant death
What is transpiration regulated by?
stomata
Leaves have high ______, which maximizes ______
SA:volume ratio
photosynthesis (but also water loss!)
Stomata (sing. Stoma)
are flanked by 2 guard cells, which control the diameter of the stomatal pore
When guard cells take in water from neighbouring cells, they become…
more turgid
Uneven cell wall thickness causes the guard cells to…
bow (becoming bean shaped)
When water is lost the guard cells become…
flaccid & the pore closes
Change in turgor is a result of…
reversible absorption and loss of K+
Accumulation of K+ inside the guard cell lowers the…
water potential, forcing water inside the guard cell (H+ leaves the cell)
Loss of K+ ions from the guard cell raises the…
water potential of the guard cell, forcing water out of the guard cell (H+ enters the cell)
Accumulation/loss of K+ both facilitated by active transport of H+ ->
results in a gradient and generation of a membrane potential which drives K+ in/out of the cells
Opening/closing of stomata regulated by what?
circadian rhythms and plant hormones (ex. ABA produced is produced during water shortages)
Most large leafed plants are what?
tropical
Most smaller leafed plants are what?
temperate
Most very small leafed plants occur where?
in harsh habitats
What are limitations in architecture due to?
finite energy to devote to shoot growth
What does Active Transport allow during transpiration?
root cortex cells to absorb and store essential minerals (K+, nitrates)
