Vascular plants and life on land Flashcards
Give an overview of the world’s terrestrial biomes
- tropical forest
- temperate forest
- boreal forest
- savanna
- grassland/shrubland
- tundra
- semi-desert/desert/ice
Which two climate axes help delineate the major biomes
- x: MAT (°C)
- y: MAP (mm)
MAT
mean annual temperature
MAP
mean annual precipitation
There is considerable diversity of plant life-forms in
- water-limited biomes
- e.g. Sonoran Desert, California, USA
List the principal plant life-forms in water-limited environments from high to low productivity
- ephemerals
- drought-deciduous shrubs
- phreatophytes
- evergreen shrubs
- leaf/stem succulents
Describe ephemerals
- short life cycle
- dormancy
Describe drought-deciduous shrubs
leaf shedding
Describe phreatophytes
deep-rooted shrubs and trees
Describe evergreen shrubs
year-round growth
Describe leaf/stem succulents
- shallow roots
- water storage
In drier climates, annual biome NPP is
an (almost) linear function of precipitation
Describe the principal plant water interaction
movement of water through the SPAC
SPAC
soil–plant–atmosphere continuum
Describe the movement of water through the SPAC
- movement from cell-to-cell across semi-permeable membranes is
osmotic (driven by differences in water potential) - movement through open conduits in the long-distance transport pathways, as well as through the soil, is in response to hydrostatic pressure gradients
What are the long-distance transport pathways of plants?
xylem and phloem
Describe the gradient across a plant
- soil pressure difference
- root water potential difference
- xylem pressure difference
- leaf air spaces water vapour concentration difference
pressure difference
delta-psi-p
water potential difference
delta-psi-W
water vapour concentration difference
delta-c-W-V
Give the equations for water potential
- psi = (muw - muw0) / Vw
- psi = P - pi
- psiW = psip + psis
water potential
- psi
- units of pressure
muw
chemical potential of water in the observed state
muw0
chemical potential of pure water
Vw
partial molal volume of water
P
turgour pressure
pi
osmotic pressure
psip
pressure potential
psis
solute (or osmotic) potential
1MPa =
- 10 bar
- 10atm
- 7.6mHg
Describe exploration of the soil by roots
mining for water and nutrients below ground
soil is a
complex, heterogeneous phase
How does water move into a plant?
- via the root hair or epidermal cell
- osmotically
- across a cell membrane
Describe the three pathways for movement into and across a root
- Apoplastic pathway
- Symplastic pathway
- Transcellular pathway
apoplastic pathway
through cell walls only, up to endodermis
symplastic pathway
via cell interior and plasmodesmata
transcellular pathway
across walls and membranes
Describe movement of water through a plant
- radially across the root cortex to the stele
- longitudinally through the xylem
stele
vascular tissue
xylem
tracheary elements
Describe flow blockage through the apoplast
hydrophobic deposits (principally lignin) in the endodermis and (sometimes) exodermis
What are a plant’s essential above-ground requirements
- sunlight
- CO2
What are a plant’s essential below-ground requirements
- H2O
- N
- K
- Ca
- Mg
- P
- S
- Si
- Cl
- Fe
- B
- Mn
- Na
- Zn
- Cu
- Ni
- Mo
What is the ratio of molybdenum to hydrogen requirement in a plant
1:60,000,000
Describe long-distance transport in the xylem
- primarily water and dissolved mineral ions
- water column is under tension
- water is in metastable state; vulnerable to cavitation
What is tension
negative hydrostatic pressure
cavitation
air seeding
Describe the tensile strength of water
great!
Describe the development of xylem elements
- procambial cells form phloem tissue, xylem parenchyma, fibres etc.
- mesophyll cell dedifferentiates and elongates
- undergoes secondary wall deposition
- undegroes apoptosis to form a mature treachery element
wood
secondary xylem
Describe the evolutionary origins of wood
- Early Devonian (~ 400 Ma)
- early tracheophytic lignophyte
- pyritised specimens with euphyllophyte affinities sim. to Psilophyton
- thick-walled tracheids in cortex interpreted as wood (occasional anticlinal divisions typical of lignophytes)
- single-walled spaces interpreted as rays (not preserved)
- possible remains of vascular cambium
- small size of specimens suggests early function of wood more likely associated with efficient water conduction rather than mechanical support
Describe fluid flow in xylem elements
- depends strongly on r of conduit
- described by Hagen–Poiseuille equation
radius
r
Give the Hagen–Poiseuille equation
volume of flow rate per tube Jv = (pir^4 / 8eta) dP/dx
The Hagen-Poiseuille equation is analogous to
Ohm’s law
For a given cross-sectional area, volume of flow rate per tube Jv =
(r^2/8eta) dP/dx
Tracheids are typical of
- gymnosperms
- some basally diverging angiosperm lineages
- many monocots
Vessels are typical of
most eudicots
What are the possible mechanisms of long-distance solution transport in the xylem?
(3) Root pressure
(4) Cohesion–tension(–adhesion) theory:
* high tensile strength of water (cohesion–tension) * strong forces of adhesion to hydrophilic surfaces * but note vulnerability of fluid under tension to
cavitation (embolism)
Give Fick’s laws of diffusion:
te = Le^2 / 4Ds
te =
time
Le
distance
Ds
diffusion coefficient of molecule (typically 10^-9m2s-1)
Give the equation for capillarity rise
(1.49x10^-5m2) / radius
Describe root pressure
- positive pressure in the xylem can result from active pumping of solutes from living, metabolically active root cells into the xylem, followed by osmotic influx of water
- responsible for guttation of water from hydathodes along leaf margins
Why isn’t root pressure valid?
- gravitational force on water column is 0.01 MPa m−1 or 0.1 MPa (i.e. 1 bar) for every 10 m
- typical root pressures = 0.01 to 0.03 MPa
- root pressure could only raise water column by 1 to 3m
Give an example of xylem sap under positive pressure
exudation of xylem sap from maple trees
Describe exudation of xylem sap from maple trees
- mobilisation of sugars into the xylem in early springtime to support bud break
- commercial use for xylem sap harvested in NE North America
embolism
cavitation
Describe the structure of a Gymnosperm pit membrane
torus-margo
Describe the structure of an angiosperm pit membrane
homogenous
Describe embolism and pit membranes
when adjacent tracheary elements are both water-filled, pit membranes are not subject to significant pressure differentials
When does embolism occur?
in response to drought stress
What happens if tension increases?
Ψ becomes more negative
Describe the propagation of air-seeding in embolism-sensitive species
if one tracheary element (on the right side) becomes embolised, and the tension increases in the adjacent fluid-filled element (on the left side), the pressure differential causes the porous pit membrane to deflect, but at a critical point the capillary seal gives way, allowing air-seeding to propagate
What are the features of pit membranes that prevent air seeding?
- increased size of the pit torus (gymnosperms)
- elaborate vestures
- thickness (angiosperms)
Describe some xylem vulnerabilities
- occlusion by microbes, e.g. Xylella fastidiosa
- occlusion by secretions e.g. tyloses, resins
- cavitation
air seeding occurs on
increasing tension
What can a plant do to cavitated elements?
refill them
Describe Sequoia sempervirens
- ## 113m high giant redwood
physiological limits of tree height…
~ 125 m
which factors affect the physiological limit of tree height?
- leaf density
- CO2
- water
- photosynthetic efficiency
Describe transfusion tracheids in cycads
Contrast with branched vascular anatomy in angiosperm leaves
Describe the relationship between both vein density and vein diameter with leaf area
- scale allometrically (logged)
Describe the relationship between vein density and leaf area
Vein density decreases with leaf area
Describe the relationship between vein diameter and leaf area
Vein diameter increases with leaf area
Describe the scale independence of finest (minor) veins
ensures that the entire leaf is hydraulically well connected in angiosperms
Describe water transport through the leaf
Water vapour exits the leaf, and carbon dioxide enters the leaf, by the same pathway
The “conquest of the land” by plants was largely
the conquest of a highly desiccating atmosphere
The higher capacity for water transport endowed by wide-diameter xylem vessels and a higher density of venation in leaves may have contributed to the evolutionary success of angiosperms.
