Plant physiology
Lecture 1 water and transport
Water has a high polarity
Ions and polar molecules dissolve easily
H- bridges: adhesion and cohesion for transport
Water potential
Potential energy of water/ability to perform mechanical work
Osmotic potential Ψs = pressure resulting from the property of water to move from
hypotonic to hypertonic solutions
Inside plants Ψs always < 0, in pure water Ψs = 0
Hydrostatic pressure Ψp = physical pressure resulting from turgor (Ψp > 0) or tension caused
by cohesion of the rising water in a vessel (Ψp < 0)
Gravitational potential Ψg = pressure exerted by the position of the water column in the
field of gravity (Ψg > 0)
Ψw = Ψs + Ψp + Ψg
Ψs = - (n/V) · R ·T (MPa)
Pa = J m-3 = N m-2
Temperature does not have a big effect – T in K (large changes in temperature in C is very
small when converted into K)
Be mindful of salts! For NaCl, 2x number of moles given
1
, Ψg = ρw · g · h
ρw = density of the liquid (in kg m-3) = 0.998 · 103 kg m-3 for water
g = Gravitational acceleration = 9.8 m s-2
h = height (in m)
5% smaller volume = 50% decrease in turgor pressure!
Driver of water transport from the roots, through the xylem, to the leaf is the difference in
Ψp. Exception: transport driven by root pressure
If Ψw is too low, the evaporation resistance is increased by (partly) closing the stomata
(stomatal conductance gs and leaf conductance gl decrease)
Opening of guard cells due to an increase in Ψp and asymmetrical enforcements of the cell
walls
Effect of transpiration on the water potential in a leaf depends strongly on the conductance
of the entire plant Annuals have a much lower resistance (wider vessels) than, for
example, conifers – more difficult to resupply water (more strongly negative leaf water
potential)
Xylem vessels have a large diameter and as such very little resistance against water
transport, but are small enough to have a capillary function
Plants can reach great heights (Ψg increases): Ψp works in the opposite direction as a result
of the evaporation in the leaves (water transport to the leaves is only possible if Ψp + Ψg <
0). Moreover, the resistance of the xylem must also be overcome.
In a tree that is 100 m tall, the potential difference between the roots and the highest
leaves can be as high as ca. – 3 MPa (30 bar!): Compare this to a bicycle tire (0.2-0.3 MPa)
In vines the thickness of the stem is limiting less xylem vessels larger diameter of
xylem vessels (do not make it too wide, as this would compromise adhesion, etc)
Due to the enormous forces on the water column in a plant, cavitation (emboly) can occur:
the water column breaks, and a xylem vessel fills with air: water transport stops
Root pressure can resolve the air bubbles in the xylem vessels: caused by active transport of
ions to the root xylem. ΔΨs increases between the cortex of the root and the vessels:
transport of water to the xylem vessels: Ψp increases: mass flow transport of water through
the xylem vessels to the leaves, decreasing – under influence of this pressure - the
cavitation bubbles (embolies)
2
, ! This only occurs when the humidity of the air is sufficiently high or when stomata are
closed, because evaporation at the leaves prevents the buildup of high ion concentrations in
the xylem.
Root pressure can also cause guttation formation of droplets at the edges of leaves
through hydathodes
Transport of water through the cortex happens through:
• the cytoplasm of cells (symplast)
• the cell walls (apoplast)
• However, transport to the vessels is always through the symplast of the endodermis –
the Casparian strip is impenetrable for water.
• Water uptake from sand is easier than from clay, but clay contains more water available
for uptake than does sand
• Uptake patterns differ: sometimes only at the root tip, other times along the entire
length of the root: depends on the extent of suberin deposition in the cell walls of the
exodermis water repellent
Need for water
Diameter of xylem vessels
Decreasing strength of wood
Uptake of water from the soil:
3
, • Through the root surface
• Root hairs increase surface area
• Passive flux
• Depends on the continuity of the H-bridges in the soil
• Depends on the matrix potential of the soil particles = their affinity to bind water through
adhesive and capillary forces (Ψp soil)
Tutorial 1
Cell wall: provides turgor/ hydrostatic pressure
In a non-turgescent cell in a plant, the hydrostatic pressure in calculating water potential is
not important, as there is so little water (no pressure!)
Hydrostatic/turgor pressure (Ψp) component of water potential is most strongly affected
compared to a plant with sufficient water
In a plant cell surrounded by a cell wall that has been placed in pure water for some time,
holds Ψp = approx. Ψ-s (use equation!)
Transport of water through xylem vessels is mainly caused by a difference in hydrostatic
pressure between the upper (more negative) and lower end of the vessel
Lecture 2 nutrients
Nitrogen
Needed in large quantities: photosynthesis, apparatus and proteins
4
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