Water-Plant Relations (Exam 2) Flashcards
water is the
universal solvent
why is water the universal solvent?
& therefore (2)
bc water is polar
& therefore has partial charges (no net charges) (solvent for other polar molecules & ions)
& allows for hydrogen bonding (approx 4 kJ/mol) (tensile strength)
what explains water movement through X?
tensile strength
what leads to tensile strength
cohesion
cohesion =
water liking itself
adhesion =
water liking others
water moves from
a higher potential energy to a lower potential energy
water moves… (2)
- downhill w/ gravity
(U = mgh) - “up” from bodies of water
water potential aka
“U”
water movement depends on
overall water potential (U) energy
U in water > U in air
water potential energy (U) is usually measured in
units of pressure
units of pressure we saw
- Bars (1 bar = 14.5 psi)
- MPa (megapascals) (1 MPa = 10 bar)
plant cell pressure is measured in
MPa (megapascals)
components for liquid water (4)
- ) H = Hydrostatic Pressure (P)
- ) Π = Osmotic Pressure (solutes)
- ) ρ(of w)gh = Pressure due to gravity
- ) Matric pressure (H2O interactions with surfaces)
Π (Osmotic Pressure) eqtn
Π = RT(Ec) E = sigma
as solute conc increases (2)
- Π (Osmotic Pressure) increases
- U (water potential energy) decreases
in trees, P =
(-)
this is why trees don’t bleed
clay soil…
small particles
“holds” H2O best
sandy soil…
big particles
“drains” H2O best
Ψsoil vs Ψroot
Ψsoil > Ψroot (for root to take up H20)
Water potential energy (U) equation
Ψ = P - Π + ρ(of w)gh + Matric Pressure
P increases with
depth
how does water move up the plant?
plants let physics do the dirty work
from leaves, water…
evaporates
why does water evaporate from the leaf?
humidity in the leaf almost = 100%
air is rarely over 50%
stomata on leaves open for
therefore
CO2 uptake,
therefore water loss in inevitable
in the xylem, water exists as
a column
tensile strength (due to cohesion) of water allows…
…water to be “pulled” up the plant through the X
what happens when transpiration > supply?
= cavitation
soil consists of
particles & open spaces (pores)
soil pores are typically occupied by
which has
air & water,
which has dissolved inorganic nutrients
availability of water in soil depends on
the interactions of water with the soil
(availability of water) sand…
…dries quickly, but when wet, most of the water is available to the plant
(availability of water) clay…
…holds water longer, but less of the water is available to the plant
what determines the water potential in soil?
unless…
hydrostatic pressure & interactions with the surfaces (matrix pressure) determine water potential (Ψ/U) more than solutes (unless soil has very high concentrations of salts)
soil water =
hygroscopic (solid particles)
roots take up
capillary water
field capacity =
max amount of water held by soil
roots take up most of their water through
&…
root hairs
& mycorrhizae can take up water too
can leaves take up water?
yes, through fog
water is not
actively absorbed
which means no energy is expended
water moves into the plant if…
Ψsoil > Ψroot
when Ψsoil < Ψroot
water can actually flow out of the plant
usually changes in the root prevent this
how do roots prevent water form flowing out of it
casparian strip
kill cortex
lignify epidermis
water in the X is
“pulled”
water that evaporates is
replaced on cell walls
what allows water to be pulled?
the cohesive properties of water
tension
tension =
(-) hydrostatic pressure
the theory of water movement in plants =
evaporation-cohesion-tension theory
