Two transport systems in plants
Xylem – transport water from soil to leaves. Under tension.
Phloem – circulating sugars
Both are bulkflow phenomenon. Xylem is tension driven, as opposed to diffusion. Need to move
things faster than can diffuse over large plant volume
Xylem volume flow is about 100x that of phloem because so much water leaving system through
Zone of elongation – not so much cell division as elongation, turgor driven growth. In the zone of
elongation, vascular tissue is functional
Primary wall construction : fiber reinforced gel
Secondary wall formation: inside primary wall.
Rigid tubes? Why? Tubes need to resist the tension that is pulling the water up a chain of
molecules from soil to leaves. So, how does that force generate?
In photosynthesis lecture, we thought about water status as relative humidity. But if want to really
understand transport in plant, need another way of thinking about availability of water
Initially, pressure is same inside and outside, but concentrations are different. So evolves to
pressure isotonic, and concentrations same if have an elastic membrane.
But if add a cell wall – cell wall resists being stretched by water trying to enter cell so only a little
water enters, this basically squeezes water, raising pressure of water, and system evolves to where
pressures and concentrations are not the same inside and outside. Instead, pressure difference
between in and out is proportional to concentration difference. This is described by RT (R= gas
constant, T= temperature). [but really temperature that is important].
This can be rearranged: water potential.
98% RH = 2.8 MPa (mega paschal) 0.2 MPa turgor (positive pressure) – 3 MPa (osmotic potential
= RTC, concentration of sugars, salts, etc)
Water flows to where it’s more negative
Xylem water is superheated fluid – should remain in a metastable liquid state if there is no pre
existing vapor phase for it to boil into.
If living in a dry environment, need to be able to support high tensions in xylem.
Problems from metastability in water column?
In bryophytes, pipes can collapse when system dehydrates. And when system rehydrates, they
regain their shape. Not particularly rigid. But not in trachiophytes. If tension becomes so great that
tubes implode, those tubes are destroyed in trachiophytes. This also provides support for plant axis
Airseeding – if an air bubble gets in, it will expand and form air embolism
Freezing (see slide)
So, a “designed leak” that preserve structural integrity of plant through airseeding is better than the
tubes collapsing – you may lose water transfer capacity but retain structural capacity.
Not just bryophytes that do collapsing and reexpanding – a Podocarp also does this. Cavitation: it’s a failure of airwater meniscus that causes the air bubble to get in. Smaller the radius
of curvature, larger the holes, but doesn’t take into account the gel phase of the membrane. We
don’t understand particularly well fracture in gels.