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Transport systems in plants
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come to the lecture!!
This lecture is about the
transport systems that
make the flows n this
figure manageable.
Water has to flow around
the plant, especially up
from ground water to
leaves.
Less obviously, sugars
and nutrients flow down
from leaves to storage
sites (mainly roots).
Sugars to ‘archive’
captured solar energy,
nutrients rescued from
The magnitude of the problem…
A single 15m high maple tree has been calculated to lose
220L of water per hour through its leaves. (A 50 gallon
drum = 189 L). How does such a volume of water get
supplied?
The tallest trees are c 110m high – how does water get so
high up? If you try to suck a column of water up by
mouth (or pump) it fails above c. 10m, leading the early
scientists to propose that nature abhors a vacuum.
The original solution (tracheids)
Sometime in the Palaeozoic, before the Silurian (440MYBP) and
extraordinary event took place. A new cell type appeared in an
early moss-like plant.
This cell was the tracheid, and it was characterised by a long thin
shape with tough, often lignified cell walls (making it
sclerenchyma). This had 2 effects:
1: It allows for long-distance transport of water and nutrients, so
roots and leaves can be well separated
2: It provides structural support, sometime that aquatic plants have
never needed. Tracheids set the scene for the invasion of land by
plants.
Tracheids are stiff, water-conducting cells.
They are covered in pits, holes, which permit
water conductance. The cell contents are
usually dead, having re-inforced the cell wall
with spiral deposits of lignin to reinforce the
‘hozepipe’.
pits
Lignin spirals
(not DNA!!)
Tracheids allowed plants to reach up towards the light,
to form multi-layered canopies (for better light
utilisation).
This allowed for the development of a branching,
independent sporophyte phase – in non-vascular plants
the sporophyte is smaller than the gametophyte and
often nutritionally dependent on it.
Now for the 1st time the sporophyte phase could
produce tall, multiple spore-dispersing structures.
Once the evolution had occurred, tracheophytes came
to dominate all but the wettest and most inhospitable
places.
Tracheids lie along side other tracheids, over-lapping
extensively, so that water can flow out of the pits of one cell
into an adjacent cell. This allows long range transfer of
water and solutes, although (since the cells are dead) the flow
has to be passive, pulled by an external force.
Water Flow (passive flow)
The driving force for this flow is hydrostatic pressure, coming
partly from root pressure (pushing up wards) but mainly from
the suction pressure created by water being evaporated from
leaves. Passive water flow in plants is upwards.
Tracheids are found in pteridophytes and
gymnosperms.
Since they are the water-conducting vessels
of the tallest trees in the world (the ancient
conifers the redwoods, Sequoia), they must
be a fundamentally good design!
Despite this, angiosperms (except Amborella) have evolved a
modification of tracheids which appear to be better engineering,
in as much as they approximate more closely to the human
design of hosepipe! These are the vessel elements, which are
also dead (sclerenchyma) lignin-reinforced cells, but instead of
overlapping these lie head-to-toe, with water-tight walls and
permeable plates (perforation plates) at each end.
Vessel elements, idealised
Vessel element, here with a open end (simple perforation plate).
A perforated
(scalariform)
perforation plate
Tracheids
Phloem and Xylem
If you understand these two transport systems you have got the
basics of fluid transport in plants. All vascular plants have both,
though the cell form can vary (tracheids or vessel elements).
Xylem is composed of dead, hollow cells (sclerenchyma) and is
passive – it carries the transpiration stream upwards from roots to
leaves.
Phloem cells are living and are used to transport sugars amino-acids
and relocated minerals, typically from leaf to roots. In other words
the opposite direction to xylem. Sugary solutions are actively
pumped into the phloem from Source Cells and pumped out by Sink
Cells.
These are held side-by-side in linear complexes called vascular
bundles.
The operation of Xylem
Xylem is dead hollow sclerenchyma cells which move water up the
plant by transpiration – tension – cohesion.
Transpiration: loss of water from the leaves (mainly through holes in
the leaf surface called stomata.
Tension – the tensions that arise in the water columns of a leaf are
transmitted down the water-bearing vessels of the whole tree. (This
would fail if there were any large air bubbles, as the tension exerted
may be many atmospheres). Dissolved gasses can come out of solution
under this tension, and tree physiologists can put sensitive listening
devices on tall trees in hot weather and hear tiny cracking noises as
each microbubble appears).
Cohesion – the water column remains cohesive, transmitting the
tension.
Pressures on xylem
flow in a tree:
Evaporation. This imparts a suction
pressure than puts immense tension
onto the water columns in a truck –
trees shrink in diameter measurably
on a hot sunny day.
Guttation –
drops on
leaf tips
Roots push upwards. This can
cause guttation (dripping of water
from leaves resulting from active
pumping), esp on damp mornings.
The elevation attained is rarely
more than a couple of metres.
(Trees can drip sap in early
spring).
Experiments on xylem flow
The German botanist Strasburger showed that 20m high trees if
stood in tubs of poison (picric acid) would transport this to the
tree top. Clearly this wasn’t pumped!
Poison still sucked up to the top of the tree
Poison
sap
We can measure the tension in a twig using a
pressure bomb: when sap is forced out of the
twig the pressure in the sealed container =
pressure in its xylem. Values are high enough to
raise water 100m.
High pressure
Xylem contains 2 types of cells (only 1 in gymnosperms):
Tracheids (explained earlier – the only tracheary tissue in
gymnosperms) and vessel elements. BOTH have annular/helical
lignin thickening. Vessel elements are confined to angiosperms.
Tracheary elements (conducting sclerenchyma)
Shape:
Tracheids
long, thin
vessel elements
short, wide
End:
pointed
ends flat
Perforations: none
one at each end
Softwoods (conifers) –
tracheids only
Hardwoods – note the larger
bore of the vessel elements
Water uptake by roots into xylem
Root hairs adhere strongly to fine soil particles and are hydrophilic,
allowing water to enter root tissue by diffusion. It flows from the root
hair through the root cortex both by apoplastic and symplastic routes.
This means that water heading towards the plant’s main water conduits
has undergone very little ‘quality control’ by living plant tissue.
Evolution has put a barrier in the way, ensuring that only purified water
(that which has passed through a symplastic route) enters the main
xylem/phloem channels.
Symplastic vs Apoplastic flow
Big words, easy idea. This concerns the path taken by water when
taken up from the soil. Does it go through cells’ cytoplasm, or in
the spaces between cells? This turns out to matter: apoplastic
means between cells, symplastic means going through cells’
plasma membranes.
Root hair
Root cortex
Symplastic flow
Apoplastic flow
This matters since the trans-membrane transport gives the plant
the ability to filter and regulate the composition of its fluids.
The flow of water into roots is controlled by a band of corky, waterimpermeable cells lining the root cortex which force water to flow into
the main vessels symplastically. This band of corky tissue (suberin +
lignin) is the casparian strip, and is present in the endodermis of the
root systems of most vascular plants.
The casparian strip ensures
that all water entering the
stele of the root (thence up to
the main stem) has passed
through a plasma membrane
so has been regulated by
transport proteins.
Casparian
strip
stele
Phloem
Phloem cells are involved in the active transport of sugars, amino
acids and other metabolites around the plant. Their operation is
very different to xylem: the main flow is downwards, and is
inhibited by metabolic poisons.
A simple if cruel experiment on this dates back to Malpighi around
1700. He girdled (ring-barked) trees and observed that the bark above
the cut swelled while the bark below died (as eventually did the tree).
This is because the cut stopped the downflow of metabolites in the
outer regions of the bark (where trees’ phloem is found).
We now know that phloem sap moves as fast as 1m per hour – too fast
for diffusion. Instead a form of bulk flow must be involved.
The model
explaining how
solutes are moved
around the phloem
is called the
pressure flow
model, and can be
explained by
consideration of
osmosis, applied to
solutions of two
sugar solutions
across a
semipermeable
membrane.
Water initially enters both ends by osmosis, but eventually the
hydrostatic pressure on the semipermeable membrane offsets the
osmotic pressure, stopping influx at the dilute (sink) end. The
pressure is greater at the top end (where conc is higher), effectively
pushing water into the conc end and out of the dilute end
Water
Initially
influx at
both ends
Conc sugar
(source end)
Dilute sugar
(sink end)
Water
Water
Net flow
of water
and
solutes
along
the tube
Membrane
bulges,
imposing
hydrostatic
pressure
Water
Phloem cells
Like xylem, phloem has 2 types of cells:
Sieve cells (elongate, sieve areas on all faces of cell) Collectively known
Sieve tube members (shorter, wider, stacked end-end as sieve elements
with sieve areas aligned)
Unlike xylem they remain alive to conduct water
and pump metabolites around the plant.
Called sieve elements because they are
interconnected by many relatively large pores.
Although alive they have the odd feature that the
nuclei have degenerated, and instead nuclear control
is supplied by adjacent cells called companion cells.
Leaf veins and their bundle
sheath cells
There is one group of companion cells which will turn out to be very
important for understanding the slightly odd photosynthesis route used
by C4 plants (more later…). These surround the veins in a plant leaf
and are called bundle sheath cells.
Upper epidermis
Mesophyll cells, with many
chloroplasts.
Bundle sheath cells, with few
chloroplasts.
Vascular bundle running along vein
Lower epidermis
To study the physiology of phloem one needs to extract liquids only
from the phloem tubes (the sieve tubes). How to do this?
It turns out to be almost impossible for humans to do, but easy for
aphids (greenflies), sap-sucking insects that unerringly insert needlelike mouthparts (a stylet) into sieve tubes. To sample phloem, let an
aphid start to feed than cut its head off!
Why do aphids select
phloem instead of
xylem?
Stylet
Secondary thickening
This process allows tree trunks to
widen every year, and explains the
growth rings in tree trunks.
In a typical tree trunk the phloem lies outside the woody core, in the
bark. Inside that is a layer of Collenchyma, then dead xylem tissue.
Once a year the collenchyma (which is a meristematic layer)
differentiates, producing a new layer of phloem on the outside and of
xylem on the inside. The xylem is laid down as new wood. The
majority of this happens rapidly in spring, leading to a thick band of
spring wood then a thin band of harder summer wood. The new xylem
cells lay down thick 2ndry walls, harden and die.
One tends to think of meristems as being the tips of a plant – shoot and
root tips – but there are also sheathing (lateral) meristematic tissues,
circling stems and roots. Most dicots, all gymnosperms, but very few
monocots undergo a widening process each year – this is secondary
thickening.
There are 2 lateral meristems involved: the vascular cambium ( which
produces new xylem = wood and secondary phloem) and the cork
cambium which makes a thick tough protective covering for stems and
roots.
Cork
Bark is everything outside
the vascular cambium,
Cork cambium
including phloem, cork
cambium and cork.
Secondary phloem
Vascular cambium
Xylem = wood
Key
C
Cambial cell
D Daughter cell
P Phloem cell
Xylem is laid down on the inside, hence
phloem always remains just under the
bark.
P
P
P
P
X Xylem cell
C
Stem centre
Production of 2ndry xylem
and phloem by the vascular
cambium.
D
P
P
P
D
P
P
P
C
C
C
C
C
C
C
C
D
X
X
X
D
X
X
X
The layers in secondary growth of a woody stem.
Primary phloem
Vascular cambium
Vascular cambium
Secondary xylem
Pith
Primary xylem
Primary xylem