Transcript Plant Biology: Roots and shoots

Plant Biology: Roots and shoots
Peter Shaw
This is the jpg-free version, for rapid downloading.
For pretty pics – come to the lecture! (Or ask me for the source
file, if you have a pressing excuse…)
Introduction
Today we are going to look at the morphology and
functions of the structural elements within plant
bodies: shoots to support leaves, and roots for
anchorage and uptake. Notice that (unlike
photosynthesis or cell anatomy) these details are
specific to the vascular plants, and do not apply to
bryophytes etc. No tracheids, no mention in this
lecture!!
Shoots
These are supportive tissues, evolved to raise leaves and/or
flowers above ground level.
(let’s expand on that and/or. Can you think of any plants
with a stalk for the flowers but not the leaves?
This is quite common, ie dandelions, asphodels…
A stalk to support the leaves but ground-level flowers?
Much harder – Aspidestra has slug-pollinated ground
level flowers (but no true stem). Some tropical forest
climbers (passion flowers) have flowers at ground level.
Aspidistra (cast iron plant) flowers, which lie
on the ground and are slug pollinated.
Terminal bud with
apical meristem
Axillary bud
An internode
A node
Cotyledons
Hypocotyl
Basic stem
anatomical terms
Buds
Stems bear buds, an embryonic shoot. Typically in a leaf axil, these
contain meristematic tissue, and any bud is able to take over if the
buds above it are mechanically removed. The only reason lateral buds
do not shoot out is hormonal suppression from other more active buds.
(More in a later lecture). Buds are the protected way that leaves
overwinter, most obviously but not only in deciduous species.
Sometimes these buds are not obvious: in the cactacea the leaves and
flowers come out of common buds, but these are known as areoles. A
cut cactus will resprout from an areole. (There is one genus,
Mammillaria, where the flowers emerge between the areoles, although
internally they still derive from bud tissue).
Potatoes
We all know potatoes, Solanum tuberosum. Most of us know their
Vavilov centre (=genetic home, where highest diversity is found:
The Andes of course.) We know that the green shoots are toxic due
to an heat-resistant alkaloid called solanine. (This is serious –
never eat potato leaves or sprouts, and best avoid green tissue
where a spud has sat in the light).
But how would you classify the tuber, the bit that you eat? The
obvious answer is a root, but roots don’t make lateral buds, which is
what potato eyes are, and in fact potato tubers are modifies stems!
Bulbs, rhizomes etc
Many plants find the soil a convenient safe place to store
energy in swollen tissues. We are all familiar with bulbs –
in fact these are accumulations of swollen leaf bases,
safely out of harms way many cm below the soil surface.
These can be remarkably water-retentive, many bulbs
surviving a year without water. Think of onions (But
never ever confuse ornamental bulbs with onions!)
Rhizomes are different. They make buds that
emerge as new shoots, so must be stems. In plants
like ginger or bracken the rhizome acts like a storage
root. In many grasses and strawberries they become
runners, creeping over the soil and putting down new
roots at intervals.
Monocot/dicot stem crosssections
In dicot stems
the vascular
bundles lie in a
ring
In monocot stems the
vascular bundles are
scattered throughout
the stem.
Roots
These organs are traditionally said to have two functions:
1: structural anchorage, providing the mechanical support that
enables the plant to put up its shoots,
2: To act as a nutrient uptake surface, collecting water and
minerals from the soil solution.
As a mycologist I must add a third function:
3: To support infections by mycorrhizal fungi.
(It is true that most plants will grow perfectly well without these
fungi. It is also true that very few wild plants actually do live
without these beneficial infections).
Types of root
system
Tap roots – deep fat roots
capable of nutrient/water
storage. Lateral roots
emerge from it. In
biennials is often edible,
accessing the energy
stored as starch.
Tap root
laterals
Fibrous roots –
numerous thin
roots, about the
same diameter.
Typical of grasses,
also
rhododendrons.
Tuberous roots –few,
short stubby roots.
Useless for nutrient
uptake! (These plants
rely heavily on their
mycorrhizal fungi, shown
here as
). Bulbs,
orchids, nonphotosynthetic parasitic
plants.
Root hairs: these
greatly increase SA
for uptake. 1 rye
plant has a root SA
of 600m2!
Vascular tissue
Root apical meristem
Root cap. Here cells are continuously
sloughed off by soil abrasion. This protects
the apical meristem and (along with mucilage)
lubricates the roots passage through the soil.
(This is also the area which senses gravity)
Endodermis
Epidermis
Stele (holds
vascular
tissue)
Cortex
Zone of cell
differentiation
Zone of cell
elongation
Zone of cell division
General root TS
Stele, containing phloem
and xylem embedded in
pericycle cells.
Endodermis. These cells contain
suberin to resist water flow,
forcing it to pass symplastically.
Cortex
Epidermis
The layout of the vascular tissues within the stele differs
between monocots and dicots. In dicots the xylem forms a star
shape (# points variable but 4-6 is typical) with phloem filling
out the intervening spaces. In monocots the central spce is
occupied by pith, with the vascular tissue in a ring.
Xylem
Phloem
Pith
c
Eudicot root
Monocot root
The apical meristems in roots give rise to lateral meristems –
side roots. These form part of the secondary body of the
plant, along with the two lateral meristems which allow tree
trunks to expand (the vascular cambium and the cork
cambium). Roots give rise to more roots, but not generally to
shoots.
Roots can pop out of odd places, such as high up on stems. These are
called adventitious roots, and are what allow gardeners to strike
cuttings.
On a larger scale, adventitious roots can act as stilts, supporting tree
trunks. The sacred banyan tree (actually a strangler fig) Ficus
benghalensis uses its adventitious roots to support the branches, and
eventually can form a covered, stilted mini-forest! The Indian prince
Gautama Siddhartha is said to have attained enlightenment under such
a Banyan tree in Bodhgaya, India. (It’s still there, albeit as a rooted
cutting).
Screwpines
(Pandanus), which
lean on their
adventitious roots.
Noteworthy roots
A few bulbous plants go to remarkable lengths to
protect their bulbs. Bluebells (Hyacintoides nonscripta) have contractile roots which pull their bulbs
deeper into the soil after germination. Mature bulbs
are often >20 deep despite have germinated at the
surface!
Epiphytic plants (the moth orchid Phaloenopsis is a
good example) have roots that adhere strongly to
their substrate, allowing them to grow high up on
tree trunks or stone surfaces. The degenerate
bromeliad Tillandsia usneoides (‘Spanish moss’)
makes a nuisance of itself by hanging off telegraph
wires!!
Mycorrhizal roots
Many plants have roots which are so short fat and stubby as t appear
useless for nutrient uptake. That’s because they are useless!
The work is done by paid helpers – mycorrhizal fungi. This trait has
evolved many times (>=5), and often results in roots utterly smothered in
hyphae.
Here we see the short, fat,
dichotomously branched tips
of a tree’s ectomycorrhizas.
Mycorrhizas
Root hairs also get infected by mycorrhizal fungi, which act as
extensions of the root system and collect nutrients (especially P) for the
plant. This is the normal state for most wild plants. In exchange the
plant supplies sugars to the fungus. (This sugar drain can be a detectable
cost, if nutrients are supplied as chemical fertilisers).
The mycorrhizal condition has evolved at least 4 times, probably closer
to 10, and can be over-simplified by shoehorning mycorrhizas into 2
groups: Sheathing (ecto) mycorrhizas, which envelop roots in a coat of
hyphae, and endomycorrhizas which penetrate inside the cells inside a
hosts’ root.
In both cases the increase in uptake area is huge – as much as 3m of
hyphae from 1cm root. There are many trials showing how much better
plants do with mycorrhizal infection in poor soils.
TS of an ectomycorrhizally infected root
Varieties of:
What you actually see
Sheath
Hartig Net
(between root
cells)
VAM
Or endomycorrhizas
vesicles - these are large and stain well.
Arbuscules are harder - really needing
EM. It is not clear that all endos have these,
casting doubt on the name VAM.
Arbuscules die and are absorbed by the
plant, only to have new arbuscules re-form.
This seems to be how P is transferred to the
plant.
Parasitic plants
Not all vascular plants have functional (green)
chloroplasts. There is a polyphyletic group of
plants called obligate parasitic plants that
cannot photosynthesise, and instead rely on
The Maltese fungus,
other plants for their energy supply.
Cynomorium coccineum
Fungus rock, Gozo, Ma
Many of these are orchids, such as the
coralroot, Corallorhiza trifida here, but others
include toothwort Laethrea, Rafflesia,
Cynomorium coccineum.
Some of these parasitic are simple root-plug-ins (called
haustoria), physically inserting the parasite’s vascular
tissue into that of its host.
The western australian christmas tree Nuytsia floribunda
(a mistletoe relative – loranthaceae) is a root parasite
whose haustoria cut into tree roots. They are so effective
that they have been reported cutting underground wires! Nuytsia
floribunda
(Like all mistletoes it lacks root hairs).
Many other plants (especially in the family
Scrophulariaceae) are hemiparasitic, sucking water and
sugars out of neighbouring plants while continuing to
photosynthesise themselves.
Witchweeds, Striga
• Striga – the witchweeds (c. 40
species) - parasitic plants that plugs
its root system into the roots of
many crops
• Is endemic in Africa, where it
significantly stunts the yield of
commercial crops. Traditionally it
was said to ‘bewitch’ the crops into
poor yields, hence its name.
• S. asiatica was accidentally
introduced into fields in Carolina noted as a pest in 1950s, affecting
175000 acres. It has been almost
eradicated here by an expensive
herbicide regime.
Mycorrhizal parasites
You will find quaint old (and not-so-old) texts saying
that the coral root orchid, ghost orchid, birds nest
orchid are ‘saprotrophs’, meaning that they feed on
decaying organic matter. We know that this is just
plain wrong. Decomposer fungi are saprotrophs.
Orchids are mycorrhizal. It’s just that some of them
give nothing back to the mycorrhiza. In fact they plug
into the mycorrhizal network which links together
forest trees and act as a sugar-sink. The orchid lives
by parasitising trees via their ectomycorrhizas.
Corallorhiza,
the coral-root
orchid. The
coralling
roots are
highly
infected by
mycorrhizal
fungi.
Orchid mycorrhizas
• These are weird!
• If a science fiction writer produced this
story, it would be rejected as being too
implausible. In fact it’s true.
• Orchids are notable for several things,
including production of vast amounts of
tiny seeds.;
• These seeds are almost invisible - too
small to carry useful reserves of energy.
So how do orchid seeds manage?
• They need to be infected by a decomposer fungus. Quite a variety
will do - Rhizoctonia is one of the commoner, but some really nasty
pathogens are used - Armillaria, Fomes, Coriolus.
• These fungi should kill the seed. In most cases they probably do. In
some cases, the seed ends up parasitising the fungus!
• Hyphae enter the plant cell, but are attacked and digested by the
orchid.
• This is the plant’s sole source of nutrients and energy until a leaf is
produced, 2-8 (15) years later.
• Some orchids never make green leaves, are purely parasitic on their
fungi.
This means that orchids can colonise
over large distances, and appear as if
by magic years after actually
colonising.
I have introduced soil + seeds,
fungus to sites in 1991 - some orchid
leaves appeared in 1995, while some
flowers appeared de novo 1996 1999. They had been there all the
time, invisible under the soil.