Transcript Leaf structure and function and stomata and leaf energy balance

Leaf structure and function and stomata and
leaf energy balance
Objectives of the lecture:
1. To describe the anatomy of leaves in relation to leaf
function and some variability between plant
types.
2. Describe the structure of stomata and control of
stomatal opening.
3. Define the energy balance of leaves.
Text book pages:
215-216,
797-798,
803.
:
Figure 23-8
Recall ...
Figure 36-11
Simple leaves have a
petiole and a single blade.
Compound leaves have
blades divided into leaflets.
Species from very cold or
Doubly compound leaves are
large yet rarely damaged by wind hot climates have needle-like
leaves.
or rain.
Blade
Petiole
Figure 36-12
Opposite leaves
Whorled leaves
Alternate leaves
Rosette
leaf vein (one vascular
bundle inside the leaf)
xylem
cuticle of upper epidermis
phloem
UPPER
EPIDERMIS
Water and
dissolved
mineral ionsDiagram of a dicot leaf
move from
roots into
stems, then
into leaf vein
(blue arrow)
PALISADE
MESOPHYLL
SPONGY
MESOPHYLL
LOWER
EPIDERMIS
Products of
Photosynthesis
(pink arrow)
enter vein and
are transported
to stems, roots)
Oxygen and water
vapor escape
from the leaf
through stomata
cuticle-coated
cell of lower
epidermis
Carbon dioxide from
the surrounding
air enters the leaf
through stomata
one stoma
(opening
across the
epidermis)
Tomato leaf, dicotyledon, C3 plant
Upper epidermis
Palisade parenchyma: chloroplasts
visible around cell periphery
Longitudinal section through a vascular
bundle
Xylem vessel: annular thickening
around cell wall
Phloem
Bundle Sheath
Spongy parenchyma
Lower epidermis
Leaf cross section of Zea mays (corn),
monocotyledon, C4 plant
Bulliform cells
Upper epidermis
Xylem
Bundle sheath cells
with chloroplasts
Parenchyma with
chloroplasts
Phloem
Lower epidermis
Leaf of a dictyledon
Coleus leaf cleared of cell contents and with xylem stained
Typically veins are
distributed such that
mesophyll cells are close
to a vein.
The network of veins
also provides a
supportive framework
for the leaf.
Leaf of a monocotyledon plant
The major venation follows the long axis of the leaf and there are numerous joining cross
veins so that, as with the dicotyledon, mesophyll cells are always close to a vein.
Leaf cross section of a conifer, Taxus (yew)
The needle is broad, but has only one
vascular bundle
The mesophyll is differentiated into palisade and spongy layers
Figure 10-21
Leaf surfaces contain stomata.
Leaf surface
Guard cells Pore
Stoma
Carbon dioxide diffuses into leaves through stomata.
Interior of leaf
O2
H2O
Leaf surface
Photosynthetic Extracellular
cells
space
CO2
Stoma
Structure of stomata
Epidermal cell
Guard cell
Nucleus
Stoma
Vacuole
Thickened wall
Chloroplast
Physiological control of stomatal opening and closing
Guard cells actively take up K causing water to enter by
osmosis. The guard cell’s walls are unevenly thickened
causing the cells to bow as they becomes turgid
Variation between species in stomatal control:
isohydric, maintains constant leaf water potential, maize, poplar;
anisohydric, leaf water potential decreases during day, sunflower, barley.
The energy budget of foliage
Radiation
input
Some radiation is
reflected and some
energy is re-radiated
If Tleaf > Tair
then the leaf
warms the air
Wind speed
and
leaf shape
The leaf boundary layer is
important in controlling heat
exchange and transpiration
Only 1-3% of
radiation is used in
photosynthesis
Evaporative cooling
of the leaf depends
upon latent heat of
evaporation
Factors affecting
transpiration
Transpiration flux, g H2O/cm2 leaf surface/second X10-7
3.0
Wind speed influences
transpiration
2.5
The boundary layer around a leaf
extends out from the leaf surface.
In it air movement is less than in
the surrounding air. It is thick in
still air, and constitutes a major
resistance to the flux of H2O from
the leaf.
2.0
1.5
A slight increase in wind speed will
reduce the boundary layer, and
increase transpiration.
1.0
Further increase in wind speed
may reduce transpiration,
especially for sunlit leaves,
because wind speed will cool the
leaf directly
0.5
Stomatal aperture, m
Thermal images of non-transpiring
leaves of sycamore and oak.
Conditions during measurement:
wind speed 0.6 m s-1,
air temperature 30.2 oC,
photo flux density 910  mol m-2 s-1
Laboratory measurement of transpiration
A laboratory potometer
1. Fill the potometer by submerging it – make sure there are no air
bubbles in the system.
2. Recut the branch stem under water and, keeping the cut end and the
potometer under water, put the cut end into the plastic tubing.
Figure 36-13
Grown in shade
Grown in sun
Leaf plasticity in response to variation in light:
Sun leaves are smaller in area (~0.5-0.6) than shade leaves
Sun leaves have 1.5 to 2.2 leaf mass/area than shade leaves
Sun leaves have up to 1.5 the density of stomata than shade leaves
Sun leaves have more Rubisco per unit chlorophyll
Sun leaves have less chlorophyll per reaction center
Coastal redwood
Sequoia sempervirens
Plasticity in foliage in relation to
water deficits
Ability to transport water to ~125m
depends upon wood structure
Reiteration of foliage from
existing branch structure
Koch et al. 2004. Nature 428, 851-854
In Taxus caespitosa and other conifers stomata are arranged in rows
Stomata with guard cells
Figure 37-16
Oleander
Adaptation
of a
xerophyte
Cross section of oleander leaf
Epidermis
Palisade
mesophyll
Waxy cuticle on
upper surface
of leaf is
especially thick
Vascular bundles
Air space
Stomata
Spongy
mesophyll
Epidermis
Epidermis
Epidermal hairs
Stomata are located in “crypts”
instead of on flat leaf surface
Things you need to know ...
1. The anatomy of leaves and variations between dicotyledons,
monocotyledons and conifers.
2. What a stoma is and UNDERSTAND how stomatal opening is
controlled and what effect it can have on transpiration.
3. Basic aspects of leaf energy budget. UNDERSTAND what the
components are and how they can be affected by environmental
variation in radiation input, air temperature, and wind speed, and
leaf shape.
4. What is meant by leaf plasticity and how it can be a response to
variation in light conditions and leaf water status.