1. 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: , ,
803. :

2. Recall ... Figure 23-8 Apical meristems are located at
specific points throughout the body.
Close-up of a shoot apical meristem
Shoot
meristems
Developing leaves
Rapidly dividing, undifferentiated
meristematic cells
Cells differentiating into ground tissue
Cells differentiating into vascular tissue
Cells differentiating into epidermal tissue
Root meristems

3. Opposite leaves Whorled leaves Alternate leaves Rosette Figure 36-11
Simple leaves have a
petiole and a single blade.
Compound leaves have
blades divided into leaflets.
Doubly compound leaves are
large yet rarely damaged by wind
or rain.
Species from very cold or
hot climates have needle-like
leaves.
Blade
Petiole
Figure 36-12
Opposite leaves
Whorled leaves
Alternate leaves
Rosette

4. cuticle of upper epidermis
leaf vein (one vascular
bundle inside the leaf)
UPPER
EPIDERMIS
LOWER
cuticle of upper epidermis
xylem
phloem
Water and
dissolved
mineral ions
move from
roots into
stems, then
into leaf vein
(blue arrow)
Diagram of a dicot leaf
PALISADE
MESOPHYLL
SPONGY
MESOPHYLL
Products of
Photosynthesis
(pink arrow)
enter vein and
are transported
to stems, roots)
cuticle-coated
cell of lower
epidermis
one stoma
(opening
across the
epidermis)
Oxygen and water vapor escape
from the leaf
through stomata
Carbon dioxide from the surrounding
air enters the leaf through stomata

5. 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

6. Leaf cross section of Zea mays (corn), monocotyledon, C4 plant
Upper epidermis
Lower epidermis
Bulliform cells
Xylem
Phloem
Bundle sheath cells with chloroplasts
Parenchyma with chloroplasts

7. 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.

8. 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.

9. 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

10. Leaf surfaces contain stomata.
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
cells
Extracellular
space
CO2
Stoma

11. Structure of stomata Epidermal cell Guard cell Nucleus Stoma Vacuole
Thickened wall
Chloroplast

12. 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.

13. The energy budget of foliage
Radiation
input
Some radiation is reflected and some energy is re-radiated
Wind speed
and
leaf shape
If Tleaf > Tair then the leaf warms the air
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

14. Wind speed influences transpiration
Stomatal aperture, m
Transpiration flux, g H2O/cm2 leaf surface/second X10-7
0.5
1.0
1.5
2.0
2.5
3.0
Wind speed influences transpiration
 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.
A slight increase in wind speed will reduce the boundary layer, and increase transpiration.
Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly 

15. Thermal images of non-transpiring leaves of sycamore and oak
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

16. 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.

17. Leaf plasticity in response to variation in light:
Figure 36-13
Grown in shade
Grown in sun
Leaf plasticity in response to variation in light:
Sun leaves are smaller in area (~ ) 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

18. Plasticity in foliage in relation to water deficits
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 Nature 428,

19. Stomata with guard cells
In Taxus caespitosa and other conifers stomata are arranged in rows
Stomata with guard cells

20. Adaptation of a xerophyte
Figure 37-16
Oleander
Adaptation of a xerophyte
Cross section of oleander leaf
Waxy cuticle on
upper surface
of leaf is
especially thick
Epidermis
Vascular bundles
Palisade
mesophyll
Air space
Stomata
Spongy
mesophyll
Epidermis
Epidermis
Epidermis
Epidermal hairs
Stomata are located in “crypts”
instead of on flat leaf surface

21. 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.