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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.
:
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Figure 23-8
Apical meristems are located atspecific points throughout the body.
Close-up of a shoot apical meristem
Shootmeristems
Root meristems
Developing leaves
Rapidly dividing, undifferentiatedmeristematic cells
Cells differentiating into ground tissue
Cells differentiating into vascular tissue
Cells differentiating into epidermal tissue
Recall ...
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Figure 36-11
Simple leaves have apetiole and a single blade.
Species from very cold orhot climates have needle-likeleaves.
Doubly compound leaves arelarge yet rarely damaged by windor rain.
Compound leaves haveblades divided into leaflets.
Petiole
Blade
Figure 36-12
Opposite leaves Alternate leavesWhorled leaves Rosette
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Diagram of a dicot leaf
one stoma(openingacross theepidermis)
cuticle-coatedcell of lowerepidermis
PALISADEMESOPHYLL
SPONGYMESOPHYLL
Carbon dioxide from the surroundingair enters the leaf through stomata
Products ofPhotosynthesis(pink arrow)enter vein andare transportedto stems, roots)
Water anddissolvedmineral ionsmove fromroots intostems, theninto leaf vein(blue arrow)
Oxygen and water vapor escapefrom the leafthrough stomata
xylem phloem
leaf vein (one vascularbundle inside the leaf)
UPPEREPIDERMIS
LOWEREPIDERMIS
cuticle of upper epidermis
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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
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Leaf cross section of Zea mays (corn), monocotyledon, C4 plant
Bulliform cells XylemUpper epidermis
Lower epidermis
Bundle sheath cells with chloroplasts
PhloemParenchyma with chloroplasts
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The network of veins also provides a supportive framework for the leaf.
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.
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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.
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Leaf cross section of a conifer, Taxus (yew)
The mesophyll is differentiated into palisade and spongy layers
The needle is broad, but has only one vascular bundle
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Figure 10-21Leaf surfaces contain stomata.
Carbon dioxide diffuses into leaves through stomata.
Guard cells Pore Stoma
StomaCO2Extracellularspace
Photosyntheticcells
O2
H2O
Leaf surface
Interior of leaf
Leaf surface
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Epidermal cell
Guard cell
Nucleus
Stoma
Vacuole
Thickened wall
Chloroplast
Structure of stomata
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Physiological control of stomatal opening and closing
Variation between species in stomatal control:isohydric, maintains constant leaf water potential, maize, poplar;anisohydric, leaf water potential decreases during day, sunflower, barley.
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
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The energy budget of foliage
Only 1-3% of radiation is used in photosynthesis
Evaporative cooling of the leaf depends upon latent heat of evaporation
Some radiation is reflected and some energy is re-radiated
Radiationinput
If Tleaf > Tair then
the leaf warms the air
Wind speedand
leaf shape
Factors affecting transpiration
The leaf boundary layer is important in controlling heat exchange and transpiration
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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.
Stomatal aperture, m
Tra
nsp
iratio
n fl
ux,
g H
2O
/cm
2 le
af s
urf
ace
/sec
ond
X10
-7
0.5
1.0
1.5
2.0
2.5
3.0Wind speed influences
transpiration
Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly
A slight increase in wind speed will reduce the boundary layer, and increase transpiration.
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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
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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.
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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
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Koch et al. 2004. Nature 428, 851-854
Reiteration of foliage from existing branch structure
Plasticity in foliage in relation to water deficits
Ability to transport water to ~125m depends upon wood structure
Coastal redwood Sequoia sempervirens
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In Taxus caespitosa and other conifers stomata are arranged in rows
Stomata with guard cells
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Figure 37-16
Cross section of oleander leaf
Oleander
Palisademesophyll
Epidermis
Spongymesophyll
Epidermis
Epidermis
Epidermal hairs
Epidermis
StomataAir space
Vascular bundles
Waxy cuticle onupper surfaceof leaf isespecially thick
Stomata are located in “crypts”instead of on flat leaf surface
Adaptation of a
xerophyte
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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.
2. What a stoma is and UNDERSTAND how stomatal opening is controlled and what effect it can have on transpiration.
Things you need to know ...
4. What is meant by leaf plasticity and how it can be a response to variation in light conditions and leaf water status.
1. The anatomy of leaves and variations between dicotyledons, monocotyledons and conifers.