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

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 ...

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

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

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 XylemUpper epidermis

Lower epidermis

Bundle sheath cells with chloroplasts

PhloemParenchyma with chloroplasts

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.

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 mesophyll is differentiated into palisade and spongy layers

The needle is broad, but has only one vascular bundle

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

Epidermal cell

Guard cell

Nucleus

Stoma

Vacuole

Thickened wall

Chloroplast

Structure of stomata

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

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

 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.

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

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

In Taxus caespitosa and other conifers stomata are arranged in rows

Stomata with guard cells

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

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.