Leaf Morphology Tree Biology-2012

Leaf Morphology Outline

• Structure & Function Review

– Function

– Epidermis, mesophyll, vascular bundles, stoma

• Environment & Physical Variations

– Stoma density & climate change

– Leaf movements

– Extrafloral nectaries

– Intra-individual variation

– Evergreen vs. Deciduous

– Pigmentation

Leaf Morphology and Photosynthesis

• Water retention

– Cuticle of wax/fatty acids

– Stoma on lower leaf surface

– Lower surface trichomes

• Capture sunlight

– Flat, broad surface

– Concentrate photosynthetic

cells and chloroplasts

towards upper surface

Leaf Morphology and Photosynthesis

• Capture CO2 /Release O2

– Leaf air spaces

– Stoma

• Reduce Heat

– Reflective surfaces or leaf

movements

– Evaporation

– Convection

• Keep leaf surface clean

– Wax and drip tips

Internal Leaf Morphology

Leaf Venation--Science Friday

Stomata

• Represent 1% of leaf surface area

• Usually only lower epidermis

• Poplars (Populus) and alders (Alnus)

tend to have stomata on both surfaces.

• Large deciduous trees may loose

40,000 liters of water through stoma in

a summer

• Stoma open in presence of light

• Close in excess heat, high CO2, or

drought stress

Climate Change and Stomatal Density

• Pinus flexilus – Fossil pack rat middens

– 15,000-12,000 years ago

• 30% increase in CO2

• 17% decrease in stoma

• 15% increase in water use

efficiency

• Salix herbacea – Two intervals of CO2 variation

over 140,000 years

– Stomatal density track

atmospheric CO2

– Fewer stoma when CO2 densities

are high

V

U

T

How many tree rings should be found at “V”?

1

2

3

4

Diurnal Leaf Movements Common in Legumes

• Response to heat or water stress – Paraheliotropism—leaves

move in response to high temperature and direct sunlight to reduce incident light;usually the leaves will orient vertically

– Diaheliotropism—leaves track the sun and tend to remain perpendicular to the angle of incident light

Morning

Noon

• Thigmonasty-leaf closure as a response to touch

Diurnal Leaf Movements Very common in Legumes

Extrafloral Nectaries extrafloral= outside of the flower

Catalpa speciosa

Extrafloral Nectaries—Populus deltoides

Intra-tree Leaf Variation

Red Mulberry-Morus rubra

Sassafras albidum

Heterophylly—having different forms of leaves on the same plant

Intra-tree Variation Sun vs. Shade Leaves

Quercus bicolor

Sun leaves will have

a thicker cuticle,

thicker palisade mesophyll,

but the leaves will be

significantly smaller

Intra-tree Variation Sun vs. Shade Leaves Abies balsamea

Evergreen vs. Deciduous

• Evergreen leaves are more abundant in southern states and the tropics

– Produce a durable leaf with heavy investment in making the leaf long-lived

• Drip tips

• Thick cuticle

• Lots of vascular tissue

Evergreen vs. Deciduous

• Deciduous leaves

– Thin, low investment to durability

– Shed on annual basis

– Winter has the upper hand

– Protect apical meristems by scales and trichomes

– Take time to recover important nutrients from leave (i.e., nitrogen)

Dry Tropical Forests Exception to Evergreen Rule

Trees lose leaves during dry season.

Leaves of Mountain Tops and Bogs short growing season, nutrient poor, and often dry

Thick cuticle,

Epidermal trichomes

Remain covered in heart of winter

Why is the North Eastern U.S. Known for Great Autumn Color?

• Species composition

– Diversity as a mixed hardwood forest

• Climate

– Distinct seasons

• Autumn weather

– Bright sunny days

– Cool nights

Leaf Pigments

Carotenoid

Chlorophyll

Anthocyanin

Tannins--Brown

• Chlorophyll is an unstable molecule (Greens)

– Sensitive to light and heat

– Continually produced during summer

– Hydrophobic tail in thylakoid; hydrophillic antennae near stroma

• Carotenoids are long-lived, stable molecules (yellow & orange)

– Always present

– Hydrophobic accessory pigments in thylakoid

• Anthocyanins—purple and red

– Seasonal variation in abundance

– Hydrophillic-vacuolar

– Color variation related to small changes in pH

– Protective function

Anthocyanins

• Phenolics-flavonoids

• Red, blue, purple

• Water soluble and found in vacuoles

• Color of many fruits

• Influenced by pH, Fe/Al, and sugar availability

Anthocyanins & Carotenoids Fall Colors

Why fall color?

• Byproduct of senescence

• Sunscreen against high light intensity and UV light

• Anti-oxidant protection for cells and membranes

• Minimizing damage from drought and frost

• A coevolved signal to minimize insect infestation

Anthocyanins in Spring /Summer Leaves

Why are young leaves red?

Also note the orientation of the youngest leafs—significance?

Why invest anthocyanins in Young and Senescing Leaves?

Many Young Tropical Tree Leaves are Red

Why invest anthocyanins in Young and Senescing Leaves?

• Anthocyanins are excellent antioxidants

• Sunscreen protection – Membranes,

– Photosynthesis pigment centers

– DNA

• Most brilliant in intense sunlight

• May also protect from insect attack—insects don’t see red

Leaf Abscission

• Triggered by short days & long nights

• Leaves produce gaseous hormone—ethylene

• Stimulate the formation of an abscission zone

– Deposition of suberin

– Small cube shape cells in separation layer

– Vascular tissue is plugged

Conditions Favoring Great Fall Color

• Nutrient poor sites

• Species of tree

• Bright sunlight

• Cool temperatures

• Why these conditions?

Sunlight is necessary to make anthocyanins.

Stress induced early color change.

Why is the left side of this tree

delayed in turning color?

1

2

3 4

5

6

7 8

Fall color along a branch of Nyssa sylvatica in Athens Georgia.

Explain this pattern.

Sunlight and Chlorophyll

are necessary for

anthocyanin production