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”?
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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?
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2
3 4
5
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Fall color along a branch of Nyssa sylvatica in Athens Georgia.
Explain this pattern.
Sunlight and Chlorophyll
are necessary for
anthocyanin production