Weber State University          Department of Botany

Autumn Leaf Drop

There are several ways to classify woody perennials
Example:  growth habit:  trees and shrubs

Another way: leaf drop habit:  evergreen vs. deciduous
    deciduous:  replace all leaves yearly.  All leaves are the same age.
    evergreen:  rate of leaf turnover is slower, from 2 years to >30 years (bristlecone pine)

Generally find the evergreens in places with longer winters, higher altitudes, northern latitudes.  Most (but not all) conifers, some angiosperms have the evergreen habit.

Among the evergreens, find features of leaf morphology and anatomy that decrease desiccation and snow holding (and therefore decrease limb damage from weight of snow)
    thin, aerodynamic shape
    thick cuticle to conserve water
    endodermis (internal tissue layer with wax in the cell walls) to conserve water

Deciduous:  lose all leaves at once (fall), grow new leaves in the spring
    broad, flat leaves =>  excellent solar collectors for photosynthesis, but the shape increases desiccation and snow holding.  Therefore, drop the leaves before winter.
    thin cuticle; no internal wax layer to conserve water
                
Programmed Cell Death (PCD)
    there is a series of steps gone through, in order, as leaves change color and drop from the trees  (other examples of PCD:  formation of aerenchyma, development of tracheary elements)
    ↓chlorophyll (veins last)
    ↓proteins and nucleic acids
    ↓organelles:  chloroplasts first, nucleus last
The tree scavenges materials, esp. N and organic C, from the leaves before leaf drop.  The materials are stored in the parenchyma cells of the wood and bark over the winter.  The vascular tissue (transport route) and nucleus (instructions) must be maintained until the salvage operation is over.  The abscission zone forms as the mobilization of leaf material ends.

Colors seen in leaves

yellows and oranges ==>  carotenoids:  carotenes and xanthophylls; fat-soluble pigments found in plastids
chlorophyll synthesis ↓ as existing chl is destroyed and not replaced; start to see the photoprotective carotenoids

reds and purples ==>  anthocyanins; water-soluble pigments found in vacuoles
    production increased with dry, sunny days and cool (not freezing) nights (need sugars to synthesize the anthocyanins)
    located in cells of the palisade layer; cells in leaves with anthocyanins experience less damage than cells in yellow leaves  (the antioxidant properties of anthocyanins).  As a consequence of less cell damage, leaves with anthocyanins are more thorough savengers than leaves without them.

Various environmental factors affect the intensity of the colors and duration of the fall color show:  temperature, moisture.

Trigger to initiate PCD ==>  increasing night length due to seasonal changes in photoperiod
The ratio of two forms of phytochrome essentially tells a plant how long it has been in the dark.

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